Stability and limit states of elastoplastic spherical shells under static and dynamic loading

The problem of elastoplastic deformation, buckling, and postcritical behavior of spherical shells is solved using a finite element method and a cross-type explicit scheme of time integration. Stability problems for hemispherical shells under external pressure and compression between rigid plates are considered. The influence of holes and boundary conditions on shell deformation is investigated. It is shown that the calculation results are in good agreement with experimental data.     

Thermal Failure of Polymeric Structural Elements under Monoharmonic Deformation

Thermal-failure problems are formulated for three-dimensional, thin-walled structural elements made of passive and piezoactive materials and subjected to monoharmonic deformation. Methods are described for calculation of critical electric and mechanical, quasistatic and dynamic monoharmonic loads and for analysis of postcritical behavior. The influence of various factors on the critical load is analyzed. Theoretical and experimental data are compared and found to be in good agreement, which is indicative of the reliability of the models used to describe the thermal failure of polymeric structural elements     

Development of inhomogeneous fields under postcritical deformation of steel specimens in extension

The paper deals with experimental studies of inhomogeneous strain fields in the “neck” region in extension of plane and cylindrical specimens at the postcritical stage, which directly precedes the fracture, by using a video system and a digital image correlation method. We consider the problems of interpretation of extension diagrams obtained for specimens of various length with strain inhomogeneity in the working section of the specimen with a “neck.”We obtain experimental data about the distribution of longitudinal, transverse, and shear strain fields and the displacement fields in the region of the plane specimen localization by using the digital image correlation method.     

On the strain hardening and texture evolution in high manganese steels: Experiments and numerical investigation

We present a systematic investigation on the strain hardening and texture evolution in high manganese steels where twinning induced plasticity (TWIP) plays a significant role for the materials' plastic deformation. Motivated by the stress–strain behavior of typical TWIP steels with compositions of Fe, Mn, and C, we develop a mechanistic model to explain the strain-hardening in crystals where deformation twinning dominates the plastic deformation. The classical single crystal plasticity model accounting for both dislocation slip and deformation twinning are then employed to simulate the plastic deformation in polycrystalline TWIP steels. While only deformation twinning is activated for plasticity, the simulations with samples composed of voronoi grains cannot fully capture the texture evolution of the TWIP steel. By including both twinning deformation and dislocation slip, the model is able to capture both the stress–strain behaviors and the texture evolution in Fe–Mn–C TWIP steel in different boundary-value problems. Further analysis on the strain contributions by both mechanisms suggests that deformation twinning plays the dominant role at the initial stage of plasticity in TWIP steels, and dislocation slip becomes increasingly important at large strains.     

Molecular mechanics method applied to problems of stability and natural vibrations of single-layer carbon nanotubes

The molecular mechanics (MM) method is used to determine the frequencies and natural vibration shapes and to determine the buckling critical parameters and the postcritical deformation shapes of single-walled carbon nanotubes with twisted ends. The following two variants of the MM method are used: the standard MM method and the mixed method of molecular mechanics/molecular structure mechanics method (MM/MSM). Computer simulation shows that the MM/MSM method allows one to obtain acceptable values of frequencies and natural vibration shapes as well as of critical angles of twist, appropriate buckling modes, and postcritical deformation configurations of nanotubes compared with the same characteristics of nanotube free vibrations and buckling obtained by the standard MM method.     

Nonlinear axisymmetric deformation of anisotropic spherical shells

A method of solving problems of nonlinear deformation of anisotropic spherical shells with consideration of critical points and postcritical behavior is outlined. The method employs the method of incremental loading in which the load increment is specified with an unknown coefficient determined as an unknown function equivalent to the other ones. The algorithm is based on the numerical discrete-orthogonalization method, which allows analyzing the deformation path for a number of shells with different anisotropy parameters     

Deformation of a Flexible Noncircular Long Cylindrical Shell Under a Nonuniform Load

The exact solution is constructed to a nonlinear problem on subcritical and postcritical deformation of a flexible long cylindrical shell with a noncircular cross-section and fixed periphery under uniform loading. The solution is represented by two relations in terms of elementary functions. The graphs demonstrate how the deflection behaves depending on the laws of change in the curvature and load distribution     

