ABSTRACT
This chapter is concerned with the type and magnitude of the deformation that results from the application of an external load. As in the case of both concrete and polymers, the load—deformation relationship in timber is exceedingly complex, resulting from these facts:
Timber does not behave in a truly elastic mode, rather its behaviour is time dependent.
The magnitude of the strain is influenced by a wide range of factors. Some of these are property dependent, such as density of the timber, angle of the grain relative to direction of load application and angle of the microfibrils within the cell wall. Others are environmentally dependent, such as temperature and relative humidity.
Under service conditions timber often has to withstand an imposed load for many years, perhaps even centuries; this is particularly relevant in construction applications. When loaded, timber will deform and a generalised interpretation of the variation of deformation with time together with the various components of this deformation is illustrated in Figure 6.1. On the application of a load at time zero an instantaneous (and reversible) deformation occurs which represents true elastic behaviour. On maintaining the load to time t 1 the deformation increases, though the rate of increase is continually decreasing; this increase in deformation with time is termed creep. On removal of the load at time t 1 an instantaneous reduction in deformation occurs which is approximately equal in magnitude to the initial elastic deformation. With time, the remaining deformation will decrease at an ever-decreasing rate until at time t 2 no further reduction occurs. The creep that has occurred during loading can be conveniently subdivided into a reversible component, which disappears with time and which can be regarded as delayed elastic behaviour, and an irreversible component which results from plastic or viscous flow. Therefore, timber on loading possesses three forms of deformation behaviour - elastic, delayed elastic and viscous. Like so many other materials, timber can be treated neither as a truly The various elastic and plastic components of the deformation of timber under constant load. (© BRE.) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429204265/2dd54dfa-b436-4e6c-ab54-6b8dc45fd540/content/fig6_1.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/> 94 elastic material where, by Hooke’s law, stress (see Section 6.2) is proportional to strain but independent of the rate of strain, nor as a truly viscous liquid where, according to Newton’s law, stress is proportional to rate of strain, but independent of strain itself. Where combinations of behaviour are encountered the material is said to be viscoelastic and timber, like many high polymers, is a viscoelastic material.