The Fracture Behavior of Carbon/Epoxy Laminates Containing Internal Cut Fibers

Abstract

Fiber breakage in a composite laminate can be caused by impact loads. A study was conducted to investigate the effect of internal fiber breakage on the tensile strength of carbon/epoxy laminates. The specimens contained internal fiber cuts, which were considered “buried cracks” in this study. Four layups, [0, ± 45,90,0₂], [90, ± 45,0]_s, [0, ± 68,0, ± 68]_s, and [0]l2, were studied. The results showed a significant strength reduction due to the internal fiber cut. Delamination occurred under load adjacent to the internal crack prior to the final fracture, as shown by ultrasonic C-scan inspection and micro-sectioning inspection. The dimensions of the delamination zones were affected by laminate layups and the length of internal cracks. Prediction of the residual strength was attempted using a principal load carrying ply failure criterion in conjunction with a 3-dimensional anisotropic finite element analysis. Laminate failure prediction was based on the behavior of individual plies. The predicted residual strength was in general agreement with measured residual strength over all the laminates and conditions investigated, indicating the applicability of the criterion.

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References

1. Rotem, A. 1988. “The Strength of Laminated Composite Materials Under Repeated Impact Loading”, Journal of Composites Technology and Research, 10(2):74–79.

2. Christoforou, A. P. and S. R. Swanson . 1988. “Strength Loss in Composite Cylinders Under Impact”, Journal of Engineering Materials and Technology, 110: 180–184.

3. Boll, D. J. , W. D. Bascom , J. C. Weidner and J. C. Murri . 1986. “A Microscopy Study of Impact Damage of Epoxy-Matrix Carbon-Fiber Composites”, Journal of Material Science, 12: 2667–2677.

4. Cairns, D. S. 1987. “Impact and Post-Impact Response of Graphite/Epoxy and Kevlar/Epoxy Structures”, Ph.D. dissertation, Massachusetts Institute of Technology.

5. Poe, C. C., Jr., W. Illg and D. P. Garber. 1986. “Tension Strength of a Thick Graphite/Epoxy Laminate After Impact by a 1/2-in.-Radius Impactor”, Nasa Technical Memorandum 87771.

6. Card, M. F. and M. D. Rhodes. 1980. “Graphite-Epoxy Panel Compression Strength Reduction Due to Local Impact”, Effect of Service Environment on Composite Materials. Proceedings of the 50th Meeting of the Agard Structures and Materials Panel. Athens. Greece. April 14–17. 1980, pp. 11–1–11–13.

7. Caprino, G. 1984. “Residual Strength Prediction of Impacted Cfrp Laminates”, Journal of Composite Materials, 18: 508–518.

8. Lai, K. M. 1982. “Prediction of Residual Tensile Strength of Transversely Impacted Composite Laminates”, Research in Structural and Solid Mechanics—1982, Nasa Conference Publication 2245, pp. 97–112.

9. Husman, G. E., J. M. Whitney and J. C. Halpin. 1975. “Residual Strength Characterization of Laminated Composites Subjected to Impact Loading”, Foreign Object Damage to Composites, Astm Stp 568, American Society for Testing and Materials, pp. 92–113.

10.

10. Bowie, O. L. 1956. “Analysis of an Infinite Plate Containing Radial Cracks Originating from the Boundary of an Internal Circular Hole”, Journal of Mathematics and Physics, 35: 60.

11.

11. Waddoups, M. E. , J. E. Eisenmann and B. E. Kaminski . 1971. “Macroscopic Fracture Mechanics of Advanced Composite Materials”, Journal of Composite Materials, 5: 446–454.

12.

12. Whitney, J. M. and R. J. Nuismer . 1974. “Stress Fracture Criteria for Laminated Composites Containing Stress Concentrations”, Journal of Composite Materials, 8: 253–265.

13.

13. Poe, C. C., Jr., W. Illg and D. P. Garber. 1985. “A Program to Determine the Effect of Low-Velocity Impacts on the Strength of the Filament-Wound Rocket Motor Case for the Space Shuttle”, Nasa Technical Memorandum 87588.

14.

14. Poe, C. C., Jr., C E. Harris and D. H. Morris. 1988. “Surface Crack Analysis Applied to Impact Damage in a Thick Graphite/Epoxy Composite”, Nasa Technical Memorandum 100600.

15.

15. Rotem, A. and Z. Hashin . 1975. “Failure Modes of Angle Ply Laminates”, Journal of Composite Materials, 9: 191–206.

16.

16. Tsai, S. W. and V. D. Azzi . 1966. “Strength of Laminated Composite Materials”, A1AA Journal, 4(2):296–301.

17.

17. Griffith, W. I., M. F. Kanninen and E. F. Rybicki. 1979. “A Fracture Mechanics Approach to the Analysis of Graphite/Epoxy Laminated Precracked Tension Panels”, Nondestructive Evaluation and Criticality for Composite Materials, Astm Stp 696. American Society for Testing and Materials, pp. 185–201.

18.

18. Poe, C. C., Jr. 1984. “Fracture Toughness of Fibrous Composite Material”, Nasa Technical Paper 2370.

19.

19. Poe, C. C., Jr . 1983. “A Unifying Strain Criterion for Fracture of Fibrous Composite Laminates”, Engineering Fracture Mechanics, 17(2):153–I71.

20.

20. Rybicki, E. F. and M. F. Kanninen . 1977. “A Finite Element Calculation of Stress Intensity Factors by a Modified Crack Closure Integral”, Engineering Fracture Mechanics, 9: 931–938.

21.

21. Tian, Z. and S. R. Swanson. “Effect of Delamination Rice Overlapping on Strain Energy Release Rate Calculations”, to appear in Composite Structure, 1991.

22.

22. Chapman, T. J. , A. J. Smiley and R. B. Pipes . 1987. “Rate and Temperature Effects on Mode Ii Interlaminar Fracture Toughness in Composite Materials”, Proceedings of Sixth International Conference on Composite Materials. Iccm & Eccm. Vol. 3. London: Elsevier Applied Science Publisher, pp. 3.295–3.304.

23.

23. Brinson, H. F. and Y. T. Yeow . 1977. “An Experimental Study of the Fracture Behavior of Laminated Graphite/Epoxy Composites”, Composite Materials: Testing and Design (Fourth Conference). Astm Stp 617, pp. 18–38.

24.

24. Harris, C. E. and D. H. Morris. 1984. “Fracture Behavior of Thick, Laminated Graphite/Epoxy Composites”, Nasa Contract Report 3784.