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Improvement of bearing strength of laminated composites
Composite Structures 76 (2006) 260–271 www.elsevier.com/locate/compstruct
Improvement of bearing strength of laminated composites A. Crosky a, D. Kelly
b,* ,
R. Li b, X. Legrand c, N. Huong a, R. Ujjin
q
b
a School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, NSW 2052, Australia Ecole Nationale des Arts et Industrie Textiles, Laboratoire GEMTEX, 9 rue de l’Ermitage – BP 30329, 59100 Roubaix, France b
c
Available online 1 August 2006
Abstract Carbon fibre reinforced composite structures generally out perform metallic ones, but this is not the case when the structure is loaded in bearing. This work examines several different strategies for improving bearing performance. The first strategy was fibre steering (directed fibre placement). Three different methods were used to determine the trajectories for the steered fibres, referred to as the principal stress method, the load path method and the genetic algorithm method. In the principal stress method the fibre trajectories were determined from the major (tensile), s11, and minor (compressive), s22, principal stresses. An improvement of 36% in bearing strength was obtained using a modified principal stress pattern in which the s11 trajectories were displaced 5 mm away from the bearing-loaded hole to reduce fibre waviness which resulted from local overcrowding of the s11 and s 22 tows directly beneath the bearing surface. In the load path method, the steered fibre trajectories followed the dominant load path. A similar improvement in bearing strength of 33% was obtained using this method. Moreover, it was found that the load path method provided surplus reinforcement against net section failure and it was possible to more than double the efficiency of carbon laminate joints by reducing the ratio of joint width to bolthole diameter w/d from the standard value of 5 to a more compact 2.5, while simultaneously maintaining the bearing strength of the laminate. The third method employed an optimisation technique in which a genetic algorithm was used to determine the trajectories for the steered fibres. The trajectories obtained were intermediate between those from the principal stress method and those from the load path method. Matrix stiffening using nanoreinforcement was also examined. In this part of the work montmorillonite clay at loadings of 7.5 and 12.5 parts per hundred resin was incorporated into the matrix resin. While the addition of the clay nanoparticles substantially stiffened the neat resin, its incorporation into the composite laminates did not increase the bearing strength due to the introduction of an alternative, premature failure mode. The incorporation of the nanoparticles did however stiffen the bearing response, indicating that the method could produce improved bearing strength if premature failure could be avoided. Through thickness reinforcement using z-pins was also examined by inserting carbon fibre z-pins, at an aerial density of 4%, locally in the vicinity of the bearing-loaded hole. This technique increased the ultimate bearing load by 7%. However this did not translate into improved bearing strength because of local thickening of the laminate in the z-pinned region due to the presence of surplus resin displaced by the z-pins. It is considered that local thickening could be readily avoided by using a caul plate in the vicinity of the z-pinned region, thereby allowing the potential of this technique to be realised. 2006 Published by Elsevier Ltd. Keywords: Bearing; Fibre steering; Directed fibre placement; Genetic algorithm; Nanocomposites; z-Pinning
1. Introduction
q This research was undertaken as part of the research program of the Cooperative Research Centre for Advanced Composite Structures Ltd. * Corresponding author. Tel.: +61 2 385 4160; fax: +61 2 966 31222. E-mail address: [email protected] (D. Kelly).
0263-8223/$ - see front matter 2006 Published by Elsevier Ltd. doi:10.1016/j.compstruct.2006.06.036
Bolted connections are commonly used in preference to other joining techniques because they allow greater freedom in assembly and repair. However, introducing a bolted joint into a laminate introduces a new potential failure mode against which the laminate is relatively weak. The bolt imposes a compression load on the laminate and this
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can result in failure in bearing by a combination of fibre buckling, shear and interlaminar splitting. The efficiency J of a single row fastened joint in a panel is the ratio of the maximum bearing load PB that can be transferred by the bolt to the laminate and the ultimate tensile load PTU for the laminate remote from the hole. It is a function of the tensile strength FTU and the bearing strength FB of the laminate, the width w and thickness t of the specimen, and the diameter d and local thickness tB of the fastener hole, as shown in Fig. 1. The joint efficiency J is given by J¼
PB PB F B dtB ¼ ¼ P TU F TU wt F TU wt
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Wu and Sun [2] made a detailed study of bearing failure initiation in 0/90 carbon fibre epoxy composites. They used the pin loaded test shown in Fig. 2 and sectioned the laminates along the contact centre line after loading to varying levels of the ultimate bearing load. This enabled them to detect the initial damage as shown in Fig. 3. They concluded that bearing failure initiated near the two flat surfaces of the laminates and then propagated inwards from each of the surfaces at an angle, Fig. 3. The failure mode in the 0 plies appeared to be a kink band resulting from fibre microbuckling, as observed in compression failure
ð1Þ
For composite laminates, the joint efficiency is substantially lower than for structural metals [1], as shown in Fig. 1. This problem has been recognised ever since composites were first considered for structural applications. The standard approach to overcome this problem in laminates is to increase the local thickness of the laminate or to add doublers in the vicinity of the joint. While doublers can increase the load carrying capacity, the bearing strength itself (determined by dividing the maximum bolt load that can be carried by the joint by the product of the hole diameter and the thickness at the hole) is often actually reduced compared to that of the laminate. This is because new failure modes involving separation of the doubler can be introduced, while any offset of the mid-plane of the laminate can introduce bending into the joint. As a result the strength does not scale directly with thickness.
Fig. 2. Schematic diagram of pin-contact test [2].
0.8
Ductile metal
0.7
d
Brittle material
PB/FTU wt
0.6
PB
Composite
0.5 0.4
Tension failure
Bearing failure
0.3 0.2 0.1 0
w
0
0.2
0.4
0.6
d/w
Fig. 1. Joint efficiency of different materials (after Hart-Smith [1]).
0.8
1
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Fig. 3. Initial damage in pin-loaded laminate [2].
16 14 12 10 E1/E2
of unidirectional composites [3], Fig. 4. From these results they concluded that bearing damage is initiated by fibre microbuckling of the 0 plies at the edges of the contact region near the free surfaces. As the load increases the damage propagates into the interior at an angle of about 30–40 until it reaches a critical length whereupon unstable growth begins and catastrophic failure occurs. From the above it is evident that increasing the compressive strength of the laminate below the contact zone should improve bearing performance. Accordingly, the present work examined the use of directed fibre placement (fibre steering) for this purpose. Increasing the bearing resistance increases the likelihood of net section failure (i.e., tensile failure at the sides of the hole) and the steered fibre patterns included an additional component to resist this mode of failure. The steered fibre tows must be accurately aligned with the load flow direction to sustain the maximum performance. As shown in Fig. 5, the effective
8 6 4 2 0 0
10
20
70
80
90
Fig. 5. Effective modulus versus aligning angle of reinforcement fibre [4].
longitudinal modulus of the reinforcement fibre in the composite declines quickly when the fibres do not line up with the load flow direction. In the present study, three different methods of establishing the trajectories were used. These are referred to as the principal stress method, the load path method and the genetic algorithm method. Wu and Sun [2] used Sun and Jun’s [5] microbuckling model to show that the critical stress in the direction of compression (rxx)cr is given by ðrxx Þcr ¼ Gep m =ð1 vf Þ Gep m
Fig. 4. Enlarged are near the edge of the cross-section shown in Fig. 3 [2].
