Dating back to the late 1980s, bulk metallic glasses (BMGs) are relatively new materials that exhibit exceptional mechanical properties (strength, hardness, fracture toughness, stored elastic energy ......), compared to those of most crystalline metallic alloys. Their apparent brittleness under uniaxial loading, however, is still a major obstacle to their industrialization. Moreover, BMGs often contain crystalline defects developed, intentionally or not, during their complex and delicate elaboration. These flaws are known to affect their fracture toughness and their plastic behavior. This paper reviews twenty years of works about this subject on Zr-based BMGs that may contain a low volume fraction of crystalline defects of different natures, e.g. dendrites or spherulites, depending on the synthesis method. Dedicated experimental set-ups, mainly bending tests on notched beams, were developed to create in the specimen a proper pre-crack by fatigue and then load it monotonically up to fracture. The measured fracture toughness and the fractographic observations allow to conclude that these crystalline defects facilitate pre-cracking, but result in an embrittlement that is more or less significant depending on their type. The loading mode of the crack - mode I, II or mixed - as well as the temperature were shown to play a key role in crack initiation and propagation, whether steadily or catastrophically, in the BMG. By means of finite element computations analyses, explanations on how the crystalline flaws presence can affect fracture toughness and perturbate crack growth, under mode I and mode II, were proposed. Finally, the relevance of these experimental techniques as well as the link between crystalline defects, fracture toughness and their consequences on the ductility of a structural component are discussed.
Indentation studies are conducted on two different bulk metallic glasses and for a variety of indenters: spherical, conical and pyramidal. Shear bands are observed around the prints for only some ...types of indenters and reasons for this are sought. Analytical models of hardness accounting or not for the pressure dependence of amorphous alloys and finite element analyses are used. It is shown that these bands are observed only when we leave the elastoplastic regime of indentation in favour of a fully plastic one. Important conclusions are drawn about hardness values. In metallic glasses, compared with, for example, crystalline alloys, because of their tremendous yield strains, determining the true hardness of a particular amorphous alloy is not straightforward; it depends on the correct choice of indenter.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Nano-indentation instrumented tests are carried out at shallow depths on PAN-based and MPP-based carbon fibres. Indentation moduli are obtained by performing the tests at ten different measured ...orientations with respect to the fibre axis. They are used to identify the elastic constants of the fibres, assuming a transversely isotropic behaviour, by minimising a cost function between measured and estimated values. Inconstancies between the identified in-plane shear and transverse moduli and reported literature values are pointed out, and some drawbacks of the nano-indentation method are highlighted. An improved method taking into account the buckling mechanisms of crystallites at stake during the indentation process, and visible in the hysteretic behaviour of force-penetration nanoindentation curves, is proposed. It allows to identify values of elastic constants that are in accordance with literature values. These elastic properties of carbon fibres are in turn used to estimate the elastic properties of epoxy matrix composites containing these fibres. Very good agreement is found with experimentally available values of unidirectional ply properties. An excellent correlation between experiments and Finite Element Analyses of the indentation response of carbon fibres is eventually found.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The mechanical behaviour of an FeCoCrMoCBY bulk metallic glass is investigated by means of indentation testing and ultrasonic echography. The elastic moduli (Young's modulus of 225
GPa, Poisson's ...ratio of 0.337), hardness (13.5
GPa), indentation toughness
(
2.2
MPa
m
)
and indentation brittleness (6
μm
−1/2) are determined. Unlike most metallic glasses, this iron-based alloy exhibits cracking features above a critical load. The cracking systems are observed by means of stepwise polishing and consist in very shallow Palmquist (or radial) cracks, which is quite uncommon in other brittle materials such as glasses and ceramics.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
In glass or carbon fibres reinforced plastics, creep or stress relaxation, arise from the polymeric nature of the matrix. Plant fibres, used in bio-composites, are also polymers. Therefore, the issue ...of their service life requires studying the viscoelastic behaviour of both the matrix and the fibres. In this study, we investigate, at different length scales, the response of elementary flax fibres to tensile tests, as well as to nano-indentation tests on their secondary cell walls. The results of these experiments are then analysed via linear viscoelastic rheological models and identification procedures. The values of the identified parameters (relaxation time, viscosity and elastic stiffness) are discussed in relation to the microstructure of the flax fibre (cellulose microfibrils, hemicelluloses and pectins). The nano-indentation technique provides much more deterministic results than tension tests on an entire fibre. The scale of the secondary wall cell is then relevant to assess the viscoelastic behaviour of the fibres.