Indentation Modulus, Indentation Work and Creep of Metals and Alloys at the Macro-Scale Level: Experimental Insights into the Use of a Primary Vickers Hardness Standard Machine

In this work, the experimental method and the calculation model for the determination of indentation moduli, indentation work, and indentation creep of metallic materials, by means of macroscale-level forces provided by a primary hardness standard machine at the National Institute of Metrological Research (INRIM) at the at room temperature were described. Indentation moduli were accurately determined from measurements of indentation load, displacement, contact stiffness and hardness indentation imaging and from the slope of the indentation unloading curve by applying the Doerner-Nix linear model; indentation work, representing the mechanical work spent during the force application of the indentation procedure, was determined by calculating the areas under the loading–unloading indentation curve, through fitting experimental data with a polynomial law. Measurements were performed with a pyramidal indenter (Vickers test). The applied force was provided by a deadweight machine, and the related displacement was measured by a laser interferometric system. Applied forces and the occurring indentation depths were simultaneously measured: the resulting loading–unloading indentation curve was achieved. Illustrative tests were performed on metals and alloy samples. Discussion and comments on the suitability of the proposed method and analysis were reported.

A Simple Method for Measuring Plastic Properties of Power Hardening Metals via the Indentation Curve with a Large Depth

The spherical indentation technique provides an easy way to evaluate the integrity of in-service structures because it is nondestructive. In this study, a simple method was proposed to measure mechanical properties such as the yield strength, the ultimate tensile strength, and the strain hardening exponent from the indentation curve at a large indentation depth, which is 0.4 times of the indenter radius. Based on finite element analyses, a simple function was proposed to relate representative stress to indentation data. Besides, representative strains at different indentation depths were identified according to the load-depth curves from simulations. The calculated plastic properties from the developed method were compared well with experimental results.

Indentation of a floating elastic sheet: geometry versus applied tension

The localized loading of an elastic sheet floating on a liquid bath occurs at scales from a frog sitting on a lily pad to a volcano supported by the Earth’s tectonic plates. The load is supported by a combination of the stresses within the sheet (which may include applied tensions from, for example, surface tension) and the hydrostatic pressure in the liquid. At the same time, the sheet deforms, and may wrinkle, because of the load. We study this problem in terms of the (relatively weak) applied tension and the indentation depth. For small indentation depths, we find that the force–indentation curve is linear with a stiffness that we characterize in terms of the applied tension and bending stiffness of the sheet. At larger indentations, the force–indentation curve becomes nonlinear and the sheet is subject to a wrinkling instability. We study this wrinkling instability close to the buckling threshold and calculate both the number of wrinkles at onset and the indentation depth at onset, comparing our theoretical results with experiments. Finally, we contrast our results with those previously reported for very thin, highly bendable membranes.

Shear Band Morphology of Indented Region in Cu-Based Metallic Glass

Plastic deformation after indentation of the metallic glass Cu47Ti35Zr11Ni6Si1 at different loading conditions was examined. Discontinuities on the loading curves were observed, the magnitude of which depends on the loading rate. The presence of these discontinuities is influenced by the precise shape of the indentation tip. At lower loading rates and using a cube corner indenter tip the discontinuities on the loading curves are more pronounced. An increase of the loading rate tends to diminish instantaneous plastic deformation as appear by pop-ins. Using a Berkovich type indenter tip the plastic deformation is more steady. It is concluded that the final morphology of the pile-up area strongly depends on the geometry of the indenter tip, whereas no correlation between discontinuities in the loading part of the indentation curve and the formation of shear band patterns was observed.

Mechanical Properties Determination of Unknown and 10Ch2MFA Steel Based on Non-Destructive Instrumented Hardness Test

The aim of the paper is to present developed methodology for evaluation of mechanical properties using nondestructive (NDT) methods. The final methodology will focus on evaluating the mechanical properties of the heterogeneous weld structure. The mechanical properties are determined by conducting NDT instrumented Vickers hardness test. The developed methodology for the basic mechanical properties determining is based on the inverse FEM modeling of instrumented hardness test and the indentation curve and the measured surface imprint are the output of indentation. The outputs from the experimental hardness test (the corresponding values from indentation curve and measured surface) are not always corresponding. These differences affect the accuracy of the developed methodology, because it is achieved of the basic material mechanical properties by comparing the outputs of the experimental instrumented hardness test and modeled hardness test using FEM.

The effect of remodelling and contractility of the actin cytoskeleton on the shear resistance of single cells: a computational and experimental investigation

The biomechanisms that govern the response of chondrocytes to mechanical stimuli are poorly understood. In this study, a series of in vitro tests are performed, in which single chondrocytes are subjected to shear deformation by a horizontally moving probe. Dramatically different probe force–indentation curves are obtained for untreated cells and for cells in which the actin cytoskeleton has been disrupted. Untreated cells exhibit a rapid increase in force upon probe contact followed by yielding behaviour. Cells in which the contractile actin cytoskeleton was removed exhibit a linear force–indentation response. In order to investigate the mechanisms underlying this behaviour, a three-dimensional active modelling framework incorporating stress fibre (SF) remodelling and contractility is used to simulate the in vitro tests. Simulations reveal that the characteristic force–indentation curve observed for untreated chondrocytes occurs as a result of two factors: (i) yielding of SFs due to stretching of the cytoplasm near the probe and (ii) dissociation of SFs due to reduced cytoplasm tension at the front of the cell. In contrast, a passive hyperelastic model predicts a linear force–indentation curve similar to that observed for cells in which the actin cytoskeleton has been disrupted. This combined modelling–experimental study offers a novel insight into the role of the active contractility and remodelling of the actin cytoskeleton in the response of chondrocytes to mechanical loading.

Analytical Modeling of Circular Glare Laminated Plates under Lateral Indentation

In this paper analytical solutions are derived to predict the static response of thin circular clamped GLARE fibre-metal laminated plates under the action of a lateral hemispherical indentor. The load-indentation curve is calculated along with the first failure load and deflection due to glass-epoxy tensile fracture. The Ritz method is employed with one, two and three-parameter Ritz approximation functions. The derived formulas are applied to GLARE 2-2/1-0.3 and to GLARE 3-3/2-0.4 circular plates with various diameters. The results converge satisfactorily in all examined cases. The calculated load-indentation curve and the first failure agree well with published experimental data for the case of a GLARE 2-2/1-0.3 plate with a radius of 40 mm (failure load within 7% and failure deflection within 3%). The same load-indentation curves are also calculated using ANSYS and by comparison to FEM results the validity of the analytical model is further verified. No analytical solution of this problem is known to the authors.