Near-Field Microwave Sensor Development for Dynamic Noncontact Thermoelastic Deformation Coupling Coefficient Measurements in Metal Plates

We hereby proposed a cavity-type near-field microwave probe (NFP) system as an alternative for noncontact and nondestructive evaluation of thermoelastic deformation in metal plates. A quasi-static near-field model combined with virtual boundary assumption and volumetric perturbation theory was developed in conjunction with an in-house fabricated NFP to quantitatively explain the variation of field intensity and frequency deviation as a result of deformation. The dynamic laser-induced thermoelastic field was analytically modeled using the Green function method and the quasi-equilibrium membrane approximation. Finite-element method was used as validation. Both theoretical and experimental results indicate a nonlinear microwave signal dependence on displacement. Strain in metal plates was found to be proportional to surface temperature through the thermoelastic coupling coefficient. Finally, the experimental data were explained analytically using nonlinear fitting which then was used to extract the coefficient. The results confirmed the capability of the NFP sensor in detecting dynamic deformations. The proposed NFP system is potential to be a comprehensive instrumentation capable of thermoelastic and dielectric property measurement in a single configuration.