30 40 50 60 aligning angle [º]
ð2Þ
where is the elastic–plastic shear modulus of the matrix and vf is the fibre volume fraction. This indicates that increasing the shear modulus of the matrix should also improve bearing performance. Increasing the modulus generally leads to a reduction in toughness for matrix resins but recent work has shown that simultaneous increases in both stiffness and toughness can be achieved in epoxy resin by
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incorporating nanoparticles into the resin [6] and this method of improving bearing strength was also examined here. It has been reported that bearing strength is also strongly affected by lateral constraint (clamping force) at the loaded hole [7,8]. The benefit of lateral constraint is also evident from examination of Fig. 3 where it can be seen that generation of the kink bands involves outward lateral displacement of the material below the kink bands. This suggests that bearing performance may be improved by through thickness reinforcement and some preliminary work using z-pinning [9] is also reported here. 2. Directed fibre placement 2.1. Principal stress method In this method the steered fibres were incorporated into near quasi-isotropic laminates with the steered fibres being placed along the trajectories defined by the maximum principal tensile stress (s11) and the minimum principal compressive stress (s22) in the vicinity of the hole. The two patterns are shown in Fig. 6 and react the net section (s11) and bearing loads (s22) respectively. MSC.PATRAN and MSC.NASTRAN were used to construct 2d models for the specific bolted joints concerned. The bolt was modelled as a rigid pin applying a compression load normal to the hole surface. The principal stress vectors were then converted to a contour using the stream trace algorithm in Techplot. Fortran and C++ codes were then used to convert this data into x–y coordinates [10], for precise placement by a robot as described in Ref. [11]. In some cases, however, the steered fibre tows were laid by hand.
Fig. 6. Principal stress trajectories for a pin-loaded hole.
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The laminates were fabricated from five plies of 180 C cure bidirectional 0/90 Fiberdux 914C-933-42 five-shaft satin weave prepreg, with Toray T300-3k-40a carbon fibre yarn, with the stacking sequence ±45/090/090/090/ ±45. Both the s11 and s22 steered fibre patterns were incorporated between each fabric ply as shown in Fig. 7. A layer of M18 film adhesive was also included between each fabric ply to assist in wet out of the steered fibres. The s11 steered fibre pattern consisted of 10 tows of Tenex 3k HTA carbon fibre while the s22 pattern consisted of 26 tows of 6k HTA fibre. It was noted that the existence of an s11 tow at the bearing edge, Fig. 8(a), caused overcrowded stacking of the steered fibre tows and increased the waviness of the s22 tows in the through-thickness direction. To avoid this, a modified s11 pattern was used where the trajectories were extended 5 mm past the hole as shown in Fig. 8(b). The resulting reduction in overstacking is shown in Fig. 9(b). On the tension side of the bolthole, the modified pattern followed the FEA s11 trajectories, retaining its reinforcing effect against net section (tensile) breakage. Laminates were hand laid up with both the steered and modified steered patterns, and also without steered fibres (baseline), then cured between caul plates in an industrial autoclave at 180 C using the material supplier’s recommended cure cycle. After curing, the laminates were cut and ground to the size illustrated in Fig. 10. A hole 20 mm in diameter was drilled and reamed in the position shown in Fig. 10 to a tight tolerance of 20 ± 0.02 mm using a numerically controlled machine. Six replicates of both sets of steered fibre specimens, as well as baseline specimens, were tested using an Instron 1185 tensile testing machine operated at a crosshead rate of 2 mm/min. The bearing tests were carried out using a modified version of ASTM D5961-96 [11] using a load pin 19.97 mm in diameter and a fastener torque of 14 N m. Full details of the test procedure are given
Fig. 7. Typical layup of steered fibre samples.
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Fig. 8. Modification of fibre steering patterns: (a) FEA s11 trajectory and (b) s11 shifted 5 mm away from edge.
Fig. 9. Reduction of fibre over stacking: (a) FEA pattern and (b) modified pattern.
Table 2 Bearing test results
Fig. 10. Dimension of specimens.
elsewhere [12]. Details of the test specimens are given in Table 1. The test results are given in Table 2. The standard steered fibre pattern produced an increase of 139% in peak load and 20% in bearing strength while the modified steered pattern produced more substantial increases of 169% (peak load) and 36% (bearing strength). It is also noted that the modified pattern produced substantially less thickening in the vicinity of the hole, Table 2. More modest improvements were obtained in specimens with robot laid steered fibres of lower loading [12]. However no significant improvement was obtained when steered
Group
Ultimate failure Peak mode load (kN)
Ultimate Weight Bearing strength (g) area (MPa) (mm2)
Baseline Steered Modified steered
Bearing [12] 9.55 367.80 Net section [14] 22.86 441.24 Net section [14] 25.65 500.40
42.50 59.75 56.38
26.40 51.83 51.27
carbon fibre s11 and s22 patterns were incorporated into glass fibre laminates. It is thought that in this case, because of their higher modulus and lower strain to failure, the steered fibres failed before the bearing strength of the fibre glass laminate was achieved, thus imparting no improvement. The failure mode typical of the onset of failure in the carbon fibre laminates reinforced with steered fibre is shown in Fig. 11. The characteristic kink band formation reported by Wu and Sun [2] is clearly evident (compare Figs. 3 and 11).
Table 1 Configuration of specimens Group
Lay-up
Description
Baseline
±45/090/090/090/±45
Steered
±45-s22/s11-090-s22/s22-090-s22/s22-090-s11/s22-±45
Modified steered
±45-s22/s11-090-s22/s22-090-s22/s22-090-s11/s22-±45
Autoclave cured 5-ply Fiberite 914C carbon fibre prepreg without steered fibres Autoclave cured 5-ply Fiberite 914C samples steered with 6k Tenex HTA tow in s22 and 3k Tenex HTA tow in s11 with 1 layer of M18 resin film Autoclave cured 5-ply Fiberite 914C samples steered with 6k Tenex HTA tow in s22 and 3k Tenex HTA tow in modified s11 with 1 layer of M18 resin film
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The forces acting on a plane with normal given by ~ n ¼ nx i þ ny j þ nz k are obtained by integrating the total stress vectors Z n dA F x ¼ V x ~ Z n dA F y ¼ V y ~ Z n dA F z ¼ V z ~
Fig. 11. Section view of bearing failure in steered fibre laminate.
2.2. Load path method In the work described above, the specimens with both the standard and modified principal stress trajectories failed ultimately by tension breakage in the net section, Table 2, while the baseline specimens failed ultimately by bearing. A new pattern was therefore trialed based on the load path trajectories developed in Refs. [10,13]. In this pattern a single layer of steered fibres was used to simultaneously enhance both the tensile strength in net-section failure and the compressive strength beneath the bearing surface, as shown in Fig. 12. 2.3. Theory of load path trajectories The concept of load flow is widely used by design engineers to describe the way a structure carries applied loads from the point of application to the point of reaction in the structure. Essentially the load path is a statically determinate path that can carry the applied load. A structure is redundant if there is more than one of these paths. In Ref. [13] it was shown that the load paths can be defined by plotting contours aligned with the total stress vectors defined by V x ¼ rxx i þ sxy j þ sxz k V y ¼ syx i þ ryy j þ syz k
where the dot indicates the vector dot product. The load path for a force in a given direction is a region in which the force in that direction remains constant. For example, if the path in Fig. 13 is to define a region in which the force Px remains constant, the requirement is to determine the curved contour forming an edge along which the normal and tangential edge loads make no contribution to force in the x direction. This requires Fx ¼ 0 R or V x ~ n dA ¼ 0 This is achieved if the normal to the surface is perpendicular to Vx as the dot product is then zero. Alternatively, this is achieved if the surface tangents are parallel to the vector Vx as indicated in Fig. 13. Fortran codes were compiled to automatically read nodal values of the stress components from a two-dimensional finite element analysis from MSC/NASTRAN or ANSYS, and calculate load path vectors for later visualisation using the stream-trace option in TECPLOT. rxx Lxx ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r2xx þ r2yy sxy Lxy ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r2xx þ r2yy ryy Lyy ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r2xx þ r2yy For X direction load path visualisation, Lxx was assigned as the stream-trace vector U in the horizontal direction, and Lxy was assigned as the stream-trace vector V in the vertical direction in TECPLOT. For Y direction load path
V z ¼ szx i þ szy j þ rzz k n Vx
B
A Px
Px C D Fig. 12. Dominant load path (lp) trajectory around a bolthole.
Fig. 13. Contours for a path along which Px is constant.
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visualisation, Lxy was assigned as the stream-trace vector U in the horizontal direction, and Lyy was assigned as the stream-trace vector V in the vertical direction in TEC-
PLOT. The load path pattern is shown in Fig. 14. The dominant load direction for the joint in Fig. 14 is the X direction.
Fig. 14. Load path trajectories.
Fig. 15. Dominant load path trajectories (hole diameter = 20 mm).
