木材抗弯强度及抗弯弹性模量试验方法的研究

A STUDY ON THE DETERMMING METHODS OF BENDING STRENGTH AND ELASTICITY

为给修订国家标准《GB 1936—80木材杭弯强度及弹性模量试验方法》提供依据,本文对现行国家标准与国际标准进行对比研究。结果表明:现在木材抗弯强度的测定不符合实际情况、应采用国际标准ISO 3349的测定方法,但加荷压头、试样支座与试样接触点之间应加钢垫片,以防止试样因压痕而产生纵向应力。ISO抗弯弹性模量测试方法,由于仅测试样加荷压头之间的弹性变形、其值甚小、加上千分表的灵敏度高、因此测计的数据不稳定,变化大,误差大,结果欠正确,应该继续采用国家标准GB 1936—80抗弯弹性模量测试方法,但加荷压头及试样支柱端头的半径应改为30mm。本文还对建国以来测试的抗弯弹性模量,提出求修正值的公式。

Based on the study of the testing method of bending strength, we have examined the advantages and disadvantages of GB Standard in comparation with ISO Standard. It has been found that the static bending and elastic test-methods of GB Standards should be reasonably revised.The bending strength (modulus of rupture (MOR)), developed by the third-point method of bending load, not only is unsuitable in practice but also doesn't coincide in static bending test-methods of ISO Standard. Therefore the testing method of static bending of Standard ISO 3349 should be instead of present standard GB 1936—80 in new revised GB Standard. The results based on Standard 1936—80 could be calculated by the following equation to convert to the values of the new Standard.б_(?)=0.775б_(GB)+15.15where б_(?)=bending strength of ISO Stan(?)ard, i. e. MOR of new GB Stardand, and 6_(GB)=present bending strength of GB Standard.During the bending testing, if the hardness of some wood species is too weak, the impresses, usually caused in contact points between the wood sample to be tested and the loading crosshead and the lower supports, develope longitudinal stress and additional deformation so that the values of testing result will be higher. To eliminate these defects, a steel plate may be mounted on the contact points between the sample and the loading crossheads and the lower supports. To determine the modulus of elasticity of GB Standard, the elastic strains of sample between lower supports are obtained from centi-dial gauge and these values of ISO Standard are also obtained but milli-dialgauge is used to measure.The milli-dial gauge is not easily to installed in exactly position and it is most sensitive to operate hardly, so that in testing the errors are commonly developed and the data collected from testing are irregular. on the other hand, the modulus of elasticity by means of GB Standard is easily and the results are more exactly. Hence the GB Standard-to determine the modulus of elasticity should comtinually be used. However, the radius of arc of loading crosshead and lower supports should be increased from 15mm to 30mm.