Thermoelastic Stress Analysis: An NDE Tool for Residual Stress Assessment of Metallic Alloys

The primary objective of this report involves studying and developing various experimental techniques for accurate measurement of the mean stress effect in thermoelastic stress analysis (TSA, also recognized as SPATE: stress pattern analysis by thermal emission). The analysis of cyclic mean stresses at the coupon level directly relates to the measurement of residual stresses in structures. In a previous study by the authors, it was shown that cyclic mean stresses significantly influenced the TSA results for titanium and nickel-based alloys, although, difficulties were encountered concerning the quantification of the mean stress effect because of large test-to-test variations. This study continues the effort of accurate direct measurements of the mean stress effect by implementing various experimental modifications. In addition, a more in-depth analysis is included which involves analyzing the second harmonic of the temperature response. By obtaining the amplitudes of the first and second harmonics, the stress amplitude and the mean stress at a given point on a structure subjected to a cyclic load can be simultaneously obtained. The rather complex analysis of the temperature response involves obtaining the first and second harmonic amplitudes for 16384 infrared detectors (128X128 focal plane array). Upon establishing a protocol for mean stress measurements in the laboratory using the TSA technique, the next step is to utilize the method to assess residual stress states in complex structures during manufacturing and life.