Abstract
本論文針對金屬缺口試桿從事雙軸向低週次疲勞壽命的實驗與分析。本文共分三大部分： 1.缺口試桿在承受基本雙軸向負荷時之裂縫形成壽命預測。 2.缺口試桿承受非正比型且具有平均應變負荷之裂縫形成壽命預測。 3.缺口試桿在受兩階段屬於不同負荷型式下的累積疲勞壽命預測。在裂縫形成壽命預測方面，局部應變法被廣泛地應用在求得缺口根部的應力應變態，以使吾人能從簡單易得之平滑試桿壽命曲線來預測缺口試桿的裂縫形成壽命。其中局部應變的求得，本文除了利用Neuber法則以外，有限元素法亦被使用。在局部之應力應變態求得後，再利用各種多軸向疲勞壽命預測模式來預測缺口試桿裂縫形成壽命。結果發現臨界平面法是預測效果最好的模式。其中Fatemi-Socie基於臨界平面，再加上考慮應力效應所提出之預測參數，在無論是否具有平均應變效應下，能有效地預測多軸向疲勞壽命。在累積疲勞方面，48種實驗情況在考慮第一階段週次比，兩階段基本壽命以及兩階段屬於不同負荷型式的變數下被測試。結果發現第二階段之扭向負荷將提高疲勞強度，而軸向負荷則對疲勞強度有害。在預測模式方面，雙線性損傷曲線是一能提供保守估計的有效預測方法。而此法中，膝點的決定也跟第二階段負荷型式有極大的關聯。The fatigue lives for notched specimens under multiaxialloading were investigated in this study. Three topics wereincluded: crack initiation life prediction for notchedspecimens under fully-reversed biaxial loading, the crackinitiation life prediction for notched specimens under biaxialnon-proportional loading paths with mean strain effect, and two-level cumulative fatigue behavior for notched specimens underdifferent biaxial loading mode sequences. In the correlationthe crack initiation lives, local strain approach was used tofind the local stress/strain states. In using local strainapproach, Neuber's estimation and elastic- plastic finiteelement method were employed. After obtaining the local stress/strain state, the prediction models for multiaxial fatiguelives were used to correlate the experimental results withreference data for smooth specimens. Lohr-Ellison model wasshown to give correlation between the fatigue lives withreference fatigue data for smooth specimens. In the study ofmean strain effect, the prediction models were extended byconsidering the stress effect. The predicted results by usingthese models were also compared and discussed. It was foundthat the critical planemodel proposed by fatemi and Sociecorrelated experimental results for notched specimens underboth proportional loading and non-proportional loading withinreasonable deviation bands. In the study of two-levelcumulative fatigue lives, several predicted models wereextended to evaluate the cumulative lives. Double linear damagerulewas found to evaluate the remaining cycle ratio of thesecond level within the reasonable deviation bands, and themajority of the predicted results were conservative.