Wood strength, environmental effects

There are two basic categories of acceptance standards performance standards and product specifications. Performance standards focus on the ability of a material, component or assembly to resist the loads or environmental effects of its intended application. Product specifications focus on aspects of material quality, which may affect strength, appearance and durability. In some instances it may be possible to use existing standards directly with newly developed products and materials in other cases it may be necessary to modify existing standards or develop new standards to assure equitable evaluation. Consensus committees comprising producer, consumer, and user groups develop performance standards, which are used to evaluate the engineering performance of wood-based panels, such as hardboard, MDF and particleboard. This standard was used because no standard exists for the evaluation of woodfibre-plastic panel materials. 

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. 

Growth Characteristics. As a fibrous product from living trees, wood is subjected to many environmental influences as it is formed and during its lifetime. These environmental influences can increase the variability of the wood material and, thus, increase the variability of the mechanical properties. To reduce the effect of this inherent variability, standardized testing procedures using small, clear specimens of wood are often used. Small, clear specimens do not have knots, checks, splits, or reaction wood. However, the wood products used and of economic importance in the real world have these defects. Strength estimates derived from small clear specimens are reported because most chemical treatment data have been generated from small clear specimens. 

C A Wood and W L Bradley, Determination of the effect of sea-water on the interfacial strength of an interlayer E-glass/graphite/epoxy composite by in-situ observation of transverse cracking in an environmental SEM , Compos Sci Technol 1997 57 1033-1043. 

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