Impact/Blast-resistance of Layered Composites



In this research, we investigate the behavior of layered steel-polymer and ceramic-polymer composites when subjected to impulsive and ballistic loadings. Materials used for making our composites include polyurea (polymer), DH36 naval structural steel and armor grade Al2O3 (ceramic). For the numerical simulations, we have used physics-based and experimentally-supported temperature- and rate-sensitive constitutive models for steel and polyurea; including, in the latter case, the pressure effects.



Steel-polyurea sandwich composites subjected to impact


A Split Hopkinson Pressure Bar (SHPB) setup is used to carry out experiments on steel-polyurea composites (figure 1). Samples are made by casting polyurea between two circular steel plates with various thicknesses and 3-inch diameter. The sandwich steel-polyurea-steel composite is placed between a cylindrical piece of polyurethane and a hollow steel ring (figure 2).


Hopkinson bar experimental setup

Figure 1. Hopkinson bar experimental setup


The gas gun is pressurized by compressed nitrogen tank. When the gun is triggered, the projectile is accelerated through the barrel and impacts the target with a high kinetic energy. Its velocity is accurately measured by two magnetic sensors at the end of the barrel. Polyurethane, which is nearly incompressible, is now confined between the projectile and the sample. Therefore it transforms the high velocity impact of the projectile to a pressure pulse on the sample. The stress history transmitted from the sample and hollow ring is measured by two strain gages mounted on the incident bar of the Hopkinson bar. This is the only quantitative measurement from the experiments which can be compared to numerical results (explained later) for accuracy verification. This measurement is also a qualitative indication of the average impact load that the sample experiences. The output bar is stopped by a clamp brake in a controlled and repetitive manner.


Exploded view of experimental setup

Figure 2. Exploded view of experimental setup


Our experiments suggest that the steel-polyurea-steel composites show a superior impact-resistance compared to steel-steel samples without polyurea cast in between. Also, unlike the bare steel, steel-polyurea bi-layers and steel-steel composites for which the fracture occurs at the center, steel-polyurea-steel composites almost never completely fail at the center. For a few samples the load-receiving steel plate remains unfractured while the steel plate on the opposite side experiences severe fracture. To further investigate these results a full-scale finite element model has been developed suitable for explicit simulations in LS-DYNA. The main part of FEM mesh is shown in figure 3.


Finite element mesh

Figure 3. Finite Element Mesh; most of output bar is not shown here. Notice that a finer mesh would give better results since the elements with larger size filter some frequency content of the pressure pulse. However, the output bar is very long and for the computational cost to be feasible, there is limitation on element size.


Pressure pulse at strain gages

Figure 4. The pressure pulse measured at strain gages


We compare the pressure pulse which is experimentally measured by strain gages to our numerical calculation of that pulse (Figure 4). Notice that some of high frequency content of this pulse is filtered by finite element calculation due to element size limitations in our model. Our numerical calculations reveal that the volume average of effective plastic strain in a small circular region at the center of the load-receiving steel plate (front plate) is much lower than that of the steel plate on the opposite side (back plate) which explains why the load-receiving side remains unfractured (Figure 5).


Volume average of effective plastic strain

Figure 5. Volume average of effective plastic strain at a circle of 10mm diameter at the center of steel plates



Performance of steel-polyurea bi-layers subjected to impact


In this research, we carried out numerical simulation of the dynamic response and deformation of 1m diameter circular DH-36 steel plates and DH-36 steel-polyurea bi-layers, subjected to impact loading. Different thicknesses of polyurea are considered and the effect of polyurea thickness on the performance of steel plates under impulsive loading is investigated. For each polyurea thickness, we have simulated three cases: 1) polyurea cast on front face (loading face); 2) polyurea cast on back face; and 3) no polyurea but an increase in steel-plate thickness such that the areal density remains the same in all three cases. Two types of loading are applied to the polyurea-steel system: (1) Direct application of pressure on the bi-layer system, (2) Application of pressure through a separate medium (polyurethane or water); whereby the differences are investigated. For the numerical simulations, we have used physics-based and experimentally-supported temperature- and rate-sensitive constitutive models for steel and polyurea; including, in the latter case, the pressure effects.


Results from the simulations reveal that in all cases, polyurea cast on the back face demonstrates superior performance relative to the other two cases. The differences become more pronounced as polyurea thickness (and the corresponding steel-plate thickness) becomes greater. Also the differences become less pronounced when direct pressure is applied.


Axi-symmetric view of the 3D finite element model

Figure 6. Axi-symmetric view of the 3D finite element model


Time-history of average effective plastic strain

Figure 7. Time-history of average effective plastic strain at a circle of diameter 10cm at the center of the steel plate for effective polyurea thickness of 3cm. The performance difference between cases becomes more pronounced when polyurea thickness is increased.



Ballistic performance of ceramic-polyurea layered composites


In this study, we investigate the effect of polyurea on ballistic efficiency of ceramic tiles. To this end, we have performed a set of penetration tests on polyurea-ceramic composites. In our experiments, a high velocity projectile is propelled to impact and perforate the ceramic-polyurea composite. The schematic view of experimental setup is illustrated in figure 8. Projectile is placed in an aluminum sabot (carrier) which is accelerated to ~900 m/s before penetrating the ceramic-polyurea composite. The velocity and mass of the projectile are measured before and after the penetration. The change in the kinetic energy of the projectile is evaluated and compared for different polyurea-ceramic configurations (e.g., polyurea on front face, polyurea on back face, polyurea between two ceramic tiles, etc.). The experimental results suggest that polyurea is not as effective as other restraining materials such as E-glass/epoxy and carbon-fiber/epoxy.


Schematic view of ballistic experimental setup

Figure 8. The schematic view of the experimental setup for ballistic tests


Ballistic tests experimental setup

Figure 9. Ballistic tests experimental setup



Ballistic performance of steel-polyurea layered composites


The same experimental procedure as above is employed to investigate the ballistic performance of steel-polyurea layered composites. Figure 10 shows different samples made for these experiments.


Steel-polyurea layered composites

Figure 10. Steel-polyurea layered composites made for ballistic experiments



Acknowledgements: This work is supported through the Office of Naval Research (ONR) grant N00014-06-1-0340 to the University of California, San Diego


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