Short-term mechanical properties

The above data represent the first from composites fabricated with fixed catalyst VGCF. A review of the data leads to the conclusion that the thermal and electrical properties of this type of carbon fiber are perhaps the most likely to be exploited in the short term. While mechanical properties of the composites are not as attractive as the thermal and electrical, it may be noted that no effort has... 

Strength and Stiffness. Thermoplastic materials are viscoelastic which means that their mechanical properties reflect the characteristics of both viscous liquids and elastic solids. Thus when a thermoplastic is stressed it responds by exhibiting viscous flow (which dissipates energy) and by elastic displacement (which stores energy). The properties of viscoelastic materials are time, temperature and strain rate dependent. Nevertheless the conventional stress-strain test is frequently used to describe the (short-term) mechanical properties of plastics. It must be remembered, however, that as described in detail in Chapter 2 the information obtained from such tests may only be used for an initial sorting of materials. It is not suitable, or intended, to provide design data which must usually be obtained from long term tests. 

Viscoelastic and rate theory To aid the designer the viscoelastic and rate theories can be used to predict long-term mechanical behavior from short-term creep and relaxation data. Plastic properties are generally affected by relatively small temperature changes or changes in the rate of loading application. 

All the mechanical properties described in tests for the conventional data sheet properties represented values of short-term application of forces, and, in most cases, the data obtained from such tests are used for comparative evaluation or as controlling specifications for quality determination of materials along with short-duration and intermittent-use design requirements. 

Plastics have the widest variety and range of mechanical properties of all materials (Figs. 1-8 and 7-1 and 7-2). They vary from basically soft to hard, rigid solids. Great many structural factors determine the nature of their mechanical behavior, such as whether a load occurs over the short term or the long... 

Fluoroelastomers Novikova et al. [32] reported unproved physico-mechanical properties of fluoro mbbers by reinforcement with chopped polyamide fibers. Other fiber reinforcements are covered by Grinblat et al. [33]. Watson and Francis [34] described the use of aramid (Kevlar) as short fiber reinforcement for vulcanized fluoroelastomer along with polychloroprene mbber and a co-polyester TPE in terms of improvement in the wear properties of the composites. Rubber diaphragms, made up of fluorosilicone mbbers, can be reinforced using aramid fiber in order to impart better mechanical properties to the composite, though surface modification of the fiber is needed to improve the adhesion between fluorosUicone mbber and the fiber [35]. Bhattacharya et al. [36] studied the crack growth resistance of fluoroelastomer vulcanizates filled with Kevlar fiber. 

PBT will absorb very little water (0.08 %), and its mechanical properties are not affected in the short term. Polyamides, on the other hand, may absorb up to 12% of water. In nylon resins, the water acts as a plasticizer it lowers the Tg, decreases the flexural modulus, and may cause part growth. Based on these criteria alone, polyesters are often a better choice than nylons for many applications (less variation of properties). 

The stability of membranes against thermomechanical and chemical stresses is an important factor in determining both their short- and long-term performance. Transport and mechanical properties of membranes affect the fuel cell performance, while the lifetime of a fuel cell is mostly dependent on the thermomechanical and chemical stability of the membrane. Thermomechanical and chemical degradation of a membrane will result in a loss of conductivity, as well as mixing of anode and cathode reactant gases. 

One feature of the response to oxidants (in particular, ozone) that has stimulated considerable interest is the apparent development of tolerance to the acute effects of short-term exposure to these agents in laboratory animals. Fairchild reviewed possible mechanisms of this phenomenon. Tolerance has been defined as the increased capacity of an organism that has been pre-exposed to oxidant to resist the effects of later exposures to ordinarily lethal (or otherwise injurious) doses of the same agent or of different agents (cross-tolerance) with similar toxicologic properties. 

Polymeric materials used for electrical insulation not only have to exhibit good electrical properties, but good mechanical properties and good appearance are also important. In addition to reliable electrical performance, thermal stability is important, mainly for miniaturized electronic and electrical equipment. Long-term stability at temperatures in the range between 90 to 125°C (194 to 257°F) is required and the short-term stability determines the resistance to melted solder and cut-through resistance. 

It is necessary to appreciate that the degree of degradation seen will vary with the property used to monitor the ageing. As with tests to measure the short term effect of temperature, ideally, the properties most relevant to service should be used to monitor the effect of ageing, but sensitivity to the degradation, reproducibility, cost, relation to fundamental changes at the molecular level and general applicability are also relevant. In practice, the simpler mechanical tests are most often used as a matter of convenience. 

From a practical point of view, the main consequence of physical ageing by structural relaxation is embrittlement (decrease in fracture resistance Chapter 12). For the other aspects of mechanical behavior, ageing has either no effect or a favourable effect (increase of relaxation times, leading to a decrease of creep or relaxation rates). This is the reason why, in most thermoset applications, the knowledge of short-term properties is considered to be sufficient for engineering design, as far as fracture and durability are not concerned. 

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