THE INTERLAMINAR MODE-I FRACTURE OF IM7 LARC-RP46 COMPOSITES AT HIGH-TEMPERATURES/

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 D CB specimens were made from unidirectional laminates and tested at thr ee high temperature levels of 232 degrees C (450 degrees F), 288 degre es C (550 degrees F) and 343 degrees C (650 degrees F) at five crosshe ad 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 t emperature, the load-displacement response at high temperatures behave s nonlinearly before reaching the maximum load. Two crack growth regio ns of slow crack extension and relatively fast crack propagation were observed and the slow crack extension caused the nonlinear load-displa cement response. The initial fracture toughness, G(IC), decreased slig htly, while the propagation value increased by nearly 30% to 100% with increased temperature. The materials exhibited the highest fracture t oughness at 288 degrees C among the three different tested temperature s. The complex combination effects of fiber bridging and the change of matrix properties and fiber/matrix interfacial adhesion at elevated t emperatures are found to be the major reason resulting in the variatio n of G(IC) with temperature. After the temperature increased beyond T- g (glass transition temperature), which is approximately 310 degrees C , the propagation fracture toughness decreased drastically and aging e ffects of the high temperature on the matrix are observed. No signific ant effects of crosshead rate on the G(IC) were observed in the range of the present rates. However, the crosshead rate is found to signific antly influenced the crack growth rate.