Evaluation of a critical impact energy in GFRP under fatigue loading

Defects in fibre reinforced polymer structures, such as impact damages, have a major influence on the fatigue behaviour. Impacts may occur during the lifetime of composite structures and lead to delaminations between adjacent layers with different fibre orientations and to matrix cracks within the layers. Specimens made of a glass fibre non crimp fabric were produced by vacuum assisted resin transfer moulding. Low velocity impact damage was introduced by use of a drop weight with a hemispherical head. The specimens were tested under fatigue loading with a stress ratio of R= -1 (tension-compression). The aim was to characterise damage development in fatigue testing of fibre reinforced polymers as a function of impact energy, lifetime and stress level. Therefore, the stiffness degradation during fatigue testing due to matrix cracks, delaminations, fibre failure and temperature development of the specimens was plotted vs. the number of load cycles. In interrupted fatigue tests with defined numbers of load cycles the influence of impact damages on matrix crack development was determined and correlated with stiffness degradation. Furthermore, thermoelastic stress analysis was performed during the interrupted tests in order to determine stress concentrations in the area of impact damage.