The Interlaminar Mode I Fracture of IM7/LaRC-RP46 Composites at High Temperatures

Abstract

In this paper, the interlaminar Mode I fracture behavior of a IM7/LaRC-RP46 composite laminate at elevated temperatures was investigated by DCB (double cantilever beam) static fracture testing. Straight-sided DCB specimens were made from unidirectional laminates and tested at three high temperature levels of 232°C (450°F), 288°C (550°F) and 343°C (650°F) at five crosshead rates from 0.51 mm/min. (0.00033 in./sec) to 6.1 mm/min. (0.004 in./sec). Unlike the linear load-displacement response observed at room temperature, the load-displacement response at high temperatures behaves nonlinearly before reaching the maximum load. Two crack growth regions of slow crack extension and relatively fast crack propagation were observed and the slow crack extension caused the nonlinear load-displacement response. The initial fracture toughness, G _IC decreased slightly, while the propagation value increased by nearly 30% to 100% with increased temperature. The materials exhibited the highest fracture toughness at 288°C among the three different tested temperatures. The complex combination effects of fiber bridging and the change of matrix properties and fiber/matrix interfacial adhesion at elevated temperatures are found to be the major reason resulting in the variation of G _IC with temperature. After the temperature increased beyond T _g (glass transition temperature), which is approximately 310°C, the propagation fracture toughness decreased drastically and aging effects of the high temperature on the matrix are observed. No significant effects of crosshead rate on the G _IC were observed in the range of the present rates. However, the crosshead rate is found to significantly influenced the crack growth rate.