Tech Briefs

This work could result in the fabrication of impact- and blast-resistant structural systems.

A thorough study through a combination of ballistic and impact experiments, microscopic failure characterization, and numerical simulations has been carried out in order to decipher the underlying mechanisms involved in the interaction between a blast and/or a blast-induced high-velocity projectile and advanced ceramic-polymer and metal-polymer composites, resulting in an improved ballistic efficiency and impact- and blast-resistant structural system.

A series of high-velocity impact and ballistic experiments has been performed on several metal-metal, metalpolyurea-metal, and polyurea-ceramic composites. New steel plate designs with different thicknesses were employed to avoid tearing of the sample at its supporting ring. New experiments support the hypothesis that the steel-polyurea sandwich samples show a noticeably better performance against blast loads, as compared with that of bare steel. The existing 3D finite element model has been improved to analyze this phenomenon in depth.

An axisymmetric model has also been generated that allows for very fine-mesh simulations. New penetration experiments have also been performed on some newly fabricated ceramic-polyurea-ceramic sandwich systems. These experimental results suggest that the use of polyurea in ceramic-polyurea composites to improve the ballistic resistance is not as effective as the use of other previously tested materials such as E-glass/epoxy or carbon-fiber/epoxy, whereas the reverse is the case for polyurea-steel layers and sandwich structures.

To study the effect of polymers on the performance of sandwich structures under blast loads, a set of experiments was performed on circular polyurea-steel sandwich samples using a 3" Hopkinson bar setup. Using an experimentally based material model for polyurea, the entire experiment was numerically modeled and analyzed in LS-DYNA, and the results were compared with the experimental results.

Three different configurations for the circular sandwich structures are considered: steel-steel, steelpolyurea-steel, and steel-polyurea-steelpolyurea. The Naval structural steel, DH-36, is used to manufacture the samples. The sandwich structure is placed between a cylindrical layer of soft polyurethane and a hollow steel ring. When the gun is triggered, high-pressure gas accelerates the projectile through a barrel towards the target. The projectile, which is made of aluminum, impacts the target at a high velocity, which is accurately measured by two magnetic sensors placed at the end of the barrel. Polyurethane, which is nearly incompressible, is now confined between the projectile and the sample. This subjects the sample to a pressure pulse. The history of transmitted force from the sample to the output bar is recorded by two strain gages mounted on the Hopkinson bar.

The experimental results reveal that when polyurea is cast between the steel plates, the performance of the sandwich structure is noticeably improved. In all cases with polyurea, the steel plate facing the blast does not experience fracture, whereas with the same amount of input energy, both front and back plates are severely fractured from the impact.

A better understanding of the sandwich-structure experiment has been achieved using computational techniques. Experimental measurements and observations include high-speed photography of the impact and penetration process, and examination of the final state of the deformed sample. However, a quantitative assessment of the entire process is highly desirable. Therefore, a 3D finite element model of the experiment has been developed that is suitable for explicit analysis in LS-DYNA. Proper material models for this model have been used that can capture the pressure, temperature, and rate sensitivities.

Simulation results show a large difference between the plastic deformation of the front plate, which faces the blast load, and the back plate. This explains why the front face survives the blast, while the back face acts like a sacrificial layer and absorbs a large amount of energy to save the front layer.

New samples with different steel plate thicknesses were made and tested. The new samples were thinner and have been tested to undergo the desired deformation when exposed to impact loadings. The new tests suggest that the performance of the sandwich structure against impact loading is consistently better than the performance of the bare steel. The finite element model has been improved to achieve a better match between the numerical and experimental results. Also, an axisymmetric model with a finer mesh has been developed, which shows similar results.

This work was done by Siavouche Nemat-Nasser of the University of California, San Diego, for the Office of Naval Research. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Materials category. ONR-0021

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Dynamic Response and Failure Mechanisms of Layered Ceramic-Elastomer-Polymer/Metal Composites (reference ONR-0021) is currently available for download from the TSP library.

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