Polyurethane (PU) chip foam was characterised for air flow resistance, using a modification of the BS 4443 method, and for compressive impact response. A finite difference model was developed for the air flow in the impacted foam, including the effects of strain on foam permeability. It successfully predicted a non-uniform stress distribution across foam block of diameter > 100 mm, and showed that air flow is responsible for some but not all the hysteresis in this distribution for impact velocities > 3 m/s. Air pressure measurements, made in narrow vertical cavities inside impacted blocks of the foam, confirm the air pressure contribution to the total stress.
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