Contact Depth Research Articles

Computational modelling of frictional deformation of bimodal nanograined metals

Extensive experiments have shown that materials with heterogeneous microstructures comprising grains with different sizes that span over at least one order of magnitude, such as the gradient and bimodal nano-grained metals, have lower coefficient of friction and wear loss. However, the deformation mechanism of the enhanced frictional performance remains unclear because of the complex grain microstructure. Here, a finite element model has been established to investigate the deformation of a bimodal structured Cu during scratch test. Our simulations show that the bimodal nanostructures can achieve a coefficient of friction (COF) as low as 0.399, which is 11% lower than the smallest COF of the homogeneous coarse-grained or nano-grained Cu, i.e., 0.446. The lowest COF of the bimodal nanostructure is achieved by decreasing the hard domain grain size and increasing the soft domain grain size and volume fraction. The reduced COF is revealed to arise from the heterogeneity of the bimodal nanostructure, which can effectively restrain the strain localization, leading to the decrease of the contact depth during the scratch process. Our calculations also show that the bimodal nanostructures possess a much better balance of high strength and low COF than the gradient ones.

Dynamic Simulation and Parameter Analysis of Contact Mechanics for Mimicking Geckos’ Foot Setae Array

According to the dynamic characteristics of the adhesion desorption process between gecko-like polyurethane setae and the contact surface, the microcontact principle of an elastic sphere and plane is established based on the Johnson–Kendall–Robert model. On this basis, combined with the cantilever beam model, microscale adhesive contact models in the case of a single and an array of setae are obtained. The contact process is numerically simulated and verified by the adhesion desorption test. After that, the effects of external preload, the elastic modulus of setae material, the surface energy, and the surface roughness on the contact force and depth during the dynamic contact process of setae are studied. The results show that the error between the simulation and test is 15.9%, and the simulation model could reflect the real contact procedure. With the increase in preload, the push-off force of the setae array would grow and remain basically constant after reaching saturation. Increasing the elastic modulus of setae material would reduce the contact depth, but have little effect on the maximum push-off force; with the increase in the surface energy of the contact object, both the push-off force between the objects and the contact depth during desorption would increase. With the increase in wall roughness, the push-off force curve of the setae array becomes smoother, but the maximum push-off force would decrease. By exploring the dynamic mechanical characteristics of the micro angle of setae, the corresponding theoretical basis is provided for the numerical simulation of the adsorption force of macro materials.

Non-seismic geophysical analysis of potential geothermal resources in the Longgang Block, Northeast China

Although geothermal energy has many clear advantages, including its sustainability and environmentally friendly nature, research into potential geothermal resources across the Longgang Block, Northeast China, has been limited. Here we present the first analysis of the potential geothermal resources in this region that employs joint geological and non-seismic geophysical methods to identify target areas that may be economically viable. We acquire and analyze high-precision gravity, magnetic, and magnetotelluric sounding data, which are constrained using the petrophysical parameters of outcropping rocks across the Longgang Block, to conduct a comprehensive evaluation of the region’s deep geological structures and their geothermal resources potential, with a focus on identifying faults, rock masses, and thermal storage structures. We find that Archean granitic gneiss and Mesozoic rock masses in the deeper section of the Longgang Block possess weak gravity anomalies and high resistivities. We also identify thermal storage structures near these deeper geological units based on their extremely low resistivities. The data are used to infer the dip and depth of known or hidden faults, to constrain the spatial distribution of intrusive rock masses, and to determine the spatial distribution of subsurface thermal storage structures. The potential of the target areas for geothermal resources exploitation is divided into three grades based on contact depths between faults and thermal storage structures, and the scale of their thermal storage structures. Our results suggest that a joint non-seismic geophysical approach can be effective in locating and evaluating geothermal resources in complex geological settings.

