Micromechanics of Crack Extension in Composites This research focuses on the characterization of matrix damage in metal matrix composites under transverse loads. A volume integral equation method (VIEM) developed as a new numerical scheme for the solution of elastodynamic and elastostatic problems in unbounded solids containing multiple inclusions and cracks is used to investigate the mechanics of the damage evolution in a unidirectional SiC/Ti composite under transverse loading. The objective of this research is to analyze and predict the failure and fracture behavior in SiC/Ti composites, and to characterize the defects through NDE and determine the materials degradation in SiC/Ti metal matrix composites.

Failure Mechanism Diagram SEM Photomicrograph

The Titanium & Carbon Fiber Composites were subjected to Periodic Loading Stiffness vs. Cycle Plot

Nondestructive Evaluation of Stiffness Degradation in Composites during Fatigue Loading We subjected cross-ply composite specimens of titanium matrix and graphite/epoxy matrix composites to tensile fatigue tests, inducing stiffness degradation. Extensional plate wave velocities in various directions, together with the quasi-shear wave velocity in the 90o direction, were used to characterize the degradation in the stiffness constants of each specimen during fatigue. The laminate stiffness was also monitored through direct measurement using an extensometer during fatigue loading. The trend in the stiffness degradation of the titanium matrix composite was identical in both measurement techniques, but the stiffness values calculated from the extensional velocity measurements were higher than those obtained from the extensometer. The discrepency was attributed to the presence of open microcracks in the static measurements.

Interaction between Ultrasound and Flaws in Metallic Alloys The main goal of this study is to improve backscattering technique in order to detect flaws and damages in large-grained metallic alloys such as titanium alloys, an alloy widely used in aerospace industry. Theoretical and numerical analyses are used to evaluate the interaction between ultrasonic waves and material microstructures as well as flaws. To do this, we use Dynamic Volume Integral Equation Method based on the integral representation of the elastodynamic field. Analytical solution of these equations has been obtained under the assumption of a single scattering and first order Born approximation.

Schematics Representation of the Backscattering Problem

Modified 9/98