Junction Growth Research Articles

Experimental investigation of junction growth of rough contacts using X-ray computed tomography

The real contact area (RCA) of randomly rough contacts has received a great deal of attention because it correlates strongly with friction, lubrication, sealing, and conductivity. Simulations have revealed that the RCA associated with deterministic normal squeezing loads increases when tangential loads are also applied, in a phenomenon called junction growth. However, experimental investigations of the junction growth of randomly rough contacts are rare. Here, we used X-ray computed tomography (CT) to measure junction growth when two aluminum alloy surfaces were in contact. A high-resolution experimental setup was used to apply loads and observe contact behaviors at a resolution of 4 µm. The RCA and average contact gaps were computed using a three-dimensional (3D) geometric model constructed from gray CT images using the Otsu thresholding method. The results showed that the RCA increased as the normal load increased. The RCA increased by 22.67% after a tangential load was applied (junction growth), and the average gap decreased by 14.01% after a tangential load was applied. Thus, X-ray CT accurately measured the junction growth as a novel quantitative method.

Research on Wear Behavior Between Crossed Parabolic Cylinders in the Early Stage of Cyclic Fretting

Abstract In practice, even the tiny relative reciprocating motion generated by external load or environmental vibration can cause interface wear. In this work, three-dimensional (3D) finite element analysis (FEA) is used to investigate the fretting wear between deformable crossed parabolic cylinders. Fretting is achieved by the application of a periodic displacement-controlled load, while the contact pair is defined as the same material, i.e., Carbon Steel/Carbon Steel, Copper Alloy/Copper Alloy, and Aluminum Alloy/Aluminum Alloy. Meanwhile, three different axis crossing angles are also studied, i.e., 45°, 60°, and 90°. Originally, a wide range of coefficients of friction (COFs) are applied to the interface without considering the damage. The results show that, when the COF is sufficiently large (µ = 1), the junction growth and the tangential load increase significantly in the first loading cycle, but then become gradually stable. Subsequently, the Archard wear model is applied to the frictional interface. Empirical equations for the initial gross slip distance and the wear volume at the end of the partial slip are established in turn. Furthermore, an empirical equation for the wear volume of a single fretting cycle is established, which is in good agreement with the FEA results. Lastly, the effect of relevant parameters on the wear volume for a single fretting cycle under elastic condition is discussed, but also the evolution of wear volume in the early stage of cyclic fretting. All the results are normalized, enabling their application to the 3D fretting wear of other material pairs.

20 kDa isoform of connexin-43 augments spatial reorganization of the brain endothelial junctional complex and lesion leakage in cerebral cavernous malformation type-3

Cerebral cavernous malformation type-3 (CCM3) is a type of brain vascular malformation caused by mutations in programmed cell death protein-10 (PDCD10). It is characterized by early life occurrence of hemorrhagic stroke and profound blood-brain barrier defects. The pathogenic mechanisms responsible for microvascular hyperpermeability and lesion progression in CCM3 are still largely unknown. The current study examined brain endothelial barrier structural defects formed in the absence of CCM3 in vivo and in vitro that may lead to CCM3 lesion leakage. We found significant upregulation of a 20 kDa isoform of connexin 43 (GJA1–20 k) in brain endothelial cells (BEC) in both non-leaky and leaky lesions, as well as in an in vitro CCM3 knockdown model (CCM3KD-BEC). Morphological, biochemical, FRET, and FRAP analyses of CCM3KD-BEC found GJA1–20 k regulates full-length GJA1 biogenesis, prompting uncontrolled gap junction growth. Furthermore, by binding to a tight junction scaffolding protein, ZO-1, GJA1–20 k interferes with Cx43/ZO-1 interactions and gap junction/tight junction crosstalk, promoting ZO-1 dissociation from tight junction complexes and diminishing claudin-5/ZO-1 interaction. As a consequence, the tight junction complex is destabilized, allowing “replacement” of tight junctions with gap junctions leading to increased brain endothelial barrier permeability. Modifying cellular levels of GJA1–20 k rescued brain endothelial barrier integrity re-establishing the spatial organization of gap and tight junctional complexes. This study highlights generation of potential defects at the CCM3-affected brain endothelial barrier which may underlie prolonged vascular leakiness.

