Assessment of mode-II fracture tests for unidirectional fiber reinforced composite laminates

Three basic mode-II test methods (ENF, JIS, and ASTM D7905M-14) are assessed using the material system AS4/8552 carbon/epoxy unidirectional composite laminate to understand similarities and differences. The modified JIS method uses a PTFE film coated stainless steel rod instead of the PTFE strip that was proposed in JIS. The ASTM D7905M-14 test method determines FEP film crack front (NPC) and shear precrack front (PC) fracture toughnesses. Alternately, wedge precracked specimens were also tested to assess the shear versus opening mode precracking on mode-II fracture toughness. The analysis and test results revealed that the JIS method is a mixed-mode I-II test and result in lower value of mode-II fracture toughness. The GI loading is about 51 J/m2 for the material tested and GIIc measured by JIS is always less than pure mode-II fracture toughness. The GIIc measured from the ASTM D7905M-14 NPC and the ENF tests are almost identical, but the ASTM test offers a compliance equation that may be beneficial in fatigue crack growth studies. As suggested in ASTM standard, shear precracked specimen is appropriate to measure mode-II fracture toughness.