Size and temperature-dependent stability of polarization vortex in PbTiO3 nanosheet under uniaxial tension or compression

Mastering the variations in the stability of a polarization vortex is fundamental for the development of ferroelectric devices based on polarization vortex domain structures. Some phase field simulations were conducted on PbTiO3 nanofilms with an initial polarization vortex under uniaxial tension or compression to investigate the conditions of vortex instability and the effects of aspect ratio of nanofilms and temperature on them. The instability of a polarization vortex is strongly dependent on aspect ratio and temperature. The critical compressive stress increases with decreasing aspect ratio under the action of compressive stress. However, the critical tensile stress first decreases and then increases with decreasing aspect ratio, then continues to decrease. There are two inflection points in the curve. In addition, an elevated temperature makes both the critical tensile and compressive stresses decline, and will also cause the aspect ratio corresponding to the inflection point to decrease. These are very important for the design of promising nano-ferroelectric devices based on polarization vortices to improve their performance while maintaining storage density.

This article examines the critical compressive stresses required for a modified fiber composite to remain straight while the fibers within it bend. It was assumed that the modified composite consists of three phases: fiber, whiskerized interfacial layer, and matrix. An example of a composite material made up of carbon fibers, a whiskerized layer of carbon nanotubes with an epoxy matrix, and an epoxy matrix was considered. Its physical parameters affecting the critical compressive stresses were assessed, and methods for determining them were proposed. The effective properties of the inclusion and binder composite material were identified using the Voigt and Reis methods. Similarly, the effective properties of the interfacial whiskerized layer were analyzed by the three-phase method. The influence of fiber wavelength and phase shift, which define the destruction of the composite material, on the critical compressive stress value was explored. The wavelengths at which the composite material is destroyed were found. The effect of the volume content of the modified inclusion on the minimum critical compressive stress value was shown. The results for the modified composites were compared with those for the classical composites with a similar volume content of inclusions.

Biaxial stress tests of carbon fiber-reinforced plastic (CFRP) laminates were performed to investigate failure criteria under biaxial loads. Specimens of unidirectional CFRP laminates were subjected to a tensile load in the longitudinal fiber direction and a compressive load in the transverse fiber direction. An exclusive jig was used to perform biaxial stress tests with a commonly used single-axis testing machine. Measurements were obtained by controlling the displacement ratio between compressive and tensile displacements. The critical tensile and compressive stresses were then calculated using a constitutive equation. The critical longitudinal tensile stress markedly dropped with increasing the compressive load. The failure criteria of the biaxial stress tests were expressed as the ellipse, of which the major and minor axes were the longitudinal tensile/transverse compressive strengths or fracture strains, respectively. Scanning electron microscope observations suggest that fiber/matrix interfacial debonding due to the compressive load could decrease the critical longitudinal tensile stress.

Cell adhesion plays a crucial role in many biological processes of cells, e.g., immune responses, tissue morphogenesis, and stem cell differentiation. An essential problem in the molecular mechanism of cell adhesion is to characterize the binding affinity of membrane-anchored receptors and ligands under different physiological conditions. In this paper, a theoretical model is presented to study the binding affinity between a large number of anchored receptors and ligands under both tensile and compressive stresses, and corroborated by demonstrating excellent agreement with Monte Carlo simulations. It is shown that the binding affinity becomes lower as the magnitude of the applied stress increases, and drops to zero at a critical tensile or compressive stress. Interestingly, the critical compressive stress is found to be substantially smaller than the critical tensile stress for relatively long and flexible receptor-ligand complexes. This counterintuitive finding is explained by using the Euler instability theory of slender columns under compression. The tension-compression asymmetry in the binding affinity of anchored receptors and ligands depends subtly on the competition between the breaking and instability of their complexes. This study helps in understanding the role of mechanical forces in cell adhesion mediated by specific binding molecules.

It is difficult to measure the mechanical properties of physical vapor deposition coatings owing to their small layer thickness. This report describes a special four-point bending device that makes it possible to determine quantitative critical tensile or compressive failure stresses and shear stresses in thin coatings. The bending test was also used to carry out measurements of hardness and critical loads (scratch test) under stress, excluding other influences such as microstructure changes. The material parameters determined form the basis of a finite element model of the indentation process, and this offers new possibilities for mechanical characterization of coated materials.

