Elastic strain versus plastic deformation

Figure 7.2. Definitions of stress, strain, and modulus. Stress is defined as force per unit area, and strain is the change in length divided by the original length. When stress is plotted versus strain, then the slope is the modulus (A). When the load is removed, any strain remaining is called permanent or plastic deformation (B). When elastic materials are loaded, they are characterized by a constant strain as a function of time, whereas viscoelastic materials have strains that increase with time (C).

Figure 2-34. Elasticity and strain, a) Basic deformation versus the time curve b) stress-strain deformation versus time (the creep effect) c) stress-strain deformation versus time (the stress-relaxation effect) d) material exhibiting elasticity and e) material exhibiting plasticity.

Fig. 7.10. Stress versus strain characteristics of three lap joints and an unbonded test coupon. Joint A, 25 mm X 12 5 mm overlap B, 25 mm X 190 mm overlap C, 25 mm X 25 0 mm overlap. Adherend mild steel, 16 gauge. Adhesive heat cured toughened epoxide (Permabond ESP105). Test coupon 25 X 75 mm, 16 gauge mild steel of the type used to fabricate the joints represented by continuous line. The classical form of the elastic/plastic deformation of the unbonded test coupon is clearly seen. (A) This specimen does not fail until after the test coupon has become plastic. (B) Although possessing 50% more bond area, the load required to fail specimen B is not very much greater than that needed to fail A however, the toughness of the adhesive and the spare capacity of the initially unloaded central area are clearly illustrated by the ability of the joint to sustain a load even though it has cracked. (C) This example emphasises the point made with specimen B. A performance such as this makes it difficult to say whether the adhesive has failed the steel or the steel has failed the adhesive.

Figure 2.3 shows deformation curves plotted in tensile strain versus tensile stress coordinates. The endpoints on the curves conform to the time of sample rupture with respective stress and strain. The elasticity modulus of the samples was calculated by the tangent of the angle of slope of the initial segments of the deformation curves. The deformation curves of filled SAN are characteristic of plastics with brittle failure. Nonfilled SAN exhibits considerable deformations. As seen in Figure 2.3 and Table 2.4, addition of up to 10% diamond carbon to SAN... 

As a plastic is subjected to a fixed stress or strain, the deformation versus time curve will show an initial rapid deformation followed by a continuous action. Examples of the standard type tests are included in Fig. 2-1. Details on using these type specimens under static and dynamic loads will be reviewed throughout this chapter. (Review also Fig. 8-9 that relates elasticity to strain under different conditions.)... 

According to the change of strain rate versus stress the response of the material can be categorized as linear, non-linear, or plastic. When linear response take place the material is categorized as a Newtonian. When the material is considered as Newtonian, the stress is linearly proportional to the strain rate. Then the material exhibits a non-linear response to the strain rate, it is categorized as Non Newtonian material. There is also an interesting case where the viscosity decreases as the shear/strain rate remains constant. This kind of materials are known as thixotropic deformation is observed when the stress is independent of the strain rate [2,3], In some cases viscoelastic materials behave as rubbers. In fact, in the case of many polymers specially those with crosslinking, rubber elasticity is observed. In these systems hysteresis, stress relaxation and creep take place. 

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