Breakage fracture measurements

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. 

The master curve shows that the life-time is thermally activated. In Fig. 28, two Arrhenius plots for PMMA have been reproduced one from the mechanical P loss peak measurements and the other from the life-time measurements. The life-time plot is the same. Many authors have already associated the P loss peak with the fracture properties Here it is shown that the fibrils breakage itself is controll-... 

A quantitative explanation of the effect requires an advance in fracture mechanics. Griffiths theory explains fracture in a perfectly elastic material as dependent on crack depth. This has been extended to cover the situation where there is a small zone of plastic deformation ahead of the crack. The problem is more difficult when the pla.stic deformation is large compared to the crack size, and, as far as I know, there has been no treatment of the situation when plastic deformation covers the whole thickness of the specimen over an appreciable length. Any analysis would also require an understanding of the transition in material from crystalline yielding to looking and chain breakage and the form of the local stress-strain curve beyond that which is measured. 

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