DETERMINATION OF FRACTURE TOUGHNESS OF BRITTLE MATERIALS BY INDENTATION

Fracture toughness is one of the crucial mechanical properties of brittle materials such as glasses and ceramics which demonstrate catastrophic failure modes. Conventional standardized testing methods adopted for fracture toughness determination require large specimens to satisfy the plane strain condition. As for small specimens, indentation is a popular, sometimes exclusive testing mode to determine fracture toughness for it can be performed on a small Bat area of the specimen surface. This review focuses on the development of indentation fracture theories and the representative testing methods. Cracking pattern dependent on indenter geometry and material property plays an important role in modeling, and is the main reason for the diversity of indentation fracture theories and testing methods. Along with the simplicity of specimen requirement is the complexity of modeling and analysis which accounts for the semi-empirical features of indentation fracture tests. Some unresolved issues shaping the gap between indentation fracture tests and standardization are also discussed

Nanoindentation investigation on the creep mechanism in metallic glassy films

Using the magnetron sputtering technique, two metallic glassy films namely Cu44.3Zr45.1Al10.6 and Cu44.2Zr43Al11.3Ti1.5 were prepared by alloy targets. The minor Ti addition effectively induces excess free volume. Upon spherical nanoindentation, the creep behaviors of both films were studied at various peak loads. As the increase of peak load, the creep deformations became more severely in both samples. Interestingly, Cu-Zr-Al-Ti film crept stronger than Cu-Zr-Al at small-load holdings (nominal elastic regimes), whereas it is opposite at high-load holdings (plastic regimes). The creep characteristic could be intrinsically related to the scale variation of the shear transformation zone (STZ) with Ti addition. Statistical analysis was employed to estimate the STZ volume, which increased by 60% with Ti addition in the Cu-Zr-Al film. The finite element modeling indicated that STZs would be activated even at the minimum load we adopted. Higher activation energy of larger STZs in Cu-Zr-Al-Ti enables less flow units, which offsets the creep enhancement by the excess free volume with Ti addition. The deeper the pressed depth of the indenter, the more contribution of the STZ operation on creep deformation. In addition, experimental observation on the creep flow rates implies that STZ could be the dominating mechanism at the steady-state creep. This study reveals that STZ volume could also be important to the time-dependent plastic deformation in metallic glass, besides as a key parameter for instantaneous plasticity. (C) 2015 Elsevier B.V. All rights reserved

An instrumented indentation method for evaluating the effect of hydrostatic pressure on the yield strength of solid polymers

The yield behavior of solid polymers may be influenced by the hydrostatic pressure, strain rate, and temperature. In the present work, we focus on evaluating the effect of hydrostatic pressure on the yield strength by instrumented indentation. Using dimensional analysis and finite element analysis, two analytical expressions were derived to relate the indentation data to the plastic properties, and a method for extracting the coefficient of internal friction which reflects the effect of hydrostatic pressure on the yield strength was established. Applications were illustrated on polypropylene (PP), polycarbonate (PC), and unplasticized polyvinyl chloride (UPVC). The coefficient of internal friction determined by this indentation method is 0.20 +/- 0.02 for PP, 0.07 +/- 0.01 for PC, and 0.10 +/- 0.01 for UPVC, which are in good agreement with the values reported in the literature. This demonstrates the proposed indentation method which is useful to evaluate the effect of hydrostatic pressure on the yield strength of solid polymers

Nanoindentation study on the creep characteristics of high-entropy alloy films: fcc versus bcc structures

Using the magnetron sputtering technique, two typical high-entropy alloy (HEA) films namely CoCrFeNiCu (Al-0) with a face-centered cubic (fcc) structure and CoCrFeNiCuAl2.5 (Al-2.5) with a body-centered cubic (bcc) structure were prepared by alloy targets. The as-deposited HEA films have a columnar-growth mode and nanocrystalline grains. The creep behaviors of both HEA films were systematically investigated by nanoindentation with a Berkovich indenter. The bcc Al-2.5 exhibited a stronger creep resistance than the fcc Al-0. In addition, with the increase of holding load and/or loading rate, the creep deformation was significantly enhanced in the fcc Al-0. Interestingly, it was almost history-independent in the bcc Al-2.5. The creep characteristics of HEA films could be related to the distinct lattice structures, which apparently affect the kinetics of plastic deformation. The strain rate sensitivity (SRS) and activation volume of the dislocation nucleation were carefully estimated for both HEA films. In view of the large differences of activation volumes between Al-0 and Al-2.5, we present discussions to explain the observed creep characteristics in HEA films. (C) 2015 Published by Elsevier Ltd

Nanoindentation investigation on creep behavior of amorphous Cu-Zr-AI/nano crystalline Cu nanolaminates

Berkovich nanoindentation experiments have been performed on amorphous /nanocrystalline nanolaminates with individual Cu-Zr-Al layers (45 nm 90 nm 225 nm) and Cu layers (7.5 nm & 15 nm). Elastic modulus hardness and indentation morphology were detected and compared to those of single Cu-Zr-Al thin film. Creep deformation was systematically investigated at various holding depths and loading rates. For the sample with thinner amorphous layer a more pronounced creep deformation was observed and it was confirmed to be due to the size effect of Cu-Zr-Al layers and the addition of Cu layers. The creep deformation was identified to be history-independent through applying various loading rates. The strain rate sensitivities were calculated from the steady-state creep and a sharp enlargement appeared as the amorphous layer reduced down to 90 nm implying a transition of creep mechanism in nanolaminates. (C) 2017 Elsevier B.V. All rights reserved

Identification of the elastic-plastic constitutive model for measuring mechanical properties of metals by instrumented spherical indentation test

Several methods for determination of elastic-plastic parameters by instrumented spherical indentation tests have been presented in the past few years. Each method was established according to a specific constitutive model. Identification of the constitutive models of new materials has become an indispensable step in order to choose an appropriate indentation method to extract the elastic-plastic parameters. In the present work, the half depth energy accumulation rate and Meyer's index were related to the elastic-plastic constitutive models via qualitative and numerical analyses. A method for identification of the elastic-plastic constitutive models by instrumented spherical indentation test was proposed

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This paper aims to obtain an analytical expression for the ratio of unloading work of indentation (W-u) to total loading work of indentation (W-t) (work recovery ratio of indentation) in instrumented spherical indentation. The expanding cavity model and Lame solution are used. Three typical stress-strain relations (elastic-perfectly plastic, linear hardening, and power-law hardening) are analyzed. The results of finite-element method coincide with the expressions. The expressions show that the work recovery ratio of indentation is just related to plastic parameters. Furthermore, elastic work (W-e) are obtained, and it is proved that W-e should be distinguished from W-u in spherical indentation