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Specimen size effect in off-axis compression tests of fiber composites
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Composites: Part B 39 (2008) 20–26 www.elsevier.com/locate/compositesb
Specimen size effect in off-axis compression tests of fiber composites Qida Bing, C.T. Sun
*
School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907-2023, USA Available online 23 February 2007
Abstract Specimen size effects in off-axis compression tests were studied by conducting experiments with small block off-axis specimens of low modulus S2 glass fiber reinforced composites and high modulus AS4 carbon fiber reinforced composites. It was found that the off-axis compressive strength of the glass/epoxy composite decreased by a small amount (<5%) when increasing either specimen width or thickness. However, an appreciable reduction in off-axis compressive strength of the high modulus carbon/epoxy composite was observed as specimen width or thickness increased with lapped specimens. However, when a thin layer of titanium was applied on both contact ends of the specimen, the contact friction was significantly reduced, leading to much smaller reductions in off-axis compressive strength as the specimen width or thickness increases. It can be concluded that specimen end surface friction can produce specimen size effect in off-axis compression tests, and the specimen size effect is negligible when the friction between the specimen end and loading element is minimized. 2007 Elsevier Ltd. All rights reserved. Keywords: A. Polymer-matrix composites (PMCs); B. Compressive strength; B. Mechanical properties; D. Mechanical testing
1. Introduction Off-axis specimens have been used to produce shear deformation and stress in fiber composites by simple axial loads. Small block off-axis specimens have been used to determine compressive composite strengths under combined loads or to determine longitudinal compressive strength [1–3]. The key advantage in using off-axis specimens lies in the fact that a combined state of stress with respect to the material principal directions can be produced simply with a uniaxial load. In testing compressive strengths of fiber composites, one often encounters the premature end failure of the specimen if 0 specimens are used. By using off-axis block specimens for compressive testing, we were able to induce microbuckling failure in off-axis carbon/epoxy and S2 glass/epoxy composite specimens at much lower compressive stresses due to the presence of in-plane shear stresses. From these data, the longitudinal
*
Corresponding author. Tel.: +1 765 494 5130; fax: +1 765 494 0307. E-mail address: [email protected] (C.T. Sun).
1359-8368/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2007.02.010
compressive strength of the composite was extracted [1,3]. The longitudinal compressive strength of AS4/3501-6 carbon/epoxy composite obtained from the off-axis compression test data at 10 5/s strain rate agreed very well with Daniel’s static test result [3,4]. Further, the small size of the off-axis block specimen makes it feasible to perform high strain rate compressive tests on a split Hopkinson pressure bar and obtain the highly rate sensitive compressive strength [3] and shear strength [2] data. A question that may naturally arise concerns the effect of the specimen size on the strength data measured. A considerable amount of research on specimen size effect on the longitudinal tensile strength of fiber reinforced composites as well as laminates has been published [5–10]. However, there have been few studies in the effect of specimen size on the compressive strength of fiber composites. This disparity may be due to the significant difficulty in testing the compressive strength of composites. Camponeschi et al. [11] performed compression tests on unidirectional fiber reinforced composites and cross-ply laminates by employing end loading combined with shear loading (via end tabs) and reported a reduction in compressive strength as the
Q. Bing, C.T. Sun / Composites: Part B 39 (2008) 20–26
size of specimen increased. Hsiao et al. [12] obtained a slight decrease in compressive strength of unidirectional fiber reinforced composites as specimen thickness increased, and noted no thickness effect in cross-ply laminates. Lee and Soutis [13] conducted compression tests in combined shear loading and end loading by employing an anti-buckling device. Their test results showed a decreased compressive strength of unidirectional composites as specimen thickness increased, and observed end failures in thick-section composite specimens. Salvi et al. [14] studied size effect using off-axis coupon tow composite specimens. In most of these studies, shear loading through the use of end tabs was utilized partially or fully in conducting compression tests. Thus, the compressive strengths measured were influenced by the presence of shear stresses. The purpose of the present work is to investigate the effect of specimen size on the compressive strength of fiber reinforced composites obtained using off-axis block specimens. This testing method does not require the assistance of shear loading during the compression test. Several surface treatments are utilized to help realize the ideal extension-shear deformation in the off-axis specimen.
21
3. Experiments 3.1. Effect of specimen width on off-axis compressive strength of S2/8853-40 glass/epoxy composite with lapped specimen surfaces Off-axis specimens were cut from a 64-ply S2/8553-40 glass/epoxy composite laminate at 10 and 15 off-axis angles. The specimen had a nominal thickness of 6.5 mm and height of 12.5 mm. Three different specimen widths (5 mm, 10 mm, and 15 mm) were considered to study the width effect on the compressive strength of this glass/epoxy composite. The specimens were lapped and tested in compression at a stroke speed of 10 3 mm/s on an MTS machine. Lubricant (G-4500 multi-purpose synthetic grease MOLYKOTE) was applied on the contact surfaces (This was also utilized in all the other tests in this study). In uniaxial compression tests performed on the MTS machine, a self-adjusting device was used in order to ensure the specimen to be in full contact with the loading element and to eliminate potential bending moments as shown in Fig. 2. Fig. 3 shows the plots of the experimental compressive strengths averaged from 4–5 tests (same number of
2. Materials and specimens Two different unidirectional fiber reinforced composites are considered in this study, namely, glass/epoxy and carbon/epoxy systems. The glass fiber reinforced composites are S2/8553-40 glass/epoxy and S2/8552 glass/epoxy. The carbon fiber reinforced composite is AS4/3501-6 carbon/ epoxy laminate. Small block specimens with different sizes were cut from the laminates at various off-axis angles by using a water jet cutting machine (FLOW). The ends of all the specimens were polished with 400 and 600 grit sand papers (BUEHLER) and lapped on a lapping machine (HYPREZ) with 15 lm and 6 lm diamond slurries to ensure that the two ends are smooth and parallel to each other. The smooth contact surface is essential in order to allow the induced shear deformation to fully develop under compression. Fig. 1 shows the off-axis specimen with offaxis angle h.
