Characterization and measurement of textile fabric properties

2 Characterization and measurement of textile fabric properties A CAUSA and A NETRAVALI, Cornell University, USA

Abstract: In the first part of this chapter ASTM methods are discussed covering the tensile, stiffness, bending and tear testing of woven, knitted and nonwoven fabrics. Special attention is given to the following items, viz., (a) the modes of deformation in fabrics under tensile, bending and tearing loads; (b) Treloar’s pioneering research on the behavior of fabrics under shear; (c) the Trapezoid (wing tip) test that allows the measurement of the tear strength of fabrics with no substantial deviation of the tear initial direction, and (d) the Kawabata Evaluation System (KES) that covers a series of tests especially designed to study the mechanical behavior of fabrics under small strains, pertinent to apparel applications. The second part of this chapter covers the basic mechanics and predictive testing of composites reinforced with fibrous assemblies, in either stiff or soft (elastomer) matrix. The study of these composites is a very important area of materials science and engineering with a vast range of applications. In this case, in addition to the properties of the reinforcement, either two- or three-dimensional, the matrix and their interphase, the engineer must consider the critically important geometrical parameters, such as the reinforcement lay-out angles and the ply stacking sequence. Three-dimensional fibrous assemblies have been shown to improve the damage tolerance to the delamination mode of failure. This chapter demonstrates that the study of composites requires a multidisciplinary approach. Key words: fabric test methods, Kawabata Evaluation System, mechanisms of deformation of fabrics, composites reinforced by fabric assemblies (two- and three-dimensional), basic mechanics of laminates – application in predictive testing.

2.1

Introduction

In the first part of this chapter a detailed description and interpretation is provided of the ASTM test methods comprising tensile, stiffness (bending) and tear testing of woven, knitted and nonwoven fabrics. In the case of shear testing, we emphasize Treloar’s pioneering work on the effect of the testpiece dimensions on the behavior of fabrics under shear as well as reference to Hearle’s detailed analysis of the forces involved in this test. Some additional observations deserve special attention, viz., (a) the Trapezoid (wing rip) test to measure the tearing strength of fabrics featuring no deviation (or decreased deviation) of the tear direction, starting with 4

Characterization and measurement of textile fabric properties

5

the initial slit or cut made on the fabric at the start of the experiment; (b) Leaf’s equation, derived by an analytical (closed-form) solution, that opens the possibility of calculating the ‘true’ shear modulus from the bending moduli in a woven fabric described by the classical Peirce’s geometrical model, and (c) the Kawabata Evaluation System (KES) covers a series of tests especially conceived to evaluate the mechanical behavior of fabrics in the domain of small strains pertinent to apparel applications. The KES system represents an important step towards the engineering design of fabrics. The second part of the chapter deals with the fibrous assemblies as reinforcement of composite structures including testing techniques and experimental characterization. The study of composite structures reinforced by fibrous assemblies is a very important and advancing area of materials science and engineering, with extensive applications in aerospace, land and sea transportation, sporting goods, civil infrastructure and biomedical products. Within the transportation industry, there is an important and often forgotten area of fiber-reinforced composites, namely, the pneumatic tire (car, truck, aircraft, off-the-road tires) and various types of conveyor belts. A polymer matrix composite, reinforced with a fibrous assembly is commonly an anisotropic material and its response to stresses is more complex than that of an isotropic material. There are also important differences between stiff vs. soft (e.g. elastomer) matrix composites. In these composites the testing as well as the interpretation of the results is more complex. We have now to consider, besides the reinforcement and matrix properties, the reinforcement/matrix interface and interphase, and the critically important geometrical parameters, such as the reinforcement layout angles and the ply-stacking sequence. Under dynamic testing conditions it is important to indicate, besides the usual factors (temperature, oxygen concentration, humidity, etc.) whether the experiment is run under stress, strain or energy control, and the type of waveform used, that is, sinusoidal, pulse, or an arbitrary waveform derived from field experience. Laboratory built composite laminates (‘coupons’) can be effectively used to study the degradation of the composite properties under ‘real-world’ excitation, viz., mechanical, thermal and chemical loading. The measurement of damage can utilize changes in dynamic properties, spectroscopic techniques, microfractography and non-destructive evaluation techniques (X-rays, ultrasound, shearography, thermography, Moiré interferometry, electronic speckle pattern interferometry and others). In addition, laboratory studies must be integrated with the testing of the whole structure. Three-dimensional (3D) fibrous assemblies have been shown to improve the damage tolerance to the delamination mode of failure. The study of composites requires the input of many disciplines including mechanical engineering, reliability engineering, physics, fiber science and others. Both

6

Structure and mechanics of textile fibre assemblies

a deterministic approach (fracture mechanics, Arrhenius model) and a stochastic approach (S-N curves) can be successfully used. Experimental mechanics must be complemented by finite element analysis and whenever possible by closed-form, analytical solutions. Clearly, the study of composites requires a multidisciplinary approach.

2.2

Tensile testing of woven fabrics

A typical load-extension curve of a woven fabric in a tensile test is presented in Fig. 2.1. Careful examination of the generalized load-extension curve for a woven fabric reveals the presence of three distinct regions. Region 1, the initial part of the curve, is dominated by interfiber friction (usually very small), that is, the frictional resistance due to thread (yarn) bending. Region 2, a region of lower modulus, is the decrimping region resulting from the straightening of the thread set in the direction of application of the load, with the associated increase in crimp in the direction perpendicular to the thread direction. This is commonly referred to as ‘crimp interchange’. Region 3, the last part of the load-extension curve, is due to yarn extension, i.e., tensile loading of threads in the direction of stress. As the crimp is decreased, the magnitude of the loading force rises very steeply, and as a result, the fibers themselves begin to be extended. In summary, in this final region, the load-extension properties of the fabric (cloth) are basically governed by the load-extension properties of the threads or the fibers. This is clearly a region of higher modulus. During the actual test, geometrical changes in the fabric are commonly observed. Due to crimp interchange effects, length-wise loading causes a width-wise contraction. Contraction in width is greatest in the middle and

Yarn extension region

Load

3

Decrimping region 2

1 Interfiber friction effect Extension

2.1 Schematic of a typical load-extension curve for a woven fabric.

Characterization and measurement of textile fabric properties

7

decreases towards the jaws. The value of this contraction is dependent on the ratio of crimp in the warp and weft threads. In the region of the jaws the stresses in the specimen are high and could cause jaw breaks. There are two standard test methods for breaking force and elongation of textile fabrics, viz., The Strip Method, ASTM D5035-95 (re-approved 2003) and The Grab Test, ASTM D-5034-95 (re-approved 2001). The strip method covers raveled strip and cut strip test procedures for determining the breaking force and elongation of most textile fabrics. The raveled strip test is applicable to woven fabrics while the cut strip test is applicable to non-woven fabrics, felted fabrics, and dipped or coated fabrics. This test method is not recommended for knitted fabrics or other textile fabrics which have high stretch (more than 11%). In fabric testing, a ‘raveled strip test’ is a strip test in which the specimen is cut wider than the specified testing width and an approximately even number of yarns are removed from each side to obtain the required testing width. A ‘cut strip test’ is a strip test in which the specimen is cut to the specified testing width. Both types of test specimens can be used in either 25 mm (1 in) or 50 mm (2 in) widths, with a length of at least 150 mm (6 in). The longitudinal (long) dimension should be accurately parallel to the direction of testing or force application. When obtaining specimens from a roll of fabric certain precautions must be observed: (a) cut specimens with their long dimensions parallel either to the warp direction or the weft direction, or cut specimens for testing in both directions as required; (b) specimens for a given fabric direction should be spaced along a diagonal of the fabric to allow for even representation of different warp and weft yarns and (c) unless otherwise specified, take specimens no nearer to the selvedge or edge of the fabric than one-tenth of the width of the fabric. The rationale for this rule is that the fabric properties are different at the edge because of the different selvedge construction (weave) and they are no longer representative of the bulk. The tensile testing machines can be of either constant-rate-of-extension (CRE) or constant-rate-of-load (CRL) type and the values for the breaking force and elongation are frequently obtained from a computer interfaced with the testing machine. Most computers will have the necessary software to record the data and to perform calculations and run the test itself. During the execution of the test, it is critical to ensure that the specimen does not slip in the jaws, or break at the edge of or in the jaws. If these conditions cannot be eliminated by adjusting the pressure in the clamps, jaw cushions or specimen tabbing may be necessary. Verification of the total operating system (loading, extension, clamping and recording or data collection) by using specimens of standard fabrics is recommended. Comparison of results from tensile testing machines operating on different principles is not desirable. When different types of machines

8

Structure and mechanics of textile fibre assemblies

are used for comparison testing, constant time-to-break at 20 ± 3 s is the established way of producing the data. The test apparatus is designed for operation at a speed of up to 300 ± 10 mm/min (12 ± 0.5 in/min) and is capable to obtain the 20 ± 3 s time-to-break. The distance between the clamps (gauge length) is 75 ± 1 mm (3 ± 0.05 in). The grab method is quite different from the strip methods described above. The grab test uses jaw faces that are consistently narrower than the fabric specimen width, thereby avoiding the need to fray the fabric to width. Hence, this method has one major advantage over the strip method, namely, that the preparation of the specimens is simpler and faster. The specimen used is 100 mm (4 in) wide by 150 mm (6 in) long but the jaws are only 25 mm (1 in) wide. The result of this set-up is that only the central 25 mm of the fabric is submitted to the tensile stress field. However, it has been found experimentally that the stressed zone of the fabric between the jaws is somewhat reinforced by the fabric on either side. Consequently, the strength measured by this method is higher than for a 25 mm (1 in) raveled strip test. To ensure that the two jaws grip the same set of threads, a line is drawn on the fabric sample 37 mm (1.5 in) from the edge to assist in the proper alignment of the two jaws. Figure 2.1 shows a schematic of a typical load-extension curve for a woven fabric. We note that the specific shapes of these curves can be affected by the type of threads used in the fabric, fabric construction, rate of elongation, temperature and relative humidity conditions and the previous loading history of the test specimen. Standard ASTM testing conditions are 21 ± 1 °C (70 ± 2 °F) and 65 ± 2% relative humidity. Also, in most cases, conditioning of the specimens for a specific period prior to testing is necessary.

