Flexural and shear properties of unidirectional pultruded composites

Flexural and shear properties of unidirectional pultruded composites R Boukhili, P Hubert and R Gauvin* Abstract - Three point bend tests were performed on pultruded half-round cross-section rods of glass fibrereinforced polyester and glass fibre-reinforced epoxy, the tests being arranged so that the specimen was unloaded as soon as the deflection corresponding to the maximum load was reached. This testing method enabled the fracture mechanisms at maximum loads to be mapped as a function of the loading rate and the span-to-depth ratios. An effect highlighted by this map is that a specimen tested at intermediate span.to-depth ratios may result in a purely tensile fracture at low loading rates and a purely shear fracture at high loading rates. This behaviour is interpreted as being due to the fact that the increase in loading rate increases the brittleness of the material, subsequently increasing the defect sensitivity and leading to a shear fracture. This explanation is supported by fracture surface observations. In addition, it was found that the shear strength increases significantly with increasing load rate up to 5000 mm rain -~ and then starts to decrease slightly. The decrease in shear strength at very high loading rates was found to correspond to multiple cracking behaviour rather than to a longitudinal fracture plane as for intermediate loading rates.

Pultrusion is an automated and continuous moulding process used to produce fibre-reinforced composites having constant cross-sectional profiles. The advantages and versatility of the process have enabled pultrusions to penetrate such market areas as land transportation, aircraft, construction, marine, etc 1. The integration of composite materials in many structural applications requires a good knowledge of their behaviour under various loading and environmental conditions. However, the method of determining the basic properties and behaviour related to static and impact loading are still a matter of discussion. Static behaviour is generally investigated by axial and bending tests performed at low loading rates and for which the specimen geometry is chosen in order to achieve the stress state and the fracture mode desired. Impact behaviour is commonly investigated by impact tests (tensile or bending) and the energy necessary to fracture a standard specimen is measured. Due to its simplicity, the three-point bending configuration is widely used for the determination of static as well as impact properties. Consequently, the idea of relating the results of impact tests to those of three-point bending tests is attractive, because the readily apparent difference between the two tests is the loading rate. This idea is particularly worthwhile in the case of pultruded composite beams which are generally used in applications where bending under impact or static loads is common ~. Unfortunately, relating impact and three-point bending tests is not an easy matter in the case of composite materials since the analysis and interpretation of the impact test itself is still unclear, as is well illustrated in Reference 4. As stated by Hayes and Adams 8, 'To achieve a full understanding of the impact response of composite materials, a test method is needed that has the capability of isolating the individual material properties. Similarly a test that would allow the chronological mapping of the failure mechanisms and the rate dependence of these failure mechanisms and material properties, would aid the designer in predicting the failure of a dynamically loaded part: *Department of Mechanical Engineering. Ecole Polytechnique de Montrdal, PO Box 6079, Station A, MontrdaL Qudbec, Canada H3C 3A, I. Fax: + 1 (514) 340 4026 CONSTRUCTION & BUILDING MATERIALS Vol. 6 No. 1 MARCH 1992

Justifications of such a need are numerous. For e0(ample, most impact-related investigations describe the fracture of impacted, three-point bending, unidirectional composite specimens as a mixed-mode failure combining interlaminar shear and tensile or compressive fracture. Since these failure mechanisms do not involve the same material properties, one could ask about the contribution of each of these mechanisms to the measured property, generally the fracture energy, as well as the physical significance of this energy. Consequently, knowledge of the failure mechanisms chronologically is a prime necessity not only for proper ranking and design of composite materials, but also for possible improvement of the impact response of these materials. Determination of which fracture mechanism had started first means, in the case of a three-point bending specimen, which of au (the tensile strength) or Tu(the shear strength) has been reached first. This is of prime interest in the case of unidirectional composites since au and Tu do not involve the same material elements (fibre and matrix). Furthermore, improvement of one of these properties, shear strength or tensile strength, does not necessarily imply the improvement of the other. Much worse, an improvement of one of these properties by a suitable design or material formulation could lead to a decrease in the other. Consequently, a misunderstanding of the first fracture mechanism will lead to improper design or material formulation. In laboratory testing, these difficulties are circumvented by using 'appropriate' specimen dimensions, which in three-point loading is the span-to-depth ratio Lid. This appropriate Lid ratio is chosen based on static tests. In order to achieve an interlaminar shear fracture, the Lid ratio should be less than 58. To achieve a tensile fracture, Lid should be no more than 167. It is also generally stated that for 5 <~ Lid ~ 16, a mixedmode failure prevails. Besides the fact that this large area of uncertainty penalizes design with composite materials, one could ask how the extent of this uncertain area depends upon test conditions such as the environment, temperature and loading rate. In this paper, an attempt is made to determine the effect of loading rate on the fracture mechanisms as a function of the span-to-depth ratio, with a particular emphasis on the case of intermediate Lldvalues and high loading rates. 0950 - 06181921010037- 06 © 1992 Butterworth-Heinemann Ltd

