STRESS RELAXATION OF WOOD AT SEVERAL LEVELS OF STRAIN.

Stress relaxation tests were performed with six tropical American species. Stress relaxation was not found to be a linear function of strain at any level of strain. At equal low levels of strain, stress relaxation in compression was much greater than in tension. A mechanical model consisting of an isolated spring in parallel with a spring and dashpot in series was used as an aid in the derivation of equations describing stress relaxation. An attempt to apply Newtonian viscous theory to the model was unsuccessful in accounting for rate of relaxation. However, when the hyperbolic sine law of viscous flow was applied, mathematically derived curves fitted the data very well. Stress relaxation appears to be related to departure strain which may be obtained readily from static stress-strain diagrams. Author