A large number of fatigue cracks and fractures of test specimens and components of various engineering alloys were investigated by means of the fractographic technique. In particular, critical experiments were performed on the observation of natural profiles of fracture plateaus and mating fracture surfaces using the scanning electron microscope. It was found that the profile of a fatigue crack appears to be a tunnel with numerous steps. In consequence, the striations on the matching surfaces are basically in register other than any kinds of symmetrical configurations such as peaks to peaks and valleys to valleys. Furthermore, the appearance of them on one surface is usually more regular and distinct than that on the matching surface due to their greater slip length and, sometimes, slipband notches or cracks. The process of fatigue crack growth is just like alternatively walking up or down the steps of a path in different areas or grains.
A more general and practical model schematically illustrating the mechanism of fatigue crack growth and striation formation, which is one of the most fundamental questions on fatigue, is proposed. According to this model, the process of fatigue crack growth under a loading cycle includes four steps: (i) crack-tip opening; (ii) shear crack growth in a very short distance by single system slip decohesion; (iii) normal crack growth by alternative conjugate slip decohesion; and (iv) crack-tip closing, resharpening and, sometimes, inducing one or two slipband notches or cracks at the very tip. It differs markedly from the previous models which, in general, describe the crack advancing under conditions of symmetric deformation at the crack tip and growing straight along the crack plane in a loading cycle. Based on the present model, the markings on the fracture surfaces and phenomena associated with the process of crack growth, such as the secondary shear cracks accompanying by the striations, the slipbands between striations, and the simultaneous growth of branching cracks, can properly be explained.