Novel Method of Reference-Free Residual Stress Measurement Around Cold Expanded Holes Utilizing Time-of-Flight and Amplitude Parameters of Ultrasonic Waves

Cold expansion (Cx) has been widely used to extend fatigue life of structural components in aerospace structures containing holes by introducing compressive hoop residual stresses that hinder crack growth at the hole edge. Reliable detection of the residual stresses introduced by Cx plays a key role in ensuring successful installation, allowing analyses to take full advantage of the process to increase inspection intervals. Previously, we had shown that a longitudinal critically refracted wave (LCR) can be used to detect presence of Cx near holes without requiring calibration with external reference material. Here we present a method expanding these capabilities by utilizing a single transducer pair to measure stress profile around the Cx holes by registering both time-of-flight (ToF) and amplitude parameters of the acoustic wave propagating through the stress region. The efficacy of this method was examined using data from 0.5 and 0.25-inch diameter holes with various stress states in test coupon made of 2024-T351 aluminum alloy.