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This study investigates the physical processes involved in picosecond pulse (20-28 ps FWHM) laser ablation of Al 6061, 316L stainless steel, and undoped crystalline Si (〈100〉) over a range of laser wavelength (355 nm and 1064 nm) and fluence (0.1-40 J/cm2). Experimental measurements of material ablation rate show enhanced removal at the 355 nm wavelength, primarily due to laser-plasma interaction (LPI) within the ablative plume that approaches an order of magnitude increase over the measured removal at 1064 nm. A transition in the ablation rate at 355 nm is identified around ∼10 J/cm2 above which the removal efficiency increases by a factor of two to three. Multi-physics radiation hydrodynamic simulations, considering LPI effects and utilizing a novel mixed-phase equation of state model, show that the transition in ablation efficiency is due to the onset of melt ejection through cavitation, where laser-driven shock heating sets the depth of melt penetration and the ensuing release wave from the ablation surface drives cavitation through the imposition of tensile strain within the melt. High-speed pump-probe imaging of the ejecta and ejecta collection studies, as well as scanning electron microscopy of the ablation craters, support the proposed cavitation mechanism in the higher fluence range. The ablation process is critically influenced by LPI effects and the thermophysical properties of the material. [ABSTRACT FROM AUTHOR]

This article investigates the use of photon attenuating inclusions (PAI) for enhancing radiographic imaging of cementitious materials in composite structures. By targeting characteristic high attenuation regions in the photon energy spectrum, PAI can improve void detection in the parent material by increasing contrast in radiographic images. Radiographic imaging of cementitious grout specimens with 5% and 15% weight fractions of candidate PAI materials (Fe, BaCO3, BaSO4) was performed to characterize X-ray attenuation. Virtual radiography simulations were then conducted to evaluate the application of PAI to grout void detection in post-tensioned concrete construction. The simulations demonstrate that grout void detection is most effective when the high energy region of the emission spectrum is near the K-edge of the PAI material. [ABSTRACT FROM AUTHOR]

Spray-applied fire-resistive material (SFRM) is prone to debonding, cracking, and spalling in steel moment-frame beam hinge regions during inelastic seismic response. This article presents an analytical case study designed to evaluate the effect of experimentally observed SFRM spall patterns on the thermomechanical response of a steel moment-frame beam-column assembly during post-earthquake compartment fire exposure. Earthquake-induced SFRM spalling is shown to significantly increase thermal degradation in the beam hinge region during fire exposure, leading to considerable temperature-induced softening of moment-rotation response for the beam-column assembly. This reduction in rotational stiffness increases the flexibility of the structural system for sideway motion, and exacerbates drift demands under the action of residual (post-earthquake) destabilizing forces. [ABSTRACT FROM PUBLISHER]

The shock imparted by a laser beam striking a metal surface can be increased by the presence of an optically transparent tamper plate bonded to the surface. We explore the shock produced in an aluminum slab, for a selection of tamper materials and drive conditions. The experiments are conducted with a single-pulse laser of maximum fluence up to 100 J/cm 2 . The pressure and impulse are measured by photon doppler velocimetry, while plasma imaging is used to provide evidence of nonlinear tamper absorption. We demonstrate a pressure enhancement of 50x using simple commercially available optics. We compare results from hard dielectric glasses such as fused silica to soft plastics such as teflon tape. We discuss the mechanism of pressure saturation observed at high pulse fluence, along with some implications regarding applications. Below saturation, overall dependencies on pulse intensity and material parameters such as mechanical impedances are shown to correlate with a model by Fabbro et al.