EXTRACTION OF PLASTIC PROPERTIES FROM THE INSTRUMENTD INDENTATION DATA: CURRENT STATUS

ABSTRACT An important aspect of the elastic-plastic analyses of sharp indentation will be the estimation of the representative strain, ε r , underneath the indenter, which varies as a function of the tip geometry. Wide range of values for ε r has been proposed in the literature. Recently, algorithms, developed on the basis of extensive large-strain finite element analyses, that enable the extraction of elastic and plastic properties from the instrumented pyramidal indentation data have been developed. Experiments that are conducted to critically assess the predictive capability of the reverse algorithms and in turn the ε r values are presented. INTRODUCTION Depth-sensing indentation has become a popular technique in the recent past for mechanical property evaluation of thin films, coatings, and biological materials. In a typical instrumented indentation test, the P-h data are continuously recorded for a complete cycle of loading and unloading and are analyzed. During indentation, the material underneath the indenter experiences a pile-up or sink-in against the faces of the indenter, depending on many material parameters. Hence, it is difficult to measure the true contact area and considerable research has been conducted to overcome this. Standardized methodologies that circumvent this problem are now available which make it possible to evaluate properties such as the elastic modulus, E, and hardness, H, of a given material routinely. Similar methods for the extraction of plastic properties such as yield stress, σ y , and the work hardening exponent, n, of metallic materials from the P-h curves, however, have been proposed. Elastic-plastic analyses of sharp indentation have been reported in the context of small strain finite element simulations. Results of such analyses have been used to develop forward and reverse analyses algorithms. While the forward algorithm predicts the P−h curve with the materials' elasto-plastic properties as input parameters, the reverse algorithm predicts materials' elasto-plastic properties from the experimentally measured P−h curves. Experimental work on a variety of metals shows that the extracted values of σ y and n deviate significantly from those measured in uniaxial compression tests, indicating that further refinements to the modeling are necessary. Recently, Dao et al

Experimental assessment of the representative strains in instrumented sharp indentation

Experimental assessment of the reverse algorithms that enable the extraction of plastic properties from the load-depth of penetration curves was conducted. Results show that they predict the stresses at 3.3% and 5.7% representative strains for Berkovich and 60 degrees cone-equivalent three-sided pyramidal indenters, respectively, with good accuracy. It was shown that the uniaxial stress-strain curves could be reconstructed from the indentation data

Three-Dimensional simulation of deformation fields underneath vickers indenter:Effects of power-law Plasticity

The effects of power-law plasticity (yield strength and strain hardening exponent) on the plastic strain distribution underneath a Vickers indenter was systematically investigated by recourse to three-dimensional finite element analysis, motivated by the experimental macro-and micro-indentation on heat-treated Al-Zn-Mg alloy. For meaningful comparison between simulated and experimental results, the experimental heat treatment was carefully designed such that Al alloy achieve similar yield strength with different strain hardening exponent, and vice versa. On the other hand, full 3D simulation of Vickers indentation was conducted to capture subsurface strain distribution. Subtle differences and similarities were discussed based on the strain field shape, size and magnitude for the isolated effect of yield strength and strain hardening exponent

Experimental assessment of the representative strains in instrumented sharp indentation

Experimental assessment of the reverse algorithms that enable the extraction of plastic properties from the load-depth of penetration curves was conducted. Results show that they predict the stresses at 3.3% and 5.7% representative strains for Berkovich and 60° cone-equivalent three-sided pyramidal indenters, respectively, with good accuracy. It was shown that the uniaxial stress-strain curves could be reconstructed from the indentation data

Plastic strain distribution underneath a Vickers indenter: role of yield strength and work hardening exponent

The effects of yield strength, σ<SUB>y</SUB>, and strain hardening exponent, n, on the plastic strain distribution underneath a Vickers indenter were explicitly examined by carrying out macro- and micro-indentation experiments on Al-Zn-Mg alloy that was aged for different times so as to obtain materials with different σ<SUB>y</SUB> but with similar n, or the same σ<SUB>y</SUB> but different n. Large Vickers indents were made (using a load of 700 N) that were subsequently sectioned along the median plane and the plastic strain distribution was determined by recourse to microindentation mapping. Comparison of the iso-strain contours shows that for similar values of n, higher σ<SUB>y</SUB> leads to a smaller deformation field in both the indentation (z) and lateral (x) directions. Higher n, at the same σ<SUB>y</SUB>, leads to a shallower deformation field. In all cases, it was found that strain fields are elliptical, with the long axis of the ellipse coinciding with the z-direction. The strain field ellipticity is sensitive to the work hardening behavior of the material, with higher n leading to lower ellipticity. While the lateral strain distribution is in agreement with the expanding cavity model, strain distribution directly ahead of the indenter tip appears to follow the Hutchison, Rice, and Rosengren fields ahead of a crack tip in an elastoplastic solid

Role of indenter angle on the plastic deformation underneath a sharp indenter and on representative strains: An experimental and numerical study

The subsurface microhardness mapping technique of Chaudhri was utilized to determine the shape, size and distribution of plastic strain underneath conical indenters of varying semi-apex angles, alpha (55 degrees, 65 degrees and 75 degrees). Results show that the elastic-plastic boundary under the indenters is elliptical in nature, contradicting the expanding cavity model, and the ellipticity increases with alpha. The maximum plastic strain immediately under the indenter was found to decrease with increasing alpha. Complementary finite-element analysis was conducted to examine the ability of simulations to capture the experimental observations. A comparison of computational and experimental results indicates that the plastic strain distributions as well as the maximum strains immediately beneath the indenter do not match, suggesting that simulation of sharp indentation requires further detailed studies for complete comprehension. Representative strains, epsilon(r), evaluated as the volume-average strains within the elastic-plastic boundary, decrease with increasing alpha and are in agreement with those estimated by using the dimensional analysis. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Hydroconversion de l'huile de Jatropha : développement de méthodes analytiques

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