Relative contact width evaluation of end hemispherical cavities rollers as an indication of Hertzian contact pressure

In a recent investigation, the end hemispherical cavities (EHC) rollers exhibited better strength against fracture than hollow rollers. Furthermore, EHC rollers looked promising from a higher fatigue life aspect than conventional solid rollers in a simulation study. Therefore, it necessitated further exploration of the EHC roller concept and to this end, in the present investigation, the contact widths of EHC rollers were relatively evaluated to judge its contact stresses' behavior with respect to the solid roller because the contact stresses are responsible for the fatigue life of rolling bearings. In the experiments, the contact footprints were obtained by forcing specimens of rollers against chemically etched surfaces and were examined by a microscope for measurement of contact widths. The experimental trials were performed with individual roller-on-plate tests and also with full-bearing samples. The etch correction factor was used to correct anomalies of real and observed contact widths due to etching film thickness. The parabolic relationships were established for roller variants which yielded constants signifying their relative ranks. The contact semi-widths, thus derived from corrected experimental results of individual roller-on-plate tests, demonstrated good agreement (<5%) with those derived from simulation results. The results of full-bearing sample tests for roller variants also ranked same as individual roller-on-plate tests. The encouraging results of contact semi-width assuredly favor the prospects of relatively higher fatigue life in case of EHC rollers.

FFT-ASSISTED ALGORITHMS FOR 3D LINE-CONTACT PROBLEMS

The line-contact is a particular type of contact with a contact length much greater than its width. Such contact scenarios can be treated in the frame of a two-dimensional plane-strain problem if the contacting surfaces can be considered nominally smooth. However, surface irregularities inherent to any manufacturing technique lead to a discontinuous contact area that differs from the one derived on the basis of the smooth profile assumption. It is therefore tantalizing to pursue the solution of a line-contact problem using an intrinsically three-dimensional (3D) model, which can only be numerical due to lack of general analytical solutions in contact mechanics. Considering the geometry of the line-contact, a major challenge in its numerical modelling is that the expected contact area is orders of magnitude larger in one direction compared to the other. This may lead to an unreasonably large number of grids in the contact length direction, which translates to a prohibitive computational burden. An alternative approach, employed in this paper, is to treat the line-contact as non-periodic in the contact width direction, but periodic in the contact length direction, with a period equal to the window required to capture and replicate the surface specific texture. This periodicity encourages the contact problem solution by spectral methods based on the fast Fourier transform (FFT) algorithm. Based on this idea, two methods are derived in this paper from the existing Discrete Convolution Fast Fourier Transform (DCFFT) technique, which was previously developed for purely non-periodic contact problems. A first algorithm variant employs a special padding technique for pressure, whereas a second one mimics the contribution of multiple pressure periods by summation of the influence coefficients over a domain a few times larger than the target domain. Both techniques are validated against the existing analytical Hertz solution for the line-contact and a good agreement is found. The advanced methods seem well adapted to the simulation of contact problems that can be approximated as periodic in one direction and non-periodic in the other.

Detection of locomotion deficit in a post-traumatic syringomyelia rat model using automated gait analysis technique

Syringomyelia (SM) is a spinal cord disorder in which a cyst (syrinx) filled with fluid forms in the spinal cord post-injury/disease, in patients syrinx symptoms include loss of pain and temperature sensation or locomotion deficit. Currently, there are no small animal models and connected tools to help study the functional impacts of SM. The objective of this study was to determine the detectability of subtle locomotion deficits due to syrinx formation/expansion in post-traumatic syringomyelia (PTSM) rat model using the recently reported method of Gait Analysis Instrumentation, and Technology Optimized for Rodents (GAITOR) with Automated Gait Analysis Through Hues and Areas (AGATHA) technique. First videos of the rats were collected while walking in an arena (using GAITOR) followed by extracting meaningful locomotion information from collected videos using AGATHA protocol. PTSM injured rats demonstrated detectable locomotion deficits in terms of duty factor imbalance, paw placement accuracy, step contact width, stride length, and phase dispersion parameters compared to uninjured rats due to SM. We concluded that this technique could detect mild and subtle locomotion deficits associated with PTSM injury, which also in future work could be used further to monitor locomotion responses after different treatment strategies for SM.

