Bistable microstructures are promising for implementation in many mictroelectromechanical system (MEMS)-based applications due to their ability to stay in several equilibrium states, high tunability and unprecedented sensitivity to external stimuli. As opposed to the extensively investigated one-dimensional curved beam-type devices of this kind, microfabrication of non-planar two-dimensional bistable structures, such as plates or shells, represents a remarkable challenge. Recently reported by us, a new moldless stamping procedure, based on pressing a soft stamp over a thin suspended metallic film, was demonstrated to be a feasible direction for the fabrication of initially curved micro plates. However, reliable implementation of this fabrication paradigm and its further development requires better understanding of the role of the process parameters, and of the effect of both the plate and the stamp material properties on the shape of the formed shell and on the postfabrication residual stresses, and therefore on the shell behavior. The need for an appropriate choice of these parameters requires the development of a systematic modeling approach to the stamping process. Here, we report on a finite element (FE)-based methodology for modeling the processing sequences of a successfully fabricated aluminum (Al) micro shell of realistic geometry. The model accounts for the elasto-plastic behavior of the plate material, the nonlinear material behavior of the foam and the contact between them. It was found that the stamping pressure and the plate material parameters are the key parameters affecting the residual shell curvature as well as its shape. Consistently with previously presented experimental results, we show that the fabrication procedure partially relieves the prestresses emerging during preceding fabrication steps, leaving a nontrivial distribution of residual stresses in the formed shell. The presented analysis approach and results provide tools for designers and manufacturers of systems including micro structural elements of shell type.

Abstract—Based on the results of multiple impact bending and static tension tests obtained using a unique technique, we determine the possible causes of the scatter of impact toughness values and their bimodal distribution for low-carbon microalloyed pipeline steel with a simulated microstructure of the heat-affected zone of a welded joint. The influence of the second phases, namely, complex nonmetallic inclusions based on titanium–niobium nitride and containing aluminum oxide and calcium sulfide, is shown.

The impact toughness, microstructure and break features of low-alloy steel weld samples produced by automatic submerged arc welding have been compared. It has been established that when a notch is applied along the weld axis, fracture occurs along large grains of grain-boundary ferrite 80–140 μm in size with an unfavorable cleavage planes orientation 001. The combination of such grains with non-metallic inclusions 3–5 μm in size facilitates the transcrystalline cleavage initiation. The effect of austenite grain size, crystallographic texture, and intergranular fracture on the impact toughness value is shown. Continuous chains of grain-boundary ferrite at the front of the main crack line up along the notch-parallel boundaries of columnar grains of axial orientation, which increases the tendency to transcrystalline cleavage. At the same time, with the lateral orientation of the columnar grains of the former austenite, the front line of the main crack intersects only certain separate areas of the grain-boundary ferrite. Offset notch results in an increase in both impact toughness and plasticity of weld metal.

The effect of distributed electrostatic force on the resonant behavior of initially flat and curved circular aluminum microplates fabricated by a mold-less stamping technique is experimentally investigated. Fabrication-related residual stress is estimated through a correlation between the experimental results and finite element analysis predictions. It is shown that the flat plates vibrating around their undeformed state exhibit hardening nonlinearity, whereas the curved plates exhibit softening nonlinearity. Curved plates vibrating around the deformed equilibrium states, induced by a steady DC voltage, manifest a much higher-frequency tunability as compared to the flat plates. In the bistable curved plates, a frequency sensitivity to the electrostatic loading is increasingly more pronounced in the vicinity of the snap-through limit point. The experimental results are consistent with the finite element model predictions.

Путем анализа результатов испытания на ударный изгиб большого количества образцов исследовано разрушение металла сварного шва, преимущественно состоящего из игольчатого феррита. Исследованный металл обладает очень широким интервалом вязко-хрупкого перехода, начинающимся при +20 и заканчивающимся ниже –60°C. В пределах исследованных температур наблюдаются 3 стабильных уровня ударной вязкости, переходы между которыми с понижением температуры определяют рассеяние работы разрушения образцов. Фрактографически вязко-хрупкий переход осуществляется за счет однократного изменения с вязкого механизма разрушения на скол. Зарождение трещин скола происходит на крупных зернах игольчатого феррита, различное взаиморасположение которых в очаге скола определяет уровень ударной вязкости образца. Путем сопоставления динамических кривых разрушения и макростроения излома показано, что трещины скола зарождаются в вершине стабильно растущей вязкой трещины. При этом на изломах можно выделить отдельные события скола, наблюдаемые на динамической кривой.