Height
Thickness Width
Compressive strength (MPa)
θ
Fig. 2. Schematic of compression test with self-adjusting system.
350
0.8%
300 4.4%
250
7.6%
200 150 100 50
10 degree off-axis
15 degree off-axis
0 0
5
10
15
20
25
Specimen width (mm)
Applied load
Fig. 1. Small block off-axis specimen with off-axis angle h.
Fig. 3. Specimen width effect on the off-axis compressive strength of S2/ 8553-40 composite.
22
Q. Bing, C.T. Sun / Composites: Part B 39 (2008) 20–26
specimens as other tests in this study) of 10 and 15 offaxis specimens with different specimen widths, as well as strength reduction percentiles when specimen width increases. It can be seen that there is a slight decrease in the compressive strengths of 10 off-axis specimens when the specimen width increases from 10 to 20 mm. A similar trend is seen for 15 off-axis specimens as the specimen width increases from 5 to 10 mm. However, there is a 7.6% strength reduction when the specimen width increases to 20 mm. All 10 off-axis specimens failed in fiber microbuckling failure mode as shown in Fig. 4. The 5 and 10 mm wide 15 off-axis specimens failed by fiber microbuckling with kink bands initiated from one of the specimen corners as shown in Fig. 5. However, several of 20 mm wide specimens failed in a different kink band pattern (see Fig. 5c), while others failed in the fiber microbuckling mode similar to that of the 5 and 10 mm wide specimens (see Fig. 5d). This may explain the larger drop in compressive strength of 20 mm wide specimens.
Fig. 4. Fiber microbuckling failure modes of 10 off-axis S2/8553-40 specimens with different specimen widths.
It is noted that the average compressive strength of 20 mm wide 15 off-axis specimens in Fig. 3 is calculated from experimental results including both kink band patterns from five tests. However, if we exclude the two test data corresponding to different kink band patterns, the average compressive strength becomes 218.1 MPa and the standard deviation is 6.2 MPa. As a result, only a small amount of reduction (3.4%) in compressive strength is found again when comparing test results of 20 to 10 mm wide specimens. 3.2. Effect of specimen width on off-axis compressive strength of AS4/3501-6 carbon/epoxy composite with lapped specimen surfaces Off-axis specimens were cut from a 48-ply AS4/3501-6 carbon/epoxy laminate at four different widths, 4 mm, 7 mm, 14 mm, and 21 mm. All block specimens were 13 mm high. After the specimen end surfaces were polished and lapped following the same procedures as glass/epoxy specimens, compression tests were conducted on an MTS machine in stroke control mode at 10 3 mm/s stroke speed. Off-axis angles 5, 10 and 15 were considered. Fig. 6 shows the experimental results of off-axis compressive strengths for different specimen widths. Significant reductions in compressive strength are evident for all the off-axis carbon/epoxy specimens as the specimen width increases. The failure modes of 7 mm, 14 mm, and 20 mm wide off-axis specimens are shown in Figs. 7–9. All 5 and 10 off-axis specimens of all widths and the 15 off-axis specimen with 7 mm width failed in fiber microbuckling mode. For the 15 off-axis specimen with 14 and 21 mm widths, failure appears to be matrix dominated shear
Fig. 5. Failure modes of 15 off-axis S2/8553-40 specimens with different specimen widths: (a) 5 mm; (b) 10 mm; (c) 20 mm and (d) 20 mm.
Q. Bing, C.T. Sun / Composites: Part B 39 (2008) 20–26
5 degree 10 degree 15 degree
800 Strength (MPa)
700
25.4%
600 7.9%
500
6.4% 13.7%
400 300
2.7%
4.1%
5
10 15 Width (mm)
9.5% 3.3% 10.5%
200 100 0 0
20
25
Fig. 6. Effect of specimen width on off-axis compressive strength of AS4/ 3501-6 composite with lapped specimen surfaces.
Fig. 7. Failure modes of lapped 5 off-axis AS4/3501-6 specimens: (a) 7 mm; (b) 14 mm and (c) 21 mm.
