## 5 Conclusions  
This study presents the first attempt to characterise the dynamic response of dyneema needle-punched nonwoven fabrics specifically designed for ballistic protection. Nonwoven fabrics present a large energy absorption capacity due to their ability to sustain load at very large strains and the frictional micromechanisms triggered during in-plane deformation, and have the capacity to improve the ballistic performance of conventional dry woven barriers acting as cushion layers. The experimental dynamic characterisation of the in-plane response of a needle-punched nonwoven fabric was conducted on a novel Split-Hopkinson Tensile Bar device specifically developed for this study. The apparatus included a high-sensitivity output bar to register the response of the soft fabrics and had a pulse duration of 1 ms to impose large deformations. The gauge length of the specimen was also long enough (35 mm) to ensure at least one representative volume element was tested. This experimental campaign offered detailed understanding on the wave propagation phenomenon in nonwoven fabrics by a combination of high-speed photography and Digital Image Correlation (DIC). The device was able to provide quantitative measurements of the wave speed, the evolution of strain fields and the wave interaction. It also presented several limitations summarised as; (i) low time resolution of the high-speed camera and (ii) a lack of force equilibrium in the bars. These limitations have been considered during the data analysis. The deformation for dynamic loading was accommodated by the same micromechanisms observed during quasi-static in-plane tensile testing and ballistic impact, which comprised fibre straightening, rotation and sliding with the loading direction. The initial wave propagation velocity of the as-received nonwoven fabric was characterised as \(c=827.5\) m/s, exceptionally slow compared to those of bulk polyethylene or ultrahigh molecular weight polyethylene fibres. The main differences between quasi-static and dynamic loading regimes was given by the heterogeneous strain gradient developed at high-strain rates, in contrast with the homogeneous deformation field observed before onset of damage in quasi-static experiments. As a result of the heterogeneous strain field, the strain rate distribution was also non constant along the specimen, with an average value of \(\dot{\varepsilon}\approx 400\) s\({}^{-1}\), four orders of magnitude higher than previous quasi-static tests. The heterogeneous strain field was a consequence of the non-linear pseudo-plastic response of the fabric and the internal energy dissipation mechanisms related to the frictional deformation processes such as fibre sliding. The load transference process was dictated by the wave propagation phenomenon between individual entangled fibres, and it depended on the current fibre orientation distribution function. Upon stretching, the fibres progressively aligned with the loading direction, increasing the tangent stiffness of the material, and therefore, the wave propagation velocity. As a result, during dynamic testing, the differences in fibre orientation distribution function and stiffness arising from the interference of reflected waves generated internal impedance mismatch fronts, preventing the propagation of larger strain waves towards the output interface. In consequence, after a certain period of time, the deformation nearby the output interface froze; meanwhile, large strain gradients and fibre pull-out developed nearby the input interface. Similar gradients have been previously reported for ballistic tests, showing large deformations and localised damage at the impact point. The output forces were analysed to determine the influence of high strain rates in the mechanical response of the nonwoven fabric. Although the strain gradients hindered the development of a theoretical equilibrium stress-state in the bars, the proposed analysis established reasonable bounds for potential dynamic stress-strain curves.