MONITORING OF FATIGUE IN COMPOSITES USING AIR-COUPLED GUIDED WAVES

The objective of this paper is to develop a non-destructive methodology for remote monitoring of fatigue induced by mechanical load in fibre reinforced plastics. The approach is based on mode conversion of air-coupled ultrasound to flexural waves. Variations in flexural wave velocity and attenuation are used as indicators of fatigue state. Glass fibre reinforced non crimp fabric plastic specimens were stepwise fatigued and measured in a single sided access configuration. The experiments conducted demonstrate that accumulated fatigue is accompanied by decrease in velocity and increase in attenuation of the flexural wave. The change in wave velocity throughout fatigue is shown to correlate closely with measurements of stiffness degradation of the composite. This parameter is found to be different for different fatigue loading protocols. The propagation of flexural waves is affected by internal interfaces such as cracks due to scattering. Hence, attenuation can be used as a measure of an increase in crack density caused by fatigue. The combination of wave velocity and attenuation is considered to be a distinctive parameter, which enables to determine whether the composite is fatigued in tension-tension or compression-tension mode.