Static and fatigue evaluation of GFRP composites in 3-point bending tests

摘要

In the present paper, the static and fatigue evaluation of a fibre glass-epoxy composite system. based on a quasi-unidirectional prepreg, in 3-point bending tests with constant amplitude of strain is made. The studied laminates, all with 16 layers, were the following ones: [0]_(16), [(0/90)_4]_s, [90]_(16), and [+-45]_(4S) [(0/+-45/90)_2]_s. The definition of end damage criterion revealed difficult, since the full rupture of the test specimen was never observed. The NX criteria were analysed (x% reduction of the initial stress) such as defined in NF T 51-120-3. The N10 criterion, in general, represents the rupture of the first ply and the beginning of the fast and catastrophic propagation over the remaining plies to the specimen neutral axis. The N80 criterion seems to represent the end of the fast damage propagation, as after this point a very slow damage evolution in the remaining plies occurred. The S-N slope curves approached for the Mandell's equation varies between 10% to 13% per decade of number of cycles for an unidirectional and an angle-ply laminates, respectively. These results are very close to the Mandell's predictions (10% for GFRP). The Young's modulus decay was studied in all laminates. In quasi-static loading varied between 5% and 30% for [0]_(16) and [90]_(16), respectively. In fatigue, the modulus decay was followed until a reduction of 10% of the initial stress. In unidirectional laminates, it was observed that the modulus reduction is load level independent, while in the remaining ones it was observed that it is load level dependent. The matrix cracking evolution was also analysed in quasi-static and fatigue loading. Evolution curves crack numbers revealed the existence of a characteristic damage state (CDS) in both test conditions. In cycle loading, the CDS is only reached for the higher load levels. A cumulative damage model based on the critical element concept of Reifsnider was used to predict the residual strength and fatigue life of cross-ply laminates. The stiffness reduction in the sub-critical element was extracted from experimental data of relevant laminates and it was used in a software with the S-N curves of the 0 °ply and the classic laminate theory. The model shows good correlation with the experimental results.