Slip softening

It is partly because of the variable effect of hydrogen (giving both softening and hardening, according to the nature of the slip) that the extrapolation of model experiments on very pure iron to predict the behaviour of commercial materials is so difficult. It is further hindered by the ability of dissolved hydrogen to modify the dislocation structure of a straining material. 

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. 

The so called plasticization effect, i.e., the depression of the glass transition temperature is also an important feature the sorbed gas acts as a kind of lubricant , making it easier for chain molecules to slip over one another, and thus causing polymer softening. 

Safety clutches usually consist of two parts the actual safety clutch and the elastic part, both of which soften vibrations and shocks and equalize any misalignment between motor and gearbox. Essentially, a distinction is made between slip clutches and mechanically disengaging clutches. Despite their higher cost, pneumatic slip clutches are normally used on larger extruders, because they are easy to handle, which means shorter downtimes in case of overload. 

Another theory of the reason for increased friction in the presence of moisture was proposed by Gao et al . They found that in a humid environment molybdenum disulphide films were more readily thinned by sliding contact, due to increased ease of interlamellar slip. They suggested that adsorption of water softened the films, and that resulting increased deformation by plowing in sliding contact led to a poorly oriented film and thus to increased friction. However, they considered that this was a short-term reversible effect which was not in conflict with theories of chemical breakdown. Gao et al also poiinted out the possibility that an increase In friction is caused by capillary pressure effects of moisture at asperity contacts. 

Non-slip finishes are mostly applied by the pad-dry process, combined as needed with hand builders, repellents or cellulose crosslinkers (easy-care finish). The combination of silicic acid sols with softeners or silicone-based water repellents is crucial - their effects are contrasting. 

Thermoplastics are polymers that can be melted and then molded into shapes that are retained when the polymer is cooled. Although they have high Tg values and are hard at room temperature, heating causes individual polymer chains to slip past each other, causing the material to soften. Polyethylene terephthalate and polystyrene are thermoplastic polymers. 

Slip point, softening point, or rising point Source PORAM Technical Brochure, 1989. 

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