Plane stress measurement of orthotropic materials using critically refracted longitudinal waves

Stress measurement in anisotropic materials attracted considerable interest from the field of engineering. This study presents an improved ultrasonic method for measuring plane stresses in orthotropic materials. This study considers the velocity variations of critically refracted longitudinal (LCR) waves induced by orthotropic anisotropy and internal stresses. The stress-acoustic relation in orthotropic materials is used as theoretical basis for the proposed ultrasonic method. To verify the accuracy and usability of these relations, measurements are performed on a 6061 aluminum alloy and a carbon fiber reinforced plastic (CFRP) composite are subjected to under tensile tests. As preconditions for testing, longitudinal wave velocity is measured and LCR wave wedges are designed for these two materials. Stress-acoustic coefficients are determined by a series of calibration tests. Finally, we confirm the accuracy of this method by comparing the acoustoelastic coefficients of the 6061 aluminum alloy with certain reference values. The acoustoelastic coefficients of CFRP composite are compared with the theoretically predicted curve to assess its usability for orthotropic materials.