Duration of load and probability of failure in wood. Part I. Modelling creep rupture

Abstract

Two models are considered for the study of wood creep rupture behaviour and the associated load duration effects on strength. Both are damage accumulation models. In model I, the damage rate is expressed as the product of a stress-dependent factor and a damage-dependent factor. In model II, the damage rate is given as the sum of a stress-dependent term and a damage-dependent term. Both models consider a stress threshold below which the damage rate vanishes. For each model, stress histories corresponding to two types of tests are considered: a step function load test and a ramp load test. The predictions of the models for these two cases are used to obtain model parameters by fitting constant and ramp load test data. Data used correspond to small, clear specimens of Douglas-fir in bending. As an application, the problem of residual strength in a ramp load test after a time of constant load is discussed.