Prediction of fatigue life of glass fiber reinforced polyester composites using modal testing

The main objective of the present work is to investigate the capability of experimental modal analysis as a nondestructive tool to characterize and quantify the fatigue behavior of laminated composite beam with different lamina orientations and cantilevered boundary condition. In the present work, experimental modal analysis was conducted on the specimens previously subjected to fatigue loading to determine the modal parameters (natural frequency, damping ratio and mode shape). This was achieved through studying the response of modal testing with different specimens of different lamina orientations as a main factor affecting fatigue life. This correlates modal parameters such as: damping ratio, natural frequency and mode shape to fatigue behavior. The composite material used in experiments is glass fiber reinforced polyester (GFRP) laminate. Plane bending fatigue tests were performed on standard fatigue specimens. The fatigue test was interrupted at different fatigue life ratios (n/Nf) and modal testing was conducted to determine the change in modal parameters. The results showed that the changes of modal parameters provide a proper means for predicting the fatigue behavior of composite structures. From the experimental results of both dynamic and fatigue tests, curve fitting technique was used to correlate modal parameters to fatigue life. An exponential and quadratic equations have been obtained which correlate fatigue life ratio to damping ratio and resonant frequency respectively. It was noticed from the curves representing exponential and quadratic equations that the value of damping ratio ξ is more noticeable than the value of frequency, which means that the extent of fatigue damage determines the damping ratio, hence damping ratio could be said to be a good indicator of the fatigue life ratio.