Strength properties environmental effects

The strength of wood can be altered by environmental agents. The changes in pH, moisture, and temperature the influence of decay, fire, and UV radiation and the adsorption of chemicals from the environment can have a significant effect on strength properties. Environmentally induced changes must be considered in any discussion on the strength of treated or untreated wood. 

Membrane materials usually get selected for building construction on the basis of properties such as mechanical strength, cost, flexibility, process-ability, cleaning behaviour, durability and resistance to UV light, fire, humidity and chemical attacks or other fouling behaviour, besides their ecological and environmental effects. 

Like mechanically fastened metal structures, composites exhibit failure modes in tension, shear and bearing but, because of the complex failure mechanisms of composites, two further modes are possible, namely cleavage and puUout. Environmental degradation of a bolted joint, after exposure to hot, wet environment is most likely to occur in the shear and bearing strength properties. The evidence shows that for fiber reinforced epoxies, temperature has a more significant effect than moisture, but in the presence of both at 127°C, a strength loss of up to 40 percent is possible. 

Strength reduction factors, environmental effects, and the effects of aging on short-term properties could also be revisited. The subsequent subsections briefly describe the current activities in this regard in ASME and JSME. 

The environmental effects on mechanical properties once again demonstrate that the interrelationships between various mechanical properties are material specific and rely on many factors, from temperature to humidity. A higher yield strength for one alloy under one environmental condition might imply better fatigue resistance however, specific supporting data are required to draw any inference to any other alloy in different circumstances. In summary, the demonstration of equivalent material durability and part life limitation in reverse engineering requires part-specific substantiation data. 

Although thermal performance is a principal property of thermal insulation (13—15), suitabiHty for temperature and environmental conditions compressive, flexure, shear, and tensile strengths resistance to moisture absorption dimensional stabiHty shock and vibration resistance chemical, environmental, and erosion resistance space limitations fire resistance health effects availabiHty and ease of appHcation and economics are also considerations. 

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. 

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