Interesting facts about residual stress measurements in Nickel-base turbine engine components

Turbine engine components are designed to operate in harsh environments that may involve static loading, high and/or low cycle fatigue, as well as significant thermal loading at elevated temperatures. Such components are typically composed of Ni and Ti alloys that were developed over decades to withstand the rigors of in-service usage in an engine. With the desire for improvements in engine efficiency and performance, new processes were introduced to enhance fatigue resistance and usable service life, while reducing overall size and weight. It is well known that the residual stresses imparted during the fabrication and processing of life limiting locations can play an important role in ensuring fatigue resistance of in-service rotating turbine engine components. Methodologies and instrumentation have been developed to accurately characterize residual stresses in a wide variety of such components using x-ray diffraction techniques. In this study several turbine engine components that were fabricated from either polycrystalline, directionally solidified, or single crystal structures of interest were characterized including: discs, spacers from different stages, drums, and turbine blades composed of Ni-base materials.

Residual stress measurements were performed on in-service components after fabrication, before assembly, and after in-service usage for various service intervals at overhaul repair depots. In addition, coupons were manufactured and tested under cyclic plastic loading to failure to study the residual stress evolution experienced during fatigue loading.

In certain instances, it was found that residual stresses were relaxed after in-service usage in very localized areas, characteristic of low cycle fatigue, with concomitant surface micro-cracks observed, also often synonymous with low cycle fatigue failures. It was found that the rate of residual stress relaxation could be approximated by a linear trend with TACS in turbine engine discs, however, the trend was found to be non-linear in fatigue coupons. Overall, the rate of residual stress relaxation was found to be gradual, suggesting that these components could be saved and replaced back into engines when residual stress levels do not exceed a set threshold, thus avoiding failure.