Analysis of the Fatigue Performance of Notched and Unnotched Graphite/Thermoplastic Composite Laminates

Abstract

The present investigation summarizes an evaluation of the effect of fatigue loading on continuous and long discontinuous fiber (LDF) composites manufactured using the AS4/PEKK (poly-ether-ketone-ketone) system. Approximately 100 static and fatigue specimens were tested in a two-phase experimental investigation. In the first phase, LDF AS4/PEKK thermoplastic specimens of 2 layups [0/90/ + 45]. were tested in static tension to determine their respective strengths. Constant amplitude fatigue tests were then conducted to evaluate the behavior of these two LDF composite laminates. Specimens surviving one million cycles were statically tested to failure. In the second phase of the investigation, continuous fiber AS4/PEKK thermoplastic laminates were tested in static tension and in fatigue to evaluate the effect of the discontinuous fiber on composite performance. The effect of a known stress concentration on both material systems was also investigated using specimens having a 0.635 cm center hole. Static tests confirmed a reduction (13%) in unnotched LDF strength compared with continuous fiber thermoplastics. One additional objective was to determine whether the presence of discontinuous fibers, and the resulting stress concentrations, would cause a severe reduction in LDF thermoplastic composite fatigue properties. This was not found to be the case.

References

1. Baker, H. 1990. "PMC's Get Tough," Advanced Materials and Processes, 138(1):39-43.

2. Croman, R. B. 1989. "Tensile Fatigue Performance of Thermoplastic Resin Composites Reinforced with Ordered Kevlar Aramid Staple," Proc. of ICCM-VII, 2:572-577.

3. Mallick, P. K. 1988. Fiber Reinforced Composites, New York: Marcel Dekker, Inc..

4. Reifsnider, K. L. and W. W. Stinchcomb . 1979. "Fatigue Damage Mechanisms in Composite Materials: A Review," ASTM STP 675.

5. Yang, S. H. 1987. "Stiffness Degradation of Composite Laminates," D.Sc. Thesis, The George Washington University, Washington, DC, June.

6. Jones, D. L. and H. A. Whitworth . 1984. "Determination of the Stiffness Reduction of [0/90/ ±45], Graphite/Epoxy Laminates under Cyclic Loading," Proc. AIAA/ASME/ASCE Twenty Fifth Structures, Structural Dynamics and Materials Conference, AIAA Paper No. 840863, pp. 428-435.

7. Stalnaker, D. O. and W. W. Stinchcomb . 1979. "Load History-Edge Damage Studies in Two Quasi-Isotropic Graphite/Epoxy Laminates," Composite Materials: Testing and Design (Fifth Conference), ASTM STP 674, pp. 620-641.

8. Reifsnider, K. L. , K. Schulte and J. C. Duke . 1983. "Long-Term Fatigue Behavior of Composite Materials." Long-Term Behavior of Composites, ASTM STP 813, pp. 136-159.

9. Russell, D. J. , K. Schulte , K. L. Reifsnider and W. W. Stinchcomb . 1984. "Characterization and Analysis of Damage Mechanisms in Tension-Tension Fatigue of Graphite/Epoxy Laminates," Effects of Defects in Composite Materials, ASTM STP 836, pp. 21-55.

10.

10. Daniel, I. M. and A. Charewich . 1986. "Fatigue Damage Mechanisms and Residual Properties of Graphite/Epoxy Laminates," Engineering Fracture Mechanics, 25:793-808.

11.

11. Whitney, J. M. and R. Y. Kim . 1977. "Effect of Stacking Sequence on the Notched Strength of Laminated Composites," Composite Materials Testing and Design (Fourth Conference), ASTM STP 617, pp. 229-242.

12.

12. Whitney, J. M.

"Fatigue Characterization of Composite Materials,"

Fatigue of Fibrous Composite Materials, ASTM STP 723, pp. 133-151.

13.

13. Reifsnider, K. L. , W. W. Stinchcomb and T. K. O'Brien . 1977. "Frequency Effects on a Stiffness Based Fatigue Failure Criterion in Flawed Composite Specimens," Fatigue of Filamentary Composite Materials, ASiM SP 636, pp. 171-184.

14.

14. Schultz, D. , J. J. Gerharz and E. Alschweig . 1981. "Fatigue Properties of Unnotched, Notched, and Jointed Specimens of a Graphite/Epoxy Composite," Fatigue of Fibrous Composite Materials, ASTM 577 723.