4. クリープ経歴を有する木材の短期圧縮強度と弾性に就て

Abstract

This report presents tne effect of sustained loading of stress well below the creep limit in compression (less than 50 percent of the static strength) on the static strength properties in compression parallel to grain. The results here reported are limited to data for Akita-Sugi (Cryptomeria from Akita district of Japan). The nominal dimensions of specimens were 3×3×6cm and the grain of wood was parallel to the length of specimen. The specimens were loaded parallel to grain. As shown in Fig. 1, the specimens were so cut in the plank that end-matched nine pairs of specimens were obtained. By this procedure it was possible to assume that specimens in each pair have the same strength properties each other. Standard static tests which were made in a mechanical testing machine of 20 tons capacity are divided into two groups; one for the control tests which were made right before creep test in order to estimate the average static strength of creep test specimens, the other for the virgin loading tests for unstressed specimens which were made right after creep test. Creep tests were made in creep testers shown in Fig. 3. After 105 days' creep test, creep test specimens were reloaded up to failure right after unloading of sustained stress. This test is called the reloading test. The effect of room conditions on the strength properties in the static loading is marked, because all the tests were not conducted under controlled conditions. That is to say, the results show that the ratios of moisture content, compressive strength and modulus of elasticity at the static loading before creep test to those after creep test were 1.4, 1/1.2 and 1/1.15, respectively. The principal object of Series 1 was to find this difference and to modify the effect of room conditions on the static strength properties. The objects of three series in this experiment are illustrated in Table 1. The resuls of tests in each series are shown in Tables 2 to 7 and Figures 4 to 6. Conclusion: Comparisons of the compressive strengths and moduli of elasticity for creep test specimens subjected to the sustained stress of less than 50 percent of the static strength in compression with those for unstressed specimens suggest that sustained loading has no effect on the compressive strength and modulus of elasticity at the static loading. (2) The proportional limit at the reloading for specimens subjected to the sustained stress is higher than that for unstressed (virgin) specimen. The strain corresponding to the maximum stress for creep test specimen is smaller than that for virgin specimen. These facts suggest that sustained loading makes specimen brittle. (3) The magnitude of sustained stress has no effect on the compressive strength and modulus of elasticity at the reloading. The proportional limit, however, is made higher as the magnitude of sustained stress is increased. The proportional limit for specimen subjected to the sustained stress of 1/3 of static strength is 125 percent of that for virgin specimen.