Nondestructive Evaluation of Bulk Stresses in Aluminum and Copper

The effect of applied stress on the temperature dependence of the longitudinal ultrasonic velocity has been investigated in commercial aluminum and copper. Velocities of 10 MHz longitudinal waves as a function of temperature were measured on ten specimens of these metals while they were subjected to external compressive stresses. In all measurements, the velocity increased linearly as the temperature was lowered in the temperature range between 280 and 200 K. Furthermore, the slope of this linear relationship was found to decrease linearly as the amount of applied compressive stress was increased within the elastic limit of the specimen under investigation. The maximum decrease in the temperature dependence of aluminum and copper were respectively 23% which occurred at a stress of 96 MPa, and 6% which occurred at 180 MPa. The linear relationship between the temperature dependence of the ultrasonic velocity and the applied stress was then used to determine the change as a function of distance of the tangential compon.ent of the stresses developed when an aluminum rod was shrunk fit into a slightly smaller hole drilled in an aluminum disc. Excellent agreement was obtained between the computed stress distribution, and that measured using the temperature dependence method.