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The effect of layer thickness on hardness and buckling behavior was investigated on Ni-Co-Cu/Cu multilayered films. The Ni-Co-Cu/Cu multilayered films were grown on annealed copper substrates by electrodeposition. We fabricated the multilayered films with various layer thicknesses ranging from 10 nm to 1000 nm. First, dependence of Vickers hardness on the Cu layer thickness was investigated. When the Ni-Co-Cu layer had the constant thickness of 75 nm and the Cu layer thickness was smaller than 75 nm, the hardness increased rapidly with decreasing Cu layer thickness. Subsequently, compressive tests were conducted on the multilayered films having the component layers ranging from100 nm to 1000 nm, where the hardness values did not change rapidly with layer thickness. The copper substrates coated with the multilayered films were compressed until 20% strain. From SEM surface observations after the compressive tests, formations of band-like structures having a certain thickness were recognized. Cross-sectional observation revealed that some band-like structures were formed as a result of local buckling of the multilayered film. The vertical thickness of the bank-like structures increased linearly with increasing component layer thickness.

A nonlinear elastic-inelastic model for a thermosetting polymer is proposed based on the molecular chain network theory. In the proposed model, the bond of the polymer chain is divided into physical and chemical bonds, and the physical bond is allowed to separate and recombine during the deformation. The decrease in the rigidity and increase in the inelastic strain are related to the development of the physical bond density to represent the characteristic deformation behavior of the polymer. A rate-form large deformation constitutive equation is then formulated based on the updated Lagrangian method. Additionally, deformation behaviors of the epoxy under monotonic and cyclic tensile tests were evaluated by experimental and computational studies. The experimental results of the monotonic tensile test displayed a gradual transition from elastic to inelastic deformation, strain softening behavior after the macroscopic yielding stress, and an increase in the initial slope and the macroscopic yielding stresses with an increasing strain rate. The computational model can represent such characteristics by controlling the development of the physical bond density. In a cyclic test, a decrease in the rigidity and an increase in the residual strain with increasing cycles were observed even in a small strain range. Such inelastic deformation depended on the strain rate and the given upper stress in the cyclic test. Further, the proposed model successfully reproduced such nonlinear behavior in the cyclic test.

Metallic materials usually have microscopically heterogeneous structures, such as polycrystalline structures, affecting macroscopic mechanical characteristics. Both macroscopic and microscopic non-uniform deformations of polycrystalline pure copper under a moderate stress gradient were investigated. In this study, macroscopic and microscopic non-uniform deformations under higher stress gradients are investigated. Uniaxial tensile tests using three-curve specimens with different curvatures and grain sizes were performed. In order to evaluate the heterogeneous strain field in the specimen surface, the development of the displacement field was measured using the digital image correlation method (DIC). The stress field was evaluated by coupling the DIC and finite-element methods. In smaller-grain specimens, a strong strain concentration was generated in the minimum cross-section area. Although a strong strain concentration was also confirmed in a larger-grain specimen, the strain field depended not only on the specimen shape but also on the microscopic heterogeneity. This microstructure-driven non-uniform deformation was also observed in the specimen with a larger curvature radius. These results indicated that the macroscopic non-uniform deformation should be estimated by the material parameter related to the microscopic heterogeneity.

Experimental evaluation and modeling are important steps in the investigation of the mechanical behaviors of hydrogels in the small- to large-strain range. In this study, the effects of cross-linking and swelling on the true stress-strain response of a specific type of hydrogel (polyacrylamide) were evaluated using a uniaxial tensile test. The development of true strain on the surface of the hydrogel was measured using the digital image correlation method. The specimens with higher cross-link density exhibited a higher initial elastic modulus and earlier orientation hardening. The initial elastic modulus was reduced by the swelling, whereas the orientation hardening occurred in an earlier strain range in the swollen hydrogel. The mechanical responses of the as-prepared and swollen hydrogels with different cross-linker contents were fitted using a non-Gaussian statistical model. The conventional model underestimated the decrease in the elasticity owing to the swelling effect and overestimated the increase in the stress in the large-strain range. The mechanical model was suitably modified to yield an accurate reproduction of the mechanical responses. The proposed model, which was characterized by five material parameters, was found to reproduce the characteristics of the mechanical responses of the as-prepared and swollen hydrogels with different cross-linker contents.

Revisiting the classical topic of the strain hardening behaviour from a perspective of the analytical model based on the evolution of dislocation density with strain, we extended the capacity of the single internal variable model towards predicting necking instability and re-examined the grain size dependence of the flow stress. An excellent agreement is observed between the model predictions of the necking strain and stress and the results of tensile testing of nickel polycrystals with grain sizes varied from sub-micrometres to hundred micrometres. The pivotal significance of the dynamic recovery in the occurrence of necking has been analysed and emphasised in the context of the discussion on the effect of grain size on flow stress. The most interesting corollary of the analysis made is that not only the simple modelling of the dislocation density evolution, which can be traced back from the very early stage of plastic flow in materials with different grain sizes, can be used for realistic approximation of the stress-strain behaviour during homogeneous deformation under constant plastic strain rate, but also for predicting the onset of necking instability with high confidence.