A Simple Solution to the Cylindrical Indentation of an Elastic Compressible Thin Layer Resting on a Rigid Substrate

In this paper, an analytical model is presented to study the contact that recedes between an elastic thin film that could be compressed and a substrate of rigidity. The surface of rigidity was formed due to cylindrical indentation. The substrate was assumed to be a rough surface without any friction. Further, the contact width of the substrate was derived, and the relationship between the compression force, compression depth, and the compression width was determined using the energy method. Finally, the obtained results were validated using finite element analysis.

Ultrahigh Strength and Shear-Assisted Decohesion of Sliding Silver Nanocontacts Studied in situ

The behavior of materials in sliding contact is challenging to determine since the interface is normally hidden from view. Using a custom microfabricated device, we conducted in situ, ultrahigh vacuum transmission electron microscope measurements of crystalline silver nanocontacts under combined tension and shear, permitting simultaneous observation of contact forces and contact width. While classically, silver exhibits substantial sliding-induced plastic junction growth, the nanocontacts exhibit only limited plastic deformation despite high applied stresses. This difference arises from the nanocontacts’ high strength, as we find the von Mises stresses at yield points approach the ideal strength of silver. We attributed this to the nanocontacts’ nearly defect-free nature and small size. The contacts also separate unstably, with pull-off forces well below classical predictions for rupture under pure tension. This provides in situ confirmation that shearing reduces nanoscale pull-off forces, consistent with recent theoretical predictions but never before directly observed.

Cell–cell contact landscapes in Xenopus gastrula tissues

Molecular and structural facets of cell–cell adhesion have been extensively studied in monolayered epithelia. Here, we perform a comprehensive analysis of cell–cell contacts in a series of multilayered tissues in the Xenopus gastrula model. We show that intercellular contact distances range from 10 to 1,000 nm. The contact width frequencies define tissue-specific contact spectra, and knockdown of adhesion factors modifies these spectra. This allows us to reconstruct the emergence of contact types from complex interactions of the factors. We find that the membrane proteoglycan Syndecan-4 plays a dominant role in all contacts, including narrow C-cadherin–mediated junctions. Glypican-4, hyaluronic acid, paraxial protocadherin, and fibronectin also control contact widths, and unexpectedly, C-cadherin functions in wide contacts. Using lanthanum staining, we identified three morphologically distinct forms of glycocalyx in contacts of the Xenopus gastrula, which are linked to the adhesion factors examined and mediate cell–cell attachment. Our study delineates a systematic approach to examine the varied contributions of adhesion factors individually or in combinations to nondiscrete and seemingly amorphous intercellular contacts.

Influence of Roll Diameter on Material Deformation and Properties during Wire Flat Rolling

The influence of roll diameter on the strain distribution, shape change, contact pressure, and damage value of a workpiece was investigated during wire flat rolling to control the material properties of the flattened wire. The flattened wires fabricated by the four different rolls were compared using finite element analysis. The strain inhomogeneity of the flat-rolled wire increased with the roll diameter; thus, the macroscopic shear bands were strengthened as the roll diameter increased during wire flat rolling. The contact width and lateral spreading of the flattened wire increased with the roll diameter; therefore, the reduction in area decreased with the roll diameter. The contour of the normal contact pressure on the wire surface exhibited a similar pattern regardless of the roll diameter. The contact pressure showed higher values at the entry, edge, and exit zones in the contact area. The distribution of the damage value varied with the roll diameter. The free surface region tended to have the peak damage value during the process; however, the center region exhibited the maximum damage value with the roll diameter. From the perspective of the damage value, the optimum roll diameter was in existence during wire flat rolling. The underlying cause of the different strain distributions, shape changes, and damage values of the flat-rolled wire was the different contact lengths originating from the different roll diameters during wire flat rolling.

