The behaviour of a rapidly moving crack around a small hole in p.m.m.a. (polymethylmethacrylate) has been investigated, by the use of ultra-high-speed photographic techniques, to measure the changes in dynamic stressintensity factor and crack velocity. Cracks were initiated by explosively driven wedge impact, and penetrated holes of diameters of 1/16 in (1.56 mm) and 1/8 in (3.12 mm). The cracks propagating after the hole show a higher velocity than those prior to the hole, though the dynamic stress-intensity factor decreases. There is no general relation between the dynamic stress-intensity factor and the surface roughness for the higher-velocity crack.
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