Elasto-plastic Material Properties Research Articles

Determination of the mechanical properties of metallic thin films and substrates from indentation tests

Abstract A procedure to obtain the elasto-plastic mechanical properties of strain-hardening materials from indentation tests, based on dimensional analysis and finite-element techniques, is proposed. The method is applicable to homogeneous materials and to coatings deposited on substrates of known mechanical properties. The Young's modulus of the material is extracted from the initial slope of the unloading indentation curve and the yield strength and strain- hardening exponent are obtained from the maximum indentation load and the contact area after unloading. The method is used to obtain the properties of a high-alloy steel and Mo and AlSi coatings deposited on a steel substrate by plasma spraying. The sensitivity of the measurement to the depth of indentation is discussed.

Numerical modelling of crack shielding and deflection in a multi-layered material system

Finite element analysis has been used to investigate the fatigue behaviour observed in testing a layered structure (representative of an automotive journal bearing). The aim of the analysis was to explain the deflection or bifurcation observed as a fatigue crack propagates through the multi-layered structure of a bearing. A fracture mechanics approach was adopted using detailed evaluations of the J-integral to assess and monitor both crack tip driving force and directional propensity with crack growth. Crack shielding or anti-shielding as well as deflection or bifurcation were conclusively linked to the difference between the fundamental elasto-plastic properties of the various constituent materials.

Materials design: a useful research focus for inelastic behavior of structural metals

An overview of emerging needs for new developments in the field of inelastic behavior of materials is given. Attention is focused principally on the application of plasticity models to support the design of durable microstructures and to engineer mesostructures to serve specified functions. Challenging issues include development of methods for characterizing maps of microstructure shakedown, reversed plasticity and ratcheting regimes, material representation for simulations, algorithms for speeding up computations, methods for meshing and representing digitized microstructures, methods for capturing the essential aspects of solutions of highly non-linear elasto-plastic and elasto-viscoplastic solutions using low degree-of-freedom representations, determining optimal microstructure configurations of multi-phase materials, combined experimental/modeling techniques for inferring in situ elasto-plastic properties of multi-phase materials, and properties of interfaces or interphases, and identification of systems-based approaches for the design of material microstructures with distributed elasto-plastic deformation.

Computational modeling of the forward and reverse problems in instrumented sharp indentation

A comprehensive computational study was undertaken to identify the extent to which elasto-plastic properties of ductile materials could be determined from instrumented sharp indentation and to quantify the sensitivity of such extracted properties to variations in the measured indentation data. Large deformation finite element computations were carried out for 76 different combinations of elasto-plastic properties that encompass the wide range of parameters commonly found in pure and alloyed engineering metals; Young's modulus, E, was varied from 10 to 210 GPa, yield strength, σ y, from 30 to 3000 MPa, and strain hardening exponent, n, from 0 to 0.5, and the Poisson's ratio, ν, was fixed at 0.3. Using dimensional analysis, a new set of dimensionless functions were constructed to characterize instrumented sharp indentation. From these functions and elasto-plastic finite element computations, analytical expressions were derived to relate indentation data to elasto-plastic properties. Forward and reverse analysis algorithms were thus established; the forward algorithms allow for the calculation of a unique indentation response for a given set of elasto-plastic properties, whereas the reverse algorithms enable the extraction of elasto-plastic properties from a given set of indentation data. A representative plastic strain ε r was identified as a strain level which allows for the construction of a dimensionless description of indentation loading response, independent of strain hardening exponent n. The proposed reverse analysis provides a unique solution of the reduced Young's modulus E*, a representative stress σ r, and the hardness p ave. These values are somewhat sensitive to the experimental scatter and/or error commonly seen in instrumented indentation. With this information, values of σ y and n can be determined for the majority of cases considered here provided that the assumption of power law hardening adequately represents the full uniaxial stress–strain response. These plastic properties, however, are very strongly influenced by even small variations in the parameters extracted from instrumented indentation experiments. Comprehensive sensitivity analyses were carried out for both forward and reverse algorithms, and the computational results were compared with experimental data for two materials.

Elastoplastic Properties of Starch-Based Materials as Revealed by Microindentation Measurements

The elastoplastic properties of injection-molded starch and starch-cement composites were investigated by load-displacement analysis from depth-sensing experiments. The creep behavior under the indenter was studied. Hardness data from the depth-sensing and imaging methods were shown to be in good agreement. The Young modulus values derived from the compliance method were studied; results are discussed in the light of the various materials used. Finally, the influence of annealing temperature on the microhardness of starch and of the starch-cement composites, processed under a water vapor atmosphere, was investigated. Results reveal a hardening of the materials on thermal treatment. This hardness increase is associated with an equilibrium water content decrease detected within the samples.

Micromechanical Investigation of the Hot Ductility Behavior of Steel.

Using a periodic microfield approach a micromechanical study is presented, which is capable to describe the ductility losses due to a film like ferrite precipitation in the high temperature region of iron based alloys by considering the two phase microstructure close to phase transformation temperatures and explicitly accounting for the elasto-plastic material properties of each present phase. For the temperature range close to the Ae3 temperature it will be demonstrated that the hot ductility behavior is directly correlated to the volume fraction of ferrite which develops along the austenitic grain boundary and the global strain rate of the specimen.