Analysis of Displacement-Controlled Fretting Between Crossed Parabolic Cylinders in Elasto-plastic Contacts

Abstract In practice, it is difficult to avoid the axis angle deviation when some regular surfaces are in micro-sliding, such as gears, machined surfaces, etc. In order to better investigate the micro-motion contact characteristics, a crossed paraboloidal contact model under frictional condition is proposed to simulate both tangential displacement-controlled fretting and the evolution of the energy dissipation in a load cycle. By deriving the theoretical of the normal and tangential contact course of the model, the load-displacement curves during initial loading, unloading and reloading stage are presented. On this basis, the hysteresis curve is then obtained by integrating the closed area surrounded by load-displacement during unloading and reloading, which also means that the empirical formulation for micro-slip in a load cycle is constructed. This study also reveals the plastic yield phenomenon under pure normal loading and plastic shakedown behavior caused by cyclic reciprocating displacement loads. In addition, the research on the junction growth, the evolution of tangential load and hysteresis curve with different COFs under multiple-cycle load is also carried out. The implications of involved parameters, such as friction coefficient, axis intersection angle, normal load and so on, are discussed with respect to hysteresis curve shape and energy dissipation. The difference about hysteresis and energy dissipation curves between the paraboloidal contact model and other classic contact models is then presented. It is discovered by comparison with other models that the paraboloidal contact model presents a relatively high energy dissipation in a load cycle.

Natural history of aortic root dilatation and pathologic aortic regurgitation in tetralogy of Fallot and its morphological variants

ObjectiveWe sought to characterize the natural history of aortic root dilatation and aortic regurgitation in tetralogy of Fallot (TOF). MethodsA single-center review of patients who underwent TOF repair from January 1960 to December 2022 was performed. Morphology was categorized as TOF-pulmonary stenosis or TOF-variant (including TOF-pulmonary atresia and TOF-pulmonary atresia-major aortopulmonary collateral arteries). Echocardiographically determined diameters and derived z scores were measured at the annulus, sinus of Valsalva, and sinotubular junction immediately before TOF repair and throughout follow-up. Linear mixed-effects models assessed trends in dimensions over time. ResultsOf 2205 patients who underwent primary repair of TOF at a median age of 4.9 months (interquartile range, 2.3-20.5 months) and survived to discharge, 1608 (72.9%) patients had TOF-pulmonary stenosis and 597 (27.1%) patients had TOF-variant. At a median postoperative follow-up of 14.4 years (interquartile range, 3.3-27.6 years; range, 0.1-62.6 years), 313 (14.2%) patients had mild or greater aortic regurgitation and 34 (1.5%) patients required an aortic valve or root intervention. The overall mean rates of annular, sinus of Valsalva, and sinotubular junction growth were 0.5 ± 0.2, 0.6 ± 0.3, and 0.7 ± 0.5 mm/year, respectively. Root z scores remained stable with time. At baseline, patients with TOF-variant had larger diameters and z scores at the annulus, sinus of Valsalva, and sinotubular junction, compared with patients with TOF-pulmonary stenosis (all P values < .05). Over time, patients with TOF-variant demonstrated relatively greater annular (P = .020), sinus of Valsalva (P < .001), and sinotubular junction (P < .001) dilatation. Patients with ≥75th percentile root growth rates had a higher incidence of mild or greater aortic regurgitation (P < .001), moderate or greater aortic regurgitation (P < .001), and aortic valve repair or replacement (P = .045). ConclusionsPatients with TOF-variant are at comparatively greater risk of pathologic root dilatation over time, warranting closer longitudinal follow-up.

Rough surface contact under creep conditions

A pressing concern in superplastic forming (SPF) is the ability to quantitatively model to effectively design tooling and assess process viability. Current finite element method simulations for SPF generally do not include the evolutionary nature of friction under creep conditions. The presence of creep strain leads to junction growth and saturated contact areas at longer time scales than conventional metal forming; therefore, friction models that ignore creep-induced strains will underpredict asperity contact and friction levels. In this work, a new micro-contact model of a rough surface in contact with a rigid flat surface is created to predict asperity flattening under creep behavior. A semi-empirical equation of the real contact area and friction at different SPF conditions is established.