Testing scenarios on geological models: Local interface insertion in a 2D mesh and its impact on seismic wave simulation

In this work, we propose a local updating method to test different contact depth scenarios and assess their impact on wave propagation in the subsurface. We propose to locally modify a 2D geological model and run time-dependent elastic simulations. The input model triangulation is conforming to geological structures. The 2D meshed model is locally updated, which means that only the reservoir compartment is modified. Several model geometries are generated by inserting a new interface, in this paper a gas–water contact that is defined by a scalar field. We quantitatively evaluate the impact of the gas–water contact depth on elastic wave propagation. We run the numerical simulations with Hou10ni2D code, which is based on a Discontinuous Galerkin method. The simulation results are compared to a reference depth by computing the L2-norm at a set of seismic receivers. Results show a consistent behavior: we observe a positive correlation between the depth difference and global L2-norm for all receivers. This approach could therefore be integrated into an inversion loop to determine the position of the fluid contact and reduce uncertainties in the reservoir model from a few seismic sensors. The algorithms are available on Github and distributed under a GPL license, allowing reproducibility.

Mechanical response of silver/polyvinyl alcohol thin film: From one-step and cyclic nanoindentation

Polymer nanocomposite and its mechanical properties are the area of immense interest for material developers in academia and industry. Present study investigates mechanical behavior of novel hexagonal bipyramidal Silver-Poly (vinyl alcohol) nanocomposites (Ag/PVA NCs) thin film fabricated using aqua-mediated in-situ reduction at room temperature. Specimen was characterized through UV-Visible (λmax = 417 nm), PL (λ(em) = 482 nm), XRD (average crystallite size 29.7 nm and preferential growth of thin film along [111] plane), TEM (hexagonal bipyramidal morphology, size ∼26.25 nm), TGA (increased thermal stability), FTIR (peaks at 945, 604 cm-1 confirms coordination of Ag and PVA matrix) and SAED. Depth sensing single-step and multi-step nanoindentation was employed to extract material’s sensitive mechanical property. Hardness (H: 0.352 to 0.192 GPa) and elastic modulus (Er: 8.718 to 6.72GPa) decreased with increasing single-step loads (from 50 μN to 10 mN), which can be attributed to indentation size effect (ISE). Cyclic nanoindentation (p∼5 mN, 23 cycles) was performed to deeply understand material sensitive degree of dislocation-structure interaction for fatigue analysis. The reduction in % elastic recovery was found at higher loads. On account of smooth surface, elastic unloading (stiffness) values linearly (y = 1.3944x + 3.8676 with R2 = 0.9835) increased from cycle 1 to 23 and contact depth increases linearly (y = 0.0329x + 1.5697, R2 = 0.9901) with the number of cycles. Interpretation of p-h profile indicated H and Er decreases with increasing load, along with proceeding cycles and no fracture or breakage/ weakening of NCs / progressive and localized structural damage was observed which evidenced that surface was completely uniform and defect free.

Mechanical properties of α-quartz using nanoindentation tests and molecular dynamics simulations

Understanding the mechanical properties of α-quartz is of vital importance to rock engineering because α-quartz is the main component of igneous, metamorphic and sedimentary rocks. Molecular dynamics simulations (MDs) of nanoindentation tests on α-quartz were performed to investigate the effects of indenter tip radius and penetration depth on the mechanical properties of α-quartz. Indentation load-penetration depth (P-h) curves were plotted, from which Reduced Young’s modulus ( E r ) , hardness ( H ) were obtained and these mechanical parameters were then compared with the laboratory nanoindentation results. The mechanical results obtained from MDs are in good agreement with the experimental values. It can be found that E r and H increase with indentation depth at shallow contact depth while they decrease with indenter tip size. To the authors’ knowledge, this is the first MDs of nanoindentation test of hard rock-forming minerals reported and we believe that this study can shed light on the precise measurement of the mechanical properties of rock minerals at micro- and nano-scales.

