A STUDY ON THE STABIRITY OF CIRCULAR SECTION STEEL STUB-COLUMNS IN THE PLASTIC-FLOW RANGE

以往给出圆截面短柱塑性范围内大象脚型屈曲强度的公式，其解中都含有切线模量值。因此，无论其直径厚度比如何，所有圆形截面短柱都在屈服平台上屈曲。平板在屈服平台上不会失去其扭转刚性。因此，平板可以被压缩到应变硬化范围内，而不会经历屈服平台上的过早屈曲。但试验结果表明，在塑性范围内，从屈曲强度的角度来看，圆形截面优于箱形截面。本研究旨在解决这一矛盾。明确了以下几点。在以往的理论中，变形理论似乎与屈服平台以外区域的试验结果相吻合。小直径厚比圆截面短柱在应变硬化前不会在屈服平台上发生过早屈曲。对于大象脚的屈曲，由于周向的曲率随直径的变化而发生，因此发生了鞍形屈曲模式。这就产生了低屈曲应力。但对弹性解的影响不到1%。对于塑性屈曲，这种影响可能以不同的形式出现，特别是在屈服平台上。

Past equations to give the buckling strength of elephant foot mode of circular section steel stub-column in the plastic range contain tangent modulus in their solutions. Therefore all circular section stub-column buckle on the yield plateau despite of their diameter- to- thickness ratio. Flat plates does not lose their torsional rigidity on the yield plateau. Therefore flat plates can be compressed into strain-hardening range without experience of premature buckling on yield plateau. But the experimental results show that circular section have an advantage over box section from the view point of buckling strength in the plastic range. This research intended to solve this contradiction. The followings were made clear.1. Among past theories, deformation theory seems to coincide with test results in the region beyond yield plateau.2. It seems that circular section stub-columns with small diameter-to-thickness ratio does not experience premature buckling on the yield plateau before strain-hardening range.3. Buckling mode like a saddle take place for elephant foot buckling because the curvature in circumferential direction occur depending on the change of diameter. This gives low buckling stress. But the effect is less than 1% for elastic solution.4. The effect 3. may appear in different form for plastic buckling especially on the yield plateau.