Impact strength is a measurement of how well a material holds up when struck, dropped, or subjected to other sharp impacts. Known as shock loads, impacts can cause considerable strain on material because of their rapidity; the material does not have time to adapt and distribute the pressure because the incident happens so quickly. Materials testing usually includes an assessment of impact strength, and the manufacturer may provide a rating with information about its resilience in the face of impacts.
Resistance to impact can be a very critical trait. Shock loads may occur in emergencies or under less than ideal conditions, and material failures would be catastrophic in these settings. Some materials remain in low impact settings until they are needed, at which point they may be subjected to immense forces and need to perform reliably. Roadside barriers, for example, don’t need very much stress resistance until a vehicle careens into them, at which point they need to be able to absorb and respond to the impact.
There are a number of techniques available to measure impact strength. In all cases, carefully calibrated equipment is used by trained technicians to achieve a quantifiable and repeatable result. One option is the use of a weighed pendulum. Technicians place the material in a cradle and drop the pendulum to see what happens. They take note of the level of impact strain required to start cracking, bending, or pitting the material, and they also record the failure point.
High impact strength can come with tradeoffs. Slightly flexible materials tend to handle impacts better because they can deform and bounce back, but this might not be desirable with things like building materials. Making the material tougher and more rigid can create brittle characteristics, which means the material may shatter under relatively low stress. Materials designers have to think about the applications of a material and develop the best compromise between available traits.
There are some drawbacks to impact strength testing. While the equipment is very controlled and the impacts are measurable, the controlled conditions can provide false information about materials. In the field, where materials may be mixed, put together under less than ideal positions, compromised by weathering, and so forth, materials may fail at pressures below the expected impact strength. Companies perform as much testing as possible, including with notched, weathered, and otherwise strained materials, to achieve the most suitable and applicable results.