Impact resistance is one of the most important properties for a part designer to consider and, without question, the most difficult to quantify. The impact resistance of a part is, in many applications, a critical measure of service life. More importantly these days, it involves the perplexing problem of product safety and liability.
One must determine:
the impact energies the part can be expected to see in its lifetime,
the type of impact that will deliver that energy, and then
select a material that will resist such assaults over the projected life span.
Moulded-in stresses, polymer orientation, weak spots and part geometry will affect impact performance. Impact properties also change when additives, e.g. colouring agents, are added to plastics.
Most real world impacts are biaxial rather than unidirectional.
Further complication is offered by the choice of failure modes: ductile or brittle. Brittle materials take little energy to start a crack, little more to propagate it to a shattering climax. Other materials possess ductility to varying degrees. Highly ductile materials fail by puncture in drop weight testing and require a high energy load to initiate and propagate the crack.
Many materials are capable of either ductile or brittle failure, depending upon the type of test and rate and temperature conditions. They possess a ductile/brittle transition that actually shifts according to these variables.