Contact Strength Research Articles

Numerical analysis of the brittle–ductile transition of deeply buried marble using a discrete approach

In high geostress areas, rocks tend to experience plastic deformation processes and show ductility. Therefore, the study of the mechanical properties of rocks, especially the post-peak behavior, is crucial for underground geotechnical engineering. For this reason, the macroscopic mechanical property of marble is studied by using the PFC/PBM numerical method. An improved numerical marble sample with random properties is used by setting different contact strengths following a bimodal Weibull distribution. Thereafter, on the basis of an improved boundary servo mechanism, the brittle–ductile transition appearing in laboratory tests on marble is successfully realized by the discrete approach, and a servo velocity threshold function of axial loading speed and confining pressure is obtained. Furthermore, the marble model and servo method are applied in the excavation of typical underground caverns. The effects of constrained velocity and geostress level on the surrounding rock failure model of the cavern are described and analyzed. In addition, mechanism of the excavation failure under high geostress is discussed.

XVII Межгосударственная конференция Термоэлектрики и их применения --- 2021" (ISCTA 2021 Санкт-Петербург, 13-16 сентября, 2021 Получение и исследование омических контактов с высокой адгезией к термоэлементам

The factors determining the adhesive strength of film coatings are considered. The functions of contacts in thermoelements used in a wide temperature range were determined. It was established that the adhesive strength of contact is a limiting factor in the mechanical strength of a thermoelement. A method of vacuum sputtering of thin-film contacts was proposed, including the surface treatment of samples of thermoelectric materials. The presence of a transition layer in the area of the metal - thermoelectric material contact, formed during the interaction of the metal with elements of the thermoelectric material, was established. The dependence of the adhesive strength of film contact on roughness of surface on which they formed was established. Thermal stable contacts for thermoelements with low resistivity of the order of 10-9 Ohm•m2 and high adhesion strength of at least 12 MPa were obtained by ion-plasma sputtering.

Proximal contact alterations between implant-supported restorations and adjacent teeth in the posterior region: A 3-month prospective study.

BackgroundInterproximal contact loss (ICL) is a multifactorial implant complication. The aims of this prospective clinical study were to evaluate proximal contact alterations between implant-supported fixed prostheses (IFPs) and adjacent teeth and to identify potential contributing factors. Material and MethodsThis study was conducted from April to September 2019 at the Department of Prosthodontics. Forty-three patients (23 females and 20 males, age range 31-70) treated with 43 posterior IFPs were recruited for this study. All proximal contacts (64) were checked visually and radiographically. Proximal contact tightness (PCT) was clinically evaluated using dental floss and measured during removal of a calibrated 0.05 mm thick metal strip previously inserted into the proximal area. Mesial and distal PCT were measured at restoration insertion (T0), 1-month follow-up (T1), and 3-month follow-up (T2). ICL was assessed in relation to the patients’ age, gender, smoking habits, implant system, proximal contact position, jaw position and restoration type of the implant prostheses. The significance level was set at P value ≤ 0.05. ResultsThe PCTs between fixed implant prostheses and adjacent teeth decreased significantly between T0 and T2. Restoration type affected the loss of interproximal contact at the mesial (free-end restorations; P = 0.008) and distal aspects (P< 0.001), whereas implant system affected only the distal aspects of the proximal contacts (P = 0.002). ConclusionsProximal contact tightness between fixed implant prostheses and adjacent teeth decreased over the 3-month observation period. Contact loss between fixed implant prostheses and adjacent teeth may be influenced by restoration type and implant system. Key words:Adjacent teeth, dental implants, implant complication, implant fixed prostheses, interproximal contact loss, proximal contact strength, proximal contact tightness.

