连续体结构拓扑优化的应力约束全局化

    Topology Optimization of Continuum Structure With Globalization of Stress Constraints

    • 摘要: 连续体结构拓扑优化面临3个困难:1)基于柔顺度为目标的连续体结构,多工况问题是多目标问题,相比较单工况而言,更难于解决;2)局部性约束问题。如单元应力极限,比全局性约束问题如位移或频率极限,要复杂的多,因为局部约束问题敏度分析计算量巨大。3)载荷病态问题。类似于结构分析中的“总刚病态”,不容易得到最终的合理的拓扑结构,因为考虑到当载荷的大小差距较大时,在迭代过程中,小载荷的传力路径有可能消失。为克服上述困难,采取了如下措施:1)利用ICM方法建立拓扑优化模型。2)基于von Mises强度理论,所有单元的应力约束转化为结构的应变能约束,即全局的约束代替了局部约束。3)载荷病态被分成3种情况:(a)各工况间有载荷病态,但工况内无载荷病态;(b)仅在某工况内部有载荷病态;(c)各工况间有载荷病态,同时某工况内也有载荷病态。4)采用应力全局化的ICM方法,基于应变能的策略,上述提到的3种载荷病态问题按照不同的方法逐个得到了解决。数值算例说明了用全局应变能约束代替局部应力约束,传力路径更容易得到。用结构应变能代替载荷作为调整系数的方法非常好地解决了载荷病态问题。

       

      Abstract: There are three difficulties in the topology optimization problem of continuum structure:1) The problem under multiple load case is not easy to be approached than that under single load case, because the former becomes a multiple objective problem on the basis of compliance objective function. 2) The problem with local constraints, such as elemental stress limit, is not easy to be solved than that with global constraints, such as displacement or frequency limits, because sensitivity analysis of the former has too expensive computation. 3) The problem with a phenomenon of ill-conditioned load, which is similar to ill-conditioned stiffness matrix in the structural analysis, is not easy to get reasonable final topological structure, because the former is difficult to consider different influences between the loads with small forces and the loads with big forces, and some of topology paths of transferring small forces may disappear during the process of iteration. To overcome the above difficulties, some measures are adopted as follows:1) Topology optimization model is established by independent continuous mapping (ICM) method. 2) Based on the von Mises criteria in theory of elastic failure, all element's stress constraints are transformed into a structural energy constraint, namely, a global constraint substitutes for lots of local constraints. 3) The phenomenon of the ill-conditioned load is divided into three cases: (a) Ill-condition exists between load cases, but not within each load case; (b) Ill-condition exists within some load case; (c) Ill-condition exists not only between load cases, but also within some load case. (4) A strategy based on strain energy is proposed to adopt ICM method with stress globalization, and the problems of the three cases of ill-conditioned load mentioned above are solved in term of different complementary approaches one by one. Numerical examples show that the topology path of transferring forces can be obtained more easily by substituting global strain energy constraints for local stresses constraints, and the problem of ill-conditioned load can be solved well by the weighting method which takes structural energy as weighting coefficient.

       

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