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In this paper, we investigate the stress fields in optimal elastic structures by the necessary conditions of optimality, studying two-phase elastic composites in two dimensions. The necessary conditions show that an optimal design is characterized by three zones: Zone 1 of pure weak Material 1; Zone 2 of pure strong Material 2; and Zone 3, where Material 1 and Material 2 mix to form an optimal microstructure. To characterize these zones, we introduce two rotationally invariant norms N
1 and N
2 of a stress tensor. The derived optimality conditions state that the inequality N
1()< constant
1 holds in Zone 1; the inequality N
2()> constant
2 holds in Zone 2; and two equalities hold simultaneously in Zone 3: N
1()= constant
1, in Material 1; N
2()= constant
2 in Material 2. Using this result, we analyze suboptimal projects and find how close the fields are to the regions of optimality. 相似文献