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Nanoindentation Under Dynamic Conditions

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Abstract

Nanoindentation has emerged as a leading technique for the investigation of mechanical properties on small volumes of material. Extensive progress has been made in the last 20 years in refining the instrumentation of nanoindentation systems and in analysis of the resulting data. Recent development has enabled investigation of materials under several dynamic conditions. The palladium-hydrogen system has a large miscibility gap, where the palladium lattice rapidly expands to form a hydrogen-rich β phase upon hydrogenation. Nanoindentation was used to investigate the mechanical effects of these transformations on foils of palladium. Study of palladium foils, which had been cycled through hydrogenation and dehydrogenation, allowed the extent of the transformed region to be determined. Unstable palladium foils, which had been hydrogenated and were subject to dynamic hydrogen loss, displayed significant hardening in the regions which were not expected to have transformed. The reason for this remains unclear. Impact indentation, where the indenter encounters the sample at relatively high speeds, can be used to probe the strain rate dependence of materials. By combining impact indentation and elevated temperature indentation, the strain rate dependence of the superelasticity of nickel-titanium was probed over a range of temperatures. Similar trends in elastic energy ratios with temperature were observed with the largest elastic proportions occurring at the Austenite finish transformation temperature. Multiple impact and scratch indentation are two modes of indentation which are thought to approximate erosive and abrasive wear mechanisms, respectively. These were utilised to investigate the wear resistance of several novel coatings formed by plasma electrolytic oxidation (PEO) of Ti-6Al4-V. Multiple impact indentation results appear to subjectively rank the erosive wear performance of both ductile and brittle materials. Comparison of normalised performance of coating systems on aluminium in abrasive wear to scratch hardness showed similar degrees of resistance. This material is based upon work supported under a National Science Foundation Graduate Research Fellowship. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Additional support for this work was provided by funding from the Atomic Weapons Establishment.
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... Several authors have shown that the strain rate at contact in the nano-impact test can be extremely high, typically in the region of 10 4 -10 5 s −1 [26][27][28]55,57]. To illustrate this, Figure 2 shows how the strain rate varies with time during a single impact event on a bulk alumina sample when impacted by a cube corner diamond probe (three repeats are shown). ...
... The applied load and accelerating distance control the impact energy delivered to the sample. Typical nano-impact test parameters that have been used for testing hard coatings are: (i) cube corner diamond impact probe, (ii) 90° impact angle, (iii) 25-150 mN applied load, (iv) 15 μm accelerating distance, (v) 0.25 Hz impact frequency, (vi) 300 s test duration (i.e., 75 impacts in total), Several authors have shown that the strain rate at contact in the nano-impact test can be extremely high, typically in the region of 10 4 -10 5 s −1 [26][27][28]55,57]. To illustrate this, Figure 2 shows how the strain rate varies with time during a single impact event on a bulk alumina sample when impacted by a cube corner diamond probe (three repeats are shown). ...
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In this review, the operating principles of the nano-impact test technique are described, compared and contrasted to micro- and macro-scale impact tests. Impact fatigue mechanisms are discussed, and the impact behaviour of three different industrially relevant coating systems has been investigated in detail. The coating systems are (i) ultra-thin hard carbon films on silicon, (ii) DLC on hardened tool steel and (iii) nitrides on WC-Co. The influence of the mechanical properties of the substrate and the load-carrying capacity (H3/E2) of the coating, the use of the test to simulate erosion, studies modelling the nano- and micro-impact test and performing nano- and micro-impact tests at elevated temperature are also discussed.
... This leads to a strain rate of indentation that remained constant with indentation depth. In the case of the nano-impact tests, however, the strain rate cannot be controlled [27], and the strain rate varied from very high values (≈10 4 s − 1 ) at the moment of impact to zero at the point of maximum depth, where the indenter stops and rebounds. ...
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Selective laser sintering (SLS) of polymers has made possible the introduction of lattice-based cells as building blocks of polymer parts, allowing to obtain optimal specific properties. The actual mechanical performance of an SLS part is strongly dependent on the printing direction and part shape. Nevertheless, macroscopic testing is not usually possible due to size restrictions of the fabricated part, as it happens, for example, in lattice-based materials. In this case, microscopic tests performed directly on the lattice surface become fundamental. In this work, the mechanical behavior of SLS PA12 under a wide range of strain rates from 10⁻³ to 10³ s⁻¹ and different printing directions is characterized using macroscopic tests on bulk samples and compared with nanoindentation performed directly on the lattice material surface. An excellent correlation was found between the rate-dependent mechanical behavior obtained at the two scales, validating microscopic techniques for characterizing the mechanical response of SLS fabricated PA12 parts.
