Analysis and Design for Impact and Fatigue Loadings

Abstract

Statically or gradually applied loadings are considered exclusively in all other chapters of this text. In this chapter, two loadings of vital interest in dynamic analysis and design are considered: impact and fatigue loadings. For example, highway and railway bridges are subjected to impact loading as vehicles cross them and allowances for such loadings are made in their design. High-speed machines contain many moving parts that are subjected to loading which varies cyclically with time. Repeated cyclical loading can lead materials to fail at stress levels substantially lower than stresses associated with static loading, provided that the number of cycles is relatively large and the range of stresses from minimum to maximum is of sizable magnitude. Such failures begin at “ stress raisers ” such as internal or surface flaws in the material or abrupt geometric changes. A small (often microscopic) crack forms and propagates until a section of a given component is too weak to resist further cycles of loading. The resulting failure is termed a fatigue failure and the associated cyclically repeated loadings are termed fatigue loadings.