Research on Life-Prediction Methods for MMCs— Phase I

A report describes the completed first phase of a NASA/industry cooperative program of research on metal-matrix composites (MMCs) as lightweight, strong, high- temperature-resistant materials for use in future aircraft engines. The first phase of the research included assessment of life- and fracture-prediction methods, determination of fracture strengths and fatigue lives, and experiments in nondestructive evaluation. The metal-matrix composite specimens used in these studies were rings made of silicon-carbide-based fibers in a titanium-alloy matrix. The particular composite material was chosen because extensive data on the material were already available and the material is representative of composites that would be used in aircraft engines. Five fracture- and life prediction analysis methods were applied to the rings; their predictions were compared with each other and with experimental data on fracture of the rings. Manufacturing defects prevented the researchers from conducting planned cyclic tests. Fatigue-life predictions ranged from 1,000 to 15,000 cycles. Fracture-stress predictions were less scattered, ranging from 25 to 40.1 kpsi (172 to 276 MPa). Low-resolution x-ray computed tomography proved to be an effective non-destructive evaluation technique.