2.3

Stiffness (bending) testing of fabrics

In most applications fabrics undergo bending. One method of assessing the stiffness or flexural (bending) rigidity of a fabric is to determine the length of fabric that bends (deflects) a fixed distance under its own weight. It is an easy test to carry out and, as expected, it is called Cantilever Bending Test and described in ASTM D1388-96 (reapproved 2002) Standard Test Method for Stiffness of Fabrics and ASTM D5732-95 (reapproved 2001) Standard Test Method for Stiffness of Nonwoven Fabrics using the Cantilever Test. This cantilever test method applies to most fabrics including woven, knitted and nonwoven fabrics, either treated or untreated. However, it is not suitable for very limp fabrics or those that show a marked tendency to curl or twist at a cut edge. The basic terminology is the same as the one used in other test procedures, viz., (a) the cross-machine direction is the direction in the plane of the fabric perpendicular to the direction of the

Characterization and measurement of textile fabric properties

9

manufacture. It refers to the direction analogous to coursewise or filling (weft) direction in knitted or woven fabrics, respectively. (b) the machine direction is the direction in the plane of the fabric parallel to the direction of manufacture. It refers to the direction analogous to walewise or warp direction in knitted or woven fabrics, respectively, and (c) in nonwoven fabrics, the term machine direction is used to refer to the direction analogous to lengthwise direction in a woven fabric. In the cantilever bending test, a horizontal strip of fabric is held at one end and the rest of the strip is allowed to hang (bend) under its own weight, for a fixed distance. Peirce’s pioneering research [1, 2] produced very important results as expressed in the following equation: B = W × C3

2.1

where B is the bending (flexural) rigidity, W is the weight of the fabric and C is the bending length. Furthermore, Peirce found that when the tip of the specimen reaches a plane inclined at 41.5° below the horizontal, the overhanging length is then twice the bending length: C=

O 2

2.2

where C is the bending length in cm and O is the length of overhang in cm. Combining the two equations we get O B = W ×⎛ ⎞ ⎝ 2⎠

3

2.3

where B is the flexural rigidity, mg.cm and W is the fabric weight per unit area, mg/cm2. In the cantilever test a specimen, held at one end, is slid at a specified rate in a direction parallel to its long dimension, until its leading edge projects from the edge of a horizontal surface. The length of the overhang is measured when the tip of the specimen is depressed under its own weight to the point where the line joining the top to the edge of the platform makes a 41.5° angle with the horizontal. From this measured length, the bending length and the flexural rigidity are calculated using the above equations. A Cantilever bending tester consists of a horizontal platform that has a smooth low-friction, flat surface such as polished metal or plastic; an indicator, inclined at an angle of 41.5 ± 0.5° below the plane of the platform surface; a movable slide; a scale and reference point to measure the length of the overhang; a motorized specimen feed unit set to 120 mm/min (4.75 in/ min) ± 5%, and a cutting die to cut test specimens 25 mm by 200 mm ± 1 mm (1 in by 8 in ± 0.04 in). The long dimension of the specimen is considered as the direction of the test. The test procedure is simple. The movable slide is removed and the specimen is placed on the horizontal platform with

10

Structure and mechanics of textile fibre assemblies

the length of the specimen parallel to the platform edge. The edge of the specimen is aligned with the line scribed on the right-hand edge of the horizontal platform. The movable slide is now placed on the specimen, being careful not to change its initial position. The tester switch is turned on and the movement of the leading edge of the specimen is carefully watched. The switch is turned off the instant the edge of the specimen touches the knife edge. The overhang length is read and recorded from the linear scale to the nearest 1 mm (0.1 in). As stated earlier, the cantilever test is not suitable for very limp fabrics or those that show a tendency to curl or twist at the cut edge. For such fabrics stiffness may be measured by forming it into a loop and allowing it to hang under its own weight. In this test, a fabric strip of a certain length L has its two ends clamped together to form a loop. The undistorted length of the loop lo, from the grip to the lowest point, has been calculated in Peirce’s classical paper [1, 2] for three different loop shapes, i.e., ring, pear and heart shapes. The heart shape is the one recommended in this standard test and it is appropriately called the ‘Heart Loop Test’. If the actual length l of the loop hanging under its own weight is measured, the stiffness can be calculated from the difference between the measured and the calculated lengths d = l − lo. The test procedure includes tables that facilitate the calculations. The modes of deformation involved in woven fabric bending have been summarized very effectively in the following manner [3]. • • •

thread bending thread twisting → fabric shear thread mobility → fabric shear.

Bending (physical) ← → fabric hand (aesthetics) Stiffness is dependent on: • •

stiffness in warp and weft directions [1, 2] and quantities related to the torsional stiffness of the yarn [4].

When a fabric specimen is subjected to a bending cycle and the results are plotted in bending moment vs. curvature coordinates, the presence of a hysteresis loop is commonly noticed. Under low-curvature bending the hysteresis is attributed to the energy loss in overcoming the frictional forces. Under high-curvature bending the viscoelastic properties of the fibers (stress relaxation) must also be considered. It is interesting to relate the equation used in calculating the flexural rigidity of the fabric to the simple theory of bending applied to a cantilever beam. The fundamental bending formula is: MR = EI

2.4

Characterization and measurement of textile fabric properties

11

where M is the bending moment, R is the radius of curvature, E is the elastic modulus and I is the moment of inertia of the cross-section, also called the ‘second moment’ of the cross-section. The bending/flexural rigidity, B, is defined as the couple required to bend a structure to unit curvature, hence B = [ M ]R =1 = EI

2.5

The curvature of the bend is R−1. The sags or deflection of beams are calculated by integrating a differential equation that expresses the local deformation of a small element of the beam. Using this approach it is possible to derive the ‘differential equation of flexure of a beam’: M = EI

d2 y = EIy ′′ dx 2

2.6

where y″ is the curvature of the beam. The equation is valid only for small deformations and in reference to y-x coordinates where the neutral plane of the beam coincides with the x-axis when the beam is unloaded (unstressed). The distance along the beam (x-axis) is positive to the right side and the deflection is positive downward (y-axis). As an example, let a simple cantilever beam be subjected to an end force P under which it sags through distance δ. Within the elastic region, P is proportional to δ, and the work done by P is 1/2 Pδ which is the energy stored in the beam. The force P acts over a distance x from the end of the beam and the total length of the beam is l. Hence, the bending moment is Px and the curvature in the beam is: y ′′ =

Px EI

2.7

The stored energy, U, is given by: U=

EI l EI ⎛ P ⎞ ( y ′′ )2 dx = ∫ 2 0 2 ⎝ EI ⎠

2 l

P2l3

2 ∫ x dx = 6EI 0

2.8

The energy is now expressed in terms of the load and therefore it is in a form suitable for the application of Castigliano’s theorem: ∂U ∂ ⎛ P 2 l 3 ⎞ Pl 3 = ⎜ ⎟= ∂P ∂P ⎝ 6 EI ⎠ 3EI

2.9

which, by Castigliano, is the work-absorbing deflection under P, that is, the vertical deflection, δ, under P. It follows then that

δ=

Pl 3 Pl 3 Pl 3 , or B = = 3EI 3B 3δ

2.10

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Structure and mechanics of textile fibre assemblies

This equation offers a rationale for the equation used in the calculation of the bending rigidity of a fabric in the cantilever bending test, where force P is the weight of the fabric and the bending distance is fixed.

2.4

Fabric shear – testing concepts

Treloar [5] published an in-depth study on the effect of the test-piece dimensions on the behavior of fabrics under shear. Previous to his work experiments were carried out almost entirely with square test pieces and two important facts were identified, viz., (a) reliable measurements of the response of fabrics under shear stresses can only be obtained as long as buckling or wrinkling of the test specimen is avoided, and (b) the strain amplitude at which wrinkling begins was shown to increase with the tensile stress applied in a direction perpendicular to the direction of shearing. Treloar’s [5] study involved woven fabrics of cotton and viscose rayon and two specimen shapes, one square and the other one rectangular, and the measurements were carried out using dead weight loading and direct microscopical observation of the fabric deformation. It was concluded that the maximum shear strain which can be applied without the occurrence of wrinkling depended not only on the applied tensile stress but also on the shape of the specimen. He also concluded that this maximum increases as the ratio of length to width of the specimen is reduced. A length : width ratio of 1 : 10 is considered to be most suitable for general measurements. In summary, the shear characteristics were found to be sensitive to the shape of the test specimen, particularly at low values of the normal tension. This sensitivity is closely related to the reduction in the amount of wrinkling with the rectangular specimen. Treloar emphasizes the importance of this reduction in the wrinkling effect in the rectangular specimen, since it allows the experimentalist to make measurements at larger strain amplitude for a given value of the normal load or alternatively, for a given amplitude, to reduce the value of the normal load. Either of these alternatives is important in practice for two reasons, viz., (a) because of the pronounced nonlinear characteristics of fabrics in shear which make extrapolation from small-strain to large strain behavior unreliable, and (b) because the behavior under small normal loads is important in the study of the fabric’s drape. We note that the expression ‘length-to-width’ ratio may be confusing and for some readers it would be preferable to use Saville’s [6] statement, i.e., ‘the errors associated with the onset of wrinkling can be reduced by the use of a narrow specimen with a reduced distance between the clamps instead of a square one. A height : width ratio of 1 : 10 is considered to be the limit for practical measurements.’

Characterization and measurement of textile fabric properties

13

Hearle [7] has done a thorough analysis of forces involved in Treloar’s shear test thereby providing an explanation for the use of an effective shear force equal to F – W tan θ. Spivak [8] adapted Treloar’s test specimen for use in a standard tester with its automatic recording capabilities. Many shear stress-strain curves over a full cycle were obtained for cotton and viscose fabrics showing, as predictable, a hysteresis loop. As mentioned before, the area within the hysteresis loop represents the energy lost in overcoming the frictional forces generated at the intersection of warp and weft. Another important parameter to measure is the ratio of the energy loss to the total work done in shearing since this ratio represents the overall response of the fabric to in-plane deformation and recovery. Spivak and Treloar [9] made an interesting study of the effect of heat setting on the shear properties of a nylon monofilament fabric of plain weave construction, with particular attention to identify the effects of two methods of heat setting, namely, with or without dimensional changes. Microscopic observations of the actual contact area at filament crossovers were used in this study. It was concluded that the properties of a nylon monofilament fabric in shear are indeed influenced by heat setting, and that the largest reduction in values of hysteresis and resistance to shearing occurs when the fabric is heat set with free contraction rather than at fixed dimensions. In the former method, due to the contraction in the dimensions of the fabric, both relaxation of internal bending stresses and change of curvature of the filaments at crossover points do occur. Associated with this change in curvature there is a considerable reduction in contact area and forces at filament crossovers. Spivak and Treloar [9] have also introduced a new dynamic method of measuring the cyclical energy loss of the fabric in shear. In this method, the fabric is mounted between two clamps, the upper one being fixed and the lower one free. If the lower clamp is displaced to produce a shear strain, and then released, a series of damped oscillations in the plane of shear will result. Mathematically, this system can be treated as a simple pendulum undergoing a damped, simple harmonic motion. The length of the pendulum is the length of the fabric between clamps, and the mass of the pendulum is the mass of the lower clamp plus any additional weights firmly attached to it. Hence, the pendulum mass is equivalent to the normal stress used in previous studies of fabric shearing. Within the context of this investigation, general agreement was found between the loss values obtained by the dynamic and the static method. However, this area deserves further study. The bending properties of the filaments were found to be unaffected by the heat setting. Spivak and Treloar [10] investigated the relation between bias extension and ‘simple shear’ for plain-woven fabrics. Bias extension refers to extension of a specimen cut at 45° to the thread directions. Their study, compris-

