The formation/nucleation of fatigue cracks in aircraft structural materials

Abstract

The issue of crack formation/nucleation and propagation related to model development for accurate fatigue life estimation/prediction of aircraft materials has been and is currently a vexing challenge. In the 1800's Sorby used the optical microscope to study fatigue deformation of materials. The early investigators of fatigue wanted to learn the mechanism(s) of fatigue crack formation/nucleation and propagation. Many investigators oriented toward developing this understanding aided progress. Many of the studies that were made were "static". That is, materials were exposed to cyclic loading and then viewed in a light microscope. The paper will illustrate the importance of understanding the formation/nucleation process and its impact on life prediction issues. A few cases will be mentioned where the formation/nucleation of cracks and its relationship to intrinsic and extrinsic factors in aircraft will be presented. The development of the scanning electron microscope aided the understanding fatigue of materials. Starting in 1970 numerous investigators developed fatigue machines that were either placed in the chamber of an SEM or were attached to the SEM and conducted "dynamic" studies. Many contributions have been made and some will be reviewed briefly in this paper. The past 45 years has seen the development of numerous in-situ systems for studying the process of fatigue crack formation/nucleation and the study of microstructural effects on the early stages of crack propagation of aircraft and other structural materials. This has led to increased understanding of fatigue crack formation/nucleation and propagation of cracks that is based on direct observation rather than speculation. Recently a new in situ SEM fatigue system has been developed at the University of Utah and work also has been done which allows accurate determination of local grain orientation effects and the role of relative orientation on crack nucleation and early crack propagation. The background of many of the above studies will be presented in the paper. Subsequently, some of the major contributions these workers have made to understand fatigue deformation in aircraft structural materials will be presented. The paper reviews progress of these studies and insights into fatigue with emphasis on aircraft structural materials. The paper concludes with a discussion of some of the opportunities available to develop a greater understanding of the fatigue crack formation/nucleation process and also to develop a greater understanding of microstructurally short crack propagation.