4 SUMMARY AND CONCLUSIONS A numerical investigation of the ballistic performance of monolithic and multi-layered targets made with either Weldox 700E steel or Al 7075-T651 or a combination of these materials against 7.62-mm APM2 projectiles was conducted, for the velocity range of 775-950 m/s and various target thicknesses. A corresponding experimental study would be prohibitively expensive and difficult. Numerical simulations demonstrated the capability of the model to predict the ballistic behaviour of Weldox 700E and Al 7075-T651, and reproduced many of the physical characteristics of the penetration process observed experimentally. The numerical model developed in this research could be used for the design of experimental testing by reducing the number of necessary tests and minimising necessary resources; however, it should be considered that the results obtained in this study are based on numerical simulations, and experimental validation should be performed. For Weldox 700E, it was observed that monolithic plates perform better than triple-layered plates, which is more noticeable for an impact velocity of 800 m/s. The difference in performance between monolithic and double-layered plates was not significant. For Al 7075-T651, the difference in performance between monolithic and multi-layered plates is not significant for thicknesses less than 20 mm, while for thickness greater than 30 mm there is a large difference in performance. Further investigation (both experimental and numerical) should be performed to assess the validity of the numerical model for Al 7075-T651 at thicknesses greater than 30 mm. Numerical and experimental results show that Al 7075-T651 has a better ballistic performance than Weldox 700E when compared for similar areal density; however, Al 7075-T651 plates have to be at least twice thicker than Weldox 700E plates to exhibit this superior performance, which could be limiting for design purposes. Multi-layered configurations made with a combination of steel and aluminium and a maximum thickness of 20 mm were also investigated. It was found that a double-layered mixed configuration with a front Al 7075-T651 plate and a Weldox 700E steel back plate (DM20AW) performs better than any other configuration with similar areal density. This finding shows that an armour shield made of two different materials may potentially perform better than the equivalent steel plate. However, the results in this study are limited and further research has to be carried out to fully understand this type of target configuration since there are few studies addressing this research topic [3]. It is concluded that multi-layered mixed plates made with a combination of high strength aluminium alloys and steels are an interesting option for protective structures because they may potentially lead to weight-savings and improvement of the ballistic performance of the structure. 17However, further numerical and experimental research has to be carried out to support the findings in this study.
