A unified approach to creep of wood

For many structural applications the most important mechanical property of wood is its resistance to deflection, including elastic and creep deflectio n. Creep includes three distinct types of behaviour, which are difficult to separate because they can all operate simultaneously. These are time-depen dent (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. Con trary to previous views, the experimental results led to the conclusion tha t time-dependent creep and mechano-sorptive creep are different means of re aching 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 th e more normal strain against time relation. It was also found that normaliz ation of both the ordinate and abscissa resulted in a single master creep c urve for all pieces from a given sample, and also, approximately, for all t est humidities. This method therefore helps to control the important contem porary problem of using juvenile wood. By using this method, the characteri zation of time-dependent creep was reduced to the measurement of a master c reep curve and the determination of the magnitude of the normalizing parame ter. 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 phenomeno n. Application of the physical-ageing theory suggests that the speed of moi sture change might be important in mechanosorptive creep, indicating a size effect that has been predicted by van der Put but otherwise usually ignore d.