Long-Term Strength Prediction of Wood Based Composites Using the Kinetic Equations

Abstract

The existing behaviour models of the structures under constant load (creep) have a fairly wide forecast horizon and low accuracy. As a rule, they consider the transition from an undestroyed state of an element to a destroyed one, in one stage. The purpose of this study is to substantiate and develop a new approach to predicting long-term strength based on kinetic equations, which, in turn, should consider the multistage nature of the process of gradual destruction of structure elements. To achieve this purpose, the study solves the tasks of creating a multistage kinetic transition of individual structure elements from an initially elastic state to a viscoelastic state, and then to a fractured state. When describing this process, the authors employed the methods of formal kinetics and the theory of continuum damage mechanics, including the method of basic diagrams. Wood-based composites were used as the materials under study. Based on the results of the conducted full-scale and computational experiments, the study discovers that a mathematical model based on kinetic equations adequately describes the behaviour of the materials under study for long-term strength; the proposed two-stage model determines the forecast horizon much more accurately than the available one-stage models. The kinetic parameters that determine the rate of transition of a structural element from an elastic state to a viscoelastic state, and then to a destroyed state, were determined based on experimental base chart. The time to fracture was determined at three-point bending at a load equal to 70% of the flexural strength at temperatures of 20°C and 60°C, constant humidity RH 65% and moisture content MC 8%. When building control charts, the load increased by another 15%. The method allows narrowing the forecast horizon and determining the moment of transition of a structure from a stationary state to a blow-up regime with a higher accuracy