Studying Deformation Behaviors in Austenitic Stainless Steels within a Temperature Range of 143 K < T < 420 K

This work deals with studying staging and macroscopic strain localization in austenitic stainless steel 12Kh18N9T within a temperature range of 143 K < T < 420 K. The visualization and evolution of macroscopic localized plastic deformation bands at different stages of work hardening were carried out by the method of the double-exposure speckle photography (DESP), which allows registering displacement fields with a high accuracy by tracing changes on the surface of the material under study and then comparing the specklograms recorded during uniaxial tension. The shape of the tensile curves σ(ε) undergoes a significant change with a decreasing temperature due to the γ-α'-phase transformation induced by plastic deformation. The processing of the deformation curves of the steel samples made it possible to distinguish the following stages of strain hardening, i.e. the stage of linear hardening and jerky flow stage. A comparative analysis of the design diagrams (with the introduction of additional parameters of the Ludwigson equation) and experimental diagrams of tension of steel 12Kh18N9T for different temperatures is carried out. The analysis of local strains distributions showed that at the stage of linear work hardening, a mobile system of plastic strain localization centers is observed. The temperature dependence of the parameters of plastic deformation localization at the stages of linear work hardening has been established. Unlike the linear hardening, the jerky flow possesses the propagation of single plastic strain fronts that occur one after another through the sample due to the γ-α' phase transition and the Portevin-Le Chatelier effect. It was found that at the jerky flow stage, which is the final stage before the destruction of the sample, the centers of deformation localization do not merge, leading to the neck formation.

The Effect of Combined Tensile-Torsional Loading Path on the Stress/Strain States of Thin-Walled Circular Tubes

In this paper, an elastoplastic analysis model of thin-walled circular tubes under the combined action of axial force and torque is discussed. Based on the von Mises yield criterion and the assumption of isotropic linear hardening, the methods of stress path and strain path loading are analyzed to study the effect of combined tensile-torsional loading path on thin-walled circular tubes. A finite element model is used to analyze the loading path effect on thin-walled circular tubes. A series of tensile and torsional tests are also carried out on 304 stainless steel thin-walled circular tubes using a universal testing machine. In addition, the consistency of the selected material with the von Mises yield criterion, the assumption of isotropic linear hardening, and other classical elastoplastic mechanics are verified. The theoretical calculation results, the numerical analysis results, and the experimental test results are analyzed and compared. The “primary effect” influenced by the stress path and the “recency effect” affected by the strain path are proved, and their application prospects are discussed. The influence of tensile-torsional loading path on the final stress and strain states of thin-walled circular tubes after entering the plastic deformation stage is concretely demonstrated, facilitating the understanding of the principles of the aforementioned two effects. The investigation for a general principle concerning the effect of loading history on the mechanical behavior of engineering materials, based on the classical plastic mechanics, has an important theoretical significance. It is of great theoretical importance for advancements in plastic yield theory and the establishment of more accurate loading conditions suitable for specific materials in engineering practice.

Study on the Expansion Force of TWIP Steel Expandable Tubular in Solid Expandable Tubular Technology

This paper focuses on the expansion process of twinning-induced plasticity (TWIP) steel tubular undergoing the large circumferential plastic deformation in expandable tubular technology. The expansion process was performed by propagating a mandrel through the tubular mechanically. This paper aimed at developing the mathematical models to predict the expansion force required for the radial expansion of the TWIP steel tubular using the rigid-perfectly plastic model and the linear hardening rigid plastic model, respectively. The volume incompressible condition together with the Tresca yield criterion was used to describe the plastic behavior of the tubular material in the expansion process. Besides, the finite element analysis of the expansion process was developed using the commercial software abaqus to validate the theoretical results and determine the scope of application of the derived expansion force formula. Further to this, the effect of the process parameters, such as the expansion ratio, friction coefficient and the cone angle, on the expansion force was investigated. It was found that the expansion force difference of two models have similar variation trend. The accuracy and applicability of the expansion force formula using the linear hardening rigid plastic model improve as the expansion ratio increases and the expansion cone angle decreases.

Improvement of J Estimation Schemes Based on Reference Stress for Linear Hardening Behavior

In Elastic-Plastic Fracture Mechanics, several J-estimation schemes are based on the reference stress approach. This approach has been developed initially for creep analyses and later on for elasto-plastic fracture assessments in 1984, then included in the R6 rule. Much later, other methods, based on the reference stress concept, were derived for 3D applications like the Js method introduced in the French RSE-M code in 1997 and the Enhanced Reference Stress (ERS) method in Korea around 2001. However, these developments are based on the J2 deformation plasticity theory and well established for a pure power hardening law. Js and ERS schemes propose some corrections for recorded behavior laws which cannot be fitted by a power law. Nevertheless, their application to materials governed by a bilinear hardening law has been called into question by several studies. One of these, carried out by M. T. Kirk and R. H. Dodds [1, 2] is of great interest since addressing the practical case of a surface cracked plate.

Numerical analysis of the counter-intuitive dynamic behavior of the elastic-plastic pin-ended beams under impulsive loading with regard to linear hardening effects

The counter-intuitive behavior where the permanent deflection of elastic-plastic beam come to rest in the opposite direction of the impulsive loading, normally appears and disappears abruptly, in certain small ranges of loading and structural parameters. One of the most important issues in the study of this phenomenon is the determination of the influence of different parameters. This work is aimed to study the effects of hardening in counter-intuitive dynamic behavior of elastic-plastic pin-ended beams under impulsive loading. This has been done by developing the proposed Galerkin numerical model and presenting a novel algorithm. The Galerkin method as well as the commercial finite element code ANSYS/LS-DYNA is applied to study this phenomenon. In order to account for the hardening effects in Galerkin method, a new algorithm is proposed. The time history curves for mid-span of the beam is studied in detail and the region of the occurrence of the counter-intuitive behavior is determined. Furthermore, using the finite element software, energy diagrams of the beam are also derived. It has been found that the counter-intuitive behavior is a phenomenon, which is very sensitive to loading, therefore it may appear with a little change in the amount of loading. The results also show that although both methods predict one continuous region of loading for the occurrence of this phenomenon in elastic-perfectly plastic beams, still there are two continuous distinct regions of loading, when considering the hardening effects, for this phenomenon. In addition, this anomalous behavior would occur in the proper ratios of kinetic to internal energy and when considering the linear hardening effects, the possibility of the occurrence of the counter-intuitive behavior exists in a wider domain of energy ratio.

Plastic deformations in the ring spherical shells

In the present work the results of the study of plastic deformations distribution in the thickness in ring spherical shells are presented. Resolving differential equations system is based on the Hirchhoff-Lave hypothesis, linear thin shells theory and small elastic-plastic deformations theory. The studying of the development area of plastic deformations in shells thickness are performed with using the results of the elastic solutions method. The basic relations of elastic solutions method that allow to determine the distribution areas of plastic deformations in shells thickness and along the generatrix are presented. The diagram of intense stress dependence from the strain intensity with linear hardening is received. The numerical solution is performed by orthogonal run method. Long and short spherical shells under the operation of three evenly distributed ring loads are observed. The shells have a tough jamming along the contour at the bottom and at the top. Dependency between tension intensity and deformations intensity is accepted for the case of a material linear hardening. Area of plastic deformations in shells thickness for three kinds of ring spherical shells are shown. The results for the loads differed by the value in twice are presented.