Kyung-Suk Kim
Brown University, Engineering, Faculty Member
How friction affects adhesion is addressed. The problem is considered in the context of a very stiff sphere adhering to a compliant, isotropic, linear elastic substrate and experiencing adhesion and frictional slip relative to each other.... more
How friction affects adhesion is addressed. The problem is considered in the context of a very stiff sphere adhering to a compliant, isotropic, linear elastic substrate and experiencing adhesion and frictional slip relative to each other. The adhesion is considered to be driven by very large attractive tractions between the sphere and the substrate that can act only at very small distances between them. As a consequence, the adhesion behavior can be represented by the Johnson–Kendall–Roberts model, and this is assumed to prevail also when frictional slip is occurring. Frictional slip is considered to be resisted by a uniform, constant shear traction at the slipping interface, a model that is considered to be valid for small asperities and for compliant elastomers in contact with stiff material. A simple model for the interaction of friction and adhesion is utilized, in which some of the work done against frictional resistance is assumed to be stored reversibly. This behavior is cons...
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We present the mechanics of folding surface-layer wrinkles on a soft substrate, i.e. inter-touching of neighbouring wrinkle surfaces without forming a cusp. Upon laterally compressing a stiff layer attached on a finite-elastic substrate,... more
We present the mechanics of folding surface-layer wrinkles on a soft substrate, i.e. inter-touching of neighbouring wrinkle surfaces without forming a cusp. Upon laterally compressing a stiff layer attached on a finite-elastic substrate, certain material nonlinearities trigger a number of bifurcation processes to form multi-mode wrinkle clusters. Some of these clusters eventually develop into folded wrinkles. The first bifurcation of the multi-mode wrinkles is investigated by a perturbation analysis of the surface-layer buckling on a pre-stretched neo-Hookean substrate. The post-buckling equilibrium configurations of the wrinkles are then trailed experimentally and computationally until the wrinkles are folded. The folding process is observed at various stages of wrinkling, by sectioning 20–80 nm thick gold films deposited on a polydimethylsiloxane substrate at a stretch ratio of 2.1. Comparison between the experimental observation and the finite-element analysis shows that the Ogde...
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We report that a graphene sheet has an unusual mode of atomic-scale fracture owing to its structural peculiarity, i.e. single sheet of atoms. Unlike conventional bond-breaking tensile fracture, a graphene sheet can be cut by in-plane... more
We report that a graphene sheet has an unusual mode of atomic-scale fracture owing to its structural peculiarity, i.e. single sheet of atoms. Unlike conventional bond-breaking tensile fracture, a graphene sheet can be cut by in-plane compression, which is able to eject a row of atoms out-of-plane. Our scale-bridging molecular dynamics simulations and experiments reveal that this compressive atomic-sheet fracture is the critical precursor mechanism of cutting single-walled carbon nanotubes (SWCNTs) by sonication. The atomic-sheet fracture typically occurs within 200 fs during the dynamic axial buckling of a SWCNT; the nanotube is loaded by local nanoscale flow drag of water molecules caused by the collapse of a microbubble during sonication. This is on the contrary to common speculations that the nanotubes would be cut in tension, or by high-temperature chemical reactions in ultrasonication processes. The compressive fracture mechanism clarifies previously unexplainable diameter-depe...
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The energy fluxes upon shrinkage of the contact area are calculated for a pair of spheres in adhesion. Various notions of energy release rate are introduced and analyzed for correlating the external work parameters and the work of... more
The energy fluxes upon shrinkage of the contact area are calculated for a pair of spheres in adhesion. Various notions of energy release rate are introduced and analyzed for correlating the external work parameters and the work of adhesion. Decomposition of the energy release rate into reversible and irreversible parts shows that the reversible part is the work of adhesion and it can be described by the cohesive response purely at the contact zone-edge. This result justifies the use of local zone-edge quantities for modeling the interaction of adhesion and friction. For specific quantitative analysis, adhesion is represented by the Dugdale model, uniform cohesive traction up to a limited separation, as an approximation to more exact inter-surface forces. Exact results are given for the entirely reversible energy release rate to the edge of the contact and the energy release rate to the cohesive zone. The latter is named the strain energy release rate and found to depend on the path in loading parameter space, while the reversible energy release rate is independent of the loading path. The solution for shear of the contact is given as well. Energy released reversibly to be converted into surface energy is identified in contrast to energy released due to slip which will be partially or totally dissipated as heat. The relevance of the results for friction is discussed and contrasted with their significance for the mixed mode fracture of a circular joint.
