1 Department of Structural Engineering and Materials, Technical University of Denmark
The DVM-theory (Damaged Viscoelastic Material) previously developed by the author to 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 (re-cycle strength) during the process of load cycling. It is concluded that number of cycles to failure is a very poor design criterion. 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 generalised also to consider non-harmonic load variations. An algorithm is presented for this purpose which might be suggested as a qualified alternative to the Palmgren-Miner’s method normally used in fatigue analysis of materials under arbitrary load variations.It is strongly emphasised throughout the paper that reliable mechanical durability studies on wood cannot be made if the influence of wood rheology is disregarded. Lifetime predicted by Palmgren-Miner methods may cause considerably overestimated time to failure, especially at low frequencies. Results from accelerated test can be used in practice only when test conditions simulate practice. It is, however, not realistic to think of experiments running under forecasted live conditions.