In-Fiber Bragg Grating Impact Force Transducer for Studying Head–Helmet Mechanical Interaction in Head Impact

In this paper, we present the first Bragg-grating-based transducer system for application to trauma biomechanics, more specifically head-helmet contact force measurements secondary to headform impact. The transducer comprises an aluminum superstructure designed to withstand typical impact forces in helmeted impact and to have resonances that allow the overall sensor system to capture all relevant spectral components of force transients in impact. Structural finite-element models and strain-optic relationships are used to predict transducer sensitivity to impact force as well as mechanical resonances. The model predictions are verified through experimental calibration, and calibration results are, on average, within 10% of model predictions of force sensitivity. The model predicted first mechanical resonance is 72 kHz. The impact force transducer is also validated for helmeted impacts using an industry standard impact experiment and test headform. Results indicate excellent repeatability: maximum standard deviation of force measurements of 0.4% of the net force applied to the impacted headform and average error in the time duration of the force transients of only 4%. Transient impact forces measured with the Bragg grating transducer are in agreement with magnitudes inferred from work of previous researchers. The presented transducer can be applied with both helmet test and anthropomorphic headforms to measure distributions of transient forces and therefore hypotheses related to helmet performance and head injury.