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A new transverse‐displacement inteferometer (TDI) is described. This interferometer makes use of intensity variations of a beam obtained by superposition of two beams diffracted symmetrically from a diffraction grating copied onto a plane... more
A new transverse‐displacement inteferometer (TDI) is described. This interferometer makes use of intensity variations of a beam obtained by superposition of two beams diffracted symmetrically from a diffraction grating copied onto a plane surface. The TDI is used to monitor the ...
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Here, we report the dynamic fracture toughness as well as the cohesive parameters of a bicontinuously nanostructured copolymer, polyurea, under an extremely high crack-tip loading rate, from a deep-learning analysis of a dynamic... more
Here, we report the dynamic fracture toughness as well as the cohesive parameters of a bicontinuously nanostructured copolymer, polyurea, under an extremely high crack-tip loading rate, from a deep-learning analysis of a dynamic big-data-generating experiment. We first invented a novel Dynamic Line-Image Shearing Interferometer (DL-ISI), which can generate the displacement-gradient time profiles along a line on a sample’s back surface projectively covering the crack initiation and growth process in a single plate impact experiment. Then, we proposed a convolutional neural network (CNN) based deep-learning framework that can inversely determine the accurate cohesive parameters from DL-ISI fringe images. Plate-impact experiments on a polyurea sample with a mid-plane crack have been performed, and the generated DL-ISI fringe image has been inpainted by a Conditional Generative Adversarial Networks (cGAN). For the first time, the dynamic cohesive parameters of polyurea have been success...
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In experimental fracture studies of viscoelastic polymers presently available optical measurement methods need to be modified to account for the rate sensitivity of the physical properties. Here, photoviscoelastic behavior is examined for... more
In experimental fracture studies of viscoelastic polymers presently available optical measurement methods need to be modified to account for the rate sensitivity of the physical properties. Here, photoviscoelastic behavior is examined for the purpose of determining the mechanical state of stress and strain at the tip of a crack moving through a viscoelastic solid. The linearly opto-mechanical constitutive relation is
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Here, we report the closure resistance of a soft-material bilayer orifice increases against external pressure, along with ruga-phase evolution, in contrast to the conventional predictions of the matrix-free cylindrical-shell buckling... more
Here, we report the closure resistance of a soft-material bilayer orifice increases against external pressure, along with ruga-phase evolution, in contrast to the conventional predictions of the matrix-free cylindrical-shell buckling pressure. Experiments demonstrate that the generic soft-material orifice creases in a threefold symmetry at a limit-load pressure of p / μ ≈ 1.20, where μ is the shear modulus. Once the creasing initiates, the triple crease wings gradually grow as the pressure increases until the orifice completely closes at p / μ ≈ 3.0. By contrast, a stiff-surface bilayer orifice initially wrinkles with a multifold symmetry mode and subsequently develops ruga-phase evolution, progressively reducing the orifice cross-sectional area as pressure increases. The buckling-initiation mode is determined by the layer's thickness and stiffness, and the pressure by two types of the layer's instability modes—the surface-layer-wrinkling mode for a compliant and the ring-...
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We report closed-form formulas to calculate the incremental-deposition stress, the elastic relaxation stress, and the residual stress in a finite-thickness film from a wafer-curvature measurement. The calculation shows how the incremental... more
We report closed-form formulas to calculate the incremental-deposition stress, the elastic relaxation stress, and the residual stress in a finite-thickness film from a wafer-curvature measurement. The calculation shows how the incremental deposition of a new stressed layer to the film affects the amount of the film/wafer curvature and the stress state of the previously deposited layers. The formulas allow the incremental-deposition stress and the elastic relaxation to be correctly calculated from the slope of the measured curvature versus thickness for arbitrary thicknesses and biaxial moduli of the film and the substrate. Subtraction of the cumulative elastic relaxation from the incremental-deposition stress history results in the residual stress left in the film after the whole deposition process. The validities of the formulas are confirmed by curvature measurements of electrodeposited Ni films on substrates with different thicknesses.