14

Structure and mechanics of textile fibre assemblies

ing both theory and experiments, concluded that it is not possible to obtain the complete stress-strain properties of a fabric in shear from a test in bias extension. One important factor contributing to this test result is that while the normal stress is constant during the test in simple shear, the normal component in the bias direction test is continually changing. Hence, the stresses applied in the two types of test are not identical and it is possible that this could result in some differences in the frictional restraints at crossover points which could contribute to the observed difference. On the other hand, reasonable agreement between the two tests was obtained for the parameter ‘relative energy loss’, defined, as previously mentioned in this chapter, as the ratio of the energy loss (area inside the hysteresis loop) to the total work done. Leaf and co-workers [11–14] have developed, by closed-form solutions, equations for tensile, shear and bending moduli for plain woven fabrics under small deformations. This type of research is one aspect of the efforts towards the engineering design of fabrics, that is, to design fabrics that meet specific mechanical requirements. As an example, we would like to use the following equation 2 ( l − k2 Dθ 2 )2 12 ( l1 − k1 Dθ 1 ) = + 2 G B1 B2

2.11

which relates various mechanical moduli. The notation used is that of Peirce’s [1, 2] classical model, i.e., the suffixes 1 and 2 refer to warp and weft directions, respectively; B is the flexural rigidity; G is the shear modulus; q is the ‘weave angle’, the angle that the centerline of the fabric makes with the tangent to the thread centerline; l is the actual thread length and l – kDq is the length of the straight section, or in reference to the warp, l1 − k1Dq.1 (we must keep in mind that the yarn path exemplified, let us say by the warp path, consists of straight lines and arcs of circle when it contacts the weft). D is, of course, the sum of the diameters d1 and d2 of the warp and weft threads. At this point we would like to quote Professor Leaf’s actual words during his lecture at the Mt. Fuji Textile Research Symposium – In the New Millennium [11]: This is a particularly interesting equation. So far as I am aware, we do not have a simple method for estimating experimentally the shear modulus of a fabric. The methods used by Treloar [5] and by KES equipment can be criticized on the grounds that they do not produce in the test specimen a uniform stress distribution of the kind envisaged in the definition of shear. But the KES equipment, for example, does allow us to make reasonable estimates of B1 and B2. Does this equation form the basis of a method for estimating the real shear modulus of a plain-woven fabric?

We note that KES refers to the Kawabata Evaluation System which is briefly described later in this chapter.

Characterization and measurement of textile fabric properties

2.5

15

Tearing strength of fabrics

Tearing of the fabric is the tensile failure – either sequentially, in bundles (groups) or a combination of both – of the yarn set perpendicular to the direction of the propagation of the tear [3]. The two major factors that influence the tearing behavior of woven fabrics are yarn tensile strength, and yarn mobility within the structure. Figure 2.2 gives a schematic view of a fabric tearing and Fig. 2.3 depicts a typical result of a tear test in a plot of load in the vertical axis and the jaw separation (extension) in the horizontal axis. The mechanism of tear consists of two major steps, viz., (a) ‘Del’ formation as yarns pulled through interlacing, leading to jamming and slippage (Peirce’s Model), and (b) as yarn is being pulled through the structure,

‘Del’ region

Load

2.2 Schematic view of a fabric tearing.

Jaw separation

2.3 A typical result of a tear test.

16

Structure and mechanics of textile fibre assemblies

frictional forces develop until the breaking strength of yarn is achieved. The factors affecting yarn tear strength are yarn tensile strength, fabric thread count, thread mobility and surface finishes which may affect the frictional forces. The terminology ‘del’ region or zone is based in the similarity one notices between this region where yarns fail and the well-known ‘del’ operator used in vector calculus. Scelzo et al. [15] provided a detailed description of the process of tear. When a displacement force is applied at a constant rate, the longitudinal yarns (i.e., those gripped in the jaws of the testing machine) are gradually loaded, stretched, and begin to lose their crimp. The transverse yarns (i.e., those initially perpendicular to the direction of tear) must locally align themselves with the applied load to bridge the gap from one tail to the other. As the load continues to increase, the transverse yarns that bridge the gap between the tails are pulled into the del zone; this also draws the outer edge of the fabric inward. Tension develops in the del yarns as they are pulled out of the specimen tails, a result of inter-yarn friction and the bending rigidity of the yarn. Longitudinal yarns are drawn together towards the cut line, and the transverse yarns are pulled into a nearly vertical plane; the size of the del zone increases. The resulting crowding of the longitudinal yarns necessarily forms a large number of frictional points of contact in a small area of fabric adjacent to the del. This effect, along with the jamming that takes place ahead of the del (in the region of untorn fabric), makes it extremely difficult for further slippage to take place without producing high local stresses. High localized stresses will lead to yarn break. As each break occurs, there is a sudden shift from one del structure to another that is less elongated and at a lower load than the previous del. This process provides a rationale for the shape of the tear load versus extension diagram. Accompanying yarn failure, where the transverse yarns fail either singly or as small groups, there is a contraction of the tails. Snapback of the tails may be large enough to cause the breakage of additional yarns. Tearing strength is important in both industrial and clothing fabrics. As indicated in the above paragraphs, thread mobility is an important factor affecting tear strength since it will facilitate the grouping or buckling of threads during tearing and therefore improve the tearing resistance as more than one thread has to be broken at a time. This grouping of threads is made easier if the yarns are smooth and can slip over each other. Special fabric finishes such as some crease-resistant finishes, which cause the yarns to adhere to one another, may reduce the tearing strength. The effect of the weave structure is also evident. Thus, a twill weave allows the threads to group better than a plain weave; hence, a twill weave will exhibit better resistance to tearing than a plain weave. Textured fabrics inhibit thread movement and reduce thread grouping and tear strength.

Characterization and measurement of textile fabric properties

2.6

17

Test methods for fabric shear

There are three important standard test methods to measure the tearing strength of fabrics, namely: 1. The tongue test, also known as the single rip or trouser test, ASTM D 2261-96 (re-approved 2002); 2. The trapezoid test, also known as the wing rip test, ASTM D 5587-96 (re-approved 2003) and 3. The falling-pendulum test procedure, ASTM D 1424-96. In addition, for nonwoven fabrics the following tests may be used. 1. ASTM D 5735-95, Tearing strength of Nonwoven Fabrics by the Tongue (Single rip) Procedure, (re-approved 2001). 2. ASTM D 5733-99, Tearing Strength of Nonwoven Fabrics by the Trapezoid Procedure. 3. ASTM D 5734-95 Tearing Strength of Nonwoven Fabrics by Falling Pendulum (Elmendorf Apparatus), (re-approved 2001). The major difference between these tests is in the geometry of the test specimen. Furthermore, while the tongue and trapezoid tests are conducted using a recording constant-rate-of-extension type (CRE) tensile testing machine, the falling-pendulum test requires a pendulum type ballistic tester, such as the Elmendorf tear tester. A brief discussion of these tests follows.

2.6.1 Tongue (single rip) test This test method applies to most fabrics including woven fabrics, knit fabrics, air bag fabrics, blankets, napped fabrics and pile fabrics. The fabrics may be untreated, heavily sized, coated, resin-treated, or otherwise treated. Nonwoven fabrics can also be tested by this procedure. Tear strength, as measured in this method, requires that the tear be initiated before testing. The reported value obtained is not directly related to the force required to initiate or start a tear but to propagate a tear. This test concept – notched test specimens – is quite similar to the one extensively used in experimental fracture mechanics of engineering materials, including elastomers and fiberreinforced composites. Two calculations for tongue tearing strength are provided: the single-peak force and the average of the five highest peak forces. The values stated in either SI units or inch-pound units are to be regarded as the standard. The inch-pound units may be approximate. The specimen in this method has a rectangular shape, with a cut in the center of a short edge to form a twotongued (trouser shaped) specimen (Fig. 2.4), in which one tongue of the

Structure and mechanics of textile fibre assemblies 75 mm (3 in)

Specimen cutting template

75 mm (3 in)

18

200 mm (8 in)

2.4 Template for cutting and making tongue tear test specimens (all tolerances ±0.5%).

specimen is gripped in the upper jaw and the other tongue is gripped in the lower jaw of a tensile testing machine. The rectangular test specimens 75 mm by 200 mm (3 in by 8 in) are cut using a die or a template and a cut to start the tear process is 75 mm (3 in) long, starting at the center of the 75 mm (3 in) width. From each fabric, five specimens are taken from the machine direction and five specimens from the cross-machine direction. The machine direction is defined as the direction in the plane of the fabric parallel to the direction of the manufacture. This term is used to refer to the direction analogous to lengthwise or warp direction in woven fabrics. The crossmachine direction is the direction in the plane of the fabric perpendicular to the direction of manufacture. This term is used to refer to the direction analogous to crosswise or weft (filling) direction in woven fabrics. During the test the separation of the jaws is continuously increased to apply a force to propagate the tear. At the same time, the force developed is recorded. The force to continue to propagate the tear is calculated from the classical autographic chart recorders, or in the modern laboratories from a dedicated computer provided with the required software. The tearing force is displayed in the form of peaks and valleys (see Fig. 2.3). The highest peaks are thought to reflect the strength of the yarn components (woven fabrics), fiber bonds or fiber interlocks (nonwoven fabrics), individually or in combination, needed to stop a tear in a fabric of the same construction. The peaks that are seen on the load-extension curve (N-mm) are more often from the breaking of a group of threads than from the individual ones. It is interesting to point out that a similar tear morphology is observed on the fracture surface of rubber and it is called stick-slip tear. At the start of the test, the distance between the clamps is set at 75 ± 1 mm (3 ± 0.05 in) and the testing speed can be set at either 50 ± 2 mm/min (2 ± 0.1 in/min) or 300 ± 10 mm/min (12 ± 0.5 in/min). Many experts prefer the higher speed based on the observation that in the ‘real world’ many tears do occur quite rapidly. The test specimen is secured in the clamp jaws with the slit edge of each tongue centered in such a manner that the origi-

Characterization and measurement of textile fabric properties

19

nally adjacent cut edges of the tongues form a straight line joining the centers of the clamps and the two tongues present opposite faces of the fabric to the operator. After the crosshead has moved to produce approximately 6 mm (0.25 in) of fabric tear, the single-peak force or multiple-peak forces are recorded. The crosshead motion is stopped after a total tear of 75 mm (3 in) or if the fabric has torn completely. If the fabric slips in the jaws or if 25% of the specimens break at a point within 5 mm (0.25 in) of the edge of the jaw, then the jaws must be modified to avoid these situations. If after making the modification(s), 25% or more of the specimens still break at a point within 5 mm (0.25 in) of the edge of the jaw, or if the specimens do not tear substantially lengthwise, then the fabric should be considered untearable by this test method. If the tear occurs crosswise to the direction of the applied force, this observation should be recorded. There are two options to calculate the tongue tearing force for the individual specimens. In option 1, for fabrics exhibiting five peaks or more, after the first 6 mm (0.25 in) of tear, the five highest peak forces are obtained from the data collection system to the nearest 0.1 mN (0.1 lbf). The average of these five highest peak forces is calculated and reported. In option 2, for fabrics exhibiting less than five peaks, the highest peak force is recorded as the single peak force to the nearest 0.1 mN (0.1 lbf). The tongue tearing strength is calculated as the average tearing force for each testing direction and condition for each laboratory sample. Standard deviation (SD) and coefficient of variation (CV) should be included in the report; in addition, if computer-processed data were used, it is recommended that the software be briefly described. If it is necessary to measure the tear of the wet fabric, the specimens are submerged in a container of distilled water at ambient temperature until thoroughly soaked. For fabrics that have a water-repellent finish, it is necessary to add a small amount of a nonionic wetting agent to the water bath. For wet testing, the specimen is removed from the water, and immediately mounted on the testing machine. The test must be performed within two minutes after removal of the specimen from the water. If more than two minutes elapse between taking the specimen out from the water bath and starting the test, the specimen is discarded and another one is obtained. Upon closing the discussion on the tearing strength of fabrics by the tongue test, it is important to keep in mind that depending on the direction the fabric is torn, the values obtained will be for the tearing strength of yarns perpendicular to the tear direction. If the direction to be torn is much stronger than the other direction, failure will occur by tearing across the tail so that it is not always possible to obtain both warp and weft results.

