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Difficulties associated with a fracture mechanics interpretation of the drop-weight nil-ductility transition temperature (DW NDTT) test are reviewed. Analytical correlations between the plane-strain fracture toughness for unstable fracture under dynamic loading Kld and DW NDTT presented in the literature usually assume that stress intensity can be calculated by postulating a small thumbnail-shaped crack subjected to dynamic yield stress loading. In practice, cracking continues to be extensive well above DW NDTT, while the region near the Hardex bead is subjected to high plastic strain. The paper presents analyses that attempt to take these factors into account, but none are felt to be entirely convincing. A more appealing explanation of why DW NDTT occurs at the temperature at which it does can be made by comparing the energy available from the specimen and tup with that required to propagate the crack. The author postulates that shear lip development may be the common factor which explains the empirically observed correlation between Kld and DW NDTT.
crack propagation, fracture mechanics, nil-ductility transition temperature, drop-weight test, ASTM standard E 208
Principal scientific officer, Admiralty,