Effects of stress wave reflection in wooden samples

Introduction. Wave propagation of stress and strain in wood as a structural material have their own characteristics. The geometric shape of wooden structural elements as a boundary between physical media, the structural shape of nodes and joints typically made of steel elements, adhesive layers, as well as layers of glued wood form complex boundary conditions for the development of a stress-strain state.

Aim. To substantiate a model of stress wave reflection from the boundary of a medium in a wooden sample with qualitative and quantitative assessments of the change in stress over time and along the sample length. The strain model of wood as a natural polymer can be used to substantiate the long-term strength of wood and wooden structures based on the kinetic strength theory.

Materials and methods. The studies of G. Kolsky, R.M. Davis, and Yu.N. Rabotnov are analyzed for substantiating the proposed model of wave processes in wood and wooden structures. Moreover, the hypothesis about the effect of wave strain propagation on the long-term strength is substantiated. The effects of stress wave reflection from the boundaries of an assumed elastic bounded medium are demonstrated in the numerical experiment. Two hypotheses are proposed. The first one assumes using the physical properties of material and geometric dimensions of the structural element. In addition, the magnitude and duration of the external load are used to determine a so-called “depth threshold,” below which the full wave effect of stress propagation within the sample begins to manifest itself. The second hypothesis considers a toroidal body with a tending to infinity length many times greater than its diameter as a cylinder of infinite length simulating an infinite medium.

Results. The stress waves propagate in the volume of the loaded sample at rest, gradually attenuating to a value of 27 MPa equivalent stress according to von Mises. The stresses on the surface where the external load is applied stabilize faster; the wave processes of stress change are characterized by a small amplitude.

Conclusions. The performed numerical and field experiments revealed a pattern confirming the previously formulated hypothesis of a significant excess in the stress wave value over the stress value for the loaded element at rest. When the sample is compressed, the reflected stress oscillates at an amplitude established relative to the value of the stress at rest.

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