Thermal softening effect

A uPVC rod of diameter 12 mm is subjected to an eccentric axial force at a distance of 3 ttun from the centre of the cross-section. If the force varies sinusoidally from — F to f at a frequency of 10 Hz, calculate the value of F so that fatigue failure will not occur in 10 cycles. Assume a safety factor of 2.5 and use the creep rupture and fatigue characteristics described in the previous question. Thermal softening effects may be ignored at the stress levels involved. 

On the basis of existing studies on the coupled HM process Wu Wenhua et al. (2002) proposed a constitutive model for coupled THM behaviour of unsaturated soils. The thermal softening effects, defined to consider the decrease in value of the pre-consolidation pressure and decrease in critical value of the suction of the SI (suction increasing), are introduced, based on the formulation of a CAP model. 

Bai [48] presents a linear stability analysis of plastic shear deformation. This involves the relationship between competing effects of work hardening, thermal softening, and thermal conduction. If the flow stress is given by Tq, and work hardening and thermal softening in the initial state are represented... 

Assuming that the cyclic waveform used in the previous section was sinusoidal then the effect of using a square wave is to reduce, at any frequency, the level of stress amplitude at which thermal softening failures start to occur. This is because there is a greater energy dissipation per cycle when a square wave is used. If a ramp waveform is applied, then there is less energy dissipation per cycle and so higher stresses are possible before thermal runaway occurs. 

Bourne, M. C. (1987). Effect of blanch temperature on kinetics of thermal softening of carrots and green beans. 

All of the chemical evidence that can be marshalled indicates that wood fiberboard manufacture exploits the thermoplastic properties of lignin. Defibering is effected by the thermal softening of lignin in the middle lamella at saturated steam pressures above 130C. Interfelted fiber mats are consolidated with or without densification pressure by the thermoplastic fusion of lignin-rich fiber surfaces at high board conversion temperatures. 

Boric acid in conjunction with APP was reported in epoxy intumescent coating.30-31 Boric acid and its derivatives were used in phenolics to impart thermal stability and tire retardancy. For example, Nisshin steel claims the use of boric acid and aluminum trihydroxide (ATH) in phenolics for sandwich panel.32 It was also reported that the small amounts of boric acid (around 0.25% by weight) in polyether imide (PEI) and glass-filled and PEI can reduce peak HRR by almost 50% in the OSU Heat Release test for the aircraft industry.33 In applications where high modulus and high strengths are needed, boric acid can be added without the softening effects of other additives such as siloxanes. 

In both these regimes of wear, high rates of power dissipation in the contact first lead to surface and then subsurface thermal softening. These thermal effects generally limit the maximum PV (pressure-velocity) product of polymeric contacts. Furthermore,... 

A very significant observation is the stress softening effect, also called the Mullins effect (Mullins and Tobin, 1965 Mullins, 1969). In this experiment, a compound sample is stretched to ei and returned to zero strain, then stretched again. For strain below si, its stress-strain curve is significantly below the first one but rejoins it at si. Stress softening is dependent on the initial strain level it can be partially reduced by thermal treatment but not be totally effaced (Figure 8.7). 

Polymers are generally processed in the molten or thermally softened states. The former represent semicrystalline polymers above the melting point, whereas the latter are amorphous polymers well above their glass temperature. The difference between them can be seen in Fig. 3-13, which represents specific heat (C ) data for both types. As can be seen, the semicrystalline materials peak at a given temperature (i.e., have a heat of fusion), but the thermaUy softened types show no such effect. 

Elastic effects can and do occur in molten or thermally softened polymer systems. This is a case of the fluid having both the characteristics of a Newtonian liquid and a Hookean elastic solid. The elastic effects will generally manifest themselves in entrance or exit situations. Hence, they can become important in capillaries. However, it should be pointed out that normally elastic effects are compensated for in the entrance length correction. 

From the foregoing, it can be seen that the effect of expansion cooling reaches a maximum where viscous dissipation reaches a minimum, and vice versa. This, together with the fact that a molten or thermally softened polymer is a poor heat... 

A set of cooling profile curves for the thermally softened polymer are given in Figs. 4-46-4-48. Once again, the effect of viscous dissipation is considerable although not apparent. This will be considered later under a discussion of Nus-selt number behavior. 

As has been pointed out, viscoelastic behavior is frequently associated with flowing polymeric systems. Usually, such behavior is especially pronounced in molten or thermally softened polymers. Elastic effects should be contrasted with the viscous effects (i.e., viscous heating). In the latter case, the energy necessary to move the fluid is dissipated as heat. In the former instance, however, energy put into an elastic system can be recovered. This suggests the possibility of a form of energy storage in a viscoelastic system. 

Fig. 4-44 Effect of viscous dissipation on heat transfer to a thermally softened polymer [44]. B is modified Brinkman number.

Permeabilities apply mainly to molecular transport in solid polymers, particularly polymer films. Also, diffusivities are usually obtained in conjunction with permeability measurements. Although such behavior is obviously important in many applications, it relates only indirectly to processing that occurs mainly with molten or thermally softened polymers. The principal role of processing with respect to permeabilities and diffusivities in solid polymers is development of the material s structural characteristics which, in turn, affect properties. As such, consideration of permeability will be deferred to Chapter 12, which deals with the effect of processing or structural parameters. 

As indicated earlier, the limiting effect of the diffusion of a gas into a molten or thermally softened polymer is its solution. Further, such behavior can be expressed in the form of Henry s law [Eq. (5-5)]. 

The flow of the molten or thermally softened polymer mass into the mold cavity is, as previously indicated, a complicated matter. A schematic diagram of the flow pattern is shown in Fig. 8-18. Several aspects of the process merit discussion. First of all, the flow is complicated by the formation of a solidified polymer layer [15-17] at and near the mold wall. Next, the molten or thermally softened polymer flows from the cavity centerline to the front of the mass and then outward toward the wall. This behavior is termed a fountain effect [14,15]. 

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