Adhesion of a Rigid Punch to a Thin Elastic Layer

Adhesion between a rigid flat cylindrical punch and an elastic layer has been investigated. FE analysis was employed to determine the layer stiffness. Linear elastic fracture mechanics was then used to determine the energy release rate, G_a, per unit of bonded area for a circular debond propagating inwards from the edge of the punch. The calculations showed a strong effect of Poisson's ratio for thin layers, small departures from complete incompressibility causing large reductions in stiffness and hence in detachment force. Experiments were performed with an aluminum punch adhered to a rubber layer using a rubber-based adhesive. The ratio of punch radius to layer thickness was varied over the range 0.07 to 3.3. Detachment forces were measured and compared with calculated values. Reasonable agreement was obtained for thick layers but not for thin ones, possibly because of a change in the mode of failure.     

Adhesion of a Rigid Punch to a Thin Elastic Layer

Adhesion between a rigid flat cylindrical punch and an elastic layer has been investigated. FE analysis was employed to determine the layer stiffness. Linear elastic fracture mechanics was then used to determine the energy release rate, G_a, per unit of bonded area for a circular debond propagating inwards from the edge of the punch. The calculations showed a strong effect of Poisson's ratio for thin layers, small departures from complete incompressibility causing large reductions in stiffness and hence in detachment force. Experiments were performed with an aluminum punch adhered to a rubber layer using a rubber-based adhesive. The ratio of punch radius to layer thickness was varied over the range 0.07 to 3.3. Detachment forces were measured and compared with calculated values. Reasonable agreement was obtained for thick layers but not for thin ones, possibly because of a change in the mode of failure.     

单搭接接头峰值应力数值模拟的正交法分析

运用正交试验法研究了几个主要力学和几何参数如泊松比,弹性模量和被粘物厚对单搭接接头Von Mises等效应力的影响。有限元分析结果的极差分析、方差分析和最优方案的工程平均等结果表明:被粘物厚对单搭接接头Von Mises等效应力影响最大,弹性模量次之,泊松比影响程度最小。分析可知:高的泊松比、低弹性模量和被粘物厚的增大会使得Von Mises等效应力值显著降低。     

HGB周期增强复合材料有效性能及界面应力场数值分析

采用周期和对称理论将空心玻璃微珠(HGB)周期增强环氧树脂复合材料模型简化为立方体元,利用有限元方法,分析了微珠填充比、壁厚两个因素对材料宏观有效弹性模量及泊松比等弹性常数的影响规律,并研究了材料界面应力场分布情况。研究发现,材料宏观有效弹性模量及泊松比均与微珠填充比、壁厚密切相关;它们之间的关系遵循提出的Φ-t等效原理。并得出界面脱黏现象主要是由界面处径向应力造成的。     

Effect of coating thickness on the stress profile at the epoxy/silicone interface subjected to pull-off loading: A finite element analysis

Previous experimental studies of silicone coatings have shown three distinct types of release behavior in the tensile flat punch test, depending on coating thickness. The mechanical response in the punch test is highly dependent upon the Poisson's ratio of the coating and its confinement ratio (punch radius divided by coating thickness). This study developed a high accuracy finite-element model of the punch test using the adaptive p-method with extensive mesh refinement to produce smooth stress profiles up to the punch edge. Stress distributions were found for a wide range of confinement parameters and Poisson's ratios. At a typical Poisson's ratio of 0.49, the highest center stress occurred for the intermediate thickness coatings—not thin or thick. Also, the thickest coatings demonstrated steadily increasing high stress towards the edge, while other thicknesses showed the steep singularity at the edge with a protective stress depression bordering inside it. The results further help explain why the critical pull-off force continues to increase as the thickness decreases, even with different release mechanisms. The stress profiles for thick coatings have almost no sensitivity to Poisson's ratio, unlike other thicknesses which show high sensitivity. Edge peeling is most likely to occur for all thick coatings, while other debonding modes are most likely for thin and intermediate thickness coatings. Together, results show the stress mechanics of the flat punch test follow three distinct types of confinement.     

