Constant contact stiffness indentation relaxation test

Nanoindentation test is of great interest to characterize small scale mechanical behavior, thus a large literature exists on the field. Nevertheless, measurements of time dependent mechanical properties by this technique is still to be improved 1. It is proposed to investigate the indentation relaxation from a different point of view. Indentation relaxation tests are usually performed keeping a constant displacement over a prescribed time duration 2. This experimental procedure is consequently very sensitive to the system drift. Hence, determination of relaxation behavior is limited to few hundreds of seconds in the best cases. Weihs and Pethica 3 and Maier et al. 4, proposed to use the continuous contact stiffness measurement as a robust measure of the contact area. Based on these studies, a novel experimental procedure has been developed. Contact stiffness is kept constant after loading to a prescribed depth, for a define period, while displacement and load are monitored. As the contact stiffness measurement is not sensitive to drift, this method allowed to perform relaxation experiments with very long hold segment. Experiments on fused silica and polymers - i.e. PMMA, PC and PS - at room temperature have been performed with a constant contact stiffness maintained up to 10 hours. It has been shown that the dispersion on the force, F, was greatly reduced (see Figure 1). This could be understood as constant contact stiffness experiments were much less affected by the system drift than constant displacement ones. This new method opens the way to time dependent mechanical characterization in a wider range of conditions, especially long time experiments and high temperature indentation tests. Please click Additional Files below to see the full abstract

Effect of hydrostatic pressure on elastic properties of ZDTP tribofilms

Previous studies have shown that the elastic properties of Zinc Dialkyl-dithiophosphate (ZDTP) tribofilms measured by nanoindentation increase versus applied pressure (Anvil effect) [1, 2]. The aim of this paper is to demonstrate that, up to 8 GPa, this increase is a reversible phenomenon. A ZDTP tribofilm has been produced on "AISI 52100" steel substrate using a Cameron-Plint tribometer. After its formation, a hydrostatic pressure of about 8 GPa was applied during one minute on the tribofilm using a large radius steel ball ("Brinell-like" test). Nanoindentation tests were performed with a Berkovich tip on pads in order to measure and compare the mechanical properties of the tribofilm inside and outside the macroscopic plastically deformed area. Careful AFM observations have been carried out on each indent in order to take into account actual contact area. No difference in elastic properties was observed between the two areas: tribofilm modulus and pressure sensitivity are the same inside and outside the residual hemispherical print. This demonstrates that Anvil effect is a reversible phenomenon in the studied pressure range

Indentation relaxation test: Opportunities and limitations

Small scale characterization of material’s mechanical behavior has been performed for fifty years using indentation tests. Many developments have been made in order to improve the reliability of both measurements and interpretations. However, determination of material’s time dependent mechanical properties by means of nanoindentation techniques is still to be enhanced 1. It is proposed to investigate the indentation relaxation – i.e. constant displacement – test as an alternative to the commonly used indentation creep – i.e. constant load – test. Effects of loading strain rate on the measured relaxation behavior are studied, analytically, from a linear viscoelastic model. It is shown that constant strain rate loading guarantees a depth-independent measure of the relaxation behavior. Moreover, indentation strain rate (ISR) affects the relaxation spectrum 2 up to a critical time constant 3 (see figure 1). These effects, highlighted analytically, are confirmed experimentally on PMMA. Limitations of the indentation relaxation test are also discussed. Two main difficulties arise from this kind of experiment. Acquisition of reliable measurements is limited, for long time characterization, by the system drift and, for short time, by the displacement control loop. A particular care has been taken in tuning the control feedback gains to limit displacement overshoot. Very low drift rate has been attained – under 0.015 nm.s-1 – This allowed for measurements at constant displacement up to 600 s. Please click Additional Files below to see the full abstract

Hydrograph separation using isotopic, chemical and hydrological approaches (Strengbach catchment, France)

The streamflow components were determined in a small catchment located in Eastern France for a 40 mm rain event using isotopic and chemical tracing with particular focus on the spatial and temporal variations of catchment sources. Precipitation, soil solution, springwater and streamwaters were sampled and analysed for stable water isotopes (18O and 2H), major chemical parameters (SO4, NO3, Cl2, Na1, K1, Ca21, Mg21, NH4, H1, H4SiO4, alkalinity and conductivity), dissolved organic carbon (DOC) and trace elements (Al, Rb, Sr, Ba, Pb and U). 18O, Si, DOC, Ba and U were finally selected to assess the different contributing sources using mass balance equations and end-member mixing diagrams. Isotopic hydrograph separation shows that the pre-event water only contributes to 2% at the beginning of the stormflow to 13% at the main peak flow. DOC associated to Si and U to Ba allow to identify the different contributing areas (upper layers of the saturated areas, deep layers of the hillslope and rainwater). The streamflow (70%) originates from the deep layers of the hillslope, the remaining being supplied by the small saturated areas. The combination of chemical (both trace and major elements) and isotopic tracers allows to identify the origin of water pathways. During the first stage of the storm event, a significant part of the runoff (30±39%) comes from the small extended saturated areas located down part of the basin (overland runoff then groundwater ridging). During the second stage, the contribution of waters from the deep layers of the hillslope in the upper subcatchment becomes more significant. The final state is characterised by a balanced contribution between aquifers located in moraine and downslopes. Indeed, this study demonstrates the interest of combining a variety of hydrometric data, geochemical and isotopic tracers to identify the components of the streamwater in such conditions

Use of a nanoindentation fatigue test to characterize the ductile-brittle transition

When considering grinding of minerals, scaling effect induces competition between plastic deformation and fracture in brittle solids. The competition can be sketched by a critical size of the material, which characterizes the ductile-brittle transition. A first approach using Vickers indentation gives a good approximation of the critical size through an extrapolation from the macroscopic to the microscopic scales. Nanoindentation tests confirm this experimental value. According to the grain size compared to the indent size, it can reasonably be said that the mode of damage is deformation-induced intragranular microfracture. This technique also enables to perform cyclic indentations to examine calcite fatigue resistance. Repeated loadings with a nanoindenter on CaCO3 polycrystalline samples produce cumulative mechanical damage. It is also shown that the transition between ductile and brittle behaviour depends on the number of indentation cycles. The ductile domain can be reduced when the material is exposed to a fatigue process.Comment: Journal of European Ceramic Society accept\'e pour publication (2008) sous-press

The understanding of silicon sequential elutriation behaviour

During the fluidization of broad PSD (Particle Size Distribution) powders, elutriation can not be avoided, but has to be process controlled. Batch elutriations of continuous PSD powders were studied in a laboratory scale fluidized bed. The reference sample was metallurgical-grade silicon powder, with non-spherical shape. The smallest elutriable fines, namely superfines (\u3c10 µm) are entrained first. However, the largest elutriable particles (Ut ~ Ug) do not begin to be entrained simultaneously, but only after a delay that is as long as the time required for the superfines to leave the bed, thus inducing sequential elutriation (Figures 1). When no superfines were present, the entrainment was not delayed. This peculiar phenomenon was observed at all of the tested gas velocities (0.05-0.2 m/s). The superfines thus seem to strongly limit the elutriation of the larger elutriable particles. This sequential behaviour is particularly interesting to separate particles according to a small and narrow PSD (Figure 2). These phenomena are related to interparticle interactions within the bed and/or the freeboard and confirm the importance of polydispersity in the elutriation behavior. Thanks to the elutriation mathematical models developed in this study, the behavior that was thought to be explained by Silicon attrition can now be explained by sequential elutriation. Please click Additional Files below to see the full abstract

About the measurement of restoration kinetics in metals using the HTSI method

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