Evaluation of a flexible force sensor for measurement of helmet foam impact performance

The association between translational head acceleration and concussion remains unclear and provides a weak predictive measure for this type of injury; thus, alternative methods of helmet evaluation are warranted. Recent finite element analysis studies suggest that better estimates of concussion risk can be obtained when regional parameters of the cranium, brain and surrounding tissues are included. Lacking, however, are empirical data at the head-helmet interface with regards to contact area and force. Hence, the purpose of this study was to evaluate a system to capture the impact force distribution of helmet foams. Thirteen Flexiforce® sensors were arranged in a 5×5 cm array, secured to a load cell. Three densities of foam were repeatedly impacted with 5 J of energy during ambient (20 °C) and cold (−25 °C) conditions. RMS error, calculated relative to the global force registered by the load cell, was <1.5% of the measurement range during individual calibration of the Flexiforce® sensors. RMS error was 5% of the measured range for the global force estimated by the sensor array. Load distribution measurement revealed significant differences between repeated impacts of cold temperature foams for which acceleration results were non-significant. The sensor array, covering only 36% of the total area, possessed sufficient spatial and temporal resolution to capture dynamic load distribution patterns. Implementation of this force mapping system is not limited to helmet testing. Indeed it may be adopted to assess other body regions vulnerable to contact injuries (e.g., chest, hip and shin protectors).