Estimation of the Influence of Compressed Hydrogen on the Mechanical Properties of Pipeline Steels

Consideration of the possibility of transporting compressed hydrogen through existing gas pipelines leads to the need to study the regularities of the effect of hydrogen on the mechanical properties of steels in relation to the conditions of their operation in pipelines (operating pressure range, stress state of the pipe metal, etc.). This article provides an overview of the types of influence of hydrogen on the mechanical properties of steels, including those used for the manufacture of pipelines. The effect of elastic and plastic deformations on the intensity of hydrogen saturation of steels and changes in their strength and plastic deformations is analyzed. An assessment of the potential losses of transported hydrogen through the pipeline wall as a result of diffusion has been made. The main issues that need to be solved for the development of a scientifically grounded conclusion on the possibility of using existing gas pipelines for the transportation of compressed hydrogen are outlined.

DETERMINATION OF THE CAUSES OF DESTRUCTION OF HIGH-STRENGTH FASTENING ELEMENTS

Intensively operated modern technology requires the use of high-strength fasteners. The article presents the results of studies of the structure and physical and mechanical properties of the material of fastening elements (threaded hardware) in order to determine the causes of their destruction during operation. The fractographic analysis of fractures of fractured bolts revealed pronounced centers of crack initiation with traces of delayed brittle fracture, which is typical for high-strength hardware that are under high load for a long time, close to the yield point. Mechanical tests of the studied fastening elements showed increased values of ultimate strength. The performed relaxation tests of the material of broken bolts also revealed an increased value of the microplasticity limit, which is responsible for the onset of plastic deformation processes in microvolumes of the material, and for the material of whole hardware, the value of the limit is included in the permissible interval. At the same time, the average hardness of the material of all products meets the regulatory requirements. The maximum spread of hardness 70 HB is fixed on the destroyed bolt. Using the experimental values of microhardness, theoretical estimates of the coefficient of plasticity, which characterize the ability of a material to perceive elastic and plastic deformations, were carried out. To ensure a sufficient level of ductility of a long-term working metal material, the ductility coefficient must be at least 0.8. The average value of the hardness of the material of the fastening elements satisfies this criterion for assessing ductility. According to the results of a step-by-step analysis of the microstructure of the samples, the inhomogeneity of the distribution of the carbide phase over the cross-section of the bolts was established. The two-phase composition (a-Fe + Fe3C) of steel was established by X-ray structural analysis, which confirms the results of microstructural studies. It should be noted that the phase composition of all studied samples is identical. Based on the results of the studies carried out, it was found that the destruction occurred due to the reduced strength and increased fragility of the material due to the presence of microdefects. The material of the whole bolts in terms of structure and mechanical properties comply with regulatory requirements.

Study of dynamic characteristics of impact of two solid deformable bodies at impact speed of up to 100 m/s

The paper analyzes the dynamic dependencies between the impact force and the depth of the indenter penetration into the obstacle. The indenter is a hardened steel ball. The target is made in the form of a rod from various types of steel, duralumin, aluminum and lead. As a result of digitizing the graphs of dependencies, interpolation formulas are obtained for different phases of the impact (the first phase of the impact is compression; the second phase of the impact is unloading). In the course of the analysis of interpolation formulas, absolute and relative data on the transformation of the initial kinetic energy of the indenter into the distribution of energies after impact are obtained: the value of the kinetic energy of the indenter after the impact, the values of the energy of elastic and plastic deformations, and the energy of shock waves. The results obtained can be used to design impact machines with an indenter impact speed against an obstacle up to 100 m/s.

Capillary forces drive buckling, plastic deformation, and break-up of 3D printed beams

Capillary forces acting at the interfaces of soft materials lead to elastic and plastic deformations and instabilities that result in buckling, coiling, and break-up of 3D printed beams.