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Steered fibre and baseline specimens were manufactured from Hexcel W3G 282 bi-directional plain weave carbon fibre fabric prepreg in a 16-ply quasi-isotropic lay-up sequence as follows: 45 =lp=0 90 =lp=0 90 =lp= 45 =lp= 45 =lp=0 90 =lp=0 90 lp= 45 =lp= 45 =lp=0 90 =lp=0 90 =lp= 45 =lp= 45 =lp=0 90 =lp=0 90 =lp= 45 where lp indicates a layer of steered fibres in the pattern obtained using the load path trajectories. Fifteen steered fibre layers of 3k Tenex HTA 5331 carbon fibre tows were placed by hand onto the surface of each of 15 prepreg plies which, together with one bare surface ply, were later laid up into a fibre steered 16-ply laminate. The specimens were autoclave cured at 180 C in a production facility using the prepreg supplier’s recommended cure cycle. Specimens were cut to the dimensions shown in Fig. 10 except that the width was 90 mm. This was necessary to avoid net section failure in the baseline specimens due to a lower fibre volume in this prepreg than in the prepreg used for the tests given in the previous sections. Three replicates of the baseline and five replicates of the steered fibre specimens were tested using an identical procedure to that described in the previous section. Full details are given in Ref. [14]. The steered specimens gave a 53% increase in peak load and a 33% increase in bearing strength. It is noted that the improved peak load translated better into improved bearing strength than for the principal stress trajectory specimens (169% increase in peak load was required to give 36% improvement in bearing strength) because thickening of the laminate was reduced since only one pattern of steered fibres was used. Moreover, contrary to what was observed for the principal stress trajectory specimens, failure still occurred in bearing, indicating that the load path patterns provided surplus net section strengthening. This presented the possibility of reducing the specimen width which, in a multi-fastened joint, would equate to a reduced fastener spacing. Accordingly, additional sets of steered fibre specimens were prepared with widths of 70, 60 and 50 mm. The contours determined for the different specimens (ratios of width to diameter from 4.5 to 2.5), including the 90 mm specimens are shown in Fig. 15. A trajectory was plotted for each proposed fibre tow, starting with a uniform distribution at the left hand end of the specimen. The number of load path trajectories (carbon fibre tows)
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in each pattern was 16, 16, 14 and 10 for the 90 mm, 70 mm, 60 mm and 50 mm wide specimens respectively. As for the 90 mm wide specimens, five replicates were tested for each of the other specimen widths. The results for all the load path specimens are given in Table 3. All specimens down to a width of 60 mm failed initially in bearing, although final failure for the 60 mm specimens was mostly net section (tension) failure. Three of the five 50 mm wide specimens also failed initially in bearing. These results indicate that the inclusion of the steered fibres shifts the transition from bearing to net section failure down to a w/d value of 2.5, compared with 4.5 for the unsteered specimens, while simultaneously maintaining (in fact slightly improving) the bearing strength (3% increase for the 50 mm wide specimens) of the laminate. 2.4. Genetic algorithm method Steered fibre trajectories were also obtained using a genetic algorithm (GA). The GA is an optimisation technique based on simulation of Darwin’s theory of life evolution. From a randomised population, the GA evolves using three operators based on several fit goals in order to get an acceptable population. The three operators are selection, cross-over and mutation. The advantage of GA’s is that they give the designer a family of near optimal designs with a small variation in their performance index instead of a single design. The domain was sampled into two-dimensional finite elements and anisotropic properties corresponding to a uni-directional carbon fibre laminate were defined for the plate. The orientations of the material axes for each element were then set as variables for the genetic algorithm. The fitness criterion was set in terms of strain energy and the population allowed to evolve to define the orientation of the fibres. The fibre orientation was assumed constant within each element and the vector showing the orientation in the element was defined at the centroid of the element. The orientation of the material axis on each element was then prescribed as an independent variable for the genetic algorithm. Full details of the procedure used are given in Ref. [15]. The result obtained is shown in Fig. 16. It is not straightforward from the GA result to determine how and where the steered fibre tow should be laid. A direct interpretation of the result shown in Fig. 16 gives the fibre steered pattern of Fig. 17(a). However, acknowledging that the carbon tow can be several millimetres wide, and the
Table 3 Bearing performance of composite joints using load path design
Peak load (kN) Ultimate strength (MPa) Ultimate failure mode
Baseline 90 mm (w/d = 4.5)
Steered 90 mm (w/d = 4.5)
Steered 70 mm (w/d = 3.5)
Steered 60 mm (w/d = 3)
Steered 50 mm (w/d = 2.5)
32.9 ± 0.2 456 ± 2 All tension
50.3 ± 2.9 605 ± 36 All bearing
46.2 ± 2.1 566 ± 25 Most bearing
41.9 ± 1.3 513 ± 9 Most tension
37.4 ± 2.0 466 ± 24 All tension
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Fig. 16. GA result.
3. Nanoreinforcement of the matrix Based on Sun and Jun’s [5] microbuckling model, stiffening of the matrix should also improve bearing performance and this was examined by incorporating nanoclay into the matrix resin. The matrix resin used was Araldite LY568, a tetraglycidyldiamino diphenylmethane (TGDDM) epoxy resin while the curing agent was Ethacure 100. The nanoclay used was Nanomer I30E, a montmorillonite clay modified with octadecyl ammonium ions. Initially nancomposites were prepared from the resin with clay loadings of 1–20 parts per hundred of resin (phr), cured and tested in uniaxial compression. Full details of the procedure used to prepare the nanocomposites are given in Ref. [16]. The compression modulus was found to increase progressively with increasing clay content with the 20 phr nanocomposite showing a 50% increase in modulus compared with the pure resin, Fig. 18. Bidirectional 0/90 laminates were prepared from 20 piles of carbon fibre woven fabric impregnated with neat resin and also with resin reinforced with 7.5 and 12.5 phr nanoclay. The laminates were hot cured in a platen press at a temperature of 195 C and a pressure of 700 kPa. The laminates were then tested using the pin-contact test developed by Wu and Sun [2], shown in Fig. 2, using a 10 mm diameter pin. The results are shown in Fig. 19. No improvement in bearing strength was obtained at either clay loading. However it was noted that the load displacement curves were steeper for the nanoclay reinforced laminates with the bearing stiffness being increased by 5% for 7.5 phr nanoclay and 15% for 12.5 phr nano-
3700 3500 3300
Modulus (MPa)
plate is not covered everywhere by steered fibre, the result shown in Fig. 17(b) is obtained. These results are of considerable interest since they are essentially intermediate between the result obtained by the principal stress method and that obtained by the load path method.
3100 2900 2700 2500 2300 2100 0
1
2.5
5
7.5
10
12.5
20
Nanoclay Content (phr)
Fig. 18. Compression modulus of TGDDM/DETDA/I30E nanocomposites.
clay. The strain to failure was correspondingly lower for the nanoclay reinforced composites. Samples of the three composites were fractured in an interlaminar mode and examined using scanning electron microscopy. It was seen that the nanoclay reinforced composites showed a much higher level of matrix failure than did the composites made with unreinforced resin, Fig. 20, indicating that the presence of the nanoclay had weakened the resin and changed the failure mode. This is consistent with the reduced strain to failure observed for the nanoclay reinforced laminates. These results indicate that incorporation of the nanoclay into the matrix resin introduced a premature failure mode. If this could be averted, increased bearing performance should be achieved. 4. Through thickness reinforcement Through thickness reinforcement using z-pins was also examined. Baseline (unpinned) and z-pinned 16 ply
Fig. 17. Two interpretations of the result shown in Fig. 16.
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269
Bearing Strength 550
Bearing strength (MPa)
500
B0: Neat resin Vf = 57%
450 400
B1: 7.5 phr 130E Vf = 57%
350
B3: 7.5 phr 130E Vf = 57%
300 250
B4: 12.5 phr 130E Vf = 52%
200 150 100 B0
B1
B3
B4
Sample
Fig. 19. Bearing strength determined from pin-contact test for laminates containing 0, 7.5 and 12.5 phr nanoclay.