20

Structure and mechanics of textile fibre assemblies

2.6.2 Trapezoid (wing rip) test This test method covers the measurement of the tearing strength of textile fabrics by the trapezoid procedure using a constant-rate-of-extension (CRE) tensile testing machine. This test method applies to most fabrics including woven, knitted and nonwoven fabrics. It is generally conceded that the trapezoid tear produces tension along a reasonably well-defined trajectory such that the tear propagates across the width of the specimen. Consequently, this test overcomes some of the problems encountered with the tongue (single rip, trouser) test as it is capable of testing most types of fabrics without causing a transfer of tear. Cross-machine direction is the direction in the plane of the fabric perpendicular to the direction of the manufacture. This term is used to refer to the direction analogous to coursewise or filling direction in knitted or woven fabrics, respectively. Machine direction is the direction in the plane of the fabric parallel to the direction of manufacture. This term is used to refer to the direction analogous to walewise or warp direction in knitted or woven fabrics, respectively. In the case of nonwoven fabrics, the terminology used is somewhat different. A nonwoven fabric is a textile structure produced by bonding or interlocking of fibers, or both, accomplished by mechanical, chemical, thermal, or solvent means, or combination thereof. Hence, for nonwovens, an easily distinguishable pattern for orientation may not be apparent, especially if removed from the roll. Care should be taken to maintain the directionality by clearly marking the direction. For nonwoven fabrics the nomenclature used is widthwise and lengthwise directions. The widthwise direction is the direction in a machine-made fabric perpendicular to the direction of movement the fabric followed in the manufacturing machine; the lengthwise direction is the direction in a machine-made fabric parallel to the direction of movement the fabric followed in the manufacturing machine. The basic test methodology is the same for woven, knitted or nonwoven fabrics: (a) An outline of an isosceles trapezoid is marked on a rectangular specimen. The specimen is slit at the center of the smallest base of the trapezoid to start the tear. The nonparallel sides of the marked trapezoid are clamped in parallel jaws of a tensile testing machine. The separation of the jaws is increased continuously to apply a force to propagate the tear across the specimen. At the same time, the force developed is recorded. The force to continue the tear is calculated from autographic chart recorders or computer data gathering system.

Characterization and measurement of textile fabric properties

21

(b) As usual, all necessary precautions must be taken to avoid slipping of the specimen from the clamps during the test. If slippage does occur, the necessary corrective actions must be implemented. (c) Rolls or pieces of fabric are considered as the primary sampling units (lot sample). For the laboratory sample, a swatch is taken extending the width of the fabric and approximately 1 m (1 yd) along the machine direction from each roll or piece in the lot sample. For rolls of fabric a sample should exclude fabric from the outer wrap of the roll or the inner wrap around the core. From each laboratory sampling unit, five specimens are taken from the machine direction (lengthwise direction for nonwoven fabrics) and five specimens from the cross-machine direction (widthwise direction for nonwoven fabrics), for each test condition. These are the test specimens which must now be cut using the templates shown in Fig. 2.5. We note that an initial slit is made 15 mm (0.625 in) long at the center of the 25 mm (1 in) edge of the isosceles trapezoid. (d) Consider the long direction as the direction of test for the woven fabric and the short direction of test for the nonwoven fabrics. (e) For woven fabrics take the specimens to be used for the measurement of machine direction with the longer dimensions parallel to the machine direction. Take the specimen to be used for the measurement of the cross-machine direction with the longer dimensions parallel to the cross-machine direction. For nonwoven fabrics, cut the specimens to be used for the measurement of the lengthwise direction with the shorter dimension parallel to the lengthwise direction. Cut the specimens to be used for the measurement of the widthwise direction with the shorter dimension parallel to the widthwise direction. Fig. 2.6 clearly illustrates the relationship of specimen orientation with respect to the test direction. (f) At the start of the test, the distance between the clamps is set at 25 ± 1 mm (1 ± 0.05 in), and the testing speed is set to 300 ± 10 mm/min (12 ± 0.5 in/min). The test is conducted in the standard atmosphere for textiles or in the atmosphere directed by the contract order. The test

75 mm (3 in)

15 mm (0.625 in)

100 mm (4 in)

150 mm (6 in)

2.5 Templates for cutting and marking trapezoid test specimen.

22

Structure and mechanics of textile fibre assemblies Lengthwise direction of fabric

Cut the specimens to be used for the measurement of the lengthwise direction

Cut the specimens to be used for the measurement of the widthwise direction

2.6 Illustration of relationship of specimen orientation with respect to test direction – nonwoven fabrics, trapezoid test.

specimen is clamped along the nonparallel sides of the trapezoid such that the end edges of the clamps are in line with the 253 mm (1 in) side of the trapezoid and the cut is halfway between the clamps. The short edge is held taut and the remaining fabric is allowed to lie in folds. The machine is started and the tearing force is recorded (forceextension curve) and it may exhibit a simple single maximum or show several maxima and minima. (g) For other details, the reader is referred to the discussion of the tongue test.

2.6.3 Falling-pendulum test (Elmendorf apparatus) This test method covers the determination of the force required to propagate a single-rip tear starting from a cut in a fabric and using a fallingpendulum type (Elmendorf) apparatus. This test method applies to woven fabrics and many other fabrics provided the fabric does not tear in the direction crosswise to the direction of the force application during the test. This method is suitable only for the warp direction tests of warp-knit fabrics; it is not suited for the course direction of warp-knit fabrics or either direction of most other knitted fabrics. A slit is centrally precut in a test specimen held between two clamps and the specimen is torn through a fixed distance. The resistance to tearing is in part factored into the scale reading of the instrument and is computed from this reading and the pendulum capacity. The test specimen is a rectangle 100 ± 2 mm (4 ± 0.05 in) long by 63 ± 0.15 mm (2.5 ± 0.005 in) wide; the critical dimension is the distance 43.0 ± 0.15 mm (1.69 ± 0.005 in) which is to be torn during the test. This tearing distance is the distance between the end of the slit and the upper edge of the specimen when the lower edge

Characterization and measurement of textile fabric properties

23

of the 63 mm (2.5 in) wide specimen rests against the bottom of the clamp. The length of the slit is 20 mm (0.787 in). The Elmendorf tear tester is a sector-shaped pendulum carrying a clamp which is in alignment with a fixed clamp when the pendulum is in the raised starting position, where it has maximum potential energy. The specimen is fastened between the two clamps with the slit centrally located between the clamps, and the force recording mechanism is set at its zero-force position. The tester may have a pointer mounted on the same axis as the pendulum to register the tearing force on a scale, or it may have a computer and associated software for automatic collection of the data and to perform the calculations. When the pendulum is released, part of its energy is lost in tearing the fabric, hence when the pendulum is on its backward swing it will not be able to reach the same height as it started from. The difference between starting height and finishing height is proportional to the energy lost in tearing the fabric specimen. The scale attached to the pendulum can be graduated to read the tearing force directly or it may give percentage of the original potential energy. It is also evident that the work done on the fabric and hence the reading obtained is directly proportional to the length of the fabric torn. The range or capacity of the instrument can be increased by using weights to increase the mass of the pendulum. Readings obtained when the specimen slips in the jaw, or where the tear deviates more than 6 mm (0.25 in) away from the projection of the original slit, must be rejected. The operator must record if puckering occurs during the test and if the tear was cross-wise to the normal (parallel) direction of tear. It is interesting to point out that the basic principle used in the Elmendorf tear test is the same as the one used to measure the impact resistance of engineering thermoplastics, that is, the classical Charpy and Izod methods. In all these tests we are using the interconversion of potential energy and kinetic energy. In all these techniques the parameter measured is the energy absorbed from the pendulum during its oscillation. This ‘impact energy’ as it is sometimes termed, may be defined as: U=

1 I (Vo2 − Vf2 ) , 2

2.12

where I is the moment of inertia of the pendulum and Vo and Vf are the velocities of the pendulum bob just before and after striking the specimen, respectively. Assuming no windage or friction losses on the pendulum V 2o = 2gho and V 2f = ghf where ho and hf are, respectively, the initial fixed height from which the pendulum is released, then, it swings down to strike and break or tear the specimen at the bottom of the swing, and then continues its swing to a measured maximum height, hf. Then

24

Structure and mechanics of textile fibre assemblies U = gI ( ho − hf ) or, more realistically, U = K ( ho − hf )

2.13

where K is a machine constant for a given system [16]. In modern research laboratories the impact resistance of thermoplastics and composites is measured in fully computerized testing machines of horizontal design and equipped with an environmental chamber. The ruptured specimens are studied by optical and/or scanning electro microscopy (fractography). The falling pendulum (Elmendorf) apparatus can be used to measure the tearing strength of most nonwoven fabrics, provided the fabric does not tear in the direction crosswise to the direction of the force applied during the test. If the tear does not occur in the direction of the test, the fabric is considered untearable in that direction by this test method. The standard Elmendorf tear tester with interchangeable pendulum is the preferred test apparatus for determining tearing strength up to 6400 g.f. For tearing strength above this value, a high capacity test instrument is available equipped with augmenting weights to increase the capacity. The nonwoven fabrics may be treated or untreated, including heavily sized, coated or resin-treated. The test specimen is the same as described before for woven fabrics, viz., a rectangular test piece 100 ± 2 mm long by 63 ± 0.15 mm wide, with a 20 mm slit, thereby leaving 43 mm to be torn. Obviously, compared to other methods for testing tearing strength this test method has the advantage of simplicity and speed since specimens are cut with a die and results are read directly from the scale on the pendulum. Furthermore, the specimens are relatively small in area and thus, require less fabric. The reading obtained is directly proportional to the length of the material torn, therefore, it is essential that the specimen be prepared to the exact size specified.

