No abstract

The microstructure of multiphase steels assisted by transformation-induced plasticity consists of grains of retained austenite embedded in a ferrite-based matrix. Upon mechanical loading, retained austenite may transform into martensite, as a result of which plastic deformations are induced in the surrounding phases, i.e., the ferrite-based matrix and the untransformed austenite. In the present work, a crystallographically based model is developed to describe the elastoplastic transformation process in the austenitic region. The model is formulated within a large-deformation framework where the transformation kinematics is connected to the crystallographic theory of martensitic transformations. The effective elastic stiffness accounts for anisotropy arising from crystallographic orientations as well as for dilation effects due to the transformation. The transformation model is coupled to a single-crystal plasticity model for a face-centered cubic lattice to quantify the plastic deformations in the untransformed austenite. The driving forces for transformation and plasticity are derived from thermodynamical principles and include lower-length-scale contributions from surface and defect energies associated to, respectively, habit planes and dislocations. In order to demonstrate the essential features of the model, simulations are carried out for austenitic single crystals subjected to basic loading modes. To describe the elastoplastic response of the ferritic matrix in a multiphase steel, a crystal plasticity model for a body-centered cubic lattice is adopted. This model includes the effect of nonglide stresses in order to reproduce the asymmetry of slips in the twinning and antitwinning directions that characterizes the behavior of this type of lattices. The models for austenite and ferrite are combined to simulate the microstructural behavior of a multiphase steel. The results of the simulations show the relevance of including plastic deformations in the austenite in order to predict a more realistic evolution of the transformation process.

The recently developed hybrid discontinuous Galerkin/extrinsic cohesive law framework is extended to the study of intra-laminar fracture of composite materials. Toward this end, micro-volumes of different sizes are studied. The method captures the debonding process, which is herein proposed to be assimilated to a damaging process, before the strain softening onset, and the density of dissipated energy resulting from the damage (debonding) remains the same for the different studied cell sizes. Finally, during the strain softening phase a micro-crack initiates and propagates in agreement with experimental observations. We thus extract a resulting mesoscale cohesive law, which is independent on the cell sizes, using literature methods.

No abstract

No abstract

The effects of grain orientation on transformation-induced plasticity in multiphase steels are studied through three-dimensional finite element simulations. The boundary value problems analysed concern a uniaxiallyloaded sample consisting of a grain of retained austenite surrounded by multiple grains of ferrite. For the ferritic phase, a rate-dependent crystal plasticity model is used that describes the elasto-plastic behaviour of body-centred cubic crystalline structures under large deformations. In this model, the criticalresolved shear stress for plastic slip consists of an evolving slip resistance and a stress-dependent term that corresponds to the projection of the stress tensor on a non-glide plane (i.e. a non-Schmid stress). For the austenitic phase, the transformation model developed by Turteltaub and Suiker (2006 Int. J. Solids Struct. at press, 2005 J. Mech. Phys. Solids 53 1747 is employed. This model simulates the displacive phase transformation of a face-centred cubic austenite into a body-centred tetragonal martensite under external mechanical loading. The effective transformation kinematics and the effective anisotropic elastic stiffness components in the model are derived from lower-scale information that follows from the crystallographic theory of martensitic transformations. In the boundary value problems studied, the mutual interaction between the transforming austenitic grain and the plastically deforming ferritic matrix is computed for several grain orientations. From the simulation results, specific combinations of austenitic and ferritic crystalline orientations are identified that either increase or decrease the effective strength of the material. This information is useful to further improve the mechanical properties of multiphase carbon steels. In order to quantify the anisotropic 1 Author to whom any correspondence should be addressed.

No abstract

While solid mechanics codes are now conventional tools both in industry and research, the increasingly more exigent requirements of both sectors are fuelling the need for more computational power and more advanced algorithms. For obvious reasons, commercial codes are lagging behind academic codes often dedicated either to the implementation of one new technique, or the upscaling of current conventional codes to tackle massively large scale computational problems. Only in a few cases, both approaches have been followed simultaneously. In this article, a solid mechanics simulation strategy for parallel supercomputers based on a hybrid approach is presented. Hybrid parallelization exploits the thread-level parallelism of multicore architectures, com- bining MPI tasks with OpenMP threads. This paper describes the proposed strategy, programmed in Alya, a parallel multiphysics code. Hybrid parallelization is specially well suited for the current trend of supercomputers, namely large clusters of multicores. The strategy is assessed through transient non-linear solid mechanics problems, both for explicit and implicit schemes, running on thousands of cores. In order to demonstrate the flexibility of the proposed strategy under advance algorithmic evolution of computational mechanics, a non-local parallel overset meshes method (Chimera-like) is implemented and the conservation of the scalability is demonstrated.