ABSTRACT of the Doctoral Thesis

Design and Computational Analysis of Compound Castings and other Multi-Material Structures

by Robert Bitsche (2009)

The multi-material lightweight design concept strives to use the "best" material and manufacturing process for each part of a structure in order to combine the advantages of different materials. Obviously, joining techniques play a major role in the manufacturing of these structures. The compound casting process allows for the joining of a casting to other parts during the casting process. That is, the casting process serves both as a production and a joining process.

The aim of this thesis is to develop computational methods for the analysis and design of compound castings and other multi-material structures. Both finite element methods and asymptotic analysis techniques are used.

During the quenching (or cooling) of a compound casting residual stresses develop due to the inhomogeneous transient temperature field and the dissimilar coefficients of thermal expansion of the materials involved. As these stresses determine the frictional connection and other important characteristics (e.g. the fatigue life) of the structure, the simulation of the quenching process is of central importance.

In the case of purely contacting interfaces, i.e., if no metallurgical bonding exists, the heat transfer at the interface is either by contact or through the gap, and the thermal contact conductance at the bimaterial interface of the compound casting depends on contact pressure and gap opening. A major finding of this thesis is that, in general, the consideration of this dependence is crucial to the simulation of the quenching process of compound castings.

During the quenching process gaps can open up at the bimaterial interface even if the structure is geometrically simple. The opening of the gap severely reduces the thermal contact conductance and forces heat to flow mainly parallel to the open gap.

Practical examples of steel-aluminum compound castings with form-locking and/or frictional connection are presented. In general, the strength of these connections could be well predicted by the finite element simulations.

Local stress concentrations can occur due to the abrupt change in material properties at the interface of a multi-material structure. Under the assumptions of linear elasticity theory, these stress concentrations can manifest themselves as stress singularities. The dependence of the order of these singularities on geometrical and material parameters is examined in a systematic way and "design charts" are developed by which the order of the stress singularity can be directly registered. Using these charts, geometry modifications can be determined that either minimize the order of the stress singularity or lead to a regular stress field. Often, great improvements can be achieved through comparatively small and local modifications of the geometry.

revised 090525 (hjb)