Michal Kursa

In metal single crystals, the observed formation of deformation banding pattern has been explained by greater latent hardening of slip systems than their self-hardening, which promotes spatial segregation of plastic slips and lamination towards single-slip domains. Numerical studies focusing on the formation of deformation bands usually involved initial imperfections, boundary-induced heterogeneity, or the postulate of minimal global energy expenditure which additionally promoted non-uniformity of deformation. This article analyses the case when no such mechanism enforcing locally non-uniform deformation is implemented in the finite element (FE) method, while the global system of equations of incremental equilibrium is solved in a standard way. The new finding in this paper is that the deformation banding pattern can appear spontaneously in FE simulations of homogeneous single crystals even in the absence of any mechanism favouring deformation banding in the numerical code. This has been demonstrated in several examples in the small strain formalism using a plane-strain model in which the twelve fcc slip systems are reduced to three effective plastic slip mechanisms. Incremental slips are determined at the Gauss-point level either by incremental work minimization in the rate-independent case or by rate-dependent regularization. In the rate-independent approach, the trust-region algorithm is developed for the selection of active slip systems with the help of the augmented Lagrangian method. Conditions under which a banding pattern appears spontaneously or is suppressed are discussed. In particular, a critical rate sensitivity exponent is identified.

Nonlinear properties of metal matrix composites (MMCs) are studied. The research combines results of loading–unloading tensile tests, microstructural observations and numerical predictions by means of micromechanical mean-field models. AA2124/SiC metal matrix composites with SiC particles, produced by the Aerospace Metal Composites Ltd. (AMC) are investigated. The aluminum matrix is reinforced with 17% and 25% of SiC particles. The best conditions to evaluate the current elastic stiffness modulus have been assessed. Tensile tests were carried out with consecutive unloading loops to obtain actual tensile modulus and study degradation of elastic properties of the composites. The microstructure examination by scanning electron microscopy (SEM) showed a variety of phenomena occurring during composite deformation and possible sources of elastic stiffness reduction and damage evolution have been indicated. Two micromechanical approaches, the incremental Mori–Tanaka (MT) and self-consistent (SC) schemes, are applied to estimate effective properties of the composites. The standard formulations are extended to take into account elasto-plasticity and damage development in the metal phase. The method of direct linearization performed for the tangent or secant stiffness moduli is formulated. Predictions of both approaches are compared with experimental results of tensile tests in the elastic–plastic regime. The question is addressed how to perform the micromechanical modelling if the actual stress–strain curve of metal matrix is unknown.

SUMMARY: We propose a new class of modular-robotic structures, intended to produce forces which scale with the number of modules. We adopt the concept of a spherical catom and extend it by a new connection type which is relatively strong but static. We examine analytically and numerically the mechanical properties of two collective-actuator designs. The simulations are based on the discrete element method (DEM), with friction and elastic deformations taken into account. One of the actuators is shown to generate forces proportional to its volume. This property seems necessary for building modular structures of useful strength and dimensions.

A collective actuator is a self-reconfigurable modular-robotic structure which produces useful mechanical work through simultaneous reconfiguration of its constituent units. An actuator is additionally called scalable if its force-to-weight ratio does not depend on the number of its member modules. In this work, we consider scalable collective actuators built from spherical catoms with two connection types: strong but fixed and weak but mobile. We investigate how to construct these actuators in such a way, as to maximize their force-to-weight ratio. We present a number of designs of high strength, whose force capacities significantly exceed those of similar actuators reported previously.

Summary: A new constitutive algorithm for the rate‐independent crystal plasticity is presented. It is based on asymptotically exact formulation of the set of constitutive equations and inequalities as a minimum problem for the incremental work expressed by a quadratic function of non‐negative crystallographic slips. This approach requires selective symmetrization of the slip‐system interaction matrix restricted to active slip‐systems, while the latent hardening rule for inactive slip‐systems is arbitrary. The active slip‐system set and incremental slips are determined by finding a constrained minimum point of the incremental work. The solutions not associated with a local minimum of the incremental work are automatically eliminated in accord with the energy criterion of path stability. The augmented Lagrangian method is applied to convert the constrained minimization problem to a smooth unconstrained one. Effectiveness of the algorithm is demonstrated by the large deformation examples of simple shear of a face‐centered cubic (fcc) crystal and rolling texture in a polycrystal. The algorithm is extended to partial kinematic constraints and applied to a uniaxial tension test in a high‐symmetry direction, showing the ability of the algorithm to cope with the non‐uniqueness problem and to generate experimentally observable solutions with a reduced number of active slip‐systems. Copyright © 2015 John Wiley & Sons, Ltd.