Justification of the parameters of the life-test bench for railway wheelsets

One of the tasks that need to be solved in order to predict the life of wheelsets is to assess the contact strength and wear of their parts. Thus, the purpose of this work is to develop a scheme and justification of the parameters of the life-test bench for railway wheelsets in conditions close to real operating ones. This bench simulates a contact of “wheel-rail” pair as a contact of “wheel-roller” pair. To ensure compliance of the bench tests conditions with the real wheelsets operation conditions we took that the maximum contact pressure in the “wheel-roller” pair is equal to the maximum contact pressure in the “wheel-rail” pair. As results of this study we obtained the dependences of the values of the roller diameter and the working force of the bench on the characteristics of the tested wheelset and proved the mutual influence of these quantities. For railway wheelsets with an axle load of 230.5 kN and wheels with a tread diameter of 957 mm, we obtained the following values of the bench parameters: roller diameter is 75 mm; working force of the bench is 8,375 kN; wheel speed is 500 rpm; number of rollers is 8; test duration is about 220 h.

Calculated Assessment of Contact Strength, Wear and Resource of Metal-Polymer Gears Made of Dispersion-Reinforced Composites

Calculated Assessment of Contact Strength, Wear and Resource of Metal-Polymer Gears Made of Dispersion-Reinforced Composites

АНАЛИЗ МЕТОДИК РАСЧЕТА ЗУБЧАТЫХ ЦИЛИНДРИЧЕСКИХ ПЕРЕДАЧ НА ГЛУБИННУЮ КОНТАКТНУЮ ПРОЧНОСТЬ

Spur gear is an integral part of transport and technological machines structure. To increase the load ability of the spur gear, chemical heat treatment of the working cogs surfaces is used. Spur gear with chemical heat treatment must be checked for deep contact strength. The article analyzes various methods (according to GOST 21354-87, methods using the generalized criterion of the limit state for structurally inhomogeneous material by Lebedev-Pisarenko, methods from the handbook describing reducers of power machines etc.) for calculating spur and helical gearing for deep contact strength. The author presents all necessary formulas and graphs for the implementation of each methods, and compares the calculations results of these methods with the results of experimental studies of cemented cylindrical rollers for deep contact destruction.

Comparative analysis of the fatigue contact strength of surfaces hardened by cementation and the ion-plasma nitriding Аvinit N

This paper reports comparative tribotechnical tests of surfaces, hardened by cementation and plasma nitriding Avinit N, for contact fatigue strength at friction in rolling with slipping. Following the cementation, the samples' hardened layer thickness was 1.2 mm; on nitriding, the thickness of the hardened layer was 0.25 mm. The tests were carried out using an acoustic emission method, which is extremely sensitive when registering the transition of tribosystems' operation from the normal (mechanochemical) wear to the initial surface destruction of a fatigue nature.The tests have shown that the average number of cycles before the initial destruction due to fatigue for the samples hardened by the Avinit nitriding technology is 1.82-time higher compared to the cementation-hardened samples. The depth of damage at the surface of the cemented samples could vary between 0.01 and 0.027 mm depending on the diameter of the damage. For the nitrided surfaces, the depth of damage did not exceed 0.003 mm.The samples' resistance to the fatigue wear (destruction) was determined by tests based on 1,000,000 cycles at contact loads σmax=1,140 MPa, typical of medium-loaded surfaces. The test results demonstrated that the integrated multicyclic resistance to fatigue wear (destruction) of the samples, hardened by nitriding, is more than 10 times higher than that of the cementation-hardened samples.The study reported here confirms the effectiveness of using the Avinit ion-plasma nitriding technology instead of cementing, to improve the contact strength of the parts' surfaces. At the same time, worth noting are the advantages of this technology to maintain the size and high quality of surface treatment, which eliminates the need to mechanically finish them after hardening

