Plastic breakage behavior

It is important to differentiate between brittie and plastic deformations within materials. With brittie materials, the behavior is predominantiy elastic until the yield point is reached, at which breakage occurs. When fracture occurs as a result of a time-dependent strain, the material behaves in an inelastic manner. Most materials tend to be inelastic. Figure 1 shows a typical stress—strain diagram. The section A—B is the elastic region where the material obeys Hooke s law, and the slope of the line is Young s modulus. C is the yield point, where plastic deformation begins. The difference in strain between the yield point C and the ultimate yield point D gives a measure of the brittieness of the material, ie, the less difference in strain, the more brittie the material. 

Indirect evidence of the likely occurrence of chain scission was proposed by Davis et al. [170—173] who studied the deformation behavior of high molecular weight low density PE (and of PTFE) at hydrostatic pressure. From the pressure sensitivity of the rate of plastic deformation they derived a zero-pressure activation volume of 0.266 nm which they suggested to correspond to the activation volume y for the breakage of a PE chain. 

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