Effect of post-curing on the fracture toughness of fiber reinforced polymer laminates and in-depth analysis of the delamination process using acoustic emission analysis

Fiber-reinforced polymer (FRP) components are widely used in applications requiring lightweight construction with high stiffness and strength. Since delamination is a dominant failure mode in FRP laminates, precise interlaminar damage models are essential. This master's thesis compares the delamination resistance of glass fiber-reinforced (GF-) and carbon fiber-reinforced (CF-) epoxy laminates manufactured with an equivalent epoxy matrix system, isolating the influence of fiber type. Mode I fracture toughness was measured using double-cantilever beam tests to determine initiation and propagation values. Mode II toughness was evaluated using end-notched-flexure tests to analyse initiation values. Both were performed according to established standards. The impact of post-curing was evaluated by testing FRPs that were either post-cured (exposed to 120 °C for one hour in a press) or non-post-cured. The effect on the glass transition temperature (Tg) was analysed using differential scanning calorimetry and dynamic mechanical analysis. Scanning electron microscopy was used to identify damage mechanisms on fracture surfaces, which were then correlated with acoustic emission (AE) cluster analysis. Three damage mechanisms were observed: matrix cracking, fiber breakage, and fiber-matrix debonding. Both materials exhibited consistent trends across testing methods and curing conditions. Post-curing raised Tg by ~20 °C for both fiber types and it also increased fracture toughness. It elevated the initiation values by a factor of ~1.4 for both fiber types, though it had insignificant effect on propagation. Mode II fracture toughness did not increase significantly. Comparing the fiber types revealed similar trends, regardless of the post-curing state. GFRP exhibited higher fracture toughness values than CFRP. The GFRP initiation Gc values determined from Mode I testing are approximately 1.3 times higher than those of CFRP and the propagation values are roughly 2.7 times higher, regardless of the post-curing state. Comparison with values of Mode II Gc showed greater differences. The non-post-cured values are 9 times higher for GFRP and 5 times for CFRP. The ratios decrease to 7 and 4, respectively, in their post-cured state. The AE cluster analysis provided better results for the CFRP because fewer AE signals were produced, resulting in better cluster separation. However, the large number of values and the merging of the GFRP measurement clusters led to unstable boundaries that shifted with each calculation iteration.