TEETH AND GEAR WHEELS DESIGN DEVELOPMENT FOR AIRCRAFT REDUCTION TRAINS

The development of designs of teeth, gear wheels and gears, which have found application in gearboxes of aircraft and helicopters, is presented. Parameters of different types of initial generating circuits, their modifications and examples of application corresponding gear wheels in gearboxes of airplanes and helicopters are considered.&#x0D; The main attention is paid to GOST 13755-2015, developed by CIAMD (Central Institute of Aviation Motor Development) and having a number of advantages over the old GOST 13755-81. The main advantage is achieved by replacing the standard tooth head height with a standard tooth tip thickness. When the teeth work along the entire active part of the gearing line with a profile angle of 250, high contact and bending strength is provided, as well as resistance against galling of the surfaces of the teeth of highly stressed gears.&#x0D; A further increase in contact strength and resource is ensured by using the initial generating circuit with a profile angle of 280. Gear trains with a guaranteed overlap ratio of 2.0 have a vibration level five times lower and, in general, their bearing capacity is approximately 1.5 times higher than that of highly stressed reduction trains with an overlap ratio less than 2.0. The asymmetrical teeth parameters were chosen to increase the density of the transmitted power, as well as to reduce the dynamic loads and vibration of the gear wheels.&#x0D; The strength of gear trains according to GOST 13755-2015 significantly exceeds the strength of gear trains according to GOST 13755-81, but the most significant advantage (more than twice) is obtained in terms of noise level. The highest strength of gears with a main circuit angle of 330 occurs at a guide circuit angle of 20 °.&#x0D; The highest stiffness of arched teeth of all designs provides arched gear trains with high durability and load capacity, which makes it possible to design reduction trains with smaller weight and size specifications.

Гистерезис в адгезионном контакте при изменении направления сдвига: эксперимент и феноменологическая модель

A phenomenological model is proposed that describes hysteretic behavior of an adhesive contact between a soft elastomer and a rigid indenter when the direction of the indenter shear is changed. The model takes into account the increase of contact strength with contact time. The dependences of the elastic force and contact radius on the indentation depth were obtained. It is shown that the adhesive strength of the contact increases with an increase in the indentation depth. An experiment on the indentation of a steel spherical indenter into a rubber sheet of a fixed thickness was carried out. A qualitative agreement between the experimental and theoretical results is shown.

Methodology of Calculation of Metal-polymer Sliding Bearings for Contact Strength, Durability and Wear

Methodology of Calculation of Metal-polymer Sliding Bearings for Contact Strength, Durability and Wear

PARAMETRIC OPTIMIZATION OF THE BEARING CAPACITY OF THE CLOSED PLANETARY MECHANISM FORMED BY TWO MECHANISMS OF TYPE ĀĪ

The dependences of the bearing capacity of a closed planetary mechanism formed from two simple planetary mechanisms of the type ĀĪ as a function of two kinematic parameters that determine the transmission ratios of simple planetary mechanisms are investigational. Dependences are obtained from the conditions of providing contact and bending strength of external gearing of simple planetary mechanisms. The influence of the number of its satellites on the choice of the kinematic parameter of a simple planetary mechanism is taken into account. A condition is given that provides the same bearing capacity of a closed planetary mechanism, taking into account both contact and bending strength, respectively. Various formulations of the optimization problem of determining the values of the kinematic parameters of simple planetary mechanisms and methods for solving it are considered. An example of decision of optimization task is made in the mathematical package Mathcad.

An advanced grain-based model to characterize mechanical behaviors of crystalline rocks with different weathering degrees

Laboratory tests revealed that by enhancing the weathering degree, a transition of pre-peak stress-strain mechanical responses of crystalline rocks under unconfined compression from approximate linearity to nonlinearity was evident, as was the weakening of macro-mechanical properties. However, thus far, very few numerical studies have been conducted to quantitatively characterize the strong-to-weak transition of the mechanical behaviors of crystalline rocks modulated by the weathering degree. We propose an advanced grain-based model (AGBM) using Universal Distinct Element Code (UDEC) to characterize mechanical characteristics of crystalline rocks with different weathering degrees. The weathering-induced deterioration of microstructures was treated as loosening of grain contacts and weakening of their properties. It was proved that the grain contact model that considered hardening nonlinear deformation in compression and linearly elastic deformation in tension or shear was feasible and applicable to characterize the mechanical behaviors of crystalline rocks with different weathering degrees. The compression hardening deformation of grain contacts significantly affected the macro nonlinear stress-strain relation and stress thresholds of crack closure, crack imitation, stable crack growth, and unstable crack growth. We acquired new insights on the weathering-induced weakening of macro-mechanical characteristics of crystalline rock, which resulted from weakening of deformation properties of the grain contact more than grain contact strength.

