Deformation-hardening coefficient

The flow stress in Figure 2.6 is given approximately by Y = Y0 + h8 where h is the deformation-hardening coefficient. It is assumed that the elastic strain is fully recovered in a hardness measurement, so it need not be considered further in this approximate treatment. Then H = 2.2h8, and since 8 = 2x/L = tan 0 = 0.414 then H = 0.89 h. Hence H — h. That is, the indentation hardness number approximately equals the deformation-hardening coefficient. 

FLD). The intersection of the limit curve with the vertical axis, which represents the plane strain deformation (S2 = 0), is an important point of the FLD and is noted FLDq. The position of this point depends mainly on the strain hardening coefficient and also on thickness. 

Let us first examine the sign of this third coefficient. If it is negative, the viscosity decreases as time goes on. If it is positive, the viscosity may deviate upwards from the baseline = git fixed by the linear modulus. In other words, it shows an upturn at a certain time from the linear baseline. Such a stress growth beyond the linear baseline caused by a large deformation is called strain hardening. 

The contribution of each mechanism to the friction coefficient depends on material properties, mechanical parameters and chemical conditions in the contact and cannot be predicted easily. Furthermore, their relative importance can change during the course of an experiment for several reasons accumulation of wear particles forming a third body change in rugosity due to plastic deformation of the asperities strain hardening of the metal near the surface increase of the contact temperature due to friction, etc. 

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