Experimental Studies on the Fracture of Tension Bracing

牙套有时在载荷迟滞的早期阶段断裂。本研究的目的是阐明它们断裂的机制。第一年，我们对斜撑进行了交变轴向加载试验。试验结果表明，断裂发生在纵向位置的局部屈曲处。可以猜测，此时截面的形状完全改变了原来的形状，当施加拉力时，只有有限的部分截面抵抗了断裂。这种断裂现象本质上与弯曲构件屈曲后断裂相同。然后，在第二年，我们用一些方形钢管等试件进行了易于加载和应变测量的交变弯曲试验。通过对局部屈曲发生的纵向位置应变的测量，推测在断裂点，纵向应变的波动是极高的拉应力和反复出现的压应力。在此基础上，利用已建立的应力-应变曲线数学模型进行了应力计算，发现该模型不能准确地表示接近断口的部位。在最后一年，我们用一些方形钢管进行了交变弯曲试验。通过测量断口处的应变迟滞控制材料进行交变拉压试验，了解断口处的应力迟滞情况。根据应力-应变曲线计算吸收能量，发现能量已达到材料的极限。综上所述，屈曲后断裂是由有效截面积减小和塑性疲劳引起的。

Braces sometimes fracture in an early phase of a load hysteresis. It is the purpose of this study to elucidate the mechanism how they fracture.In the first year, we conducted alternating axial loading tests to angle braces. As a result of these tests, the fracture occurred in the longitudinal position of there local buckling. It was guessed that at this point, the shape of section totally changed from the original shape, and when they were put tensile force, only limited part of the section resisted the fracture. A phenomenon of this fracture is essentially the same as bending members post-buckling fracture.Then, in the next year, we conducted alternating bending tests which was easy to laod and measure of strains, by means of such test pieces as some square-shape steel tubes. As a result of the measurements of strains at the longitudinal position of local buckling occurred, it was guessed that at the point of the fracture, the fluctuation of the longitudinal strains was extremely high tensile stress and compression stress occurred repeatedly. Then we tried to calculate the stress by means of an accomplished mathematical models of stress-straln curve, we found that we could not express precisely the parts close to the fracture by these models.In the final year, we conducted alternating bending tests with some square-shape steel tubes. We conducted alternating tensile-compression tests of the material which controled by the strain hysteresis measured at the point of fracture, so as to know the stress hysteresis at the point. After calculating the absorbed energy from the stress-strain curve, we found that the energy had reached to the material's limit.In conclusion of these tests, post-buckling fracture caused by reduction of effective sectional area and plastic fatigue.