Showing papers on "Contact area published in 2022"

The Role of Block-Type Support Structure Design on the Thermal Field and Deformation in Components Fabricated by Laser Powder Bed Fusion

Abstract: Successful production of overhang features, with no underlying deposited layers or substrate, in a Laser Powder Bed Fusion (LPBF) process requires sacrificial structures, known as support structures. However, these structures add extra time and cost to the manufacturing process of a component. Optimisation of support structure design should reduce materials waste (which affects the time and cost of the build process) while minimising bleed-throughs and residual deformation. Optimisation of support structure design requires an understanding of their impact on the thermal field and mechanical constraints. In this research, a combination of experiments and numerical modelling was used to quantify the contribution of various support structure design elements on component deformation. The results indicate that the total contact area of teeth and the total support base area alter thermal fields within the produced components. Increasing the area for heat conduction to the start plate was found to reduce the peak temperature in the cantilevered plate. Additionally, increasing the contact area between teeth and the cantilevered plate reduces the peak temperature and the vertical temperature gradient in the overhang platform, both of which are consistent with reducing in-situ plastic strain accumulation and residual part deformation.

Evolution and flow maps of the oil layer in successive rolling point contact systems: Bearing as a case

Abstract: It is of great relevance to maintain an adequate and continuous lubricant oil supply to each contact region in successive rolling point contact systems, such as rolling bearings, rolling guides, and ball screws. As a case study, we implemented laser-induced fluorescence observations of the dyed oil in a ball bearing. It is found that there exists an oil layer evolution as the rotating speed increases, especially when it reaches some critical levels. The oil layer will break up and migrate out of the rolling contact traces, while the oil distribution changes. In addition, a series of air–oil two-phase flow simulations based on computational fluid dynamics models and the volume-of-fluid method are run to reveal the forming mechanism. Flow maps are of construction with the flow patterns and the oil distribution. The critical points of the flow pattern transitions and the oil distribution changes are highly coincident with the contour lines of Ca. The force extraction shows that the pressure gradient force caused by Sommerfeld pressures has different directions at the outlet and inlet zones. They tend to attract the outlet meniscus into the low-pressure area while pushing the inlet meniscus out of the high-pressure area. As a result, the oil layer will leave and migrate out of the rolling contact traces. The capillary force will always work against the pressure gradient force. Their competition and equilibrium may determine the flow pattern transitions, the oil distribution changes, and their close relationships with Ca.

A new analytical model for the flattening of Gaussian rough surfaces

Abstract: This paper advances an analytical incremental contact model for the purely elastic or elastic-perfectly plastic Gaussian rough surfaces. The contact is modelled by the accumulation of identical circular contacts with radius given by the total truncated area at varying heights divided by the contact patch number. The contact area-load relationship is derived analytically, showing approximate linearity for the contact fraction up to 10%. Good agreement is found between the new proposed model and the direct finite element simulations. To characterize the influence of plastic deformation, a dimensionless plasticity parameter is introduced as the ratio of yield strain to root mean square gradient of the rough surface. It is demonstrated that the general elastic-plastic contact response would approach to the limit of purely elastic as the plasticity parameter increases.

Analytical modeling of complex contact behavior between rock mass and lining structure

Abstract: Based on the nondestructive test data of operating railway tunnels in China, this paper summarizes the basic characteristics of the complex contact behavior between the rock mass and lining structure. The contact modes are classified into dense contact, local non-contact, and loose contact. Subsequently, the corresponding mechanical model for each contact mode is developed according to its mechanical characteristics using the complex variable method. In the proposed mechanical model, a special algorithm is introduced to detect whether the local non-contact zone is re-contacted. Besides, a novel conformal mapping method is also proposed to accurately calculate the mechanical response of the concrete lining. Finally, the accuracy of the proposed method is verified by comparing it with the finite element method (FEM). Several parameter investigations are conducted to analyze the effects of different contact modes on the rock–lining interaction. The results show that: (i) the height of the local non-contact area does not have a significant effect on the contact stress distribution if no re-contact occurs; (ii) backfill grouting can reduce the local stress concentration caused by poor contact modes; and (iii) reducing the friction coefficient of the interface can lead to a more uniform distribution of internal forces in the concrete lining.