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A novel approach to quantitative deformation characterization of high-resolution electron microscopy (HREM) defect images has been developed. The essential principle of this technique, called Computational Fourier Transform Deformation... more
A novel approach to quantitative deformation characterization of high-resolution electron microscopy (HREM) defect images has been developed. The essential principle of this technique, called Computational Fourier Transform Deformation (CFTD) analysis, is to extract an accurate displacement field about a defect from its HREM image using Fourier transformation procedures. The methodology's unique feature is to digitize the defect image and compute the Moire pattern, from which the displacement field is obtained, without the need for an external reference lattice image, normally associated with the interference phenomena. Details of the image processing steps are described elsewhere. The motivation is that from this data, the displacement gradient can be calculated which yields much information on the experimental deformation mechanics of some solid undergoing a specific growth process or mechanical testing. One question that has arisen is whether different imaging conditions of t...
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As a sequel of part I (Kothari et al. 2018 Proc. R. Soc. A 474 , 20180054), we present a general thermodynamic framework of flexoelectric constitutive laws for multi-layered graphene (MLG), and apply these laws to explain the role of... more
As a sequel of part I (Kothari et al. 2018 Proc. R. Soc. A 474 , 20180054), we present a general thermodynamic framework of flexoelectric constitutive laws for multi-layered graphene (MLG), and apply these laws to explain the role of crinkles in peculiar molecular adsorption characteristics of highly oriented pyrolytic graphite (HOPG) surfaces. The thermodynamically consistent constitutive laws lead to a non-local interaction model of polarization induced by electromechanical deformation with flexoelectricity–dielectricity coupling. The non-local model predicts curvature and polarization localization along crinkle valleys and ridges very close to those calculated by density functional theory (DFT). Our analysis reveals that the non-local model can be reduced to a simplified uc-local or e-local model (Kothari et al. 2018 Proc. R. Soc. A 474 , 20180054) only when the curvature distribution is uniform or highly localized. For the non-local model, we calibrated and formulated the layer-...
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The shear force required to emit circular dislocation loops from the edge of a circular adhesive-contact zone is calculated analytically as a function of contact-zone radii. The emission condition is based on the balance of the... more
The shear force required to emit circular dislocation loops from the edge of a circular adhesive-contact zone is calculated analytically as a function of contact-zone radii. The emission condition is based on the balance of the configurational force and the Peierls force on a dislocation loop initiated at the edge of the adhesive contact zone. The analysis suggests that there is a transition, for a nanometer-scale single-asperity contact, from concurrent (mobile- dislocation-free) slip to single-dislocation-assisted (SDA) slip. The nanometer-scale friction stress (shear force required for slip/contact area), which experimentally is observed independent of normal loading and contact-zone size, is believed to be the stress required for concurrent slip. The analysis also predicts a second transition from SDA slip to multiple-dislocation-cooperated (MDC) slip at the scale of tens of micrometers in contact size. The friction stress at this large length scale has also been observed experi...
The practice of forming electrical conduction paths in an insulating material by filling cylindrical holes with molten metal can result in high residual stresses when the metal cools. Residual stress is greatest near the metal-insulator... more
The practice of forming electrical conduction paths in an insulating material by filling cylindrical holes with molten metal can result in high residual stresses when the metal cools. Residual stress is greatest near the metal-insulator interface, and stress relaxation by means of de-adhesion is possible. Another failure mode that poses greater practical difficulties is the growth of cracks along paths which spiral away from the interface into the brittle material. Such cracks may occur singly or in pairs, and their lengths can be sufficiently great to provide links with adjacent conduction paths. Such cracks are considered from the fracture mechanics point of view. The residual stress field is relaxed by the growth of spiral cracks which are modeled as continuous distributions of dislocations. It is assumed that these cracks grow so that the stress state on the prospective fracture plane just ahead of the crack tip is purely tensile. The paths are determined by means of an incremen...
