Impact testing is testing an object's ability to resist high-rate loading. An
impact test is a test for determining the energy absorbed in fracturing a test
piece at high velocity. Most of us think of it as one object striking another
object at a relatively high speed.
Why is Impact Testing Important?
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,
- select a material that will resist such assaults
over the projected life span.
Molded-in stresses, polymer orientation, weak spots
(e.g. weld lines or gate areas), and part geometry will affect impact
performance. Impact properties also change when additives, e.g.
coloring agents, are added to plastics.
Ductile vs. Brittle
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.