Monitoring Early Damage Initiation of Very High Cycle Fatigued Composite Material Using a Nonlinearities Based Inverse Approach

During the operational life of composite structures while subjected to repeated loading, there often lies the possibility of fatigue damage initiation that can lead to catastrophic failure in case no precautions are made. Identification of those possibly barely visible damages may indulge costly non-destructive inspection techniques, where structural health monitoring (SHM) could be shown to be rather economical and promising. The widely practiced linear vibration based techniques, however, fail to account for the bilinear nature of damages calling for the adoption of nonlinear methods to justify the stiffness changes due to crack closure. In this paper, a 2D-FEM model using COMSOL for a three point bending test of a CFRP sample with the presence of fatigue-breathing delamination damage is simulated and their frequency transforms of the time domain signals for various crack sizes and crack locations are studied. The numerical studies show that the presence of breathing cracks in the frequency domain exhibits higher harmonic peaks due to the opening/closing action of the crack. Once the features of the frequency domain response as functions of damage size and location are formulated, numerical inversion can be applied to the experimental data to obtain the damage parameters such as the size and its location. The simulated results are then validated with the experimental data obtained from Very High Cycle Fatigue (VHCF) testing of CFRP (Polyphenylenesulfide- PPS) test samples. Good agreement between the simulated and experimental results ensures the robustness of the approach showing high potential in SHM for detecting early nonlinear-type damages in structures. doi: 10.12783/SHM2015/168