ABSTRACTA novel approach to quantitative interpretation of high-resolution electron microscopy images of defects in materials has been developed. The emphasis of this paper is on the methodology, which has been named Computational Fourier... more
ABSTRACTA novel approach to quantitative interpretation of high-resolution electron microscopy images of defects in materials has been developed. The emphasis of this paper is on the methodology, which has been named Computational Fourier Transform Moiré Analysis. The essential principle of this technique is to extract an accurate displacement field about a defect from its near-atomic-resolution picture using digital Fourier transformation procedures. From this data, the displacement gradient can be calculated which yields much information on the experimental deformation mechanics of the material under investigation. As a by-product, we produce the computational Moiré pattern without the need of an external perfect reference lattice image normally associated with the interference phenomena. This method is illustrated using a bounding Frank partial dislocation for a Frank loop of the vacancy type. Results are presented on its strain field, Burgers vector and dislocation core shape an...
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ABSTRACTRough surface contact plasticity at microscale and nanoscale is of crucial importance in many new applications and technologies, such as nano-imprinting and nano-welding. This paper summarizes our recent progress in understanding... more
ABSTRACTRough surface contact plasticity at microscale and nanoscale is of crucial importance in many new applications and technologies, such as nano-imprinting and nano-welding. This paper summarizes our recent progress in understanding contact plasticity from a multiscale point of view, and also presents our perspectives. We first discuss a contact model based on fractal roughness and continuum plasticity theory. Interestingly, our simple, elastic-plastic contact model of the Weierstrass-Archard type gives rise to many practical scaling relations of contact pressure, contact compliance etc. The usefulness of those predictions is discussed for experimental measurements of the thermal/electrical contact resistance. A material length scale can be introduced by a nonlocal plasticity theory, or implicitly by dislocation mechanics modeling. The recent work on micro-plasticity of surface steps gives a variety of surface yielding and hardening behaviors, depending on interface adhesion, r...
... 1988Printed in Great Britain.0020-7683/88 $3.00+ .00 1988 Pergamon Press pieELASTOPLASTIC ANALYSIS OF THE PEEL TESTK-S. KIMDepartment of ... is used to relate an experimentally measured peel force tothe specific fracture energy.1.... more
... 1988Printed in Great Britain.0020-7683/88 $3.00+ .00 1988 Pergamon Press pieELASTOPLASTIC ANALYSIS OF THE PEEL TESTK-S. KIMDepartment of ... is used to relate an experimentally measured peel force tothe specific fracture energy.1. INTRODUCTIONThe peel test is a ...
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The incremental in-plane Green-Lagrange strain tensor was measured near a stationary crack tip in a cyclically work-hardened copper single crystal. Measurements were made on the surface of a four-point bend specimen, using a... more
The incremental in-plane Green-Lagrange strain tensor was measured near a stationary crack tip in a cyclically work-hardened copper single crystal. Measurements were made on the surface of a four-point bend specimen, using a finite-deformation laser moiré interferometer. The measurement showed the existence of a narrow asymptotic field beyond a distance of 300 μm from the crack tip. The inner boundary of the asymptotic zone was almost fixed at a characteristic distance ahead of the crack tip. This length scale is thought to arise from a microstructural evolution near the crack tip. The inhomogeneous hardening due to glide-band clustering and patchy slip in a small volume near the crack tip triggered such an evolution. The outer boundary of the asymptotic zone radially grew with the increasing load. The deformation field was found to be very sensitive to additional mode II loading.