2.7

Kawabata evaluation system (KES)

Professor Emeritus Sueo Kawabata (Kyoto and Shiga Prefecture Universities, Japan) [17] developed a comprehensive program to study fabric handle (hand) to replace the subjective assessment of fabrics by experts, with an objective, laboratory instrument-based system, capable of providing consistent and reproducible results. The KES is especially designed to study the mechanical behavior of fabrics in the domain of small strains, pertinent to apparel applications. In this case, as previously indicated, decrimping is the most important region of the load-extension curve since the load is rarely high enough for thread extension to take place. The KES has also the capability to characterize the energy loss (hysteresis loop) in the process of the

Characterization and measurement of textile fabric properties

25

mechanical deformation and recovery cycle. Some examples are briefly discussed below:

2.7.1 Tensile properties Fabric tensile properties are measured by plotting the force-extension curve from zero to a maximum force of 500 gf/cm (4.9 N/cm) as well as the recovery curve. As a result of this loading/unloading cycle, the recovery curve does not return to the origin, i.e., a residual strain remains (permanent set), which reflects the viscoelastic nature of the component fibers. The area inside the hysteresis loop denotes the energy lost during the load-unload cycle. A typical load-extension for KES showing one deformation cycle is presented in Fig. 2.7. From these curves the following values are calculated [6]: Tensile energy WT = the area under the force-extension curve (load increasing) and represents the work done in tensile deformation WT • Linearity = area triangle OAB defines the extent of non-linearity of the force-extension curve. The triangle OAB is obtained by drawing a 45 ° straight line from 0 (origin of the y-x rectangular coordinates) to point A, with a y value (ordinate) of 500 gf/cm. Point B is the corresponding value of the extension on the x axis (abscissa) [6]. area under load decreasing curve • Resilience RT = × 100% WT •

The relation between these parameters and the wearing performance of the fabric is the following: Load

A

O

C

B

Extension

2.7 Load-extension curves for KES showing one deformation cycle (loading-unloading). Line OA represents perfect linear elasticity.

26

Structure and mechanics of textile fibre assemblies

• WT, tensile energy: a lower value causes hard feeling in extension • LT, linearity in extension: a higher value causes a stiff feeling • RT, resilience: a lower value causes inelastic behavior.

2.7.2 Shear properties Shear properties of the fabric are measured using a rectangular specimen with a width of 20 cm and a height of 5 cm, clamped along the two long opposite edges and free on the other two edges. On this specimen, a constant tension of 10 gf/cm (98.1 mN/cm) is applied along the clamped sides of the fabric in the x direction to avoid buckling of the fabric. During the test, the fabric is subjected to shear forces on the clamped edges which undergo relative displacements along the y axis as a result of the applied shear forces. The angle θ represents the rotation of a point on the moving edge of the tested specimen. A schematic of the test is given in Fig. 2.8. The reader should also refer to Treloar’s [5] pioneering research in this area. The maximum angle of rotation in this test is 8 ° which corresponds to the wearing condition of fabrics. The following quantities are measured: •

• •

Shear modulus or modulus of rigidity G = average slope of the linear region of the hysteresis curve (shear force-shear strain curve) to ±2.5 ° shear angle. Shearing hysteresis, 2HG = average widths of the shear hysteresis loop at ±0.5 ° shear angle. Shearing hysteresis, 2HG5 = average widths of the shear hysteresis loop at ±5 ° shear angle.

Note that shearing hysteresis is also called Force hysteresis. The relation between these parameters and the wearing performance of the fabric is the following: 20 cm A

B

θ

θ

D

5 cm

C

Shear forces

x

Tensile forces y

2.8 Schematic of the KES fabric shear test.

Characterization and measurement of textile fabric properties • • •

27

G, shear stiffness or rigidity: a larger value makes the fabric stiff and paper-like 2HG, shear hysteresis at 0.5 ° shear angle: a larger value causes inelastic behavior in shearing 2HG5, shear hysteresis at 5 ° shear angle: a larger value causes inelastic property in shearing and wrinkle problems.

Hu [18] states that the KES shear tester does not produce a pure shear state deformation in the tested specimen. On the other hand, a pure shear state is indeed achieved in the conventional shear test for a stiff engineering material in which a rod of circular cross-section is subjected to torsional deformations. As previously stated, the angle θ represents the rotation of a point on the moving edge of the tested specimen, but not the shearing strain.

2.7.3 Bending properties The fabric specimen is bent between the curvatures −2.5 and 2.5 cm−1, the radius of the bend being the reciprocal of the curvature. A curve of bending moment vs. curvature is obtained by continuously monitoring the bending moment required to produce this range of curvatures. The measured bending parameters are: •

B, bending stiffness (rigidity), the average slope of the linear regions of the bending hysteresis curve between the radius of curvature of 0.5 cm−1 and 1.5 cm−1. • 2HB, bending hysteresis, the average width of the bending hysteresis loop at ±0.5 cm−1 curvature. The relation of the obtained bending parameters with the wearing performance of the fabric is the following: • •

B, bending stiffness. A larger value makes the fabric stiff. 2HB, bending hysteresis. A larger value causes inelastic behavior in bending.

2.7.4 Compression properties Compression properties are measured by subjecting the fabric specimen, between two plates, to increasing pressure while following the change in the fabric thickness, up to a maximum pressure of 50 gf/cm2. The load is then slowly reduced and the recovery process is measured. The parameters LC, WC and RC are obtained using the same criteria as in the tensile properties.

28

Structure and mechanics of textile fibre assemblies

•

LC, linearity of the compression-thickness curve which is a measure of the deviation of the deformation curve from a straight line. Higher values of LC mean a higher initial resistance to compression. • WC, compression energy, is the work done in compression as measured by the area under the compression curve (gf.cm/cm2). • RC, compressive resilience, is the ability to recover from compression deformation and is expressed as a percentage of the work recovered to the work done under compression deformation. The relation between these parameters with the wearing performance of the fabric is the following: • • •

LC, linearity in compression. A higher value causes a hard feeling in compression. WC, compression energy. A lower value causes a hard feeling in compression. RC, resilience. A lower value causes inelastic compression property.

2.7.5 Surface properties In addition to the properties briefly discussed above, KES system includes the measurements of the fabric surface frictional coefficient and surface frictional roughness. The relation of these parameters with the wearing performance of the fabric is the following: • • •

MIU, mean frictional coefficient. Too high and too low values yield unusual surface feeling. MMD, surface frictional roughness. A higher value causes roughness; mean deviation of MIU. SMD, surface geometrical roughness. Too high and too low values make unusual feeling surface.

Additional parameters in the KES system are fabric thickness (mm) and fabric weight per unit area (mg/cm2). Instruments capable of measuring the longitudinal tensile, axial compression, transverse compression and torsional characteristics of single fiber/ filaments of an average diameter of 15 micrometres have also been developed. These parameters have become very useful in correlating mechanical properties to the fiber molecular structure and microstructure (morphology), as well as in selecting fibers or fiber assemblies as reinforcement of composite materials. In summary, Prof. Kawabata’s extensive research has provided an important contribution towards the development of a true engineered design of textile and apparel performance.

Characterization and measurement of textile fabric properties

2.8

29

The FAST system: fabric assurance by simple testing

FAST comprises test methods and instruments designed by the CSIRO division of Wool Technology, Australia, to measure the tailoring performance of the fabric and thereby identify problems that may be encountered in converting a fabric into a garment. Both KES and FAST methods are designed for measuring the low stress (or deformation) mechanical properties of fabrics, but some differences exist in the testing philosophy. Thus, while the KES bending tester uses the principle of pure bending to measure the bending property, the FAST bending tester is based on the cantilever principle. In the measurement of the shear property, the KES measures the simple shear while the FAST shear tester uses the principle of bias extension. There appears to be a consensus of opinion among the experts that the FAST system is more readily applied to industrial production, while the KES is preferred in a research laboratory environment [18].

2.9

Detailed study of a fabric’s compressional property

Matsudaira and Qin [19] developed a theoretical model for the compressional deformation of a fabric and confirmed the model experimentally. The fabric is considered as an assembly of yarns and/or fibers and with a space (air) between them. Three stages can be identified in the fabric compression and recovery curves. First stage – in the first stage the compression plate comes in contact with fibers protruding from the fabric surface (‘hairs’) and the resistance to compression comes from the bending of these fibers. Second stage – in the second stage the compression plate makes contact with the surface of the yarn, hence inter-yarn and/or inter-fiber friction provides the resistance to compression until the fibers are all in contact with one another. Third stage – in the third stage the resistance to compression comes from the lateral compression of the fibers themselves. The authors opine that the first and third stages of the compressional curve can be approximated by a linear equation of the type y = a + bx since elastic deformation predominates, while the second stage of the compressional curve can be regressed by an exponential equation of the type y = a exp(bx) + c, where y is the compressional force (gf/cm2), x is the deformation (mm) and a and b are the regression constants. In this second step frictional forces predominate.

30

Structure and mechanics of textile fibre assemblies

In the recovery curve, the first step can be approximated by the linear equation, but the second step is regressed by an exponential equation. The third step of the recovery curve is the region where instantaneous recovery is impossible. The area inside the hysteresis loop is the part of the compressional energy lost to internal friction. For the experimental confirmation of the theoretical model the KES compression tester was used. While in the KES the maximum compressive stress is 50 gf/cm2, in this study the maximum pressure was 250 gf/cm2. The fabrics used were all wool, all silk, all polyester (filament), all polyester (spun) and all cotton. The fabric thickness (mm) was obtained at the pressure of 0.5 gf/cm2 as in the KES procedure, and all the experiments were carried out at 20 ± 0.3 °C temperature and 65 ± 3% relative humidity. Very good agreement was noted between the calculated and experimental compressional and recovering curves.

2.10 Mechanisms of deformation of fabrics – summary [3] Woven

Knitted

Nonwoven

Crimp removal Fiber slippage Fiber straightening Fiber extension Yarn flattening Yarn bending Thread shearing Crimp interchange

Crimp removal Fiber slippage Fiber straightening Fiber extension Yarn flattening Yarn bending Thread shearing Change in spacing

Fiber deformation Bond deformation

2.11 Fibrous assemblies as reinforcement of composite structures The study of composite structures reinforced by fibrous assemblies is a very important area of materials science and engineering, with applications in aerospace, land and sea transportation, sporting goods, civil infrastructure and biomedical products. Of particular importance is the increased use of carbon fiber reinforced composites in structural components of large passenger airplanes. Fibers are commonly used for reinforcement of polymer, metal and ceramic matrices and the fibers themselves can be polymeric, metallic or ceramic. Reinforcing fibers can also be produced with different cross-sectional shapes and crystalline microstructure and can be engineered in the form of two-dimensional or three-dimensional architectures in order to optimize composite properties.

