Lifetime and Residual Strength of Wood Subjected to Static and Variable Load

摘要

The DVM-theory (Damaged Viscoelastic Material) previously developed by the authorto predict lifetime of wood subjected to static loads is further developed in this paper such that harmonic load variations can also be considered. Lifetime (real time or number of cycles) is predicted as a function of load amplitude, load average, fractional time under maximum load, and load frequency. The analysis includes prediction of residual strength during the process of load cycling. It is concluded that number of cycles to failure is a poor design criterion. A simple time criterion is much better. The theory is successfully compared with data from experiments representing different wood products. Master graphs are developed which can be used in fatigue design of wood products in general. These graphs are valid for any creep behavior (relaxation, moisture content) and materials quality (grading, strength level). It is demonstrated how the theory developed can be generalized also to consider some non-harmonic load variations. An algorithm is presented for this purpose which might be suggested as qualified alternative to the Palmgren-Miner's method normally used in fatigue analysis of materials under arbitrary load variations. The Palmgren-Miner's method has the disadvantage of not considering creep which may cause considerably overestimated time to failure, especially at low frequencies. Finally it is discussed how the theory can be generalized also to consider other materials such as concrete and other aging viscoelastic materials. (Copyright (c) 1996 by Lauge Fuglsang Nielsen.)