Non-destructive evaluation of longitudinal elastic modulus of freestanding thin material layers using a waveguide-based method

Abstract

The need for a relatively limited wavelength in ultrasonic testing of thin (i.e. sub-wavelength thick) material layers tends to conflict with the determination of material homogenized mechanical properties such as Young's modulus. Aiming at coping the difficulties brought by the characterization of thin material layers using conventional mechanical techniques, the present work introduces an experimental method allowing a non-destructive assessment of Young's modulus of freestanding thin material layers. The abovementioned conflict is addressed by the use of transverse wave ultrasonic transducers. The proposed setup allowed the generation and detection of a relatively low-frequency (here 250 kHz) in-plane longitudinal ultrasonic wave (LUW). Based on the LUW velocity measurement, Young's moduli of a wide panel of materials (various metals, a ceramic, a natural and a synthetic organic materials), whether isotropic or anisotropic, with thickness-to-wavelength ratios ranging from 0.15 to 0.0006, were determined. The proposed method is discussed and validated by comparison with Young's modulus values reported in the literature.