Dynamic Shear and Void Collapse in PolymethylmethacrylatedissertationsPilvelait, Thomas JamescreatorGuduru, PradeepAdvisorHenann, DavidReaderZecevic, MilovanReaderZuanetti, BryanReaderBrown University. Engineering: Mechanics of Solidssponsor2026xxii, 203 p.born digitalThesis (Ph. D.)--Brown University, 2026thesesAbstract of "Dynamic Shear and Void Collapse in Polymethylmethacrylate'', by Tom Pilvelait, Ph.D., Brown University, May 2026 Polymer-based materials form a fast-growing area of materials in engineering applications, which increasingly involve extreme environments, including in aerospace, blast protection, and energetic materials testing. Under these dynamic conditions, polymers demonstrate history-dependent thermomechanical responses that are highly sensitive to strain level, strain rate, temperature, and pressure. To disentangle the complex mechanisms leading to severe deformation and failure, careful experimental and computational treatments that simplify the problem geometry and boundary conditions are required. To this end, a new plate impact facility has been constructed at Brown University with custom-made Photon Doppler Velocimetry diagnostics. Nanosecond resolution measurements of surface velocities are used to extract stress histories of samples loaded simultaneously in compression and shear. Pressure-shear plate impact (PSPI) experiments were conducted on the thermoplastic polymethylmethacrylate (PMMA) at normal stresses between 2.5 and 10 GPa and at strain-rates of 10^5-10^6/s. The dynamic shear strength of PMMA is found to match existing lateral stress gauge measurements, which together describe an initially strong pressure dependence followed by a loss of shear strength at higher pressures.f To further probe the dynamic behavior of PMMA, the interaction between shock waves and cylindrical voids in PMMA was observed with high-speed x-ray imaging. Shock waves were generated via flyer plate impact, and shock pressures ranging from 0.35 to 4 GPa were imposed. Digital Image Correlation (DIC) was used to extract the transient displacement and strain fields under plane strain conditions, allowing for a quantitative 2D analysis of the deformation processes. A thermodynamically consistent constitutive model was introduced, which captures viscoelastic effects, strain softening, strain hardening, and damage based on statistical crack mechanics. The model gives rise to key insights into the nature of deformation and failure under such extreme loading conditions.Solid MechanicsMechanicsExperimental mechanicsEnglishRPB20260516