Dynamic Measurement of Crack-Tip Opening Displacement

Crack opening displacements were measured at two positions very close to the tip of a crack in three-point bend specimens that were loaded explosively. Indentations were placed across the fatigue precrack at locations 100 (i.m and 200 n,m behind the tip. These indentations, when illuminated with a laser, generate fringe patterns in space, and the motion of these fringes is proportional to the displacement between them. The fringe motion was monitored with photomultiplier tubes and an oscilloscope and converted to crack-tip opening displacement (CTOD). Explosive loading of the specimens generated stress-intensity factors increasing at a rate of 3 x 10* MPa m"/s. A soft tool steel was used as a prototype material for this technique development because its material response is similar to the tough materials that are to be studied in the future. Both CTOD and crack-tip opening angle were measured in a search for an appropriate measure of resistance to dynamic fracture. Quasi-static tests were also conducted using the same measurement techniques plus a clip gage. A dynamic critical stress-intensity factor (not plane strain) was determined using a slope reduction procedure based on the static tests. The results show Uttle change in dynamic fracture toughness for this material with increasing loading rate. The experimental techniques show promise for further dynamic studies.