非均勻應力分佈板材中貫穿型疲勞裂縫之進展行為

The Propagating Behaviors of Cracked Sheets in Non-Uniformed Stress Distribution Fields

本文針對貫穿型平板疲勞裂縫在板厚方向的應力分佈不均勻狀態下之進展行為，以應力分析與實驗方式進行檢討。在CCT（center crack tension）試驗片表面，沿著疲勞裂縫進展方向，導入形狀不同的圓弧，使得板材在板厚方向的應力分佈呈現非均勻狀態。依據此試片尺寸，分別建立二次元與三次元有限元素模型，分析其應力集中狀態、裂縫前緣塑性區域尺寸與應力強度因子（stress intensity factor, SIF）等影響裂縫進展行為的數據。並由分析所得到之數據，配合疲勞裂縫進展實驗的結果，檢討非均勻應力分佈對於疲勞裂縫進展壽命的影響。結果顯示，在相同的外力作用下，非均勻應力分佈場下的疲勞進展壽命比較長，而這個現象係由裂縫先端在厚度方向的塑性區域尺寸差異所導致。 The propagating behavior of penetrated fatigue cracks in non-uniformed stress distribution fields is studied. Two kinds of center crack tension (CCT) specimens having geometric discontinuity that induces the non-uniform stress distribution along the thickness direction were designed. Stress concentration factor, crack tip plastic zone and stress intensity factor were assessed through finite element analysis (FEA). In order to identify the effect of the non-uniformed stress, fatigue crack propagating tests under the same loading conditions were carried out. As the results, the specimens with non-uniformed stress distribution have longer propagating life than that of the common CCT specimen also it is found that the slower propagating rate is derived from the non-uniformed plastic zone size.