... One of them is the strain rate of indentation. As an impact test, the strain rate rapidly changes from very high values in the moment of first contact (10 4 − 10 5 s −1 ) to zero at the point of maximum depth when the indenter is arrested due to the material's resistance (Wheeler, 2009). Another challenge is the presence of a secondary oscillation superimposed on the main indenter motion due to pendulum oscillation. ...
Thesis
Fibre-reinforced polymer composites are nowadays the material of choice in applications requiring high structural performance and low weight. However, there are still areas where composite materials have not reached their full potential, such as in components subjected to impacts and high speed events, which are often seen in the aerospace and automotive industry. This is mostly due to the insufficient knowledge about their dynamic behaviour and the lack of accurate analysis methodologies. In view of this, this work proposes the use of a physically-based multiscale simulation strategy based on computational micromechanics to study the effect that the strain rate has on the initiation of failure within the composite ply. Virtual testing based on bottom-up multiscale modelling aims at reducing the cost of certification test campaigns by providing high-fidelity simulations that can complement the experiments at all levels of the test pyramid. This work represents an extension of this state of the art simulation strategy to the high strain rate regime. One of the singularities of the proposed multiscale simulation strategy is the use of micromechanical testing for the in situ mechanical characterization of the matrix and the fibre-matrix interface, which is carried out by instrumented nanoindentation. To this end, the first step of this work involved the development of a novel micromechanical testing methodology to extract properties of the composite constituents over a wide range of strain rates. A commercially available nanoindenter with the in-built capability of performing impact tests at the nano-/microscale was modified to add force sensing capability. This enabled the determination of a force-displacement curve, which is the essential output of nanoindentation. In parallel, a FEM-based inverse analysis methodology was devised to transform the direct outputs of the combined nanoindentation and nano-impact tests into material parameters needed to calibrate the rate dependent constitutive model of the matrix. The inverse analysis was validated against materials (Cu and PMMA) with well known rate dependent mechanical behaviour. The predictions of the analysis correlated well with the literature results. However, the predictions for the matrix material of the two composite systems under study (IM7-8552 and IM7-M91) deviated significantly from test results at the macroscale. As a result, an additional technique for matrix characterization, the micropillar compression test, was proposed. The microscale tests provided accurate values of the compression flow stress over a wide range of strain rates (10^-3-10^2 s^-1) that were in good agreement with the results obtained at the macroscale. In addition, it was confirmed that the new experimental setup allows for the implementation of push-in tests for the characterization of the fibre-matrix interface at impact conditions. However, the acquisition system of the setup must be improved to obtain results free of experimental noise. The inputs from micromechanical testing, complemented by macroscale testing and literature values, were used to calibrate the material models of a simulation tool based on computational micromechanics that uses the representative volume element (RVE) approach to represent the composite ply. The stress-strain response at the ply level was simulated for two matrix-dominated deformation modes, transverse compression and in-plane shear, over a wide range of strain rates and compared with macroscale test results. The numerical predictions were in good agreement with the experiments. Furthermore, the comparison revealed a change in failure mechanism with strain rate. At low strain rates, failure initiated due to the combined effect of fibre-matrix debonding and matrix plasticity. At high strain rates, failure initiation was caused primarily by matrix plasticity.
... Nanoindentation mechanical testing technique presses a indenter tip with standard geometries, such as Berkovich and Cube Corner, into material surface with controlled force or controlled displacement and then withdraw. The indented displacement and force data are continuously measured, which allows the mechanical properties to be directly extracted from the loaddisplacement curves based on Oliver-Pharr theory [6][7]. Mechanical properties of coating deposited over a surface treatment that changes the original material surfaces properties on nano-to micro-scale depth can be better characterized by instrumented indentation testing method because of its simple preparation and automation [8][9][10]. ...
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... In order to guarantee the accuracy and repeatability of the nanoindentation results, the nanoindentation systems require calibration and calibration standards. Three calibration measurements are required for all nanoindentation systems (Wheeler, 2006): ...
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