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Title: On the evaluation of cyclic behaviour for a mechanical structure under variable mechanical and thermal loads
Author: Chen, Weihang
ISNI:       0000 0004 2744 5376
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2012
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This thesis investigates and develops computational procedures for the assessment of the structural integrity and lifetime of mechanical structures subjected to cyclic variable loads. The Linear Matching Method (LMM), which has been used in design and life assessment calculations for a number of years, is adopted for investigating the above problems. In order to produce sequences of linear solutions with spatially varying linear moduli, LMM matches the behaviours of a non-linear material to that of a linear material. The developed iterative programming algorithms, when implemented within the finite element scheme, ABAQUS, generate a monotonically reducing sequence of upper bounds that ultimately converges to the least upper bounds. There are three main objectives of the present study. The first is to investigate the overall response of mechanical structures under the combined actions of mechanical and thermal loads without the presence of creep. Shakedown and ratchet limit interacti on diagrams of different types of mechanical structures identifying the regions of shakedown, reverse plasticity, ratchetting and plastic collapse mechanism are presented and parametric studies of different types of defective component are investigated. The results for different types of mechanical structures shown in the parametric studies, confirm the applicability of LMM on complex structures under cyclic loadings without the effect of creep. The second objective is to bridge the gap between Continuum Mechanics and Fracture Mechanics through the LMM by calculating the cyclic J-integral (ΔJ). The derivation of ΔJ based on the potential energy expression for a single edge cracked plate subjected to cyclic uniaxial loading and cyclic bending moment conditions using LMM is formulated in this study. The results of the proposed model have been compared to the cases obtained from the Reference Stress Method for a single edge cracked plate. They indicate that the estimates provide a relatively easy method for estimating ΔJ considering the complete accumulated cycle effects. The last objective is to further develop the current LMM for the evaluation of the cyclic behaviour for mechanical structures when the effect of creep is taken into consideration. The creep strain and plastic strain range are obtained to be used in creep damage and fatigue assessments, respectively. A benchmark example of a Bree cylinder subjected to cyclic thermal load and constant mechanical load is analysed to verify the applicability of the LMM in creep fatigue damage case. The cyclic responses for different loading conditions and dwell time periods within the Bree boundary are obtained. In order to show the efficiency and effectiveness of LMM method for defective structures, a three dimensional plate with a hole and a welded pipe subjected to cyclic thermal loads and constant mechanical loads are analysed. The results of both examples show that the presence of creep changes sign ificantly the cyclic responses. The LMM procedure provides a general purpose technique for the evaluation of cyclic behaviour and the determination of the plastic strain range and creep strain for the creep fatigue damage assessment with creep fatigue interaction.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available