Showing papers on "Contact area published in 2021"

A Two-Finger Soft-Robotic Gripper With Enveloping and Pinching Grasping Modes

Abstract: This article presents a two-finger soft-robotic gripper with enveloping grasping (EG) and pinching grasping (PG) modes. The proposed soft-robotic gripper is based on a two-finger design that combines two dual-module pneumatic actuators with a variable chamber height (DMVCHA). The prototype of the DMVCHA has been fabricated by molding of silicone rubber. In the PG mode, the high passive compliance of the DMVCHA allows large contact area and realizes a vertically plane contact with the object, which improves the grasping reliability and is suitable for grasping objects with a small and medium size. In the EG mode, the DMVCHA becomes the pneumatic network actuator (PneuNet) with a variable chamber height, which provides greater grasping force and is suitable for grasping objects with larger and hollow size. Compared with the traditional PneuNets, the DMVCHA has a greater output force. The bending angle and output force of actuators with different structures have been analyzed by finite-element analysis. The grasping performance of the proposed and universal soft grippers is studied by experiments. The result shows that with only two fingers, the gripper can reliably grasp objects with various shapes and volume, especially small and fragile objects.

Dual textured carbide tools for dry machining of titanium alloys

Abstract: During dry machining of titanium alloys, severe friction exists at the tool-chip interface, which causes excessive heat generation and tool wear. To limit the tool-chip interface friction and to change tribological characteristics, texturing of the tool rake surface is one of the feasible solutions. In this study, innovative dual texture geometries consisting of grooves in the sticking and dimples in the sliding regions over the tool-chip contact area were fabricated. The other combinations included grooves or dimples spread over the entire tool-chip interface and the non-textured tools. Turning experiments showed that the dual textured tools reduced cutting and thrust forces by 31.2% and 16.2%, respectively, over non-textured tools. Surface morphology and elemental dispersive spectroscopy of dual textured tools rake surface had lower workpiece adhesion and less built-up-edge formation due to wear debris entrapment inside the textures. Dual textured tools produced almost continuous chips with very thin segmentation at low feeds compared with non-textured tools which produced highly segmented chips. The analytical model captured the effect of textures on the tool surface in the same way as the reduced contact area tools, and it was in good agreement with corresponding experimental cutting forces.

Are Contact Angle Measurements Useful for Oxide-Coated Liquid Metals?

Abstract: This work establishes that static contact angles for gallium-based liquid metals have no utility despite the continued and common use of such angles in the literature. In the presence of oxygen, these metals rapidly form a thin (∼1-3 nm) surface oxide "skin" that adheres to many surfaces and mechanically impedes its flow. This property is problematic for contact angle measurements, which presume the ability of liquids to flow freely to adopt shapes that minimize the interfacial energy. We show here that advancing angles for a metal are always high (>140°)-even on substrates to which it adheres-because the solid native oxide must rupture in tension to advance the contact line. The advancing angle for the metal depends subtly on the substrate surface chemistry but does not vary strongly with hydrophobicity of the substrate. During receding measurements, the metal droplet initially sags as the liquid withdraws from the "sac" formed by the skin and thus the contact area with the substrate initially increases despite its volumetric recession. The oxide pins at the perimeter of the deflated "sac" on all the surfaces are tested, except for certain rough surfaces. With additional withdrawal of the liquid metal, the pinned angle gets smaller until eventually the oxide "sac" collapses. Thus, static contact angles can be manipulated mechanically from 0° to >140° due to hysteresis and are therefore uninformative. We also provide recommendations and best practices for wetting experiments, which may find use in applications that use these alloys such as soft electronics, composites, and microfluidics.

Contact with coupled adhesion and friction: Computational framework, applications, and new insights

Abstract: Contact involving soft materials often combines dry adhesion, sliding friction, and large deformations. At the local level, these three aspects are rarely captured simultaneously, but included in the theoretical models by Mergel et al., (2019). We here develop a corresponding finite element framework that captures 3D finite-strain contact of two deformable bodies. This framework is suitable to investigate sliding friction even under tensile normal loads. First, we demonstrate the capabilities of our finite element model using both 2D and 3D test cases, which range from compliant tapes to structures with high stiffness, and include deformable–rigid and deformable–deformable contact. We then provide new results on the onset of sliding of smooth elastomer–glass interfaces, a setup that couples nonlinear material behavior, adhesion, and large frictional stresses. Our simulations not only agree well with both experimental and theoretical findings, they also provide new insights into the current debate on the shear-induced reduction of the contact area in elastomeric contact.

