| Summary: | 博士 === 國立交通大學 === 機械工程系 === 88 === 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.
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