Validated and predictable simulation model for penetration processes in adobe walls

Endballistic research investigates the penetration behavior of projectiles in materials. Over the course of many decades, the occurring physical processes have been investigated by means of scientific methods for a wide range of experimental conditions and parameter ranges and, in most cases, have been transferred into model descriptions. As a result, penetration processes of projectiles are predictable with relatively high accuracy, particularly where conventional protective materials such as armor steels are considered. However, there has recently been a need for research on the ballistic response behavior of building materials. While concrete has been intensively characterized for a long time in the context of civilian applications, such as the protection of buildings or industrial facilities against bomb attacks, the missions of the German Armed Forces (Bundeswehr) have raised new questions regarding the ballistic protection of building elements and building materials. In this context, for example, the evaluation of the protective properties of masonry is of great interest in order to ensure the protection of civilian population and deployed soldiers in the areas of operation. An example of a building material used in the Bundeswehr’s possible fields of operation are adobe bricks (Figure 1). Despite a low density of one gram per cubic centimeter to two grams per cubic centimeter and also a low compressive strength of typically one megapascal to five megapascals, these show a very complex behavior towards impacting projectiles, in particular with regard to failure and fragmentation.

In order to be able to understand and evaluate the response behavior of adobe walls during penetrating impact processes, Fraunhofer EMI is pursuing a scientifically founded approach. It is based on the development and use of material models for the application in numerical simulation calculations. Laboratory tests are used to validate the simulation models so that on the basis of these, predictive simulations are possible. Another important aspect is that the material parameters required for the models used at Fraunhofer EMI can be measured with quasistatic and dynamic test methods. For the material discussed here, it has been shown in particular that it is of central importance to fundamentally record the compacting behavior, in particular the associated pore collapse, using high-dynamic test methods such as planar-plate impact tests.

The starting point for the modeling of adobe material at Fraunhofer EMI was the RHT material model for concrete, which was developed within the framework of a doctoral thesis at the institute about 15 years ago and since then has established itself as a standard for the modeling of building materials at great deformation speeds. For adobe, an RHT model parameter set was first derived from our own material tests and from ones found in literature.

Subsequently, penetration processes of steel bullets were simulated at different impact velocities and compared with available experimental data so that a validated simulation model for adobe could be derived. Figure 2 exemplarily shows the comparison of the experiment and the simulation of the residual velocities of steel bullets with a diameter of 13.5 millimeters after the perforation of adobe targets with thicknesses of 71 millimeters and 142 millimeters. The observed consistency is excellent. On the basis of the validated model, predictable simulations are now possible for the penetration of different projectiles into adobe walls. An example of this is shown in Figure 3: Due to the special nose shape, the penetration of the projectile is not stable, so that it follows a curved trajectory within the wall. It is in particular visible that the projectile emerges laterally in the fourth stone in the simulation, and in the experiment. This very complex penetration behavior observed in experiments is reproduced qualitatively and quantitatively by the simulation over a large speed range. The damage of penetrated stones is also predicted by the model (Figure 4).

Fraunhofer EMI has therefore developed a predictive hydrocode simulation model for adobe walls, which is now available for applications in the framework of research projects and for evaluation questions. Thus, the penetration behavior, which as a whole is sensitive to impact conditions and material properties, can be analyzed under specific conditions.