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PROCEEDINGS OF THE EIGHTH INTERNATIONAL CONFERENCE ON COMPUTATIONAL STRUCTURES TECHNOLOGY
Edited by: B.H.V. Topping, G. Montero and R. Montenegro
Finite Element Analysis of the Fiber Offsetting Effects on the Creep Behaviour of MMC Composites
M. Mondali, A. Abedian and M. Pahlavanpour
Department of Aerospace Engineering, Sharif University of Technology, Tehran, Iran
M. Mondali, A. Abedian, M. Pahlavanpour, "Finite Element Analysis of the Fiber Offsetting Effects on the Creep Behaviour of MMC Composites", in B.H.V. Topping, G. Montero, R. Montenegro, (Editors), "Proceedings of the Eighth International Conference on Computational Structures Technology", Civil-Comp Press, Stirlingshire, UK, Paper 16, 2006. doi:10.4203/ccp.83.16
Keywords: FEM, fiber offsetting, creep, metal matrix composite, debonding.
In recent years, the high temperature creep behavior of discontinuous SiC-reinforced aluminum alloys (whiskers and particulates) has been the subject of many creep investigations that aimed at assessing the potential of these materials for the use in high temperature applications. So far, the finite element (FE) studies on the creep of composites have not been so successful as a result of the dependence of the modeling on the mathematical creep relationships used for the matrix, the amount of fiber-matrix debonding, the distribution and arrangement of fibers in the matrix, and the manufacturing defects such as voids [1,2,3].
In the previous research, the effects of the first two phenomena, i.e. the type of mathematical model of creep and the amount of interface debonding on the creep behavior of the SiCw/Al6061 composite have been studied [4,5]. The results showed that the power law model of creep (which is widely considered for the matrix) is not sufficient to explain the creep deformation of the composite. Also, they indicated that the FE model must include the material imperfections such as the fiber-matrix debonding to produce better creep estimations. Moreover, it appeared that the value of fiber-matrix debonding parameter could only changes in the range of zero (no debonding on the fiber side surface but full debonding at the fiber end) to 0.5 (i.e. full end debonding and side debonding of half a fiber length) . However, the results of FE modeling using the exponential type law of creep for the matrix and considering fiber-matrix debonding were found to be in a good agreement with the experimental results. In fact, the discrepancy between the experimental and numerical results showed a good reduction for the case with fiber-matrix partial debond compared to the studies using perfect bond or with only fiber end debond.
In the present research work, the effects of the offsetting (i.e. the first case for an irregular fiber arrangement where the ends of fibers in every two parallel rows of fibers overlap) on the creep rate of metal matrix composites is studied. This phenomenon has been studied before by other researchers using 2D plain strain models . However, the results indicated that the model was unsuccessfull. It must be noted that even with no fiber offsetting, the plain strain model have not been successful either. In fact, the creep strain rate calculated by the model is four times grater than the results obtained by the studies using axisymmetric models. Therefore, considering the accuracy problem of the plain strain model and inability of the axisymmetric model for modeling of the offsetting phenomenon, the use of 3D models is inevitable.
Since the geometrical parameters required for modeling the composite are found from SEM photos, it is so important to define the percentage of the offsetting ratio (OR) in real composites in order to choose the best model for the composite under consideration. However, based on the results obtained here, the offsetting ratio of OR=0.21, is considered as the critical ratio.
In another attempt, since the fiber/matrix debonding affects the creep strain rate, the effects of fiber offsetting in presence of debonding was also tested. Comparison of the available experimental creep data with the results obtained from the three dimensional FE model shows that the suggested modeling modifications (i.e. debonding and fiber offsetting) and the material modeling (i.e. incorporation of the exponential creep law) play an important role in precise prediction of the creep deformation of the metal matrix composite. Interestingly, the results obtained highly match the existing experimental results.
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