ABSTRACT of the Doctoral Thesis

Micro- and Macromechanical Models for Hybrid, Selectively Reinforced Structures

by Christian M.Chimani (1997)

In modern research and development numerical simulation plays an important role for obtaining a better understanding of complex correlations in natural and technical processes and thus reducing expensive trial and error loops in experimental development procedures. Based on research in the field of micromechanics of materials, in the present work numerical simulation techniques are employed and developed that make it feasible to investigate the thermo-mechanical behavior of light metal matrix composites and selectively reinforced structures.

An overview of selected micromechanical methods for the analytical and numerical description of heterogeneous materials is followed by an examination whether these methods, especially the Mori-Tanaka method, give reliable results modeling composites containing curved fibers, as found e.g. in circumferentially reinforced axisymmetric composites.

An introduction and comparison of some mechanical properties is given for fiber reinforced magnesium and aluminium based composites. This material characterization can provide information, from the mechanical point of view, for a proper selection of constituents used for the experimental development of processing routes for the manufacturing of selectively reinforced components.

At the intersection of a material interfaces and the free surface in multi material structures complex tri-axial stress states occur which are expected to be critical with respect to damage in the envisaged applications. These free edge effects are studied in terms of a bimaterial wedge problem. The stress singularities typically predicted when homogeneous material descriptions are used are determined analytically and numerically. Based on the analytical solution techniques rules for an optimized interface design are derived.

Introducing a micromechanical embedding technique that explicitly accounts for the micro scale heterogeneity of the composite material the previously mentioned singular solutions are reconsidered. It is found that the singular solution disappears for several important applications.

Finally a thermo-elasto-plastic analysis of a selectively, circumferentially reinforced axisymmetric structures is presented. Two modeling techniques, the incremental Mori-Tanaka approach and a hexagonal cell tiling approach, are employed for comparison. The stress distribution after cooling down from the manufacturing temperature is studied on the macro and micro level.

revised 980114 (hjb)