Dynamic fracture toughness measurements on small charpy specimens

A technique for measuring dynamic fracture toughness on small specimens is described and preliminary data are presented. Dynamic crack tip opening displacements (CTODs) are measured with laser interferometry over times as short as 10 mu s. The measured displacement is converted to stress intensity values via the equations of linear elastic fracture mechanics and the peak value obtained is taken as the dynamic fracture toughness. Two materials were tested at two different loading rates; a brittle plastic at 10 (exp 3) and 10 (exp 4) MPa-m(exp 1/2)/s and a ceramic 5 X 10 (exp 4) and 10 (exp 6) MPa-m(exp 1/2)/s. The results from this new technique were compared with simultaneous results from an instrumented Charpy hammer and, in some cases, from foil gages on the specimens. At the slower rates, the agreement among the results is acceptable. At the higher rates, the instrumented hammer technique is no longer valid. The technique is potentially useful on small, brittle specimens at room and high temperatures.