Search

In soft lithography roof collapse is frequently observed on the stamp surface consisting of rectangular micro-grooves which yields unwanted contact between the sagged surface and the substrate. Deep understanding of the adhesive contact behavior is a key to design of high-performance collapse-resistant stamp. Both theoretical and numerical studies are presented for a single rectangular micro-groove in the middle of the stamp surface under applied pressure. The JKR-type adhesion solution is established by the principle of superposition and equivalent energy release rate, with a series of closed-form expressions derived which are consistent with previous studies. Finite element analysis (FEA) is performed based on virtual crack closure technique (VCCT) to investigate the roof collapse mechanism with interfacial adhesion and applied pressure considered. Comparison of theoretical, numerical, and experimental results demonstrates that both size effect and constitutive nonlinearity of the stamp on the self-collapse contact width are not obvious for shallow grooves but become prominent for deep grooves, so that the obtained analytical solution is limited to shallow grooves due to its assumptions of half-plane and linear elasticity adopted for the stamp. For deep grooves, the present FEA method shows potential to capture more accurate results. [ABSTRACT FROM AUTHOR]

The prediction of tensile strength properties of powder compacts remains an important industrial issue. In particular, one of the main problems of the powder compaction process is the failure of compacts. Indeed, some powder compacts exhibit cracks which appear during compaction. Such defects occur due to localised tensile or shear stresses, for example close to geometrical singularities. They are also related to the ability of a powder to create enough adhesion at contacts between particles to withstand tensile stresses. Therefore, cracks are a consequence of phenomena occurring at the particle scale and below, down to the molecular scale. To help understanding this mechanism, a particle-scale, numerical method called the multi-particle finite-element method was developed using the finite-element software suite Abaqus (Abaqus 6.14 Documentation, 2016). Such a method allowed to explicitly model the microstructure of a granular media idealised as an assembly of elastic–plastic spheres. The particles were meshed such that their deformations were fully taken into account, using a continuum-mechanics-based material model. The interactions between particles were managed using finite-element contact formulations. A multi-scale, adhesive contact model was developed based on the literature and implemented into the multi-particle finite-element code. The contact model was based on a surface energy formulation weighted by the roughness model developed by Pullen and Williamson (1972). It introduced a novel aspect, the development of adhesion under the effect of external mechanical loads, which is consistent with the cold compaction process. This model was then applied to predict the mesoscopic properties of a packing of spheres, i.e. its response to mechanical stresses of any type, in particular strongly deviatoric stress paths. Such a method intends to be a help toward the development of an efficient continuum model for the modelling of the powder compaction process. • Contact adhesion was implemented into the multi-particle finite-element method. • A new adhesive contact model including the effect of surface roughness was developed. • It was validated against existing adhesive, elastic–plastic contact models. • Adhesion had an influence on yield properties under tensile and shear loading paths. [ABSTRACT FROM AUTHOR]

By incorporating the adhesive force as a body force in the finite element method, the adhesive contact between a diamond sphere and the iron substrate is studied. The adhesive force is derived from Morse potential by integrating the pair potential function over the spherical domain. Based on the tensile behaviors of iron nanowires, the elastic and elastoplastic material parameters are obtained for the substrate. As the radius of the sphere is larger than 320 nm and the maximum depth of indentation is one-tenth of the radius of the sphere, the adhesion caused by interatomic interaction between diamond and iron is small enough to ignore. Then take the sphere whose radius is 20 nm as an example, the results of a single load–unload cycle and multiple load–unload cycles are discussed. In the contact process between the sphere and the surface, the phenomena jump-to-contact and jump-off-contact were observed. Further, the stress distributions inside the substrate and the pressure distribution are investigated. With the analysis of the results, the proposed method can be used to simulate the mechanical behaviors under micro-nano scale contact. [ABSTRACT FROM AUTHOR]

A three-dimensional surface adhesive contact formulation is proposed to simulate macroscale adhesive contact interaction characterized by the van der Waals interaction between arbitrarily shaped deformable continua under finite deformation. The proposed adhesive contact formulation uses a double-layer surface integral to replace the conventional double volume integration to compute the adhesive contact force vector. Considering nonlinear finite deformation, we have derived the surface stress tensor and the corresponding tangent stiffness matrix in a Galerkin weak formulation. With the surface stress formulation, the adhesive contact problems are solved in the framework of nonlinear continuum mechanics by using the standard Lagrange finite element method. Surface stress tensors are formulated for both interacting bodies. Numerical examples show that the proposed surface contact algorithm is accurate, efficient, and reliable for three-dimensional adhesive contact problems of large deformations for both quasi-static and dynamic simulations. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The stress and displacement fields of a complete contact problem are investigated using a finite element analysis (FEA) and a Moiré experiment, respectively. Asymptotic analysis of an adhered contact condition is preceded for comparison. Aluminum and copper alloys are used as the contact materials, from which specimens of different contact angles are fabricated. The FEA scheme is validated through comparison with the displacement distributions that are measured in the Moiré experiment. In turn, the generalized stress intensity factors are calculated using the validated FE models. These factors are used to evaluate the dimensional stress distributions in the asymptotic analysis, which are then compared with the FEA results. Excellent agreement in stress components is found between the asymptotic and FE methodologies. The relative error of the stress components σ rr and σ θθ between the methodologies was less than 3.8% within the range from the contact edge to 1 mm, and it increased gradually as the distance from the contact edge increased. In addition, a slight discrepancy is found in the contact of an elastically dissimilar case. [ABSTRACT FROM AUTHOR]