Fig. 8. Failure modes of lapped 10 off-axis AS4/3501-6 specimens: (a) 7 mm; (b) 14 mm and (c) 21 mm.
imens could be partially attributed to the contact friction resulting from the contact between stiff carbon fiber ends and loading surfaces. It has been shown that, in compression tests with AS4/3501-6 carbon/epoxy off-axis specimens, endings of stiff carbon fibers could make significant indentations on the contact surface and, thus, restrain the shear deformation from taking place fully [15]. To reduce this friction, Bing and Sun [3] applied a titanium coating on the contact surfaces of the off-axis AS4/3501-6 carbon/epoxy specimen and showed that the added titanium coating can greatly reduce contact frictions, allowing a fully developed extension–shear coupling. Two different widths (7 mm, and 14 mm) were used in this study. After off-axis specimens (13 mm high) were cut from the 48-plies laminate, they were polished and lapped, and then a titanium layer of 1.2 lm thick was applied on both ends of the off-axis specimen using a Varian E-Beam evaporator. Compression tests in stroke control mode at 10 3 mm/s were conducted on these coated off-axis specimens and the experimental results are shown in Fig. 10. A substantial decrease of specimen width effect on off-axis compressive strength was observed in coated composite specimens in contrast to that in uncoated but lapped specimens. It can be concluded that the surface friction induced by stiff carbon fiber ends in lapped but uncoated AS4/3501-6 off-axis specimens can produce appreciable size effect in compression tests. Figs. 11–13 show the failure modes of coated AS4/3501-6 off-axis specimens. At 5 and 10 off-axis angles, coated specimens broke into pieces with kind bands present. Coated 15
Off-axis compressive strength (MPa)
900
600 500
2.1%
400
5.0%
300
2.5%
200
5 degree off-axis 10 degree off-axis 15 degree off-axis
100 0 0
Fig. 9. Failure modes of lapped 15 off-axis AS4/3501-6 specimens: (a) 7 mm; (b) 14 mm and (c) 21 mm.
23
5 10 Specimen width (mm)
15
Fig. 10. Specimen width effect on the compressive strength of coated AS4/ 3501-6 composite. The percentiles indicate the differences between the two widths.
failure. However, kink band can be observed on the failed surfaces as shown in Fig. 9c. 3.3. Effect of specimen width on off-axis compressive strength of AS4/3501-6 carbon/epoxy composite with coated specimen surfaces We suspected that the fairly significant width effect on off-axis compressive strength in lapped carbon/epoxy spec-
Fig. 11. Failure modes of coated 5 off-axis AS4/3501-6 specimens: (a) 7 mm and (b) 14 mm.
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Q. Bing, C.T. Sun / Composites: Part B 39 (2008) 20–26
Fig. 12. Failure modes of coated 10 off-axis AS4/3501-6 specimens: (a) 7 mm and (b) 14 mm.
After off-axis specimens were polished and lapped following the previously described procedures, compression tests were conducted on an MTS testing machine at 10 3 mm/s stroke speed. Fig. 14 shows the experimental results. Little difference in off-axis compressive strength of S2/8552 glass/epoxy composite is observed as specimen thickness changes except for the 5 off-axis angle. This difference may be explained by the fact that the off-axis compressive strength of smaller off-axis angles is more sensitive to the precision of the fiber orientation in the specimen. All the specimens with off-axis angles 5, 10, and 15 failed by fiber microbuckling mode (compressive failure), and the 30 off-axis specimens failed by matrix shear failure for both specimen thicknesses as shown in Figs. 15 and 16. 3.5. Effect of specimen thickness on off-axis compressive strength of AS4/3501-6 carbon/epoxy composite with lapped specimen surfaces
specimens failed in matrix shear failure mode as shown in Fig. 13. By comparing the specimen width effect on off-axis compressive strength of lapped glass/epoxy composite specimens and coated carbon/epoxy composite specimens, the trend of 0–5% compressive strength decrease as specimen width is doubled agrees well with the fact that the two different composite systems have nearly the same loading surface conditions. 3.4. Effect of specimen thickness on off-axis compressive strength of S2/8852 glass/epoxy composite with lapped specimen surfaces The effect of specimen thickness on off-axis compressive strength of unidirectional fiber reinforced composites was first studied with S2/8552 glass/epoxy composite. Two S2/8552 laminates of 48 plies and 80 plies were made and block specimens were cut from these laminates at off-axis angles 5, 10, 15, and 30 with nominal dimensions of 9 mm in width and 14 mm in height. The specimen thickness was measured and the average laminate thicknesses are listed in Table 1. The thickness ratio is consistent with the ratio of the ply numbers of the two laminates. The consistency is important because it indicates that the fiber volume fractions in the two laminates are the same, so that the compressive strength will not be affected by fiber volume fraction variations.
Block specimens with 7 mm in width and 13 mm in height were cut from two AS4/3501-6 laminates (48-plies and 72-plies). After the specimen end surfaces were polished and lapped, these specimens were tested in compression at 10 3 mm/s stroke speed. The experimental results are plotted against off-axis angles in Fig. 17. Large compressive strength decrease is found when specimen
Compressive strength (MP)
Fig. 13. Failure modes of coated 15 off-axis AS4/3501-6 specimens: (a) 7 mm and (b) 14 mm.
