FATIGUE OF METALS UNDER NONUNIFORM STRESSED STATE.PART 1. STRESSED STATE ASSESSMENT METHODS AND RESULTS OF INVESTIGATION

摘要

One of the main factors that govern the fatigue limit of materials is the stressed state in a test specimen or a structural component [1-5].The stressed state in a structural component depends, on the one hand, on the state of stress at the maximum-stressed point, which is represented by the magnitude of either maximum normal (shear) stresses or equivalent stresses allowing for multiaxiality of loading and calculated by a given criterion, and, on the other hand, on the pattern of variation of stresses versus geometrical dimensions, primarily the cross section. A stress gradient is usually used as a characteristic of nonuniform stressed state at a point under consideration.There have been numerous publications addressing the stressed state in nonuniformly stressed structural components under elastic deformation, including the case with a stress concentration, and the corresponding stress gradients.The assessment of stress-strain state and stress gradients in nonuniformly stressed structural components under inelastic deformation becomes much more complicated. A distinction should be made between the nominal stresses, which are calculated ignoring the inelasticity and stress concentration (in the case with stress raisers), and the effective stresses calculated with regard for stress concentration and inelasticity. Abundant experimental evidence shows that the fatigue characteristics under uniform stressed state substantially differ from those under nonuniform stressed state. Specifically, the fatigue limits of metals and alloys on a base of 10~7 bending cycles can exceed the same parameters under axial loading by 50% and more; local stresses at the tip of a notch, which correspond to the fatigue limit, are much higher than the fatigue limit of smooth specimens for the same number of cycles. Meanwhile, the nominal stresses in the notch section, which correspond to the fatigue limit, are far below the fatigue limit for smooth specimens.