Apparatus and Method to Detect Residual Stress Around Cold Expanded Holes Using Longitudinal Critically Refracted Waves

A method for detecting residual stress induced by cold expansion (Cx) (Cx stress) at a hole in a structure, comprising:

• using a first transducer pair to transmit and receive ultrasonic waves into the structure in a pitch-catch configuration along a first path parallel to a tangent of the hole;
• wherein the first path is a first distance from the hole where Cx stress will be present if the material has undergone Cx;
• using a second transducer pair to transmit and receive ultrasonic waves into the structure in a pitch-catch configuration along a second path parallel to the tangent of the hole;
• wherein the second path is a second distance from the hole where Cx stress will have little or no presence if the material has undergone Cx;
• wherein the first transducer pair and the second transducer pair are arranged such that their incident angle into the structure provides longitudinal critically refracted waves (LCR) within the structure;
• generating LCR waves within the structure along the first path and the second path;
• measuring the time-of-flight (ToF) of the LCR waves along the first path and the second path;
• comparing the ToF measurements along the first path and the second path, thereby obtaining a ToF difference;
• and
• using acoustoelasticity calculations to determine if Cx stress is present at the hole.