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The treatment of very short crack growth in low cycle fatigue and creep fatigue

机译：在低周疲劳和蠕变疲劳中非常短的裂纹扩展的处理

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摘要

Given total endurances of low cycle fatigue specimens, several empirical formulae enable an estimate to be made of 'initiation' cycles to a specific crack depth in a zone ranging from 20 to 400 mm from the surface. For greater depths, the cyclic growth behaviour of cracks can be calculated directly from relations using established constants (including the effects of creep dwell) for a wide range of alloys. However, below a critical depth of 200 mm or thereabouts, these empirical growth laws, expressed in terms of total strain range, break down. Early crack growth can take place discontinuously, as the crack tip encounters grain boundaries or other obstacles. Such information arises from replica takings, or in situ microscopic observations of surfaces during continuous cycling and creep-fatigue tests. Striation spacings on the fracture surface and the first observed striation spacing provide valuable indications of early crack growth. In this region, the cyclic rate of growth depends on (i) crack shape which can change during advance; (ii) whether or not embryo cracks link up (coalescence); (iii) whether or not there is a small starter defect (deliberately introduced or otherwise); (iv) the degree of creep or oxidation damage; and (v) the repeat distance of obstacles or barriers. Creep effects can overtake these considerations by bulk damage accruing with consequent acceleration in growth. In this survey, crack growth characteristics in ferritic and austenitic steels up to 625 degrees C and superalloys up to 1000 degrees C are considered. Surface behaviour is connected to penetration into the depth. As part of a sensitivity analysis, various 'stop-start' models are examined and their related growth cycles calculated in the early region before the macroscopic growth law takes over. These are compared with an upper bound constant growth rate law. Alternatively, it is shown that crack growth can be described over the whole range by a single exponential function.
机译：给定低循环疲劳样品的总耐久性，几个经验公式可以估算从表面到20至400 mm范围内特定裂纹深度的“初始”循环。对于更大的深度，可以使用各种合金的既定常数（包括蠕变停留的影响）从关系直接计算出裂纹的循环生长行为。但是，在200mm左右的临界深度以下时，以总应变范围表示的这些经验增长定律会失效。由于裂纹尖端遇到晶界或其他障碍，裂纹的早期生长会不连续地发生。这些信息来自复制品，或在连续循环和蠕变疲劳测试过程中对表面进行的原位显微镜观察。裂缝表面的条纹间距和首次观察到的条纹间距为早期裂纹扩展提供了有价值的指示。在该区域，循环的增长率取决于（i）裂纹形状，该形状在前进过程中可能会发生变化; （ii）胚胎裂纹是否连接在一起（聚结）; （iii）起动器是否有小缺陷（故意引入或以其他方式）; （iv）蠕变或氧化破坏的程度; （v）障碍物或障碍物的重复距离。蠕变效应可能会因大量损坏而导致增长加速，从而超过这些考虑因素。在这项调查中，考虑了高达625摄氏度的铁素体和奥氏体钢以及高达1000摄氏度的超合金的裂纹扩展特性。表面行为与渗透到深度有关。作为敏感性分析的一部分，在宏观增长定律接管之前的早期区域，研究了各种“停止-启动”模型并计算了它们的相关增长周期。将这些与上限恒定增长率定律进行比较。替代地，显示出裂纹扩展可以通过单个指数函数描述在整个范围内。

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