Characterization of Damage Progression and Its Correlation to Residual Strength in a Sigma/Ti-6242 Composite Using Nondestructive Methods

In this study, a novel approach of both global and localized damage assessment in metal matrix composites, by means of nondestructive evaluation and damage mechanics, was used to correlate NDE data and the residual tensile strength of a six ply, unidirectional BP Sigma-1240 SiC/Ti-6Al-2Sn-4Zr-2Mo composite after being isothermally fatigued. Baseline tension and fatigue curves were generated since minimal information on this particular metal matrix composite was available in the literature. Information obtained from these tests was used to pinpoint load levels and interruption points for subsequent interrupted fatigue tests. The following nondestructive evaluation techniques were used to evaluate the test specimens before and after fatigue testing: (1) High Frequency Scanning Acoustic Microscopy; (2) Oblique Incidence Shear Waves; (3) Global ‘Reflector Plate’ Ultrasonic Scanning; (4) Ultrasonic Surface Waves (10 MHz and less); (5) In Situ Surface and Longitudinal Waves; and (6) X-Ray Radiography. After the inspections were completed, the specimens underwent a tension test to determine the residual strength. Scanning electron microscopy was used throughout this study to examine fracture surfaces to gain a better understanding of the damage mechanisms present during isothermal fatigue loading. In addition to scanning electron microscopy, metallography and other destructive methods were used in the correlation and verification of fatigue damage. The work has demonstrated the need for developing quantitative correlations between nondestructive evaluation results and the material behavior of metal matrix composites.