Characterization and measurement of textile fabric properties

31

Within the transportation industry, we must also remember an important fiber-reinforced composite, namely, the pneumatic tire (car, truck, aircraft, off-the-road tires). In the rubber industry, polymeric and metallic reinforcing fibers are predominantly utilized in the form of a cord, a plied twisted filament structure, since this type of structure can be readily varied to provide a rather wide range of alternative selections of strength, stiffness and fatigue. A simple rule of thumb is that higher twist results in better fatigue resistance at the expense of reduced stiffness and strength. From a materials science viewpoint, a pneumatic tire can be defined as a compliant, viscoelastic cord-rubber composite structure which undergoes variable periodic deformations during service. The modern radial tire features a stiff, almost inextensible belt package connected to the rim by a tough, flexible radial casing, thereby ensuring optimum traction and directional stability. Fig. 2.9 shows a schematic of the internal structure of a radial tire for a car. The tire cords are the load-carrying constituent of the tire while the rubber in the plies transmits the load to the cords via shearing stresses at the cord-rubber interface. This, of course, requires an optimum level of adhesion between the cords and the rubber matrix. Quasi-static, time dependent and dynamic mechanical properties of cord and rubber must be measured using suitable test specimens. Dynamic mechanical properties are of primary importance and comprise the measurement of elastic modulus, loss modulus, rate of heat generation (hysteresis) and fatigue as well as dependence of these properties on temperature, strain (or stress) amplitude, frequency and material microstructure. For the rubber matrix in particular, crack growth properties and network microstructures must be fully

Overlay (cap ply)

Tread

Belt (breaker) plies

Body (carcass) ply

Sidewall Beads

2.9 Schematic of the internal structure of a radial tire for a car.

32

Structure and mechanics of textile fibre assemblies

characterized. We must keep in mind that the rubber matrix itself contains non-fibrous particulate reinforcing fillers such as carbon black and silica. The properties of the rubber cord-interface are particularly difficult to study since the interface is not a simple boundary surface but it is actually an interfacial zone (‘interphase’) consisting of the surface layers of fiber (polymeric or metallic) and rubber and adhesive layer(s) between these surfaces. Why are the dynamic mechanical properties, that is, the multi-cycle fatigue strength of the cord-rubber composite so important in assessing its suitability for use in the body and belt plies of radial tires? The answer is simple if we keep in mind that 40,000 miles of passenger tire travel subjects each cord to approximately 30 million fatigue cycles! Cords in heavy duty truck tires commonly experience over one billion fatigue cycles even before retreading!

2.11.1 Fatigue of reinforced polymer composites The response of fiber reinforced, polymer composites to stress is more complex than that of an isotropic material. This is due to the fact that there are many variables that affect the composite behavior and fatigue modes. The fracture behavior of the composite is affected by the following variables: (a) type of fiber and fiber assembly construction; (b) type of matrix; (c) fiber-matrix interfacial bond strength and toughness; (d) fiber orientation and ply stacking sequence; (e) presence of flaws or discontinuities; (f) mode and rate of loading, and (g) environment (heat, moisture, chemicals). Several possible damage and failure modes can be observed in fiber reinforced polymer composites: (a) matrix fracture; (b) fiber-matrix interfacial bond failure or interfacial area failure; (c) fiber fracture; (d) crack growth from flaws or at materials and geometric discontinuities, and (e) delamination. In a multi-directional composite under complex loading one may find one or several of these damage modes, and it is sometimes difficult to identify a single dominant crack that controls the composite failure.

2.11.2 Stiff polymeric matrix composites versus elastomeric matrix composites Elastomeric matrix composites behave quite differently from stiff polymer matrix composites in the following major ways: 1. Elastomeric matrix composites have a much larger elastic deformation range than that of stiff polymer composites. Hence, the geometric changes of the configuration must be taken into account. This statement

Characterization and measurement of textile fabric properties

33

means that the stress can be defined by the force per either undeformed area (Lagrangian stress) or deformed area (Eulerian stress). 2. Elastomeric matrix composites have low shear modulus and hence exhibit large shear deformation, which allows the fibers to change their orientation under loading. 3. The stiffness of an elastomeric matrix composite (lamina or laminate) is extremely sensitive to the fiber orientation. Hence, elastomeric matrix composites are highly anisotropic. It is apparent therefore that the conventional linear elastic theory, based on the infinitesimal strain assumption for stiff polymer matrix composites is not applicable to elastomeric composites under finite deformation. Furthermore, it can be readily deduced from all of the preceding statements that although optical and scanning electron microscopy remain the key tools to pinpoint microfailure modes, in order to understand the root cause of failure, scientists and engineers must have a sound knowledge of the properties of the fiber or fiber assembly, rubber and rubber-fiber interfaces.

2.11.3 Fundamentals of predictive testing The experimental characterization of composite materials can be done on several scales, viz., micromechanical, macromechanical and structural. The testing of composite materials has three major objectives: 1. Determination of properties of the unidirectional lamina (ply) and of laminated structures. 2. Investigation and verification of predictions of mechanical behavior derived either from analytical (closed form) solutions or numerical (finite element analysis) procedures. 3. Independent experimental study of material and structural behavior for specific geometries and loading conditions [20]. The major objective of predictive testing is, as expected, to predict the lifetime of the composite structure. There are several requirements to achieve this goal: • •

sound knowledge of application understand and quantify the mechanism of failure and cumulative damage • relate to material and design • develop scaling concept in time and geometry to relate laboratory tests to actual field performance. A typical predictive accelerated testing method is the ‘step stress testing’. The engineer begins with a fairly large number of specimens and conducts

34

Structure and mechanics of textile fibre assemblies

the test for a fixed time interval starting at a low stress level. At the end of the time period, the stress is increased and the good parts remaining from the previous step are subjected to the increased stress for the same time period. The process is repeated until the onset of failure that can be caught at its initial stages using various non-destructive evaluation techniques. Stresses are applied at a constant rate or frequency and there can be a single dominant stress applied alone or in combination with other stresses depending on our knowledge of the ‘real-world’ situation. The interaction of two or more stresses has the added advantage that it may result in a a reduction in test time. If many levels of accelerated stress are employed, it becomes possible to plot a stress vs. life cycles curve (S-N curve). The S-N curve represents the mathematical model that displays the way life varies by the application of different stress levels. We will return to this topic when we describe an example of fatigue testing. Since the application of accelerated stresses may result in degradation through higher temperatures, the use of the Arrhenius model seems quite logical. This is particularly true of elastomeric matrix composites due to the viscoelastic nature of the rubber matrix. The Arrhenius model is based on the classical equation that describes the reaction rate of a chemical process, but stated in a slightly different manner. The Arrhenius approach implies that a linear relationship exists between the logarithm of the time to a certain magnitude of material property change and the reciprocal of the absolute temperature. The energy of activation is obtained from the slope of the line. Another useful technique is the Palmgren-Miner Cumulative Damage Rule. This rule states that the number of stress cycles imposed on a material or structure, expressed as a percentage of the total number of stress cycles of the same amplitude necessary to cause failure, gives the fraction of expended fatigue life. If ni is the number of cycles corresponding to the ith block of constant stress amplitude si in a sequence of m blocks, and if Nfi is the number of cycles to failure at si, then the PalmgrenMiner damage rule can be expressed mathematically in the following manner: i=m

∑

i =1

ni =1 N fi

2.13

The Palmgren-Miner’s rule has one important shortcoming; it implicitly suggests that the order in which the stress blocks of different amplitudes are imposed does not affect the fatigue life. This is not in agreement with experimental facts.

Characterization and measurement of textile fabric properties

35

2.12 Basic mechanics of laminates: application on testing Laminated composites are extensively used in engineering applications featuring, in most cases, unidirectional reinforcement. The basic mechanics of angle-ply laminates can be explained by simple graphical representations. Let us consider two separate one-ply systems, with one ply at a reinforcement angle of +θ and the other ply at −θ as shown in Fig. 2.10. If a uniform tensile stress is applied at each end of these specimens at an angle to the reinforcing fibers (off-axis loading), an in-plane shear strain will develop and the plies will undergo shear deformations of opposite signs. In fact, the deformation patterns of these plies are mirror images of each other. When the two plies are now bonded together to form a ±θ angle ply laminate, the oppositely directed in-plane shearing stresses in each ply will produce two major effects: (a) interlaminar shear stresses will develop in the matrix layer between the two plies (or laminae) and the moment produced by these stresses is equilibrated by intralaminar shear stresses within each lamina, and (b) an out-of-plane twisting of the laminates as shown in Fig. 2.11. The structure is said to exhibit in-plane to out-of-plane coupling, that is, an inplane stress causes an out-of-plane deformation. The coupling stresses arise because the laminate is not symmetrical about its center plane. When the two plies are bonded together to form a −θ/+θ angle-ply laminate, that is a laminate with reversed stacking sequence, the application of

σx (−)

σx (+)

σx T = –α

σx (+)

σx (−)

σx T=α

Ply 1 Ply 1

Ply 2

Ply 1 Ply 1

Ply 2

Ply 2

σx

σx In-plane to out-of-plane coupling caused by laminate design

σx

Ply 2

σx

σx

σx

Reversed coupling caused by reversing the stacking

2.10 Schematic of the in-plane to out-of-plane coupling of one-ply systems.

36

Structure and mechanics of textile fibre assemblies σx

+θ –θ –θ +θ

Interlaminar shear

Intralaminar shear

Direction of fiber rotation in +θ lamina σx

(a)

(b)

2.11 (a) Shear stresses resulting from tensile loading of an angle-ply (±θ) laminate and (b) laminate with no in-plane to out-of-plane coupling.

the same tensile stress will produce a twist of the same magnitude but opposite direction with respect to the ±θ laminate. It can then be readily deduced that a four-ply laminate with a stacking sequence +θ/−θ/−θ/+θ. Designated [+θ/−θ]s, will possess a plane of symmetry and will not exhibit in-plane to out-of-plane coupling. The laminate responses to unidirectional tensile stresses as described above can be demonstrated experimentally and a mathematical description is available using finite element analysis and laminate theory.