Fig. 20. Fracture surfaces of unreinforced (neat resin) and nanoclay reinforced (7.5 phr 130E) laminates.
quasi-isotropic laminates, with a (±45/090/090/)2s stacking sequence, were prepared from Hexcel W3G 282 bi-directional plain weave fabric prepreg. All samples were cured (without a caul plate) in an industrial autoclave at
180 C using the prepreg supplier’s recommended cure cycle. The z-pins were 0.28 mm diameter carbon fibre z-pins with an aerial density of 4%. They were supplied as 20 mm by 40 mm performs and were inserted into the
z-pin
final thickness
Stage One
Stage Two
bulge
laminate
Stage Three
pin is cut
Stage Four
Fig. 21. Z-Pinning procedure.
remaining plies added
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Z- pins overlapping hole by 4 mm Rectangular z-pin preform
40 mm 20 mm
Fig. 22. Z-Pin perform and positioning of z-pins relative to hole.
Table 4 Bearing test results
improved bearing strength (in fact a reduction of 6% occurred) because the z-pinned laminates had bulked out in the region of the z-pins, relative to the rest of the laminate, when they were cured. This extra bulk was resin which had been displaced by the z-pins. Sections taken through the baseline and z-pinned specimens are shown in Fig. 23(a) and (b). It can be seen that the z-pins have substantially altered the failure mode. In particular, the damage has not penetrated as deep below the hole. Although this initial study did not show an improvement in bearing strength, the increase in bearing load achieved does suggest that the technique should improve bearing performance if local thickening of the laminate is avoided. Since the thickening was due only to surplus resin, avoidance of thickening should be possible using a caul plate. 5. Conclusions
Specimens
Baseline
Z-Pinned
Change (%)
Thickness (mm) Peak load (kN) Ultimate bearing strength (MPa)
3.54 ± 0.01 18.7 ± 2.9 528 ± 82
3.91 ± 0.03 20.0 ± 2.6 496 ± 62
10.6 7.2 6.1
laminates with an ultrasonic gun, before curing, using the procedure illustrated in Fig. 21. After curing the laminates were cut to the size shown in Fig. 10. However the hole diameter was reduced to 10 mm. The hole was drilled so as to penetrate 4 mm into the z-pinned area as shown in Fig. 22. Seven replicates of both the baseline and the z-pinned laminates were tested using the procedure described earlier for the steered fibre specimens, except that a 10 mm load pin was used. The results are given in Table 4. The z-pins increased the ultimate load by 7% however this did not translate into
Fibre steering, matrix stiffening and through thickness reinforcement using z-pins have been examined as ways of improving bearing performance in laminated fibre reinforced composites. The principal findings are as follows: • Bearing strength was improved by 36% using a modified pattern derived from the principal stresses. This pattern consisted of two sets of steered fibres defined respectively by the s11 (tensile) and s22 (compressive) principal stresses, as obtained from finite element analysis. The modification to the pattern involved shifting the FEAderived s11 trajectories 5 mm away from the hole to reduce overstocking of the steered fibre tows in the vicinity of the hole and thereby reduce the waviness of the s22 steered tows in this region. This almost doubled the improvement in bearing strength of 20% obtained with the unmodified steered fibre pattern.
Fig. 23. Section view of bearing failure in (a) baseline and (b) z-pinned laminates.
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• Bearing strength was improved by 33% using steered fibre trajectories defined by the dominant load path. Only a single set of fibres was used compared to the two separate sets necessary for the principal stress trajectories. As a result the reinforcement was more efficient in improving bearing strength. • The dominant load path trajectories provided surplus net section strength. It was therefore possible to reduce the sample width to almost one half that of the unsteered sample before the onset of net section failure occurred. This equated to a reduction in the w/d ratio for the transition from bearing failure to net section failure from 4.5 to 2.5. Since tension breakage governs the separation between fasteners in a multi-fastener design, the reduced w/d ratio for the onset of net section failure indicates that by using the load path design, more fasteners could be included in a joint of a given width. This would then increase the load capacity of the joint as each fastener transfers a load defined by the bearing strength. The joint efficiency J could therefore be more than doubled by fibre steering using the load path design. • The addition of clay nanoparticles to the matrix resin substantially stiffened the neat resin, but its incorporation into composite laminates did not increase the bearing strength due to the introduction of a different failure mode to that which occurred in the laminates made from the neat resin. The incorporation of the nanoparticles did however stiffen the bearing response, indicating that the method could produce improved bearing strength if introduction of the new and premature failure mode could be avoided. • Through thickness reinforcement using z-pins increased the ultimate bearing load by 7%. However this did not translate into improved bearing strength because of local thickening of the laminate in the z-pinned region due to the presence of surplus resin displaced by the zpins. It is considered that local thickening could be readily avoided by using a caul plate in the vicinity of the zpinned region, thereby allowing the potential of this technique to be realised.
Acknowledgements Part of this work was supported by the Office of Naval Research, Arlington USA, through GRANT
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N000140010583 to the Cooperative Research Centre for Advanced Composite Structures Ltd. Y.D.S. Rajapakse was the Technical Monitor. The authors are grateful to Hawker de Havilland, Sydney, Australia for assistance with specimen preparation and to Professor Adrian Mouritz, RMIT University, Melbourne, Australia for assistance with the z-pinning. References [1] Hart-Smith LJ. Joints. Engineered materials handbook. Composites, vol. 1. Metal Park, OH: ASM International; 1987. p. 479–95. [2] Wu PS, Sun CT. Modeling bearing failure initiation in pin-contact of composite laminates. Mech Mater 1998;29:325–35. [3] Camponeschi Jr ET. Compression response of thick-section composite materials. DTRC-SME-90/60. David Taylor Research Center, 1990. [4] Daniel IM, Ishai O. Engineering mechanics of composites materials. New York: Oxford University Press; 1994. p. 65. [5] Sun CT, Jun AW. Compressive strength of unidirectional fiber composites with matrix nonlinearity. Compos Sci Technol 1994;52:577–87. [6] LeBaron PC, Wang Z, Pinnavaia TJ. Polymer-layered silicate nanocomposites: an overview. Appl Clay Sci 1999;15(1–2):11–29. [7] Tsujimoto Y, Wilson D. Elasto-plastic failure analysis of composite bolted joints. J Compos Mater 1986;20:236–52. [8] Eriksson LI. On the bearing strength of bolted graphite/epoxy laminates. J Compos Mater 1990;24:1246–69. [9] Freitas G, Magee C, Dardzinski P, Fusco T. Fiber insertion process for improved damage tolerance in aircraft laminates. J Adv Mater 1994;25(4):36–43. [10] Tosh MW, Kelly DW. On the design, manufacture and testing of trajectorial fibre steering for carbon fibre composite laminates. Compos Part A 2000;31:1047–60. [11] Kelly DW, Willgoss R, Li R, Crosky A. A manufacturing procedure for composite laminate production using precision fibre placement. In: Repecka L, Saremi FF, editors. Proceedings, 48th international SAMPE symposium. Long Beach; 2001. p. 2495–506. [12] Kelly DW, Li R, Willgoss R, Crosky A. Improvement of bearing strength of mechanically fastened composite joints using fibre steering. In: Repecka L, Saremi FF, editors. Proceedings, 48th international SAMPE symposium. Long Beach; 2001. p. 415–26. [13] Kelly DW, Hsu P, Asadullah M. Load paths and load flow in finite element analysis. Eng Comput 2001;18(1/2):304–13. [14] Li R, Kelly D, Crosky A, Schoen H, Smolich L. Joint efficiency of fibre steered composite joint using load path trajectories. J Compos Mater, in press. [15] Legrand X, Crosky A, Kelly D, Crepin D. Optimisation of fibre steering in composite laminates using a genetic algorithm. Compos Struct 2006;75:524–31. [16] Huong N, Crosky A, Qi B, Kelly D, Li DC. Effect of nanoclay content on mechanical behaviour of TGDDM epoxy nanocomposites. In: Proceedings, Nanotech 2005, vol. 2. Anaheim, USA; 200. p. 79–88.