Дослідження особливостей руйнування шаруватої кераміки в її мікрооб’ємах методами індентування

Technology and modes of ZrB2―SiC layered ceramic composites manufacturing have been developed. The structures, elastic characteristics and strength properties of the materials under investigation have been studied. Effect of internal stress fields on fracture processes in the indentation area and mechanical properties of the ceramics in its microvolumes has been investigated both in layers and at their interfaces. Using values of contact tensile strength along different directions in layers of the composites, effective residual thermal stresses have been calculated (≈180 MPa). The obtained data on contact tensile strength and effective crack resistance, taking into consideration the contributions of residual stresses to their values, have been used for estimations of contact strength and crack resistance of the layer materials themselves. The fracture toughness measured by the three-point bending method is 3,3—3,7 MPa · m1/2. Analysis of the data obtained indicates that the spark notch provides a greater sharpness of the crack tip and better conditions for measuring K1c, while processing with a blade picks up a sharp thermal crack in the notch tip. The elastic properties of the multilayer system (SiC—15% ZrB2) + (SiC—30% ZrB2) were studied using ultrasonic research methods. The values of the velocities of sound and elastic characteristics are sufficiently large and close to those expected from the models of the composite, which does not contain noticeable porosity and microcracks in the layers themselves and in the region of their boundaries. For directions along and across the plane of the layers, the values of Young's moduli differ by about 6%. For the directions of propagation of an ultrasonic wave along and across the layers, anisotropy of ultrasonic velocities of ~5% and elastic moduli of ~10—12% is observed, which may be due to the texture that develops in the structure of the layers during hot pressing. Keywords: layered ceramics, indentation, strength properties, thermal stresses.

Structural and Functional Disorders in Glaucoma: Prospects for Preclinical Diagnosis. Part 2. Electrophysiological Markers of Early Neuroplastic Events

Analysis of the literature on the problem of structural and functional relationships in the development and progression of glaucomatous optical neuropathy (GON) shows that the search for a single primary factor may lead to an erroneous exaggeration of its role in the pathogenesis of GON. A more promising approach may be to search for clinically significant combinations of current markers of changes in structure, function, and ocular blood flow, and to expand our fundamental understanding of the processes underlying these changes, designed to improve their interpretation radically. The discussed in this review data of recent studies showed that the earliest event in the development of GON is the weakening and loss of synapses, even with the preserved dendritic branching. We assume that the loss of synapses on dendrites and axon terminals, being a manifestation of synaptic plasticity, may occur simultaneously with the change in anterograde transport in axons of retinal ganglion cells (RGC), or, ahead of it. Early changes in the discharge timing of the RGCs associated with a decrease in the strength of synaptic contacts and the elimination of synapses on dendrites can be a target for neuroprotective therapy. The review analyzes the tests of modern electroretinography, which can serve as markers of early events in the development of GON, including plastic changes in the retina at the preclinical stage of glaucoma, and provides physiological rationales for their selective possibilities for clinical practice.

The use of computational contact mechanics approaches to assess the performance of parts bearing stress concentrators

Purpose: The purpose of this work is to build new computational schemes for assessing the strength parameters of parts with inhomogeneous properties of surface layers in the presence of stress concentrators. Design/methodology/approach: Using the developed approaches of mathematical modeling and open software for calculating the structures of the FEM - FEniCS, the required thickness of the hardened zones of parts has been established, which ensures their minimum softening during operation, depending on the characteristics of the stress concentrator. Findings: It is shown that for each size of the surface stress concentrator there is a critical value of the hardening thickness, the excess of which does not affect the operational strength of the parts, but increases the cost of technological operations. Research limitations/implications: In this article proposes a method for calculating the influence of the dimensional characteristics of hardening zones on the contact strength of parts with stress concentrators under conditions of prevailing power loads. Practical implications: The results obtained in this work were used to determine the technological modes of plasma hardening, which ensure an increase in the contact strength of parts with stress concentrators, depending on their dimensional characteristics. Originality/value: Using the approaches of computational mechanics and mathematical and computer modeling, methods for controlling the contact strength of parts with inhomogeneous non-local properties in the presence of a surface stress concentrator are proposed for the first time.

