An experimental method to determine the longitudinal intralaminar fracture toughness of high tensile strength fibre-reinforced unidirectional composites

The high tensile strength of a variety of commercially-available carbon fibres presents a considerable challenge in determining the fibre-dominated longitudinal intralaminar tensile fracture toughness of cured unidirectional composite plies. The use of standard compact tension specimens leads to premature buckling and crushing at the unloaded end of the specimen. A non-standardized grooved Compact Tension (CT) specimen as well as an external fixture was developed to limit these damage zones and promote crack propagation from the precrack location. Nonetheless, problems were still encountered with extensive sublaminate delamination followed by fibre extension and pull-out leading to fibre fracture. This made it difficult to measure the crack length. To mitigate this shortcoming, a novel measurement technique based on the fibre's elongation and non-destructuve inspection (NDI) was developed to determine an accurate crack length. An IMS60/epoxy composite system was used to manufacture cross-ply specimens [(90/0)_s]₄. Two different data reduction schemes, compliance calibration and the area method, were used to determine the fibre-dominated initiation and propagation fracture toughness values. Propagation values of fracture toughness were measured at 774.9 kJ/m² (compliance calibration ) and 768.5 kJ/m² ( area method ), respectively. A computational model with an in-house VUMAT subroutine for ABAQUS/Explicit was developed to assess the accuracy of the measured fracture toughness. Good agreement was obtained between numerical and experimental results.