Determination of the mode I crack resistance curve of polymer composites using the size-effect law

This paper presents a new method to measure the crack resistance curve associated with the longitudinal failure of polymer composites reinforced by unidirectional fibres. Rather than using compact tension test specimens, the identification of the size-effect law of double edge notched specimens is used to obtain the crack resistance curve. Special emphasis is placed on the appropriate calculation of the stress intensity factor of the specimens when using quasi-isotropic or cross-ply laminates. For this purpose, both analytical closed-form solutions and numerical methods are investigated. Four different carbon-epoxy material systems, T800/M21, IM7/8552, T700/AR-2527, and T700/ACE are tested and the corresponding size effect laws and R-curves are measured. A good correlation between the crack resistance curve obtained using the size effect law and that previously measured for one of the material systems using the compact tension test is obtained. The highest value of the longitudinal fracture toughness was obtained for the T800/M21 material.