Dependence of crack acceleration on the dynamic stress-intensity factor in polymers

The caustics method in combination with high-speed photography was employed to study velocity effect on the dynamic-stress-intensity factor of fast cracks in polymethyl methacrylate and in Araldite D. The specimen geometry was so determined that both the accelerating and decelerating crack propagation occurred noticeably in one fracture event. Instantaneous crack velocity as well as its acceleration were expressed as a function of the crack length by using polynomials of the ninth order which were given on the basis of the least-square method. The results show that the dynamic-stress-intensity factor depends not only on the crack velocity but also on crack acceleration, and that the accelerating crack has a smaller value stress-intensity factor than the decelerating crack at the same velocity.