A material is said to exhibit elastic behavior if it returns to its previous shape and size once an applied force is removed. This is called reversible deformation. Simple materials such as elemental metals typically display purely elastic behavior. These tend to stretch to a certain point and then experience sudden fracture if the stress is not removed. Materials such as these are described as being brittle.
More complicated materials such as polymers, proteins, biomaterials and some inorganic amorphous solids exhibit elastic behavior until a certain stress is exceeded (yield strength). Beyond this point, less force is required to induce further deformation, and the material is unable to recover its size and shape once the load is removed. This phenomenon is referred to as irreversible deformation, plastic deformation, or permanent set. The applied force causes something to change inside the substance at a molecular level that causes it to become fundamentally different in its physical structure. The change can be a rearrangement of crystalline lattice structure from one type to another, shifting or slippage of molecular alignment in an amorphous or semi-crystalline material, change of protein tertiary structure, or breaking of bonds in polymeric compounds. Materials with this property are referred to as being ductile or having greater toughness than brittle substances.
Plastic deformation is particularly relevant to the health hair and its appearance. If excessive force is used to style or comb hair, the yield strength can easily be exceeded, and the hair can no longer bounce back when it is pulled out of shape. This can adversely affect its ability to hold a style or retain curl and can result in shapeless, frizzy hair.
Additionally, special caution should be taken with wet hair. Hair saturated with water is fragile and can stretch much more easily than when it is dry. It is very easy to exceed the yield strength when hair is wet and permanently diminish its elasticity, or even cause breakage. For this reason, it is crucial to use extreme care when handling and combing wet hair. The use of a good conditioner helps protect wet hair from plastic deformation by decreasing combing forces (less force is required to get the comb through tangles).