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Elastic deformation is a reversible change to a material's shape, where it bends out of position and snaps back after the strain is released. A common example can be seen in glasses with memory frames, which the wearer can bend or crumple in the hand before releasing, allowing them to spring back to their normal shape. This property can be seen in a variety of materials, and assessing the dividing line between elastic and plastic deformation, where the change in shape becomes permanent, is an important part of materials testing.
For many materials, some elasticity is desirable. The ability to snap back from relatively low stress can keep materials durable and useful longer. Memory plastics, foams, metals, and gels all exhibit this trait. The elasticity of the material allows it to recover from stress to restore normal functionality, although over time and in certain conditions, the material can become brittle and may not exhibit as wide a range of elastic deformation. Materials often become less pliable when cold or subjected to hardening chemicals that interfere with their elasticity.
With plastic deformation, the material changes shape permanently, although it does not fail. This can also be a useful trait with some materials; highway barriers, for instance, are sometimes designed to give under the pressure of a car while staying in place, to stop cars without rebounding them back into the roadway. When the material fails because of stress, this can pose a risk. Materials with a low failure point are said to be brittle.
Materials testing can determine how much, if any, elastic deformation a material displays. It may be possible to tweak the configuration of the material to increase the elasticity. With polymer plastics, for instance, special softening materials can be added to the mix to allow it to bend and give under pressure without permanently changing shape. The elastic deformation under pressure can be useful for things like memory foams in seats, which can give under weight and spring back up to accommodate a different sitter.
Too much elastic deformation can become an issue. With some materials, elasticity is not a desirable trait, and the material is made more brittle, but stronger, so it can resist strain. In a building, for instance, a small amount of give can prevent problems, but if girders bend and flex wildly, it could damage other structural components in the building. This could create cracks in the cladding, damage in electrical systems, uneven walls and floors, and so forth.