In-situ ultrasonic surface wave assessment of mechanical fatigue damage accumulation in metal matrix composites

This study demonstrates that an in-situ nondestructive, ultrasonic surface wave technique can successfully detect the onset and extent of matrix cracking fatigue damage in a titanium metal matrix composite (MMC). A quasi-isotropic [0/±45/90]_sSCS-6/Timetal® 21S MMC material was used for room temperature fatigue tests and the resultant matrix cracking damage was ultrasonically monitored in situ as a function of cycle count. Damage accumulation in the material was successfully correlated with decreases in ultrasonic pitch catch amplitude and verified through the use of immersion ultrasonic C-scans and metallographic techniques. Damage initiation and progression was tracked through the use of complementary nondestructive and destructive techniques. The in-situ surface wave data show that the higher the fatigue stress level, the more quickly damage occurs; conversely, the lower the stress level, the slower the damage initiation. The in-situ surface wave technique proved to be more sensitive to the accumulating damage than standard load-displacement modulus measurements. The surface wave technique also indicated a change in material properties after only one fatigue cycle. The data acquired show that a better understanding of damage initiation and accumulation can be gained using the in-situ surface wave technique in comparison to current load-displacement modulus measurements.