Determination of crack resistance curves under static and dynamic loading by analysis of load displacement relationship

The key curve method according to Ernst et al. ( Fracture Mechanics, ASTM STP 677, pp. 581–599, 1979) was applied for the determination of J R -curves. Steels with different yield strength and toughness were investigated. The method requires a calibration curve, which has to be determined for each material and specimen type. It was shown for CT-specimens that crack extensions calculated by the key curve method are in good agreement with those observed on the fracture surfaces. The J 1-values determined coincided well with values derived from ‘stretched-zone’ measurements and the equation for the ‘blunting-line’ according to Shih ( J. Mech. Phys. Solids 29, 305–326,1981). It was demonstrated that the key curve method can be applied even at high loading rates. 1 CT-25-specimens were tested at ram speeds up to 4 m s . First results of instrumented charpy impact tests showed that crack propagation can be monitored in charpy V-notch specimens, too. On the other hand, crack initiation is difficult to detect in the latter case because of superimposed oscillations in the load signal.