Evaluation of the Split Cantilever Beam for Mode III Delamination Testing

A test fixture for testing a thick split cantilever beam for SClSsonng delamlnatlon (Mode III)fracture toughness was developed A three-dimensional finite element analysis was conducted on the test specimen to determine the strain energy release rate, G, distribution along the delammatlon front The virtual crack closure technique was used to calculate the G com- ponents resulting from interlammar tension, G~, mterlammar sliding shear, Gn, and interlamlnar teanng shear, Gm The finite element analysis showed that at the delamlnatIon front no G~ component existed, but a Gu component was present in addition to a Gm component Furthermore, near the free edges, the Gn component was significantly higher than the Gm component The GH/ Gm ratio was found to increase with delamlnatlon length but was msensttwe to the beam depth The presence of Gn at the delamlnatlon front was verified experimentally by examination of the failure surfaces At the center of the beam, where the failure was in Mode III, there was significant fiber bridging However, at the edges of the beam where the failure was in Mode II, there was no fiber bndgmg and Mode II shear hackles were observed Therefore, it was concluded that the split cantilever beam configuration does not represent a pure Mode III test The experimental work showed that the Mode II fracture toughness, Gur must be less than the Mode III fracture tough- ness, Gmr Therefore, a conservative approach to characterizing Mode IlI delamlnatIon is to equate GIIIr to Gnr