On the detection of fatigue damage in composites by use of second harmonic guided waves

In composite structures cyclic loading leads to micro-structural damage even at an early stage. This damage grows over lifetime and may finally cause catastrophic failure. In this study nonlinear wave propagation based on the cumulative second harmonic Lamb wave generation is used to detect micro-structural damage and the corresponding material degradation experimentally. It is shown that an increasing relative acoustical nonlinearity parameter indicates a decreasing Young's modulus. This result is analyzed more in detail by numerical simulations of the nonlinear wave propagation as well as by Young's modulus determination. Therefore, horizontal cracks, detected by microscopy in damaged material, are added to the waveguide with contact boundary conditions. The numerical analysis shows a higher harmonic mode generation due to the interaction of the waves and the cracks and an increasing nonlinear effect with increasing crack sizes and numbers. Since the stiffness is not affected by horizontal cracks the decreasing Young's modulus is assumed to be related to non-visible damage like material degradation, leading also to an increasing nonlinear effect of the wave propagation. In conclusion, the relative acoustical nonlinearity parameter can be used as an indicator of the material degradation and, therefore, for monitoring of micro-structural damage.