Positioning of the Moving and Static Contacts of the Switch Machine Based on Double-Layer Mask R-CNN

With the continuous development of rail transit, the maintenance of the switch machine becomes more and more important, and the contact depth of the moving contact and static contact in the switch machine is a key part of it. At present, the manual measurement method is the main measure of contact depth, which has the problems of low efficiency and strong subjectivity. The measurement of contact depth based on machine vision includes two steps: moving and static contact positioning and distance conversion. The positioning result will have an important impact on distance measurement. Therefore, a positioning method for moving and static contact based on double-layer Mask R-CNN (DLM) is proposed in this paper: first, the moving contact is roughly positioned by Mask R-CNN to obtain the predicted target area; second, the subgraph of the target area is preprocessed; finally, the precise positioning is used to determine the precise position of the moving and static contact. The accuracy and robustness of the proposed DLM are verified by the internal image of the switch machine.

Effect of plasticity and adhesion on the stick-slip transition at nanoscale friction

When an external force is suddenly applied to the material interface, how much the critical force is required for the onset of slip? To answer this, we used molecular simulations to study the slip initiation during a nano-scratch. The shear load is applied by a very soft spring to achieve the constant force boundary. Using this model, we studied how much plasticity and adhesion affect the critical shear load. Results show that the slip initiation depends on contact depth because the dominated factor for stick-slip changes from interfacial adhesion in shallow contact to dislocation plasticity in deep contact. Also, the initiation stress of slip depends on temperature; the critical shear stress follows a temperature dependence of ~TlnT when dislocation plasticity prevails.

Silicon carbide coatings on zirconium alloy for light water reactor fuel cladding studies

Coating of zirconium alloys is a promising approach for improving various properties of the cladding of fuel elements, including physical, mechanical properties, corrosion resistance and reduction of radiation damage. In this paper, SiC coatings with thicknesses of 100, 200, and 300 microns were deposited by selective laser sintering (SLS) on Zr-1Nb alloy substrates at a laser power of 150 W. The 200 μm layer deposition registered the least contact depth during macro-scratch test to a maximum depth of about 25 μm into the coating. The uniformity and less porosity of the coatings is observed on the transverse sections in the entire variant. Analysis of the phase composition of the coating revealed the formation of silicon carbide phases (6H & 4H), with protective oxides of Al2O3, Y2O3, SiO2 and YAlO3. High coating adhesion was noticed with results of the scratch tests showing no delamination regardless of the higher loads used. However, partial chippings and transverse cracks were observed. The results obtained indicate the need for further research to improve the porosity of the surface structure of the coating and to conduct other necessary tests.

Stratigraphic identification with airborne electromagnetic methods at the Hanford Site, Washington

Stratigraphic units can influence the fate and transport of subsurface contaminants within groundwater. Units having coarse-grained sediments act as preferential flow pathways, and therefore can accelerate the transport of contaminants to reach human and ecological receptors. At legacy waste sites, detailed knowledge of subsurface stratigraphy can be used for effective monitoring and remediation planning to help minimize risk to human health and the environment. Airborne electromagnetic (AEM) methods can non-invasively provide information on kilometer-scale or larger subsurface stratigraphic features and fill informational gaps in directly sampled data from sparsely located boreholes. In this paper, we present inversion results of a 412 line-km frequency-domain AEM survey to delineate subsurface stratigraphic features at the Hanford Site, located in southeastern Washington State. The inversion was performed using a massively parallel 3D electromagnetic modeling and inversion code, where the modeling is based on solving frequency-domain Maxwell's equations using an unstructured-mesh finite-element method and the inversion employs a Gauss-Newton optimization scheme. The results are compared to an underlying geologic framework model (GFM), built by interpolating contact depths of stratigraphic units interpreted from site borehole datasets. In areas with good borehole coverage, the inversion results show a good match with the GFM to a depth of about 60 m. Outside of these areas, the inversion results exhibit inconsistencies from the assumptions made to create the GFM, demonstrating that the AEM survey results can be used to improve the understanding of the geological conceptual model.