Dependency of Contact Angles on Three-Phase Contact Line: A Review

Abstract: The wetted area of a sessile droplet on a practical substrate is limited by the three-phase contact line and characterized by contact angle, contact radius and drop height. Although, contact angles of droplets have been studied for more than two hundred years, there are still some unanswered questions. In the last two decades, it was experimentally proven that the advancing and receding contact angles, and the contact angle hysteresis of rough and chemically heterogeneous surfaces, are determined by interactions of the liquid and the solid at the three-phase contact line alone, and the interfacial area within the contact perimeter is irrelevant. However, confusion and misunderstanding still exist in this field regarding the relationship between contact angle and surface roughness and chemical heterogeneity. An extensive review was published on the debate for the dependence of apparent contact angles on drop contact area or the three-phase contact line in 2014. Following this old review, several new articles were published on the same subject. This article presents a review of the novel articles (mostly published after 2014 to present) on the dependency of contact angles on the three-phase contact line, after a short summary is given for this long-lasting debate. Recently, some improvements have been made; for example, a relationship of the apparent contact angle with the properties of the three-phase line was obtained by replacing the solid–vapor interfacial tension term, γSV, with a string tension term containing the edge energy, γSLV, and curvature of the triple contact line, km, terms. In addition, a novel Gibbsian thermodynamics composite system was developed for a liquid drop resting on a heterogeneous multiphase and also on a homogeneous rough solid substrate at equilibrium conditions, and this approach led to the same conclusions given above. Moreover, some publications on the line energy concept along the three-phase contact line, and on the “modified” Cassie equations were also examined in this review.

Adhesion and friction in hard and soft contacts: theory and experiment

Abstract: This paper is devoted to an analytical, numerical, and experimental analysis of adhesive contacts subjected to tangential motion. In particular, it addresses the phenomenon of instable, jerky movement of the boundary of the adhesive contact zone and its dependence on the surface roughness. We argue that the “adhesion instabilities” with instable movements of the contact boundary cause energy dissipation similarly to the elastic instabilities mechanism. This leads to different effective works of adhesion when the contact area expands and contracts. This effect is interpreted in terms of “friction” to the movement of the contact boundary. We consider two main contributions to friction: (a) boundary line contribution and (b) area contribution. In normal and rolling contacts, the only contribution is due to the boundary friction, while in sliding both contributions may be present. The boundary contribution prevails in very small, smooth, and hard contacts (as e.g., diamond-like-carbon (DLC) coatings), while the area contribution is prevailing in large soft contacts. Simulations suggest that the friction due to adhesion instabilities is governed by “Johnson parameter”. Experiments suggest that for soft bodies like rubber, the stresses in the contact area can be characterized by a constant critical value. Experiments were carried out using a setup allowing for observing the contact area with a camera placed under a soft transparent rubber layer. Soft contacts show a great variety of instabilities when sliding with low velocity — depending on the indentation depth and the shape of the contacting bodies. These instabilities can be classified as “microscopic” caused by the roughness or chemical inhomogeneity of the surfaces and “macroscopic” which appear also in smooth contacts. The latter may be related to interface waves which are observed in large contacts or at small indentation depths. Numerical simulations were performed using the Boundary Element Method (BEM).

Percolation and Reynolds Flow in Elastic Contacts of Isotropic and Anisotropic, Randomly Rough Surfaces

Abstract: In this work, we numerically study the elastic contact between isotropic and anisotropic, rigid, randomly rough surfaces and linearly elastic counterfaces as well as the subsequent Reynolds flow through the gap between the two contacting solids. We find the percolation threshold to depend on the fluid flow direction when the Peklenik number indicates anisotropy unless the system size clearly exceeds the roll-off wave length parallel to the easy flow direction. A critical contact area near 0.415 is confirmed. Heuristically corrected effective-medium treatments satisfactorily provide Reynolds fluid flow conductances, e.g., for isotropic roughness, we identify accurate closed-form expressions, which only depend on the mean gap and the relative contact area.

