A unified approach to creep of wood

Abstract

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, mechano–sorptive (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 mechano–sorptive creep, indicating a size effect that has been predicted by van der Put but otherwise usually ignored.