Investigation of the load sequence effects in fatigue of damaged composite laminate & characterization of fatigue damage

摘要

Carbon fiber reinforced composites are widely used today in various areas and specially in aerospace industry for structural applications. This investigation focuses on the effect of different load sequencing and impact damage on the fatigue behaviour of CFC laminates. The specimens made from plain CFC laminates and low energy impact damaged CFC laminates were subjected to a typical flight block loading sequence and the fatigue strength degradation was monitored through stiffness measurement using load displacement data obtained during block loading. Three different stress/strain levels were used in testing. All the tests were performed using a computer controlled 100 kN servo-hydraulic test machine in load mode at room temperature and in lab air atmosphere on undamaged and low energy impact damaged composite laminates. Fatigue tests were performed with a sinusoidal waveform at 3 Hz. It was observed that lower strain levels did not show any significant effect on the fatigue properties in both the type of loading i.e. low to high and in high to low block loading in case of both the undamaged and impact damaged CFC specimens. Significant.reduction in stiffness was seen at higher strain level i.e. 6500me in both the undamaged and impact damaged CFC specimens. The low energy impact damaged specimens showed early failure at higher strain levels compared to undamaged specimens. The specimens were observed to have delaminated in the high stress fatigue cycling. The observed stiffness reduction due to fatigue cycling and the presence of delamination provide a means of macroscopic identification of fatigue strength degradation in composite materials. The energy plots appear useful tool to assess the damage growth.