Mechanical characterization of the enamel microstructure treated with non alcoholic drinks

In previous studies, it has been shown that the microstructure of prismatic dental enamel presents differences between the external and internal zone. Radial enamel is found in the outer third of enamel and has higher microhardness values than enamel with Hunter- Schreger Bands (HSB) that occupies the inner 2/3. Our aim was to evaluate the mechanical behavior of radial enamel and HSB due the action of a non-alcoholic beverage in vitro. Longitudinal sections of dental crowns were embedded in polymer, worn and polished with sandpapers of decreasing granulation. The samples were immersed in a flavoured natural water for 12 minutes. Nanohardness tests (Triboindenter Hysitron) were performed on the radial and HSB enamel before and after the exposure to the drink. Hardness determinations "H", reduced modulus "Er" and contact depth "hc" were obtained. The percentage of reduction of hardness was determined. The values found in healthy radial enamel were H: 5.48±0.23 GPa; Er: 86.97±8.11 GPa; hc: 149.73±4.25 nm and in HSB H: 4.24±0.43 GPa; Er: 75.24±7.09 GPa; hc: 176.36±11.29 nm. After exposure to beverage, it was found in the radial enamel H: 2.22±0.31 GPa; Er: 58.73±10.79 GPa; hc: 270.29±21.22 nm, and in HSB H: 1.54±0.42 GPa; Er: 48.11±6.54 GPa; hc: 350.10±63.33 nm. After the drink action, the values of hardness of the radial enamel and HSB decreased and the trend observed in healthy enamel remained, where the highest values corresponded to the radial enamel. The percentage reduction of H in the radial enamel was 59.48% and in the HSB enamel it was 63.67%. The contact depth increased by about 50%.The decrease in hardness is related to the mineral loss produced by the acids contained in the drink. We conclude that the action of the non-alcoholic beverage produces a decrease in the mechanical properties in both the radial enamel and the HSB. The lower values in the reduced module Er indicate the formation of a superficial softened layer, the enamel with HSB being more vulnerable.

IoTouch: whole-body tactile sensing technology toward the tele-touch

In the era of social distancing, humans tend to interact more with digital edges/devices and robotic avatar. Users mostly interact with cyber-physical systems through audiovisual devices, while tangible interfaces are limited to touch panels. Touch perception not only brings sense of physical contact, texture; but also conveys the intention and emotion of users. Here, we introduce a novel soft tangible device, called IoTouch, as a combination of soft robotics and deep learning, that can act as both robotic skin or haptic sensing interface. The technique aims to implementation of touch sensing with a vision-based approach. We showcase on barrel shape (ellipsoid with two flattened ends) of the IoTouch, where a dataset of tactile images was collected through an autonomous process, in order to train a network for recognition of physical contact states (contact location, contact depth). A Finite Element Method model of the barrel skin was also setup for data acquisition of skin's deformation. The proposed method can detect multiple contacts simultaneously on the entire area of the barrel skin. Obtained result promises a platform for creation of robotic avatars that can sense diverse states of physical touch, through which the users' experience and communication of haptics can be enhanced.

Effect of surface detection error due to elastic–plastic deformation on nanoindentation measurements of elastic modulus and hardness

Elastic modulus and hardness are commonly measured using nanoindentation. Calculating these properties from measured force–displacement curves typically requires knowledge of the contact depth of the indenter into the specimen. However, surface detection methods in many nanoindentation experiments can lead to an error in the contact depth measurement and, subsequently, the measured properties. Here, the contributions of elastic and plastic deformations to surface detection errors in nanoindentation experiments are examined through experiments and modeling. The model is used to quantify errors in elastic modulus and hardness measurements due to elastic–plastic deformation during surface detection as a function of the specimen properties, indenter geometry, preload, and contact depth. Nanoindentation measurements on polystyrene, an aluminum alloy, and fused silica specimens with a Berkovich indenter are used to illustrate the effects of surface detection error and are compared to the model. The experiments and model both demonstrate that surface detection error can lead to measurement of apparent depth-dependent properties in homogenous materials.

