Aleš Materna

Characterization of Anisotropy in Hardness and Indentation Modulus by Nanoindentation

Abstract This paper provides a useful guide how to characterize material anisotropy by nanoindentation. Hardness and indentation modulus of austenitic stainless steel (grade A304) were characterized by instrumented indentation at the grain scale (at low indentation load and depth of penetration). We applied the grid indentation method on an area containing several grains with different crystallographic orientation which was simultaneously characterized by electron back-scatter diffraction. Hardness and indentation modulus dependencies on crystallographic orientation were then evaluated and compared with single crystal Young’s modulus and finite element simulations.

A Numerical Estimation of Fatigue Crack Tip Plastic Zone Size and Shape under Mixed-Mode Loading Condition

A 2D elastic-plastic FEM simulation of growing fatigue crack under combined mode I and II loading was performed. An inclined fatigue crack propagated in a sheet of an Al-alloy D16CT1. The effect of increasing mode-mixity on cyclic zone size, shape and the amount of dissipated energy was investigated.

Some Issues in Relations between Microstructure and Indentation Measurements

Case study on copper-tungsten metal-matrix composite was performed. The influence of presence of an interface on the distribution of measured hardness and/or modulus was incorporated by a statistical distribution taking into account a progressive change of materials behavior as a function of depth of penetration. Unbiased (intrinsic) material properties (hardness and Young’s modulus) were then successfully extracted from the experimental grid indentation data.

Investigation of Indentation Parameters Near the Interface between Two Materials

The paper participates in a development of composites. A composite tungsten-steel is studied as materials suitable for a first wall of tokamak, a composite FeAl + Al2O3 is a possible material for fourth generation of a nuclear power plant and a composite Al2O3 + YSZ is a potential implant material. The focus of our study is change of material properties near the interface and a determination of area size which is influenced by the adjacent material. Material properties are investigated by nanoindentation. The task is simulated using finite element method. Simulated specimen is composed of a tungsten part and a steel part. The sharp boundary between materials is a plane which is located parallel to the loading force direction. Elastic modulus is determined in dependence of a distance between the interface and a tip of the indenter. The simulated results are verified experimentally.

A Numerical Investigation of the Effect of Cubic Crystals Orientation on the Indentation Modulus

The elastic nite element computations of the indentation process with the Berkovich indenter are performed to examine the e ect of cubic crystal and indenter orientation on indentation moduli of anisotropic material. Three metals with a cubic crystal lattice and various degree of elastic anisotropy from 1 to 9 are studied: tungsten, AISI 304 steel and β-brass. Di erences in anisotropy ratios expressed by the Young moduli on the one side and by the indentation moduli on the other side are quanti ed.

Numerical Model of Instrumented Indentation by a Rounded Cone Indenter Using Finite Element Method

The paper serves as an introduction to investigation of mechanical properties of functionally graded materials and deals with elastic nanoindentation numerical models. The models were based on the finite element method. Youngs moduli were estimated by Oliver-Pharr method. The indenter geometry for which numerical solutions were accomplished was a rounded cone indenter. The effect of tip sharpness was examined by applying an increasing spherical tip radius. The results show that the apparent Youngs modulus and the hardness increase linearly with increasing radius of the tip. The effect of approaching interface between two elastic materials on the apparent hardness and indentation modulus was identified in 3D model. The specimen consisted of two materials. First, the interface was linear and parallel to the direction of indentation, so that the Youngs modulus changed suddenly. Second, the Youngs modulus was continuously changing. The dependence on various boundary conditions of the specimen was also considered.

Determination of the Elastic-Plastic Properties of 15Kh2MFA Steel with Austenitic Cladding

The spherical indentation response of pressure vessel reactor steel with austenitic cladding is investigated both experimentally and numerically. The instrumented indentation test was performed for both materials at a sufficient distance from the bi-material interface, thus the results can be compared with the bulk data obtained from the standard tensile and compression tests. The stress – plastic strain curve for austenitic cladding estimated by a simplified inverse analysis of the indentation load – penetration curve is shifted to a harder response compared with that determined from the tensile test. One of the possible reasons, anisotropy of the cladding metal, was experimentally observed during the compression tests performed in the longitudinal orientation of the tensile test specimens and in the transverse orientation identical with the direction of the material indentation.

Effect of Crystallographic Orientation on Hardness and Indentation Modulus in Austenitic Stainless Steel

The most commonly used method for the analysis of instrumented indentation test (Oliver-Pharr) is based on isotropic elastic solution of contact problem which is not necessarily valid when indenting at the scale of one (anisotropic) grain. In this paper, we performed the grid indentation method at the sub-micron scale (at low indentation load and depth of penetration) on an area containing several grains with different crystallographic orientation which was simultaneously characterized by electron back-scattered diffraction. Measured dependencies of hardness and indentation modulus on crystallographic orientation were compared with analytical solution and finite element simulations.

Effect of Neighboring Phase Properties on Measured Indentation Data

The paper focuses on the variation of mechanical properties near the interface of two materials. The aim is to determine the properties of the individual phases and the size of the region influenced by the adjacent material. Nanoindentation measurements were performed in the vicinity of a sharp interface between tungsten and steel, which was a plane located parallel to the loading force direction. The probability of indentation in the affected area as a function of depth of penetration and distance from the interface was fitted by a proper statistical distribution function. The intrinsic properties of the individual phases can subsequently be extracted from the experimental indentation data.

Plastic Zone around a Fatigue Crack Determined by the FEM and a Nanoindentation Technique

Plastic zone around a fatigue crack in AISI 304 stainless steel was studied experimentally using nanoindentation and numerically by the finite element analysis. Results obtained from one experimental observation of crack propagating under constant amplitude loading showed that nanohardness can be correlated to strain hardening caused by the cyclic deformation in the vicinity of the crack. However, for the material chosen for this study, exact plastic zone shape is hard to evaluate due to the scattering of experimental results.