2.12.1 Application in testing The angle-ply laminate featuring metallic or polymer cords as reinforcement and an elastomer as matrix is an excellent test specimen to study the fatigue endurance of the belt structure of a radial tire. As expected, numerous investigations on this topic have been conducted and the results are available either in scientific publications or as part of patent applications. A review article is also available [21]. The fatigue studies are conducted in either an electromechanical or a servohydraulic testing machine provided with suitable data acquisition system with special software. A two-ply, +θ/−θ cord rubber laminate can be used since a device is available to allow the testing equipment to prevent the twisting of the sample. An environmental chamber and an infra-red

Characterization and measurement of textile fabric properties

37

σ

σ

Stiff

Stiff Soft

Soft A

B

A

σ1

B

C 0

C

D (a)

ε

0

ε1 (b)

ε

2.12 Stress-strain curves of stiff and soft materials tested under (a) stress and (b) strain control.

camera are needed due to the critical importance of temperature on the fatigue durability of the composite. The composite laminate must be sufficiently long so that in the region far away from the ends, end effects (due to the grips) are negligible by virtue of the Saint Venant’s principle. In addition to the environmental factors (temperature, oxygen, ozone) it is important to know the testing conditions, viz., the mode of deformation control and the type of excitation waveform used in the fatigue study. Observation of the stress-strain curves of two hypothetical compounds 1 & 2, stiff and soft, in Fig. 2.12 shows that for displacement (strain) control (b), that is, a fixed strain, the low modulus compound should be better for fatigue durability, since its strain energy density, let us call it U2 (area under its stress-strain curve) is smaller than the area U1 for the higher modulus compound. This conclusion is correct if the crack growth rate corresponding to the strain energy release rate for the U2 condition is also smaller than that which corresponds to the strain energy release rate for U1. On the other hand, if the compound is tested under load (stress) control (Fig. 2.12 (a)), we observe that the higher modulus compound will now have a lower strain energy density, U1, and therefore a longer fatigue life. This conclusion assumes that the crack growth rate corresponding to the strain energy release rate for the U1 condition is lower than that corresponding to the strain energy release rate for U2. It is also possible to run fatigue experiments under energy control [22]. Sinusoidal and pulse waveforms are commonly used in dynamic experiments, but many other waveforms can also be used. We must keep in mind that the pulse excitation has a relaxation period which is not present in the sinusoidal curve. Note that the word ‘compound’ is used in the rubber

Structure and mechanics of textile fibre assemblies

Displacement

38

Load

Cycles

Cycles

2.13 Schematic of the evolution of fatigue life of an angle-ply laminate.

industry to designate the rubber (natural or synthetic elastomer) with the addition of sulfur and an accelerator to vulcanize the rubber, plus carbon black and/or silica, and antioxidants. Figure 2.13 shows in a schematic manner the evolution of the cumulative damage, as a function of fatigue life (time), for a ±23 ° angle-ply laminate, reinforced with high tensile steel cords in a matrix of natural rubber (cis1,4-polyisoprene). The fatigue test was run under load control using a sinusoidal waveform with a frequency of 10 Hz. The width of the test piece (often called ‘coupon’) was 25.4 mm with a gauge length of 254 mm; no external heat was applied to the sample [22]. The first damage observed is in the form of small cracks in the rubber at the edge of the cords; microfractography indicates that this failure agrees with Bikerman’s concept of a weak boundary layer fracture. This edge cracking is also known as ‘socketing’ [23]. In later stages these initial cracks propagate and connect with one another not only along the length of the sample but also through the rubber layer between the two cord reinforcing layers. As predictable, the infra-red camera shows that this rubber layer is the hottest spot in the sample. The final stage shows the interply fracture, that is, the well-known delamination mode of failure. Observation of the progress of the damage in this experiment reveals an interesting difference between composites and metals. In metals much of the fatigue life is spent before cracks appear. In a composite structure much of the fatigue life is spent after the appearance of the first crack, and the cumulative damage is quite complex [24].

Stress amplitude

Characterization and measurement of textile fabric properties

39

Fatigue or endurance limit

10

102

103

104

105

106

Number of cycles to failure

2.14 Schematic representation of an S-N curve (fatigue life curve).

In addition to fracture mechanics based on the strain energy release rate, the S-N curve is another classical approach to interpret the experimental fatigue studies of composites. The fatigue life data can be conveniently presented as plots of applied stress (stress amplitude, stress range) vs. the number of cycles to failure (N), usually on a log-log scale. A schematic representation of a fatigue life curve (Woehler curve) is shown in Fig. 2.14. These plots show that the life steadily increases with decreasing stress until the horizontal asymptote is reached that defines the ‘fatigue or endurance limit’ below which the life becomes infinitely long, that is, failure does not occur on any realistic time scale. However, to define the asymptote may require lengthy experiments with increased scatter of the individual data points. Consequently, it is more appropriate to use the concept of ‘fatigue strength’, that is, the stress level at which the material will live a specified number of cycles. Stress (or strain) cyclic lifetime curves have been used for a great number of years in the study of fatigue in metals and in rigid matrix composites, and continue to be an important engineering design tool.

2.12.2 Importance of geometrical parameters Our previous statements have indicated the critical importance that geometrical parameters, such as ply stacking sequence and angles, play in the mechanical behavior of the composite structure. To emphasize this point, let us provide an example that has historical importance. Figure 2.15 shows the load-elongation behavior of two laminates with different stiffness values. The laminate which is less stiff (more compliant) has the construction we have already discussed, viz., a +/− angle-ply geometry, while the stiffer laminate has the same +/− angle-ply layout but under it lies another ply reinforced with cords lying transverse to the direction of the applied load. X-ray

40

Structure and mechanics of textile fibre assemblies Stiffness increase due to body ply cords Triangulated 90°/+q/–q laminate Pantographing

Load

+q/–q laminate

Elongation

2.15 Typical load-elongation behavior of two laminates with different stiffness values.

photographs reveal different deformation patterns for these two constructions. The less stiff laminate shows a pantographic network of criss-crossing cords which mechanically respond as deformable rhomboids, while the stiffer laminate shows a cord network made up of relatively undeforming triangles. This stiffness increase is well known to civil engineers and is called ‘triangulation’, and it was recognized in the patent filed by the Michelin Tire Company on June 4, 1946, in Paris, France, under the signature of Pierre Marcel Bourdon [25]. The reader should remember that the +/−θ angle-ply laminate represents the belt of the radial tire, while the laminate with the cords lying transverse to the direction of the applied load simulates the body ply of the tire. In a passenger tire, the former is reinforced with steel cords and the latter with polymer cords, most often, polyester. The increased stiffness resulting from triangulation provides increased rigidity to the tread and hence better treadwear, improved stability of the tire on the road and lower rolling resistance. The belt structure as defined above, that is, belt plus body plies must be characterized by its in-plane bending (flexural) rigidity and its in-plane shear stiffness (Iosipescu test method), the most important parameters controlling the cornering characteristics of the tire. The out-of-plane bending rigidity (stiffness) must also be measured since it controls the ride characteristics of the radial tire. Measurements under biaxial loading are often required. An article published in the November 2004 issue of High performance Composites provides another interesting example of the use of a geometrical parameter to obtain a specific mechanical response from a laminate structure [26]. The author states that when a laminate of the usual ‘balanced or mirrored’ construction is submitted to an axial bending force, the lami-

Characterization and measurement of textile fabric properties

41

nate will respond uniformly along its axis. On the other hand, when the laminate with an ‘unbalanced’ construction is subjected to the same axial bending force, it will shift some of the force to the off-axis direction, and the laminate will twist around its axis. This study was funded by Sandia National Laboratories, Albuquerque, NM, USA, and it was conducted by a team of companies that specialize in structural design and materials. The objective was to develop what they call an ‘adaptive wind blade’ to be used in wind turbines. As wind speed goes up (wind gust), the bending force on the blade increases, but the blade twists along its longitudinal axis thereby reducing the bending load and avoiding damage to the turbine system. ‘Adaptive blades’ are also called ‘twist coupled blades’. The blades are made of an epoxy matrix reinforced by a special carbon/glass hybrid fabric. The article emphasizes that design and materials were selected based on an extensive use of finite element analysis; therefore a prototype must be built and tested to confirm the theoretical predictions. An additional example of the importance of the ply stacking sequence in the area of elastomer matrix composites is illustrated in the patent literature, for example, US Patent No. 4,688,615, August 25, 1987 and US Patent 6,668,889, December 30, 2003, both to The Goodyear Tire and Rubber Company. In the preceding paragraphs we have discussed the elementary mechanics of the shear-deformable angle-ply laminates that function as the belt package of a radial tire. We have also covered the dynamic testing of this structure and its failure mechanism involving edge cracking and delamination. In the earlier patent mentioned above, the author states that the belt edge delamination is not altered by the constraint of the body ply. In other words, a cord-rubber composite specimen which simulates belt plies bonded to a body ply, that is a cord angle sequence of (90/+23/−23) degrees may still be prone to edge delamination between the belt plies. It was found experimentally that with this construction there is a considerable mismatch of Poisson’s ratios between the belt ply and body ply resulting in a decrease in fatigue strength. A new construction is proposed that consists in positioning a third ply between the original belt plies and, of course, bonded thereto. This third ply includes a plurality of parallel cords which form a zero degree angle with respect to the midcircumferential centerplane of the tire (also known as the ‘equatorial’ plane). The ply stacking sequence of this new construction is therefore, as a typical example, 90/+23/0/−23 °, but it is not limited to a belt angle of 23 °. This sequence of plies has been found to reduce strain gradients near the edges of the laminate and it also enables adjacent plies to have closer values of Poisson’s ratio. In addition, the positioning of a zero degree middle belt between the two adjacent but oppositely angled belts of the belt structure also strengthens the interply region between the belt plies. The overall result is a substantial improvement in

42

Structure and mechanics of textile fibre assemblies

fatigue life. The major drawback of this construction is that in many tire designs it did not produce a tire of the desired uniformity. In this patent the zero degree middle ply was made of a plurality of continuous parallel cords that were more extensible and of lower tensile strength than the cords in the conventional angled belts. In the more recent patent (Dec. 2003), the zero degree middle ply was constructed with discontinuous parallel cords resulting in the production of a uniform tire in most tire designs, with improved durability and handling properties versus a commercial control tire. In actual production it is easier to achieve improved belt edge fatigue durability and enhanced high-speed performance by positioning a ‘cap ply’ or ‘overlay’ between the belt and tread of a radial tire. The cap ply contains either continuous or discontinuous cords oriented in a circumferential direction, that is, it falls in the category of a zero degree ply as shown earlier in Fig. 2.9.