Effect of composition and high-temperature annealing on the local deformation behavior of silicon oxycarbides

Silicon oxycarbides with varying compositions were investigated concerning their elastic and plastic properties. Additionally, the impact of thermal annealing on their elastic properties was assessed. Phase separation of SiOC seems to have no significant impact on Young’s modulus (high values of β-SiC compensate the low values of the vitreous silica matrix) and hardness. However, it leads to an increase in Poisson’s ratio, indicating an increase in the atomic packing density. The phase composition of SiOC significantly influences Young’s modulus, hardness, brittleness and strain-rate sensitivity: the amount of both β-SiC and segregated carbon governs Young’s modulus and hardness, whereas the fraction of free carbon determines brittleness and strain-rate sensitivity. Thermal annealing of SiOC glass-ceramics leads to an increase in Young’s modulus. However, the temperature sensitivity of Young’s modulus and Poisson’s ratio is not affected, indicating the glassy matrix being stable during thermal annealing. A slightly improved ordering of the segregated carbon and the β-SiC nanoparticles upon thermal annealing was observed. It is suggested that this is responsible for the increase in Young’s modulus.     

On the prediction of graphene’s elastic properties with reactive empirical bond order potentials

The elastic properties of graphene as described by the reactive empirical bond order potential are studied through uniaxial tensile tests calculations at both zero temperature, with a conjugate gradient approach, and room temperature, with molecular dynamics simulations. A perfect linear elastic behavior is observed at 0 K up to ≈0.1% strain. The Young’s modulus and Poisson’s ratio obtained with this potential are of ≈730 GPa and 0.39, respectively, with little chirality effects. These values differ significantly from former estimations, much closer to experimental values. We show that these former values have certainly been obtained by neglecting the effect of atomic relaxation, leading to a severe inaccuracy. At larger strains, an extended apparent linear domain is observed in the stress–strain curves, which is relevant to Young’s modulus calculations at finite temperature. Our molecular dynamics simulations at 300 K have allowed obtaining the following, chirality dependent, apparent Young’s moduli, 860 and 761 GPa, and Poisson’s ratios, 0.12 and 0.23, for armchair and zigzag loadings, respectively.     

Longitudinal Strain Dependent Variation of Poisson’s Ratio for HTPB Based Solid Rocket Propellants in Uni‐axial Tensile Testing

Poisson’s ratio of HTPB based composite propellant is estimated at break using double dumbbell specimens as per ASTM D638 Type IV standard and its value obtained by change in the volume of specimens is calculated as approximately 0.25. This major finding contradicts the behaviour of solid rocket propellants in respect of Poisson’s ratio, which is reported to be 0.5. Further, Poisson’s ratio varies almost linearly with strain even in linear portion of stress‐strain curve in uni‐axial tensile testing as per theoretical calculations. It must be noted that no change in volume does not necessarily indicate constant Poisson’s ratio equal to 0.5. SEM scan indicates that the rate of reduction of Poisson’s ratio with longitudinal strain accelerates after dewetting due to the formation of vacuoles. Bilinear variation of Poisson’s ratio with longitudinal strain is observed. One slope is valid in pre‐dewetting region, calculated from close form solution and other slope is valid for post‐dewetting region, which is measured at break. Measurement of Poisson’s ratio at various longitudinal strains indicates uni‐linear variation and not a bilinear variation with a kink. It is also observed that Poisson’s ratio is different along different lateral directions of the propellant specimen. Poisson’s ratio in two orthogonal directions perpendicular to longitudinal axis is calculated as 0.17 and 0.30. As ASTM Specimen has rectangular cross‐section of approximate size 6×4 mm, the directional behavior of Poisson’s ratio may be attributed to initial dimensions. Prismatic propellant specimen with square cross‐section of 115×6×6 mm dimension do not show any variation in respect of Young’s modulus, tensile strength and percentage elongation as compared to ASTM specimen. Directional behavior of Poisson’s ratio with almost similar numerical value is again observed, thus ruling out dependence of this behavior on different initial dimensions of propellant cross‐section. The propellant slurry flow during vacuum casting, directional curing and orientation of specimen with respect to web of the cast propellant are mainly responsible for this directional behaviour of Poisson’s ratio for the composite propellants. Composite propellants behave as compressible material in most of the region and near failure region or at higher strains; Poisson’s ratio is not anywhere close to 0.5, instead it is close to 0.25.     

An analytical model of the mechanical properties of bulk coal under confined stress

This paper presents the development of an analytical model which can be used to relate the structural parameters of coal to its mechanical properties such as elastic modulus and Poisson’s ratio under a confined stress condition. This model is developed primarily to support process modeling of coalbed methane (CBM) or CO₂-enhanced CBM (ECBM) recovery from coal seam. It applied an innovative approach by which stresses acting on and strains occurring in coal are successively combined in rectangular coordinates, leading to the aggregated mechanical constants. These mechanical properties represent important information for improving CBM/ECBM simulations and incorporating within these considerations of directional permeability. The model, consisting of constitutive equations which implement a mechanically consistent stress–strains correlation, can be used as a generalized tool to study the mechanical and fluid behaviors of coal composites. An example using the model to predict the stress–strain correlation of coal under triaxial confined stress by accounting for the elastic and brittle (non-elastic) deformations is discussed. The result shows a good agreement between the prediction and the experimental measurement.     

