Studies on Stable Crack Growth

The goal of this work is to experimentally investigate the stable crack growth (SCG) fracture behavior of AISI 4340 alloy steel. A series of mode I and mixed mode SCG fracture tests were carried out on 8 mm thick compact tension (CT) specimens subjected to quasistatic loading. The wire cutting technique was used to introduce a pre-notch/ pre-crack of 0.05 mm root radius to the specimen. Five different loading angles Ψ between the loading axis and the crack surface were employed; 90° (mode I), 75°, 65°, 60° and 50°. Five different ratios of original crack length to specimen width ao/w were also employed, 0.41, 0.42, 0.43, 0.44 and 0.45. Different combinations of Ψ and ao/w were used. 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 fracto graphs were obtained. A noticeable blunting was observed prior the crack initiation. Although the crack commences its growth from a pre-notch / pre-crack of a straight front it has a considerable tunneling at each stage of stable crack growth. In mixed mode, the crack takes place along a straight-line path initially, inclined with the main crack at an angle equal to the direction of crack extension. The loading angle Ψ and the initial crack length to the specimen width ao/w ratio affect the SCG fracture behavior significantly. The direction of initial stable crack extension was determined through an elastic finite element analysis. There was reasonably good agreement between the experimental and the predicted results