Ultrasonic attenuation-based assessment of interlayer slip in pre-tightened cushion structures.

Interlayer slip in pre-tightened cushion structures changes the local interfacial contact state within enclosed metal-polymer interfaces. Existing ultrasonic studies mainly address discrete defects or spatially homogeneous imperfect interfaces, leaving sub-aperture mixed contact states insufficiently quantified. This study develops an ultrasonic attenuation-based method for assessing interfacial contact variations associated with interlayer slip. The mixed interface comprising polymer-contact regions and air gap within the transducer footprint is represented by a normalized contact area. An area-averaged reflection model is coupled with a multiple-echo attenuation model to relate this parameter to the effective attenuation slope. Experiments on a planar aluminum/silicone foam sandwich structure show prediction errors below 15 % and an a90/95 value of 0.45. Validation on a spherical pre-tightened cushion structure further demonstrates the applicability of the framework to curved geometries. These results provide a quantitative basis for inferring interlayer slip from ultrasonic boundary responses.