Sufficient Discrete—Integral Criterion of Rupture Strength

The behavior of the atomic structure in the vicinity of the crack tip is modeled. The loss of stability and postcritical deformation of a triatomic cell in a close–packed atomic layer in tension are studied. For macrocracks in single crystals, the concept of the generalized Burgers vector is introduced. A sufficient discrete—integral strength criterion is proposed for normal–rupture cracks in the case where the stress fields have a singular component. In accordance with Novozhilov's hybrid model, this criterion is formulated with the use of a new class of solutions that differs from solutions used in formulating the classical sufficient strength criterion. In the limiting case where the energy characteristics of the postcritical deformation of the cell can be ignored, the sufficient criterion proposed admits a limiting passage to the necessary criterion. The critical loads calculated by means of the sufficient criterion differ substantially from those determined with the use of the necessary criterion; this makes it possible to describe the Rehbinder effect.     

Relaxation mechanism of rotational type in fracture of weld joints for austenic steels

Studied is the mesoscale behavior of plastic deformation and fracture of weld joints for austenic steels. Large-scale rotational modes of deformation in heat affected zones are identified with the first stage of formation of band structures; it is related to the “traveling neck” phenomenon in the base metal. The second stage is connected with crack generation and the state of affairs in the heart-affected zone.     

Method of calculating the fields of residual stresses and plastic strains in cylindrical specimens with allowance for surface hardening anisotropy

A mathematical model is proposed for calculating the stress-strain state of a cylindrical specimen, which arises as a result of plastic surface hardening leading to material deformation anisotropy. The model adequacy is verified through comparisons with experimental data for cylindrical specimens made of 30KhGSA and St. 45 steels. A method of identifying model parameters on the basis of results of a fundamental experiment is developed. Good agreement of the calculated and experimental data is demonstrated.     

The Effect of a Geometrical Parameter on the Deformation of a Hinged Flexible Noncircular Cylindrical Shell

The exact analytical solution of a nonlinear boundary-value problem is used to study the effect of a generalized geometric parameter, which characterizes thickness and curvature, on the subcritical and postcritical deformation of a hinged infinite noncircular cylindrical shell. The load on the shell is nonuniformly distributed over its cross section. The deflection of the shell is plotted for various values of the geometric parameter     

States of Equilibrium and Secondary Loss of Stability of a Straight Rod Loaded by an Axial Force

A general analytical solution of the problem of postcritical deformation of a straight incompressible rod loaded by an axial force is given. The bending of the rod is studied under various boundary conditions, and new states of equilibrium the occurrence of which is due to the secondary loss of stability are found. It is shown that, for simply supported and clamped rods, the solution bifurcates when the ends meet.     

Notch-root plastic response by temperature measurement

It is an experimental fact that the majority of the irreversible work of plastic deformation in metals is converted to heat. Under adiabatic conditions the spatial distribution of heating is indicative of the spatial distribution of plastic work. Tiny thermocouples welded directly to a metal surface were found to provide adequate measurement speed and sensitivity to detect the temperature change associated with plastic deformation. An array of thermocouples was used with some success in determining plastic-zone size. The heating response was used to study notch-root plastic deformation following overloads and following various post-overload heat treatments. The notch-root plastic response was correlated directly with the notch-root residual-stress level. The degree of stress relief as a function of time and temperature found in the notched members studied agrees well with predictions made using the Larson-Miller parameter and relaxation data on other steels. The deformation response in notched-member tests following stress relief is in qualitative agreement with that predicted by mechanics analysis of cracked bodies done by others.     

异种不锈钢激光焊缝材料的动态本构关系

利用等应变测试法获取了304及316L激光焊接焊缝材料的准静态应力应变曲线,发现焊缝材料 具有明显的细晶硬脆化趋势。利用SHTB技术对304、316L及焊接构件材料高温动态力学性能进行了研究。 根据动态实验数据对不锈钢304及316L母材应变率及温度相关的Johnson-Cook本构方程参数进行了拟合。 利用LS-DYNA建立了SHTB动态拉伸实验数值模型,发现了在应力波加载初始阶段由于结构效应及材料 阻抗不匹配引起的应力不平衡现象。通过动态实验与数值模拟相结合的方法确定了焊缝材料的应变率相关 本构参数。     

Constitutive model for plastic deformation of nanocrystalline materials with shear band

A phase mixture model was used to study the plastic deformation behaviors in hardening stage of nanocrystalline materials. The material was considered as a composite of grain interior phase and grain boundary (GB) phase. The constitutive equations of the two phases were determined in term of their main deformation mechanisms. In softening stage, a shear band deformation mechanism was presented and the corresponding constitutive relation was established. Numerical simulations have shown that the predications fit well with experimental data. The investigation using the finite-element method (FEM) provided a direct insight into quantifying shear localization effect in nanocrystalline materials.     