In polycrystalline materials, with the increase in grain size relative to the macrostructure, the collective behavior of crystal grains affects their macroscopic deformation field. To investigate the effect of relative size on the mechanical behavior of polycrystalline materials, the interaction between the microstructure-induced non-uniform deformation and the specimen shape-induced non-uniform deformation was evaluated based on experimental and numerical studies of uniaxial tensile tests of polycrystalline copper specimens with a curved gauge section. The effects of the macroscopic stress gradient and grain size on the strain field of the specimen were evaluated using specimens with different curvature radii obtained from different thermal treatment conditions. The development of strain distribution was measured using the digital image correlation method. A high strain concentration was observed at the minimum cross-section region in the specimen with smaller grains, whereas such strain concentration was relaxed in the specimen with larger grains because a random strain distribution occurred owing to the polycrystalline structure. A full-scale crystalline plasticity finite element method simulation, under conditions similar to those in the experiment, was then performed. Deformation concentrated zone, in which the length and width depended on the grain size, occurred in the polycrystalline specimen. The cross-section of the specimen was locally reduced when the deformation concentrated zone reached the free surface, and the tensile force became smaller for the specimen with larger grains. To discuss the relative specimen size effect, the plastic strain was divided into local and nonlocal plastic strains. Both the experimental and simulation results clarified that the nonlocal plastic strain gradient evaluated in the finite volume region increased with the region-averaged stress. From these results, we proposed a constitutive equation for the plastic strain as a function of local stress and finite volume averaged stress. The nonlocal plastic work for the evaluation region, which is estimated using the nonlocal strain gradient, increased with the stress during the strain-hardening stage in both the simulation and experimental results.

An Al foil/AlN junction is fabricated by bonding a 30-μm Al foil and a 650-μm AlN plate at room temperature and 473 K. Their bonding strength, which is measured by the 180° peel test, is estimated to be ~30 and ~60 N/m for junctions fabricated by the room-temperature and 473-K bonding, respectively.

Polymer foam is widely used in the engineering field because of various functions such as lightweight, shock absorbing, insulation and so on. However, the strength of the material dramatically decreases with the increase in the volume fraction of the foam. Furthermore, deformation behavior of the material strongly depends on the microscopic heterogeneous structure consisting of foam and polymer wall. In the present study, a micro-to macroscopic deformation behavior of polymer foam is modeled using the rate-form second-order homogenization method. To model the microscopic structure consisting of foam and polymer wall, a hexagonal shape periodic structure was used, and the molecular chain network theory was used to represent an elastic-viscoplastic deformation behavior of polymer wall. The effect of the sizes of macroscopic and microscopic structures on the mechanical behavior of LDPE foam was investigated by computational simulation of bending of cantilever. The bending resistance, in other words, resistance to the strain gradient clearly depended on the comparative size between microscopic and macroscopic structures. For smaller foam model, the strain distributions were point symmetry, and those for different sizes of specimens were same. On the other hand, in larger foam model, strain field was not symmetry, and the magnitude of the asymmetry was emphasized for small specimen. Large difference in the macroscopic strain increasing with the microscopic size induced these asymmetry, and it caused size-dependent bending resistance of the LDPE foam.

A phenomenological dislocation-based approach is proposed to account for the necking phenomenon during tensile deformation of metals and alloys. The critical strain corresponding to the onset of tensile instability is predicted in a simple explicit form based on the Kocks-Mecking dislocation kinetics approach. The model strongly suggests that uniform elongation is controlled primarily by the rate of dislocation recovery. The role of the stain rate sensitivity in stabilizing uniform plastic flow is also elucidated. Model predictions are found to be in excellent agreement with experimental data obtained for ultrafine grained 316L steel produced by severe plastic deformation. The approach presented provides general ques for designing materials with enhanced ductility, including ultrafine grained and bulk nanostructured metals and alloys. The proposed recipe is based on microstructural control of the rate of dynamic recovery of dislocation.

Engineering materials usually exhibit heterogeneity such as that observed in the polycrystalline structure of metals, and this heterogeneity affects the nonuniform deformation of a material. In this study, the micro- to macroscopic nonuniform deformation of polycrystalline copper specimen with a curved gage section is evaluated by a finite element method (FEM) simulation based on the second-order homogenization method (2nd-HM). The effects of the microstructure size and macroscopic stress gradient on the nonuniform deformation of the material are then investigated by comparing the simulation and experimental results. A two-dimensional plane strain polycrystalline microstructure was periodically applied to all the integration points in the macrostructure; the anisotropic deformation of the crystal grains is represented by the conventional crystalline plasticity constitutive equation. The computational results indicate that the interaction between nonuniform deformation on the micro and macroscopic scales induces a slight size effect in the material. However, the FEM simulation based on the 2nd-HM could not predict the decrease in the macroscopic strain concentration in the specimens with large crystalline grains, which was observed in the experimental studies, because of random strain localization resulting from the microscopic heterogeneity.

Aiming at better understanding of the deformation mechanisms involved into plastic deformation of duplex stainless steels, acoustic emission (AE) measurements were performed during room temperature tensile deformation of model steels and alloys with different phase structure including single phase i Fe­30Cr and £ SUS316L steels and duplex i­£ SUS329J4L, Fe­24Cr­6Ni alloys. The real time AE investigations were complemented by microscopic investigations of deformed microstructures. The quantitative AE analysis revealed different AE patterns which were correlated with underlying deformation mechanisms and microstructural transformations. The cluster analysis of AE power spectral densities has been proven effective in revealing specific groups of signals (clusters) which appear on different deformation stages in different steel variants, which can be correlated with involved source mechanisms of plastic deformation and microstructural processes including dislocation slip in either fcc or bcc lattice of £ or i phase, respectively, twinning in the SUS 316L steel and £­iA martensitic transformation in the Fe­24Cr­6Ni model alloy. [doi:10.2320/matertrans.MBW201203]