Variation of Fracture Surface Marks due to Static and Dynamic Strain Rate

Abstract

Fracture tests of unsaturated polyester resin on large, single-edge notched-bend specimens were conducted under static and dynamic bend tests at room temperature. The velocity of a loading point in the three-point bending was changed from 8.3×10-6mm/s to 7×105 times its value. The two types of fracture marks of the both-ends-closed mark (like ellipses) and the one-end-opened mark (like parabolas) were observed and they were expressed by the equation of fracture marks. A rocket mark, defined here, was also often observed in the dynamic bending test and was well-approximated by the equation. The experimental number, percentage, relative interference appearing distance and the calculated relative critical distance of the marks changed with the crack propagation distance and the testing speed. The velocity of a primary crack propagation was measured and approximated by a mixed-condition relation proposed by us. This change and the gammer-letter relation between velocity and a dynamic stress intensity factor KD (or a dynamic strain energy release rate, DD) gives the latter (KD or DD) as a function of the crack propagation distance. This provides a good theoretical explanation of the above variations of fracture marks. © 1988, The Japan Society of Mechanical Engineers. All rights reserved.