This study benchmarks a relatively new axisymmetric specimen concept for use in threshold testing, where the crack initiates and grows from the corner of a flat-bottomed circular hole in a cylindrical specimen while the bottom of the hole is cyclically loaded with a flat-ended plunger. The Flat-Bottom Hole (FBH) specimen is configured so the stress intensity naturally decreases as the crack grows radially outward at constant cyclic load, favoring self-centering, highly circular crack growth, and thus enabling tightly repeatable results especially for threshold testing. Because the specimen achieves K-shedding without load shedding, the opening at every point along the crack is constantly increasing during constant load amplitude growth, minimizing the threat of remote closure. Contrary to most conventional specimens, the circular crack front makes no intersection with a free surface, resulting in a crack front of uniform constraint and, where applicable, a uniform crack closure state. In practice, the specimen can be made quite small in size compared to other specimens of similar function.

Two configurations are described, one for ambient testing, and one for elevated temperature testing. The K-solution and displacement solution obtained using FRANC2D are presented. Test results include data for Ti 6A-4V forging and Inconel 718 forging.

这项研究确定了用于阈值测试的相对较新的轴对称试样概念的基准，在该概念中，裂纹从圆柱形试样中平底圆形孔的角开始并扩展，而孔的底部周期性地加载有平头柱塞。构造平底孔（FBH）样品，以使应力强度随着裂纹在恒定循环载荷下径向向外扩展而自然降低，从而有利于自定中心，高度圆形的裂纹扩展，因此可实现紧密可重复的结果，尤其是用于阈值测试。由于样品在不发生负荷减少的情况下实现了K脱落，因此在恒定的负荷振幅增长过程中，沿着裂纹的每个点的开口都在不断增加，从而最大程度地减少了远距离闭合的威胁。与大多数常规标本相反，圆形裂纹前缘与自由表面不相交，从而导致裂纹前缘具有均匀的约束，并在适用时具有均匀的裂纹闭合状态。在实践中，与具有类似功能的其他标本相比，标本的尺寸可以做得非常小。

描述了两种配置，一种用于环境测试，一种用于高温测试。介绍了使用FRANC2D获得的K解和位移解。测试结果包括Ti 6A-4V锻件和Inconel 718锻件的数据。