37

Testing of unidirectional pultruded composites

Fig 2

Three-pointbending fixture and specimen loading

O'max = 1.311Prnax/-/r3

(1)

rrnax = 0.466Pmax/r2

(2)

where Pro,=is the maximum load, L is the support span and r is the specimen radius.

Pultruded unidirectional glass/polyester: (a) typical cross-section view; (b) a resin-rich region F_xp rimantal procedures Materials Two commercial, unidirectional pultruded composite materials were used in this study: a glass-fibre reinforced polyester (glass/polyester) from Trench Company and a glass fibrereinforced epoxy (glass/epoxy) from Mitsubishi Company. The glass fibre diameter was about 20/Lm in the glass/polyester and about 10#m in the glass/epo~. In each material, the glass content was 80% by weight and the materials were received in the form of half-round rods of 6.7 mm radius (r) and 400 mm length. The exact formulation of the resins used is not available from the suppliers. Some sample cross-sections of the materials used were viewed in a Jeol 820 scanning electron microscope (SEM) in order to obtain an insight into the distributions of microvoids and rasin-rich regions. In Fig l(a), which shows a typical general view of the cross-section in glass/polyester, the curved lines indicate the resin-rich regions. Such a pattern of lines is characteristic of pultruded unidirectional composites. Fig l(b) shows a resin-rich region at a high magnification along with associated microvoids. Three-point bending fixture and specimens A three-point pending fixture was designed to allow the testing of the half-round specimens with support span lengths varying between 25 and 200 mm. The three-point bend specimens were cut at eight different lengths L to obtain L/r ratios ranging from 4,5 to 17. The specimens were loaded at the mid-span of the flat surface, with the round surface down as shown in Fig 2. The support anvils were similar to that required by the ASTM standard D4475-858. The maximum tensile stress or~x prevailing in the extreme fibres of the round surface and the maximum shear stress ~%=xprevailing at the neutral plane were calculated using the beam theory, and for the case of half-round rods, they are given by, respectively: 38

Testing The tests were performed on a servohydraulic MTS fatigue machine at nine constant cross-head speeds ranging from 1 to 20 000 mm min-~. The load/displacement curves were recorded using a data acquisition system linked to an IBM computer on which all the necessary computations were made. The use of a fatigue machine was particularly useful for this investigation for two reasons: • It allows an instantaneous unloading of the specimen at any load or displacement for any loading rate. Particularly, a test can be run so that the specimen is unloaded as soon as the displacement at the maximum load is reached. This will allow a precise determination of the fracture mechanism corresponding to the maximum load; and • the fatigue machine used in this study permits the reaching of a constant loading rate as high as 20 000 mm min-', which is almost a low rate impact. However, this is different from an impact test where only the incident energy can be controlled. Finally, additional tests were performed on an instrumented pneumatic impact machine (Dynatup GRC-8250) equipped with an anti-rebound system.

.....

!°°_!

......

~5

Time F~g3

Schematic representation of the triangular displacement/time waveform used and the resulting load/time response.

CONSTRUCTION& BUILDINGMATERIALSVol. 6 No. 1 MARCH1992

R Boukhili et sl

Results end discussion Fig 3 is a schematic representation of the triangular displacement/time waveform used and the resulting load/time response. To determine the displacement at which the specimen will be unloaded, a first set of at least six samples are loaded well above the maximum load and the displacement dc corresponding to this maximum load measured. Then the triangular waveform is readjusted so that the pro-imposed displacement will correspond to the displacement at the maximum load and a second set of specimens is teated to determine the fracture mode corresponding to this maximum load. Fracture mechanisms mapping