EZTM experience in designing and manufacturing of rolling mill main drive gearboxes

The experience of the Electrostal Heavy Machinery Plant (EZTM) in designing and production of special gearboxes and gear stands for main drive of various rolling mills is summed up. The specifics of section and ball-rolling mills, piercing and reduction mills, mills for cold rolling of tubes and mills for the production of reinforcing bars are considered. It was shown that at designing of special gearboxes for the main drives of rolling mills, the designer, taking into account the parameters of the mill, must build a cyclogram of their loading, which depends both on the range of rolled products and on the variability of loads within the rolling cycle of one billet. In the Designing Department of EZTM, the strength calculation of gears of different types is carried out in the REDUK 4.3 software package, which allows performing strength calculations of gears taking into account the expected loading cyclogram. At the same time, the program provides the initial data for calculating the shafts and bearings, i.e, the forces in engagement and the rotational speed of each of the shafts for the first stage of the cyclogram. The geometrical parameters of the gears are optimized according to the criterion of contact and flexural strength, both between the stages and within each of them.The load sequence diagram depends both on the range of rolled products, and variable loadings in a rolling cycle of a workpiece. Geometric parameters of gears are optimized by contact and bending equal strength, as between the stages as within each of them. Carburizing, hardening and grinding of teeth are provided and secured technologically. Currently, the gearboxes manufactured by the plant are successfully operated at many metallurgical enterprises, both in Russia and abroad. Among them, the gearboxes of main drive of the piercing mill of the Seversk Pipe Plant, the mills for cold rolling of tubes in the USA and China, the section mills at the Yartsevo Casting and Rolling Plant, the reduction mill at the Dnepropetrovsk Metallurgical Plant, etc.

Increased Contact Time and Strength Deficits in Runners With Exercise-Related Lower Leg Pain.

Exercise-related lower leg pain (ERLLP) is common in runners. To compare biomechanical (kinematic, kinetic, and spatiotemporal) measures obtained from wearable sensors as well as lower extremity alignment, range of motion, and strength during running between runners with and those without ERLLP. Case-control study. Field and laboratory. Of 32 young adults who had been running regularly (>10 mi [16 km] per week) for ≥3 months, 16 had ERLLP for ≥2 weeks and 16 were healthy control participants. Both field and laboratory measures were collected at the initial visit. The laboratory measures consisted of alignment (arch height index, foot posture index, navicular drop, tibial torsion, Q-angle, and hip anteversion), range of motion (great toe, ankle, knee, and hip), and strength. Participants then completed a 1.67-mi (2.69-km) run along a predetermined route to calibrate the RunScribe devices. The RunScribe wearable sensors collected kinematic (pronation excursion and maximum pronation velocity), kinetic (impact g and braking g), and spatiotemporal (stride length, step length, contact time, stride pace, and flight ratio) measures. Participants then wore the sensors during at least 3 training runs in the next week. The ERLLP group had a slower stride pace than the healthy group, which was accounted for as a covariate in subsequent analyses. The ERLLP group had a longer contact time during the stance phase of running (mean difference [MD] = 18.00 ± 8.27 milliseconds) and decreased stride length (MD = -0.11 ± 0.05 m) than the control group. For the clinical measures, the ERLLP group demonstrated increased range of motion for great-toe flexion (MD = 13.9 ± 4.6°) and ankle eversion (MD = 6.3 ± 2.7°) and decreased strength for ankle inversion (MD = -0.49 ± 0.23 N/kg), ankle eversion (MD = -0.57 ± 0.27 N/kg), and hip flexion (MD = -0.99 ± 0.39 N/kg). The ERLLP group exhibited a longer contact time and decreased stride length during running as well as strength deficits at the ankle and hip. Gait retraining and lower extremity strengthening may be warranted as clinical interventions in runners with ERLLP.

