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Complex oxides exhibit a wide range of fascinating functionalities, such as ferroelectricity, piezoelectricity, and pyroelectricity, which are indispensable for cutting‐edge electronics, energy, and information technologies. The intriguing physical properties of these complex oxides arise from the complex interplay between lattice, orbital, charge, and spin degrees of freedom. Here, it is reviewed how electromechanical properties can be achieved/improved by artificially breaking the symmetry of centrosymmetric oxides via engineering thermodynamic variables such as stress, strain, electric field, and chemical potentials. The mechanisms that have been utilized to break the inherent symmetry of conventional materials that lead to novel functionalities and applications are explored. It is highlighted that access to “hidden phases,” which otherwise are prohibited, could uncover opportunities to host exotic properties, such as piezoelectricity, pyroelectricity, etc. This review not only reports how to engineer intrinsically nonpolar and centrosymmetric oxides for emergent properties, but also has implications for manipulating polar functional materials for better performance. Electromechanical properties can be achieved by breaking the symmetry of the centrosymmetric oxides via engineering thermodynamic variables such as stress/strain, electric field, and chemical potentials. The access to “hidden phases” which are prohibited in the equilibrium phase diagram, could offer opportunities to host exotic properties, such as piezoelectricity, pyroelectricity, etc.

Shear banding is a ubiquitous phenomenon of severe plastic deformation, and damage accumulation in shear bands often results in the catastrophic failure of a material. Despite extensive studies, the microscopic mechanisms of strain localization and deformation damage in shear bands remain elusive due to their spatial−temporal complexities embedded in bulk materials. Here we conducted synchrotron-based X-ray microdiffraction (μXRD) experiments to map out the 3D lattice strain field with a submicron resolution around fatigue shear bands in a stainless steel. Both in situ and postmortem μXRD results revealed large lattice strain gradients at intersections of the primary and secondary shear bands. Such strain gradients resulted in severe mechanical heterogeneities across the fatigue shear bands, leading to reduced fatigue limits in the high-cycle regime. The ability to spatially quantify the localized strain gradients with submicron resolution through μXRD opens opportunities for understanding the microscopic mechanisms of damage and failure in bulk materials.

Nonlocal strain gradient continuum mechanics is a methodology widely employed in the literature to assess size effects in nano-structures. Notwithstanding this, improper higher-order boundary conditions (HOBC) are prescribed to close the corresponding elastostatic problems. In the present study, it is proven that HOBC have to be replaced with univocally determined boundary conditions of constitutive type, established by a consistent variational formulation. The treatment, developed in the framework of torsion of elastic beams, provides an effective approach to evaluate scale phenomena in smaller and smaller devices of engineering interest. Both elastostatic torsional responses and torsional-free vibrations of nano-beams are investigated by applying a simple analytical method. It is also underlined that the nonlocal strain gradient model, if equipped with the inappropriate HOBC, can lead to torsional structural responses which unacceptably do not exhibit nonlocality. The presented variational strategy is instead able to characterize significantly peculiar softening and stiffening behaviors of structures involved in modern nano-electro-mechanical systems.

BackgroundA significant amount of uniaxial tensile tests has been carried out using Gleeble systems to investigate the viscoplastic deformation of boron steel (22MnB5) under hot stamping conditions. However, due to heat loss through the end clamps, a temperature gradient in the reduced parallel section of dog-bone shaped specimens is inevitable.ObjectiveIn the work reported in this paper, the effect of temperature gradient on measured outcomes is examined.MethodsUniaxial tensile tests on 1.5 mm thick boron steel specimens are carried out, under hot stamping conditions and strain fields are quantified using the digital image correlation (DIC) technique. The effect of gauge length on the properties of boron steel, as calculated from observed test results, is determined.ResultsCompared with the test at room temperature, a bell-shaped strain distribution occurs within the gauge length even before the appearance of the maximum load. Also, average strain within the gauge length, especially in the later stages, changes with gauge length within the investigated range, and thus, different engineering stress-strain curves and fracture strains are determined. In addition, normalized strain rate is significantly dependent on gauge length, which results in over 16% difference among the computed flow stresses by using a unified constitutive model.ConclusionsThe characterized properties of the material are dependent on gauge length and thus, a testing standard for measuring thermal-mechanical data of materials by using a Gleeble need to be defined.

A comprehensive constitutive theory for the thermo-mechanical behaviour of generalized continua is established within the framework of continuum thermodynamics of irreversible processes. It represents an extension of the class of generalized standard materials to higher order and higher grade continuum theories. It reconciles most existing frameworks and proposes some new extensions for micromorphic and strain gradient media. The special case of strain gradient plasticity is also included as a contribution to the current debate on the consideration of energetic and dissipative mechanisms. Finally, the stress gradient continuum theory emerges as a new research field for which an elastic-viscoplastic theory at finite deformations is provided for the first time. This article is part of the theme issue ‘Fundamental aspects of nonequilibrium thermodynamics’.

