Physical and Statistical Aspects of Fatigue

Abstract

This chapter discusses the physical and statistical aspects of fatigue. Structural fatigue has established itself as a major factor in modern aircraft design. The increase of design stresses resulting from the use of metal of increasing static strength but decreasing relative fatigue resistance has reduced the fatigue life of aircraft structures by approximately four orders of magnitude. In conventional fatigue tests, at constant stress amplitude, the range of variation of number of cycles to failure is at least one, more frequently two, occasionally even three orders of magnitude. The fatigue crack progresses quickly under a high intensity of the cyclic force, but take years to develop under low force amplitude. An inverse relation exists between the force or deformation amplitude and their specific intensities expressed in terms of nominal stress or strain and the “fatigue life”—the number of repetitions of the stress or strain cycle producing fractures at the applied amplitude. The fatigue life of a material at a given stress amplitude and mean stress is controlled by a number of variables defining the structure of the material, the geometry and size of the specimen, and the properties of its surface and of its environment.