Digital Repository, The Annual Postgraduate research Student Conference - 2015

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An elastoplastic model simulation to calculate local stress-strain sequences under uniaxial/multiaxial constant/variable amplitude cyclic loading
N. Zuhair Faruq

Last modified: 2015-04-09


The demand for advanced design method to accurately analyse fatigue damage of complex systems under multiaxial variable amplitude loading condition, in conjunction with the minimum safety factors due to more emphasis on lighter structures, saving materials and cost reduction has been extremely increasing. The correct prediction of fatigue lifetime under multiaxial cyclic loading seriously depends on the description of local elastoplastic stress-strain sequences at the critical point on a component [1]. Furthermore, it’s universally agreed that in all engineering components local elastoplastic stress/strain will vary according to the geometrical features and degree of multiaxiality. Consequently, estimating these sequences will be more complicated by the presence of notch under multiaxial loads. Accordingly, calculating the local elastoplastic stress-strain properties with sufficient accuracy will result in a reliable and realistic fatigue assessment. However, procedural standards to plot the local elastoplastic stress-strain properties without performing experiment work are not yet available when the reversal stress/strains are involved. For the sake of accuracy, people come to believe that performing an experimental investigation is the best way to accurately estimate the local stress-strain response of components under multiaxial loading. In contrast, engineering designers argued that time and monetary requirements in the experimental investigation, in addition to the economic consideration or sometimes difficulty in accessing an accurate experimental testing machine parallel to the development of modern programing have increased attention on the use of numerical simulation model analysis. In the light of the above well-known fact, the present paper summarises an attempt to correctly formalise a novel model to be used to predict fatigue life time of unnotched components. The proposed elastoplastic model was analysed by using finite element (FE) program system ANSYS under reversed constant/variable amplitude uniaxial and torsion, as well as in-phase and out-of-phase multiaxial (tension-torsion) strain controlled loading at room temperature. From a reliability and safety point of view, a systematic validation exercise is followed by: First, using 38 experimental data sets from other technical literatures [6,7,8,9,10 &11] generated by testing 6 different materials under various loading conditions. Second, comparing the predicted local stress-strain properties with the result obtained from Jiang’s model [2] that was built based on the material properties and constants obtained from the experimental test. To conclude, the local predicted hysteresis loops are compared with their experimentally determined counterparts and the results from Jiang’s model. Such an extensive exercise showed that the proposed model has the capability of describing the local elastoplastic deformation features of materials under different cyclic loading. The overall scientific goal beyond this research is to formalise and verify a novel systematical study to simulate an accurate model, which can be used to estimate fatigue life time of notched metallic components under multiaxial variable amplitude loading case that will be outlined in the next steps of this research.

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