Probing bonded interfaces with finite-element resonant ultrasound spectroscopy

Resonant ultrasound spectroscopy (RUS) has long been applied for non-destructive quantification of elasticity, geometry, and density in material samples. Improvements in mode imaging and finite element analysis now provide validation and predictive models for a growing range of sample types. We build on recent work which applied finite-element RUS to characterize layered samples. Our study takes a closer look at the utility of RUS to evaluate the bonded interface between layers, with extension to thin films. Forward modeling of bond properties provided insights into mode-dependent frequency shifts for various types of homogeneous bond changes such as thickness and stiffness. Experimental work consisted of RUS testing on Al-Ti samples diffusion-bonded at different pressures. Despite identical base materials, samples bonded at 2 MPa and 15 MPa exhibit spectral differences not explained by geometry variance alone, indicating sensitivity to the bond interface. Though highly geometry and material dependent, the sensitivity of RUS to interface properties appears to be roughly an order of magnitude smaller than to the constitutive material properties, emphasizing careful experimentation.