Enhanced frictional performance in gradient nanostructures by strain delocalization

Extensive experiments have demonstrated that gradient metals have significantly lowered coefficient of friction (COF) as compared to their homogeneous nano-grained or coarse-grained counterparts. However, the mechanism remains unclear due to the complex gradient microstructure. Here, a two-dimensional finite element model has been established to simulate the scratch test of gradient nanograined Cu by a conical indenter. The mechanical properties of each layer with different grain sizes of the gradient Cu are described by a dislocation density-based constitutive relation. The apparent COF is obtained by calculating the ratio of the frictional force to the normal one between the indenter and the gradient substrate. The results show that the apparent COF of the gradient structure can be significantly lowered as compared with its homogeneous counterparts with various grain sizes. The COF can be further minimized by tuning the gradient microstructures. Our simulations clearly revealed that the lowered COF of the gradient Cu originates from the strain delocalization induced by the co-deformation of the gradient structure. The strain delocalization produces smaller contact depth between the indenter and the gradient substrate as compared with the homogeneous substrate. A design map is also provided for designing gradient nanostructured Cu with high strength and low COF.

Indentation size effects in graphene oxide under suspended nanoindentation

In this study, measurements of the elastic properties and the pre-stress of free-standing multilayers graphene oxide (GO) sheets were performed by suspended nanoindentation tests, using two indenter radius with values of approximately 50 nm and 636 nm, in an atomic force microscope and a triboindenter system respectively. The GO sheets were obtained by a modified Hummers’ method and deposited by drop-casting method over a specialized transmission electronic microscopy (TEM) grid. The GO sheets had about 3 layers, an average area of 3 μm2, and a roughness average, ra, of approximately 3 nm. The results showed that the GO sheets had differences in the magnitude of the mechanical properties, according to the number of layers. However, once the measurements were normalized based on their thickness, the bulk elastic modulus from both types of nanoindentation systems had a different value of approximately 130 GPa. Thus, it can be argued that also the mechanical behavior was influenced by the roughness as well as by magnitude of contact depth according to the tip radius size used. Besides, finite element (FE) simulations of both nanoindentations performed on a representative GO sheet, were carried out to verify the elastic modulus obtained experimentally, and then to approximate the pre-stresses involved in the regarding suspended nanoindentations. It was demonstrated that GO sheet roughness, as the tip radius, were also fundamental factors that played a significant role in the different mechanical properties obtained by the considered experimental and computational procedures.

Optimization of AL6061-T6 Tube End Forming Process Using Response Surface Method

Tube end closing is a metal forming process that replaces welding processes while closing tubes ends. It depends on deforming a rotating tube using a roller, and therefore, it is also called tube end spinning. The process involves many parameters like contact depth, roller inclination angle, roller diameter, mandrel curvature, and tube rotational speed. This study develops a finite element model (FE-model) for this process and validates it through experimental results. The numerical and experimental results have shown minor deviation of 1.87%. The FE-model is then employed to carry out a statistical analysis based on the response surface method (RSM). The analysis of variance (ANOVA) and regression analysis have proved the accuracy of the obtained mathematical model. The contact depth has proved to have the most significant effect in the process responses, while the roller diameter has the least effect. Finally, an optimization analysis is carried out to select the finest conditions for the process.

Effect of Sample Tilt on Indentation and Scratch Behavior of Single Crystal Copper

Inclined factors bring a lot of errors to indentation and scratch experiments. The aim of this paper is to investigate the effect of sample tilt on indentation and scratch behavior of single crystal copper. Angled indentation and scratching experiments were conducted on the flat of 5°,10°,15° and 30°, and spherical-conical indenter was used to study the effect. In scratching simulation with SPH method, normal and lateral forces during scratching process increased when tilt. Contrary to simulation result, when contact depth was held as a constant, the forces declined as tilt angle increased. In geometry model, the actual indentation depth was less than the theoretical contact depth and the projected area of the tilt indenter was an enclosed area like an elapse with two circular arcs. Residual impression images of indentation and scratching indicated that not only the sphere section of the tip of indenter, but also the conical surface penetrated into sample and squeezed a slope. The results showed that the size of spherical-conical indenter has significant effects on the size and shape of wear debris and could not be neglected during indentation and scratching tests with sample tilt.

