Ultra-high molecular weight polyethylene (UHMWPE) fibre composites are considered to be state-of-the-art materials for penetration and ballistic impact protection applications. The composites are made of strong UHMWPE fibres with a soft compliant matrix. The extreme anisotropy caused by the mismatch between the stiffness and strength of the fibres and the matrix resulted in unique deformation and failure mechanisms which cannot be found in conventional engineering materials. Therefore, the thesis contributes towards understanding the governing mechanisms of UHMWPE composites that resulted in their high penetration and impact resistance, as well as characterizing their mechanical response under dynamic loading.

In the first part of the thesis, we focus on the quasi-static penetration response of UHMWPE composites by sharp-tipped punches. It is shown that the punch penetrates the composites without fibre fracture but by the formation of mode-I cracks along the fibre directions. The results indicate that the high penetration resistance of the composites by sharp-tipped punches is attributed to the high toughnesses of the composites. In the second part of the thesis, failure mechanism maps are developed to illustrate the mechanisms by which failure can initiate in UHMWPE composite beams impacted by blunt projectiles. We reveal that beams with low shear strengths fail by the indirect tension mode at high impact velocities while beams with high shear strengths fail by the bending mode at significantly lower impact velocities. The study thus provides a mechanistic understanding of the experimental observations that high ballistic performance composites require low matrix shear strength. Finally, in the third part of the thesis we investigate the dynamic in-plane compressive response of the composites. It is revealed that compressive deformation of the composites occurs by ply level kink band formation. Additionally, the study shows that the composites become strongly strain rate dependent at strain rates above 100 s^{-1} and the observed strain rate dependency is mainly attributed to that of the matrix.

The findings presented throughout this thesis reveal the key mechanisms and material parameters of the UHMWPE composites which governs their impact and penetration resistance, hence open new avenues and additional routes towards the design of composite materials with ultimate performance.