Analysis of impact bending test results for a large number of specimens is used to investigate fracture of a weld metal consisting predominantly of acicular ferrite. The metal has a very broad ductile-to-brittle transition interval: from +20 to below –60°C. In the temperature range studied, we observe three stable impact toughness levels. Transitions between them with decreasing temperature determine the scatter in the work done to fracture the specimen. According to fractography results, the ductile-to-brittle transition is due to a single-step change from a ductile fracture mechanism to cleavage. Cleavage cracks nucleate at large acicular ferrite grains. Their different arrangements on a cleavage site determine the impact toughness level. Comparison of dynamic fracture curves and the macroscopic structure of fracture surfaces demonstrates that cleavage cracks nucleate at the tip of a stably growing ductile crack. Examination of fracture surfaces makes it possible to identify individual cleavage events observed in the corresponding dynamic curve.

The degree of microstructural factors influences on the impact strength of four K60 strength class steels after simulating a thermal welding cycle in the coarse-grained area of the weld-affected zone is investigated. It has been shown that the differences between steels with different impact strength KCV–20 are mainly caused by the differences in the content and structure of titanium nitrites. In steels prone to brittle fracture, titanium nitrides act as sources of cleavage cracks origin. In the case of blocking the transition of the cleavage crack from nitride to the matrix, other less significant factors affect the impact strength. It is shown that high-angle grain boundaries stop cleavage cracks by limiting their size, which leads to a decrease in the probability of a crack crossing the grain boundary. The correlation of the impact strength with the volume fraction of the MA component particles and the residual austenite in the studied microstructures is weakly expressed. Such particles cannot be considered as localized sources of cleavage cracks. Differences in the level of Cr + Ni + Cu alloying in the studied steels, which are not characterized by the cleavage cracks origin on titanium nitride inclusions, can be considered as an important factor of impact strength. The mechanism of this influence may be associated with a change in the tendency to cleavage of the ferritic matrix.

A simulated coarse-grained heat affected zone microstructure formation mechanism is established in high strength low alloy steels using electron backscatter diffraction (EBSD). The governing effect of dispersion and ratios between different types of ferritic structural constituents on variation in impact strength is demonstrated. It is assumed from results of simulating a heat affected zone coarse-grained area that a reduction in welding energy input leads to a shift in ductile-brittle transition temperature towards a lower temperature.

Microstructural mechanisms reducing the impact strength values of a coarse grained heat-affected zone are studied for two K60 microalloyed steels. Research is conducted on specimens subjected to simulation of a heat-affected zone. The microstructure and fracture surface are studied using scanning electron microscopy and backscattered electron diffraction. It is shown that titanium nitride inclusions have the greatest influence on impact strength of the heat-affected zone whose cleavage within large bainite packages may cause macro-brittle specimen fracture. The risk of such non-metallic inclusions depends on the their structural features. In particular, presence of aluminum oxide and calcium sulfide blocking contact of the ferritic matrix with nitride leads to an increase in cleavage stress and hinders macroscopic brittle fracture in the early stages of impact bending.

In the paper, the deformation features of the propagation of cleavage cracks in low-alloy low-carbon steel with a ferritic-pearlite microstructure by example of steel 09G2S after hot rolling are investigated. The studied cleavage cracks were obtained in impact bending tests at temperatures in the ductile to brittle transition interval. The study was carried out by transmission electron microscopy, transmission Kikuchi diffraction, and electron backscattered diffraction. It is shown that the deformation accompanying the growth of the cleavage crack in the ferritic-pearlite microstructure is formed when the joints between the cracks propagating in parallel planes break. The crack growth within a single plane occurs without any recorded deformation. The joints are broken by a ductile mechanism according to the scheme of mixed loading by opening and shear. Overlapping of the cleavage cracks determines the relationship between the shear mode and the opening mode during joint deformation, which controls the shape and depth of the plastic deformation zones.

In this paper, we study the morphology of the bainite component of the microstructure of rolled products and the mimic coarse-grained region of the thermal effect zone of K60–K65, 09G2S, and 08KhN2MFB low-carbon steels by methods of optical microscopy and electron backscatter diffraction. In the homogeneous microstructure of the former austenite—both in the coarse-grained heat-affected region and in rolled products—the density of high-angle grain boundaries is shown to be higher in lath bainite than that in granular bainite. In the case of a substantially inhomogeneous microstructure of the former austenite, the size effect of the initial grain structure on the effective grain size (high-angle boundaries) prevails over the change in the bainite morphology. The lower-temperature component, lath bainite, turns out to be coarser-grained (in the case of its formation from large-sized austenite grains) than granular bainite. The impact viscosity and cold resistance of a metal, the microstructure basis of which is a mixture of granular and lath bainite, increased at an increase in the density of high-angle boundaries, which in turn is determined by both the grain size of the initial austenite and the morphology of bainite.