Improvement of bearing strength of laminated composites A. Crosky a, D. Kelly
b,* ,
R. Li b, X. Legrand c, N. Huong a, R. Ujjin
q
b
a School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, NSW 2052, Australia Ecole Nationale des Arts et Industrie Textiles, Laboratoire GEMTEX, 9 rue de l’Ermitage – BP 30329, 59100 Roubaix, France b
c
Available online 1 August 2006
Abstract Carbon fibre reinforced composite structures generally out perform metallic ones, but this is not the case when the structure is loaded in bearing. This work examines several different strategies for improving bearing performance. The first strategy was fibre steering (directed fibre placement). Three different methods were used to determine the trajectories for the steered fibres, referred to as the principal stress method, the load path method and the genetic algorithm method. In the principal stress method the fibre trajectories were determined from the major (tensile), s11, and minor (compressive), s22, principal stresses. An improvement of 36% in bearing strength was obtained using a modified principal stress pattern in which the s11 trajectories were displaced 5 mm away from the bearing-loaded hole to reduce fibre waviness which resulted from local overcrowding of the s11 and s 22 tows directly beneath the bearing surface. In the load path method, the steered fibre trajectories followed the dominant load path. A similar improvement in bearing strength of 33% was obtained using this method. Moreover, it was found that the load path method provided surplus reinforcement against net section failure and it was possible to more than double the efficiency of carbon laminate joints by reducing the ratio of joint width to bolthole diameter w/d from the standard value of 5 to a more compact 2.5, while simultaneously maintaining the bearing strength of the laminate. The third method employed an optimisation technique in which a genetic algorithm was used to determine the trajectories for the steered fibres. The trajectories obtained were intermediate between those from the principal stress method and those from the load path method. Matrix stiffening using nanoreinforcement was also examined. In this part of the work montmorillonite clay at loadings of 7.5 and 12.5 parts per hundred resin was incorporated into the matrix resin. While the addition of the clay nanoparticles substantially stiffened the neat resin, its incorporation into the composite laminates did not increase the bearing strength due to the introduction of an alternative, premature failure mode. The incorporation of the nanoparticles did however stiffen the bearing response, indicating that the method could produce improved bearing strength if premature failure could be avoided. Through thickness reinforcement using z-pins was also examined by inserting carbon fibre z-pins, at an aerial density of 4%, locally in the vicinity of the bearing-loaded hole. This technique increased the ultimate bearing load by 7%. However this did not translate into improved bearing strength because of local thickening of the laminate in the z-pinned region due to the presence of surplus resin displaced by the z-pins. It is considered that local thickening could be readily avoided by using a caul plate in the vicinity of the z-pinned region, thereby allowing the potential of this technique to be realised. 2006 Published by Elsevier Ltd. Keywords: Bearing; Fibre steering; Directed fibre placement; Genetic algorithm; Nanocomposites; z-Pinning
1. Introduction
q This research was undertaken as part of the research program of the Cooperative Research Centre for Advanced Composite Structures Ltd. * Corresponding author. Tel.: +61 2 385 4160; fax: +61 2 966 31222. E-mail address: [email protected] (D. Kelly).
0263-8223/$ - see front matter 2006 Published by Elsevier Ltd. doi:10.1016/j.compstruct.2006.06.036
Bolted connections are commonly used in preference to other joining techniques because they allow greater freedom in assembly and repair. However, introducing a bolted joint into a laminate introduces a new potential failure mode against which the laminate is relatively weak. The bolt imposes a compression load on the laminate and this
A. Crosky et al. / Composite Structures 76 (2006) 260–271
can result in failure in bearing by a combination of fibre buckling, shear and interlaminar splitting. The efficiency J of a single row fastened joint in a panel is the ratio of the maximum bearing load PB that can be transferred by the bolt to the laminate and the ultimate tensile load PTU for the laminate remote from the hole. It is a function of the tensile strength FTU and the bearing strength FB of the laminate, the width w and thickness t of the specimen, and the diameter d and local thickness tB of the fastener hole, as shown in Fig. 1. The joint efficiency J is given by J¼
PB PB F B dtB ¼ ¼ P TU F TU wt F TU wt
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Wu and Sun [2] made a detailed study of bearing failure initiation in 0/90 carbon fibre epoxy composites. They used the pin loaded test shown in Fig. 2 and sectioned the laminates along the contact centre line after loading to varying levels of the ultimate bearing load. This enabled them to detect the initial damage as shown in Fig. 3. They concluded that bearing failure initiated near the two flat surfaces of the laminates and then propagated inwards from each of the surfaces at an angle, Fig. 3. The failure mode in the 0 plies appeared to be a kink band resulting from fibre microbuckling, as observed in compression failure
ð1Þ
For composite laminates, the joint efficiency is substantially lower than for structural metals [1], as shown in Fig. 1. This problem has been recognised ever since composites were first considered for structural applications. The standard approach to overcome this problem in laminates is to increase the local thickness of the laminate or to add doublers in the vicinity of the joint. While doublers can increase the load carrying capacity, the bearing strength itself (determined by dividing the maximum bolt load that can be carried by the joint by the product of the hole diameter and the thickness at the hole) is often actually reduced compared to that of the laminate. This is because new failure modes involving separation of the doubler can be introduced, while any offset of the mid-plane of the laminate can introduce bending into the joint. As a result the strength does not scale directly with thickness.
Fig. 2. Schematic diagram of pin-contact test [2].
0.8
Ductile metal
0.7
d
Brittle material
PB/FTU wt
0.6
PB
Composite
0.5 0.4
Tension failure
Bearing failure
0.3 0.2 0.1 0
w
0
0.2
0.4
0.6
d/w
Fig. 1. Joint efficiency of different materials (after Hart-Smith [1]).
0.8
1
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Fig. 3. Initial damage in pin-loaded laminate [2].
16 14 12 10 E1/E2
of unidirectional composites [3], Fig. 4. From these results they concluded that bearing damage is initiated by fibre microbuckling of the 0 plies at the edges of the contact region near the free surfaces. As the load increases the damage propagates into the interior at an angle of about 30–40 until it reaches a critical length whereupon unstable growth begins and catastrophic failure occurs. From the above it is evident that increasing the compressive strength of the laminate below the contact zone should improve bearing performance. Accordingly, the present work examined the use of directed fibre placement (fibre steering) for this purpose. Increasing the bearing resistance increases the likelihood of net section failure (i.e., tensile failure at the sides of the hole) and the steered fibre patterns included an additional component to resist this mode of failure. The steered fibre tows must be accurately aligned with the load flow direction to sustain the maximum performance. As shown in Fig. 5, the effective
8 6 4 2 0 0
10
20
70
80
90
Fig. 5. Effective modulus versus aligning angle of reinforcement fibre [4].
longitudinal modulus of the reinforcement fibre in the composite declines quickly when the fibres do not line up with the load flow direction. In the present study, three different methods of establishing the trajectories were used. These are referred to as the principal stress method, the load path method and the genetic algorithm method. Wu and Sun [2] used Sun and Jun’s [5] microbuckling model to show that the critical stress in the direction of compression (rxx)cr is given by ðrxx Þcr ¼ Gep m =ð1 vf Þ Gep m
Fig. 4. Enlarged are near the edge of the cross-section shown in Fig. 3 [2].
30 40 50 60 aligning angle [º]
ð2Þ
where is the elastic–plastic shear modulus of the matrix and vf is the fibre volume fraction. This indicates that increasing the shear modulus of the matrix should also improve bearing performance. Increasing the modulus generally leads to a reduction in toughness for matrix resins but recent work has shown that simultaneous increases in both stiffness and toughness can be achieved in epoxy resin by
A. Crosky et al. / Composite Structures 76 (2006) 260–271
incorporating nanoparticles into the resin [6] and this method of improving bearing strength was also examined here. It has been reported that bearing strength is also strongly affected by lateral constraint (clamping force) at the loaded hole [7,8]. The benefit of lateral constraint is also evident from examination of Fig. 3 where it can be seen that generation of the kink bands involves outward lateral displacement of the material below the kink bands. This suggests that bearing performance may be improved by through thickness reinforcement and some preliminary work using z-pinning [9] is also reported here. 2. Directed fibre placement 2.1. Principal stress method In this method the steered fibres were incorporated into near quasi-isotropic laminates with the steered fibres being placed along the trajectories defined by the maximum principal tensile stress (s11) and the minimum principal compressive stress (s22) in the vicinity of the hole. The two patterns are shown in Fig. 6 and react the net section (s11) and bearing loads (s22) respectively. MSC.PATRAN and MSC.NASTRAN were used to construct 2d models for the specific bolted joints concerned. The bolt was modelled as a rigid pin applying a compression load normal to the hole surface. The principal stress vectors were then converted to a contour using the stream trace algorithm in Techplot. Fortran and C++ codes were then used to convert this data into x–y coordinates [10], for precise placement by a robot as described in Ref. [11]. In some cases, however, the steered fibre tows were laid by hand.
