Air-coupled guided waves combined with thermography for monitoring fatigue in biaxially loaded composite tubes

Non-destructive methodologies for remote monitoring of fatigue induced by mechanical load in fibre reinforced plastics are presented. Hollow cylinders (glass fibre winding) were stepwise biaxially fatigued and measured in single-sided access configurations. Based on conversion of air-coupled ultrasound to guided waves, it is shown that accumulated fatigue damage is accompanied by decrease in phase velocity and increase in attenuation. The change in wave velocity caused by fatigue is shown to correlate closely with measurements of stiffness degradation of the composite. The attenuation of guided waves is affected by crack density which is visually traceable in the transparent composite. Monitoring of cyclic loading of the specimens by thermal imaging and a high-speed camera revealed that the initiation of final failure in the specimens coincides with spots of increased temperature. Air-coupled guided wave area scans allow for observing the development of these areas and other local damage in the composite.