The aim of the article is mathematical modeling of behavior of hyper viscoelastic particulate reinforced composite materials. Three dimensional hyper viscoelastic constitutive equations, which include an inherent damage parameter and its functions, have been developed. The model was used to demonstrate physical phenomenon such as the so called dewetting effect. New strain rate, damage rate and stress state sensitive dewetting criterion is proposed. At the same time dewetting criterion is the first order ordinary differential equation with respect to damage parameter and serves as its evolution law. The softening of the material due to dewetting during the loading is considered by means of the strain energy density function augmented by the damage parameter and its functions. The proposed material model simulates volumetric strains caused by the hydrostatic pressure, as well as the dilatation caused by the deviatoric part of the stress, namely deviatoric-bulk coupling is considered. The material model proposed in the framework of this article takes into account the behavior of particulate reinforced hyper viscoelastic composite materials such as strong nonlinearity, dewetting, complex coupling of the deviatoric bulk behavior. Mullins effect and temperature dependence have not been covered by this research. The proposed material model has been implemented into Finite Element package MSC. Marc by means of FORTRAN user subroutines “elevar” and “uelastomer” and it has been shown good correlation with test results.

— We have experimentally demonstrated continuity of hypoeutectoid and pearlite ferrites in aggregates in the microstructure of low-carbon, low-alloy ferrite–pearlite steel. Such ferrite aggregates of various origins have been repeatedly observed along with hypoeutectoid ferrite and pearlite grains. We propose that aggregates of hypoeutectoid and pearlite ferrites should be regarded as a characteristic microstructural feature and this should be taken into account in microstructural characterization.

The behavior of the metal of welded joints and low-alloy low-carbon steel in the process of impact bending is studied from the point of view of energy partitioning due to crack growth process. It is shown that for metal with different microstructures there are significant differences in the degree of deformation before the formation of a brittle crack. The relationship between the work spent before the formation of a brittle crack and the overall work of fracture is shown. Statistical data confirming the increased susceptibility of the metal of welded joints to early formation of brittle cracks are presented. It is shown that the destruction of the metal of welded joints takes place with a large spent of energy on the final fracture of the specimen relative to the base metal.

— Using commercial low-alloy steel as an example, we have studied the nature of the increased scatter in impact toughness in the temperature range of the ductile-to-brittle (D–B) transition by multiple impact toughness tests and examined the microstructure of the steel by electron backscatter diffraction (EBSD). The results demonstrate that the scatter in the impact toughness of the low-alloy steel in the case of fracture in the temperature range of the D–B transition is due to toughness nonuniformity in the plastic zone. To assess the local toughness nonuniformity of the plastic zone in the microstructure of particular specimens fractured in the range of the B–D transition, we evaluated the percentage of the total length of ductile/brittle microcracks. We have demonstrated conceptual feasibility of developing a method for separating the contributions of local plastic deformation preceding and accompanying fracture for particular specimens in the range of the D–B transition using EBSD.

The connection between occurrence of remarkable impact toughness scattering in ductile-to-brittle transition region and microstructure features of low carbon microalloyed steel was established by means of multiple impact toughness tests and electron backscatter diffraction microstructure measurements. The phenomenon of local inhomogeneity in ductility of plastic zone was established on the base of different microcracks nature revealed by electron backscatter diffraction. The residual microcracks occur in local embrittled regions of material as a result of cleavage at the early stage of fracture. Pre-strained regions with local ductility variations are fractured by the formation of splittings and by the ductile tearing. Pancaked parent austenite microstructure, various ferritic microstructures and martensite-austenite constituent along with the significant grain size variations can be considered as the sources for the occurrence of remarkable impact toughness scattering in low carbon microalloyed steels during fracture in ductile-to-brittle transition region.

This research proposes new metal structural elements based on metamaterials as a replacement of conventional enforcement bars, which reduce metal consumption and increase stiffness and strength. Modern manufacturing machines based on lasers has a capability of fast one-side welding. This type of machines are standard now, what makes this procedure fast and cheap. Another aspect of the nowadays usage of metamaterials for construction works is the possibility to model them with all small details. Ten or twenty years ago modelling of numerous small elements covered by concreate was too expensive, even for one slab not to mention the whole building or a bridge. This work demonstrates one variant of metal reinforcement of concreate plate based on pyramid type elements. This type of pyramid reinforcements was inspired by the analysis of topology optimization of maximum stiffness of structure in case of bending. It was shown numerically, that under condition of three point bending such kind of plate has up to two times higher strength capabilities then conventional plate with equivalent metal consumption.