Foam materials are critical safety elements in impact protection systems such as helmets, pads, and playing surfaces.  Often materials with different performance windows are layered to address the breadth of expected impact times and energies. Recently, the design of padding systems has moved beyond ‚Äúcomfort‚Äù and ‚Äúprotective‚Äù layers to optimizing structure and viscoelasticity enabling tunable dynamic transfer functions. Current dynamic test methods focus on scalar system response characteristics (e.g., maximum transmitted force and peak acceleration) without providing a framework to quantify material response at the sub-system level. This leaves a gap in the structure-performance design process that is often filled by modeling the system response. This can reduce accuracy and limit innovation. 

Here, a structure-property-performance approach is initiated with an eye toward material data frameworks. Structural information, e.g., relative density, pore anisotropy, and tortuosity, are quantitatively determined from micro-computed tomography via custom image processing scripts and research software developed by the porous solids community.  Dynamic performance is quantified via impact testing of monolithic and bi-layer foam stacks. Figures of merit are built around time-resolved surface strain measurements for meso-scale strain distributions and global mechanical impedance measurements that include force and acceleration. 

We focus on two elastomeric closed-cell vinyl-nitrile foams (cf., to vary relative density) and a polyurethane-based ‚Äúmemory‚Äù-type foam (cf., to vary viscoelasticity) as base elements. Preliminary results of stacking indicate that a stiff-soft stacking of vinyl-nitrile foams with soft foam on the impact face yields a lower impactor acceleration and peak transmitted forces. This runs counter-intuitive to putting soft ‚Äúcomfort‚Äù layers closer to the wearer. Surface strain evolution confirms that deformation response of the layers is also influenced by the stacking orientation ‚Äì typically a softer top layer will lead to increased involvement of both layers.  Finally, we explore methods to couple this measurement framework into an engineering design strategy.  

Micro-computed tomography and instrumented drop weight testing for structure-performance measurements of layered foams

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