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 degrees and 90 degrees) of the quasi-orthotropic composite. Based on mode conversion of air-coupled ultrasound to Lamb waves, variation in a(0)-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 a(0)-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 a(0)-mode velocities from single-ply properties whose stiffness degradation was determined using the observed crack densities and a finite element based model. (C) 2012 Elsevier Ltd. All rights reserved.