Two important things we measure are ultimate elongation and elastic elongation Ultimate elongation is important for any kind of material. It is nothing more than the amount you can stretch the sample before it breaks. Elastic elongation is the percent elongation you can reach without permanently deforming your sample. That is, how much can you stretch it, and still have the sample snap back to it's original length once you release the stress on it. This is important if your material is an elastomer .
Elastomers have to be able to stretch a long distance and still bounce back.
Modulus Elastomers need to show high elastic elongation. But for some other types of materials, like plastics , it usually better that they not stretch or deform so easily. If we want to know how well a material resists deformation, we measure something called modulus . To measure tensile modulus, we do the same thing as we did to measure strength and ultimate elongation. This time we measure the stress we're exerting on the material, just like we did when we were measuring tensile strength.