The effects of moisture and temperature on the interlaminar delamination toughness of a carbon/epoxy composite

HTA/6376C composite has been investigated for influence of temperature and moisture content on the interlaminar delamination toughness in mode I, mode II and mixed mode conditions. Dry and moisture-saturated specimens were tested over the temperature range -50 degrees C to 100 degrees C. Evaluation methods based on load/displacement and load measurements were employed. In pure mode II the critical strain-energy release rate drops with moisture content and increase in temperature. In mixed mode the critical strain-energy release rate also decreases with moisture content, but no general trends in the dependence on temperature is observed. The critical strain-energy release rate in pure mode I is unaffected by changes lit moisture content and was found to increase slightly at elevated temperatures. During crack propagation, enhanced fiber bridging due to increases in temperature and moisture content promotes R-curve behavior in the pure mode I tests. The resulting mode mixity of the mixed-mode bending (MMB) tests is found to be severely affected by the evaluation methods. Methods based an load measurements only are considered to give unreliable strain-energy release rates as the measured compliance/displacement relationships were found to he non-linear even prior to crack growth. Further studies are needed to assess the mixed-mode ratio in the MMB test. (C) 1998 Elsevier Science Ltd. All rights reserved.