正確地探討材料因往復循環應力而造成變形及破壞是重要的，傳統上皆利用疲勞試驗中裂縫生長的長度、裂縫生長速度以及應力循環周期數作為預測。然而試片遭受較大拉伸應力或較大壓縮應力後，裂縫生長速度會產生減速或者加速的現象，其現象與機制已被廣泛的發現及探討，然而至今並無一完整的理論可對於此議題作完整且正確的描述。而中子由於具有高穿透性、不帶電、與原子核而非電子進行交互作用以及具有磁矩的性質，近二十年來已成為研究工程材料性質的一重要的工具，其中中子繞射在分析材料微結構性質變化應用相當廣泛。
本實驗利用中子繞射的方式探討疲勞試驗中，在不同施加應力條件下﹝一般疲勞狀況、施加一較大拉伸應力﹞，疲勞裂縫尖端其殘餘應變及應力的分布，調查裂縫生長速率變化的可能因素。並且，量測三個軸向﹝裂縫生長方向、施力方向、厚度方向﹞的應力應變分布，有利於提供傳統上利用有限元素法模擬的實際實驗結果。此外，利用中子繞射最大的優勢，即利用及時量測的方式，分析當材料承受一拉伸應力狀態下，其應力及應變隨著施加應力的大小以及分布變化。

To discuss the deformation and fracture characteristics around the crack tip accurately is of importance to elucidate the fatigue crack growth mechanism and to further develop the life prediction methodology. The overload during constant-amplitude fatigue crack growth result in the retardation in the crack-growth rate, making it difficult to predict the crack-propagation behavior and fatigue lifetime. Although there have been numerous investigations to account for these transient crack growth behavior, the phenomena are still not understood precisely. Neutron diffraction measurements were employed to investigate these transient crack-growth mechanisms; which help us to gain a thorough understanding of the crack-tip deformation and fracture behaviors under applied loads; and to establish a quantitative relationship between the crack-tip-driving force and crack-growth behavior. In the recent study, a fatigue crack under constant-amplitude cyclic loading is studied on a 304L stainless steel compact-tension specimen. Neutron diffraction is employed to measure the distribution of three-orthogonal-direction strain fields directly with 1-mm3 spatial resolution as a function of distance from the crack tip. The mapping results provide insights into the fatigue crack tip which indicate the through-thickness direction residual-strain distributions around the crack tip are abnormal plane strain-like behavior which was observed at the mid-thickness position. Hence, the residual-strain distribution would be orientation dependent. Furthermore, in-situ measurements give the internal-strain evolution during the loading condition, the tensile/compressive strains transitions are also shown in this study.

ASTM Standard E647-99: Standard Test Method for Measurement of Fatigue Crack-Growth Raters.
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