ABSTRACT
Fatigue is a critical design consideration in many airframe components. As improvements are sought in o‘verall performance, maintenance requirements and total service life, the demand for fatigue resistant materials and structures is increased. In this respect it is important to recognize that fatigue performance is highly sensitive to applied load levels, with the fatigue life of a built-up airframe structure containing stress concentrations (particularly fasteners) being inversely proportional to the load range raised to a power of about seven, i.e. small increases in working load levels place disproportionate demands on fatigue resistance. It is telling that whilst tremendous effort is expended in predicting fatigue performance, and significant progress has been made in the scientific understanding of fatigue processes, aircraft structures are still verified by expensive structural tests at every design level (i.e. ranging from simple single panels and joints, through to entire airframes). This may, at least in part, be related to the complex, multi-mechanistic character of fatigue failure, where numerous competitive and synergistic interactions arise from changes in load parameters such as applied stress state (e.g. proportional and non-proportional multi-axial loads), load history (simple transients, block loads, etc.) and chemical environment. Unlike primary structural properties such as elastic modulus and yield strength, there is no guarantee that any one fatigue performance parameter established in the laboratory will be meaningful or accurate under service conditions. In terms of detailed microstructurebased fatigue research and understanding, most reports within the literature have been biased towards relatively simple load environments, rendering detailed quantification of microstructural effects on fatigue in realistic
difficult [1]. Whilst a full discussion of all the the scope of this chapter, it is clear that quanti-and optimization of realistic commercial micro-of in-service fatigue performance are as yet largely untenable. This chapter considers the underlying physical principles associated with this problem, identifying important aspects for future scientific assessment, along with general technical issues relevant to the future aluminium airframe design.
