Temperature dependence, yield strength

In place of a complete evolutionary expression of normal softening and yield-strength recovery, Swegle and Grady [13] propose a temperature-dependent yield strength F(7]) in qualitative agreement with expected behavior... 

Measurements of stress relaxation on tempering indicate that, in a plain carbon steel, residual stresses are significantly lowered by heating to temperatures as low as 150°C, but that temperatures of 480°C and above are required to reduce these stresses to adequately low values. The times and temperatures required for stress reUef depend on the high temperature yield strength of the steel, because stress reUef results from the localized plastic flow that occurs when the steel is heated to a temperature where its yield strength is less than the internal stress. This phenomenon may be affected markedly by composition, and particularly by alloy additions. 

Sufficient tensile stress. Sufficiency here is difficult to define since it depends on a number of factors such as alloy composition, concentration of corrodent, and temperature. In some cases, stresses near the jdeld strength of the metal are necessary. In other cases, the stresses can be much lower. However, for each combination of environment and alloy system, there appears to be a threshold stress below which SCC will not occur. Threshold stresses can vary from 10 to 70% of yield strength depending on the alloy and environment combination and temperature (Fig. 9.6). 

Many properties are temperature dependent. For example up to 100°C the yield stress drops with temperature at a faster rate than does the yield stress of polypropylene however, it retains some strength up to 160°C. 

As shown in Sect. 2, the fracture envelope of polymer fibres can be explained not only by assuming a critical shear stress as a failure criterion, but also by a critical shear strain. In this section, a simple model for the creep failure is presented that is based on the logarithmic creep curve and on a critical shear strain as the failure criterion. In order to investigate the temperature dependence of the strength, a kinetic model for the formation and rupture of secondary bonds during the extension of the fibre is proposed. This so-called Eyring reduced time (ERT) model yields a relationship between the strength and the load rate as well as an improved lifetime equation. 

For the investigation of the time and the temperature dependence of the fibre strength it is necessary to have a theoretical description of the viscoelastic tensile behaviour of polymer fibres. Baltussen has shown that the yielding phenomenon, the viscoelastic and the plastic creep of a polymer fibre, can be described by the Eyring reduced time (ERT) model [10]. The shear deformation of a domain brings about a mutual displacement of adjacent chains, the... 

Ry Ratio of the average temperature-dependent trend curve value of yield strength to the room temperature yield strength ... 

If up to 40% of ESI is blended with LDPE then foamed, the foam properties are closer to those of LDPE foams. Ankrah and co-workers (33) showed that the ESI/LDPE blends have slightly lower initial compressive yield strengths than the LDPE alone, allowing for the density of the foam. The temperature dependence of the yield stress is similar to that of LDPE foam (Figure 3). Although the yield stress is higher than EVA foam of the same density, the compression set values are lower. The ESI/LDPE foams have improved impact properties, compared with EVA foams of similar density. Analysis of creep tests shows that air diffuses from the cells at a similar rate to EVA foams of a greater density. 

In Figure 10.1 the time course of thermodynamically and kinetically controlled processes catalysed by biocatalysts are compared. The product yield at the maximum or end point is influenced by pH, temperature, ionic strength, and the solubility of the product. In the kinetically controlled process (but not in the thermodynamically controlled process) the maximum yield also depends on the properties of the enzyme (see next sections). In both processes the enzyme properties determine the time required to reach the desired end point. The conditions under which maximum product yields are obtained do not generally coincide with the conditions where the enzyme has its optimal kinetic properties or stability. The primary objective is to obtain maximum yields. For this aim it is not sufficient to know the kinetic properties of the enzyme as functions of various parameters. It is also necessary to know how the thermodynamically or the kinetically controlled maximum is influenced by pH, temperature and ionic strength, and how this may be influenced by the immobilization of the biocatalysts on different supports. 

Eigure 5.70 shows plots of (Xy/T against log Sy for polycarbonate for a series of temperatures between 21.5°C and 140°C, where the Tg of polycarbonate is 160°C. Note that the predictions of the Eyring model are obeyed, namely, that Oy/T increases linearly with log By, and at constant log Sy,OylT increases with decreasing temperature. Thus, the yield strength is dependent on both strain rate and temperature. 

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