Even though many studies have dealt with the effect of spanto-depth ratio °,1° and loading rate 5," on the fracture mechanism, no literature where both effects are studied simultaneously seems to exist. That is, the effect of span-todepth ratio is investigated at a given loading rate, or the effect of loading rate is investigated for a given specimen length, which is usually a tensile specimen. In this investigation both effects are investigated at the same time - eight L/r ratios at nine loading rates - and, as mentioned previously, the three-point bending tests were arranged so that the fracture mechanism responsible for the first fracture could be determined. Table 1 maps the fracture mechanisms observed, shear (S) or tensile (T), for glass/polyester and glass/epoxy and the corresponding strengths. This means if a shear fracture is observed then the shear strength is calculated using Equation (2), and if tensile fracture is observed it is the flexural strength which is calculated using Equation (1). When tensile and shear fractures coexist, shear and tensile strengths are calculated. It should be mentioned that the results obtained with glass/polyester at low loading rates and small/Jr ratios are not taken into account since they correspond to a compression fracture mode. In Table 1, the cases where both tensile and shear (T/S) fracture modes are reported correspond to situations where both ultimate strengths seem to be reached at the same time. In fact these are cases where some specimens fail in tension and others fail in shear. Up to 14 specimens were retested under the same conditions to determine whether this was a random effect; however, the number of the specimens failing in a given mode was too large to be neglected. A possible explanation of this situation is that, depending on the amount of processing defects (resin-rich regions and microvoids) in each specimen, a fraction of the specimens tested fail in shear, another fraction fail in tension, and a third fraction may fail in a mixed mode. The latter case can be avoided by a better adjustment of the preset maximum deflection. In the following sections and from Table 1, two points will be discussed: first, the effect of the loading rate on the apparent interlaminar shear strength (ILS) and on the flexural strength, and the role of processing defects; second, the change in fracture mechanism as a function of loading rate for intermediate L/r ratios. Dependence of shear and tensile strength on loading rate

The effect of loading rate on the shear strength could be descdbed by any of the L/r ratios which gives a shear fracture in a large range of loading rates, since this effect is the same for all of them. From Table 1,/jr = 5.5 seems to give the best compromise (the larger loading rates range) for the two materials investigated and the results are plotted in a semilogarithmic diagram in Fig 4 along with the scatter bars. From Fig 4, it can be seen that the shear strength of both materials increases significantly with loading rate and reaches a maximum value at 5000 mm min -1. Above 5000 mm min -1 the shear strength decreases slightly and this decrease seems to be supported by the additional impact tests. However, the CONSTRUCTION & BUILDING MATERIALS Vol. 6 No. 1 MARCH 1992

Table 1 Fracture mechanisms mapping representing the fracture mechanism compression (C), shear (8) or tensile (T) and the measured strength Tu or ou in MPa, as a function of the loading rate and the span-to-depth ratio Glass fibre-reinforced epoxy Cross-head speed (mm min -1) 1 10 100 1000 10000 20000

L/r 4.6

4.75

5.5

6.2

S 70.1 S 75 S 77.2 S 91.8 S 87.8 S 87.6

S 75.3 S 78.1 S 84.3 S 88 S 80 S 81

S 72.1 S 76.7 S 80.4 S 87.2 S 85.2 S 80.5

S 74.5 S 77.4 S 80.6 S 84 S 81.3 S 82.1

6.9

8.3

16

17,6

T/S T T T 1318/6,51 1 5 5 7 1518 1544 T/S T T T 1385/68.5 1649 1631 1629 T/S T/S T T 1509/74.5 1700/62.3 1695 1771 S T/S T T 79.2 1757/64.4 1747 1759 S S T T 79.4 66.5 1796 1810 S S T T 80 58,8 1807 1818

Glass fibre-reinforced polyester Cross-head speed (mm min -1)

4.5

4.75

5.5

1

C

C

10

C

C

100

S 60.9 S 6,56 S 69 S 61.4

S 59.5 S 67.4 S 66.3 S 62.6

S 46.1 S 49.1 S 55.5 S 62 S 66.5 S 63

1000 10000 20000

L/r 5.9

8.75

8.9

16

S T T T 45.6 939 1118 1185 S TIS T T 54.6 1048/41 1150 1276 S T/S TIS T 5 5 . 9 1214/47,1 1230/46.9 1338 S S T/S T 60.9 53.7 1403/53.6 1421 S S S T 65.2 56 55.2 1593 S S S T 62.8 56 56 1595

17 T 1202 T 1355 T 1495 T 1557 T 1546 T 1631

IO0 • Gloss/epoxy 90

o Gloss/polyester

8O

f

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,

iO-I

i ......

i

IOO

i

,

. . . . . ,i

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IO2

IO5

IO4

IO5

IO6

Cross-heod speed (ram rain -I )

F/g4

Variation of shear strength as a function of loading rate for L/r = 5.5

overall effect of the loading rate is larger in the case of glass/polyester than in the case of glass/epoxy. This is well illustrated in Fig 5 where the relative increase in the shear strength, given by: A on r/~- = (~n - ~'o)/7o where ~o is the ILS at loading rate of 1 mm min -1 and Tn is the ILS at another loading rate n, is plotted against the loading rate. The larger loading rate effect in the case of glass/polyester can be related to the fact that the polyester resin, being more ductile than the epoxy, is more prone to viscoelastic effects and consequently is more rate sensitive. 39