AP2 Regulates Thickveins Trafficking to Attenuate NMJ Growth Signaling in Drosophila.

Compromised endocytosis in neurons leads to synapse overgrowth and altered organization of synaptic proteins. However, the molecular players and the signaling pathways which regulate the process remain poorly understood. Here, we show that σ2-adaptin, one of the subunits of the AP2-complex, genetically interacts with Mad, Medea and Dad (components of BMP signaling) to control neuromuscular junction (NMJ) growth in Drosophila. Ultrastructural analysis of σ2-adaptin mutants show an accumulation of large vesicles and membranous structures akin to endosomes at the synapse. We found that mutations in σ2-adaptin lead to an accumulation of Tkv receptors at the presynaptic membrane. Interestingly, the level of small GTPase Rab11 was significantly reduced in the σ2-adaptin mutant synapses. However, expression of Rab11 does not restore the synaptic defects of σ2-adaptin mutations. We propose a model in which AP2 regulates Tkv internalization and endosomal recycling to control synaptic growth.

Ultrahigh strength and shear-assisted separation of sliding 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 conduct 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 silver classically 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 attribute 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 strongly indicates that shearing reduces nanoscale pull-off forces, predicted theoretically at the continuum level, but not directly observed before.

Ultrasonic additive manufacturing of nanocrystalline laminated composites

Ultrasonic additive manufacturing has been used to fabricate laminated composites of commercially pure aluminum and a nanocrystalline nickel–cobalt (nc-NiCo) alloy. The nc-NiCo alloy would not weld to itself but readily welded to aluminum. Thus, by alternating between foils of nc-NiCo and Al, we achieved multi-material laminates with strong interlayer bonding. Electron microscopy showed that the nanoscale grain structure of the nc-NiCo was preserved during deposition and that the nc-NiCo/Al weld interface was decorated with comminuted surface oxides as well as Al–Ni–Co intermetallics. These findings are considered in light of process models of junction growth, interdiffusion, and grain growth, which together reveal how the different pressure- and temperature dependences of these phenomena give rise to a range of processing conditions that maximize bonding while minimizing coarsening and intermetallic formation. This analysis quantitatively demonstrates that using a soft, low melting point interlayer material decouples junction growth at the weld interface from grain growth in the nc-NiCo, expanding the range of optimal processing conditions.Graphical abstract

Effects of different selenium sources on duodenum and jejunum tight junction network and growth performance of broilers in a model of fluorine-induced chronic oxidative stress

The protective effects and underlying molecular mechanisms of sodium selenite (SS) and selenomethionine (SM) against chronic oxidative stress-induced duodenum and jejunum tight junction (TJ) network disturbance and growth inhibition of broilers were investigated in the current experiment. At the age of 1 d, 720 Lingnan Yellow broiler chicks were allocated to 4 experimental diets (with 6 replicates per diet and 30 birds per replicate) and offered either a control diet (fluorine [F] 23 mg/kg, control [CoN] group) or test diets (800 mg/kg F, high F [HF] group; 800 mg/kg F+0.15 mg selenium [Se]/kg as SS [SS group] or SM [SM group]) for 56 d. The results showed that HF group could induce chronic oxidative stress and subsequently increased (P < 0.05) proinflammatory cytokines levels of duodenum and jejunum in comparison with the CoN group. Increased proinflammatory cytokines levels of HF group promoted myosin light chain kinase (MLCK) transcription, thus leading to a decrease (P < 0.05) in TJ proteins expression of duodenum and jejunum when compared with the CoN group. A reduction of TJ proteins expression destroyed the TJ structures in the HF group, which in turn increased intestinal mucosal permeability of duodenum and jejunum and ultimately induced growth inhibition of broilers. Dietary Se supplementation could ameliorate HF-induced duodenum and jejunum TJ network impairment and growth retardation of broilers, potentially by increasing (P < 0.05) the glutathione peroxidase and thioredoxin reductase activities, reducing (P < 0.05) the reactive oxygen species and malondialdehyde levels, regulating the secretion of proinflammatory cytokines, and mediating the transcription level of MLCK in the duodenum and jejunum. Additionally, our data also suggested that the protective effects of SM were superior to those of SS. This study will provide a theoretical basis for developing SM into an efficient protective agent for intestinal mucosal barrier in poultry.

