Mechanical Characterization of Nonlinear Soft Materials Using Surface Waves

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This grant will fund research that enables on-the-fly characterization of nonlinear properties of soft materials, with application to additive manufacturing, tissue engineering, food processing, and biomedical diagnostics, thereby promoting the progress of science and advancing the national prosperity, health, and welfare. Soft materials occur naturally in biological tissue and find increased use as load-bearing elements in engineered systems, including robotic devices and 3d-printed biomaterial scaffolds. Their accurate mechanical characterization could enable in-vivo diagnosis of tumor formation, ensure reliable performance of flexible actuators, and support successful tissue growth for transplantation. Existing techniques for characterization, such as shear-wave elastography and ultrasonics, rely on body waves and monitor linear material response. Consequently, they fall short of capturing amplitude- and rate-dependent effects associated with large deformations and the complex constitution of typical soft materials. In this project, a new characterization technique will be developed that relies on surface waves, which propagate longer distances than body waves and interact constructively with the nonlinear mechanical properties of the underlying material. The research will demonstrate how a surface-wave-based methodology can achieve reliable, quick, non-contact, and non-destructive characterization of soft materials under normal operating conditions. Wave-based material characterization techniques and instrumentation will be presented at a workshop targeting local industry and as part of interactive exhibits at annual outreach programs for K-12 students and the public.This research aims to uncover mechanisms whereby surface waves on soft materials interact with constitutive nonlinearities, trigger internal resonances, and generate higher-order harmonics that propagate long distances and are detectable by appropriate instrumentation. Numerical and experimental studies will be performed to correlate surface-wave characteristics to rate-dependent nonlinear properties and establish how those correlations depend on preloads in the soft materials due to thermal, mechanical, and chemical effects that are residual from fabrication or induced during operation. The direct outcome of the research will be experimentally validated physics-based correlation maps between constitutive nonlinearities and dispersion, attenuation, and acoustoelasticity of surface waves. In addition, the project will advance the current measurement and analysis techniques for mechanical waves via fracture-induced transient excitations and advanced multipoint laser vibrometry.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.