Testingof unidirectionalpultruded composites 0.5 • Gloss/epoxy

o

o

o o

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102

103

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105

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Cross-head speed ( mm min-I)

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Relative increase in shear strength as a function of loading rate

Fig8

Resin.rich region in a glass~epoxy as a preferential crack initiation site at high loading rate (20 000 mm min-,)

2000

ill}

• Gloss/epoxy

1800

o Glass/polyester

•

i

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°nl40C

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i00

I01 102 I03 104 Cross-heed speed (ram min -I )

105

106

Fig 9

Typicalinduced impact multiple cracking in a cross-section of glass-polyester

Fig6

Variation of tensile strength as a function of loading rate for L/r = 16

0..5 • Gloss/epoxy 0.4

o Gloss/polyester

ooo OtD

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,

, IH,,

106

Relative increase in tensile strength as a function of loading rate

The effect of loading rate on the flexural strength is described using the L/r ratio of 16 since the observed fracture is purely tensile independently of the loading rate. As opposed to the shear strength, the flexural strength seems to increase continuously over the whole range of loading rate used, as shown by Fig. 6. The additional impact results confirm this tendency, although a large scatter is observed for these tests. The increase of shear strength and tensile strength as a function of loading rate may be related to the viscoelastic behaviour of the polymeric matrix. Nevertheless, the viscoelastic effects are expected to be more pronounced for the shear strength than for the flexural strength, since the former is a matrix-controlled property. Actually, the loading rate is found to affect 7. and % by approximately the same amount for both materials as shown in Figs 5 and 7. A probable explanation of such behaviour is to consider that the positive effect of the loading rate increase on 7. is diminished by a deleterious effect of the pultrusion defects, that is: • The materials contain processing defects such as resinrich regions and microvoids (Fig l(b)); • the brittleness of the matrix increases as the loading rate increases; and • a brittle matrix is more sensitive to the presence of defects and consequently the material is more sensitive to the processing defects at high loading rates. CONSTRUCTION & BUILDING MATERIALS Vol. 6 NO. 1 MARCH 1992

R Boukhili et al

To support this interpretation, cross-sections of some specimens loaded to their ultimate load were viewed in the SEM. These observations show that at high loading rates the propagating crack initiates preferentially in the resin-rich regions close to or at the neutral plane as shown in Fig 8. The above-mentioned defect sensitivity also explains the slight decrease of T= recorded at very high loading rates and impact tests (Figs 4 and 5). As a matter of fact, many of the specimens teated at impact velocities showed a multiple cracking pattern rather than a shear plane, as shown in Fig 9 for a giaas/polyester specimen. Fracture mechanisms as a function of loading rate

The dependence of the fracture mechanisms on the spanto-depth ratio is mainly controlled by the ratio of the tensile strength to the shear strength, air ¢~o. Based on Equations (1) and (2), for the half-circle cross-section used, it can be found that: G/T = 2.94L/r

(3)

Supposing that the values of the shear strength and flexural strength are known, the L/r ratio corresponding to a mixedmode failure (shear and flexure) can be deduced. Since in

6000

this investigation it was found that both the tensile and the shear strengths increase with increasing loading rate by approximately the same ratio, the ratio d~. could be considered as a constant. In the case of glass/polyester the olT ratio is approximately 25. According to Equation (3), this corresponds to an approximate critical L/r ratio of about 8*5 and, consequently, specimens tested at this L/r ratio should fail in a mixed mode. According to Table I this is only true for intermediate loading rates. Actually, the fracture mechanisms change progressively from purely tensile at low loading rates to a mixed-mode fracture at intermediate loading rates and then purely shear at high loading rates (see Fig 10). The evolution of the fracture mechanisms towards shear with increasing loading rate is again basically related to the interaction between the loading rate, the brittleness and the sensitivity to defects, as detailed in the previous section. Alternatively, the role of defects can be highlighted by the decrease of shear strength with increasing/Jr ratio. Indeed, considering the L/r ratios between 4.5 and 9 at loading rates above 103 mm min -1, all these specimens fail in shear and the recorded shear strengths decrease as the L/r ratio increases (Fig 11). Since r is constant, the increase of L/r ratio implies an increase of the loaded volume and this implies an increase in the probability to find defects. This in turn explains the decrease in the shear strength. Conclusions The major feature of this investigation is that the deflection at the maximum load is pre-imposed and the specimen is unloaded as soon as this deflection is reached. This method has enabled us to map the fracture mechanisms involved at the maximum load as a function of the loading rate and the span-to-depth ratio. From this mapping, the following conclusions can be drawn.