Pneumatic Extension Actuators With Kirigami Skins

Soft pneumatic actuators have found many applications in robotics and adaptive structures. Traditionally, these actuators are constructed by wrapping layers of reinforcing helical fibers around an elastomeric tube. This approach is versatile and robust, but it suffers from a critical disadvantage: cumbersome fabrication procedures. Wrapping long helical filaments around a cylindrical tube requires expensive equipment or excessive manual labor. To address this issue, we propose a new approach towards designing and constructing pneumatic actuators by exploiting the principle of kirigami, the ancient art of paper cutting. More specifically, we use “kirigami skins” — plastic sleeves with carefully arranged slit cuts — to replace the reinforcing helical fibers. This paper presents an initial investigation on a set of linear extension actuators featuring kirigami skins with a uniform array of cross-shaped, orthogonal cuts. When under internal pressurization, the rectangular-shaped facets defined by these cuts can rotate and induce the desired extension motion. Through extensive experiments, we analyze the elastic and plastic deformations of these kirigami skins alone under tension. The results show strongly nonlinear behaviors involving both in-plane facet rotation the out-of-plane buckling. Such a deformation pattern offers valuable insights into the actuator’s performance under pressure. Moreover, both the deformation characteristics and actuation performance are “programmable” by tailoring the cut geometry. This study lays down the foundation for constructing more capable Kirigami-skinned soft actuators that can achieve sophisticated motions.

Stabilization of the response of cyclically loaded lattice spring models with plasticity

This paper develops an analytic framework to design both stress-controlled and displacement-controlled T -periodic loadings which make the quasistatic evolution of a one-dimensional network of elastoplastic springs converging to a unique periodic regime. The solution of such an evolution problem is a function t->(e(t),p(t)), where ei(t) and pi(t) are the elastic and plastic deformations of spring i, that satisfies the initial condition (e(t0),p(t0)). After we rigorously convert the problem into a Moreau sweeping process with a moving polyhedron C(t) in a vector space E of dimension d, it becomes natural to expect (based on a result by Krejci) that the elastic component t->e(t) always converges to a T-periodic function. The achievement of the present paper is in spotting a class of loadings where the Krejci's limit doesn't depend on the initial condition (e(t0),p(t0)) and so all the trajectories approach the same T-periodic regime. The proposed class of sweeping processes is the one for which the normal vectors of any d different facets of the moving polyhedron C(t) are linearly independent. We further link this geometric condition to mechanical properties of the given network of springs. In this way we obtain an analogue of the Frederick-Armstrong theorem from continuum mechanics.

Determination of the Mechanical Properties of PIN–PMN–PT Bulk Single Crystals by Nanoindentation

The present study aimed to experimentally evaluate the mechanical properties of Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN–PMN–PT) bulk single crystals with different crystallographic directions using the nanoindentation technique. The load–indentation depth curves, elastic and plastic deformations, hardnesses, and Young’s moduli of [100]- and [110]-oriented 0.28PIN–0.43PMN–0.29PT bulk single crystals were investigated. Our results show that with an increase in the maximum indentation depth hmax, the plastic residual percentage increased for both the [100]- and the [110]-oriented single crystals. At each hmax, the plastic residual percentage of the [100]-oriented PIN–PMN–PT single crystals was less than that of the [110]-oriented PIN–PMN–PT single crystals. At hmax from 500 nm to 2000 nm, the plastic deformation was larger than the elastic deformation, and the plastic residual percentage was larger than 50% for both the [100]- and the [110]-oriented single crystals. This means that the plastic deformation dominated in the indentation process of PIN–PMN–PT single crystals. The indentation size effect on the hardness of the PIN–PMN–PT single crystals was apparent in the nanoindentation process. Both the hardness and the Young’s modulus of the [100]-PIN–PMN–PT single crystals were greater than those of the [110]-PIN–PMN–PT single crystals, which indicates that the PIN–PMN–PT single crystals had anisotropic mechanical characteristics.