Topography characteristics and formation mechanism of the bolt-hole contact interface during the bolt installation of interference-fit composite structure

Abstract: During the bolt installation process of interference-fit composite structure, the bolt will come to contact with the composite, forming the contact interface. Understanding the topography characteristics and formation mechanism of the contact interface is of great significance to ensure the load-bearing function of the structure. In this paper, bolt installation experiments were conducted with considering the interference-fit length and interference-fit size. The micro topography characteristics of the flat contact surface and bolt-hole contact surface were analyzed. The main topography characteristics of the flat contact surface is fiber bending and delamination, while the main topography characteristics of the bolt-hole contact surface is material shedding. Based on the topography of the bolt-hole contact interface, formation mechanism of the bolt-hole contact interface is revealed. Energy spectra results show coatings has crucial effect on the formation of bolt-hole contact interface. The main components of the bolt-hole contact interface are coating debris and composite debris, with little bolt debris. The thickness of the bolt-hole contact interface is positive related to the friction length and interference-fit size. This paper provides potential application for the determination of leakage channel and corrosion position of interference-fit composite bolted structure.

Is there more than one stickiness criterion?

Abstract: Abstract Adhesion between an elastic body and a smooth, rigid substrate can lead to large tensile stresses between them. However, most macroscopic objects are microscopically rough, which strongly suppresses adhesion. A fierce debate has unfolded recently as to whether local or global parameters determine the crossover between small and large adhesion. Here, we report simulations revealing that the dependence of the pull-off force F n on the surface energy γ does not only have two regimes of high and low adhesion but up to four regimes. They are related to contacts, which at the moment of rupture consist of (i) the last individual Hertzian-shaped contact, in which is linear in γ , (ii) a last meso-scale, individual patches with super-linear scaling, (iii) many isolated contact patches with extremely strong scaling, and (iv) a dominating largest contact patch, for which the pull-off stress is no longer negligible compared to the maximum, microscopic pull-off stress. Regime (iii) can be seen as a transition domain. It is located near the point where the surface energy is half the elastic energy per unit area in conformal contact. A criterion for the transition between regimes (i) and (ii) appears difficult to grasp.

Fractal geometry of contacting patches in rough elastic contacts

Abstract: Many naturally formed and processed surfaces are rough over a broad range of length scales. Surface roughness reduces the area of contact between solids, with ramifications for phenomena that depend on the geometry of the interface and the amount of direct contact, including friction and adhesion. In this work, we employ large-scale boundary-element simulations for nonadhesive, elastic solids to study the size dependence of contact patch mean pressure and geometry for patches formed between solids with self-affine fractal surface roughness across seven decades in patch area. Contact patches with diameters smaller than a crossover length scale of order the minimum wavelength of roughness are generally compact with simple geometries and bear pressures well described by Hertz theory. The patch pressure in contact patches larger than the crossover scale rises logarithmically before saturating at a finite value. Furthermore, the largest contact patches formed during our simulations are ramified and populated with regions out of contact, or bubbles, which reduce patch area and increase patch perimeter. As a result, we show that the mean contact diameter of the largest patches saturates, indicating that the patch contact area is proportional to the total patch perimeter. We quantify the effects of bubbles on patch area and perimeters as a function of Hurst exponent and contrast our findings with results of comparable bearing-area model calculations. The slow evolution of the mean patch pressure with patch size in our large-scale calculations explains the common observation that the global mean contact pressure depends on the structure of the roughness, the contact area, and even on system size.

