Nonlinear ultrasonic testing of carbon fibre reinforced plastics in the very high cycle fatigue regime

Carbon fibre reinforced plastics (CFRP) are applied as a light-weight construction material. In their lifespan in service CFRP components are exposed to variable loads, which can amount to 1011 load cycles. It is therefore important to examine the fatigue of CFRP materials in the very high cycle regime (more than 107 loading cycles). In a joint project a three point bending ultrasonic fatigue testing system was developed at WKK. A high amplitude sinusoidal vibration is added to a static load. The operating frequency of the system f = 20 kHz is used for sample load case and at the same time as input for online ultrasonic monitoring of the fatigue process. CFRP plate material with a thickness of 4 mm was investigated. During the fatigue process the vibrations of CFRP samples were measured with a laser vibrometer and a microphone. The analysis methods fast Fourier, short time Fourier, and Hilbert Huang transformation were used to evaluate the linear and nonlinear spectral content of the signals. Increase of nonlinearity with increasing amount of fatigue cycles was observed. Non-destructive testing methods were used to examine the initial state as well as the different fatigue states of the samples during the loading intervals.