Seongmin Yoon

Adhesive fracture in layered structures is governed by subsurface crack evolution that cannot be accessed using surface-based diagnostics. Methods such as digital image correlation and optical spectroscopy measure surface deformation but implicitly assume a straight and uniform crack front, an assumption that becomes invalid for interfacial fracture with wide crack openings and asymmetric propagation. In this work, terahertz time-domain spectroscopy (THz-TDS) is combined with double-cantilever beam testing to directly map subsurface crack-front geometry in opaque adhesive joints. A strontium titanate-doped epoxy is used to enhance dielectric contrast. Multilayer refractive index extraction, pulse deconvolution, and diffusion-based image enhancement are employed to separate overlapping terahertz echoes and reconstruct two-dimensional delay maps of interfacial separation. The measured crack geometry is coupled with load–displacement data and augmented beam theory to compute spatially averaged stresses and energy release rates. The measurements resolve crack openings down to approximately 100 μm and reveal pronounced width-wise non-uniform crack advance and crack-front curvature during stable growth. These observations demonstrate that surface-based crack-length measurements can either underpredict or overpredict fracture toughness depending on the measurement location. Fracture toughness values derived from width-averaged subsurface crack fronts agree with J-integral estimates obtained from surface digital image correlation. Signal-to-noise limitations near the crack tip define the primary resolution limit. The results establish THz-TDS as a quantitative tool for subsurface fracture mechanics and provide a framework for physically representative toughness measurements in layered and bonded structures.

This study utilizes in situ multipoint time-gated Raman spectroscopy to simultaneously measure molecular structural changes along with the corresponding changes in mechanical properties in shock-compressed sucrose crystals. The measurements successfully captured full-field shock compression wave propagation, leading to visualization of the elastic and plastic phases of the shock compression. The shock wave initially generates an elastic front, increasing pressure and temperature, followed by a larger elastoplastic zone marked by peak shifts and new peaks. The crystal then transitions through a metastable phase before undergoing shock-induced phase transformation, eventually reverting to a stable phase, which differs from its original undeformed state. The Raman measurements focused on CH and CH2 peak shifts under shock loading, enabling the calculation of pressure (∼5 GPa) and temperature (>1000 °C) spatial maps as a function of time during shock wave propagation. For an impact velocity of 1.2 ± 0.1 km/s, the width of the shock pulse was determined to be 125 ± 5.2 μm, and the elastic shock velocity was 6.25 ± 0.52 km/s. The energy density dissipation, derived from shifts in the CH and CH2 stretch modes, was 6.67 ± 2.11 × 107 kJ/m3 and evenly distributed relative to the pressure and temperature fields. The primary mode of failure was found to be micromechanical crushing by the forward shock wave; however, in some cases, the compaction followed by crushing allowed for the return of a rarefaction wave, leading to separation along the spall plane.