The Influence of Matrix Fracture Strain and Interface Strength on Cross-Ply Cracking in CFRP in the Temperature Range of -100°C to +100°C

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Abstract
Cross-ply cracking in specimens out of the laminate 0/904/0 was in vestigated at -100°C, -40°C, RT, 60° and 100°C. The thermal strain in the transverse ply at these temperatures was determined with the thermal expansion coefficients in fibre direction and transverse to the fibre of a unidirectional laminate. These thermal expansion coefficients were found to vary approximately linearly with the temperature. Multiple fracture of the 90°-ply of 0/904/0 specimens was registered with the aid of a piezo-electric transducer in connection with a computer during a displacement-controlled quasi-static tensile test. The 904-ply is divided in a number of elements N all of which can break. The registered crack data ( n cracks, n < N, with their respective fracture strains) can be described by two-parameter Weibull distributions of the fracture strain ( = thermal plus mechanical strain). From the Weibull distributions and an available model for the transverse fracture strain distribution it was found that at increasing test temperature two phenomena influence the transverse fracture strain. These are the fracture strain (and duc tility) of the matrix and the strength of the interface, which at increasing temperature im proves and reduces the transverse fracture strains respectively. First ply failure proved to be dominated by the increasing fracture strain of the matrix from -100°C to RT. Above RT, however, the influence of the decrease in interface strength dominates over the in fluence of the increased matrix fracture strain.