Fracture roughness evolution during mode I dynamic crack propagation in brittle materials

The evolution of roughness RMS during dynamic fracture in thin plates of epoxy (Araldite B) was studied in relation to important fracture parameters such as crack velocity ȧ, acceleration and deceleration ä and mode I stress intensity factors (K_ID). Dual-focus high speed photography was carried out to evaluate values of K_ID by the caustic (K_ID(C)) and photoelastic (K_ID(P)) methods simultaneously. A specially designed jig applied successive tensile loadings to specimens which resulted in cyclic change of ȧ and ä. The changes of ȧ, RMS and K_ID are shown to be qualitatively similar with respect to the value of a. Quantitatively, a typical result showed that the position of the first maximum value of ȧ came 7 mm behind the first loading axis, while that of K_ID (P), RMS and K_ID, (C) appeared on the fracture surface 7 mm, 15 mm, and 25 mm behind the axis, respectively. The agreement between K_ID (C) and K_ID (P) was acceptable except for data around the first loading axis. Discussions are given on the uniqueness problem of ȧ and K_ID.