This research was conducted to analyze the features of the critical tensile stresses at the top and bottom of the concrete slab in the jointed concrete pavement (JCP) when subjected to both the environmental and vehicle loads. First, the stress distribution in JCP was analyzed when the system was subjected to only the environmental loads or the vehicle loads by using the finite element model of JCP. Then, the stresses were analyzed when the system was subjected to the environmental and vehicle loads at the same time. From this study, it was found that the critical tensile stresses at the slab bottom under the vehicle loads were almost constant regardless of the loading positions once the loads were applied at the positions having some distance from the transverse joint. The critical tensile stresses at the slab bottom could be obtained using the model consisting of normal springs for underlying layers by adding the critical stresses due to the environmental loads and the vehicle loads for the curled-down slab, and by subtracting the critical stress due to the environmental loads from that due to the vehicle loads for the curled-up slab. The critical tensile stresses at the top of the slab could be obtained using the model consisting of tensionless springs for underlying layers by adding the critical stress due to the environmental loads and the stress at the middle of the slab under the vehicle loads applied at the joint for the curled-up slab. An alternative to obtain the critical stresses at the top of the slab for the curled-up slab was to use the critical stresses under only the environmental loads obtained from the model having normal springs for underlying layers.

The effect of elastic, plastic and elastoplastic models on the accuracy of the calculated critical stresses and strains of copper strips hot-rolled in the edging rolls of a universal two-high mill is studied. It is shown that using elastoplastic models improves the accuracy of the calculated critical stresses by a factor of 150 to 200 and the accuracy of setting up the sheet mill to ensure rolling stability. Analytical equations for calculating the critical compressive stresses for various rolling conditions are derived. The critical compressive stresses and strains are calculated to find their permissible values ensuring the stability of a copper strip hot-rolled in the edging rolls of an 850 × 1000 universal mill.

Critical compressive stresses were computed and were compared with measured values for 26 different aluminum-alloy and magnesium-alloy extrusions. Critical stresses for local instability were computed, considering the flanges as plates of uniform thickness rigidly clamped along one edge. Critical stresses for primary instability were computed from Kappus ' theory of torsional instability, taking account of the stiffening effect of fillets and the effect on the torsion bending constant of bending stresses varying across the thickness of the wall. An estimate of the effect of plastic yielding was made by reducing the critical stresses in the ratio of reduced modulus, for a column of rectangular section, to Young's modulus. The measured and computed critical stresses for 120 of the 125 specimens differed less than 5,000 lbs. per sq.in. Comparison of strength-weight ratios for buckling and for failure showed that in general the Z-sections were most efficient. A procedure is outlined for calculating buckling stresses of unsupported extrusions of one material from measured buckling stresses of similar extrusions of a different material. The procedure was checked by comparing calculated buckling stresses with measured buckling stresses for extruded T-sections, L-sections, and Z-sections of several aluminum and magnesium alloys. The differences between the measured and calculated buckling stresses did not exceed 10,000 lbs. per sq.in. The average difference was lees than 5,000 lbs. per sq.in.

Hydroforming of aluminum extrusion tubes for automotive applications. Part I: buckling, wrinkling and bursting analyses of aluminum tubes

Three fracture test methods: uniaxial compression, uniaxial tension and torsion were examined by interpreting results using theories upon which the methods were based. In each of these tests, the fracture of gels can occur as a result of shear, compression or tension. The fracture properties determined from uniaxial compression and tension were compared with torsion testing, a suitable reference technique. Shear stress and strain in uniaxial compression were comparable with shear stress and strain in torsion. However, the tensile stress in compression is not in agreement with that in torsion. Tensile stress or shear stress values in uniaxial tension were generally comparable with tensile or shear stress values in torsion, while the strain levels in uniaxial tension were typically much lower than those in uniaxial compression or torsion. This result could be related to the fracture strain being a function of elongation necessary to reduce the specimen cross section to an area producing the critical fracture stress. The comparison among different methods revealed shear stress and strain can be the fracture criteria for uniaxial compression, and tensile stress can be the fracture criterion for uniaxial tension, whereas the fracture strain criterion in uniaxial tension cannot be specified. Possible mechanisms for differences among methods are discussed in the manuscript.

