摘 要

本文應用實驗及數值解析的方法來檢討，鋼板因為表面開槽而引發斷面上的應力分佈不均勻狀況下，其疲勞裂縫進展的行為。主要目的在於釐清此形狀鋼板的疲勞裂縫進展行為與母材間的差異，並對於含有凹槽或凸槽之鋼板提供一個預測疲勞裂縫進展速度的方法。

首先對於含有不同凹槽半徑及凸槽之鋼板進行疲勞實驗，求出在相同荷重作用下之疲勞壽命、a-N曲線與da/dN- K曲線，分析疲勞裂縫在不同凹槽或凸槽狀況下進展的實際情形。在實驗過程中，適當的導入beach mark，藉此觀察疲勞裂縫進展過程中，裂縫尖端在試片厚度方向所顯現的形狀。對於含不同凹槽半徑及凸槽之鋼板，其疲勞進展速度不同的原因，在將量測得到的開口荷重加入考慮後可以得到合理的解釋。

此外，本文利用有限元素計算套裝軟體ANSYS.Code5.5，根據實驗條件計算各組模型的應力分佈，並配合破壞力學進行裂縫尖端塑性區域的計算，作為預估模式的基本數據。最後，根據實驗數據及有限元素法模擬分析結果，利用斷面上的塑性區域分佈情形來解釋不同凹槽半徑及凸槽之鋼板其開口荷重不同的原因。

最後利用Wheeler遲滯模型之觀念，建立一個可預測裂縫進展壽命之模式，作為往後預測貫穿裂縫在此類型鋼板之進展速度的依據。

ABSTRACT

The propagating characteristics of thickness-through-cracks in irregular stress distribution fields are discussed in the present paper. Three kinds of specimen having different size of defects on the both surfaces or steel sheets are designed (hereafter referred or as “defective specimen”) to produce irregular stress distribution fields. Using defective specimens and common CCT specimens (referred to as “flat specimen”) fatigue crack propagating tests were carried out.

During the tests, crack growth fates and closure load were measured, and the beach marks used to record the shape of crack tip were introduced. For an analysis purpose, stress distribution and plastic zone of defective specimens are calculated be finite element analysis.

As the results, although all of the specimens are subjected to the same cyclic loading, defective specimens have higher closure loads and longer fatigue propagation life than that of flat specimens. Taking the closure loads into account and using effective stress intensity range, , to assess the propagation life, a precise prediction can be obtained. Also, the shape of beach marks obtained from defective specimens differs from that of flat specimen and is highly stress distribution dependent.

Finally, an approach to estimate the growth rate of thickness-through-cracks in irregular stress distribution fields is proposed.

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