Differential ultrasonic stress–strain measurements

Abstract

Fatigue tests are often encumbered by lack of the specimen’s exact length due to mechanical grip effects and by environmental noise factors. Such issues may be mitigated when differential measurements are undertaken between consecutive data states. Our principle-monitoring tool is an ultrasonic pulse-echo process where the primary and secondary echoes along the principle specimen’s axis provide both length and gage length, respectively [D. Hazony, Circuit Systems Signal Process. 14(4), 525–538 (1995)]. The process is also useful for crack detection [I. Mostafa et al., Int. J. Fracture 85, 99–109 (1997)]. Compared with absolute measurements, differential stress-strain monitoring is shown to be highly reproducible and more sensitive providing changes in Young’s modulus and Poisson’s ratio as well as a focus for the detection of an emerging crack. [Work supported by NASA.]