During defeat of high velocity bullets and fragmentation, large deformations have been shown to occur on the inside of non-metallic helmet shells. If these deformations contact the underlying head there is the potential to cause injury, termed as Behind Helmet Blunt Trauma (BHBT). A method of assessing BHBT is required to fully understand the protective capability of a helmet system. Existing methods using synthetic head models are limited in their ability to predict injury outcome as a result of a lack of available injury criteria. The aim of this study was to improve the prediction of cranial fracture outcomes associated with BHBT, focusing on the effect of impact curvature and flat face diameter.
Representative impact curvatures and flat face diameters were derived from helmet back face deformation measurements in free-air. An instrumented projectile was used to deliver a repeatable impact to a Bovine Scapula Model (BSM); the BSM being a fracture analogue for the cranium. Cranial fracture risk curves were developed using BSM fracture outcomes. These were developed for the different curvatures and flat face diameters tested, using the parameters of peak impact force, impact velocity, bone thickness considered in conjunction with impact velocity, and the Blunt Criterion (BC).
It was concluded that flat impact diameter and radius of curvature should be measured within BHBT assessment to improve the accuracy of cranial fracture prediction. To support application of the risk curves to BHBT assessment methods, it was recommended that fracture outcome should be investigated in terms of the effect of using a rigid projectile when compared to helmet deformation. BHBT assessment methods should consider how to achieve consistent and representative stand-off. Testing should also be completed on full helmet systems as opposed to flat material samples
