Time-Related Flexural Behavior of Small Douglas-Fir Beams Under Prolonged Loading

The mechanical behavior of wood depends to a considerable degree on the conditions under which mechanical forces are imposed. Wood exhibits both elastic and viscous deformation characteristics and this aspect of mechanical behavior is ultimately related to its intricate anatomical composition. Wood deforms instantly under load in relation to the stress imposed; however, deformation at any time is not linearly related to the stress producing the deformation. This study was designed to explore the load deflection characteristics of small wood beams under center loading with three general types of load-time relationships: 1) constant loads, 2) cyclic loading, and 3) progressive loading. Douglas-fir samples were prepared and brought to equilibrium under one of three temperature-relative humidity conditions and were tested with the different load, types. Time to failure decreased as stress level was increased, although the relationship appeared to be independent of moisture content. Analysis of the data indicated that time to failure was more closely related to average modulus of rupture of the control specimens than to proportional limit stress or any other property estimated from the control data. Successive deflections in the intermittent-load tests traced a curve in three parts with a point of inflection apparently of the same type as the constant load tests. These test specimens required approximately twice, as long as those under constant load to develop failure at the same stress level. Invariably, however, a point was reached where the elastic strain began to increase with successive loading and unloading cycles. Under increasing strain rate, all of the properties measured showed a tendency to increase in direct proportion to the strain rate.

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