Fig. 6. Principal stress trajectories for a pin-loaded hole.
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The laminates were fabricated from five plies of 180 C cure bidirectional 0/90 Fiberdux 914C-933-42 five-shaft satin weave prepreg, with Toray T300-3k-40a carbon fibre yarn, with the stacking sequence ±45/090/090/090/ ±45. Both the s11 and s22 steered fibre patterns were incorporated between each fabric ply as shown in Fig. 7. A layer of M18 film adhesive was also included between each fabric ply to assist in wet out of the steered fibres. The s11 steered fibre pattern consisted of 10 tows of Tenex 3k HTA carbon fibre while the s22 pattern consisted of 26 tows of 6k HTA fibre. It was noted that the existence of an s11 tow at the bearing edge, Fig. 8(a), caused overcrowded stacking of the steered fibre tows and increased the waviness of the s22 tows in the through-thickness direction. To avoid this, a modified s11 pattern was used where the trajectories were extended 5 mm past the hole as shown in Fig. 8(b). The resulting reduction in overstacking is shown in Fig. 9(b). On the tension side of the bolthole, the modified pattern followed the FEA s11 trajectories, retaining its reinforcing effect against net section (tensile) breakage. Laminates were hand laid up with both the steered and modified steered patterns, and also without steered fibres (baseline), then cured between caul plates in an industrial autoclave at 180 C using the material supplier’s recommended cure cycle. After curing, the laminates were cut and ground to the size illustrated in Fig. 10. A hole 20 mm in diameter was drilled and reamed in the position shown in Fig. 10 to a tight tolerance of 20 ± 0.02 mm using a numerically controlled machine. Six replicates of both sets of steered fibre specimens, as well as baseline specimens, were tested using an Instron 1185 tensile testing machine operated at a crosshead rate of 2 mm/min. The bearing tests were carried out using a modified version of ASTM D5961-96 [11] using a load pin 19.97 mm in diameter and a fastener torque of 14 N m. Full details of the test procedure are given
Fig. 7. Typical layup of steered fibre samples.
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Fig. 8. Modification of fibre steering patterns: (a) FEA s11 trajectory and (b) s11 shifted 5 mm away from edge.
Fig. 9. Reduction of fibre over stacking: (a) FEA pattern and (b) modified pattern.
Table 2 Bearing test results
Fig. 10. Dimension of specimens.
elsewhere [12]. Details of the test specimens are given in Table 1. The test results are given in Table 2. The standard steered fibre pattern produced an increase of 139% in peak load and 20% in bearing strength while the modified steered pattern produced more substantial increases of 169% (peak load) and 36% (bearing strength). It is also noted that the modified pattern produced substantially less thickening in the vicinity of the hole, Table 2. More modest improvements were obtained in specimens with robot laid steered fibres of lower loading [12]. However no significant improvement was obtained when steered
Group
Ultimate failure Peak mode load (kN)
Ultimate Weight Bearing strength (g) area (MPa) (mm2)
Baseline Steered Modified steered
Bearing [12] 9.55 367.80 Net section [14] 22.86 441.24 Net section [14] 25.65 500.40
42.50 59.75 56.38
26.40 51.83 51.27
carbon fibre s11 and s22 patterns were incorporated into glass fibre laminates. It is thought that in this case, because of their higher modulus and lower strain to failure, the steered fibres failed before the bearing strength of the fibre glass laminate was achieved, thus imparting no improvement. The failure mode typical of the onset of failure in the carbon fibre laminates reinforced with steered fibre is shown in Fig. 11. The characteristic kink band formation reported by Wu and Sun [2] is clearly evident (compare Figs. 3 and 11).
Table 1 Configuration of specimens Group
Lay-up
Description
Baseline
±45/090/090/090/±45
Steered
±45-s22/s11-090-s22/s22-090-s22/s22-090-s11/s22-±45
Modified steered
±45-s22/s11-090-s22/s22-090-s22/s22-090-s11/s22-±45
Autoclave cured 5-ply Fiberite 914C carbon fibre prepreg without steered fibres Autoclave cured 5-ply Fiberite 914C samples steered with 6k Tenex HTA tow in s22 and 3k Tenex HTA tow in s11 with 1 layer of M18 resin film Autoclave cured 5-ply Fiberite 914C samples steered with 6k Tenex HTA tow in s22 and 3k Tenex HTA tow in modified s11 with 1 layer of M18 resin film
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The forces acting on a plane with normal given by ~ n ¼ nx i þ ny j þ nz k are obtained by integrating the total stress vectors Z n dA F x ¼ V x ~ Z n dA F y ¼ V y ~ Z n dA F z ¼ V z ~
Fig. 11. Section view of bearing failure in steered fibre laminate.
2.2. Load path method In the work described above, the specimens with both the standard and modified principal stress trajectories failed ultimately by tension breakage in the net section, Table 2, while the baseline specimens failed ultimately by bearing. A new pattern was therefore trialed based on the load path trajectories developed in Refs. [10,13]. In this pattern a single layer of steered fibres was used to simultaneously enhance both the tensile strength in net-section failure and the compressive strength beneath the bearing surface, as shown in Fig. 12. 2.3. Theory of load path trajectories The concept of load flow is widely used by design engineers to describe the way a structure carries applied loads from the point of application to the point of reaction in the structure. Essentially the load path is a statically determinate path that can carry the applied load. A structure is redundant if there is more than one of these paths. In Ref. [13] it was shown that the load paths can be defined by plotting contours aligned with the total stress vectors defined by V x ¼ rxx i þ sxy j þ sxz k V y ¼ syx i þ ryy j þ syz k
where the dot indicates the vector dot product. The load path for a force in a given direction is a region in which the force in that direction remains constant. For example, if the path in Fig. 13 is to define a region in which the force Px remains constant, the requirement is to determine the curved contour forming an edge along which the normal and tangential edge loads make no contribution to force in the x direction. This requires Fx ¼ 0 R or V x ~ n dA ¼ 0 This is achieved if the normal to the surface is perpendicular to Vx as the dot product is then zero. Alternatively, this is achieved if the surface tangents are parallel to the vector Vx as indicated in Fig. 13. Fortran codes were compiled to automatically read nodal values of the stress components from a two-dimensional finite element analysis from MSC/NASTRAN or ANSYS, and calculate load path vectors for later visualisation using the stream-trace option in TECPLOT. rxx Lxx ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r2xx þ r2yy sxy Lxy ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r2xx þ r2yy ryy Lyy ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi r2xx þ r2yy For X direction load path visualisation, Lxx was assigned as the stream-trace vector U in the horizontal direction, and Lxy was assigned as the stream-trace vector V in the vertical direction in TECPLOT. For Y direction load path
V z ¼ szx i þ szy j þ rzz k n Vx
B
A Px
Px C D Fig. 12. Dominant load path (lp) trajectory around a bolthole.
Fig. 13. Contours for a path along which Px is constant.
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visualisation, Lxy was assigned as the stream-trace vector U in the horizontal direction, and Lyy was assigned as the stream-trace vector V in the vertical direction in TEC-
PLOT. The load path pattern is shown in Fig. 14. The dominant load direction for the joint in Fig. 14 is the X direction.
Fig. 14. Load path trajectories.
Fig. 15. Dominant load path trajectories (hole diameter = 20 mm).
