Residual stresses (RS) are crucial to the strength, fatigue life and corrosion resistance of structural components, and are often inherent to fabrication processes like welding, forging and heat treatments. They primarily arise from the contraction of neighbouring material and differences in thermal expansion.
The magnitude of RS at expected failure locations is included in engineering critical assessments (ECAs), thus the more accurate the residual stress measurement, the less conservative the ECA can be. This can lead to increased load limit state and fatigue life, enabling greater confidence and/or more severe loading conditions to be accounted for. In addition, if the fatigue life could be extended, the time between inspection periods could be lengthened, leading to reduced maintenance costs as a result of minimising repairs.
Unless the RS field can be measured reliably, the magnitude of RS to be input to an ECA is assumed.
In order to maintain lower operating costs and prevent catastrophic failures, it is crucial that RS are measured accurately, allowing decisions on component repair or replacement to be made efficiently. Several RS measurement methods exist currently, classified as: destructive – contour mapping, block removal, splitting and layering (BRSL), slitting and boring; semi-destructive – hole-drilling; and non-destructive – neutron, synchrotron and X-ray diffraction. These techniques are well validated with some providing high-resolution RS distributions. However, they are often costly and time consuming to perform with most requiring a laboratory with dedicated facilities. For on-site measurements, hole-drilling can be used, but it provides only a surface measurement and the site often requires repair after the measurement is performed. To date, no on-site, through-thickness RS measurements can be routinely made.