Nonlinear Mechanical Behavior of Wood

Creep and recovery of hard maple caused by longitudinal tension stress were examined over time intervals of 1,000 and 100 minutes, respectively, for preselected values of the independent variables in the temperature range from 30 – 70?C, moisture content from 4-12 percent, and stress from 20-80 percent of the short time stress at failure. Creep and recovery results are presented in the form of multivariable least-square regression equations. The mechanical behavior is shown to be clearly nonlinear viscoelastic. As a first approximation, the response of wood to longitudinal tensile stress may be considered elastic, as a second approximation as linear viscoelastic, and as a third approximation as nonlinear viscoelastic. The higher orders of approximation are needed as moisture, temperature, and stress increase. Time – temperature superposition according to Ferry’s “reduced variables method” is not believed applicable. A survival distribution is presented for those specimens that failed during the test at the highest relative stress level. It appears that mechanical work to failure may be constant for certain stress histories. Both recovery and flow compliances are time dependent.

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