Light Protective Armors Thanks to Newly Developed Elastomer.
ELASTOMER, LIGHTWEIGHT CONSTRUCTION, ARMORING
Multi-layered armoring constitutes a cost-effective and weight-efficient alternative to traditional steel armoring, since synergy effects can be created by combining the most diverse materials. Due to their viscoelastic properties and ability to convert kinetic energy to heat, elastomers are promising candidates for use as an interlayer in armoring to protect against small-caliber projectiles in civil applications.
Lighter and safe
Disruptor-absorber armoring systems consist of a hard front plate (high-strength armor steel or ceramic), which is used to destroy incident projectiles, thereby reducing their kinetic energy and distributing it over a larger surface. A ductile rear panel (ductile steel or aluminum) has an absorbent effect and converts the kinetic energy of the fragments into heat and deformation energy. By inserting an elastomer interlayer, its damping effect is harnessed, which further reduces the kinetic energy. The effectiveness of such multi-layered armoring can therefore be increased to such an extent that this results in a significant weight reduction compared to standard steel armoring.
The project aims to clarify the mechanism of this type of armoring in conjunction with the material properties of the elastomer. On this basis, an elastomer formulation with improved properties is being developed in order to display an optimal protective effect. A practical validation of the improved material is also being carried out using ballistic experiments.
The viscoelastic properties of an elastomer and therefore its damping depend on the deformation rate of an applied load. In the area of glass transition, the damping gets maximized, enabling the development of an optimized elastomer for a certain load. The glass transition may be triggered in two ways. If the temperature is so low that the thermal energy is not sufficient to overcome energetic barriers from certain chain redistributions, the elastomer behaves in a brittle and glass-like manner. This is also the case if the deformation rate is so high that the polymer chains cannot follow quickly enough. Therefore, the location of the glass transition can be equivalently described through a temperature or frequency (time-temperature superposition).
A model to extrapolate the glass transition was created using material parameters attained by means of Dynamic Mechanical Analysis (DMA). By comparing these extrapolated values with deformation rates which occur in the targeted protection class in accordance with VPAM standard, it was possible to carry out a material verification. It was found that a self-developed butyl rubber compound meets the requirements and is therefore a suitable material for the application.
Figure 1: Required glass temperature in accordance with the strain rates induced on impact compared to the DMA curve of the formulation tested. (© Fraunhofer LBF)
An exact knowledge of the material and the capacity to produce elastomers with defined properties are paramount in order to test the influence of various material properties on the ballistic effectiveness of elastomers. Design of experiments was used to investigate the range of material properties of the selected elastomer, which can be achieved by varying the formulation. This in turn provided information on the influence and interaction of the individual formulation parameters.
Based on the results attained in the project, the composition of the formulation was optimized for an improved ballistic protective effect of the elastomer. In addition, the foundation was set out in order to identify suitable materials for damping applications and to optimize these individually for specific applications.
Figure 2: Influence of carbon black and oil content of the formulation on glass temperature, tan δ and hardness. (© Fraunhofer LBF)
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