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Amorphous carbon films grown by ion beam deposition from hydrocarbon precursors on compliant polymer substrates are shown here to undergo spontaneous self-assembled folding during growth. When deposited up to 30 min, the... more
Amorphous carbon films grown by ion beam deposition from hydrocarbon precursors on compliant polymer substrates are shown here to undergo spontaneous self-assembled folding during growth. When deposited up to 30 min, the deposition-induced stretch strain of an amorphous carbon film on poly(dimethylsiloxane) (PDMS) with a Young’s modulus of 1–2 MPa reached more than 50%, which is much higher than usually observed compressive mismatch strains of approximately 1–2% on silicon. During deposition of the carbon film, compliant PDMS substrates allowed large amplitude film buckling to let in lateral growth of the film with the significant compressive mismatch strain. The film wrinkled at a low strain of approximately 1% at an early stage of deposition. Then, the wrinkled film was observed to transform its configuration through two different nonlinear modes; formations of ridges and asymmetric localized folds. Due to the biaxial nature of the deposited thin film, the wrinkled film showed her...
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ABSTRACTA previous dislocation model analysis predicts that nano-scale contacts of surface steps induce nucleation of dislocations leading to pro-load and anti-load dislocation segregation near the contact surface. Such dislocation... more
ABSTRACTA previous dislocation model analysis predicts that nano-scale contacts of surface steps induce nucleation of dislocations leading to pro-load and anti-load dislocation segregation near the contact surface. Such dislocation segregation generates a sub-layer of tensile residual stress in a much thicker layer of compressive residual stress near the surface. The sub-layer thickness is expected to be about 50 to 100 times the step height. In order to verify the predictions of the model analysis, experiments are carried out on polycrystalline aluminum surface to determine the existence of the tensile sub-layer. The variation of the residual stress along the thickness direction is measured using a newly developed high sensitivity curvature-measurement interferometer. The interferometer measures the curvature change of the back surface of a plate specimen of about 1.9 mm thickness while the contact-loaded front surface is chemically etched. The residual stress distribution measured...
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Two different types of experimental methods have beeen developed for measuring lateral interaction forces between two solid surfaces for nano- and micro-meter scale contacts. One is the type of direct measurement methods which typically... more
Two different types of experimental methods have beeen developed for measuring lateral interaction forces between two solid surfaces for nano- and micro-meter scale contacts. One is the type of direct measurement methods which typically utilize AFM instrumentations. In the direct lateral force measurements some size-scale effects are commonly observed due to the effects of adhesion and surface roughness. A recent development of a fine AFM lateral force calibration method, a diamagnetic lateral force calibrator, has made it possible to study such size-scale effects systematically. The other type is the field projection method which requires a high resolution measurement of a deformation field near the edge of a contact. For such measurements a comprehensive map of deformation measurement techniques is introduced in a domain of spatial and strain resolutions. This technique provides a way of assessing the non-uniform distribution of the surface interaction forces for nano and micro-me...
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Electric-current (EC) induced failure of pre-cracked, wide unpassivated interconnections is studied on a 0.2 mmthick, pure aluminum film. The film is deposited by high vacuum evaporation coating (HVEC) and patterned into test structures... more
Electric-current (EC) induced failure of pre-cracked, wide unpassivated interconnections is studied on a 0.2 mmthick, pure aluminum film. The film is deposited by high vacuum evaporation coating (HVEC) and patterned into test structures of varying widths. The specimens were electrically loaded to failure under EC via step, and slow or fast ramp loading. The EC induced failure was observed as localized crack growth or spread of a damage zone, depending on the rate of EC loading and the uncracked ligament width. Both crack and damage zone grew or spread towards the positive electrode. The experimental measurements of EC to failure showed a square root dependence on the uncracked ligament width, irrespective of the details of the failure mechanics. Based on this observation, the macroscopic critical current intensity factor is proposed to be used as the material property of the failure criterion, similar to the fracture toughness KIC for brittle fracture.
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... eg, [Ueda et al., 1975] and [Prime and Hill, 2006] ) or shape changes (eg, [Jensen et al., 1990] , [Choi and Kim, 1992] and ... methods has been actively developed over the past several decades (eg, [Groetsch, 1993] , [Robertson,... more
... eg, [Ueda et al., 1975] and [Prime and Hill, 2006] ) or shape changes (eg, [Jensen et al., 1990] , [Choi and Kim, 1992] and ... methods has been actively developed over the past several decades (eg, [Groetsch, 1993] , [Robertson, 1998] and [Ben Abdallah and Bonnet, 2000] ), and ...
