Stresses other than temperature

Studies of salt inhibition of the halophilic shrub Kochia gave a different pattern of salt inhibition of metabolic rates [86]. Metabolic rates decrea.sed nearly linearly with increased salt concentration. Possibly this respon.se by the halophile reflects a continuous use of metabolic energy to pump out salt, as opposed to barley which may exclude salt until it is no longer successful and is injured. 

Salt inhibition of Chlorella has also been studied by calorimetric methods by Loseva et al. [87], Addition of low salt concentrations causes an increase in metabolic rate. The increased metabolic rate is not sufficient to offset energetic losses and growth rate of the Chlorella is reduced. 

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The common factor to the subjects considered in this chapter is that they arc all concerned with evaluating the resistance of materials to exposure to some environmental agent other than temperature (and air). Durability involves many factors mechanical, thermal, and electrical stresses including creep, fatigue, and abrasion. These can be difficult enough to measure and are greatly complicated by the effects of nonambient temperatures, but when the effects of environments are superimposed, the problems can increase in a quantum leap. 

Physical Factors. Unsatuiated elastomers must be stretched for ozone cracking to occur. Elongations of 3—5% are generally sufficient. Crack growth studies (10—18) have shown that some minimum force, called the critical stress, rather than a minimum elongation is required for cracking to occur. Critical stress values are neady the same for most unsaturated mbbers. However, polychloroprene has a higher critical stress value than other diene mbbers, consistent with its better ozone resistance. It has been found that temperature, plasticization, and ozone concentration have httie effect on critical stress values. 

Plastic Pipe In contrast to other piping materials, plastic pipe is free from internal and external corrosion, is easily cut and joined, and does not cause galvanic corrosion when coupled to other materials. Allowable stresses and upper temperature limits are low. Normal operation is in the creep range. Fluids for which a plastic is not suited penetrate and soften it rather than dissolve surface layers. Coefficients... 

A single bar (I) in these stress tables indicates that there are conditions other than stress which affect usage above or below the temperature as described in otlier referenced notes. A double bar (II) after a tabled stress indicates that use of the material is proliibited above that temperature. 

Since SCC is often dependent on environmental factors other than stress and exposure to a specific corrodent, appropriate alteration of these other factors may be effective. For example, a reduction in metal temperature, a change in pH, or a reduction in the levels of oxygen or oxidizing ions may reduce or eliminate SCC. 

It is important to note that the state determined by this analysis refers only to the pressure (or normal stress) and particle velocity. The material on either side of the point at which the shock waves collide reach the same pressure-particle velocity state, but other variables may be different from one side to the other. The material on the left-hand side experienced a different loading history than that on the right-hand side. In this example the material on the left-hand side would have a lower final temperature, because the first shock wave was smaller. Such a discontinuity of a variable, other than P or u that arises from a shock wave interaction within a material, is called a contact discontinuity. Contact discontinuities are frequently encountered in the context of inelastic behavior, which will be discussed in Chapter 5. 

Contact discontinuity A spatial discontinuity in one of the dependent variables other than normal stress (or pressure) and particle velocity. Examples such as density, specific internal energy, or temperature are possible. The contact discontinuity may arise because material on either side of it has experienced a different loading history. It does not give rise to further wave motion. 

Silver-copper-palladium alloys with liquidus temperatures of 800-1 000°C have very low vapour pressures combined with good wetting and flow characteristics and are widely employed in vacuum work. They exhibit a lower tendency to stress corrosion than silver-copper, and do not form brittle alloys with other metals. 

Although Bowman and Seery s results would, at first, seem to refute the suggestion by Fenimore that prompt NO forms by reactions other than the Zeldovich mechanism, one must remember that flames and shock tube-initiated reacting systems are distinctively different processes. In a flame there is a temperature profile that begins at the ambient temperature and proceeds to the flame temperature. Thus, although flame temperatures may be simulated in shock tubes, the reactions in flames are initiated at much lower temperatures than those in shock tubes. As stressed many times before, the temperature history frequently determines the kinetic route and the products. Therefore shock tube results do not prove that the Zeldovich mechanism alone determines prompt NO formation. The prompt NO could arise from other reactions in flames, as suggested by Fenimore. 

The stress-optical coefficient of PE networks is calculated, and results are compared with experimental data. Observed temperature coefficients of AT and the optical anisotropy for unswollen samples are much larger than those calculated using acceptable values of E(g), the energy of the gauche conformation, relative to that of Vans. It is concluded that observed temperature coefficients should Include some contributions other than those implied in the theory, i.e., those arising from the conformational change with temperature. 

Intergranular stress-corrosion cracking (IG-SCC) can occur in some sensitized materials when placed under tensile stress. Thus DL-EPR has been used to study the effects of aging time on the susceptibility of Alloy 600 to IGSCC, as shown in Fig. 41 (39). This work also shows the need to modify the experimental parameters of the test to achieve optimal correlation for alloys other than Type 304SS, in this case lowering the KCNS concentration and the temperature while raising the peak potential and the scan rate. 

Type 304 is listed because it Is the least costly of the acceptable materials. Other 300 series SS s may be needed for considerations other than low temperature. For example, low-carbon grades are desirable for seacoast environments to avoid intergranular stress corrosion cracking during the periods when the material is not at cryogenic temperature,... 

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