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The effect of fatigue pre-cracking forces on fracture toughness

机译：疲劳预裂纹力对断裂韧性的影响

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Testing procedures for the determination of the fracture toughness of a material by monotonic loading of fatigue pre-cracked specimens are well established in standards such as BS 7448, BS EN ISO 15653, ISO 12135, ASTM E1820 and ASTM E1921. However, a review of these standards indicates a wide range of permitted fatigue pre-cracking forces, whilst the underlying assumption in each standard is that the pre-cracking conditions do not affect the fracture toughness determined. In order to establish the influence of different fatigue pre-cracking forces on the fracture toughness, tests were carried out on specimens from an API 5L X70 pipeline steel. Single-edge notch bend specimens of Bx2B geometry were notched through thickness and tested at temperatures of +20 ℃, -80 ℃ and - 140℃ to show the fracture behaviour in different regions of the fracture toughness ductile-to-brittle transition curve. Fatigue pre-cracking was conducted on a high-frequency resonance fatigue test machine over a range of pre-cracking forces permissible within the various standards and beyond. The results showed that an excessively high pre-cracking force can result in a significant overestimation of the value of fracture toughness for material exhibiting brittle behaviour, whilst very low fatigue pre-cracking forces appeared to result in an increase in scatter of fracture toughness. A review of standards indicated that there was a possibility to misinterpret the intention of the ISO 12135 standard and potentially use excessively high pre-cracking forces. Suggested clarifications to this standard have therefore been proposed to avoid the risk of overestimating fracture toughness.
机译：BS 7448，BS EN ISO 15653，ISO 12135，ASTM E1820和ASTM E1921等标准已经很好地建立了通过疲劳预破裂试样的单调加载确定材料的断裂韧性的测试程序。但是，对这些标准的审查表明，允许的疲劳预开裂力范围很广，而每个标准中的基本假设是，预开裂条件不会影响所确定的断裂韧性。为了确定不同的疲劳预破裂力对断裂韧性的影响，对API 5L X70管线钢的样品进行了测试。对Bx2B几何形状的单边切口弯曲试样进行厚度切割，并在+20℃，-80℃和-140℃的温度下进行测试，以显示断裂韧性在韧性到脆性过渡曲线不同区域的断裂行为。疲劳预裂纹是在高频共振疲劳试验机上在各种标准及更高标准所允许的一系列预裂纹力下进行的。结果表明，过高的预开裂力会导致显着高估表现出脆性的材料的断裂韧性值，而非常低的疲劳预开裂力似乎会导致断裂韧性的散布增加。对标准的审查表明，可能会误解ISO 12135标准的意图，并可能使用过高的预裂纹力。因此，建议对此标准进行澄清，以避免过高估计断裂韧性的风险。

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