A soft thumb-sized vision-based sensor with accurate all-round force perception

Abstract: Vision-based haptic sensors have emerged as a promising approach to robotic touch due to affordable high-resolution cameras and successful computer-vision techniques. However, their physical design and the information they provide do not yet meet the requirements of real applications. We present a robust, soft, low-cost, vision-based, thumb-sized 3D haptic sensor named Insight: it continually provides a directional force-distribution map over its entire conical sensing surface. Constructed around an internal monocular camera, the sensor has only a single layer of elastomer over-molded on a stiff frame to guarantee sensitivity, robustness, and soft contact. Furthermore, Insight is the first system to combine photometric stereo and structured light using a collimator to detect the 3D deformation of its easily replaceable flexible outer shell. The force information is inferred by a deep neural network that maps images to the spatial distribution of 3D contact force (normal and shear). Insight has an overall spatial resolution of 0.4 mm, force magnitude accuracy around 0.03 N, and force direction accuracy around 5 degrees over a range of 0.03--2 N for numerous distinct contacts with varying contact area. The presented hardware and software design concepts can be transferred to a wide variety of robot parts.

A Soft Variable-Area Electrical-Double-Layer Energy Harvester.

Abstract: The technological promise of soft devices-wearable electronics, implantables, soft robotics, sensors-has accelerated the demand for deformable energy sources. Devices that can convert mechanical energy to electrical energy can enable self-powered, tetherless, and sustainable devices. This work presents a completely soft and stretchable (>400% strain) energy harvester based on variable-area electrical-double-layer (EDL) capacitors (≈40 µF cm-2 ). Mechanically varying the EDL area, and thus the capacitance, disrupts equilibrium and generates a driving force for charge movement through an external circuit. Prior EDL capacitors varied the contact area by depressing water droplets between rigid electrodes. In contrast, here, the harvester consists of liquid-metal electrodes encased in a hydrogel. Deforming the device by ≈25% strain generates a power density ≈0.5 mW m-2 . This unconventional approach is attractive because: (1) it does not need an external voltage supply to provide charge; (2) the electrodes themselves deform; and (3) it can work under various modes of deformation such as pressing, stretching, bending, and twisting. The unique ability of the harvester to operate underwater shows promising applications in wearables that contact sweat, underwater sensing, and blue energy harvesting.

Dynamics of contact electrification.

Abstract: Although the electrical charging of objects brought into contact has been observed for at least 2000 years, the details of the underlying mechanism are still not yet fully understood. The present paper deals with the very basic process of contact electrification between two metals. We have developed an experimental method to follow the charge of a small sphere bouncing on a grounded planar electrode on a time scale down to 1 μs. It reveals that the sphere is discharged in the moment of contact, which lasts about 6 to 8 μs. However, at the very moment of disruption of the electrical contact, it regains charge far beyond the expectation according to the contact potential difference. The excess charge rises with increasing contact area.

Effect of shear stress on adhesive contact with a generalized Maugis-Dugdale cohesive zone model

Abstract: The interplay between interfacial shear stress and adhesion has been an active but controversial subject of adhesive contact mechanics, which is currently plagued by diverse, sometimes contradicting, predictions. Recently, McMeeking et al. showed that a reversible interface slip parameter plays an essential role in determining how interfacial shear stress affects adhesion for a Johnson-Kendall-Roberts (JKR) contact interface. In this paper, adhesive contact between a rigid spherical indenter and an elastic half-space is studied with a generalized Maugis-Dugdale (M-D) model, where a constant frictional shear stress presents in the intimate contact area while a constant adhesive stress exists in a cohesive zone near the contact edge. The model solution predicts that the contact behavior is governed by a non-dimensional reversible shear index α τ ¯ 2 as well as the Maugis parameter λ . More specifically, it is found that the impact of interfacial shear stress on adhesion is most significant when the model approaches the JKR limit, and it gets less pronounced in the transitional regime and eventually becomes negligible in the Derjaguin-Mulller-Toporov (DMT) limit. Such behavior is in distinct contrast to Johnson's phenomenological solution. Finally, the proposed model is experimentally validated by adhesion tests on contact interfaces with varying Maugis parameters, where the reversible slip factor is experimentally extracted for the first time.

