The objective of this dissertation is to provide, for a fiber reinforced MMC, an understanding of the evolution of strains as a result of thermo-mechanical loadings and the relationship of the deformation evolution to microstructural factors. Both analytical and finite element approaches were used to predict global and local properties. The analytical approach involved rigorous modifications to the shear-lag model to account for fiber end effects. It was demonstrated that the modification not only results in a correct prediction of the modulus increases in the small aspect ratio regime, but is also able to correctly predict the values of local stress variations in the matrix and whisker. The elasto-plastic behavior of the composite including fiber/fiber interactions was investigated in detail. Cyclic stress-strain hysteresis and Bauschinger effect due to the presence of fibers was also analyzed. It was found that plastic constraint generates a triaxiality in the matrix and so gives a substantial increase of fiber axial stress and an elevation of composite flow stress. Detailed deformation evolution was evaluated both under monotonic and fatigue conditions. Role of thermal residual stresses due to thermal mismatch in MMCs were investigated using the thermo-elasto-plastic FEA with temperature dependent matrix properties. It was found that the spatial distribution of the residual stresses during cooling is sensitive to constraint effects. The evolution of the "caging" plasticity surrounding fibers during cooling including the shape and size of the plastic zones were determined. While FEA solutions give good results, the application of FEA to composites requires careful attention to the geometry of the optimum mesh used in the analysis. The optimization strategy was based on an error energy norm calculation for global convergence and traction differential approach for local convergence at the fiber/matrix interface. It was shown that this optimization approach provides the optimum mesh with a much more rapid convergence than conventional automatic codes. The mesh patterns generated are also shown to be significantly different from using this approach. A converged local property values can be obtained using a significantly lower degree of freedom than by conventional methods.
| Identifer | oai:union.ndltd.org:UMASS/oai:scholarworks.umass.edu:dissertations-6669 |
| Date | 01 January 1992 |
| Creators | Kim, Hong Gun |
| Publisher | ScholarWorks@UMass Amherst |
| Source Sets | University of Massachusetts, Amherst |
| Language | English |
| Detected Language | English |
| Type | text |
| Source | Doctoral Dissertations Available from Proquest |
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