Numerical Assessment of Non Lethal Projectile Thoracic Impacts
Anti personnel Non Lethal Weapons (NLW) are weapons that are designed to impart sufficient effect onto a person in order to deter uncivil, suspect or hazardous behaviour with a low probability of severe or fatal injury. They are used both by military and law enforcement in situations of low intensity conflicts like riot control, access denial, peacekeeping missions i.e. situations where the use of lethal force is not appropriate nor desired. The most used of these weapons are the Kinetic Energy Non Lethal Weapons (KENLW) The underlying principle of KENLW is to launch a projectile with a mass varying between 5 g and 140 g at initial velocity up to 160 m/s, which by a mechanical action on the target, will cause enough physical pain to incapacitate or repel the target while minimising the risk of severe injuries. But their use is not without risk as in practice, the impacts of Kinetic Energy Non Lethal (KENL) projectiles on the human thorax resulted in injuries, some of the them were severe even fatal. Therefore there is a need to develop methods of assessment. These assessment methods are essential in order to help deciders in charge of nonlethal weapon procurement with technical information so they can choose the best product (weapon or projectile) available on the market; to give relevant information to the manufacturers in either developing new weapons or projectiles that are more effective, or improving the existing ones, and finally to the end users (military or police forces) of these weapons, information on operational distance of engagement. In practice, it has been observed that the thorax is the body region of where the impacts of KNL projectiles led to more significant injuries than other parts of the body apart from the head which is never targeted at. Therefore in the present thesis, only assessment of thoracic impacts will be investigated. Besides tests on PMHS (Post Mortem Human Subjects) animals or human surrogates, one powerful tool that is used nowadays to assess the thoracic impacts is the finite element method (FEM) It has many advantages like the capability of accounting for complex geometries or complex material modelling and its cost effectiveness. It also gives insight on physical variables (stresses, strains, inside the material vii which are inaccessible by other means. It helps for a better understanding of the injury mechanism. Moreover, it helps to reduce cadavers or animal testings. In the present thesis, only FEM will be considered as tool for injury risk assessment. On the one hand, a thorax finite element (FE) model, the SHTIM (Surrogate Human Thorax Impact Model) has been developed for the injruy risk assessment. Number of assumptions has been made relative to the thorax geometry and the material characteristics are based on literature. The model has been validated thanks to the results of experiments carried out by professor C. Bir. A viscous injury criterion were defined as the parameter relevant for the occurrence of the thorax skeletal injury. This criterion is used for the prediction of the thoracic injury outcome. On the other hand, FE models of six projectiles have been developed where most of material characteristics were taken from the literature. For the 40 mm sponge grenades, a new method of characterising the deformable nose has been developed. The projectile FE models were validated by comparing numerical results to experimental results obtained from real firing testings of the projectile against a rigid wall structure. These testings have been performed within the Department ABAL. Good correspondence was found. Once the thorax FE model and the projectile FE models validated, numerical simulations of the impact between the thorax and the projectile were performed. Using the viscous injury criterion, risk assessment of the thorax impacts was carried out. For each projectile, a critical velocity was determined which thanks to the retardation can be linked to a minimum firing distance, the safe distance. This is the distance below which an impact will result in a higher risk of skeletal thoracic injury. This information is very important for the end users (military, police) Moreover comparison of the performance of different KENL projectiles was carried out. Few years ago, the Department ABAL acquired a thorax mechanical surrogate, the 3RBID (3 Rib Ballistic Impact Dummy) for the prediction of thoracic injury. It was an opportunity to compare both surrogates and to see if the SHTIM results are consistent with the 3RBID results. Good correspondence has been found especially for projectiles with larger diameter for example the 40 mm sponge grenades.