800 48 Plies
10.7%
80 Plies
600 -3.8% 400 5.1% 1.6% 200 0
5
10 15 20 Off-axis angle (o)
25
30
Fig. 14. Specimen thickness effect on the off-axis compressive strength of S2/8552 composite. The percentile for each off-axis angle indicates the difference between the strengths of the two types of specimens.
Table 1 Thickness of S2/8552 glass/epoxy laminates Number of plies
Thickness (mm)
Standard deviation (mm)
48 80
3.91 6.58
0.12 0.32
Fig. 15. Fiber microbuckling failure in the 10 off-axis S2/8552 composite for both (a) 48-plies and (b) 80-plies thicknesses.
Q. Bing, C.T. Sun / Composites: Part B 39 (2008) 20–26
25
Compressive Strength (MPa)
800 3.3%
600
2.8% 400
-1.7% -2.2%
200 30 layers
48 layers
0 0 Fig. 16. Matrix shear failure in the 30 off-axis S2/8552 composite for both (a) 48-plies and (b) 80-plies thicknesses.
600 Off-axis strength (MPa)
48Layers 16.7%
72Layers
400 24.3% 23.8%
8.1%
200
10.2%
6.1%
0 0
5
10
15
20
25
30
35
Off-axis angle (o)
Fig. 17. Specimen thickness effect on off-axis compressive strength of AS4/3501-6 composite with lapped end surfaces.
thickness increases for both compressive failure and matrix shear failure, showing a significant thickness effect on offaxis compressive strength of lapped AS4/3501-6 specimens. It is also noticed that the thickness effect decreases as specimen off-axis angle increases. 3.6. Effect of specimen thickness on off-axis compressive strength of AS4/3501-6 carbon/epoxy composite with coated specimen surfaces The result of the study in Section 3.5 shows that surface friction induced by stiff carbon fiber ends can significantly change the specimen width effect on off-axis compressive strength of carbon fiber composites. To study how the surface friction affects the specimen thickness effect on off-axis compressive strength, compression tests with coated specimens were conducted. Off-axis block specimens were cut from two AS4/3501-6 laminates with 30-plies and 48 plies, respectively. The thicknesses of those two laminates were 3.67 mm and 5.74 mm, respectively, and both had the same fiber volume fractions. The nominal specimen dimensions were 9 mm · 14 mm with off-axis angles 5, 10, 15, and 30. After specimen surfaces were sanded and lapped, a 1.2 lm thin layer of titanium was coated on both ends of the off-axis specimens
10
20
30
40
Off-axis angle (o) Fig. 18. Specimen thickness effect on off-axis compressive strength of AS4/3501-6 composite with coated end surfaces.
using the vapor deposition technique. Fig. 18 shows the test results of off-axis compressive strength obtained from coated specimens of AS4/3501-6 with different specimen thicknesses. Little compressive strength variation is found as specimen thickness increases. Again, the greater thickness effect in lapped AS4/3501-6 specimens is mainly caused by contact friction. It is clear that the presence of Ti coating can greatly reduce surface friction in compression tests and, hence, minimize the effect of thickness on off-axis compressive strength of coated specimens. 4. Summary and conclusions Compression strength tests were performed on off-axis specimens of carbon/epoxy and glass/epoxy composites with different specimen widths and thicknesses at various off-axis angles. Greater decreases in compression strength for lapped carbon fiber composite were observed when specimen width or thickness increased. However, specimen width and thickness showed little effect in off-axis tests with lapped S2 glass/epoxy composite specimens. It was also shown that by applying a thin layer of titanium coating on the contact surfaces of carbon/epoxy composite specimen, the size effect also diminished. Moreover, it was shown that by lapping the contact surfaces of the specimen and applying lubricant, it was possible to essentially eliminate contact friction in the compression tests with glass/ epoxy specimens, and the same surface condition can also be obtained for carbon/epoxy composite specimens with titanium coatings. It can be concluded that the variation in compressive strength data obtained from off-axis compression tests can be minimized by reducing the contact friction between the loading element and the specimen. Acknowledgements This work was supported by ONR Solid Mechanics Program Grant No. N00014-1-0552. Dr. Yapa D.S. Rajapakse was the Program Manager.
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Q. Bing, C.T. Sun / Composites: Part B 39 (2008) 20–26
References [1] Tsai J, Sun CT. Dynamic compressive strengths of polymeric composites. Int J Solids Struct 2004;41(11–12):3211–24. [2] Tsai J, Sun CT. Strain rate effect on in-plane shear strength of unidirectional polymeric composites. Compos Sci Technol 2005;65(13):1941–7. [3] Bing Q, Sun CT. Modeling and testing strain rate-dependent compressive strength of carbon/epoxy composites. Compos Sci Technol 2005;65(15–16):2481–91. [4] Daniel IM, Ishai O. Engineering mechanics of composites. New York: Oxford University Press; 1994. [5] Wisnom MR. Size effects in the testing of fibre-composite materials. Compos Sci Technol 1999;59:1937–57. [6] Phillips H, Shenoi R, Lewis S. Effect of specimen size on the strength scaling of GFRP laminates. Mater Lett 1994;21:229–38. [7] Jackson K, Kellas S. Effect of specimen size on the tensile strength of geometrically scaled [+hn/ + hn/902n]s composite laminates. In: US army symposium on solid mechanics, 1993, Plymouth MA.
