Abstract
Cracking behavior has been investigated through successive observations on the surfaces of five steels, an alpha brass, an age-hardened Al-alloy and a pure titanium, with particular emphasis on the relation between the crack initiation process and the notch sensitivity in fatigue.Two types of crack initiation process were observed. One was the process where cracking starts at a very small region compared with a grain diameter, as observed on an Al-alloy. The other was the one where a fatigue crack initiates not at a point but at a definite region having the size comparable to the grain diameter, as observed on other metals.The difference in cracking characteristics was related to the thickness of surface layer. The notch sensitivity based on fatigue crack initiation decreased with increasing thickness. An Al-alloy with the thinnest thickness had the highest sensitivity, whereas a pure titanium the lowest. For a sharp notch, however, a non-propagating crack appeared, so that the fatigue limit did not decrease drastically with increasing notch sharpness, leading to rather low sensitivity. The fatigue strength of the notched members can be uniquely assessed from the relation between the reciprocal of notch root radius and the elastic maximum stress at the fatigue limit, based on the new “Linear Notch Mechanics” concept.
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