Shear localization

The second is the absorbed hydrogen-enhanced local plasticity mechanism (HELP). This is based on the fact that the local decrease of the flow stress by hydrogen leads to highly localized failure by ductile processes, while the local macroscopic deformation remains small. Shear localization results from local hydrogen absorption, giving a macroscopically brittle fracture related to microscopic localized deformation.95... 

In the SFA experiments there is no way to determine whether shear occurs primarily within the film or is localized at the interface. The assumption, made by experimentalists, of a no-slip flow boundary condition is invalid when shear localizes at the interface. It has also not been possible to examine structural changes in shearing films directly. MD simulations offer a way to study these properties. Simulations allow one to study viscosity profiles of fluids across the slab [21], local effective viscosity inside the solid-fluid interface and in the middle part of the film [28], and actual viscosity of confined fluids [29]. Manias et al. [28] found that nearly all the shear thinning takes place inside the adsorbed layer, whereas the response of the whole film is the weighted average of the viscosity in the middle and inside the interface. Furthermore, MD simulations also allow one to examine the structures of thin films during a shear process, resulting in an atomic-scale explanation [12] of the stick-slip phenomena observed in SFA experiments of boundary lubrication [7]. 

Gu, Y. and Wong, T.-f. 1994. Development of shear localization in simulated quartz gouge. PAGEOPH, 143 387-423. 

Figure 2. Shear localization in Ti four times compressed and additionally strained by 15% at 400°C deformation relief (a) and TEM image of dislocation boundary scattering (b).

At the macroscopic scale, shear localization flow in the alloy develops during initial increments of deformation. Softening and globularization of structure in the macro shear band lead to realization of deformation at mesoscopic scale. In this case the mesoscopic scale deformation is determined by cooperative grain boundary sliding leading to superplastic flow. Superplastic flow results in deformation accumulation in the central area of the sample and impedes in structure transformation in periphery regions. 

It focuses more on the observed behavior, less so on its cause. Catastrophic thermal shear leads to chips with a segmented or serrated or sawtooth form when viewed from a direction normal to the cutting tool s cutting edge (see Application for examples). It is theory and applications of shear localization, leading to segmented, serrated, or sawtooth chip formation, that is the subject of this entry. 

Strain softening, shear localization, and shear banding are all associated with adiabatic shearing, but they are not synonyms as they can also occur, for other reasrais, in isothermal conditions. 

Shear localization or banding due to thermal softening does not require truly adiabatic (i.e., no heat flow) conditions. All that is required is a conditiOTi in which enough heating occurs. The term catastrophic thermal shear covers this. 

Strain softening is necessary for shear localization to occur but is not always due to adiabatic shear. Figure 2a, b show example dependencies... 

Developing a quantitative theory of the conditions for shear localization is the subject of ongoing numerical (finite element-based) research. Key earlier papers are (Recht 1964) in which the instability criterion dx/dy = 0 was first applied, (Semiatin and Rao 1983) in which it was argued that dx/dy needed to be substantially negative and (Hou and Komanduri 1997) in which the complexities of temperature distributions in shear localized chips were examined in more detail than in previous work. Adiabatic shearing has been the subject of a number of general reviews, for example, Walley (2007), and books, for example, Bai and Dodd (1992). These mention but do not have a main focus on machining. Walley (2007) mentions nine earlier reviews. 

Once shear localization is initiated, flow of heat from the shear band is predominantly normal to its surface. From the theory of heat diffusion and given that a shear band is active (Fig. 1) for a time (h/Vc), the minimum width over which conditions may be considered adiabatic (a[h/Vc])° This is expected to be the minimum width 5 of an adiabatic shear band, or (S/h)mm (a/[hVc])° . For typical machining conditions, 5mm is of the order of 10 pm. Experiments generally do show 8 to be of this order and reducing with reducing h/Vc but not always to the power of 0.5. 

Table 1 lists, for a range of metals of interest to machining and from published literature, experimentally observed minimum values of the product of h and v. for serrated chip formation due to adiabatic shear localization. They are to be regarded only as indicative as in fact they depend on rake angle, a factor not considered in collecting material for the table and on material heat treatment, sometimes not recorded in the literature. Furthermore, transitions from one chip form to another are not sharp. 

Bai Y, Dodd B (1992) Adiabatic shear localization occurrence, theories and applications. Pergamon Press, Oxford... 

Semiatin SL, Rao SB (1983) Shear localization during metal cutting. Mater Sci Eng 61 185-192 Shaw MC (2004) Metal cutting principles, 2nd edn. 

Butterworth Heinemann, Newton, Ch. 11 Walley S (2007) Shear localization a historical overview. Metall Mater Trans A 38(ll) 2629-2654... 

The temperature and the chip formation have a radical influence on the forces and the cutting power. Shear localization and material failure mechanisms with increased cutting speed lead to a change in chip formation, resulting in a reduction of process forces (Tonshoff et al. 2005a). 

Since shear deformation has a habit of undergoing localization in the form of shear bands by interaction of STs, the size of the RVE at times becomes difficult to establish and can become quite large. We consider shear localization in Section 7.8, but concentrate here first on the quasi-homogeneous process of flow by relatively non-interacting additions of STs under stress. 

The mechanics of localized vs. homogeneous shear in metallic glasses was considered first by Spaepen (1977) and later, in more detail, by Argon (1979), who developed a perturbation model of shear localization in the context of flow by repeated STs. A more formal model was presented later by Steif et al. (1982). Similar flow-dilatancy-based models had actually been considered much earlier, independently, for flow of cohensionless media, such as sand and soils (for an overview of these see Anand and Gu (2000)). 

---