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Steered fibre and baseline specimens were manufactured from Hexcel W3G 282 bi-directional plain weave carbon fibre fabric prepreg in a 16-ply quasi-isotropic lay-up sequence as follows: 45 =lp=0 90 =lp=0 90 =lp= 45 =lp= 45 =lp=0 90 =lp=0 90 lp= 45 =lp= 45 =lp=0 90 =lp=0 90 =lp= 45 =lp= 45 =lp=0 90 =lp=0 90 =lp= 45 where lp indicates a layer of steered fibres in the pattern obtained using the load path trajectories. Fifteen steered fibre layers of 3k Tenex HTA 5331 carbon fibre tows were placed by hand onto the surface of each of 15 prepreg plies which, together with one bare surface ply, were later laid up into a fibre steered 16-ply laminate. The specimens were autoclave cured at 180 C in a production facility using the prepreg supplier’s recommended cure cycle. Specimens were cut to the dimensions shown in Fig. 10 except that the width was 90 mm. This was necessary to avoid net section failure in the baseline specimens due to a lower fibre volume in this prepreg than in the prepreg used for the tests given in the previous sections. Three replicates of the baseline and five replicates of the steered fibre specimens were tested using an identical procedure to that described in the previous section. Full details are given in Ref. [14]. The steered specimens gave a 53% increase in peak load and a 33% increase in bearing strength. It is noted that the improved peak load translated better into improved bearing strength than for the principal stress trajectory specimens (169% increase in peak load was required to give 36% improvement in bearing strength) because thickening of the laminate was reduced since only one pattern of steered fibres was used. Moreover, contrary to what was observed for the principal stress trajectory specimens, failure still occurred in bearing, indicating that the load path patterns provided surplus net section strengthening. This presented the possibility of reducing the specimen width which, in a multi-fastened joint, would equate to a reduced fastener spacing. Accordingly, additional sets of steered fibre specimens were prepared with widths of 70, 60 and 50 mm. The contours determined for the different specimens (ratios of width to diameter from 4.5 to 2.5), including the 90 mm specimens are shown in Fig. 15. A trajectory was plotted for each proposed fibre tow, starting with a uniform distribution at the left hand end of the specimen. The number of load path trajectories (carbon fibre tows)
267
in each pattern was 16, 16, 14 and 10 for the 90 mm, 70 mm, 60 mm and 50 mm wide specimens respectively. As for the 90 mm wide specimens, five replicates were tested for each of the other specimen widths. The results for all the load path specimens are given in Table 3. All specimens down to a width of 60 mm failed initially in bearing, although final failure for the 60 mm specimens was mostly net section (tension) failure. Three of the five 50 mm wide specimens also failed initially in bearing. These results indicate that the inclusion of the steered fibres shifts the transition from bearing to net section failure down to a w/d value of 2.5, compared with 4.5 for the unsteered specimens, while simultaneously maintaining (in fact slightly improving) the bearing strength (3% increase for the 50 mm wide specimens) of the laminate. 2.4. Genetic algorithm method Steered fibre trajectories were also obtained using a genetic algorithm (GA). The GA is an optimisation technique based on simulation of Darwin’s theory of life evolution. From a randomised population, the GA evolves using three operators based on several fit goals in order to get an acceptable population. The three operators are selection, cross-over and mutation. The advantage of GA’s is that they give the designer a family of near optimal designs with a small variation in their performance index instead of a single design. The domain was sampled into two-dimensional finite elements and anisotropic properties corresponding to a uni-directional carbon fibre laminate were defined for the plate. The orientations of the material axes for each element were then set as variables for the genetic algorithm. The fitness criterion was set in terms of strain energy and the population allowed to evolve to define the orientation of the fibres. The fibre orientation was assumed constant within each element and the vector showing the orientation in the element was defined at the centroid of the element. The orientation of the material axis on each element was then prescribed as an independent variable for the genetic algorithm. Full details of the procedure used are given in Ref. [15]. The result obtained is shown in Fig. 16. It is not straightforward from the GA result to determine how and where the steered fibre tow should be laid. A direct interpretation of the result shown in Fig. 16 gives the fibre steered pattern of Fig. 17(a). However, acknowledging that the carbon tow can be several millimetres wide, and the
Table 3 Bearing performance of composite joints using load path design
Peak load (kN) Ultimate strength (MPa) Ultimate failure mode
Baseline 90 mm (w/d = 4.5)
Steered 90 mm (w/d = 4.5)
Steered 70 mm (w/d = 3.5)
Steered 60 mm (w/d = 3)
Steered 50 mm (w/d = 2.5)
32.9 ± 0.2 456 ± 2 All tension
50.3 ± 2.9 605 ± 36 All bearing
46.2 ± 2.1 566 ± 25 Most bearing
41.9 ± 1.3 513 ± 9 Most tension
37.4 ± 2.0 466 ± 24 All tension
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Fig. 16. GA result.
3. Nanoreinforcement of the matrix Based on Sun and Jun’s [5] microbuckling model, stiffening of the matrix should also improve bearing performance and this was examined by incorporating nanoclay into the matrix resin. The matrix resin used was Araldite LY568, a tetraglycidyldiamino diphenylmethane (TGDDM) epoxy resin while the curing agent was Ethacure 100. The nanoclay used was Nanomer I30E, a montmorillonite clay modified with octadecyl ammonium ions. Initially nancomposites were prepared from the resin with clay loadings of 1–20 parts per hundred of resin (phr), cured and tested in uniaxial compression. Full details of the procedure used to prepare the nanocomposites are given in Ref. [16]. The compression modulus was found to increase progressively with increasing clay content with the 20 phr nanocomposite showing a 50% increase in modulus compared with the pure resin, Fig. 18. Bidirectional 0/90 laminates were prepared from 20 piles of carbon fibre woven fabric impregnated with neat resin and also with resin reinforced with 7.5 and 12.5 phr nanoclay. The laminates were hot cured in a platen press at a temperature of 195 C and a pressure of 700 kPa. The laminates were then tested using the pin-contact test developed by Wu and Sun [2], shown in Fig. 2, using a 10 mm diameter pin. The results are shown in Fig. 19. No improvement in bearing strength was obtained at either clay loading. However it was noted that the load displacement curves were steeper for the nanoclay reinforced laminates with the bearing stiffness being increased by 5% for 7.5 phr nanoclay and 15% for 12.5 phr nano-
3700 3500 3300
Modulus (MPa)
plate is not covered everywhere by steered fibre, the result shown in Fig. 17(b) is obtained. These results are of considerable interest since they are essentially intermediate between the result obtained by the principal stress method and that obtained by the load path method.
3100 2900 2700 2500 2300 2100 0
1
2.5
5
7.5
10
12.5
20
Nanoclay Content (phr)
Fig. 18. Compression modulus of TGDDM/DETDA/I30E nanocomposites.
clay. The strain to failure was correspondingly lower for the nanoclay reinforced composites. Samples of the three composites were fractured in an interlaminar mode and examined using scanning electron microscopy. It was seen that the nanoclay reinforced composites showed a much higher level of matrix failure than did the composites made with unreinforced resin, Fig. 20, indicating that the presence of the nanoclay had weakened the resin and changed the failure mode. This is consistent with the reduced strain to failure observed for the nanoclay reinforced laminates. These results indicate that incorporation of the nanoclay into the matrix resin introduced a premature failure mode. If this could be averted, increased bearing performance should be achieved. 4. Through thickness reinforcement Through thickness reinforcement using z-pins was also examined. Baseline (unpinned) and z-pinned 16 ply
Fig. 17. Two interpretations of the result shown in Fig. 16.
A. Crosky et al. / Composite Structures 76 (2006) 260–271
269
Bearing Strength 550
Bearing strength (MPa)
500
B0: Neat resin Vf = 57%
450 400
B1: 7.5 phr 130E Vf = 57%
350
B3: 7.5 phr 130E Vf = 57%
300 250
B4: 12.5 phr 130E Vf = 52%
200 150 100 B0
B1
B3
B4
Sample
Fig. 19. Bearing strength determined from pin-contact test for laminates containing 0, 7.5 and 12.5 phr nanoclay.