The effects of restraining force on root cracking of various high strength steel welds, ranging rom 50 to 80 kg/mm2, were investigated by the NRIM TRC test (Tensile Restraint Cracking test). Crittical transverse tensile stress, which is necessary to initiate a root crack in the first layer of a weld, was determined for each weld under several welding conditions. Effects of cooling process below 300°C, preheating temperature and hydrogen content on critical stress were discussed. The conclusions obtained in the study are summarized as follows:(1) The behavior of root cracking in the TRC tests on high strength steel welds is a delayed failure type and practically identical to that in the slotted groove restraint cracking tests(2) From the TRC tests of various high strength steels of tensile strength grades of HT 50 to HT 80, the values of critical tensile stresses were obtained below which no root cracking occurred.(3) The value of critical tensile stress is generally increased with an increase of weld heat input or preheating temperature and a decrease of diffusible hydrogen content. The value is generally greater, the lower is the tensile strength of the steel.(4) The welding conditions and preheating temperatiure for which the TRC critical tensile stress is approximately equal to the yield stress of weld metal, are satisfactory to prevent root cracking in slotted groove restraint cracking tests.(5) The cooling process below 300°C has considerable effect on the value of critical tensile stress of HT 80 steel welds, but liftle on HT 60 steel welds. Faster cooling is harmful.(6) The correlation among the value of critical tensils stress σc, (kg/mm2), preheating temperature Τ0 (°C) and diffusible hydrogen content [Η] (cc/100 gr) of an HT 80 steel 8 E weld is summarized by the following experimental formula :logσc=1.845-(200-Τ0){0.02log([Η]+1)-0.003}(7) The correlation, like as above formula, in an HT 60 steel 6 F weld is summarized by the following experimental formula :logσc=1.64-0.156log([Η]+1)+0.001Τ0(8) A little content of diffusible hydrogen is detrimental to root cracking in HT 80 welds, but not in HT 60 welds, and preheating is much more effective for the prevention of root cracking in HT 80 welds than in HT 60 welds.

Consideration is given to the strain stability at axial compression of the tubular instrumentation tube guide connecting the top nozzle and the bottom nozzle of the fuel assembly of the nuclear reactor WWER-1000. The influence of the grip conditions of the edges of the instrumentation tube guide on the values of critical compressive forces and stresses and also on the shape of the bend at the stability loss has been studied. The changes in the compressive stress and force conditioned by the stochastic nature of the change in the parameters during the operation of the fuel assemblies of nuclear reactors can be comparable with the obtained relatively low critical values of 1.8 to 7.5 MPa of the compressive stress and the values of 6 to 24 kgf (kilogram-force) of the compressive force at which the bending of instrumentation tube guides is possible due to the loss of stability. Due to the insignificant minimum value and the difference between different values of the critical compressive force answering the loss of stability of the guide channel like a long compressed rod it is quite possible that the shape of the instrumentation tube guide bent due to the loss of stability during the operation will have several inflection points that will considerably hamper the motion of the regulating elements of the reactor protection and control system. Due to this fact it is reasonable to develop design diagrams that allow for the determination of critical compressive forces and axial stresses in the instrumentation tube guides taking into consideration their interaction with the top nozzle and the bottom nozzle and the spacer grid of the fuel assembly and this problem requires additional studies.

The laws of variation of the strains and critical stresses for smooth thin circular cylindrical glass-reinforced plastic shells based on EDT-10P resin have been experimentally investigated at different orientations of the fabric reinforcement. The results of the tests are compared with the theoretical data obtained from the relations of the theory of elasticity of an orthotropic body and orthotropic shells.

Griffith's theory of brittle fracture in three dimensions

The present work is a photoelastic investigation of the stress distribution in deep beams with and without web openings. The general form of stress diffusion has been established and the critical zones have been identified. The critical tensile and shear stresses have been evaluated and their sensitivity to various span‐to‐depth ratios and opening positions along the span have been established. Based on stress flow pattern and contour lines of principal tensile stresses, failure mechanisms have been predicted and recommendations have been made for the design of reinforced concrete deep beams.