Effects of Stress Level on Wood Creep under Bending
The stress level in the structure largely influences on the rate of wood creep under the steady load. Previously deformation properties were investigated in a lesser extent, in the stage of nonlinear creep. Therefore, there are no experimental data of long-term tests at low stress levels, allowing us to obtain the “stress - strain” dependence. The purpose of the experimental studies is to obtain the quantitative estimates of wood creep, expressed in total (elastic and residual) strain at the design stress levels of various rates. Long-term tests of four spruce samples, sawn from one board, were carried out in the mode of “bending” at different stress levels of 2 uA … 13 uA. The tests were carried out in a dry and warm room under the stable thermal and moist conditions. Samples moisture content during the tests was 6 %. The test validity, reduced to 12 % of moisture by the known formulas, was estimated by comparing the calculated values of short-term and long-term modulus of elasticity and the standard values. The study results confirmed the prerequisite about the impact of the stress level on the value and period of the creep process. At low levels of normal stress the creep attenuation in the midspan occurred within the first month. At stresses close to the calculated resistance the creep attenuation in the samples occurred in six months. During the seventh month the increase of deflection was not observed. According to the measurements of deflections at the test period the actual values ​​of total strains were calculated. These values were used for “total strain - hold time” plotting for each sample. These graphs were the basis of the presented in the paper dependence “stress - total strain”, which can be used to determine the modulus of deformation. It is concluded, that for the construction of reliable empirical dependence and further approximation it is necessary to continue the research by increasing the number of samples and the range of stress levels.