Local strain-hardening

Surface layers interfere with the motion of dislocations near surfaces. Among other effects, this causes local strain-hardening, creating a harder surface region which thickens with further deformation, and eventually affects an entire specimen. A specific way in which this happens is through curving... 

Clearly, the homogeneous deformation response shown in Fig. 12.17 also applies in inhomogeneous flow where notches and other local strain concentrations are present. There, through the double effect of enhancement of the local strain hardening and of the strain rate, global embrittlement will result. 

When plastic deformation occurs, crystallographic planes sHp past each other. SHp is fackitated by the unique atomic stmcture of metals, which consists of an electron cloud surrounding positive nuclei. This stmcture permits shifting of atomic position without separation of atomic planes and resultant fracture. The stress requked to sHp an atomic plane past an adjacent plane is extremely high if the entire plane moves at the same time. Therefore, the plane moves locally, which gives rise to line defects called dislocations. These dislocations explain strain hardening and many other phenomena. 

In the unstrained material far from the center of an indentation, dislocations can move freely at much lower stresses than in the material near the center where the stress (and the deformation) is much larger. Thus, local plastic shear bands can form at the edges of the indenter, and do (Chaudhri, 2004). The lengths of these shear bands are often several times the size of an indentation. The leading dislocations in these bands move in virgin (undeformed) material, so they can move at lower stresses than the dislocations in the strain-hardened material near the center of an indentation.. The patterns they form are called rosettes. ... 

It should be understood that the positive effects of autofrettage are always based on two different sources compressive residual stresses, which permit a larger amplitude of the operational stress and, depending on the material, the improved local strength of the material owing to strain-hardening by local plastification. 

Qian, S., Zhou, J., Rooij, M.R., Schlangen, E., Ye, G. and Breugel, K.V. (2009). Self-healing behaviour of strain hardening cementitious composites incorporating local waste materials. Cement and Concrete Composites, 31 613-621. 

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