冷擴孔周圍殘留應力之研究

Study of Residual Stresses around Cold-expanded Holes

賈德(Khurram, Amjad)

指導教授 ： 王偉中

國立清華大學/工學院/動力機械工程學系/博士(2017年)

摘要

為了增強機體結構中緊固件孔的疲勞特性，開縫襯套擴孔是一個目前廣泛應用於航太工業中之技術。本研究觀察在結構件於擴孔後，周圍產生的壓縮殘餘應力引起疲勞行為產生的裂紋。為了探討擴孔後所產生之疲勞裂紋行為，並且同時探討裂紋周圍之殘留應力，本研究主要探討的有兩方向：一為探討擴孔後之變形行為，二為擴孔後之裂紋疲勞行為。
應變分析採用立體數位影像相關（Digital Image Correlation, DIC）法量測0.25 與1 的兩種厚度/孔洞直徑比的鋁合金於擴孔後之應變變化，經DIC 法量測能獲得擴孔後所產生塑性區域的全域形變與擴孔後的孔徑形狀與尺寸。結果顯示傳統開縫襯套擴孔法並無法有效的在厚壁與薄壁緊固件孔周圍產生軸對稱的壓縮殘餘應力，而實驗之薄壁試片採用與飛行器之機身與機翼蒙皮相同厚度，因此需要重新檢視傳統開縫襯套擴孔法之作業程序。
本論文以利用DIC法分析多層板試片於擴孔後之全域應變，由實驗結果可得知使用多層板之結構確實可改善結構。因此，本論文建立一可有效分析機體結構中之緊固件孔與鉚釘固定連結後結構強度之判斷方法。此外，利用熱彈應力（Thermoelastic Stress Analysis, TSA）方法探討構件於擴孔後孔邊之疲勞裂紋成長；同步輻射X 光繞射（Synchrotron X-ray Diffraction, SXRD） 技術分析裂紋周圍之殘留應力。
長久以來許多文獻探討如何利用降低壓縮殘留應力抑制疲勞裂紋成長有著不同之論點，本論文為了探討此問題利用TSA 與SXRD 進行量測並探討疲勞裂紋成長與殘留應力間之關係，但由實驗結果可得知疲勞裂紋之形成與成長並無明顯之關聯性。實驗結果可觀察到構件受力條件與殘留應力有效降低之關係，由此對於建立一準確之理論模型為非常之重要。因此，本論文提出的方法與實驗結果有利於航太工業提供一精準分析與量測擴孔製程後之技術，並於製造機體時有效判斷最佳之加工狀態以提高構件之疲勞耐久性。

關鍵字

開縫襯套擴孔 ； 緊固件孔 ； 殘留應力 ； 疲勞裂紋 ； 數位影像相關 ； 熱彈應力 ； 同步輻射X光繞射 ； 疲勞

並列摘要

Split sleeve cold expansion is one of the most widely used methods in the aerospace industry to enhance the fatigue performance of fastener holes in airframe structures. The initial motivation, which led to this research programme, was to develop an understanding of the behaviour of fatigue cracks emanating from cold-expanded holes, with a particular emphasis on the influence of these cracks on the surrounding compressive residual stresses. There are two strands of the research presented in this thesis: first being related to the study of hole deformation resulting from split sleeve cold expansion; and the second one focused on the fatigue behaviour of cracks emanating from cold-expanded holes.
The strain fields developed from cold expansion were measured using stereoscopic digital image correlation (DIC) technique in aluminium specimens of two different thicknesses giving thickness to hole diameter ratio of 0.25 and 1. The capability of DIC in providing full-field strain data was exploited to determine the shape and size of the plastic zones developed from cold expansion. The results showed that the existing split sleeve cold expansion process is not as effective in creating an axisymmetric compressive residual elastic stress field around the fastener holes in thin as it is in the thick specimens. The thin specimens used in this investigation were equivalent in thickness to sheet material commonly used in an aircraft fuselage or wing skins and the results indicate that there is a need to review the use of cold expansion process using a split sleeve and mandrel for holes in thin sheets.
A simple approach utilising DIC was presented to analyse the strain fields resulting from cold expansion in stacked specimens. The results showed that stacking offers some improvement in the cold expansion of thin sheet components. They also demonstrated the workability of this approach which can be applied effectively to analyse cold expansion of fastener holes associated with a real joint configuration in an airframe.
The propagation of fatigue cracks initiating from the cold-expanded holes was investigated by employing the thermoelastic stress analysis (TSA) technique and their influence on the surrounding residual stresses was determined using synchrotron x-ray diffraction (SXRD) technique. A long-standing ambiguity in the literature regarding the potential relaxation of beneficial compressive residual stresses, as a result of fatigue crack propagation, was addressed; and it was established, from TSA and SXRD results, that the formation or propagation of a fatigue crack does not cause any significant relaxation of these residual stresses. The results also clearly identify the loading conditions under which the residual stresses are expected to relax. This information is important in improving the theoretical models for fatigue life assessment of cold-expanded holes. The results should also be useful for the engineers in the aerospace industry to realise the full potential of the cold expansion process and to utilise it more effectively in the manufacturing of airframes leading to improved fatigue endurance under different loading conditions.

並列關鍵字

Cold Expansion ； Fastener Holes ； Residual Stress ； Fatigue Cracks ； Digital Image Correlation ； Thermoelastic Stress Analysis ； Synchrotron X-Ray Diffraction ； Fatigue

參考文獻

1. Gran R, Orazio FD, Paris P, Irwin G and Hertzberg R. Investigation and Analysis Development of Early Life Aircraft Structural Failures. Ohio US: Air Force Flight Dynamics Laboratory, Wright-Patterson Air Force Base; 1971. Available from: www.dtic.mil

2. Champoux R. An overview of hole cold expansion methods. In: Fracture Prevention and Design: International Conference Proceedings (ed: Barnby J) Amsterdam: EMAS; 1986.

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4. Maximov J, Duncheva G and Amudjev I. A novel method and tool which enhance the fatigue life of structural components with fastener holes. Eng Failure Anal 2013; 31: 132-143.

5. Process specification 8101J: Cold expansion of holes using the standard split sleeve system and countersink cold expansion. Seattle US: Fatigue Technology Inc; 2014. Available from http://www.fatiguetech.com