[8] Cunningham M, Schoulz S, Toth JM. Effect of end tab design on tension specimen stress concentrations. [9] Gurvich M, Pipes R. Strength size effect of laminated composites. Compos Sci Technol 1995;55:93–105. [10] Bullock R. Strength ratios of composite materials in flexure and in tension. J Compos Mater 1974;8:200–6. [11] Camponeschi JET, Gillespie JJW, Wilkins DJ. Kink-band failure analysis of thick composites in compression. J Compos Mater 1993;27:471–90. [12] Hsiao HM, Daniel IM, Wooh SC. A new compression test method for thick composites. J Compos Mater 1995;29(13):1789–806. [13] Lee J, Soutis C. Thickness effect on the compressive strength of T800/ 924 C carbon fibre-epoxy laminates. Compos Part A-Appl S 2005;36(2):213–27. [14] Salvi AG, Waas AM, Caliskan A. Specimen size effect in the off-axis compression test of unidirectional carbon fiber tow composites. Compos Sci Technol 2004;64:83–97. [15] Ninan L, Tsai J, Sun CT. Use of split Hopkinson pressure bar for testing off-axis composites. Int J Impact Eng 2001;25:291–313.
Composites: Part B 39 (2008) 20–26 www.elsevier.com/locate/compositesb
Specimen size effect in off-axis compression tests of fiber composites Qida Bing, C.T. Sun
*
School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN 47907-2023, USA Available online 23 February 2007
Abstract Specimen size effects in off-axis compression tests were studied by conducting experiments with small block off-axis specimens of low modulus S2 glass fiber reinforced composites and high modulus AS4 carbon fiber reinforced composites. It was found that the off-axis compressive strength of the glass/epoxy composite decreased by a small amount (<5%) when increasing either specimen width or thickness. However, an appreciable reduction in off-axis compressive strength of the high modulus carbon/epoxy composite was observed as specimen width or thickness increased with lapped specimens. However, when a thin layer of titanium was applied on both contact ends of the specimen, the contact friction was significantly reduced, leading to much smaller reductions in off-axis compressive strength as the specimen width or thickness increases. It can be concluded that specimen end surface friction can produce specimen size effect in off-axis compression tests, and the specimen size effect is negligible when the friction between the specimen end and loading element is minimized. 2007 Elsevier Ltd. All rights reserved. Keywords: A. Polymer-matrix composites (PMCs); B. Compressive strength; B. Mechanical properties; D. Mechanical testing
1. Introduction Off-axis specimens have been used to produce shear deformation and stress in fiber composites by simple axial loads. Small block off-axis specimens have been used to determine compressive composite strengths under combined loads or to determine longitudinal compressive strength [1–3]. The key advantage in using off-axis specimens lies in the fact that a combined state of stress with respect to the material principal directions can be produced simply with a uniaxial load. In testing compressive strengths of fiber composites, one often encounters the premature end failure of the specimen if 0 specimens are used. By using off-axis block specimens for compressive testing, we were able to induce microbuckling failure in off-axis carbon/epoxy and S2 glass/epoxy composite specimens at much lower compressive stresses due to the presence of in-plane shear stresses. From these data, the longitudinal
*
Corresponding author. Tel.: +1 765 494 5130; fax: +1 765 494 0307. E-mail address: [email protected] (C.T. Sun).
1359-8368/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2007.02.010
compressive strength of the composite was extracted [1,3]. The longitudinal compressive strength of AS4/3501-6 carbon/epoxy composite obtained from the off-axis compression test data at 10 5/s strain rate agreed very well with Daniel’s static test result [3,4]. Further, the small size of the off-axis block specimen makes it feasible to perform high strain rate compressive tests on a split Hopkinson pressure bar and obtain the highly rate sensitive compressive strength [3] and shear strength [2] data. A question that may naturally arise concerns the effect of the specimen size on the strength data measured. A considerable amount of research on specimen size effect on the longitudinal tensile strength of fiber reinforced composites as well as laminates has been published [5–10]. However, there have been few studies in the effect of specimen size on the compressive strength of fiber composites. This disparity may be due to the significant difficulty in testing the compressive strength of composites. Camponeschi et al. [11] performed compression tests on unidirectional fiber reinforced composites and cross-ply laminates by employing end loading combined with shear loading (via end tabs) and reported a reduction in compressive strength as the
Q. Bing, C.T. Sun / Composites: Part B 39 (2008) 20–26
size of specimen increased. Hsiao et al. [12] obtained a slight decrease in compressive strength of unidirectional fiber reinforced composites as specimen thickness increased, and noted no thickness effect in cross-ply laminates. Lee and Soutis [13] conducted compression tests in combined shear loading and end loading by employing an anti-buckling device. Their test results showed a decreased compressive strength of unidirectional composites as specimen thickness increased, and observed end failures in thick-section composite specimens. Salvi et al. [14] studied size effect using off-axis coupon tow composite specimens. In most of these studies, shear loading through the use of end tabs was utilized partially or fully in conducting compression tests. Thus, the compressive strengths measured were influenced by the presence of shear stresses. The purpose of the present work is to investigate the effect of specimen size on the compressive strength of fiber reinforced composites obtained using off-axis block specimens. This testing method does not require the assistance of shear loading during the compression test. Several surface treatments are utilized to help realize the ideal extension-shear deformation in the off-axis specimen.
