S.M. Keralavarma

A discrete dislocation analysis of strengthening in bilayer thin films

A two-dimensional (2D) discrete dislocation plasticity framework, which incorporates some three-dimensional mechanisms through constitutive additions, is used to analyse the response to uniaxial tension of nanoscale bilayer thin films. Frank–Read sources, modelled as junction dipoles in 2D, act as sources of dislocations. Infinite, homogeneous medium fields of the discrete dislocations are superposed with a non-singular complementary field that enforces the boundary conditions and accounts for image stresses arising from the difference in elastic properties between the layers. The resulting boundary value problem is solved using the finite element method. Analysis has been carried out for fully coherent bilayer Al/Cu and Cu/Ni films oriented for double slip. The analysis accounts for the effects of three key mechanisms: resistance to dislocation nucleation and motion due to elastic modulus mismatch (e.g. Koehler barrier); single-dislocation bow-out within layers (Orowan process) and slip blocking at interfaces (Hall–Petch mechanism). The relative importance of each mechanism is studied as a function of the bilayer thickness. The results indicate a significant strengthening with decreasing bilayer thickness. Conclusions are drawn regarding the possible causes of the observed strengthening.

Quantum-to-continuum prediction of ductility loss in aluminium–magnesium alloys due to dynamic strain aging

Negative strain-rate sensitivity due to dynamic strain aging in Aluminium-5XXX alloys leads to reduced ductility and plastic instabilities at room temperature, inhibiting application of these alloys in many forming processes. Here a hierarchical multiscale model is presented that uses (i) quantum and atomic information on solute energies and motion around a dislocation core, (ii) dislocation models to predict the effects of solutes on dislocation motion through a dislocation forest, (iii) a thermo-kinetic constitutive model that faithfully includes the atomistic and dislocation scale mechanisms and (iv) a finite-element implementation, to predict the ductility as a function of temperature and strain rate in AA5182. The model, which contains no significant adjustable parameters, predicts the observed steep drop in ductility at room temperature, which can be directly attributed to the atomistic aging mechanism. On the basis of quantum inputs, this multiscale theory can be used in the future to design new alloys with higher ductility.

Strength distribution of large unidirectional composite patches with realistic load sharing

Monte Carlo simulations of the failure of unidirectional fiber composites in a plane transverse to the fiber direction are performed on much larger patches than in previous works, assuming a realistic load redistribution scheme from broken to intact fibers. Computational effort involved in these simulations is substantially reduced using an algorithm based on the quadtree data structure. The empirical strength distribution obtained from the simulations has a weak-link character, regardless of the variability in fiber strengths. The empirical strength distribution is well captured by a probabilistic model based on the growth of a tight cluster of fiber breaks. It is also well captured by regarding composite patch failure as the failure of the weakest equal load-sharing bundle of a certain size, following Curtin [Phys. Rev. Lett. 80, 1445 (1998)PRLTAO0031-900710.1103/PhysRevLett.80.1445]. The approximate coincidence of these two predictions identifies the dominant failure mechanism underlying Curtins empirical scaling relationship.

A fast algorithm for the elastic fields due to a single fiber break in a periodic fiber-reinforced composite

The stress state in a shear-lag model of a unidirectional fiber composite with an arbitrary configuration of fiber breaks is obtained by the weighted superposition of the stress state due to a single broken fiber. In a periodic patch comprised of N fibers located at the points of a regular lattice, a method to determine the stress state due to a single break was proposed by Landis et al. (J Mech Phys Solids 48(3):621–648, 2000). This method entails the determination of the eigenspace of an

A constitutive model for plastically anisotropic solids with non-spherical voids

N\times N

Work hardening in micropillar compression: In situ experiments and modeling

N×N matrix, at a computational cost of

Void growth and coalescence in anisotropic plastic solids

O(N^3)