Short answer
Elastic deformation disappears when the applied stress is removed; plastic deformation leaves a nonrecoverable change. The key distinction is recovery, not whether the stress–strain curve is straight: some materials behave elastically without a linear relationship. 1
On this page
At a glance
Scroll to compare all columns →
| Question or attribute | Elastic deformation | Plastic deformation |
|---|---|---|
| What happens after unloading? | Original configuration is recovered | A nonrecoverable change remains |
| Must stress and strain be proportional? | No; nonlinear elasticity exists | Loss of proportionality alone is not decisive |
| Can one test involve both? | Yes | Yes |
These distinctions follow NIST’s recovery descriptions and its treatment of combined elastic–plastic indentation. 1
What each thing is
Both terms describe material deformation under stress. Elastic deformation is recoverable; plastic deformation is permanent in the sense of not being recovered on unloading. Penn State explains metallic plasticity through bonds breaking and reforming with different atomic neighbors, leaving a changed form after stress relief. 1 2
Key differences
For most metals at low tensile stress, elastic strain is proportional to stress, with Young’s modulus describing that relationship. Beyond a certain strain, most metals develop plastic deformation. These are different responses to loading, not necessarily labels for different materials: a metal can exhibit each in different loading ranges. 1
How to tell them apart
Use unloading as the practical identification rule: recovery indicates elastic deformation; a remaining change indicates a plastic contribution. Do not substitute a curve’s departure from a straight line for this rule. Its practical limit is locating the precise onset of plasticity, which Penn State notes can be difficult. 1 2
Where they overlap
Elastic and plastic deformation need not describe mutually exclusive tests. NIST discusses indentation methods that measure both during loading. Thus, evidence of permanent deformation does not mean the entire measured response was plastic; the test can contain an elastic contribution as well. 1
Edge cases
Nonlinear elasticity is an important exception to the familiar straight-line picture. NIST describes tangent and secant moduli for materials whose initial elastic response is nonlinear. Another exception is a conspicuous yield point: only a subset of metals show the sharp drop and subsequent nearly constant stress described in the report. 1
Why the distinction exists
The distinction separates recoverable changes from changes that remain. Yield strength helps describe the transition in testing, but identifying it can require a measurement convention. Penn State presents a line offset by 0.002 strain to identify yield strength; that construction is not a universal definition of the first permanent change. 1 2
Common misconceptions
Plastic deformation does not mean the material has broken. Penn State distinguishes yielding from the later maximum tensile stress and fracture point. Nor does a curved stress–strain response automatically prove plasticity: NIST explicitly allows nonlinear elastic behavior. Both misconceptions confuse a useful curve feature with the recovery distinction. 1 2
Examples
Two hypothetical cases apply the distinction. First, a metal specimen stretches under low tensile stress and returns to its original configuration on unloading: that is elastic deformation. Second, an indenter leaves a permanent impression in a metal surface: the remaining impression demonstrates plastic deformation, even if the indentation test also measures elastic response. 1