Упругопластическое состояние полого шара

Для модели сжимаемого идеального изотропного упругопластического тела рассмотрена задача о полом шаре, на внешнюю и внутреннюю границы которого заданы разные давление и температура. Решается несвязанная задача в рамках теории малых деформаций. Принимается, что полные деформации равны сумме упругих и пластических деформаций, пластические деформации и напряжения связаны соотношениями ассоциированного закона течения. Упругие деформации определяются из соотношений закона Дюамеля–Неймана. Выбирается условие пластичности, не зависящее от первого инварианта тензора напряжений. При определении напряжения и деформации в пластической области рассматривается квазистатический подход, т.е. не указывается закон изменения внешних параметров воздействия (давление и температура на границах шара) до значений, принимаемых при вычислениях. В настоящей работе определены границы изменения перепада давления и температуры, для которых шар будет находиться в упругом состоянии. Установлено, что в зависимости от значений внешних параметров воздействия пластическая зона может зарождаться на внутренней или на внешней границе шара, или на внутренней и внешней границах одновременно, или между границами шара. В качестве примера приведены графики распределений напряжений, деформаций, перемещений, когда пластическая зона занимает некоторую часть, расположенную между внутренней и внешней границей шара. Приводятся графики годографа напряжений, которые являются элементами верификации решения задачи. В настоящей работе учитывается внешнее и внутреннее давление на полый шар, а также температура внешней и внутренней границы шара, что приводит к более общей постановке задачи и необходимости построения разных алгоритмов ее решения. Также определены области изменения внешних параметров, при которых полый шар находится в определенном состоянии. Ключевые слова: упругопластическое тело, теория пластического течения, температурные напряжения, полый шар, допустимые значения внешних параметров, термоупругопластичность, упругопластическая граница, эквивалентное напряжение. The problem of a hollow sphere with different pressure and temperature levels on the external and the internal boundaries has been considered for the model of an ideal compressible isotropic elastic body. An independent problem is solved within the theory of small strains. It is assumed that total deformations are equal to the sum of elastic and plastic deformations, plastic deformations and stresses are related by the relations of the associated flow law. Elastic deformations are determined by their correlation under the DuhamelNeumann law. The plasticity condition which is independent of the first invariant of the stress tensor is considered. In determination of stress and strain in the plastic range a quasi-static approach is considered, that is, the law of changing the external parameters of the action (pressure and temperature at the boundaries of the sphere) to the values assumed in the calculations is not indicated. In this article, the boundaries of the pressure drop and temperature for which the sphere will be in an elastic state are determined. It has been established that depending on the values of the external parameters of the impact, the plastic zone can occur on the inner or outer boundary of the sphere, or on the inner and outer boundaries simultaneously, or between the boundaries of the sphere. As an example, the graphs of the distributions of stresses, deformations, displacements when the plastic zone occupies a certain part located between the inner and outer boundary of the sphere are given. The stress hodograph graphs which are integral elements of verification of the solution of the problem are given. Keywords: elastoplastic body, theory of plastic flow, temperature stresses, hollow sphere, permissible values of external parameters, thermoelastic elasticity, elastic-plastic boundary, equivalent stress.

High-force magnetic tweezers with hysteresis-free force feedback

ABSTRACTMagnetic tweezers based on solenoids with iron alloy cores are widely used to apply large forces (~100 nN) onto micron-sized (~5 μm) superparamagnetic particles for mechanical manipulation or microrheological measurements at the cellular and molecular level. The precision of magnetic tweezers, however, is limited by the magnetic hysteresis of the core material, especially for time-varying force protocols. Here, we eliminate magnetic hysteresis by a feedback control of the magnetic induction, which we measure with a Hall sensor mounted to the distal end of the solenoid core. We find that the generated force depends on the induction according to a power-law relationship, and on the bead-tip distance according to a stretched exponential relationship. Together, both relationships allow for an accurate force calibration and precise force feedback with only 3 calibration parameters. We apply our method to measure the force-dependence of the viscoelastic and plastic properties of fibroblasts using a protocol with stepwise increasing and decreasing forces. We find that soft cells show an increasing stiffness but decreasing plasticity at higher forces, indicating a pronounced stress stiffening of the cytoskeleton. By contrast, stiff cells show no stress stiffening but an increasing plasticity at higher forces. These findings indicate profound differences between soft and stiff cells regarding their protection mechanisms against external mechanical stress. In summary, our method increases the precision, simplifies the handling and extends the applicability of magnetic tweezers.SIGNIFICANCEMagnetic tweezers are widely used, versatile tools to investigate the mechanical behavior of cells or to measure the strength of receptor-ligand bonds. A limitation of existing high-force magnetic tweezer setups, however, is caused by the magnetic hysteresis of the tweezer core material. This magnetic hysteresis requires that the tweezer core must be de-magnetized (de-Gaussed) prior to each measurement, and that flexible force protocols with decreasing forces are not possible. We describe how these limitations can be overcome with a force feedback though direct magnetic field measurement. We demonstrate the applicability of our setup by investigating the visco-elastic and plastic deformations of fibroblasts to forces of different amplitudes.