Fatigue Design for Powder Metallurgy

Abstract

Powder metallurgy, among several competing mass production methods, plays an important role, not only as a means of saving material and energy, but also as a technique, delivering materials with good fatigue properties which are able to substitute for conventional materials. However, it is necessary to understand the interaction between strength behaviour and design determining factors such as service loading, component geometry, and manufacturing. After presenting definitions of fatigue strength, which is divided into the areas of low cycle (endurance limit N<5 × 10⁴), finite (5 × 10⁴<N<2 × 10⁶), and high cycle (N>2 × 10⁶) fatigue under constant amplitude loading and the area of variable amplitude fatigue with 10⁴<N< 10⁹, and after the introduction of parameters which describe and determine S–N or fatigue life curves, relevant design data such as S–N curves, the effect of notches and mean stresses, and fracture mechanical properties are considered. The influence of manufacturing parameters (alloy composition, density, sintering temperature, post-sintering, mechanical and thermal surface treatments, heat treatment) on the fatigue properties of sintered steels is discussed. The need to determine material properties under different loading modes, i.e. axial, bending, different mean stresses, stress concentrations, and gradients, is outlined against the background of various service conditions and component geometries. Fatigue design starts by determining the local stresses and their distributions due to the service loading in the most critical areas of the part. For fatigue life assessment, the local stresses must then be compared with material properties to select design, material, and technology and to decide whether production can be started or further optimisation is necessary. Within the design procedure, the importance of the scatter of material properties, manufacturing, and service loading, which determine the safety margin, is also discussed from the standpoint of component reliability and economy. The design procedure for PM components is illustrated by the example of an angle lever. PM/0537