Relationships between the J-integral and the crack opening displacement for stationary and extending cracks

Relationships between the J-integral and the crack opening displacement δ_t are obtained by exploiting the dominance of the Hutchinson-Rice-Rosengren singularity in the crack-tip region. The coefficient d_n that relates J to δ_t, is dependent on the material deformation properties and is independent of crack configuration under small-scale yielding conditions. For low hardening materials, d_n appears to be configuration dependent in the fully yielded state. Similarly, the slope of the J-resistance curve is relatable to an operationally defined crack opening angle if J-controlled crack growth conditions are met. These relationships are corroborated by finite element results for the complete regime of elastic-plastic deformation and experimental data for A533B steels, HY-80 steels and several other ductile metals.     

Prediction of flow stress at hot working condition

A mathematical model has been developed to determine flow stress at hot deformation condition. The proposed model is capable of including work softening due to dynamic phase transformations as well as the effect of temperature and strain rate variation on flow stress utilizing the additivity rule for strain. To verify the model, hot compression tests for two grades of steels has been carried out. The comparison between the experimental and theoretical results confirms the validity of the model.     

一个综合模糊裂纹和损伤的混凝土应变软化本构模型

本文研究就变软化材料的本构关系，提出了一个考虑损伤的粘塑性模型，损伤不仅影响材料的临界应力，而且影响材料的粘塑性，为模拟材料的应变软化行为，假设受损混凝土的破坏局部区域由模糊裂纹和损伤所统治，软化模量和局部区域尺度参量依赖于模糊裂纹扩展时释放的断裂能的参变量，用文中提出的模型计算了混凝土单轴压缩时不同应变速率下的瞬时应力应变响应以及等应力长期作用下的徐变，均得到很的结果。     

On the evolution of lattice deformation in austenitic stainless steels—The role of work hardening at finite strains

In this work, a three dimensional crystal plasticity-based finite element model is presented to examine the micromechanical behaviour of austenitic stainless steels. The model accounts for realistic polycrystal micromorphology, the kinematics of crystallographic slip, lattice rotation, slip interaction (latent hardening) and geometric distortion at finite deformation. We utilise the model to predict the microscopic lattice strain evolution of austenitic stainless steels during uniaxial tension at ambient temperature with validation through in situ neutron diffraction measurements. Overall, the predicted lattice strains are in very good agreement with those measured in both longitudinal and transverse directions (parallel and perpendicular to the tensile loading axis, respectively). The information provided by the model suggests that the observed nonlinear response in the transverse {200} grain family is associated with a competitive bimodal evolution of strain during inelastic deformation. The results associated with latent hardening effects at the microscale also indicate that in situ neutron diffraction measurements in conjunction with macroscopic uniaxial tensile data may be used to calibrate crystal plasticity models for the prediction of the inelastic material deformation response.     

Modeling hydrogen attack effect on creep fracture toughness

The effect of high temperature hydrogen attack on creep crack growth rates in steels is studied by modeling the interaction between creep deformation and gaseous pressures generated by hydrogen and methane. The equilibrium methane pressure as a function of hydrogen pressure, temperature and carbide types for carbon steels and Cr–Mo steels is calculated. This gaseous driving force is incorporated into a micromechanics model for void growth along grain boundaries of a creeping solid. Growth and coalescence of voids along grain boundaries is modeled by a microporous strip of cell elements, referred to as the fracture process zone. The cell elements are governed by a nonlinear viscous constitutive relation for a voided material. Two rate sensitivities as well as two types of grain boundaries are considered in this computational study. Simulations of creep crack growth accelerated by gaseous pressures are performed under conditions of small-scale and extensive creep. The computed crack growth rates at elevated temperatures are able to reproduce the trends of experimental results.