Dynamic crack propagations in PMMA and epoxy specimens were studied using the method of caustics in combination with a Cranz-Schardin type high-speed camera. Single-edge-cracked tensile specimens were fractured under pin-loading conditions so that cracks could experience acceleration, deceleration and re-acceleration stages in one fracture process. The dynamic stress intensity factor KID, crack velocity ȧ and acceleration ä were evaluated in the course of crack propagation to examine the effects of ȧ and ä on KID. Results showed that ȧ and ä were important factors in changing the values of KID, and for a constant ȧ the decelerating crack had a larger value of KID than the accelerating or re-accelerating crack. Also, it was found that KID could be expressed as two parametric functions of ȧ and ä for PMMA and epoxy specimens.
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