機械構件之破壞模態分析及其應用

Abstract

本文將應用理論和實驗方法來研究一些重要機械或結構構件的破壞模態和行為，並由分析結果研擬改善這些構件的功能。其中包括研究易脆性複合材料發射筒端蓋在承受均勻靜態內壓、靜態外壓及動態內壓的破壞模態及強度。此發射筒端蓋以四片類似板狀的複合材料板塊膠合而成，此設計的用意不但是為了在端蓋受到衝擊內壓時會沿著預先設計的破壞模式破壞，並且要使它所能承受的靜態外壓破壞強度比所能承受的靜態內壓破壞強度大數倍。文中以有限元素法求取發射筒端蓋之應力分佈，並以適合的破壞準則為基礎驗證端蓋的破壞情況。在實驗方面，製作一些複合材料發射筒端蓋以測試靜態內壓、靜態外壓及動態內壓的破壞強度值，研究其破壞模式並以實驗結果驗證理論的預測值，由觀察得知，實驗與理論分析結果相當接近，故證明本文所設計的發射筒端蓋可以達成設計的目標。 另外，本研究應用破裂力學理論來建立一評估汽機元件疲勞壽命的方法。建立分析元件應力分佈的有限單元模式是評估元件疲勞壽命的一個重要步驟，本研究應用三維單元和表面接觸單元來建立葉片和其卡榫之有限單元模式，並藉此模式來探討葉片根部之應力分佈、裂縫形成位置、影響應力分佈之幾何參數和邊界條件及計算裂縫前緣之應力集中因子。由分析結果得知用此有限單元模式所預測裂縫形成之位置與實際葉片根部所發生者相同。裂縫前緣之應力強度因子是用裂縫閉合法來求得，裂縫之成長則藉Paris定律來計算。為了驗証所用有限單元模式及破裂分析方法之正確性，本研究對類似葉片的試片進行靜力和疲勞試驗，比較實驗和有限單元分析結果得知兩者之誤差相當小，可証所建立之有限單元模式確實可行。如此可證，本文所使用的破裂力學理論及裂縫成長率公式預估元件之殘餘壽命是正確的。另外，本研究也利用非破壞評估方法建立另外一種評估汽機元件受損程度和預測其殘餘壽命的方法。汽機元件受損後，其自然振動頻率會下降，首先應用有限單元法建立裂縫長度與元件基本振頻之間的關係，並探討裂縫對元件基本振頻的影響。為了驗證所建立之有限單元模式能準確預測含裂縫汽機元件的振動行為和自然頻率，故對一擬似葉片進行振動試驗，並比較理論預測與實驗量測之自然頻率，結果顯示，所建立之有限單元模式可獲合理之結果。然後量測含單一裂縫葉片之自然振頻以建立反算其裂縫長度的方法，根據量測到的自然振頻可判別裂縫產生的長度。若葉片內含有兩條裂縫，則再量測兩條裂縫間之應變變化，即可判定第二條裂縫的裂縫長度。最後再次應用破裂力學的理論來計算元件的殘餘壽命。 最後，應用理論及實驗方法研究以複合材料修補片修補受損元件在承受週期負荷後之破壞行為及其疲勞壽命之變化。在理論方面，修補元件的應力分佈和應力強度因子大小由有限單元方法求取，然後利用破壞準則判斷膠合層及修補片的破壞情況，並根據Paris定律求取受損元件經過修補後的疲勞壽命。在實驗方面，是以受損緊密張力試片(Compact Tension Specimen，簡稱C-T試片)進行雙面修補工作及對其作疲勞試驗。比較理論和試驗預測之疲勞壽命後得知所建立之分析模式能相當準確地預測修補件之疲勞壽命。

The failure modes and behaviors of a number of mechanical and structural components are studied via the theoretical and experimental approaches. The results obtained in the study are used to improve the performance of the components. Herein, the failure modes and strength of a frangible laminated composite canister cover subjected to uniform static internal pressure, static external pressure and dynamic internal pressure are studied via both theoretical and experimental approaches. The frangible canister cover, which is fabricated with four plate-like laminated composite parts, is designed in such a way that it will fail in a predetermined pattern when subjected to an impulsive internal pressure and its external failure pressure is much higher than its internal failure pressure. The stress distribution in the canister cover is determined using the finite element method and the failure of the cover identified on the basis of an appropriate failure criterion. A number of laminated composite canister covers were fabricated and subjected to uniform static internal pressure, static external pressure and dynamic internal pressure testings. The failure modes of the frangible covers are studied and the experimental results used to verify the theoretical predictions. Close agreements between the experimental and theoretical results have been observed. The present study shows that the design of frangible covers can be achieved. On the other hand, the theory of fracture mechanics is used to construct a method for the life assessment of the components of steam turbine. A finite element model using brick elements and surface contact elements is constructed for the stress analysis of turbine blades. Different geometric parameters and boundary conditions that can affect the stress distribution at the roots of the blades are studied. It has been found that a proper finite element model can produce reasonable results, ie, the location of crack initiation can be predicted as what has been observed in real blades. The modified crack closure technique is used to calculate the stress intensity factor at the crack tip. The Paris law is used to predict the crack propagation and residual life of the blades. Experimental investigation is performed using blade like aluminum specimens to study the stress distribution, deformation, and crack propagation at the roots of the specimens. The experimental results are then used to validate the finite element model and crack propagation law. The residual lives predicted by the proposed method match the experimental results well. A nondestructive evaluation method is also developed for the crack size identification and fatigue life prediction of damaged turbine blades. The present nondestructive evaluation method utilizes the feature that the natural frequencies of a damaged component are different from those of the component without damage. The finite element method is used to construct the relations between natural frequency and crack size for a turbine blade with different cracking conditions. The accuracy of the adopted finite element model has been verified against the experimental results obtained for a blade like specimen. Vibration testing of the damaged turbine blade is performed. The response of the blade is measured and the natural frequencies of the blade extracted. In the identification process, each of the possible cracking conditions is studied via a number of the finite element analyses of the blade. If the blade includes two cracks, the second crack length can be predicted by measuring strain between the two cracks. Finally, failure behavior and fatigue life of damaged components symmetrically repaired with bonded composite patches subjected to cyclic loading is studied via both theoretical and experimental approaches. In the analytical study, the stress distribution and stress intensity factor in the repair of damaged components is determined using the finite element method and the failure of composite patches and adhesive identified on the basis of an appropriate failure criterion. The applicability of Paris law in fatigue life prediction of damaged components repaired with bonded composite patches are investigated. Fatigue tests of C-T specimens with bonded repairs are performed to validate the accuracy and feasibility of the proposed method.