21
3. Experiments 3.1. Effect of specimen width on off-axis compressive strength of S2/8853-40 glass/epoxy composite with lapped specimen surfaces Off-axis specimens were cut from a 64-ply S2/8553-40 glass/epoxy composite laminate at 10 and 15 off-axis angles. The specimen had a nominal thickness of 6.5 mm and height of 12.5 mm. Three different specimen widths (5 mm, 10 mm, and 15 mm) were considered to study the width effect on the compressive strength of this glass/epoxy composite. The specimens were lapped and tested in compression at a stroke speed of 10 3 mm/s on an MTS machine. Lubricant (G-4500 multi-purpose synthetic grease MOLYKOTE) was applied on the contact surfaces (This was also utilized in all the other tests in this study). In uniaxial compression tests performed on the MTS machine, a self-adjusting device was used in order to ensure the specimen to be in full contact with the loading element and to eliminate potential bending moments as shown in Fig. 2. Fig. 3 shows the plots of the experimental compressive strengths averaged from 4–5 tests (same number of
2. Materials and specimens Two different unidirectional fiber reinforced composites are considered in this study, namely, glass/epoxy and carbon/epoxy systems. The glass fiber reinforced composites are S2/8553-40 glass/epoxy and S2/8552 glass/epoxy. The carbon fiber reinforced composite is AS4/3501-6 carbon/ epoxy laminate. Small block specimens with different sizes were cut from the laminates at various off-axis angles by using a water jet cutting machine (FLOW). The ends of all the specimens were polished with 400 and 600 grit sand papers (BUEHLER) and lapped on a lapping machine (HYPREZ) with 15 lm and 6 lm diamond slurries to ensure that the two ends are smooth and parallel to each other. The smooth contact surface is essential in order to allow the induced shear deformation to fully develop under compression. Fig. 1 shows the off-axis specimen with offaxis angle h.
Height
Thickness Width
Compressive strength (MPa)
θ
Fig. 2. Schematic of compression test with self-adjusting system.
350
0.8%
300 4.4%
250
7.6%
200 150 100 50
10 degree off-axis
15 degree off-axis
0 0
5
10
15
20
25
Specimen width (mm)
Applied load
Fig. 1. Small block off-axis specimen with off-axis angle h.
Fig. 3. Specimen width effect on the off-axis compressive strength of S2/ 8553-40 composite.
22
Q. Bing, C.T. Sun / Composites: Part B 39 (2008) 20–26
specimens as other tests in this study) of 10 and 15 offaxis specimens with different specimen widths, as well as strength reduction percentiles when specimen width increases. It can be seen that there is a slight decrease in the compressive strengths of 10 off-axis specimens when the specimen width increases from 10 to 20 mm. A similar trend is seen for 15 off-axis specimens as the specimen width increases from 5 to 10 mm. However, there is a 7.6% strength reduction when the specimen width increases to 20 mm. All 10 off-axis specimens failed in fiber microbuckling failure mode as shown in Fig. 4. The 5 and 10 mm wide 15 off-axis specimens failed by fiber microbuckling with kink bands initiated from one of the specimen corners as shown in Fig. 5. However, several of 20 mm wide specimens failed in a different kink band pattern (see Fig. 5c), while others failed in the fiber microbuckling mode similar to that of the 5 and 10 mm wide specimens (see Fig. 5d). This may explain the larger drop in compressive strength of 20 mm wide specimens.
Fig. 4. Fiber microbuckling failure modes of 10 off-axis S2/8553-40 specimens with different specimen widths.
It is noted that the average compressive strength of 20 mm wide 15 off-axis specimens in Fig. 3 is calculated from experimental results including both kink band patterns from five tests. However, if we exclude the two test data corresponding to different kink band patterns, the average compressive strength becomes 218.1 MPa and the standard deviation is 6.2 MPa. As a result, only a small amount of reduction (3.4%) in compressive strength is found again when comparing test results of 20 to 10 mm wide specimens. 3.2. Effect of specimen width on off-axis compressive strength of AS4/3501-6 carbon/epoxy composite with lapped specimen surfaces Off-axis specimens were cut from a 48-ply AS4/3501-6 carbon/epoxy laminate at four different widths, 4 mm, 7 mm, 14 mm, and 21 mm. All block specimens were 13 mm high. After the specimen end surfaces were polished and lapped following the same procedures as glass/epoxy specimens, compression tests were conducted on an MTS machine in stroke control mode at 10 3 mm/s stroke speed. Off-axis angles 5, 10 and 15 were considered. Fig. 6 shows the experimental results of off-axis compressive strengths for different specimen widths. Significant reductions in compressive strength are evident for all the off-axis carbon/epoxy specimens as the specimen width increases. The failure modes of 7 mm, 14 mm, and 20 mm wide off-axis specimens are shown in Figs. 7–9. All 5 and 10 off-axis specimens of all widths and the 15 off-axis specimen with 7 mm width failed in fiber microbuckling mode. For the 15 off-axis specimen with 14 and 21 mm widths, failure appears to be matrix dominated shear
Fig. 5. Failure modes of 15 off-axis S2/8553-40 specimens with different specimen widths: (a) 5 mm; (b) 10 mm; (c) 20 mm and (d) 20 mm.
