Slip line

Figure 3.10. Rosenhain s micrograph shosviiig slip lines in lead grains.

The early understanding of the geometry and dynamics of dislocations, as well as a detailed discussion of the role of vacancies in diffusion, is to be found in one of the early classics on crystal defects, a hard-to-find book entitled Imperfections in Nearly Perfect Crystals, based on a symposium held in the USA in 1950 (Shockley et al. 1952). Since in 1950, experimental evidence of dislocations was as yet very sparse, more emphasis was placed on a close study of slip lines (W.T. Read, Jr.,... 

Fig. 20.33 (top) Transmission electronmicrograph showing dislocation tangles associated with precipitates in an Al-Cu-Mg-Si alloy (x 24 000, courtesy S. Blain) and (bottom) light micrograph showing slip lines in pure lead (x 100)... 

Dislocations are readily visible in thin-film transmission electron micrographs, as shown in Figs. 20.28 (top) and 20.33 (top). The slip step (Fig. 20.31c) produced by the passage of a single dislocation is not readily apparent. However, for a variety of reasons, a large number of dislocations often move on the same slip plane or on bands of closely adjacent slip planes this results in slip steps which are very easily seen in the light microscope, as shown by the slip lines in Fig. 20.33 (bottom). 

This is similar to the analysis obtained by Ainsley and Smith (see Chhabra, 1992) using the slip line theory from soil mechanics, which results in a dimensionless group called the plasticity number ... 

If slip-line fields do not control plastic indentation, what does The answer is not the beginning of the plastic deformation, but the end of it. The end means after deformation hardening has occurred. That is, it is not the initial yield stress, Y0, that controls indentation, but the limiting yield stress, Y. This is... 

In textbooks, plastic deformation is often described as a two-dimensional process. However, it is intrinsically three-dimensional, and cannot be adequately described in terms of two-dimensions. Hardness indentation is a case in point. For many years this process was described in terms of two-dimensional slip-line fields (Tabor, 1951). This approach, developed by Hill (1950) and others, indicated that the hardness number should be about three times the yield stress. Various shortcomings of this theory were discussed by Shaw (1973). He showed that the experimental flow pattern under a spherical indenter bears little resemblance to the prediction of slip-line theory. He attributes this discrepancy to the neglect of elastic strains in slip-line theory. However, the cause of the discrepancy has a different source as will be discussed here. Slip-lines arise from deformation-softening which is related to the principal mechanism of dislocation multiplication a three-dimensional process. The plastic zone determined by Shaw, and his colleagues is determined by strain-hardening. This is a good example of the confusion that results from inadequate understanding of the physics of a process such as plasticity. 

Plastic deformation (strain). When two surfaces of ductile materials are placed in contact and the load exceeds the elastic limit of one of the two materials, plastic deformation or strain occurs. The plastic deformation of one surface when two surfaces are in solid-state contact can occur in the presence or absence of lubricants. In fact, in some instances, the presence of lubricants can increase the deformability of the solid surfaces by a mechanism such as the Rehbinder effect. Plastic deformation of the solid surface is, therefore, observed in the presence of lubricants. Plastic deformation is accommodated by the generation of slip lines for dislocation flow in the solid surface. Dislocations are line defects in the solid and they are site of higher energy state on the surface. Thus, they interact or react more rapidly with certain chemical agents than do the bulk surfaces (Buckley, 1981 Lunarska and Samatowicz, 2000). 

The common defects can be seen with optical microscopes, and they become more clearly visible after suitable etches.20 The most common defects are stacking faults and spikes (see Figure 17). These can be caused by local surface imperfections as well as surface particulates. Another defect frequently occurring is the slip lines shown in Figure 18. 

Figure 18 Slip lines with stacking faults.19...

Fig. 2. Taper sections of a brass surface abraded on l/0 grade emery paper. Taper ratio 8.2. (a) Etched in ferric chloride reagent and showing the inhomogeneous distribution of the deformation close to the surface. X 100G before reduction for publication, (b) Etched to develop slip-line traces, showing the full extent of the plastically deformed layer. X 25(4 before reduction for publication. The fragmented layer, which is more clearly shown in Fig. 1, is also discern-able in these micrographs.

Fig. 7 Sequential micrographs of the evolution of the damage in a SiO 4.5 wt.% P film deposited on an Al substrate subjected to a tensile test (system C, Figure 6). The black arrows show the tensile direction, (a) Networks of primary and secondary cracks perpendicular to the tensile axis (e = 11%). The white arrows show a secondary crack which stops when getting close to primary cracks, (b) decohesion and buckling of the strips of film. Slip lines are observed on the Al surface under the buckled strips, and (c) transverse rupture of the buckled zones along the directions of maximum shear of the substrate (e = 19%).

The initial acceleratory stage of the reaction was attributed to the rapid formation of nuclei at surface lattice defects, identified [64] as slip lines. The applicability of... 

The early observation of macroscopic slip lines in deformed ice single crystals by Nakaya (1958) or Readings and Bartlett (1968)" indicated the simultaneous and correlated motion of many dislocations. More recently, acoustic emission analyses performed on ice single crystals during deformation revealed the scale-free intermittent motion of dislocations through dislocation avalanches. 

Figure 3. Copper deformed by 6% (courtesy O. Kolednik). The circle marks slip lines different from the main set in grain A.

Cross slip is the process where a dislocation - a screw dislocation -changes its slip plane. The concept of cross slip was introduced on the basis of slip-line observations on polished surfaces [1] at an early stage of the investigation of the behavior of dislocations... 

According to the test, five specimens (each 50 mm long, 100 mm wide, and 1 mm thick) are conditioned at ambient temperature as specified in the test, and placed in a forced draft oven equipped with a biaxial rotator. Specimens should be attached to the rotator by metal slips lined with fluoropolymer film and should not directly contact with the metal clips or metal parts of the oven. The frequency of rotation about the horizontal and vertical axes of the rotator should be 1-3 min . The time to failure is determined by regular visual examination of the specimens as the number of days after which the specimen shows localized crazing, crumbling, or discoloration, or a combination thereof. According to the standard procedure, the oven temperature shall be 150°C (302°F). 

Careful examination of a notched region, after a slight impact load has started the yielding process, reveals a pattern of shear bands (Fig. 9.6a) in some glassy plastics. The plastic strain occurs inhomogeneously. The shear strain is about 1 within the shear bands, and zero between them. The overall pattern is remarkably similar to a particular slip line field pattern (Fig. 9.6b). 

Slip line fields are used to analyse metal plasticity under plane strain conditions (see Appendix C). The slip line field consists of two families of logarithmic spirals, with equations in polar coordinates r, d... 

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