  Engineering fatigue is a very prevalent and dangerous phenomenon that limits
the useful life span of mechanical structures. It can be found in most
engineered structures and machinery that are made from metallic or composite
materials. The life prediction methodology under variable amplitude loading
(VAL), however, has not reached maturity due to the complexity of the loading.
This complexity results in load sequence effects, which significantly alter the
fatigue life. Since most of the realistic loading experienced by engineered
structures is of variable amplitude nature, and the corresponding responses at
the defect site are highly irregular, characterization of the VAL and its
resulting damage is paramount. Fatigue life estimation under VAL conventionally
uses methods based on the Miner's Rule or the cumulative damage rule (CDR) in
conjunction with the rainflow counting method. However, CDR does not account
for the load interaction/sequence effects induced by local residual stress (or
crack opening stress) variations.
