Effects of thickness on the fatigue-behavior of quasi-isotropic carbon/epoxy composites before and after low energy impacts

The effects of laminate thickness on the tension-compression fatigue behavi or of [0/45/90/-45](ns) (n = 2,4,6) T300/976 carbon/epoxy composites damage d by low-energy impact have been investigated. The penetration impact prope rties and the residual tensile strength under Various low-energy impacts we re tested. The initial slope during penetration impact reveals that the thi cker laminate has a higher stiffness than the thinner one. Moreover, the re sults also show that the total energy required to penetrate the composite i s not proportional to the thickness. The static tensile strength measuremen ts and tension-compression fatigue tests at various stress levels were also carried out on the impacted and unimpacted laminates. The median-rank meth od and Weibull distribution function were applied to predict the failure pr obability of the composites under given applied loading and fatigue cycles. The relationship between stress levels and fatigue life (S-N curve) was es tablished and reductions in fatigue life of the impacted laminates at vario us stress levels n ere compared. The different slopes of the S-N curves imp ly that different fracture mechanisms may occur, depending on the laminate. The S-N curves of the composites with different thicknesses were also comp ared. Ultrasonic C-scanning was used to examine the damage zones in composi tes subjected to various impact energies and fatigue loadings. (C) 1999 Els evier Science Ltd. All rights reserved.