Non destructive evaluation of bi-material structures: a case study

Abstract

Due to their specific elastic properties (high stiffness to mass ratio), regular microstructure materials (architectural materials), are widely used in industry. This work deals with nondestructive evaluation of a 3D bi-material structure made of an architectural ABS polymer embedded in an Epoxy resin. The difficulty of applying ultrasonic techniques to bi-material structures lies in the fact that different phenomena coexist, such as diffraction effects caused by the periodicity and the nonlinear effects caused by the adhesion properties between matrix and fibers. A combination of techniques have, therefore, been applied. For this purpose, a series of measurements in reflection mode were first employed to measure the characteristic Bragg spectrum of a healthy sample and after compressive loadings. The results show that the damage disturbs the periodicity of the structure and consequently alters the Bragg spectrum. Then the nonlinear effects, possibly due to adhesion, are investigated using cross-modulation spectroscopy technique. The transducers used were chosen according to the thickness and the periodicity scale of the sample. Based on the preliminary results, higher harmonics and sidebands increase in amplitude with increasing damage has been realized. Finally, a total difference frequency distortion parameter TDFD was introduced as a damage indicator for quantifying the damage.