Aircraft and other structures subjected to cyclic loading are prone to developing fatigue cracks, particularly at holes, that if not detected could lead to catastrophic failure.  Holes are routinely cold expanded to induce a beneficial residual stress zone around the hole, shielding it from the effect of cyclic loading.  Life enhancement and/or damage tolerance benefit from the process is not taken advantage of in determination of life improvement or inspection intervals because there has been no economical method or system to verify the process was correctly applied or that the residual stress state has not changed over time. 

A practical method of measuring and recording the residual stress state of materials, including cold expanded holes, has recently been developed by Direct Measurements Inc. (DMI).   Using innovative optically read “gauges”, similar to a bar code strategically placed around holes or at critical regions, gauge reference lengths are measured before and after cold expansion, providing a relative measure of residual stress. This reading can then be used to determine any change in the residual stress state as a result of a crack growth after a period of cyclic loading.  Furthermore, monitoring of the gauge readings could be used to establish a fatigue index of the aircraft which would then be used for future life extension determination as a measure of the structural health of the specific aircraft.  It would be a critical tool for ongoing life management based on individual tail number tracking.