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The mechanical response of amorphous silica (or silica glass) under hydrostatic compression for very high pressures up to 25GPa is modelled via an elastic–plastic constitutive equation (continuum ...mechanics framework). The material parameters appearing in the theory have been estimated from the ex situ experimental data of Rouxel et al. Rouxel T, Ji H, Guin JP, Augereau F, Rufflé B. J Appl Phys 2010;107(9):094903. The model is shown to capture the major features of the pressure–volume response changes from the in situ experimental work of Sato and Funamori Sato T, Funamori N. Phys Rev Lett 2008;101:255502 and Wakabayashi et al. Wakabayashi D, Funamori N, Sato T, Taniguchi T. Phys Rev B 2011;84(14):144103. In particular, the saturation of densification, the increase in elasticity parameters (bulk, shear and Young’s moduli) and Poisson’s ratio are found to be key parameters of the model.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We report on the difficulties of extracting plastic parameters from constitutive equations derived by instrumented indentation tests on hard and stiff materials at shallow depths of penetration. As a ...general rule, we refer here to materials with an elastic stiffness more than 10 % of that of the indenter
and
a yield strain higher than 1 %, as well as to penetration depths less than ∼ 5 times the characteristic tip defect length of the indenter. We experimentally tested such a material (an amorphous alloy) by nanoindentation. To describe the mechanical response of the test, namely the force-displacement curve, it is necessary to consider the combined effects of indenter tip imperfections and indenter deformability. For this purpose, an identification procedure has been carried out by performing numerical simulations (using Finite Element Analysis) with constitutive equations that are known to satisfactorily describe the behaviour of the tested material. We propose a straightforward procedure to address indenter tip imperfection and deformability, which consists of firstly taking account of a deformable indenter in the numerical simulations. This procedure also involves modifying the experimental curve by considering a truncated length to create artificially the material’s response to a perfectly sharp indentation. The truncated length is determined directly from the loading part of the force-displacement curve. We also show that ignoring one or both of these issues results in large errors in the plastic parameters extracted from the data.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
The determination of the contact area is a key step in deriving mechanical properties such as hardness or an elastic modulus by instrumented indentation testing. Two families of procedures are ...dedicated to extracting this area: on the one hand, post-mortem measurements that require residual imprint imaging, and on the other hand, direct methods that only rely on the load vs. penetration depth curve. With the development of built-in scanning probe microscopy imaging capabilities such as atomic force microscopy and indentation tip scanning probe microscopy, last-generation indentation devices have made systematic residual imprint imaging much faster and more reliable. In this paper, a new post-mortem method is introduced and further compared to three existing classical direct methods by means of a numerical and experimental benchmark covering a large range of materials. It is shown that the new method systematically leads to lower error levels regardless of the type of material. The pros and cons of the new method vs. direct methods are also discussed, demonstrating its efficiency in easily extracting mechanical properties with enhanced confidence.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Indentation and scratching tests are carried out on a ZrCuAlNi bulk metallic glass. The bonded interface technique is used to characterize the plasticity mechanisms underneath the indentation. ...Finite-element analyses are conducted with a Drucker-Prager behaviour law to challenge the indentation experimental data. The relevance of the bonded interface technique, in terms of quantitative evaluation, is discussed. It is also reported that the angle value, for which radial bands intersect at the surface or underneath it, is not a constant value and depends on the indenter geometry. Finally, it is shown that a simple Drucker-Prager model can describe most of the indentation mechanical response but fails in predicting completely the indentation morphology.
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► The diametral compression (Brasilian test) is employed to characterise the plasticity in a Zr-based bulk metallic glass. ► Finite Element modelling with constitutive models with ...growing complexity is employed to match experimental and computational results. ► Most models succeed in matching global experimental data. The need for local information to discriminate models is highlighted.
Multiaxial and heterogeneous mechanical experiments, by means of the diametral compression test (Brazilian test), were carried out on Zr-based Bulk Metallic Glass. Attention was notably focused on obtaining the displacement field in the area of interest (large strains) employing a Digital Image Correlation device, in addition to the measurement of the usual global load–displacement curve. In order to reproduce the behaviour of the BMG at stake, several constitutive equations were considered with growing complexity: von Mises, Drucker–Prager (pressure dependence), free volume based model as well as a viscoplastic Coulomb–Mohr type model. The two latter were implemented as user-material in a Finite Element computation code. The results of these investigations including experiments and computational simulations are discussed.
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