500C IOOmm~ 400C

sooo o

2000

The shear strength and flexural strength increase with the loading rate and follow the same general pattern of behaviour. However, at loading rates exceeding 5000 mm min -1 the shear strength tends to decrease slightly due to the sensitivity to processing defects.

i mmmin_t

I000 0

Fig 10

I

i

I

I

0

I

I

I

2 Displacement(ram)

I

3

4

Typicalload~displacement responses for Ur = 8.75 at different loading rates for glass~polyester

For intermediate span-to-depth ratios, the maximum load results in a tensile fracture at low loading mtas and a shear fracture at high loading rates. This behaviour was interpreted as being due to the fact that the increase in loading rate increases the brittleness of the material, thereby increasing the sensitivity to existing defects and leading to a shear fracture rather than to a tensile fracture.

I00 o Gloss/ polyester • Glass/epoxy

90

t

t

A readily apparent implication which could be raised from a mapping such as that shown in Table 1 is that in the design of pultrudad unidirectional composite beams, the designer should be aware of the fact that the fracture modes, and hence the design parameters, are not necessarily the same at low and high loading rates. Finally, the role of pultrusion processing defects at high loading rates and impact velocities is a subject which should be further clarified, particularly with the increasing application of pultrudad structures.

8O ~-770

60 5 N ,

-4

Fig 11

,

I

5

I

The loading rate effect is greater in glass/polyester than in glass/epoxy. This is due to the fact that the polyester resin is more ductile than the epoxy resin and, consequently, more rate sensitive.

I

I

I

6 7 S~n-to- depthratio

,

I

8

I

9

Variation of shear strength as a function of span-to-depth ratio at high loading rate (10 000 mm min -I)

CONSTRUCTION & BUILDINGMATERIALSVol. 6 No. 1 MARCH1992

Acknowledgements The authors wish to thank the Natural Sciences and Engineering Research Council of Canada and the Fonds pour la Formation des Chemheurs et de rAide b la Rechemhe for their financial support. 41

Testing of unidirectional pultruded composites References 1 Martin, J D and Sumerak, J E. Pultrusion. Engineering Materials Handbook, Vol2: Engineering Plastics ASM Int (1989) 389-398 2 Sims, G D, Johnson, A F and Hill, R D. Mechanical and structural properties of GRP pultruded section Composite Struct 8 (1987) 173-187 3 Bank, L C. Flexural and shear moduli of full-section fibre reinforced plastic (FRP) pultruded beams J Testing and Evaluation 17 (1989) 40-45 4 Kesslert S L, Adams, G C, Ddsooll, S B and Ireland, D R (Eds) Instrumented impact testing of plastics and composite materials ASTM STP 936 American Society for Testing and Materials, Philadelphia (1987) 5 Hayes, S V and Adams, D F. Rate sensitive tensile impact properties of fully and partially loaded unidirectional composites J Testing and Evaluation 10 (2) (1982) 61-68 6 Apparent intedeminar shear strength of parallel fibre composites by short-beam method ASTM D 2344-84, ASTM Standards and Literature for Composite Materials, First Edition American

7

8 9 10

11

Society for Testing and Materials. Philadelphia (1987) Flexural properties of unreinforced and reinforced plastics and electrical insulating materials AS'I'M D 790-86, Annual Book of ASTM Standards Vol 08, American Society for Testing and Materials, Philadelphia Apparent horizontal shear strength of pultruded reinforced rods by the short-beam method ASTM D 4475-85 idem Rossnssft, M and Marom, G. Evaluation and bending test methods for composite materials J Composite Technol & Res 7 (1) (1985) 12-16 Sattar, S A and Kellog, D H. The effect of geometry on the mode of failure in short beam shear test Composite Materials: Testing and Design, ASTM STP 460 American Society of Testing and Materials, Philadelphia (1969) 62-71 Lhymn, G, Lyhmn, Y, Peckens, P and Young, J. Effect of loading rate on tensile strength of fibrous composites Composites 19 (1988) 295-299

This article first appeared in Composites, 22 (1), January 1991, 39-45.

Kessler, S L, Adams, G C, Drlscoll, S B and Ireland, D R (Eds)

42

CONSTRUCTION& BUILDING MATERIALSVol. 6 No. 1 MARCH 1992