Fig. 20. Fracture surfaces of unreinforced (neat resin) and nanoclay reinforced (7.5 phr 130E) laminates.
quasi-isotropic laminates, with a (±45/090/090/)2s stacking sequence, were prepared from Hexcel W3G 282 bi-directional plain weave fabric prepreg. All samples were cured (without a caul plate) in an industrial autoclave at
180 C using the prepreg supplier’s recommended cure cycle. The z-pins were 0.28 mm diameter carbon fibre z-pins with an aerial density of 4%. They were supplied as 20 mm by 40 mm performs and were inserted into the
z-pin
final thickness
Stage One
Stage Two
bulge
laminate
Stage Three
pin is cut
Stage Four
Fig. 21. Z-Pinning procedure.
remaining plies added
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Z- pins overlapping hole by 4 mm Rectangular z-pin preform
40 mm 20 mm
Fig. 22. Z-Pin perform and positioning of z-pins relative to hole.
Table 4 Bearing test results
improved bearing strength (in fact a reduction of 6% occurred) because the z-pinned laminates had bulked out in the region of the z-pins, relative to the rest of the laminate, when they were cured. This extra bulk was resin which had been displaced by the z-pins. Sections taken through the baseline and z-pinned specimens are shown in Fig. 23(a) and (b). It can be seen that the z-pins have substantially altered the failure mode. In particular, the damage has not penetrated as deep below the hole. Although this initial study did not show an improvement in bearing strength, the increase in bearing load achieved does suggest that the technique should improve bearing performance if local thickening of the laminate is avoided. Since the thickening was due only to surplus resin, avoidance of thickening should be possible using a caul plate. 5. Conclusions
Specimens
Baseline
Z-Pinned
Change (%)
Thickness (mm) Peak load (kN) Ultimate bearing strength (MPa)
3.54 ± 0.01 18.7 ± 2.9 528 ± 82
3.91 ± 0.03 20.0 ± 2.6 496 ± 62
10.6 7.2 6.1
laminates with an ultrasonic gun, before curing, using the procedure illustrated in Fig. 21. After curing the laminates were cut to the size shown in Fig. 10. However the hole diameter was reduced to 10 mm. The hole was drilled so as to penetrate 4 mm into the z-pinned area as shown in Fig. 22. Seven replicates of both the baseline and the z-pinned laminates were tested using the procedure described earlier for the steered fibre specimens, except that a 10 mm load pin was used. The results are given in Table 4. The z-pins increased the ultimate load by 7% however this did not translate into
Fibre steering, matrix stiffening and through thickness reinforcement using z-pins have been examined as ways of improving bearing performance in laminated fibre reinforced composites. The principal findings are as follows: • Bearing strength was improved by 36% using a modified pattern derived from the principal stresses. This pattern consisted of two sets of steered fibres defined respectively by the s11 (tensile) and s22 (compressive) principal stresses, as obtained from finite element analysis. The modification to the pattern involved shifting the FEAderived s11 trajectories 5 mm away from the hole to reduce overstocking of the steered fibre tows in the vicinity of the hole and thereby reduce the waviness of the s22 steered tows in this region. This almost doubled the improvement in bearing strength of 20% obtained with the unmodified steered fibre pattern.
Fig. 23. Section view of bearing failure in (a) baseline and (b) z-pinned laminates.
A. Crosky et al. / Composite Structures 76 (2006) 260–271
• Bearing strength was improved by 33% using steered fibre trajectories defined by the dominant load path. Only a single set of fibres was used compared to the two separate sets necessary for the principal stress trajectories. As a result the reinforcement was more efficient in improving bearing strength. • The dominant load path trajectories provided surplus net section strength. It was therefore possible to reduce the sample width to almost one half that of the unsteered sample before the onset of net section failure occurred. This equated to a reduction in the w/d ratio for the transition from bearing failure to net section failure from 4.5 to 2.5. Since tension breakage governs the separation between fasteners in a multi-fastener design, the reduced w/d ratio for the onset of net section failure indicates that by using the load path design, more fasteners could be included in a joint of a given width. This would then increase the load capacity of the joint as each fastener transfers a load defined by the bearing strength. The joint efficiency J could therefore be more than doubled by fibre steering using the load path design. • The addition of clay nanoparticles to the matrix resin substantially stiffened the neat resin, but its incorporation into composite laminates did not increase the bearing strength due to the introduction of a different failure mode to that which occurred in the laminates made from the neat resin. The incorporation of the nanoparticles did however stiffen the bearing response, indicating that the method could produce improved bearing strength if introduction of the new and premature failure mode could be avoided. • Through thickness reinforcement using z-pins increased the ultimate bearing load by 7%. However this did not translate into improved bearing strength because of local thickening of the laminate in the z-pinned region due to the presence of surplus resin displaced by the zpins. It is considered that local thickening could be readily avoided by using a caul plate in the vicinity of the zpinned region, thereby allowing the potential of this technique to be realised.
Acknowledgements Part of this work was supported by the Office of Naval Research, Arlington USA, through GRANT
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N000140010583 to the Cooperative Research Centre for Advanced Composite Structures Ltd. Y.D.S. Rajapakse was the Technical Monitor. The authors are grateful to Hawker de Havilland, Sydney, Australia for assistance with specimen preparation and to Professor Adrian Mouritz, RMIT University, Melbourne, Australia for assistance with the z-pinning. References [1] Hart-Smith LJ. Joints. Engineered materials handbook. Composites, vol. 1. Metal Park, OH: ASM International; 1987. p. 479–95. [2] Wu PS, Sun CT. Modeling bearing failure initiation in pin-contact of composite laminates. Mech Mater 1998;29:325–35. [3] Camponeschi Jr ET. Compression response of thick-section composite materials. DTRC-SME-90/60. David Taylor Research Center, 1990. [4] Daniel IM, Ishai O. Engineering mechanics of composites materials. New York: Oxford University Press; 1994. p. 65. [5] Sun CT, Jun AW. Compressive strength of unidirectional fiber composites with matrix nonlinearity. Compos Sci Technol 1994;52:577–87. [6] LeBaron PC, Wang Z, Pinnavaia TJ. Polymer-layered silicate nanocomposites: an overview. Appl Clay Sci 1999;15(1–2):11–29. [7] Tsujimoto Y, Wilson D. Elasto-plastic failure analysis of composite bolted joints. J Compos Mater 1986;20:236–52. [8] Eriksson LI. On the bearing strength of bolted graphite/epoxy laminates. J Compos Mater 1990;24:1246–69. [9] Freitas G, Magee C, Dardzinski P, Fusco T. Fiber insertion process for improved damage tolerance in aircraft laminates. J Adv Mater 1994;25(4):36–43. [10] Tosh MW, Kelly DW. On the design, manufacture and testing of trajectorial fibre steering for carbon fibre composite laminates. Compos Part A 2000;31:1047–60. [11] Kelly DW, Willgoss R, Li R, Crosky A. A manufacturing procedure for composite laminate production using precision fibre placement. In: Repecka L, Saremi FF, editors. Proceedings, 48th international SAMPE symposium. Long Beach; 2001. p. 2495–506. [12] Kelly DW, Li R, Willgoss R, Crosky A. Improvement of bearing strength of mechanically fastened composite joints using fibre steering. In: Repecka L, Saremi FF, editors. Proceedings, 48th international SAMPE symposium. Long Beach; 2001. p. 415–26. [13] Kelly DW, Hsu P, Asadullah M. Load paths and load flow in finite element analysis. Eng Comput 2001;18(1/2):304–13. [14] Li R, Kelly D, Crosky A, Schoen H, Smolich L. Joint efficiency of fibre steered composite joint using load path trajectories. J Compos Mater, in press. [15] Legrand X, Crosky A, Kelly D, Crepin D. Optimisation of fibre steering in composite laminates using a genetic algorithm. Compos Struct 2006;75:524–31. [16] Huong N, Crosky A, Qi B, Kelly D, Li DC. Effect of nanoclay content on mechanical behaviour of TGDDM epoxy nanocomposites. In: Proceedings, Nanotech 2005, vol. 2. Anaheim, USA; 200. p. 79–88.