Abstract The present work aims to present a new complete procedure to characterize fracture toughness in terms of stress intensity factor K or the J − integral for Mode I of loading using four-point bend specimens. Four-Point Bend (4PB) specimens are shown to be valid configurations for fracture toughness characterization tests with certain advantages when compared to traditional three-point bend geometries and single edge notch tension SE(T) specimens under clamped conditions. Fracture toughness is a key parameter in defining critical flaw sizes in a fitness-for-service (FFS) procedure based upon fracture mechanics principles. Accurate estimation of fracture toughness is therefore necessary to assess the integrity of critical engineering structures by following the failure assessment diagram concept. Extensive 3D finite element analyses were performed to obtain the analytical relationships necessary to calculate the fracture toughness parameters. New formulas are provided for the elastic compliance, stress intensity factor, and eta factor for calculating fracture toughness valid for four-point bend specimens. Using the distance between inner and outer roller supports as a normalization parameter, the stress intensity factor and the normalized elastic compliance equations are shown to be independent of the specimen supporting configuration. The eta factor based upon the CMOD is dependent on the distance between external and internal roller supports as well as a ∕ W . Experimental fracture tests were carried out to check the practicality of the methodology. J -R curves obtained by four-point bend specimens were compared to resistance curves derived from three-point SE(B) specimens and clamped SE(T) samples which are standardized specimens. The results show that the four-point bend specimens provide reliable fracture toughness values somewhat higher than three-point bend geometries.

中文翻译：

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使用四点弯曲试样进行断裂韧性表征的新公式

摘要 目前的工作旨在提出一种新的完整程序，根据应力强度因子 K 或使用四点弯曲试样加载模式 I 的 J - 积分来表征断裂韧性。与传统的三点弯曲几何形状和夹紧条件下的单边缘缺口张力 SE(T) 试样相比，四点弯曲 (4PB) 试样被证明是断裂韧性表征测试的有效配置，具有某些优势。在基于断裂力学原理的适用性 (FFS) 程序中，断裂韧性是定义关键缺陷尺寸的关键参数。因此，为了通过遵循失效评估图概念来评估关键工程结构的完整性，必须准确估算断裂韧性。进行了广泛的 3D 有限元分析以获得计算断裂韧性参数所需的分析关系。为弹性柔量、应力强度因子和 eta 因子提供了新公式，用于计算对四点弯曲试样有效的断裂韧性。使用内外滚子支撑之间的距离作为归一化参数，应力强度因子和归一化弹性柔量方程显示为独立于试样支撑配置。基于 CMOD 的 eta 系数取决于外部和内部滚子支架之间的距离以及 ∕ W 。进行了实验性断裂试验以检查该方法的实用性。通过四点弯曲试样获得的 J-R 曲线与从三点 SE(B) 试样和夹紧的 SE(T) 试样得出的电阻曲线进行比较，这些试样是标准化试样。结果表明，四点弯曲试样提供了比三点弯曲几何形状略高的可靠断裂韧性值。