PLASTIC DEFORMATION AND STRESS-STRAIN CURVES

After a material has reached its elastic limit, or yielded, further straining will result in permanent deformation. After yielding not all of the strain will be recovered when the load is removed. Plastic deformation is defined as permanent, non-recoverable deformation. Plastic deformation is not linear with applied stress. Recall if a material experiences only elastic deformation, when the stress is removed the elastic strain will be recovered. If a material is loaded beyond its yield point it experiences both elastic and plastic strain. After yielding the rate of straining is no longer linear as the applied stress increases. When the stress is removed, only the elastic strain is recovered; the plastic strain is permanent. Elastic deformation occurs as the interatomic bonds stretch, but the atoms retain their original nearest neighbors and they "spring back" to their original positions when the load is removed. Clearly in order to have permanent deformation there must be permanent movement in the interatomic structure of the material. Although some of the atoms move away from their original nearest neighbors not all of the interatomic bonds
are broken (this is evident because we can achieve permanent deformation without fracture of the material). The mechanism for permanent deformation is called slip.