Nondestructive evaluation of damage in silicon carbide/aluminum metal matrix composite using X-ray tomographic microscopy

A fundamental understanding of damage evolution will be required before metal matrix composites (MMC) can be utilized safely for structural applications. Although macroscopic mechanical response to cyclic loading has been monitored in many mechanical and thermal test programs, little is known about the nucleation and growth of damage in MMC's. The goal of the present work is to improve the understanding of damage accumulation in SiC/Al using a new microscopic nondestructive volume-imaging technique, X-ray Tomographic Microscopy (XTM), which has resolution comparable to optical microscopy.; Correlation of damage initiation and accumulation mechanisms and the macroscopic mechanical response of samples are discussed for continuous fiber SiC/Al MMC's. A series of mechanical tests were performed on a continuous, aligned fiber SiC/Al MMC, and the ensuing three-dimensional damage state was nondestructively characterized using XTM to map the x-ray absorptivity within the sample. The types of damage detected include: fiber fracture (SiC sheath and C core), fiber-matrix interface microcracking, intra-ply matrix voids and cracks. Quantitative three-dimensional measurements of damage are reported in as-fabricated, monotonically loaded and mechanically fatigue loaded SiC/Al.; The XTM results indicate that increases in observed macroscopic structural stiffness during monotonic loading and the first few fatigue cycles of an MMC coupon correspond to elimination of processing-related matrix porosity and to displacement of the fibers from a somewhat irregular arrangement into a more nearly hexagonal array. The XTM of monotonically loaded samples also show that the carbon cores begin to fracture at or below 828 MPa, that is, at loads far less than those for fracture of the entire fiber. The fracture of the SiC sheath appears to be significantly affected by the fracture of the C cores. The fiber fracture surface topography and sub-surface damage modes observed provide insight into the overall monotonic loading fracture process of this MMC. Structural changes due to the high and low cycle fatigue processes are illustrated. The implication of these results and the use of in situ loading for damage quantification are discussed for mechanical modeling.