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Creep-fatigue damage accumulation and interaction diagram based on metallographic interpretation of mechanisms

机译：基于机制金相分析的蠕变疲劳损伤累积与相互作用图

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The deleterious effect of creep damage upon fatigue endurance in elevated temperature low cycle fatigue is well established and has become known as the 'creep-fatigue interaction'. This has been identified as a narrow region where a transgranular crack becomes intergranular as creep damage develops. In other cases transgranular fatigue cracks and intergranular creep cavitation damage to some extent can take place separately ('competitive' mode) so that interaction is less pronounced. These phenomena have been incorporated into 'damage interaction diagrams' which feature in many high temperature Code cases (ASME, R5, RCC-MR etc). Fractional creep and fatigue damage are considered separately, and failure is conceded when their sum is unity ('additive' mode). For some materials the sum is set to a value < 1. It appears that purely empirical arguments have been used to arrive at total damage values < 1. Tt is shown that such deviations from linearity can be simply predicted, assuming that creep damage accumulates uniformly during a given test. For maximum interaction, the resulting assessment curve is shown to pass through the points (total fatigue damage = 0.4; total creep damage = 0.4), or very nearly. The influence of fatigue upon creep damage is less well known. Examples are provided, and when included in the calculation, produce a more damaging curve passing through the point (0.33, 0.33). By use of 'interaction coefficients' less damaging curves for the 'competitive' model can be produced. The calculation may be simplified by use of an effective fatigue damage cycle number for use as a reference. Based on metallographic examples from power plant operation and laboratory testing, the 'competitive', 'additive' and 'true interaction' modes of creep and fatigue damage are identified. These can be related to practical applications of base load operation, fast start-up and shut-down procedures, slow start-up and shut-down procedures etc. Components might operate under base load conditions (creep) followed by temperature cycling (fatigue). These sequential events may not be interactive (as has been demonstrated in the laboratory) and thus use of the current assessment diagram would be pessimistic. Comment is made on a proposed damage diagram for the advanced ferritic steels, which is highly conservative and does not reflect their good performance in service and laboratory tests.
机译：蠕变损伤对高温低周疲劳中的疲劳承受力的有害影响已得到充分证实，并已被称为“蠕变疲劳相互作用”。这已被确定为一个狭窄的区域，随着蠕变损伤的发展，该处的跨晶裂纹变为晶间裂纹。在另一些情况下，晶间疲劳裂纹和晶间蠕变空化损伤在一定程度上可以分开发生（“竞争”模式），因此相互作用不太明显。这些现象已被并入“损伤相互作用图”，在许多高温规范情况下（ASME，R5，RCC-MR等）都具有这种特征。分数蠕变和疲劳损伤是分开考虑的，并且当它们的和为1时（“加和”模式），将承认失败。对于某些材料，总和设置为值<1。似乎纯粹的经验论据已用于得出总损伤值<1。Tt表明，可以假设与蠕变损伤一致地累积，可以简单地预测线性偏差。在给定的测试中。为了获得最大的相互作用，显示出的评估曲线穿过点（总疲劳破坏= 0.4;总蠕变破坏= 0.4）或非常接近。疲劳对蠕变损伤的影响鲜为人知。提供了示例，当包含在计算中时，会产生一条更具破坏性的曲线通过点（0.33，0.33）。通过使用“相互作用系数”，可以为“竞争性”模型生成更少的破坏曲线。通过使用有效的疲劳损伤循环次数作为参考，可以简化计算。根据电厂运行和实验室测试的金相实例，确定了蠕变和疲劳损伤的“竞争性”，“加性”和“真实相互作用”模式。这些可能与基本负载操作，快速启动和关闭程序，慢速启动和关闭程序等的实际应用有关。组件可能在基本负载条件（蠕变）下运行，随后温度循环（疲劳）运行。这些连续事件可能不是交互的（如在实验室中已经证明的），因此使用当前评估图将是悲观的。对建议的高级铁素体钢的损伤图进行了评论，该图非常保守，不能反映其在服务和实验室测试中的良好性能。

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