SUMMARYA new upper bound formulation of limit analysis of two-and three-dimensional solids is presented. In contrast to most discrete upper bound methods the present one is formulated in terms of stresses rather than velocities and plastic multipliers. However, by means of duality theory it is shown that the formulation does indeed result in rigorous upper bound solutions. Also, kinematically admissible discontinuities, which have previously been shown to be very efficient, are given an interpretation in terms of stresses. This allows for a much simpler implementation and, in contrast to existing formulations, extension to arbitrary yield criteria in two and three dimensions is straightforward. Finally, the capabilities of the new method are demonstrated through a number of examples.

The application of conic programming to some traditionally difficult plasticity problems is considered. Convenient standard forms for conic programming of both limit and incremental elastoplastic analysis are given. The types of yield criteria that can be treated by conic programming is discussed and it is shown that the three-dimensional Mohr-Coulomb criterion can be cast as a set of conic constraints, thus facilitating efficient treatment by dedicated algorithms. Finally, the performance of a number of mixed finite elements is evaluated together with a state-of-the-art second-order cone programming algorithm.

SUMMARYThe problem of small-deformation, rate-independent elastoplasticity is treated using convex programming theory and algorithms. A finite-step variational formulation is first derived after which the relevant potential is discretized in space and subsequently viewed as the Lagrangian associated with a convex mathematical program. Next, an algorithm, based on the classical primal-dual interior point method, is developed. Several key modifications to the conventional implementation of this algorithm are made to fully exploit the nature of the common elastoplastic boundary value problem. The resulting method is compared to state-of-the-art elastoplastic procedures for which both similarities and differences are found. Finally, a number of examples are solved, demonstrating the capabilities of the algorithm when applied to standard perfect plasticity, hardening multisurface plasticity, and problems involving softening.

SUMMARYThe non-linear programming problem associated with the discrete lower bound limit analysis problem is treated by means of an algorithm where the need to linearize the yield criteria is avoided. The algorithm is an interior point method and is completely general in the sense that no particular ÿnite element discretization or yield criterion is required. As with interior point methods for linear programming the number of iterations is a ected only little by the problem size.Some practical implementation issues are discussed with reference to the special structure of the common lower bound load optimization problem, and ÿnally the e ciency and accuracy of the method is demonstrated by means of examples of plate and slab structures obeying di erent non-linear yield criteria.

SUMMARYRecently, Krabbenhøft et al. (Int. J. Solids Struct. 2007; 44:1533-1549) have presented a formulation of the three-dimensional Mohr-Coulomb criterion in terms of positive-definite cones. The capabilities of this formulation when applied to large-scale three-dimensional problems of limit analysis are investigated. Following a brief discussion on a number of theoretical and algorithmic issues, three common, but traditionally difficult, geomechanics problems are solved and the performance of a common primaldual interior-point algorithm (SeDuMi (Appl. Numer. Math. 1999; 29:301-315)) is documented in detail. Although generally encouraging, the results also reveal several difficulties which support the idea of constructing a conic programming algorithm specifically dedicated to plasticity problems.

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