Energy consistent modified molecular structural mechanics model for the determination of the elastic properties of single wall carbon nanotubes

A new, modified molecular structural mechanics model for the determination of the elastic properties of carbon nanotubes is presented. It is designed specifically to overcome drawbacks in existing molecular structural mechanics models, which are not consistent with their underlying chemical force fields in terms of energy. As a result, modifications are motivated, developed and implemented in order to create a new, energy consistent molecular structural mechanics model. Hence, the new model leads to a better prediction of the material parameters for single wall carbon nanotubes, while the simple applicability of the approach is maintained. The results calculated for the elastic constants (Young's modulus, Poisson ratio) of armchair and zig-zag CNTs are given and discussed. Both elastic constants were found to be dependent on the chirality as well as on the carbon nanotube diameter. An asymptotic value of approximately 800 GPa was obtained for the Young's modulus and a value of approximately 0.28 for the Poisson ratio.     

Adhesive Failure of Model Acrylic Pressure Sensitive Adhesives

An axisymmetric adhesion apparatus was used to characterize the adhesive and viscoelastic properties of acrylic block copolymer layers that behave as model pressure sensitive adhesives. The mechanisms of deformation were summarized and related to the structure and linear viscoelastic response of each model adhesive. In cases where the area between the adhesive layer and adhering surface remained circular and shrunk uniformly during detachment, the adhesive failure criterion can be quantified and compared to predictions from linear elastic fracture mechanics. The nature of adhesive failure can not be reconciled with these traditional, low-strain approaches, but is consistent with models of large strain elasticity, provided that the finite thickness of the adhesive layer is taken into account. A dimensionless ratio involving the adhesive strength, elastic modulus and adhesive layer thickness can be used to define the regime in which the adhesive failure criterion can be quantified with linear elastic fracture mechanics.     

Adhesive Failure of Model Acrylic Pressure Sensitive Adhesives

An axisymmetric adhesion apparatus was used to characterize the adhesive and viscoelastic properties of acrylic block copolymer layers that behave as model pressure sensitive adhesives. The mechanisms of deformation were summarized and related to the structure and linear viscoelastic response of each model adhesive. In cases where the area between the adhesive layer and adhering surface remained circular and shrunk uniformly during detachment, the adhesive failure criterion can be quantified and compared to predictions from linear elastic fracture mechanics. The nature of adhesive failure can not be reconciled with these traditional, low-strain approaches, but is consistent with models of large strain elasticity, provided that the finite thickness of the adhesive layer is taken into account. A dimensionless ratio involving the adhesive strength, elastic modulus and adhesive layer thickness can be used to define the regime in which the adhesive failure criterion can be quantified with linear elastic fracture mechanics.     

Prediction of elastic properties of carbon fibers and CVI matrices

The mechanical properties of carbon materials are highly dependent upon nanostructure and orientation distribution of graphitic layer planes. A model of deformations based upon theory of elasticity for anisotropic solids is proposed. Then it is used for prediction of elastic modulus and Poisson coefficient from intrinsic elastic constants for particles and orientation distribution of graphitic planes. It was applied to carbon fibers and to carbon matrices produced via Chemical Vapor Deposition based techniques. The orientation distribution of graphitic planes was determined using the distribution of intensity of X-ray scattering I(?). The predictions were compared to Young’s moduli measured on single fibers and matrices deposited on single fibers (microcomposites). The results underline the key role played by the modulus for shear between the graphitic layer planes. Influence of graphitic layer Poisson coefficient and Young’s modulus and nanostructure parameters is discussed.     

Elastic properties of porous oxide ceramics prepared using starch as a pore-forming agent

The elastic properties, in particular the tensile modulus (Young's modulus) and Poisson ratio, of porous alumina, zirconia, and alumina–zirconia composite ceramics are studied using the resonance frequency method and the results compared with theoretical predictions. Starch is used as a pore-forming agent, so that the resulting microstructure is essentially of the matrix-inclusion type (with large bulk pores, connected by small throats when a percolation threshold is exceeded). It is found that for this type of microstructure the porosity dependence of the Young's modulus is significantly below the upper Hashin–Shtrikman bound and the power-law prediction; it corresponds well, however, to a recently proposed exponential relation and to an empirical volume-weighted average of the upper and lower Hashin–Shtrikman bounds. Results for all three types of ceramics indicate that – in the porosity range considered, i.e. up to approximately 50% – the Poisson ratio depends only slightly on porosity.     

