Elastic Environmental

In contrast, various sensors are expected to respond in a predictable and controlled manner to such diverse parameters as temperature, pressure, velocity or acceleration of an object, intensity or wavelength of light or sound, rate of flow, density, viscosity, elasticity, and, perhaps most problematic, the concentration of any of millions of different chemical species. Furthermore, a sensor that responds selectively to only a single one of these parameters is often the goal, but the first attempt typically produces a device that responds to several of the other parameters as well. Interferences are the bane of sensors, which are often expected to function under, and be immune to, extremely difficult environmental conditions. 

As an example, for room-temperature applications most metals can be considered to be truly elastic. When stresses beyond the yield point are permitted in the design, permanent deformation is considered to be a function only of applied load and can be determined directly from the stress-strain diagram. The behavior of most plastics is much more dependent on the time of application of the load, the past history of loading, the current and past temperature cycles, and the environmental conditions. Ignorance of these conditions has resulted in the appearance on the market of plastic products that were improperly designed. Fortunately, product performance has been greatly improved as the amount of technical information on the mechanical properties of plastics has increased in the past half century. More importantly, designers have become more familiar with the behavior of plastics rather than... 

Material behavior have many classifications. Examples are (1) creep, and relaxation behavior with a primary load environment of high or moderate temperatures (2) fatigue, viscoelastic, and elastic range vibration or impact (3) fluidlike flow, as a solid to a gas, which is a very high velocity or hypervelocity impact and (4) crack propagation and environmental embrittlement, as well as ductile and brittle fractures. 

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. 

It is well known that LCB has a pronounced effect on the flow behavior of polymers under shear and extensional flow. Increasing LCB will increase elasticity and the shear rate sensitivity of the melt viscosity ( ). Environmental stress cracking and low-temperature brittleness can be strongly influenced by the LCB. Thus, the ability to measure long chain branching and its molecular weight distribution is critical in order to tailor product performance. 

Tensile Modulus. Tensile samples were cut from the 0.125 in. plates of the compositions according to Standard ASTM D638-68, into the dogbone shape. Samples were tested on an Instron table model TM-S 1130 with environmental chamber. Samples were tested at temperatures of -30°C, 0°C. 22°C, 50°C, 80°C, 100°C and 130°C. Samples were held at test temperature for 20 minutes, clamped into the Instron grips and tested at a strain rate of 0.02 in./min. until failure. The elastic modulus was determined by ASTM D638-68. Second order polynomial equations were fitted to the data to obtain the elastic modulus as a function of temperature for each of the compositions. 

Fibers (qv) have been defined by the Textile Institute as units of matter characterized by flexibility, fineness, and a high ratio of length to thickness (3). For use in textile applications, fibers should have adequate temperature stability, strength, and extensibility. Other important qualities include cohesiveness or spinability and uniformity. There are also several secondary characteristics that improve customer satisfaction and therefore may be desirable. These include cross-sectional shape, specific gravity or density, moisture regain, resiliency, luster, elastic recovery, and resistance to chemicals, environmental conditions, and biological organisms. 

The need to maintain elasticity of rubber is of paramount importance under any serious and severe environmental conditions. The most stable rubbers in radiation environments are polyurethanes and phenyl silaxanes which are usable at well above 108 rads (106 Gy). Butyl rubber liquefies and neoprene evolves hydrochloric acid at similar dose levels. Most polyurethane rubber foams can be used at a dose level of 109 rads (107Gy) in vacuum at temperature levels of between -85°C to +250°C. Silicone and polysulphide sealants are probably less tolerant to ionizing radiation in a nuclear plant where chemical processes are being carried out. A schematic graphical representation of the tolerance of rubbers to ionizing radiation in nuclear plant is shown below in figure 7.4. 

The physical properties of barrier dressings were evaluated using the Seiko Model DMS 210 Dynamic Mechanical Analyzer Instrument (see Fig. 2.45). Referring to Fig. 2.46, dynamic mechanical analysis consists of oscillating (1 Hz) tensile force of a material in an environmentally (37°C) controlled chamber (see Fig. 2.47) to measure loss modulus (E") and stored modulus (E ). Many materials including polymers and tissue are viscoelastic, meaning that they deform (stretch or pull) with applied force and return to their original shape with time. The effect is a function of the viscous property (E") within the material that resists deformation and the elastic property (E )... 

Reinforcements in the form of continuous fibres, short fibres, whiskers or particles are available commercially. Continuous ceramic fibres are very attractive as reinforcements in high-temperature structural materials. They provide high strength and elastic modulus with high temperature-resistant capability and are free from environmental attack. Ceramic reinforcement materials are divided into oxide and non-oxide categories, listed in Table 3.1. The chemical compositions of some commercially available oxide and non-oxide reinforcements are given in Table 3.2 and Table 3.3. 

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