Strength environmental factors

General description. Incomplete penetration describes the condition in which the weld fails to reach the bottom of the weld joint, resulting in a notch located at the root of the weld (Fig. 15.12). This critical defect can substantially reduce the intrinsic mechanical strength of the joint and can combine with environmental factors to produce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). 

Environments. Among the environmental factors that can shorten life under thermal fatigue conditions are surface decarburization, oxidation, and carburization. The last can be detrimental because it is likely to reduce both hot strength and ductility at the same time. The usual failure mechanism of heat-resistant alloy fixtures in carburizing furnaces is by thermal fatigue damage, evidenced by a prominent network of deep cracks. 

Designers of most structures specify material stresses and strains well within the pro-portional/elastic limit. Where required (with no or limited experience on a particular type product materialwise and/or process-wise) this practice builds in a margin of safety to accommodate the effects of improper material processing conditions and/or unforeseen loads and environmental factors. This practice also allows the designer to use design equations based on the assumptions of small deformation and purely elastic material behavior. Other properties derived from stress-strain data that are used include modulus of elasticity and tensile strength. 

So far, we have focused on how differences in molecular structure affect the solubilities and activity coefficients of organic compounds in pure water at 25°C. The next step is to evaluate the influence of some important environmental factors on these properties. In the following we consider three such factors temperature, ionic strength (i.e., dissolved salts), and organic cosolutes. The influence of pH of the aqueous solution, which is most important for acids and bases, will be discussed in Chapter 8. 

Several factors must be considered when the experimental assay condi tions are developed. The reaction rate depends on the concentrations ol substrate, enzyme, and necessary cofactors. In addition, the reaction rate is under the influence of environmental factors such as pH, temperature, anti ionic strength. Enzyme activity increases with increasing temperature until the enzyme becomes denatured. The enzyme activity then decreases until all enzyme molecules are inactivated by denaturation. During kinetic mea surement, it is essential that the temperature of all reaction mixtures lx maintained constant. 

The conditions used in an enzyme assay depend on what is to be accomplished by the assay. There are two primary applications of an enzyme assay procedure. First, it may be used to measure the concentration of active enzyme in a preparation. In this circumstance, the measured rate of the enzyme-catalyzed reaction must be proportional to the concentration of enzyme stated in more kinetic terms, there must be a linear relationship between initial rate and enzyme concentration (the reaction is first order in enzyme concentration). To achieve this, certain conditions must be met (1) the concentrations of substrate(s), cofactors, and other requirements must be in excess (2) the reaction mixture must not contain inhibitors of the enzyme and (3) all environmental factors such as pH, temperature, and ionic strength should be controlled. Under these conditions, a plot of enzyme activity (p-rnole product formed/minute) vs. enzyme concentration is a straight line and can be used to estimate the concentration of active enzyme in solution. 

The functionality of a protein is dictated by the molecular properties of the protein as modified by processing treatments, environmental factors, and interactions with other components. Environmental conditions, such as pH, ionic strength, type of salts, moisture content, and oxidation-reduction potential, may alter the functional properties of a protein in a food. Protein functional properties are also influenced by unit operations during processing... 

The rate of an enzymatic reaction is affected by a number of environmental factors, such as solvent, ionic strength, temperature, pH, and presence of inhibitor/activator. Some of these effects are described below. 

It may be deduced from KP = Koc x foc that partition coefficients of hydro-phobic organic compounds in general are dependent upon the chemical of interest (compound-specific properties affect the value of Koc) and the matrix properties of the medium in which it resides. In addition to the fraction of organic carbon present in the sorption phase, additional environmental factors affect partitioning. These factors include temperature, particle size distribution, the surface area of the sorbent, pH, ionic strength, the presence of suspended material or colloidal material, and the presence of surfactants. In addition, clay minerals may act as additional sorption phases for organic compounds. Nevertheless, organic carbon-normalized partition... 

Environmental factors such as pH, ionic strength, and temperature affect enzyme activity. They must be controlled when making in vitro measurements of enzyme activity and heeded in vivo when abnormal conditions such as acidosis, alkalosis, and fevers may exist. 

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