Q. Bing, C.T. Sun / Composites: Part B 39 (2008) 20–26
5 degree 10 degree 15 degree
800 Strength (MPa)
700
25.4%
600 7.9%
500
6.4% 13.7%
400 300
2.7%
4.1%
5
10 15 Width (mm)
9.5% 3.3% 10.5%
200 100 0 0
20
25
Fig. 6. Effect of specimen width on off-axis compressive strength of AS4/ 3501-6 composite with lapped specimen surfaces.
Fig. 7. Failure modes of lapped 5 off-axis AS4/3501-6 specimens: (a) 7 mm; (b) 14 mm and (c) 21 mm.
Fig. 8. Failure modes of lapped 10 off-axis AS4/3501-6 specimens: (a) 7 mm; (b) 14 mm and (c) 21 mm.
imens could be partially attributed to the contact friction resulting from the contact between stiff carbon fiber ends and loading surfaces. It has been shown that, in compression tests with AS4/3501-6 carbon/epoxy off-axis specimens, endings of stiff carbon fibers could make significant indentations on the contact surface and, thus, restrain the shear deformation from taking place fully [15]. To reduce this friction, Bing and Sun [3] applied a titanium coating on the contact surfaces of the off-axis AS4/3501-6 carbon/epoxy specimen and showed that the added titanium coating can greatly reduce contact frictions, allowing a fully developed extension–shear coupling. Two different widths (7 mm, and 14 mm) were used in this study. After off-axis specimens (13 mm high) were cut from the 48-plies laminate, they were polished and lapped, and then a titanium layer of 1.2 lm thick was applied on both ends of the off-axis specimen using a Varian E-Beam evaporator. Compression tests in stroke control mode at 10 3 mm/s were conducted on these coated off-axis specimens and the experimental results are shown in Fig. 10. A substantial decrease of specimen width effect on off-axis compressive strength was observed in coated composite specimens in contrast to that in uncoated but lapped specimens. It can be concluded that the surface friction induced by stiff carbon fiber ends in lapped but uncoated AS4/3501-6 off-axis specimens can produce appreciable size effect in compression tests. Figs. 11–13 show the failure modes of coated AS4/3501-6 off-axis specimens. At 5 and 10 off-axis angles, coated specimens broke into pieces with kind bands present. Coated 15
Off-axis compressive strength (MPa)
900
600 500
2.1%
400
5.0%
300
2.5%
200
5 degree off-axis 10 degree off-axis 15 degree off-axis
100 0 0
Fig. 9. Failure modes of lapped 15 off-axis AS4/3501-6 specimens: (a) 7 mm; (b) 14 mm and (c) 21 mm.
23
5 10 Specimen width (mm)
15
Fig. 10. Specimen width effect on the compressive strength of coated AS4/ 3501-6 composite. The percentiles indicate the differences between the two widths.
failure. However, kink band can be observed on the failed surfaces as shown in Fig. 9c. 3.3. Effect of specimen width on off-axis compressive strength of AS4/3501-6 carbon/epoxy composite with coated specimen surfaces We suspected that the fairly significant width effect on off-axis compressive strength in lapped carbon/epoxy spec-
Fig. 11. Failure modes of coated 5 off-axis AS4/3501-6 specimens: (a) 7 mm and (b) 14 mm.
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Q. Bing, C.T. Sun / Composites: Part B 39 (2008) 20–26
Fig. 12. Failure modes of coated 10 off-axis AS4/3501-6 specimens: (a) 7 mm and (b) 14 mm.
After off-axis specimens were polished and lapped following the previously described procedures, compression tests were conducted on an MTS testing machine at 10 3 mm/s stroke speed. Fig. 14 shows the experimental results. Little difference in off-axis compressive strength of S2/8552 glass/epoxy composite is observed as specimen thickness changes except for the 5 off-axis angle. This difference may be explained by the fact that the off-axis compressive strength of smaller off-axis angles is more sensitive to the precision of the fiber orientation in the specimen. All the specimens with off-axis angles 5, 10, and 15 failed by fiber microbuckling mode (compressive failure), and the 30 off-axis specimens failed by matrix shear failure for both specimen thicknesses as shown in Figs. 15 and 16. 3.5. Effect of specimen thickness on off-axis compressive strength of AS4/3501-6 carbon/epoxy composite with lapped specimen surfaces
specimens failed in matrix shear failure mode as shown in Fig. 13. By comparing the specimen width effect on off-axis compressive strength of lapped glass/epoxy composite specimens and coated carbon/epoxy composite specimens, the trend of 0–5% compressive strength decrease as specimen width is doubled agrees well with the fact that the two different composite systems have nearly the same loading surface conditions. 3.4. Effect of specimen thickness on off-axis compressive strength of S2/8852 glass/epoxy composite with lapped specimen surfaces The effect of specimen thickness on off-axis compressive strength of unidirectional fiber reinforced composites was first studied with S2/8552 glass/epoxy composite. Two S2/8552 laminates of 48 plies and 80 plies were made and block specimens were cut from these laminates at off-axis angles 5, 10, 15, and 30 with nominal dimensions of 9 mm in width and 14 mm in height. The specimen thickness was measured and the average laminate thicknesses are listed in Table 1. The thickness ratio is consistent with the ratio of the ply numbers of the two laminates. The consistency is important because it indicates that the fiber volume fractions in the two laminates are the same, so that the compressive strength will not be affected by fiber volume fraction variations.
