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A7085鋁合金Ⅰ-Ⅱ復合型疲勞裂紋擴展及其數值模擬

Ⅰ-Ⅱ mixed-mode fatigue crack propagation of A7085 aluminum alloy and its numerical simulation

  • 摘要: 為探究不同加載角度下A7085鋁合金Ⅰ-Ⅱ復合型疲勞裂紋擴展機理,在MTS疲勞試驗機上采用緊湊拉伸剪切試件(CTS)對A7085鋁合金進行不同加載角度的疲勞實驗;用有限元分析計算不同裂紋擴展長度的裂紋尖端應力強度因子,通過七點遞增多項式法對數據進行處理,計算出A7085鋁合金Paris公式中的參數Cm.結果表明不同加載角度的裂紋基本沿著與外載荷垂直的方向擴展,裂紋擴展路徑近似為一條直線,裂紋擴展角測量結果基本符合最大環向拉應力理論;Ⅰ-Ⅱ復合型裂紋一旦發生擴展,Ⅱ型應力強度因子K所占比例急劇減小,Ⅰ型應力強度因子K不斷增大,此后K遠遠小于K,有效應力強度因子(KK的組合)基本等于K,相當于裂紋擴展主要受Ⅰ型應力強度因子控制,研究結果有助于對Ⅰ-Ⅱ復合型疲勞裂紋擴展機理的理解.

     

    Abstract: Compared with other types of aluminum alloys, A7085 aluminum alloy has a series of excellent properties such as high strength, high toughness, and high fatigue resistance. These advantages meet the requirements of aircraft performance; thus, A7085 aluminum alloy is widely used for fabricating aircraft components. The shell cracks in aeronautical structures are often mixed-mode cracks, i. e., comprising open type and sliding type, and they are also known as the Ⅰ-Ⅱ compound crack. It has been found that fatigue fracture is the main reason for the failure of most specimens. At present, most studies on fatigue crack are focused on mode Ⅰ crack, but the load on the specimen is usually not a single pure type Ⅰ, Ⅱ, or Ⅲ mode. It is usually a combination of these three kinds of loads. When the crack is subjected to Ⅰ-Ⅱ mixed-mode loads, its crack growth rate and crack growth path are affected by the loading conditions. To investigate the mechanism of Ⅰ-Ⅱ mixed-mode fatigue crack growth of A7085 under different loading angles, mixed-mode (Ⅰ-Ⅱ) fatigue crack growth tests were performed on compact tension shear (CTS) specimens using a servo-hydraulic fatigue testing machine. The stress intensity factor of the crack tip was calculated by finite element analysis. Furthermore, C and m in the Paris law were calculated using seven-point incremental polynomial methods. The results show that when under different loading angles, cracks will extend along the vertical direction of the external load. Moreover, the path seems to be a straight line. The results of experiments agree with the maximum tensile stress theory. Once the crack expands, type Ⅱ stress intensity factor K basical-ly remains at 0, while type Ⅰ stress intensity factor K increases gradually. The stress intensity factor amplitude is almost equal to K Ⅰ, and crack propagation is mainly controlled by K. The result is helpful to understand the mechanism of the Ⅰ-Ⅱ fatigue crack propagation.

     

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