Noncontact Measurement of Elastic Constants of Anisotropic CFRP Laminate Using Orthogonal Cross-Scan Point Source-Point Receiver Laser Ultrasound

Elastic constants are key parameters to quantitatively evaluate material strength of anisotropic carbon fiber-reinforced plastic (CFRP) that is increasingly utilized in commercial aircraft. One most widely used approach to nondestructively determine elastic constants is to measure quasi-longitudinal (QL) and quasi-shear wave group or phase velocities in diverse directions. However, such ultrasonic methods are limited in operating flexibility since they are usually performed by rotating particularly prepared samples under water-immersed condition and require prior knowledge of fiber direction and material thickness to inversely calculate anisotropic wave velocity distribution. In this work, an orthogonal cross-scan point source-point receiver (PS-PR) laser ultrasound (LU) method is proposed, where through-transmission QL and quasi-shear waves are generated by one point laser source scanning in two orthogonal directions and captured by one point laser receiver fixed on the opposite side surface. An inversion approach is developed to infer actual material parameters that yield the best agreement between experimentally measured LU wavefront and ultrasonic travel times theoretically predicted using iteratively updated material parameters. This LU method is numerically and experimentally validated on a 4.45-mm-thick unidirectional CFRP laminate. The results show that elastic constants, fiber direction, and material thickness are always simultaneously determined with relative errors lower than 4.76%, 0.29%, and 0.55%, respectively, at arbitrary scan direction, demonstrating high inversion precision and robustness. The LU method is adaptable to various practical scenarios owing to its noncontact and nondestructive nature and allows “no-reference measurement” where fiber orientation and material thickness involved in the inversion are not known a prior.