Berkovich nanoindentation of Zr55Cu30Al10Ni5 bulk metallic glass at a constant loading rate

Berkovich nanoindentation experiments were conducted on Zr55Cu30Al10Ni5 bulk metallic glass under a constant loading rate. Indentation hardness decreased linearly with contact depth, which defied the direct application of conventional indentation size effect model for crystalline material. Elastic modulus remained invariant under small loads, but decreased linearly under large loads. The scaling relationships between indentation variables such as contact depth, permanent depth, the maximum indentation displacement, plastic energy, the total work of indentation and their ratios were investigated. The area of shear band circle, the projected contact area at the maximum load, and the residual projected area were all found to be proportional to one another. A new methodology to determine fracture toughness was proposed based on the total work of indentation, provided that the critical indentation displacement at the onset of fracture corresponds to zero value of elastic modulus extrapolated from curve fitting.

Determination of elastic moduli of elastic-plastic microspherical materials using nanoindentation simulation without mechanical polishing.

When using the Oliver–Pharr method, the indented specimen is assumed to be a perfectly flat surface, thus ignoring the influences of surface roughness that might be encountered in experiment. For nanoindentation measurements, a flat surface is fabricated from curved specimens by mechanical polishing. However, the position of the polished curved surface cannot be controlled. There are no reliable theoretical or experimental methods to evaluate the mechanical behavior during nanoindentation of an elastic–plastic microsphere. Therefore, it is necessary to conduct reliable numerical simulations to evaluate this behavior. This article reports a systematic computational study regarding the instrumented nanoindentation of elastic–plastic microspherical materials. The ratio between elastic modulus of the microsphere and the initial yield stress of the microsphere was systematically varied from 10 to 1000 to cover the mechanical properties of most materials encountered in engineering. The simulated results indicate that contact height is unsuitable to replace contact depth for obtaining the indentation elastic modulus of microspherical materials. The extracted elastic modulus of a microsphere using the Oliver–Pharr method with the simulated unloading curve depends on the indentation depth. It demonstrates that nanoindentation on microspherical materials exhibits a “size effect”.

Influence of Sr/Nd partial replacement on fundamental properties of Bi-2223 superconducting system

This comprehensive work aims to examine the change in flux pinning mechanism, physical, mechanical, and structural characteristics of pure and Sr-site Nd-substituted Bi1.8Pb0.35Sr1.9−yNdyCa2.2Cu3Ox (Bi-2223) systems. The magnetoresistivity performances for all the samples are carried out by magnetotransport experiments in the existence of external magnetic field strength intervals 0–7 T. It is found that the increment of Nd/Sr substitution amount in bulk Bi-2223 system retrogrades the pinning capability of thermal flux motions for interlayer Josephson junction between the isolated grains. Similarly, the coupling probabilities of copper pairs and potential energy barriers are significantly diminished by increasing Nd impurity. This is in association with the enhancement of permanent structural problems in the crystal structure. Therefore, the excessive Nd inclusions improve the reattached linear/split pancake-like nature. In this regard, the best magnetic performance quantities are obtained for the pure sample. Besides, the SEM images show that the grain connectivity and surface morphology damage significantly with the Nd impurity. Additionally, the experimental microhardness findings conducted at various external loads (0.245–2.940 N) display that the Nd purity in the superconducting system degrades dramatically the key design mechanical features. Besides, we analyze the mechanical characteristic properties founded on the theoretical approaches with the proportional sample resistance, elastic/plastic deformation, and Hays–Kendall methods. The results obtained show that the Nd purity causes the indentation size effect behavior to decrease dramatically for all the samples. Furthermore, the findings of Hays–Kendall method are noticed to much more agree with the real hardness parameters. Thus, the Hays–Kendall model is the best methods to find the load-independent Vickers hardness values for the Sr-site Nd-substituted Bi1.8Pb0.35Sr1.9−yNdyCa2.2Cu3Ox (Bi-2223) systems. Moreover, in the dynamic microhardness measurements, the contact depth (hc), elastic modulus (Er), and load (Pmax) of all the samples are experimentally recorded for the first time. The results reveal that the mechanical properties depend strongly on the load and Nd impurity level.