Block specimens with 7 mm in width and 13 mm in height were cut from two AS4/3501-6 laminates (48-plies and 72-plies). After the specimen end surfaces were polished and lapped, these specimens were tested in compression at 10 3 mm/s stroke speed. The experimental results are plotted against off-axis angles in Fig. 17. Large compressive strength decrease is found when specimen
Compressive strength (MP)
Fig. 13. Failure modes of coated 15 off-axis AS4/3501-6 specimens: (a) 7 mm and (b) 14 mm.
800 48 Plies
10.7%
80 Plies
600 -3.8% 400 5.1% 1.6% 200 0
5
10 15 20 Off-axis angle (o)
25
30
Fig. 14. Specimen thickness effect on the off-axis compressive strength of S2/8552 composite. The percentile for each off-axis angle indicates the difference between the strengths of the two types of specimens.
Table 1 Thickness of S2/8552 glass/epoxy laminates Number of plies
Thickness (mm)
Standard deviation (mm)
48 80
3.91 6.58
0.12 0.32
Fig. 15. Fiber microbuckling failure in the 10 off-axis S2/8552 composite for both (a) 48-plies and (b) 80-plies thicknesses.
Q. Bing, C.T. Sun / Composites: Part B 39 (2008) 20–26
25
Compressive Strength (MPa)
800 3.3%
600
2.8% 400
-1.7% -2.2%
200 30 layers
48 layers
0 0 Fig. 16. Matrix shear failure in the 30 off-axis S2/8552 composite for both (a) 48-plies and (b) 80-plies thicknesses.
600 Off-axis strength (MPa)
48Layers 16.7%
72Layers
400 24.3% 23.8%
8.1%
200
10.2%
6.1%
0 0
5
10
15
20
25
30
35
Off-axis angle (o)
Fig. 17. Specimen thickness effect on off-axis compressive strength of AS4/3501-6 composite with lapped end surfaces.
thickness increases for both compressive failure and matrix shear failure, showing a significant thickness effect on offaxis compressive strength of lapped AS4/3501-6 specimens. It is also noticed that the thickness effect decreases as specimen off-axis angle increases. 3.6. Effect of specimen thickness on off-axis compressive strength of AS4/3501-6 carbon/epoxy composite with coated specimen surfaces The result of the study in Section 3.5 shows that surface friction induced by stiff carbon fiber ends can significantly change the specimen width effect on off-axis compressive strength of carbon fiber composites. To study how the surface friction affects the specimen thickness effect on off-axis compressive strength, compression tests with coated specimens were conducted. Off-axis block specimens were cut from two AS4/3501-6 laminates with 30-plies and 48 plies, respectively. The thicknesses of those two laminates were 3.67 mm and 5.74 mm, respectively, and both had the same fiber volume fractions. The nominal specimen dimensions were 9 mm · 14 mm with off-axis angles 5, 10, 15, and 30. After specimen surfaces were sanded and lapped, a 1.2 lm thin layer of titanium was coated on both ends of the off-axis specimens
10
20
30
40
Off-axis angle (o) Fig. 18. Specimen thickness effect on off-axis compressive strength of AS4/3501-6 composite with coated end surfaces.
using the vapor deposition technique. Fig. 18 shows the test results of off-axis compressive strength obtained from coated specimens of AS4/3501-6 with different specimen thicknesses. Little compressive strength variation is found as specimen thickness increases. Again, the greater thickness effect in lapped AS4/3501-6 specimens is mainly caused by contact friction. It is clear that the presence of Ti coating can greatly reduce surface friction in compression tests and, hence, minimize the effect of thickness on off-axis compressive strength of coated specimens. 4. Summary and conclusions Compression strength tests were performed on off-axis specimens of carbon/epoxy and glass/epoxy composites with different specimen widths and thicknesses at various off-axis angles. Greater decreases in compression strength for lapped carbon fiber composite were observed when specimen width or thickness increased. However, specimen width and thickness showed little effect in off-axis tests with lapped S2 glass/epoxy composite specimens. It was also shown that by applying a thin layer of titanium coating on the contact surfaces of carbon/epoxy composite specimen, the size effect also diminished. Moreover, it was shown that by lapping the contact surfaces of the specimen and applying lubricant, it was possible to essentially eliminate contact friction in the compression tests with glass/ epoxy specimens, and the same surface condition can also be obtained for carbon/epoxy composite specimens with titanium coatings. It can be concluded that the variation in compressive strength data obtained from off-axis compression tests can be minimized by reducing the contact friction between the loading element and the specimen. Acknowledgements This work was supported by ONR Solid Mechanics Program Grant No. N00014-1-0552. Dr. Yapa D.S. Rajapakse was the Program Manager.
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