The effect of pre-damage on the penetration resistance of composite materials
Creators: name: EDWARDS, CERISE AUTUMN
Publisher: name: University of Surrey
Year: 2021

description: Military personnel use protective armour systems that are frequently exposed to low-level damage, such as non-ballistic impact, wear-and-tear from everyday use, and damage during storage. The extent to which such mechanical low-level pre-damage could affect the performance of an armour system is unknown. To maintain a given level of protection, armour is recalled periodically, which is necessarily conservative, but costly. A greater understanding of the types and degree of composite damage which negatively affect armour performance could inform a less conservative approach, whilst maintaining assurance of protection. The research conducted in this work is an initial examination of this issue to ultimately inform armour applications such as composite backing in body armour and spall liners in vehicle armour.
description: This research considers two issues: the effects of fatigue pre-damage on the residual quasi-static penetration resistance of fully resin-infused (‘solid’) composites and the effects of tensile pre-damage on the residual ballistic penetration resistance of resin-starved composites.
description: The first approach introduced fatigue pre-damage into solid eight-layer glass fibre-reinforced polymer composites and assessed the residual penetration resistance using quasi-static indentation tests. Fatigue pre-damage was considered for fully infused glass-fibre composites to introduce different levels of controlled damage, as non-critical damage can be imparted through cyclic loading in vehicle armour materials. Quasi-static indentation tests were used to assess the penetration resistance to observe the global impact response of the material to low-velocity impact. The level of pre-damage introduced was monitored by reducing the tangent stiffness by 5%, 10% and 15%. Through microscopic characterisation and detailed pre-damage analysis, it was found that additional fibre fractures and large micro-delaminations - up to 30% of the plan-view area - were required to produce a significant change in peak load and drop in energy absorption.
description: The second approach introduced tensile pre-damage into resin-starved eight-layer aramid fibre-reinforced polymer composites and assessed the residual penetration resistance using ballistic testing. Tensile pre-damage was considered for resin-starved aramid-fibre composites commonly used in body armour, to introduce comparatively higher levels of non-critical damage because of their known superior fatigue resistance. Ballistic testing was used to assess the penetration resistance because of the required ballistic fragment resistance for body armour. The composites used in this work were thinner than those used in body armour, thus also need to be checked against thicker samples for applicability. Microscopic characterisation showed that the pre-damage introduced included matrix fractures and delaminations. While no difference was indicated between the ballistic limit of the pre-damaged and a control group of specimens, a difference in the residual out-of-plane deformation as a consequence of ballistic testing was observed. The results of this approach highlighted the importance of backface deformations as large backface deformation can lead to behind armour blunt trauma and an increase in sustained injuries of the user.
description: Recommendations for future work with regards to solid laminates include studies on ballistic performance and backface deformation of pre-damaged fully-infused laminates. To determine whether these conclusions are applicable to armour systems, it is recommended that standard thickness armour systems are assessed in this manner. In addition, it is recommended that an assessment and characterisation of dynamic backface deformations, following ballistic impact of pre-damaged specimens, is carried out. Recommendations include focusing on the internal analysis (i.e. damage mapping) of real-life non-critical damage to enable the comparison of laboratory induced pre-damage and damage in real-life applications. These results could lead to the further enhancement of existing numerical simulations to predict the ballistic response of composite armour materials.