Chapter 5 - Simple mechanical tests

Abstract

This chapter considers compressive, tensile, and shear tests on foams as well as their interpretation. The time scale varies from days for creep tests, to a few milliseconds for impact tests. There are some specialized tests, such as indentation force deflection tests for soft foams and fracture toughness tests for rigid foams. Although tensile tests are common for other materials, they are rarely used for foams. This is partly because of the difficulty of gripping foams to apply tensile loads, and partly because few applications involve tensile loads; most foams are weak in tension and fracture easily. Compressive loading is common in foam cushions and packaging foams loaded by the weight of the contents. Shear occurs if the applied force vector lies in the surface plane of the foam block. Bending and torsion often occur in foam products. Stresses in bending or torsion vary with the distance from the neutral surface or axis of twisting of the specimen; therefore, only a small region is under high stress. In compression tests, stresses are intended to be uniform. Bead foam moldings usually vary in density from the skin to the core, so have inhomogeneous microstructures. Consequently the Young's modulus and strength vary with position in specimens cut from these moldings.