Results of a Round Robin ballistic load sensing headform test series

The majority of methods to assess the behind armour blunt trauma (BABT) risk for ballistic helmets is based on plastic deformable headforms. An alternative, the Ballistic Load Sensing Headform (BLSH) can record the dynamic contact force between helmet back face and the skull. Helmet BABT methods are still under development and outcomes are highly sensitive to test procedures. A test method for the BLSH has been developed, as there was no method available, and evaluated in a round robin series performed by three laboratories, using the same procedure and their own headform, data acquisition and processing. Helmets were tested three times at front, rear and both lateral sides. Peak contact force is used as performance criterion. The peak force varies between 3.536 kN and 10.110 kN depending on location. This is to a large extend caused by a significant variation in stand-off which occurs despite the strict positioning procedure of the helmet. Results become more consistent by statistically correlating peak force and stand-off. However a significant spread within each laboratory remains, which needs further exploration. It is expected that peak force is too sensitive as it is a high frequency phenomena and the tested helmets are not designed to control nor optimize the BABT contact force. It appears that there is systematic difference between laboratories which is not understood. The helmet BABT risk assessment method needs further development to allow inter laboratory comparison and is therefore not acceptable for a test standard. However it is very useful for research purposes and helmet optimisation knowing the vulnerable and sensitive aspects. The scattered response requires a statistical approach and consequently multiple helmet test are required to allow for meaningful conclusions.