三維疲勞裂紋形狀之演進

Crack Shape Evolution of Three-Dimensional Fatigue Cracks

張揚揮(Yang-Hui Chang)

指導教授 ： 侯建元

國立暨南國際大學/科技學院/土木工程學系/碩士(2014年)

https://doi.org/10.6837/NCNU.2014.00341

摘要

銲接鋼構件受反覆載重後常於銲趾處產生眾多微小表面裂紋。這些裂紋起先各自獨立成長，而後結合成為一個較大的裂紋，欲掌握這些裂紋的成長行為，須對裂紋的形狀演變進行正確的模擬。本研究建立一套電腦自動化程序以模擬三維表面半橢圓狀疲勞裂紋的形狀發展，其中不同形狀的裂紋以其短軸對長軸之比表示。分析結果發現，不論半圓狀的起始裂紋或是十分狹淺的起始裂紋，發展至最後其短軸對長軸比之值均介於0.6~0.8間。文中亦分析每次裂紋向前成長之最大距離 、試體寬度b、高度h對形狀演變的影響，結果均顯示這些參數對於模擬之裂紋形狀均無明顯的影響。本文亦探討對稱的二個共平面半橢圓裂紋在結合後的形狀演變，模擬結果顯示半橢圓之中心與對稱面距離愈大，則二裂紋的前緣成長為單一裂紋所需之距離亦較大。

關鍵字

疲勞裂紋 ； 有限元素 ； 裂紋成長 ； 裂紋結合

並列摘要

Fatigue is the main reason that causing heavy casualties in steel structures. In many cases of steel structure which have been fatigue failure, most of the fatigue crack initiate at the toe of weld. Because the irregular geometry of weld toe, small cracks will start from weld toe after the force loading of the members. The rate of crack growth will be faster after various cracks combined, also the crack shape will change drastically. This study use finite element method with the previous theory and establish a set of automated computer program to simulate the foregoing growth process. The result showed that whether the initial crack shape is semicircular or very shallow, the value of aspect ratio is between 0.6 and 0.8 in the last stage of growth. And the maximum length of crack growth, , model width, b and height, h, are insignificant on crack shape growth simulation. This study also discuss the crack shape evolution of two symmetrical coplanar semi-elliptical after crack combined. The simulation results show that the greater distance between the center of the semi-elliptical and plane of symmetry, the required distance of the leading edge of two cracks grows into a single smooth curve is also larger.

並列關鍵字

Fatigue cracks ； Finite element ； Crack growth ； Crack combine

參考文獻

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