Dynamic fracture test is performed for bend specimen of SCM4 by drop weight test machine, which is designed and manufactured for this study. Resistance strain gages are used to measure the strains near the propagating crack tip and velocities of the crack. The dynamic fracture initiation toughness, $K_{Id}$ is obtained from load-time record and is found to be 37.6 MPa$\sqrt{m}$. The dynamic fracture propagation toughness, $K_{ID}$ is also measured experimentally. The range of the loading rate used is $5\times 10^5-1.2＆times10^6 MPa\sqrt{m}S^{-1}$. The crack velocity obtained is about 3.4 - 10% of the Rayleigh wave speed of the material.
It is found that the dynamic fracture initiation toughness, $K_{Id}$ is larger than the static fracture toughness $K_{IC}$ and the dynamic fracture propagation toughness, $K_{ID}$ is smaller than $K_{IC}$. The strain signal from the gage near the crack tip shows the point of inflection, which Corran insists is caused by the effect of the combined stress wave and external load.
