For many structural applications the most important mechanical property of wood is its resistance to deflection, including elastic and creep deflection. Creep includes three distinct types of behaviour, which are difficult to separate because they can all operate simultaneously. These are time-dependent (viscoelastic) creep, mechanosorptive (moisture-change) creep, and the pseudo-creep and recovery that has been ascribed to differential swelling and shrinkage. This paper describes how two special techniques can be used to help assess the relative importance of the three types of behaviour. Contrary to previous views, the experimental results led to the conclusion that time-dependent creep and mechano-sorptive creep are different means of reaching the same creep result. This finding led, logically, to a new way of characterizing wood creep, namely, plotting data in the form of strain rate against strain. Solution of this differential equation can then lead to the more normal strain against time relation. It was also found that normalization of both the ordinate and abscissa resulted in a single master creep curve for all pieces from a given sample, and also, approximately, for all test humidities. This method therefore helps to control the important contemporary problem of using juvenile wood. By using this method, the characterization of time-dependent creep was reduced to the measurement of a master creep curve and the determination of the magnitude of the normalizing parameter. The effects of humidity changes require the additional measurement of an increased 'activity' associated with the molecular destabilization, and its relaxation-time constant, associated with the physical-ageing phenomenon. Application of the physical-ageing theory suggests that the speed of moisture change might be important in mechanosorptive creep, indicating a size effect that has been predicted by van der Put but otherwise usually ignored.

Physical ageing means the 'time-dependent approach of a polymer to thermodynamic equilibrium'. This phenomenon has been studied for some years in artificial polymers, but has been little considered in wood, in spite of the fact that wood is also polymeric, and experiences time-dependent behaviour. The concept of physical ageing could help in explaining some of this behaviour.Work on the subject of physical ageing has been published extensively by Struik [1]. This assumes that many rate-phenomena depend on the degree of molecular packing. The equilibrium packing can be disturbed by, for example, temperature changes, moisture-content changes or tensile straining. The resulting increase in free volume gives a destabilized condition that slowly returns to equilibrium. This asymptotic return to equilibrium is known as 'ageing'.A feature of Struik's papers was a series of creep tests at various ageing intervals following a temperature 'quench'. These showed a strong effect of ageing time on rigid PVC, following a quench from 90 °C to 20 °C: giving shift factors on the log-time scale of more than four orders of magnitude, for ageing times between 0.3 and 1000 days. He also gave evidence that large strains caused destabilization followed by ageing. Evidence that moisture reduction has a similar effect on the creep rates of nylon 6,6 (a hygroscopic polymer) have been given by Hunt and Darlington [2].Besides static creep behaviour, molecular destabilization and ageing of hygroscopic materials are expected to affect dynamic mechanical behaviour, moisture diffusion and equilibrium, swelling and shrinkage, and electrical properties; together with chemical behaviour and calorific properties.Struik's original ideas concerning physical ageing have more recently been modified by McCrum [3], who interpreted Struik's results in terms of sequential-ageing theory; but nevertheless the theory of physical ageing is a useful first approach to explaining some anomalous types of behaviour.If such a physical ageing phenomenon exists in wood, it could be associated with observations such as mechano-sorptive creep (enhanced creep during humidity changes), the relatively small amount of recovery following removal of creep loads, and the acceleration of recovery by moisture cycling. It 80 would also modify the interpretation of creep results of very long-term tests, for instance.It is well known that moisture in wood does not appear to obey Fick's Law. According to the physical-ageing theory, moisture diffusion should be easier and quicker while the wood is in an unstable condition. This fits in with observations by Christensen and Hergt [4], by Downes and Mackay [5] and by Christensen and Kelsey [6], that rates of sorption of water vapour and the final equilibrium values depended strongly on the moisture history. They also found that any such anomalous effects could be reversed by first saturating and then redwing the wood.During hygroscopic swelling and shrinkage, the dimensions at any given moisture content would be expected to slowly decre...

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