Keywords: topology optimization, stress constraints, mixed finite elements.
The main motivation behind this paper resides in three apparently independent issues that are conversely tightly connected:
 the still openproblem of obtaining stressconstrained optimal solutions. Most existing contributions in the field of topology optimization do not even introduce
any stress constraint and the consequent optimal solutions are likely to be practically unfeasible because of local stress violations. As pointed out in [2] two main questions
need be addressed at this regard, namely the computational method should be able to handle many local constraints and a suitable stress criterion should be devised capable to handle
intermediate density materials;
 the wellknown checkerboard pattern problem that is likely be encountered unless suitable remedies are adopted. Traditionally, one resorts to some filtering approach
imitating more or less what is done in the area of image processing [3];
 the spacediscretization scheme chosen to approximate the problem. In most applications, classical displacementbased finiteelement approaches are adopted. By so doing, one
gains numerical tractability but the use of such a simple scheme has two main drawbacks. On the one hand stresses are not primary variables of the formulations and therefore imposing
a stress constraint calls for the computation of an approximate strain field that is converted to the stress field by the adopted constitutive law. Secondly traditional displacement
schemes do not permit the modelling of extreme materials such as incompressible ones because of the wellknown locking problem.
The aim of this paper is to propose a novel formulation for stressconstrained topology optimization based on a mixedfiniteelement approximation scheme. The variational principle of HellingerReissner
is used and two dual formulations proposed. The first one is simpler in that classical polynomial finiteelements may be used to approximate a regular displacement field and
piecewise discontinuous stress field. Results from this formulation are presented in this paper. The dual formulation, often refereed to as "trulymixed" in the literature
[
1], has the appealing feature of passing the infsup condition even in the case of incompressible materials.
Figure:
Example 1. Optimal density. No stress constraint (L)  With stress constraint (R)

Figure:
Example 1. von Mises stress  No stress constraint (L)  With stress constraint (R)

 1
 Brezzi F. and Fortin M. 1991. Mixed and Hybrid Finite Element Methods. SpringerVerlag, New York.
 2
 Duysinx P. and Bendsøe M. Topology optimization of continuum structures with local stress constraints. International Journal for Numerical Methods in Engineering 1998; 43:14531478. doi:10.1002/(SICI)10970207(19981230)43:8<1453::AIDNME480>3.0.CO;22
 3
 Sigmund O. and Petersson J. Numerical instabilities in topology optimization: A survey on procedures dealing with checkerboards, meshdependencies and local minima. Struct. Multidisc. Optim. 1998; 16(1):6875. doi:10.1007/BF01214002
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