Risk homozygous haplotype regions for autism identifies population-specific ten genes for numerous pathways

BackgroundRecessive homozygous haplotype (rHH) mapping is a reliable tool for identifying recessive genes by detecting homozygous segments of identical haplotype structures. These are shared at a higher frequency amongst probands compared to parental controls. Finding out such rHH blocks in autism subjects can help in deciphering the disorder etiology.ObjectivesThe study aims to detect rHH segments of identical haplotype structure shared at a higher frequency in autism subjects than controls to identify recessive genes responsible for autism manifestation.MethodsIn the present study, 426 unrelated autism genotyped probands with 232 parents (116 trios) were obtained from Gene Expression Omnibus (GEO) Database. Homozygosity mapping analyses have been performed on the samples using standardized algorithms using the Affymetrix GeneChip® 500K SNP Nsp and Sty mapping arrays datasets.ResultsA total of 38 homozygous haplotype blocks were revealed across sample datasets. Upon downstream analysis, 10 autism genes were identified based on selected autism candidate genes criteria. Further, expressive Quantitative Trait Loci (QTL) analysis of SNPs revealed various binding sites for regulatory proteins BX3, FOS, BACH1, MYC, JUND, MAFK, POU2F2, RBBP5, RUNX3, and SMARCA4 impairing essential autism genes CEP290, KITLG, CHD8, and INS2. Pathways and processes such as adherens junction, dipeptidase activity, and platelet-derived growth factor—vital to autism manifestation were identified with varied protein-protein clustered interactions.ConclusionThese findings bring various population clusters with significant rHH genes. It is suggestive of the existence of common but population-specific risk alleles in related autism subjects.

A strong basis for friction as the origin of size effect in cutting of metals

A study has been made of the phenomenon of size effect in cutting of metals, namely increase in the specific energy with decreasing chip thickness, in the absence of edge radius effects and under ideal two-dimensional cutting conditions. Freshly cleaved glass knives are used to achieve cutting edges that are sharp on an atomic level. An instrumented ultramicrotomy apparatus is described in which it is possible to achieve two-dimensional orthogonal cutting at small cutting depths in the range of 100 nm to 3 micrometers. Based on observations with three metals (aluminum, copper and zinc), new evidence is presented in support of tool-chip friction as the primary source of the size effect. It is shown that size effect arises as a result of non-proportional decrease of the friction component with the underlying length scale. Calculation of the shear flow stress during cutting showed that this was approximately constant for a given material, independent of the depth of cut. Size dependence of tool-chip friction and contact size is interpreted in terms of the junction growth and adhesion models. Cutting experiments are also carried out with tools covered with solid and liquid contaminants. These results show that contaminant films effectively diminish adhesion and lead to a drastic reduction in the size effect, suggesting practical benefits for industrial manufacturing processes.

Tribological performance of boron-based superhard coatings sliding against different materials

A superhard boron-based ceramic coating was deposited on a cylindrical surface of a steel disk by high-energy short-pulse laser melting. During severe sliding friction, the coating can act as a solid lubricant with an ultralow coefficient of friction (COF). In combination with different counterface materials, the coating was subjected to sliding wear tests under boundary lubrication conditions. The obtained results show that resistance to severe adhesive wear and associated surface damage (galling) correlate with chemical inertness of tested materials with respect to boron carbide, which is the main component of the superhard coating. Chemically inert materials such as bronzes and babbitt alloys, while sliding against the boron-based coating, exhibit a transition to low friction behaviour at high normal loads and high sliding rates. In this case, the low COF was attributed to the formation of the B–O bonds. The bond appearance was confirmed by the X-ray photoelectron spectroscopy. On the other side, sliding of aluminium alloys against the boron-based superhard coatings leads to galling due to the reaction of B4C with aluminium at the interface and, associating with this, the extent of Al–B junction growth that occurs on further sliding, so this combination of materials is incompatible.

