Mechanical properties of interleaved PMR-15 composites

The addition-type thermosetting polyimide PMR-15 has a relatively high T g and attractive high-temperature composite mechanical properties after post curing. PMR-15 is also a fairly brittle material characterized by a low fracture toughness. One method that has been used to improve the toughness of a brittle laminate involves interleaving a tough thermoplastic resin between selected plies. However using interleaves may affect other mechanical properties of a composite. In this study, a thermoplastic polyimide (Matramid 5218) was used to interleaf between plies of PMR-15/graphite composites. The interleaving was done with the thermoplastic in powder and film form. Standard 0° flex and short beam shear tests were performed on interleaved and noninterleaved composites at room and elevated temperatures. Microcracking was examined before and after 500 hours of exposure at 316°C. The powder interleaves were nonuniform in thickness and did not arrest crack growth as effectively as the uniformly thick film interleaves