In this paper, free vibration analysis and temperature-dependent buckling behavior of porous functionally graded magneto-electro-thermo-elastic material consisting of cobalt ferrite and barium titanate were modeled and analyzed. A high-order sinusoidal shear deformation theory was used to accurately model the anisotropic material behavior. The study examined the porosity role variation across thickness in the buckling and free vibration behavior of nanobeams, as well as the effects of magneto-electro-elastic coupling, thermal stresses, nonlocal properties, externally applied electric and magnetic field potential, and porosity volume fraction.

In this paper, a nonlocal strain gradient isogeometric model based on the higher order shear deformation theory for free vibration analysis of functionally graded graphene platelet-reinforced composites (FG GPLRC) plates is performed. Various distributed patterns of graphene platelets (GPLs) in the polymer matrix including uniform and non-uniform are considered. To capture size dependence of nanostructures, the nonlocal strain gradient theory including both nonlocal and strain gradient effects is used. Based on the modified Halpin–Tsai model, the effective Young’s modulus of the nanocomposites is expressed, while the Poisson’s ratio and density are established using the rule of mixtures. Natural frequencies of FG GPLRC nanoplates is determined using isogeometric analysis. The effects played by strain gradient parameter, distributions of GPLs, thickness-to-length ratio, and nonlocal parameter are examined, and results illustrate the interesting dynamic phenomenon. Several results are investigated and considered as benchmark results for further studies on the FG GPLRC nanoplates.

This paper considers the applicability of high-velocity forming by investigating the compound explosion process of electrical and chemical energy in electric-pulse-triggered energetic materials loading (ETEL). The deformation behavior of DP600 steel sheets during ETEL and wire explosion loading (WEL) is studied by experiments, numerical simulation, and microstructure analysis. The increment of quantized energetic materials (EMs) (0.5 g) on the deformation volume of the sheet is reduced during ETEL, indicating that triggering more EMs at the same discharge energy reduces its energy utilization. The percentage contribution of different discharge energies triggering the same mass of EMs to the deformation volume of the sheet has no effect, with 1.0 g EMs contributing about 35.5%. Compared with WEL specimens of a similar bulging height, an appropriate increase in discharge energy in ETEL is beneficial to improve the energy utilization. The relationships between the kinetic energy, plastic strain energy, stress, and strain of the sheets during ETEL/1.5 g and WEL/5472 J are analyzed by numerical simulation. The results show that ETEL has a smaller stress gradient difference than WEL, indicating that ETEL changes the stress distribution in the deformation zone and expands the strain distribution range. Compared with WEL/5472 J, the low angle grain boundary (LAGB) fraction of ETEL/1.5 g increases, and dislocations are uniformly distributed in the grains and disperse the strain concentration at the grain boundaries. The fracture specimens of ETEL also show multiple necking bands, large and deep dimples, and thicker tear ridges, indicating a significantly higher degree of deformation.

For the first time, buckling behavior of functionally graded (FG) nanoplates made of anisotropic material (beryllium crystal as a hexagonal material) is investigated. Also, it is the first time that the size-dependent behavior of nanostructured systems is studied for buckling response of the graded anisotropic material. The properties of graded material are assumed vary exponentially through the z-direction. Nonlocal strain gradient theory is utilized to predicate the size-dependent buckling behavior of the nanoplate. The nanoplate is modeled by a higher order shear deformation refined plate theory in which any shear correction factor not used. Governing equations and boundary conditions are obtained using a virtual work of variational approach. To solve the buckling problem for different boundary conditions, Galerkin’s approach is utilized. Finally, the influences of different boundary conditions, small-scale parameters, geometry parameters and exponential factor are studied and discussed in detail. It is hoped that the present numerical results can help the engineers and designers to understand and predict the buckling response of FG anisotropic materials.

With the aid of the non-uniform rational B-spline (NURBS)-based isogeometric technique, for the first time, the size-dependent geometrically nonlinear bending characteristics of microplates made of porous functionally graded materials (FGMs) having a central cutout with different shapes are studied. The nonlocal strain gradient continuum elasticity within the framework a hybrid higher-order quasi-3D plate theory is adopted to describe the kinematic relations via only four unknowns. To capture the effective material properties, a porosity-dependent rule of mixture is employed. The nonlocal strain gradient nonlinear load–deflection responses are obtained corresponding to various geometrical and material parameters as well as different boundary conditions. It is revealed that the significance of both the nonlocal and strain gradient reduces. This prediction is the same for all values of the material property gradient index as well as the porosity index. Also, it is demonstrated that a central cutout leads to change the trend of load–deflection response, and this change occurs at a specific value for the applied distributed load which depends on several parameters such as the cutout geometry and boundary conditions. In addition, it is displayed that corresponding to different maximum deflections, the significance of the strain gradient size effect in the absence of nonlocality on the nonlinear flexural stiffness of a porous FGM microplate is more than that of the nonlocal size effect in the absence of the strain gradient size dependency.