Materials ageing and structural reliability

Byline: D. Gary Harlow, Robert P. Wei Material ageing adversely affects the reliability of engineered structures. It impacts the costs of operation, maintenance, and warranty as well as long-term safety of complex structures, e.g. aircraft and automobiles. The typical empirically based design methodologies, which are often developed from accelerated life testing, are inadequate. These are appropriate for interpolation, but not for extrapolation. Effective extrapolation, beyond the range of conditions employed in obtaining supporting data and assessments of risk, must be based upon mechanistic models that capture the function, dependence on all the key external and internal variables. This undertaking requires multidisciplinary research, pertinent to industrial concerns, that addresses the chemical and micromechanical processes that control damage evolution in materials and quantifies the stochastic aspects of these processes. The purpose of this paper is to demonstrate the use and applicability of the mechanistically based probability methodology and to provide a framework for other applications. The approach is illustrated by the time dependent evolution and distribution of damage in aluminium alloys used in aircraft construction. The accuracy of estimates for the computed distribution of damage is demonstrated by comparisons with tear-down inspection data from a transport aircraft that had been in commercial service for about 24 years.