Lamb waves for non-contact fatigue state evaluation of composites under various mechanical loading conditions

Methodologies for non-destructive evaluation of mechanically induced fatigue in fibre reinforced polymers are discussed. Specimens made of non-crimp glass fabric are fatigued using three different load ratios (tension–tension, tension–compression, and compression–compression). The investigation involves two loading directions (0° and 90°) of the quasi–orthotropic composite. Based on mode conversion of air-coupled ultrasound to Lamb waves, variation in a0-mode velocity is measured in a non-contact and single-sided access configuration. The velocity measurements are performed within and outside the servo-hydraulic test rig used for inducing fatigue damage. Formation of cracks monitored in the transparent composite results in degradation of stiffness observed by the test rig. Decrease in a0-mode velocity caused by fatigue is shown to correlate closely with stiffness degradation for all loading ratios and directions. The correlation is studied by calculating a0-mode velocities from single-ply properties whose stiffness degradation was determined using the observed crack densities and a finite element based model.