Published: Jan 1967
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Studies of fatigue fractures have indicated that crack propagation takes place by two stages. The first stage propagates at 45 deg with respect to the stress axis, while the second does so at 90 deg. The evidence for crack growth is reviewed, and it is concluded that the Stage II mode occurs by plastic blunting of the crack tip during the tensile part of a fatigue cycle followed by resharpening of the crack in the compression part. The mechanism of Stage I growth occurs on too small a scale to be studied directly but is deduced to be essentially the same as Stage II. The softness of fatigue slip bands with respect to the bulk of the material, however, provides an easy path for crack propagation in Stage I and thus causes propagation at about 45 deg to the stress axis. On the other hand, in Stage II, the larger stress concentration associated with the longer crack dominates the structure at the crack tip and orients the path of crack propagation at 90 deg to the stress axis. The morphological details of fracture surfaces are considered in the light of these mechanisms. The role of microstructure in influencing the mechanisms is treated from two aspects. First, it is shown that the various kinds of hardened structures merely act to control the degree of deformation at a crack tip in relaxation, and thus the kinetics of crack propagation. Otherwise, microstructures hardened to extremes by cold-working or by precipitation can be the exception to the generalization, acting in conjunction with other testing variables to change the mode of propagation so as to introduce cleavage or quasi-cleavage fracture to the crack propagation process.
microstructure, fatigue (materials), crack propagation, plastic flow, slip bands, fracture
Ford Motor Co., Dearborn, Mich.