Development of A Reliability-based Method for Remaining Life Evaluation Using Extremes Statistics Analysis of Short Fatigue Cracks

本研究旨在发展一种基于责任的剩余寿命评估方法。该方法利用极值统计和图像处理的最大裂纹长度估计。本研究内容包括建立一套高温疲劳试验系统（主要是使用微机的控制系统和应变测量系统），在650 ℃空气中进行奥氏体不锈钢SUS 316 L的蠕变-疲劳试验，引入图像处理技术，包括开发一些新的算法，测量大量短疲劳裂纹的尺寸，基于应变循环的恒定间隔下的极值统计并与测量值进行比较来估计最大裂纹长度，并开发基于可靠性的剩余寿命评估方法。主要结果如下：首先，图像处理成功地应用于大量疲劳短裂纹的尺寸测量，并应用了极值统计。可以大大减少测量所消耗的时间和精力。在相对误差为10秒的范围内，可以测量大于30 gm的裂纹长度<plus-minus>，发现了蠕变疲劳中最大裂纹长度估算的局限性，即在裂纹贯通之前，估算的最大裂纹长度与测量的最大裂纹长度是一致的，而在贯通之后，估算的最大裂纹长度与测量的最大裂纹长度是偏离的。最后，提出了“修正重现期”的新概念，并利用该概念建立了基于可靠性的剩余寿命评估方法。该方法能够预测对应于低故障概率（或高可靠性）的剩余寿命。

This research was conducted aiming at development of a liability-based method for remaining life evaluation. The method makes use of the maximum crack length estimated by means of statistics of extremes and image processing. The research involves construction of a high temperature fatigue testing system (mainly, a control system using a microcomputer and a strain measurement system), execution of creep-fatigue tests of the austenitic stainless steel SUS316L at 650 ﾟC in air, introduction of image processing technique including development of some new algorithms, size measurement of a large number of short fatigue cracks, estimation of the maximum crack length based on statistics of extremes at a constant interval of strain cycles and comparison with the measured value, and development of a reliability-based method for remaining life evaluation. The principal results are summarized as follows.First, image processing was successfully applied to size measurement of a large number of short fatigue cracks to which statistics of extremes was applied. The time and efforts consumed in measurement could be greatly reduced. It was possible to measure the crack length larger than 30 gm within the relative error of <plus-minus>10Second, a limitation of estimating the maximum crack length in creep-fatigue was found. That is, it was found that up to occurrence of crack coalescence, the estimated maximum crack length was in agreement with the measured one, whereas after occurrence of coalescence the former deviated from the latter. This is important in remaining life evaluation based on the maximum crack length.Finally, the new concept of "modified return period" was proposed , and a reliability-based method for remaining life evaluation was developed using this concept. This method enables to predict the remaining life corresponding to a low failure probability (or high reliability).