Determination of the conditions of fracture of plutonium and its alloy with gallium under impact loading

We present the results of numerical analysis of the conditions of cleavage fracture of plutonium and its alloy containing 1.6 wt.% of gallium under impact loading. On the basis of the experimental data obtained earlier, we determine the critical tensile stresses corresponding to the initial stage of macroscopic cleavage fracture of specimens. In our calculations, we take into account the elastoplastic properties of materials and polymorphic transformations in the alloy at normal and elevated (315°C) temperatures.

Numerical Analysis of Wood Chipping

Normally, in pulp wood chipping, the chips desired are thinner and have less fiber damage than those currently being produced. However, experiments to develop chippers are rather laborious, so there is a need for a mathematical model to complement the experiments. A two-dimensional finite element model was developed in order to be able to predict chip thickness and chip damage, the model incorporated a fictitious crack model and elasto-plastic material properties. The sensitivity of the model to changes in material data was controlled, and the results of the calculations were compared to experimental observations reported in the literature. Finally, the influences of the geometry of the cutting knife and of the friction between knife and wood were calculated. The general impression of the modelled chip formation was that the results of the finite element model were in reasonable agreement with experimental observations and that the model yields qualitative results that are trustworthy. The calculations indicate that there is a strong interaction - which greatly influences chip formation - between the knife angle and the coefficient of friction between the knife and the wood. The results suggest that a chipper knife should have a small knife angle complemented with a bevel, and should have a small coefficient of friction to wood.

Analysis Of Time Of Collapse Of Steel Columns Exposed To Fire

Unprotected steel columns exhibit a rapid deterioration in strength with increasing temperature. A method for the analysis of unprotected steel columns in fire conditions is presented. Beam finite elements are used to model the columns that are concentrically loaded with initial out-of-straightness and temperature-dependent elasto-plastic material properties. The accuracy of the model was determined by comparing it with available experimental results for column strength at elevated temperature and for time of collapse with time-varying temperatures. A study of the time to collapse of unprotected steel columns was undertaken for a range of load levels, slenderness ratios and column section sizes.

Assessment of thermal stresses and ratchetting in reactor vessels

Reactor vessels are often subject to “hot spots” developing at certain positions on the shell, which, if they are of a cyclic nature, may result in strain growth and fatigue failure if the amplitude of the cyclic strains are not restricted. The paper deals with an investigation of a particular vessel with the object of determining the characteristics of “hot spots” that would cause cyclic plasticity or ratchetting in order to determine the limiting criteria to avoid or restrict such effects onder continuous operational conditions. The basis of the investigation is finite element analysis. Temperature dependent elasto-plastic material properties are taken into account and the mesh is refined in selected “hot spot” positions in order to obtain accurate strain and stress states under cyclic loading conditions. The assessment criteria are discussed which are the basis of recommendations for continuous operation under cyclic conditions.

ON Computational Diagnostics of Constructional Medium

The method of definition of elastoplastic properties of constructive materials on the basis of the diagram of inculculation in the process of loading of material is suggest.

Finite elements for capturing localized failure

In the paper, we examine the ability of different finite element enhancements to capture localized failure in elasto-plastic solids. Altogether seven variations of four noded elements are studied, from the standard bilinear quadrilateral up to recent mixed strain-displacement expansions. The weak form of localization determines whether an element is capable to reproduce discontinuous bifurcation for elasto-plastic material properties which exhibit a singularity of the localization tensor in the element domain. Both, discontinuous tensile splitting and shear banding are examined in square and quadrilateral element, geometries within the frame of associated and non-associated elasto-plastic Rankine-and Drucker-Prager descriptions. Aside from spectral studies of discontinuous bifurcation on the element level, the effect of progressive failure computations is examined with the help of two boundary value problems.

Development of a new finite element formulation for the elasto-plastic analysis of fiber-reinforced composites

An analysis model is presented for the elasto-plastic analysis of continuous fiber-reinforced composite structures. The model uses the material properties of each constituent separately instead of the smeared global properties obtained from the lamination theory. Both fiber and matrix stresses are computed using the analysis model. Thus, the elasto-plastic deformation of a metal-matrix composite, composed of elastic fibers and elasto-plastic matrix, can be analyzed using the yield function and the flow rule suitable for each constituent. If each constituent is homogeneous and isotropic, then the usual von Mises yield function and the Prandtl-Reuss flow rule can be used. There is no need to obtain the global elasto-plastic material properties using the rule of mixtures. The formulation for the finite element matrix equation is given for the analysis model. Some numerical examples are solved using the method.

Kinetic approach to description of the elastoplastic properties of materials in shock loading

Kinetic approach to description of the elastoplastic properties of materials in shock loading

Physicomechanical model of the elastoplastic properties of materials taking account of the structural levels of deformation and the kinetic properties of real crystals

On the basis of an earlier-proposed translational-rotational model of a continuous medium, reflecting a hierarchy of structural levels of deformation and destruction, the influence of diverse temperature-velocity conditions of loading on the properties of materials is investigated. The applicability of the theory for the description of unified equations of the mechanical properties of crystals in nonsteady conditions of deformation is established. Analysis and comparison of the calculations results with experiment is undertaken for uniaxial loading.