In this work, we have developed a novel multiscale computational contact formulation based on the generalized Derjuguin approximation for continua that are characterized by atomistically enriched constitutive relations in order to study macroscopic interaction between arbitrarily shaped deformable continua. The proposed adhesive contact formulation makes use of the microscopic interaction forces between individual particles in the interacting bodies. In particular, the double-layer volume integral describing the contact interaction (energy, force vector, matrix) is converted into a double-layer surface integral through a mathematically consistent approach that employs the divergence theorem and a special partitioning technique. The proposed contact model is formulated in the nonlinear continuum mechanics framework and implemented using the standard finite element method. With no large penalty constant, the stiffness matrix of the system will in general be well-conditioned, which is of great significance for quasi-static analysis. Three numerical examples are presented to illustrate the capability of the proposed method. Results indicate that with the same mesh configuration, the finite element computation based on the surface integral approach is faster and more accurate than the volume integral based approach. In addition, the proposed approach is energy preserving even in a very long dynamic simulation. [ABSTRACT FROM AUTHOR]

An adhesive unilateral contact between visco-elastic bodies at small strains and in a Kelvin–Voigt rheology is scrutinized, neglecting inertia. The flow-rule for debonding the adhesive is considered rate independent, unidirectional, and non-associative due to dependence on the mixity of modes of delamination, namely Mode I (opening) needs (= dissipates) less energy than Mode II (shearing). Such mode-mixity dependence of delamination is a very pronounced (and experimentally confirmed) phenomenon typically considered in engineering models. An efficient semi-implicit-in-time FEM discretization leading to recursive quadratic mathematical programs is devised. Its convergence and thus the existence of weak solutions is proved. Computational experiments implemented by BEM illustrate the modeling aspects and the numerical efficiency of the discretization. [ABSTRACT FROM PUBLISHER]

The size effects and physical mechanisms governing small-scale mechanical behavior have important influences on the atomic force microscopy (AFM) measurement. Typically, high surface to volume ratio associated with the small-scale system leads to an appreciable surface force; such that the adhesion of AFM indenter to specimen should be taken into consideration for the nanoindentation and nanoscratch testing. In this work, a finite element method (FEM) model was developed to simulate the adhesive contact between spherical indenter and half-space, in which the adhesive stress is obtained based on the Lennard–Jones potential. Shallow indentation and scratching under different contact conditions were investigated with the proposed FEM model. For adhesion-included cases, the load drop events were found during the indent process, which is demonstrated as a manifestation of local plastic yielding caused by the adhesive stress in contact edge. Adhesion-included scratch load was found sharply increased in the beginning, and rapidly decreased once the scratch displacement reaches to a critical value. Such a predicted stick-slip behavior was interpreted as the rupture of adhesive bond with increasing of scratching distance. [ABSTRACT FROM AUTHOR]

Abstract: A multi-scale model enables investigation of the effect of surface roughness on energy loss in adhesive contact. Fully atomistic simulation of nano-scale indentation predicts size-dependent force–distance trends that are introduced as roughness into a micro-scale finite element model through randomly distributed non-linear hysteretic springs. The multi-scale model predicts that the energy loss, quantified by the hysteresis loop formed by loading and unloading data, increases with increasing indentation depth and surface roughness. This behavior is discussed in terms of an analytical model of a simply connected system. [Copyright &y& Elsevier]

Abstract: In the last decade, instrumented macro- and nano-indentation tests have become useful tools for probing mechanical properties of materials. In this context, little in-depth work has been done for soft bio-materials like biofilms, tissues, gels, cells, etc. Such materials show large elastic strains when subjected to relatively fast loading (excluding viscoelastic phenomena). The present work focuses on the adhesive contact of such materials. Flat punches of circular imprint are used and are subjected to pullout normal forces or bending moments. The materials are modeled as hyperelastic (fast-testing conditions), using the Mooney–Rivlin (M–R) strain energy density function. We examine both incompressibility and compressibility issues. The contact problem of half-space materials interacting with rigid indenters is solved explicitly for moderate deformations and is compared with finite element results. Experiments are conducted with an artificial material (gel, reinforced with talc powder) that is modeled as a hyperelastic material. The present work is expected to extend indentation testing to important technologies like medical applications (health monitoring of tissues) and food industries (quality control of various production stages). It is concluded that the adhesive contact can be used to estimate the initial elastic modulus of soft substrates, but not all the material constants required by the hyperelastic models. [Copyright &y& Elsevier]

Abstract: The paper aimed to study the effect of large deformation and material nonlinearity on the adhesive contact between a smooth rigid spherical indenter and a Neo-Hookean layer of finite thickness, for the cases of the layer thickness/indenter radius ratio between 1 and 2. Our analysis was based on the large-deformation JKR (LDJKR) theory, which models the adhesive contact of two elastic solids in large-deformation regime by knowing the solution of the corresponding non-adhesive contact problem. In this paper, the non-adhesive contact between a spherical indenter and a Neo-Hookean layer was solved by finite element analysis. Combined these numerical results and the LDJKR theory, approximate analytic expressions of the applied load and displacement of adhesive contact of Neo-Hookean layers were obtained. The effects of layer thickness were also discussed. [Copyright &y& Elsevier]

A finite element technique for analysis of adhesive contact is developed in which the adhesive force is modeled as a body force derived from Lennard–Jones 12–6 potential. Adhesive contact of an elastic hemispherical asperity with the plane surface of a semi-infinite rigid body is analyzed. Variations of the interaction force and contact radius during approach and withdrawal, and the dependence of pull-off force on the asperity radius are shown to be in good agreement with those of Maugis–Dugdale model. Analysis results reveal that smaller asperity is superior for preventing stiction and for reducing adhesive friction, but is subject to more severe adhesive wear. It is anticipated that this technique can be utilized in designing a low-adhesion surface profile for MEMS applications since the effect of various surface geometries can be examined. [Copyright &y& Elsevier]