It is important to avoid buckling during low-cycle fatigue testing. The buckling load is dependent on the specimen shape, material properties, and the testing machine. In the present investigation of hourglass-shaped specimens the importance of the diameter to radius of curvature is examined. Diameters of 5 and 7 mm are examined with a ratio of radius of curvature to diameter of 4, 6, and 8. The machine used is an Instron 8800 with elongated rods for a climate chamber. This leads to a reduced stiffness of the machine during compression testing. A finite element model (in Abaqus) is developed to identify the critical buckling force. For hourglass-shaped specimens, buckling means onset of sideways movement, without a drop in the applied load which is typical for conventional Euler buckling. The onset of sideways movement is identified experimentally by analysis of the data from extensometer and the load cell. This model is verified by experiments and fits within 0.6 to − 11% depending on the specimen diameter and diameter to radius of curvature ratio. The smallest deviations are obtained for the 7-mm-diameter specimen with deviation varying from 0.6 to − 3.3% between the model and the experiments. The current investigation is done with a commercially available hot rolled structural steel bar of Ø16 mm.

中文翻译：

---

沙漏形试样屈曲力的预测

在低周疲劳试验期间避免屈曲很重要。屈曲载荷取决于试样形状、材料特性和试验机。在目前对沙漏形试样的研究中，检验了直径对曲率半径的重要性。直径为 5 和 7 毫米，曲率半径与直径之比为 4、6 和 8。使用的机器是 Instron 8800，带有用于气候室的细长杆。这导致压缩测试期间机器的刚度降低。开发了一个有限元模型（在 Abaqus 中）来识别临界屈曲力。对于沙漏形试样，屈曲意味着开始侧向运动，而施加的载荷没有下降，这是传统欧拉屈曲的典型特征。通过分析引伸计和称重传感器的数据，通过实验确定侧向运动的开始。该模型已通过实验验证，根据试样直径和直径与曲率半径之比，拟合度在 0.6% 到 - 11% 之间。最小偏差是针对 7 毫米直径样本获得的，模型和实验之间的偏差从 0.6% 到 - 3.3% 不等。目前的调查是使用 Ø16 毫米的市售热轧结构钢筋完成的。模型和实验之间的 3%。目前的调查是使用 Ø16 毫米的市售热轧结构钢筋完成的。模型和实验之间的 3%。目前的调查是使用 Ø16 毫米的市售热轧结构钢筋完成的。