Strain Hardening

When certain materials reach the point of ultimate stress, their cross-sectional areas decrease. This change may not be visually noticable without technological aids.

When loading is carried beyond the yielding region, the load needs to increase for additional strain to occur. This effect is called strain hardening, and it is associated with an increased resistance to slip deformation at the microscale (for polycrystalline materials). Eventually, the stress-strain curve reaches a maximum at the point of ultimate stress. For many materials, the decrease in the cross-sectional area of the specimen is not readily visible to the naked eye until this limit point is passed.

Figure 6b: Small changes in cross-sectional area are invisible to the naked eye.