A newly designed DCB specimen was introduced fro the study of Mode I delamination fracture toughness of fiber composites at high crack propagation speeds. A strip of FM-73 adhesive film was placed at the tip of a pre-existing crack during laminate lay-up. The presence of the adhesive strip increased the crack initiation toughness substantially so that after initiation an unstable crack propagation was generated and high crack propagation speed could be achieved. Specimens were made from 30-ply unidirectional laminates and were loaded at quasi-static deflection rate on an MTS machine. Crack extension was monitored by means of a conductive line circuit applied along the path of the propagating crack. Continuous recrods were obtained of the load, deflection and crack extension. The strain energy release rate was calculated by using plate finite elemments. It was found that the critical dynamci strain energy release rate is a constant and equals to the critical static energy strain rate with crack speed up to 250 m/s, thereafter, it increases slightly with increasing crack speed.
展开▼
