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RISS 인기검색어
- 검색키워드
- 저자 : 후마윤 셰이크 무하마드 카비르 (검색결과 1 건)
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재료 매개 변수의 특성과 저사이클 피로 : CHARACTERIZATION OF MATERIAL PARAMETERS AND LOW CYCLE FATIGUE
후마윤 셰이크 무하마드 카비르 울산대학교 2011 국내박사
In the field of plasticity and low-cycle fatigue studies, the necessity of characterization of hardening parameters of constitutive models, study of the influence of temperature and strain rate on mechanical properties and fatigue life, and the evaluation of fatigue approaches concerning mean stress and mean strain effects have become important research arenas. And, continuously rising energy costs combined with the demand on high performance has necessitated that the materials in mechanical components become more efficient yet low priced and lightweight. Thus, for accurate understanding of the underlying physical mechanism and a methodology of aforementioned research arenas, monotonic and strain-controlled low-cycle fatigue tests results of some selected ferritic and austenitic stainless steels are used as the basis of this study. These steels are developed and competent for the use in exhaust manifold system of automobiles. This thesis proposes the procedure of characterization of material parameters of constitutive models and life prediction methodology of structural steels. The one aim of this thesis is to correlate the hardening parameters of plasticity models, and determine the unified material parameters evocatively to model the inelastic behavior of stainless steels. On the characterization of parameters, for better description of cyclic hardening/softening observed in the experiment, unified stress variation ratio and two isotropic hardening variables are introduced to the materials of interest. An improved algorithm to find the distribution of sub-elements effectively is also employed in Overlay model. The models are integrated using the implicit integration scheme and calibrated with the experimental stabilized loops available. The stabilized cyclic behavior of a material is of particular interest in low cycle fatigue life prediction since the crack initiation prediction is generally based on it. Therefore, determination of optimum parameters to find the stabilized hysteresis loop as well as other responses in an elasto-plastic steel behavior is an important step. Attempts are made to simulate the stabilized hysteresis loops in ABAQUS by means of UMAT subroutine for the considered elasto-plastic constitutive models using the determined material parameters. The predicted stabilized loops with the determined parameters show good agreement with the experimental results signifying the validity of the parameter determination scheme and correlation of hardening parameters. The other endeavor of this thesis is to reveal the effect of dynamic strain ageing on a ferritic stainless steel with detail relation to monotonic and cyclic responses over a wide range of temperatures. For assessing the effect of strain rate on mechanical properties, tensile test results are studied at two different strain rates. Serrations in monotonic stress-strain curves, anomalous dependence of tensile properties with temperatures, prominent hardening in low cycle fatigue curves, and negative temperature dependence of half-life plastic strain amplitude at a specific temperature regime are attributed to the dynamic strain ageing effect. The regime for dependence of marked cyclic hardening lies within the dynamic strain ageing regime of anomalous dependence of flow stress and dynamic strain hardening stress with temperature and negative strain rate sensitivity regime of monotonic response. It is believed that shortened fatigue life observed in the intermediate temperature is mainly due to the adverse effect of dynamic strain ageing. A life prediction model is addressed for as-received material to consider the effect of temperature on fatigue life. The numbers of load reversals obtained from experiment and predicted from fatigue parameter are compared and found to be in good agreement. In this thesis, another attempt is made to extend the total strain energy approach for predicting the fatigue life subjected to mean stress under uniaxial state. The effects of means stress on the fatigue failure of a ferritic stainless steel and high pressure tube steel are studied under strain-controlled low cycle fatigue condition. Based on the fatigue results from different strain ratios, modified total strain energy density approach is proposed to account for the mean stress effects. The proposed damage parameters provides convenient means of evaluating fatigue life with mean stress effects considering the fact that the definitions used for measuring strain energies are the same as in the fully-reversed cycling . A good agreement is observed between experimental life and predicted life using proposed approach. Two other mean stress models are also used to evaluate the low cycle fatigue data. Based on a simple statistical estimator, the proposed approach is compared with these models and is found realistic. Finally, strain-controlled fatigue data of an austenitic stainless steel subjected to uniaxial state at different strain ratios with tensile and compressive mean strain are investigated to understand the fatigue phenomena when non-zero mean straining is involved. The fatigue test result indicates that mean stresses relax to very low level and the material experiences elastic-plastic response throughout the life. Moreover, the material has higher fatigue resistance under tensile mean strain loading condition than that of completely reversed loading and compressive mean strain cycling at the same strain amplitude which might be attributed to the micro-structural deformation mechanism. The capabilities of several damage parameters to characterize the non-zero mean strain effect on fatigue life are examined based on fatigue data and are found inappropriate for the austenitic stainless steel. Therefore, a fatigue damage approach with a power law relationship between modified damage parameter and reversals to failure, considering a mean strain function in stress-strain-based fatigue damage module, is introduced to the material which shows better correlation with the uniaxial fatigue data under mean strain loading compared to that of some established fatigue models.
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