Elevated temperature adhesion and junction growth mechanisms in Al–Mg alloys sliding in argon and air

During hot forming of Al sheets, sliding contact against the forming die causes material transfer and adhesion to die surfaces. This work uses in-situ microscopy methods during sliding experiments conducted inside an environmentally controlled high temperature pin-on-disk tribometer to investigate both formation of junctions during initial contact and growth of adhesively transferred material. For this purpose, AA5083 (Al-0.1 wt%Cu-4.8 wt%Mg) pins placed in contact with a flat quartz glass counterface mounted on to the motorized stage of an optical microscope were tested at 573 K and 673 K in argon and ambient air atmospheres. The instantaneous changes in the contact area size and morphology were recorded along with the tangential force. Evidence for plastic deformation during junction formation and growth was provided by the formation of slip lines and grain boundary sliding. Junction growth occurred continuously in argon without separation of the adhesive junctions moving into the steady state friction stage, whereas in air, initial junction growth was interrupted due to oxidation, giving rise to a stick-and-slip behaviour, and the tangential force at the onset of junction break was determined as 1.75 ± 0.15 N at 573 K and 1.00 ± 0.08 N at 673 K. The junction growth rate in the absence of oxide formation followed an Arrhenius-type relationship in the temperature range investigated with an activation energy in the range of 144 ± 2 kJ mol-1, implying that the plastic deformation (creep) during junction growth was controlled by diffusional flow.

Thermally Activated Jamming in Ultrasonic Powder Compaction

Herein, the role of thermal softening in ultrasonic powder compaction is assessed by comparing the densification behaviors of nominally pure Cu and a thermally stable CuTa alloy. These materials have similar thermal properties, but pure Cu softens at much lower temperatures than does the CuTa alloy. Using a specialized ultrasonic powder compaction setup, in situ measurements of relative density, sonotrode power consumption, and temperature are collected, which together provide a time‐dependent geometric hardening parameter that reflects the structure of the compact. The geometric hardening data for the pure Cu powder reveal three distinct stages of densification: an initial particle rearrangement stage; a jamming transition where strong junctions develop between particles; and a final stage characterized by compatible plastic deformation. By contrast, the geometric hardening data for the thermally stable CuTa powder show that it remains a weak fluidized granular medium, despite experiencing higher normal pressures, oscillation amplitudes, and temperatures. The contrasting behaviors of the Cu and the CuTa powders suggest that a thermally activated jamming transition drives interparticle junction growth and densification in ultrasonic powder compaction.

Junction growth and interdiffusion during ultrasonic additive manufacturing of multi-material laminates

Ultrasonic additive manufacturing is a promising approach for making net-shaped multi-material laminates from material combinations difficult to process with fusion-based additive manufacturing techniques. The properties of these multi-material laminates depend sensitively on the interface between the constituents, which can be decorated with pores as well as thin intermetallic layers. Here, we develop process models for junction growth and interdiffusion during ultrasonic additive manufacturing of dissimilar metals. These process models are validated against published experimental data, then used to generate process diagrams which reveal that high normal loads and high sonotrode velocities can reduce intermetallic growth while maintaining strong interlayer bonding.

Evolution of real contact area during stick-slip movement observed by total reflection method**Project supported by the National Natural Science Foundation of China (Grant No. 11872033) and the Beijing Natural Science Foundation, China (Grant No. 3172017).

We build an experiment system based on total reflection (TR) method to observe the evolution of real contact area of polymethyl methacrylate (PMMA) in the continual stick-slip movement. The bilateral friction is adopted to overcome the bending moment in the lateral friction movement. Besides some classical phenomena of stick-slip movement such as periodical slow increase of frictional force in sticking phase and a sudden drop when slipping, a special phenomenon that the contact area increases with the tangential force is observed, which was called junction growth by Tabor in 1959. Image processing methods are developed to observe the variation of the junction area. The results show that the center of the strongest contact region will keep sticking under the tangential force until the whole slipping, the strongest point undergoes three stages in one cycle, which are named as sticking stage, fretting stage, and cracking stage, respectively. The combined analysis reveals a physical process of stick-slip movement: the tangential force causes the increase of the real contact area, which reduces the pressure between the contact spots and finally leads to the slipping. Once slipping occurs, the real contact area drops to the original level resulting in the pressure increase to the original level, which makes the sticking happen again.

