Plastic behaviours

Polymer elasticity is determined by the reversible deformation of the chain molecules as we saw in the previous section. Polymers can also deform plastically, with chain molecules sliding past each other over large distances as 

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Load-extension curves for non-elastic (plastic) behaviour... 

The plastic behaviour of a material is usually measured by conducting a tensile test. Tensile testing equipment is standard in all engineering laboratories. Such equipment produces a load/displacement (F/u) curve for the material, which is then converted to a nominal stress/nominal strain, or cT l , curve (Fig. 8.10), where... 

Cellular materials can collapse by another mechanism. If the cell-wall material is plastic (as many polymers are) then the foam as a whole shows plastic behaviour. The stress-strain curve still looks like Fig. 25.9, but now the plateau is caused by plastic collapse. Plastic collapse occurs when the moment exerted on the cell walls exceeds its fully plastic moment, creating plastic hinges as shown in Fig. 25.12. Then the collapse stress (7 1 of the foam is related to the yield strength Gy of the wall by... 

The free volume model seems to be more adequate to describe the plasticization behaviour of the systems of lower amine content. According to Eq. (5), the higher is the change of the expansion coefficient the lower is the influence of the diluent volume fraction. The three TGDDM-DDS mixtures cured with 20, 30 and 50 PHR of hardener were characterized l2) by changes of the expansion coefficient at the glass transition, respectively, of 0.63, 1.08 and 2.94x 10 3 °C l. The more dense and stiffer resin crosslinked with 50 PHR of DDS should be, in principle, the less... 

The zinc polycarboxylate cement sets within a few minutes of mixing and hardens rapidly. Strength is substantially developed within an hour. However, even when fully hardened the cement exhibits marked plastic behaviour. Its most important property is its ability to bond permanently to untreated dentine and enamel. 

These cements have marked creep characteristics and flow under pressure even when fully set. In this they contrast markedly with the rigid phosphate cements (Wilson Lewis, 1980). This plastic behaviour explains why such cements provide a good seal despite a high setting shrinkage and thermal expansion of 35 x 10 °C" (Civjan Brauer, 1964). 

Mechanical properties per se concerns with the qualities which determine the behaviour of a material towards applied forces. The ability to support weight without rupture or permanent deformation, to withstand impact without breaking, to be mechanically formed into different shapes - all these depend upon a combination of mechanical properties characteristic of metals. Four types of behaviour of a material under stress are very important linear or elastic behaviour, plastic behaviour, creep behaviour and fatigue behaviour. 

In a further development of the continuous chain model it has been shown that the viscoelastic and plastic behaviour, as manifested by the yielding phenomenon, creep and stress relaxation, can be satisfactorily described by the Eyring reduced time (ERT) model [10]. Creep in polymer fibres is brought about by the time-dependent shear deformation, resulting in a mutual displacement of adjacent chains [7-10]. As will be shown in Sect. 4, this process can be described by activated shear transitions with a distribution of activation energies. The ERT model will be used to derive the relationship that describes the strength of a polymer fibre as a function of the time and the temperature. 

The transition from ideal elastic to plastic behaviour is described by the change in relaxation time as shown by the stress relaxation in Fig. 66. The immediate or plastic decrease of the stress after an initial stress cr0 is described by a relaxation time equal to zero, whereas a pure elastic response corresponds with an infinite relaxation time. The relaxation time becomes suddenly very short as the shear stress increases to a value equal to ry. Thus, in an experiment at a constant stress rate, all transitions occur almost immediately at the shear yield stress. This critical behaviour closely resembles the ideal plastic behaviour. This can be expected for a polymer well below the glass transition temperature where the mobility of the chains is low. At a high temperature the transition is a... 

Fig. 66 The elastic, viscoelastic and plastic behaviour can be described by changing the relaxation time <5...

Figure 1.5 (a) Power law behaviour from a 12% polyvinyl pyrrolidone solution (b) Bingham plastic behaviour from a 14% w/v sodium kaolinite dispersion... 

Metals and glass generally support higher temperatures than polymers, which present a more or less plastic behaviour under stresses, leading to ... 

TPOs are handicapped by a weak elasticity and a marked plastic behaviour even at ambient temperature risks of creep, relaxation and permanent set, the more so the higher the temperature rises (high compression set at 70°C for certain grades) swelling in mineral oils and oily products limited thermal behaviour. 

The term "clay" refers to in nature present materials which mainly consist of fine granular minerals, which generally exhibit plastic behaviour at certain water contents and which harden after drying or baking. Although clay usually consists of phyllosilicates, it can also contain other materials which do... 

The reason why clay exhibits plastic behaviour can be found in the packing of clay sheets with water molecules and hydrated ions in between. At the surface of such a clay sheet the water molecules are tightly bound by means of H-bridges between the water molecules and the surface charge of the sheet. Further away from the sheet the water molecules are interconnected by means of H-bridges but these forces are weaker and consequently those water molecules can move more easily than the ones which are bound to the clay sheet. In that movement both the loose water molecules and the hydrated ions act as a lubricant between the sheets and can move parallelly in relation to each other the plastic behaviour (figure 9.9). 

Ceramic materials have strong ionic or covalent bonds and that is why sliding processes as occur in metals are not or only slightly possible. They lack the necessary plastic behaviour. 

Comparison of Plastic Behaviours of CMIMx and MGIMx Copolymers... 

It is interesting to analyse the effect of comonomer nature on the plastic behaviour of the copolymers. 

Finally, in BPA-PC, intermolecular cooperativity controls the temperature (and pressure) dependence of the plastic behaviour, in particular the strain softening amplitude reflected in nSSA. 

Thixotropy is the time-dependent analogue of shear-thinning and plastic behaviour, and arises from somewhat similar causes. If a thixotropic system is allowed to stand and is then sheared at a constant rate, the apparent viscosity decreases with time until a balance between structural breakdown and structure re-formation is reached. If the sheared system is then allowed to stand, it eventually regains its original structure. A thixotropic hysteresis loop (Figure... 

From Fig. 3g, the slope is fip = 6.4 Ns/m2 and the graph confirms Bingham plastic behaviour. 

In this paper the recently developed techniques to characterize the mar resistance of coating systems were presented. The techniques base on methods that create a single scratch onto a surface. Characteristic values like the critical load as a measure for the transition from plastic behaviour to brittle fracture can be determined and used to rank different clearcoat systems and to compare these results with other physical properties. In the field of mar resistance the cross-linking density of the... 

Fig. 5a,b Schematic representation of a the tip-sample contact upon high loading b the according compliance curve. In the case of perfectly plastic response the unloading curve is identical to the vertical line intersecting with the abscissa at hmax. In general, some viscoelastic recovery occurs and the residual impression depth hy is smaller than hmax. The difference hc—hy represents the extent of viscoelastic recovery. Ap and Ae denote the dissipated and the recovered work, respectively. Ap=0 for perfect elastic behaviour, whereas Ae=0 for perfect plastic behaviour. The viscoelastic-plastic properties of the material may be described by the parameter Ap(Ap+Ae) l. The contact strain increases with the attack angle 6. Adapted from [138]... 

Only in the last three years have some researchers begun to pay their attention to the structured surface and to parameters influencing the lithography with force-displacement curves (FDI) [266-268]. The big advantage of FDI is the possibility of gaining knowledge about the whole indentation process during FDI, the force and the indentation are known at every point, and not only stiffness and hardness, but also other important properties such as density, elasto-plastic behaviour, adhesion, time behaviour, etc. can be measured and calculated. 

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