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ABSTRACTA nonlinear field projection method has been developed to study nanometer scale mechanical properties of grain boundaries in nanocrystalline FCC metals. The nonlinear field projection is based on the principle of virtual work, for... more
ABSTRACTA nonlinear field projection method has been developed to study nanometer scale mechanical properties of grain boundaries in nanocrystalline FCC metals. The nonlinear field projection is based on the principle of virtual work, for virtual variations of atomic positions in equilibrium through nonlocal interatomic interactions such as EAM potential interaction, to get field-projected subatomic-resolution traction distributions on various grain boundaries. The analyses show that the field projected traction produces periodic concentrated compression sites on the grain boundary, which act as crack trapping or dislocation nucleation sites. The field projection was also used to assess the nanometer scale failure processes of Cu Σ5 grain boundaries doped with Pb. It was revealed that the Pb dopants prevented the emission of dislocations by grain boundary slip and embrittles the grain boundary.
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The mechanics aspects of the peel test are analyzed. In this analysis, the limitation and applicability of the peel test for thin film adhesion have been investigated. Firstly, the effect of plasticity in the peel test of thin metallic... more
The mechanics aspects of the peel test are analyzed. In this analysis, the limitation and applicability of the peel test for thin film adhesion have been investigated. Firstly, the effect of plasticity in the peel test of thin metallic films is analyzed. The analysis has given a closed form solution for the partitioning of peel strength between the energy expenditure rate for the decohesion process and the internal work expenditure rate caused by plastic deformation (bending) of the film. The results predicted the variation of peel strength as a function of the film thickness, the film yield stress and the compliance of the substrate. This analysis shows that the peel strength is very sensitive to the thickness of the film and the yield stress of the film. As observed in the experiment, the peel strength has a peak value at a certain thickness of the film. This is explained for both L and T peel tests. Unlike the L peel test, the peak phenomenon in the T peel test (of metallic films...
ABSTRACTTheoretical and experimental studies on the measurement of the mechanical constitutive relation and the interfacial strength for thin polymeric film structures are presented. For the measurement of the constitutive relation, a new... more
ABSTRACTTheoretical and experimental studies on the measurement of the mechanical constitutive relation and the interfacial strength for thin polymeric film structures are presented. For the measurement of the constitutive relation, a new innovative testing technique based on the J-integral concept is introduced. This method measures the stressstrain relationship in the thickness direction, while the stress-strain relationship in the lateral direction can be measured with conventional techniques. For the measurement of interface strength the cut test and the ball rolling test were used to construct the failure criteria, based on the interface fracture mechanics concept. The interface between polyimide and glass (SiO2) has been tested and analyzed in detail. This shows that the toughness of the PMDA-ODA polyimide/glass interface was measured as 2.2 J/m2 in pure opening mode and as 15 J/m2 for pure sliding mode. It was also observed that the PMDA-ODA/SiO2 interface toughness is insens...
ABSTRACTA process of self-assembly induced by electro-chemical etching was used to produce nano and micrometer scale pyramid-structures on (100) surfaces of gold. The pyramids grew in a self-similar fashion with the facets aligned in... more
ABSTRACTA process of self-assembly induced by electro-chemical etching was used to produce nano and micrometer scale pyramid-structures on (100) surfaces of gold. The pyramids grew in a self-similar fashion with the facets aligned in (114) plane. Using the unique characteristics of the self-similar pyramid structure, plastic compression of the pyramids by a flat-surface platen was performed to study length scale effects in the plastic deformation. A continuum limit analysis and a finite element simulation as well as molecular dynamics simulations were carried out to predict the deformation and load-displacement behavior of the pyramid compression. The limit analysis predicts that the load of compression is proportional to the square of the contact-compression displacement. The continuum analysis provides estimation on the asymptotic behavior of the elastic-plastic load-deflection response of the pyramid under compression for a large value of displacement. The three dimensional molec...