The Tricellular Junction Protein Sidekick Regulates Vertex Dynamics to Promote Bicellular Junction Extension

Remodeling of cell-cell junctions drives cell intercalation that causes tissue movement during morphogenesis through the shortening and growth of bicellular junctions. The growth of new junctions is essential for continuing and then completing cellular dynamics and tissue shape sculpting; however, the mechanism underlying junction growth remains obscure. We investigated Drosophila genitalia rotation where continuous cell intercalation occurs to show that myosin II accumulating at the vertices of a new junction is required for the junction growth. This myosin II accumulation requires the adhesive transmembrane protein Sidekick (Sdk), which localizes to the adherens junctions (AJs) of tricellular contacts (tAJs). Sdk also localizes to and blocks the accumulation of E-Cadherin at newly formed growing junctions, which maintains the growth rate. We propose that Sdk facilitates tAJ movement by mediating myosin II-driven contraction and altering the adhesive properties at the tAJs, leading to cell-cell junction extension during persistent junction remodeling.

An elastoplastic finite element study of cylindrical plane-strain contact for steel/steel and Inconel 617/Incoloy 800H undergoing unidirectional sliding

The current work employs a two-dimensional plane strain finite element analysis to investigate the unidirectional sliding contact between a deformable half-cylinder and a deformable flat block. The unidirectional sliding is governed by a displacement-controlled action where the materials of the two contacting bodies are first set to identical steels at 20 ℃ and then to Inconel 617 and Incoloy 800H at 800 ℃. First, a normal interference (indentation) is applied, which is followed by unidirectional sliding. The von Mises stress distribution, plastic strain distribution, junction growth, normal force, tangential force, effective coefficient of friction, and scars on the surface of the block are obtained during the sliding motion. The leading edge of the contacting area and the bulk material under the leading edge experience large von Mises stresses. The large plastic strain is found on the surface of the block, and forms a “pocket” shape under the surface. The junction growth is also investigated, showing the direction of the growth is in the same direction of the tangential force that the weaker material experiences. The forces and the effective coefficient of friction are found to stabilize after a certain sliding distance, and the effective coefficient of friction converges to the coefficient of friction used in the model. Pileup is found on the surface of the block after a sufficient unidirectional sliding distance.

Fretting Wear Modeling of Cylindrical Line Contact in Plane-Strain Borne by the Finite Element Method

This is the first study to develop an empirical formulation to predict fretting wear (volume removal) under frictional conditions for plane-strain line contacts as borne out by the finite element analysis (FEA). The contact is between a deformable half-cylinder rubbing against a deformable flat block. The FEA is guided by detailed physical conceptions, with results that subsequently lead to the methodical modeling of fretting wear. The materials in contact are first set to steel/steel, then to Alloy617/Alloy617, and finally to copper/copper. Various coefficients of friction (COFs) and the Archard Wear Model are applied to the interface. Initially, pure elastic conditions are investigated. The theoretical predictions for the wear volume at the end of the partial slip condition in unidirectional sliding contact agree very well with the FEA results. The empirical formulation for the initial gross slip distance is constructed, again revealing results that are in good agreement with those obtained from the FEA for different materials and for various scales. The Timoshenko beam theory and the tangential loading analysis of a half elastic space are used to approximate the deflection of the half-cylinder and the flat block, respectively. That theory supports well the empirical formulation, matching closely the corresponding FEA results. The empirical formulation of the wear volume for a general cycle under fretting motion is then established. The results are shown to be valid for different materials and various COFs when compared with the FEA results. Finally, plasticity is introduced to the model, shown to cause two phenomena, namely junction growth and larger tangential deformations. Wear is shown to either increase or decrease depending on the combined influences of these two phenomena.