An asperity-based statistical model for the adhesive friction of elastic nominally flat rough contact interfaces

Abstract: Contact mechanics-based models for the friction of nominally flat rough surfaces have not been able to adequately capture certain key experimentally observed phenomenona, such as the transition from a static friction peak to a lower level of sliding friction and the shear-induced contact area reduction that has been observed in the pre-sliding regime especially for soft materials. Here, we propose a statistical model based on physically-rooted contact mechanics laws describing the micromechanics of individual junctions. The model considers the quasi-static tangential loading, up to full sliding, of the contact between a smooth rigid flat surface and a nominally flat linear elastic rough surface comprising random independent spherical asperities, and accounts for the coupling between adhesion and friction at the micro-junction level. The model qualitatively reproduces both the macroscopic shear-induced contact area reduction and, remarkably, the static friction peak without the need to explicitly introduce two different friction levels. It also demonstrates how the static friction peak and contact area evolution depend on the normal load and certain key microscale interface properties such as surface energy, mode mixity and frictional shear strength. "Tougher" interfaces (i.e. with larger surface energy and smaller mode mixity parameter) are shown to result in a larger real contact area and a more pronounced static friction peak. Overall, this work provides important insights about how key microscale properties operating at the asperity level can combine with the surface statistics to reproduce important macroscopic responses observed in rough frictionalsoft contact experiments.

Contact Characteristics at Interface in Three-Body Contact Conditions with Rough Surfaces and Foreign Particles

Abstract: Nanoparticles as lubricant additives under a certain average diameter and concentration may reduce wear, friction and scuffing damage. However, atmospheric dust particles affect not only human health but also the efficiency of components, and even cause component failures. Therefore, the contact characteristics at interfaces with foreign particles require careful investigation. In this work, a 3-body microcontact mechanics concept is used to analyze the effects of wear debris and foreign particles on real contact area, contact mode, asperity deformation type and separation at interface. The results show that the relationship profile between dimensionless real contact area (At*) and dimensionless normal contact load (Ft*) is wedge-shaped in a 3-body contact interface. Using surface-to-surface 2-body contact area as upper bound and surface-to-particle 3-body contact as lower bound, the 3-body hybrid contact situation is in between upper and lower bounds. As the dimensionless normal contact load increases, At* increases gradually as well. The order of contact mode is p-s contact, hybrid contact and then s-s contact. If the 3-body contact interface is in hybrid contact mode, the decrease in the hardness and average third body diameter will cause the At* to increase significantly at the same Ft*. Conversely, the separation and real contact area ratio of plastic deformation decrease gradually. The turning point of contact area (TPCA) occurs when the contact mode is within hybrid contact mode and the ratio of average third body diameter to the composite equivalent surface RMS roughness is about 50–70% for foreign particles and wear debris. When the Ft* is slightly larger than Ftpca*, the third body and surface share the total interface load approximately equally which will help reduce the real contact pressure and plastic contact area to improve surface performance.

Analysis of Contact Stress Distribution between Rolling Element and Variable Diameter Raceway of Cageless Bearing

Abstract: The change in contact state between the rolling elements and raceway of a cageless bearing with a variable diameter raceway affect the wear of the bearing, which leads to discrete motion failure of the rolling elements. For this purpose, the contact characteristics as contact form and contact stress between the rolling elements and raceway were determined. A numerical method is proposed to determine the three-dimensional contact stress of a cageless bearing. First, combined with the variable diameter raceway structure characteristics and the motion of rolling elements, the rolling elements and raceway contact stress model was established, and the influence factors of contact stress and the maximum stress distribution were determined. Based on the rolling contact theory, the relative position of the stick-slip region and the tangential stress distribution of the contact area were analyzed. The stress equations for the three-dimensional between rolling elements and variable diameter raceway were obtained by the principle of superposition, and the stress component characteristics of the contact area were numerically simulated. The results show that the main influencing factors of contact stress are: load, structure of variable diameter raceway, spindle speed, friction coefficient µ and the ratio of the stick region and the slip region k. Taking a cageless bearing as an example, the influence of the contact curvature Ri on the contact stress is smaller than that of ri. Increasing ri to make it larger than 1.5 mm and controlling the speed to be lower than 13,950 r/min, the maximum stress appears in the conventional raceway, which is beneficial to alleviate the failure of the variable diameter raceway. There are a slip region and a stick region in the contact area, reducing the friction coefficient µ and increasing the stick-slip coefficient k appropriately can ensure the discrete movement of the rolling elements and reduce the wear of the variable diameter raceway. The error of the stress distribution model is less than 15%, which can predict and characterize the contact stress distribution between the rolling elements and the variable diameter raceway. The theoretical guidance for the development and application of cageless bearings is provided.