Stable crack extension through AISI 4340 steel: Experimental investigation

The stable crack growth (SCG) through AISI 4340 steel alloy was investigated experimentally under mode-I and mixed-mode (I and II) loading. A pre notch of 0.05 mm root radius was introduced in the specimen through electro discharge machining (EDM) wire cutting technique. Different combinations of loading angles ψ and original crack length to specimen width (a _o/W) ratio were examined. Data concerned with direction of initial crack extension load-load line displacement (L-LLD) diagrams, initiation and maximum loads, range of stable crack growth, crack blunting, crack front geometry, fracture surfaces, and scanning electron microscope fractographs were obtained. A noticeable blunting was observed prior to the crack initiation. Although the crack initiates from a straight front, a considerable crack front tunnelling was observed as the crack proceeds. In a mixed mode, the crack extension takes place initially almost along a straight path inclined with the main crack. The loading angle and initial crack length affect the initiation and maximum load significantly, however the ratio between P_max and P_i remains almost constant.