Impact resistance is one of the most important properties for material engineers or designers to consider. One of the most common tests of the physical characteristics of materials is the notched izod impact test as specified by ASTM D 256 Standard Test Method. In Europe, ISO 180 methods are used and results are based only on the cross-sectional area at the notch (J/m2). The dimensions of a standard specimen for ASTM D256 are 63.5 × 12.7 × 3.2 mm (2.5 × 0.5 × 0.125 in). The most common specimen thickness is 3.2 mm (0.125 in), but the width can vary between 3.0 and 12.7 mm (0.118 in and 0.500 in).
The result of the Izod test is reported in energy lost per unit of specimen thickness (such as ft-lb/in or J/m) at the notch. In Europe, ISO 180 methods are used and results reported based only on the cross-sectional area at the notch (J/m²). The higher the resulting numbers the tougher the material. The impact resistance of a specific commercial grade of a polymer is a function of the base resin plus the presence of any impact modifiers and reinforcing agents that may be added by the manufacturer or compounder.
Environmental factors other than temperature also play a role in impact resistance. For example, nylons generally experience higher impact strength in the conditioned state in equilibrium with atmospheric moisture than in a dry-as-molded state because of the plasticizing effect of absorbed moisture.