Detection of Locomotion Deficit in a Post-Traumatic Syringomyelia Rat Model Using Automated Gait Analysis Technique

Syringomyelia (SM) is a spinal cord disorder in which a cyst (syrinx) filled with fluid forms in the spinal cord post-injury/disease, in patients syrinx symptoms include loss of pain and temperature sensitivity or locomotion deficit. Currently, there are no small animal models and connected tools to help study the functional impacts of SM. The objective of this study was to determine the detectability of subtle locomotion deficits due to syrinx formation/expansion in post-traumatic syringomyelia (PTSM) rat model using the recently reported method of gait analysis instrumentation, and technology optimized for rodents (GAITOR) with automated gait analysis through hues and areas (AGATHA) technique. First videos of the rats were collected while walking in an arena (using GAITOR) followed by extracting meaningful locomotion information from collected videos (using AGATHA software). PTSM injured rats demonstrated detectable locomotion deficits in terms of duty factor imbalance, paw placement accuracy, step contact width, stride length, and phase dispersion parameters compared to uninjured rats due to SM. We concluded that this technique could detect mild and subtle locomotion deficits associated with PTSM injury, which also in future work could be used further to monitor locomotion responses after different treatment strategies for SM.

Mechanisms of Confining Pressure Dependence of Resistivity Index for Tight Sandstones by Digital Core Analysis

Summary Resistivity measurements are a major input into hydrocarbon reserve estimation and are usually described by Archie’s laws. In this study, we use digital rock physics to analyze the mechanisms of non-Archie and Archie behavior of formation factor (FF) and resistivity index (RI) of low-porosity Fontainebleau (FB) sandstone for ambient conditions and under high confining pressure, respectively. FB sandstone was imaged by micro-X-ray computed tomography (micro-CT) at a resolution of 1 µm. Subresolution details of the grain contact width distribution along with their length were extracted from a set of scanning electron microscope (SEM) images. The nanoscale aperture of grain contacts, which is below tomogram resolution, is accounted for in micro-CT-based numerical calculations by assigning effective porosity and conductivity to individual voxels of the extracted grain contact network. A porosity reduction of grain contacts and open pore space as a function of applied confining pressure is introduced, capturing the pressure dependence. The concept was implemented by grain contact labeling, introducing an additional phase derived from a Euclidean distance transform (EDT). Subvoxel stress-strain effects were incorporated by attributing all compressibility effects to the pore space (open pore space and grain contacts), treating the solid phase as perfectly rigid. Voxel-scale input conductivities are assigned using Archie’s law followed by solving the Laplace equation for sample-scale rock resistivity and RI directly on the segmented image using the finite element method. For the numerical modeling of the FF and RI of low-porosity FB sandstone as a function of confining pressure, which depends on subresolution features, a set of hypotheses were tested. These are based on two segmentation scenarios incorporating the measured contact aperture distribution from SEM analysis—a homogeneous aperture-based segmentation by assuming all grain contacts as an average constant value and a heterogeneous aperture-based segmentation assigning two groups of grain contact apertures. The segmentation scenarios enable homogeneous and heterogeneous morphological change of grain contacts due to confining pressure effects. Furthermore, partial saturation of grain contacts is considered. In all cases, strong water-wetness was assumed, and discretization effects were analyzed carefully. The numerical results highlight the relative contribution of each of two conductive components of FB sandstone (open pores vs. grain contacts) over the full range of partial saturations. Of importance is the connectivity of the system, with discretization effects having a significant effect on FF, but a small effect on the RI. Grain contacts and confining pressure are found to have a significant impact on RI behavior of low-porosity FB sandstone. Both the grain contact network with homogeneous aperture and the heterogeneous grain contact network are able to describe experimental observations. However, it is not sufficient to assume a homogeneous change in contact area, and an inhomogeneous deformation of grain contact zones is required to match the experiment.

An investigation of the elastic cylindrical line contact equations for plane strain and stress considering friction

This work presents a finite-element model-based study of elastic cylindrical contact. The aim is to evaluate the transition between the plane stress and plane strain-based Hertz solutions when each assumption is most applicable. To accomplish this, a range of curvatures, widths, Poisson’s ratios, and friction coefficients are considered. The finite-element model results for the elastic cylindrical contact cases are compared with the Hertz contact model when assuming plane stress or plane strain. Perhaps, surprisingly, the finite-element model predictions show little dependence on Poisson’s ratio and friction coefficient. The finite-element model predictions of force as a function of deflection agree relatively well with the plane stress Hertz prediction for all cases considered. The finite-element model predictions of contact width as a function of force actually fall below all the analytical Hertz predictions. Therefore, an adapted version of the Hertz equations is provided, which shows better agreement with the cases considered in this work.