Origin of friction hysteresis on monolayer graphene

Abstract: Load-dependent friction hysteresis is an intriguing phenomenon that occurs in many materials, where the friction measured during unloading is larger than that measured during loading for a given normal load. However, the mechanism underlying this behavior is still not well understood. In this work, temperature-controlled friction force microscopy was utilized to explore the origin of friction hysteresis on exfoliated monolayer graphene. The experimental observations show that environmental adsorbates from ambient air play an important role in the load dependence of friction. Specifically, the existence of environmental adsorbates between the tip and graphene surface gives rise to an enhanced tip-graphene adhesion force, which leads to a positive friction hysteresis where the friction force is larger during unloading than during loading. In contrast to positive friction hysteresis, a negative friction hysteresis where the friction force is smaller during unloading than during loading is observed through the removal of the environmental adsorbates upon in situ annealing. It is proposed that the measured friction hysteresis originates from the hysteresis in the contact area caused by environmental adsorbates between the tip and graphene. These findings provide a revised understanding of the friction hysteresis in monolayer graphene in terms of environmental adsorbates.

Detailed wheel/rail geometry processing with the conformal contact approach

Abstract: This paper proposes a new way of considering wheel–rail contact in multibody systems simulation that goes beyond the traditional planar constraint and elastic approaches. In this approach, wheel–rail interaction is modelled as a force element with pressures and shear stresses distributed over a contact area that may be curved, supporting conformal contact situations. This by-passes the selection of the contact reference location and reference angle, which are delicate aspects of planar contact approaches. The idea is worked out introducing the curved reference surface as the new backbone for the computations, instead of the tangent plane used previously in planar contact approaches. The steps are described by which the curved reference is constructed in CONTACT, using generic facilities for markers, grids, and coordinate transformations, by which generic wheel/rail configurations can be analyzed in a fully automated way. Numerical results show the capabilities of the new method for measured, worn profiles, suppressing discontinuities in the forces when multiple contact patches split or merge. A further application concerns the evaluation of strategies used in planar contact approaches. There we find that the tangent plane’s inclination is of the biggest importance. This should be defined in an averaged way to achieve maximum correspondence to the more detailed curved contact approach.

Formability and microstructure of TC4 titanium alloy hollow shafts formed by cross-wedge rolling with a mandrel

Abstract: The formability and microstructure of TC4 titanium alloy hollow shafts formed by cross-wedge rolling (CWR) are being investigated to ensure that products manufactured for utilisation in the aviation sector are lightweight. The flow behaviour of the TC4 alloy was studied via isothermal hot compression tests. The constitutive equations in different phase regions were then established and applied to a finite element (FE) model to study the effect of process parameters on the ellipticity of the TC4 alloy hollow shafts formed by CWR. Corresponding CWR experiments were conducted to validate the FE model; further, the microstructure of the TC4 alloy hollow shafts was investigated. The results demonstrate that forming angle, stretching angle and area reduction considerably affect the ellipticity of the TC4 alloy hollow shafts by varying the contact area between a die and a workpiece. The ellipticity evidently increases as the relative wall thickness decreases, as the flattening deformation increases. An increase in the deformation temperature will result in a decrease in the deformation resistance of the TC4 alloy and an increase in the ellipticity. Moreover, the effect of the deformation temperature, area reduction and wall thickness of the workpiece on the microstructure of the TC4 alloy hollow shafts formed by CWR was investigated. The degree of kink or globularisation of the strip alpha phase increases with the above parameters. The volume fraction of the beta phase increases with the deformation temperature. The microstructure is typically equiaxed when the deformation temperature is 950 °C.

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

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