Compression tests of Japanese cedar in its L, R, and 45-degree directions under various stress rates were done in order to investigate the viscoelastic effect on the stress-strain curves with a focus on the proportional limit. The tangent modulus decreased gradually with the elevation of stress either for any direction of loading to the grain and for any stress rate of slow to fast loading. This suggests that the so-called proportional limit does not exist in an exact sense. A three-element model interpreted the viscoelasticity of the tested wood such that the spring constant is free from stress rates as well as stress repetitions like the Hookean body, while the dashpot is sensitive to them unlike the Newtonian linear fluid.
