Evaluation and prediction of fatigue resistance of metals from the kinetics of variation in surface properties

Abstract

The paper studies the fatigue resistance of metallic samples subjected to high-cycle loading and microhardness measurement. The fatigue damage of materials during loading is identified as decrease in the thickness of the barrier surface layer, which prevents fatigue failure. It is shown that the thickness of this layer is independent of the plastic characteristics of the material and the level of stress. A method to evaluate accumulated fatigue damage is developed. Kinetic curves of damage accumulation are analyzed. Methods to predict fatigue characteristics are proposed Keywords: high-cycle loading, fatigue strength, microhardness, damage accumulation, surface layer, barrier layerIntroduction. The overwhelming majority of modern machine parts serve under cyclic loads that commonly lead to fatigue failure. In this connection, the fatigue strength of materials is one of the major criteria for the evaluation of the structural strength of many machine parts [22,23,25].It was shown in [8, 15, 20, etc.] that fatigue damage of metals is not uniform over the volume, even with a homogeneous stress state, and is initiated and more intensively develops in the surface layer. If the stress state is inhomogeneous, the role of the surface will be more important due to the stress gradient.The dependence of fatigue life on the factors responsible for the state of the surface layer of the material is now well-understood [17]. The effects of surface condition, stress state, corrosive media, and other factors were established. However, many issues related to the role of the surface in fatigue damage remain unresolved. Of current importance is to make relevant assessments to formulate fatigue failure criteria as a basis for the improvement of design methods. This, in turn, calls for the development of a method for analysis of surface behavior and selection of necessary techniques and means.One of the most readily available and efficient methods for analyzing the mechanical properties of surfaces is the microhardness (H m ) test methods. They produce results that are characterized by high reproducibility and experimentally validated correlation with the mechanical properties of the material. The chief advantage of microhardness tests is the possibility of obtaining quantitative data on the material properties within microvolumes and thin layers, where other methods fail. The methods help to study elastoplastic deformation, brittle fracture, variation and anisotropy of mechanical properties, phase transitions, and contact-induced wear [2,4,8,10,18]. Microhardness test data correlate well with data produced by other physical methods [8].The paper presents microhardness test results on the properties of a surface layer subject to cyclic loading. To simplify the procedure of H m measurement, i.e., to make it unnecessary to remove the sample from the fatigue testing machine, a special test setup was created (see [5] for a detailed description). It excluded the errors due to mounting/removal of the sample, which is especially important in...