Environmental considerations for fatigue cracking

An essential aim for the study of fatigue cracking is the development of science-based methods for reliability and service life assessments as a part of the life-cycle design and management of engineered systems. In this context, a critical comparison between a mechanistically based probability and the more traditional, empirically based statistical approaches for reliability analysis and life prediction is presented. The comparison serves to highlight the need for quantitative, mechanistic understanding and modeling and for the adoption of a multidisciplinary approach. Current understanding of the influences of gaseous and aqueous environments on fatigue cracking, particularly with respect to the effects of frequency and temperature, is briefly summarized, and special considerations for fatigue at very high frequencies are discussed. The efficacy of the mechanistically based probability approach is illustrated through a comparison of the estimated and observed distribution of corrosion and corrosion fatigue damage in Boeing 707 aircraft that had been in commercial service for 24 years.