Investigations on the mode I translaminar failure and determination of fracture toughness in woven-ply carbon fibers thermoplastic composites at high temperatures

The purpose of this study is to investigate the mode I translaminar fracture energy for initiation and propagation of 5-harness satin weave carbon fabrics reinforced PolyPhenylene Sulphide (PPS) laminates subjected to tensile loadings at T > Tg (glass transition temperature) when matrix toughness is significantly enhanced. Specimens with a quasi-isotropic stacking sequence and a single-edge-notch geometry are characterized by an elastic-brittle response resulting from transverse matrix cracking and fiber breakage near the notch tip. From the macroscopic response standpoint, crack initiation and propagation virtually occur at the same time corresponding to ultimate failure making therefore difficult the assessment of corresponding strain energy release rates. The values obtained for crack onset were determined by means of three different technics: (i) the Crack Tip Opening Displacement (CTOD) method based on the outer surface full-field measurements - (ii) cracking gauges bonded to the outer surface - (iii) monitoring of the Acoustic Emission (AE) activity associated with the prominent damage mechanisms (transverse matrix cracking and fibers breakage). Within the framework of Linear Elastic Fracture mechanics (LEFM), the initiation KI_init and critical KIc mode I fracture toughness were calculated from the strain energy release rate GI in the case of quasi-isotropic laminates. Through the comparison with the value calculated from semi-empirical solutions developed for isotropic materials, the capability of these methods to provide reproducible and accurate values was validated. Among the different techniques implemented in the present work, the monitoring of AE signals is particularly relevant as it provides a reliable approach to detect the crack onset at the microscopic scale.