Theoretical study on shaft-loaded blister test technique: Synchronous characterization of surface and interfacial mechanical properties

In this paper, the existing shaft-loaded blister test technique was improved and a theoretical study on synchronous characterization of mechanical properties of coating thin-film and film/substrate interface was presented. Problems considered include the exact analytical solution to the problem of axisymmetric deformation of a blistering film and the theoretical derivation of expressions to determine Poisson׳s ratios, Young׳s modulus, the work done by the applied external load, the elastic energy stored in a blistering film, and energy release rate. Some relative issues such as how to control the blistering film as free as possible from plastic yielding and the influence of changing the loading-shaft radius on the membrane stress distribution were discussed. Moreover, an experiment was conducted to verify the presented theoretical work.     

Modeling of elastic properties and conductivity of partially sintered ceramics with duplex microstructure and different grain size ratio

The elastic constants and conductivity of partially sintered single-phase and two-phase ceramics (exemplified by alumina ceramics and alumina-zirconia composites, respectively) with different grain size ratio (from 1:1 to 1:4) are investigated by numerical modeling. The relative elastic moduli of partially sintered two-phase ceramics are shown to be relatively similar to those of single-phase ceramics, whereas the relative conductivity is significantly lower, because of the higher phase contrast. The more the grain size ratio deviates from unity, the higher is the initial packing fraction, and the lower are the relative elastic moduli and conductivity of the partially sintered ceramics. The porosity dependence of the Poisson ratio shows a decreasing trend which is only very weakly affected by the grain size ratio. Correlations between relative Young’s modulus and relative conductivity lie between upper and lower cross-property bounds. For single-phase materials the correlation lies below, for two-phase materials above, the Pabst-Gregorová cross-property relation.     

Mesoscopic Nonlinear Elastic Modulus of Thermal Barrier Coatings Determined by Cylindrical Punch Indentation

Cylindrical punch indentations are performed to determine the effective modulus of a plasma-sprayed ZrO₂–8 wt% Y₂O₃ thermal barrier coating (TBC) as a function of coating depth. Cylindrical punch indentations offer significant advantages over pointed (Vickers, Berkovich, or Knoop) indentations for materials that do not exhibit linear elastic behavior. Cyclic loading with a cylindrical punch clearly shows the TBCs to exhibit nonlinear elastic behavior with significant hysteresis that is related to the compaction and internal sliding within the plasma-spray splat microstructure. Also, the effect of a high-heat-flux laser treatment is shown to produce a gradient both in the effective TBC modulus and degree of loading/unloading hysteresis with depth.     

Experimental study of the Mode I adhesive fracture energy in DCB specimens bonded with a polyurethane adhesive

The Mode I fracture energy of a polyurethane adhesive with low Young’s modulus was investigated. Metal adherends in standardized double cantilever beam (DCB) tests are typically too stiff for soft adhesives, making it difficult to measure the fracture energy accurately. However, soft adhesives, such as a single-component polyurethane adhesive tested in this paper, are in high demand in the automobile industry. Thus, accurate measurement techniques must be established. Flexible substrates composed of spring steel were used for the DCB tests to accommodate the deformation of the adhesive layer. First, the applicability of the flexible substrates was discussed using specimens bonded with an epoxy adhesive. For soft adhesives, however, the deformation of the adhesive layer must be considered in the calculation methods of the fracture energy. Although the deformation effect on the DCB tests has been discussed with Winkler’s elastic foundation, the crack length must be measured along with the load and displacement. To overcome the difficulty of measuring the crack length, a calculation method based on Winkler’s elastic foundation was introduced applying the compliance-based beam method (CBBM). Finally, the fracture energy of the polyurethane adhesive was discussed by comparing the calculation methods with and without measuring the crack length.     

Flexibility and Poisson effect on detachment of gecko-inspired adhesives

Geckos and some insects can easily adhere to and detach from surfaces with micro/nanoscale hair structures on their foot, called setae and spatulas. Here, a model is developed to describe the detachment of the seta. In this model, the seta is assumed to be a beam whose tip adheres to a surface. When normal and tangential forces are applied to the root of the beam, a moment is generated at the contact tip and detachment occurs. The detachment conditions depend heavily on flexibility of the hair. The effects of Young's modulus and aspect ratio of length versus thickness of the beam on the detachment condition are theoretically investigated. The Poisson effect on the detachment conditions was also examined with the experimental results using fabricated silicone rubber beam arrays.