Environmental Effects in Composite Laminates with Voids

Abstract

This work presents an experimental program aimed at investigating some aspects of environmental effects on carbon/epoxy laminates with voids. The objective is to obtain experimental data to subsidize acceptance criteria for ultrasonic inspection of composite parts for aerospace applications. A fabrication process that produces laminates with reasonably uniform distribution of voids and nearly constant fibre fraction is described. The interlaminar shear strength of laminates with three different levels of void content was measured at four different conditions: RTD (room temperature, dry), RTW (room temperature, wet), ETW (elevated temperature, wet), and LTW (low temperature, wet). Also, the possibility of extensive degradation of the laminate for a small number of thermal cycles was investigated. Since aerospace applications are considered, a group of specimens was submitted to four cycles of thermal shock between -54°C and 65°C. The experimental results are consistent with the fracture criterion for laminates with voids proposed by Almeida and Nogueira Neto. They postulated mat the presence of voids does not have detrimental effects on the laminate strength unless the void content is above a certain critical level. For laminates not submitted to thermal shock, it was concluded that humidity increases the laminate toughness but reduces its critical void content level. The effect of temperature is to decrease laminate toughness but increase the critical void content level. The four cycles of thermal shock between -54°C and 65°C only have a significant effect on the laminates in RTD condition. For this test condition, it has an effect similar to that of humidity, increasing the laminate toughness but reducing its critical void content level. After thermal shock, laminates under conditions RTD, RTW, and LTW had approximately the same interlaminar shear strength.