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Abstract.Recent experiments involving near-contact underwater explosions on air-backed plates suggest the following failure mode categories: (1) holing and petaling, (2) complete or partial edge tearing due to shock only, (3) complete or partial edge tearing due to shock and bubble collapse, and (4) large deformation without rupture. Finite-element analysis was used to further investigate the detailed response and failure of the plates, and determine the limit between center plate holing and edge failure. When compared with experimental results, finite-elements showed good agreement with the failure modes of the plates and reasonable agreement with the experimental deformations. It was found that the linear interaction criteria (LIC) failure between plastic strain and through-thickness shear stress produced results closer to experiments than the quadratic interaction criteria (QIC). For the 18 gauge specimens it was found that the through-thickness shear dominated the failure initiation for very close proximity charges, with the direct strain becoming dominant as the standoff was increased. For the thinner 20 and 22 gauge plates the direct strain was always found to be the dominant factor in the failure criteria.

The critical conditions for initiation of reaction by shock loading in various compositions that produce little or no gas upon reaction were investigated. Shock recovery experiments using Mn+S were first carried out in two different apparatus geometries and for two different initial sample densities. In one geometry, the sample was subjected to a planar shock followed by interactions with the confining walls. In the other geometry, a curved shock free of wall interactions was delivered to the sample. The low‐density (55 % TMD) Mn+S was found to be significantly more sensitive to the curved shock than to the planar shock with wall interactions. For high‐density (90 % TMD) Mn+S samples however, shock sensitivity was the same in both apparatuses. Next, the reaction onset temperature and the critical initiating shock pressure were determined for a number of powder mixtures using DTA and shock recovery (in the geometry producing planar shocks with interactions with the confinement walls), respectively. For the majority of the mixtures tested, the minimum shock energy required to cause the entire sample mixture to react was found to be much less than the enthalpy of the sample at its reaction onset temperature, with no significant correlation between these two parameters. The process of arrested ball‐milling, which results in a reduction of the reaction onset temperature of a mixture, may lead to an increase in shock sensitivity. Additionally, thermal sensitivity in the particular mixtures considered was not increased when they were first shock‐compacted by sub‐critical shocks.

The damage sustained by rigidly-clamped square steel plates when subjected to close-proximity underwater explosions has been investigated. The test specimens consisted of plates 0.76 mm and 1.21 mm thick made of either ASTM A1008 mild steel or 350 WT structural-grade steel with a low-temperature notch-toughness requirement. The explosively-loaded area of the plates was square, with dimensions of 254 mm X 254 mm. High-explosive charges from 1.1 g to 50 g were used at different standoff distances to obtain different shock strengths and bubble collapse intensities. Although the main impulsive load on the plate was due to the shock impact, because the standoff distances were less than twice the maximum free-field bubble radius, a strong interaction between the detonation product bubble and the target plate caused a rapid water jet to impinge on the plate, resulting in additional loading and damage. As a result, four main regimes of loading and damage were identified: a) holing/petaling due to shock loading, b) edge tearing due to shock loading only, c) edge tearing due to the cumulative loading from shock and bubble collapse, and d) large deformation due to shock and bubble collapse loading. The damage mechanisms and dynamic response of the plates were measured using dynamic displacement sensors, pressure gauges, and high-speed video. A fracture analysis was performed on the damaged plates to analyze the mechanisms of failure. Finally, finite-element analysis using a failure criterion based on normalized shear stress and effective strain has been used to examine the failure limits.

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