Nataliya A. Sakharova

Effect of shoulder cavity and welding parameters on friction stir welding of thin copper sheets

Abstract The aim of this investigation was to study the influence of shoulder cavity and welding parameters on torque, defect formation, microstructure and mechanical properties of friction stir welds in very thin sheets of deoxidised copper. Three types of tools were used: a flat shoulder tool and two tools with conical shoulder cavities of 3 and 6° respectively. The welding parameters analysed were tool rotation and traverse speeds. It was observed that the torque, the microstructure and hardness and the formation of defects in the welds are influenced mainly by tool rotation speed and, to a lesser extent, by the traverse speed and shoulder cavity. The tensile properties of welds carried out at high rotation speeds are little affected by the shoulder cavity.

Inverse Strategies for Identifying the Parameters of Constitutive Laws of Metal Sheets

This article is a review regarding recently developed inverse strategies coupled with finite element simulations for the identification of the parameters of constitutive laws that describe the plastic behaviour of metal sheets. It highlights that the identification procedure is dictated by the loading conditions, the geometry of the sample, the type of experimental results selected for the analysis, the cost function, and optimization algorithm used. Also, the type of constitutive law (isotropic and/or kinematic hardening laws and/or anisotropic yield criterion), whose parameters are intended to be identified, affects the whole identification procedure.

Young's modulus of thin films using depth-sensing indentation

A new methodology for the determination of Youngs modulus of thin films, using a single hardness test measurement of film/substrate composites, has been developed. It is based on a recently proposed weight function, the reciprocal of the Gao function. Firstly, results of three-dimensional numerical simulation of the Vickers hardness tests on several fictitious composites are considered. Then, the methodology is checked experimentally by using depth-sensing indentation results on real composite materials.

On the identification of kinematic hardening with reverse shear test

Abstract An inverse analysis methodology for determining the parameters of the kinematic law of sheet metals is proposed. The sensitivity of the load versus displacement curves, obtained by reverse shear tests of rectangular and notched specimens, to the kinematic law parameters are studied following a forward analysis, based on finite element simulations. Afterwards, an inverse analysis methodology using a gradient-based Levenberg–Marquardt method is established, by evaluating the relative difference between numerical and experimental results of the shear test, i.e. the load evolution in function of the displacements of the grips. The use of a notched specimen is proposed in order to allow an easy and suitable numerical representation of the boundary conditions of the shear experimental test. This methodology has proven to be appropriate for determining the parameters of the kinematic hardening law.

How to Combine the Parameters of the Yield Criteria and the Hardening Law

The numerical simulation of sheet metal forming processes needs the accurate identification of the material parameters, for a given constitutive model. This identification can follow different methodologies and different sets of experimental data can be used, which lead to distinct sets of material parameters. In order to accurately compare the results of several methodologies, it is necessary to guarantee uniformity of their presentation. In this work, the correspondence between sets of parameters of the Hill’48 criterion is explored. The meaning of the “isotropic values” of the parameters associated with the out-of-plane stresses components is discussed and a required condition is proposed, in order to properly compare numerical simulation results obtained by using different input sets of constitutive parameters, identified by different procedures. Finite element simulations of complex shaped forming process, involving strain-path changes, are carried out in order to support the analysis.

Strain Path Change Effect on Deformation Behaviour of Materials with Low-to-Moderate Stacking Fault Energy

Stacking fault energy (SFE) plays an important role in face centred cubic (f.c.c.) metals and alloys in determining the prevailing mechanisms of plastic deformation. Low SFE metals and alloys have a tendency to develop mechanical twinning, besides dislocation slip, during plastic deformations. Deformation behaviour and microstructure evolution under simple and complex strain paths were studied in 70/30 brass, with small and intermediate grain sizes, which corresponds to a f.c.c. material with low SFE. Simple (rolling and tension) and complex (tension normal to previous rolling) strain paths were performed. The macroscopic deformation behaviour of materials studied is discussed in terms of equivalent true stress vs. equivalent true strain responses and strain hardening rates normalized by shear modulus (dσ/dε)/G as vs. (σ – σ0)/G (σ0 is the initial yield stress of the material and G is the shear modulus). The mechanical behaviour is discussed with respect to dislocation and twin microstructure evolution developed in both, simple and complex strain paths.

Dislocation Microstructure in Copper Multicrystals Deformed under the Sequences: Rolling - Tension and Tension - Rolling

The microstructure evolution of copper multicrystalline sheets, undergoing plastic deformation in the sequences of strain paths rolling – tension and tension – rolling, was studied in the present work. For both sequences, two different types of change of strain path were studied: the tensile and rolling directions were parallel and normal to each other. Samples submitted to these four complex strain paths were investigated by transmission electron microscopy (TEM). TEM observations have shown the typical dislocations microstructures for the prestrain paths in tension and rolling. The dislocation microstructures observed during the second path were analysed and discussed as a function of the sequence and of the type of strain path change (parallel and normal sequential paths). Special microbands features were observed during the second path, for both sequences, rolling – tension and tension – rolling. The appearance of such microstructural features is discussed in terms of the sequence and type of strain path change and it is linked with the slip activity during the second deformation mode.

Numerical Simulation of the Mechanical Behaviour of the Multi-Walled Carbon Nanotubes

The mechanical behaviour of non-chiral multi-walled carbon nanotubes under tensile and bending loading conditions was investigated. For this purpose, a simplified finite element model of armchair and zigzag multi-walled carbon nanotubes, which does not take into account the van der Waals forces acting between layers, was tested in order to evaluate their tensile and bending rigidities, as well as the Young’s modulus. The current numerical simulation results are compared with data reported in the literature. The robustness of the simplified model for evaluation of the Young’s modulus of multi-walled carbon nanotubes is discussed.

Numerical Simulation of the Mechanical Behaviour of Single-Walled Carbon Nanotube Heterojunctions

The study of the mechanical behaviour of single-walled carbon nanotube heterojunctions has been carried out, implementing nanoscale continuum approach. A three-dimensional finite element model is used in order to evaluate the elastic behaviour of cone heterojunctions. It is shown that the bending rigidity of heterojunctions is sensitive to bending conditions. The torsional rigidity does not depend on torsion conditions. Both rigidities of the heterojunction are compared with those of the thinner and thicker constituent nanotubes.

The Effect of Vacancy Defects on the Evaluation of the Mechanical Properties of Single-Wall Carbon Nanotubes: Numerical Simulation Study

A three-dimensional finite element model is used in order to evaluate the tensile and bending rigidities and, subsequently, Young’s modulus of non-chiral and chiral single-walled carbon nanotubes containing vacancy defects. It is shown that the Young’s modulus of single-walled carbon nanotubes with vacancies is sensitive to the presence of vacancy defects in nanotube: it decreases with increasing of the density of vacancy defects.