ABSTRACT
This chapter deals with elastic deformation as representing a very good approximation of what happens in practice. Timber may undergo dimensional changes solely on account of variations in climatic factors; on the other hand, deformation may be due solely to the effects of applied stress. Frequently stress and climate interact to produce enhanced levels of deformation. When a sample of timber is loaded in tension, compression or bending, the instantaneous deformations are approximately proportional to the values of the applied load. The material may still behave elastically, though not linearly elastically; the prime criterion for elastic behaviour is that the stress–strain curve is truly reversible, that is no permanent deformation occurs on release of the load. However, where stresses are applied for a period of time, viscoelastic behaviour will be experienced and, while it is possible to apply elasticity theory with a factor covering the increase in deformation with time, this procedure is at best only a first approximation.