Meshing and Bearing Analysis of Nutation Drive with Face Gear

Background: Nutation drive is being extensively investigated due to its ability to achieve a high reduction ratio with a compact structure and the potential for low vibration, high efficiency and design flexibility. However, many problems including the difficulty to process the inner bevel gear, less number of teeth in engagement and not being suitable for high-power transmission have restricted its development. Objective: The purpose of this paper is to analyze the contact strength of a patent about a new nutation drive developed based on meshing between two face gears, which has the advantages of both face gear and nutation drive, including large transmission ratio, large coincidence, small size, compact structure and strong bearing capacity. Methods: Based on the meshing principle and basic structure of the nutation face gear drive, the contact strength of nutation face gear transmission is analyzed by the Hertz contact analysis method and FEM method. Results: The maximum stress values of nutation face gear teeth are compared by two methods, which verify the accuracy of Hertz contact analytical method in calculating the contact strength of nutation face gear teeth. Furthermore, nine groups of three-dimensional models for the nutation face gear drive with a transmission ratio of 52 and different cutter parameters are established. Conclusion: The study analyzes the contact stress of fixed and rotary face gears in meshing with planetary face gears, and obtains the distribution law of contact stress and the influence of the number of teeth and parameters of the cutter on the load-carrying capacity.

Numerical Characterization of Cohesive and Non-Cohesive ‘Sediments’ under Different Consolidation States Using 3D DEM Triaxial Experiments

The Discrete Element Method has been widely used to simulate geo-materials due to time and scale limitations met in the field and laboratories. While cohesionless geo-materials were the focus of many previous studies, the deformation of cohesive geo-materials in 3D remained poorly characterized. Here, we aimed to generate a range of numerical ‘sediments’, assess their mechanical response to stress and compare their response with laboratory tests, focusing on differences between the micro- and macro-material properties. We simulated two endmembers—clay (cohesive) and sand (cohesionless). The materials were tested in a 3D triaxial numerical setup, under different simulated burial stresses and consolidation states. Variations in particle contact or individual bond strengths generate first order influence on the stress–strain response, i.e., a different deformation style of the numerical sand or clay. Increased burial depth generates a second order influence, elevating peak shear strength. Loose and dense consolidation states generate a third order influence of the endmember level. The results replicate a range of sediment compositions, empirical behaviors and conditions. We propose a procedure to characterize sediments numerically. The numerical ‘sediments’ can be applied to simulate processes in sediments exhibiting variations in strength due to post-seismic consolidation, bioturbation or variations in sedimentation rates.

Analysis of interface interaction of aggregate-asphalt system and its effect on shear-slip behavior of asphalt mixture

The interface interaction of aggregate-asphalt system is a key factor affecting the strength formation and deterioration of the asphalt mixture, which has a great influence on the distress and durability of pavement. To analyze the influence of the interface interaction of aggregate-asphalt system on the shear-slip behavior of asphalt mixture, two self-developed testers were adopted to measure the parameters of particle contact and interface shear-slip, and the effective evaluation methods were proposed on the mesoscopic level. The results indicate that the interface interaction is from the coupling effect of the particle contact and the bonding and lubrication of asphalt. Among involved aggregate-asphalt systems, the AC-13L system has a strong contact strength, while its skeleton structure is easily affected by the lubrication of asphalt. The contact and interaction parameters of involved aggregate-asphalt systems except for AC-13L exhibit a good linear correlation with the shear-slip parameters of asphalt mixtures. The AC-20M system has the strongest contact effect and easily forms a multilevel dense interlocking structure interacting with asphalt, which shows the best resistance to shear-slip deformation. The interference of fine aggregates and the lubrication of asphalt can adversely affect the resistance to slip deformation of asphalt mixtures.