Experimentally separating fluid and matrix contributions to polymeric foam behavior

A new experimental procedure to determine the loads carried by the fluid (air) and matrix components of a polymeric foam is presented. Testing is carried out in a sealed chamber equipped with a differential pressure transducer to measure changes in the chamber air pressure and a load cell to measure the load applied to the specimen. Multiexposure photographs are used to determine lateral specimen expansion at various degrees of compression. From these data the amount of air trapped and compressed within the foam can be determined. Theoretical analyses suggest and tests confirm that for the strain rates used here the trapped air undergoes isothermal compression. By treating compression of the air trapped in the specimen as an isothermal process, an equivalent volume-average pore pressure can be determined, and the load carried by the fluid phase calculated. The load carried by the polymer matrix component is the difference between the total response and the fluid component. The energy input into each phase during compression can then be calculated.The effectiveness of the procedure is demonstrated by displacement-controlled compression tests of 50×100×100-mm semi-rigid, polyurethane foam specimens. Two types of foam were compressed to 75-percent strain at nominal strain rates of 1.4/s and 14/s. Calculated values show a high degree of repeatability.