2.13 Three-dimensional fibrous assemblies for structural composites A textile structural composite is defined as a composite reinforced by textile structures (preforms) dedicated for load-bearing structural applications. The fiber preforms are produced by textile forming techniques, such as knitting, braiding, weaving and stitching and can be processed using automated techniques such as RTM (resin transfer molding). Of particular interest are the three-dimensional (3D) textile preforms, since by offering some fibers in an out-of-plane orientation, they will provide enhanced stiffness and strength in the thickness direction. This architecture will result in improved damage tolerance to delamination failure mode. In addition, 3D preforms offer the possibility of near-net-shape design and manufacturing of composite components with complex shapes at reduced cost. A study conducted by Ding and Jin at Dong Hua University in Shanghai [27] provides a good example of the importance of the architecture of the fibrous reinforcement in the fatigue behavior of composites. It also demonstrates that through-thickness reinforcement helps avoid abrupt fatigue delamination failure. In this study an 11-layer 3D woven preform was made with continuous fiberglass and an unsaturated polyester as the resin matrix. RTM was employed to inject the resin into the mold and woven composites in plate form were fabricated. Ten rectangular specimens, 70 mm × 15 mm × 2.8 mm were cut from the composite plate with the warp direction parallel to the longitudinal edge of the specimens. The fiber volume fraction of the specimens was approximately 44%. For comparison purposes, ten specimens of a unidirectional (UD) laminate were fabricated using the same fiber/resin system and conditions as the 3D woven test specimens. The fiber volume fraction of the

Characterization and measurement of textile fabric properties

43

UD-laminate was approximately 40%. As it is customary, quasi-static flexural tests were performed prior to fatigue testing in order to obtain information on failure locations and mechanisms. Flexural fatigue testing was conducted on an electro-mechanical universal testing machine using a three-point bending configuration in deflectioncontrol mode. The span was set at 16 times the thickness of the specimens (S/t ratio); the load ratio (R-ratio), that is, the ratio of minimum to maximum load, was set at 0.1, and the testing frequency was 4 Hz. The value of the frequency was selected to minimize hysteresis that would raise the temperature of the sample and decrease its fatigue life. The stiffness loss was monitored continuously by the decrease of the applied load necessary to keep the given deflection constant over the fatigue life of the test specimens. A specimen was considered to have failed when its stiffness had reduced to 70% of its initial value. Static flexural tests were conducted using a three-point bending fixture, the S/t ratio was 16, loading speed was set at 5 mm/min, and ambient temperature 20 °C and 65% relative humidity. Both the 3D woven composite and the UD laminate were tested under the same conditions. The authors in this study present the results in three plots, viz., 1. quasi-static flexural test in the form of stress (MPa) vs. strain% 2. stiffness loss E/Eo vs. N/Nf in the dynamic flexural test. The number of testing cycles has been normalized by the fatigue life, Nf and the stiffness by the initial value of Eo and 3. dD/dN, damage rate vs. N/Nf under flexural fatigue testing. The damage parameter D is defined by this simple relation: D = 1 − E Eo

2.14

where E is the stiffness which is a function of fatigue testing cycles and is calculated according to the values of applied load and resulting deflection of the test sample, and Eo is the initial stiffness of the test sample in the undamaged state, at the start of the fatigue testing. The conclusion from this study is clear, namely, that although microcracks are formed in the initial stage of the flexural fatigue test, the 3D reinforcement resists the propagation of the initial cracks and thereby prevents the onset of global delamination failure. On the other hand in the UD laminates global delamination is clearly observed in the failed specimens.

2.14 Sources of further information and advice Recommendations for further reading on this expanding field of materials science and engineering are contained in the references at the end of this chapter.

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Structure and mechanics of textile fibre assemblies

2.14.1 List of ASTM test methods covered in this chapter Tensile testing Breaking Force and Elongation of Textile Fabrics (Strip Method), ASTM D 5035-95 (reapproved 2003). Breaking Strength and Elongation of Textile Fabrics (Grab Test), ASTM D 5034-95 (reapproved 2001). Stiffness (bending) testing Stiffness of Fabrics, ASTM D 1388-96 (reapproved 2002). Stiffness of Nonwoven Fabrics Using the Cantilever Test, ASTM D 5732-95 (reapproved 2001). Tearing strength (Same test concepts for nonwoven fabrics) Tearing Strength of Fabrics by the Tongue (Single Rip) Procedure (ConstantRate-of-Extension Tensile Testing Machine), ASTM D 2261-96 (reapproved 2002). Tearing Strength of Fabrics by Trapezoid Procedure, ASTM D 5587-96 (reapproved 2003). Tearing Strength of Fabrics by Falling-Pendulum Type (Elmendorf) Apparatus, ASTM D 1424-96. Test concepts for nonwoven fabrics Tearing Strength on Nonwoven Fabrics by the Tongue (Single Rip) Procedure (Constant-Rate-of-Extension Tensile Testing Machine), ASTM D 5735-95 (reapproved 2001). Tearing Strength of Nonwoven Fabrics by the Trapezoid Procedure, ASTM D 5733-99. Tearing Strength of Nonwoven Fabrics by Falling-Pendulum (Elmendorf Apparatus), ASTM D 5734-95 (reapproved 2001).

2.14.2 Suggested additional reading Adams, D. F., Carlsson, L. A. and Pipes, B. R., Experimental Characterization of Advanced Composite Materials, 3rd edition, CRC Press, Boca Raton, Florida, USA, 2003. Chou, T. W., Microstructural design of fiber composites, Cambridge University Press, 1992. Harrison, P. W., The tearing strength of fabrics, I., A review of the literature, Journal of Textile Institute, 51, T91, 1960.

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Hull, D., An Introduction to Composite Materials, Cambridge University Press, 1992. Ko, F. K. and Du, G. W., Processing of textile preforms, Chapter 5, in Advanced Composites Manufacturing, Gutowski, T. G. (ed.), John Wiley & Sons, NY, 1997. Liao, T. and Adanur, S., 3-D structural simulation of tubular braided fabrics for netshape composites, Textile Research Journal, 70, 297–303, 2000. Suresh, S., Fatigue of Materials, Cambridge University Press, 1991. Van Vuure, A. W., Ko, F. K. and Beevers, C., Net-shape knitting for complex preforms, Textile Research Journal, 73, 1–10, 2003. Weissenbach, G., Issues in the analysis and testing of textile composites with large representative volume elements, Doctoral Thesis, University of Ulster, March, 2003, Dissertation.com, Boca Raton, Florida, USA, 2004. (This discusses pioneering work by Bogdanovich, Pastore and Gowayed). Proceedings of the 30th Textile Research Symposium at Mt. Fuji in the New Millennium (2001), Fuji Educational Training Center, Shizuoka, Japan, July 30–31 and August, 1, 2001. (This contains a wealth of information from lectures delivered by experts in the area of textiles applications in both apparel and in structural composites.) Two magazines • •

High Performance Composites, Ray Publishing Inc. Journal of Advanced Materials, SAMPE (Society for Advancement of Material and Process Engineering)

2.15 Acknowledgements The authors want to thank Yuzo Yamamoto, Xiaosong Huang of Cornell University and Yao-Min Huang of Goodyear Tire and Rubber Co. for their help in the preparation of this chapter.

2.16 References 1. Peirce, F. T., The handle of cloth as a measurable quantity, Journal of the Textile Institute, 21, T377, 1930. 2. Peirce, F. T., The geometry of cloth structure, Journal of the Textile Institute, 28, T45, 1937. 3. Schwartz, P., Prof., Cornell University Notes, TXA 639, Mechanics of Fibrous Assemblies, 1992–1999. 4. Cooper, D. N. E., The stiffness of woven textiles, Journal of the Textile Institute, 51, T317, 1960. 5. Treloar, L. R. G., The effect of test-piece dimensions on the behavior of fabric is shear, Journal of the Textile Institute, 56, T533, 1965. 6. Saville, B. P., Physical Testing of Textiles, Woodhead Publishing Ltd., Cambridge, UK, 2004, pages 270 and 284–288. 7. Hearle, J. W. S., in Structural Mechanics of Fibers, Yarns, and Fabrics, Vol. 1, Chapter 12, Wiley-Interscience, 1969, page 378.

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8. Spivak, S. M., The behavior of fabrics in shear, Part I: Instrumental methods and the effect of test conditions, Textile Research Journal, 35, 1056–1063, 1966. 9. Spivak, S. M. and Treloar, L. R. G., The behavior of fabrics in shear, Part II: Heat-set nylon monofil fabrics and a new dynamic method for the measurement of fabric loss properties in shear, Textile Research Journal, 36, 1038–1049, 1967. 10. Spivak, S. M. and Treloar, L. R. G., The behavior of fabrics in shear, Part III: The relation between bias extension and simple shear, Textile Research Journal, 37, 963–971, 1968. 11. Leaf, G. A. V., Analytical woven fabrics mechanics, Invited lecture, Proceedings of the 30th Textile Research Symposium at Mount Fuji, in The New Millennium, 25–34, 2001. 12. Leaf, G. A. V. and Sheta, A. M. F., The initial shear modulus of plain woven fabrics, Journal of the Textile Institute, 75, 157–163, 1984. 13. Leaf, G. A. V., Chen, Y., and Chen, X., The initial bending behavior of plainwoven fabrics, Journal of the Textile Institute, 84, 419–427, 1993. 14. Chen, X. and Leaf, G. A. V., Engineering design of woven fabrics for specific properties, Textile Research Journal, 70, 437–442, 2000. 15. Scelzo, W. A., Backer, S. and Boyce, M. C., Mechanistic role of yarn and fabric structure in determining tear resistance of woven cloth Part I: Understanding tongue tear, Textile Research Journal, 64, 291–304, 1994. 16. Reed, P. E., Impact performance of polymers, in Developments in Polymer Fracture – 1, Andrews, E. H. (ed.) Chapter 4, Applied Science Publishers Ltc., England, 1979. 17. Kawabata, S, Niwa, M. and Yamashita, Y., Recent developments in the evaluations in the technology of fibers and textiles: Toward the engineered design of textile performance, Journal of Applied Polymer Science, 83, 687–702, 2002. 18. Hu, J., Structure and Mechanics of Woven Fabrics, Woodhead Publishing Ltd., Cambridge, UK, 2004. 19. Matsudaira, M and Qin, H., Features and mechanical parameters of a fabric’s compressional property, Journal of the Textile Institute, 86, 476–486, 1995. 20. Daniel, I. M. and Ishai, O., Engineering Mechanics of Composite Materials, Chapter 8, Oxford University Press, 1994. 21. Causa, A. G., Borowczak, M. and Huang, Y. M., Some observations on the testing methodology of cord-rubber composites: A review, in Progress in Rubber and Plastics Technology, Heath, R. J. (ed.), Vol. 15 (4), RAPRA Technology, Ltd., 1999. 22. Causa, A. G., Perspectives on testing methodology for fibers and fiber-reinforced rubber-matrix composites, Goodyear Corporate Research Division, Akron, OH, USA, Lecture delivered at the Fiber Society Fall Symposium, Ithaca, NY, October 11, 2004. 23. Breindenbach, R. F. and Lake, G. J., Rubber Chemistry Technology, 52, 96, 1979. 24. Causa, A. G., Keefe, R. L., Failure of rubber-fiber interfaces, in Fractography of Rubbery Materials, Bhowmick, A. K. and De, S. K. (eds), Chapter 7, 247–276, Elsevier Applied Science, London, 1991.

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25. Walter, J. D., The Firestone Tire and Rubber Co., The role of cord reinforcement in radial tires, lecture delivered at the Akron Rubber Group Meeting, October 27, 1988. 26. Mason, K. F., Composite anisotropy lowers wind-energy costs, High Performance Composites, 12, 44–46, 2004. 27. Ding, X. and Jin, H., Flexural performance of 3-D woven composites, Journal of Advanced Materials, 35, 25–28, 2003.