Prediction of actual limit stresses in a thin-walled pipe loaded with internal pressure and axial tension

If a thin-walled pipe loaded with internal pressure and tension allows the appearance of plastic trains, then the uniform plastic stability loss with the emergence of a local plastic deformation zone is considered the limit state, the corresponding stresses are considered as the limit. Correct prediction of the stress-strain state at the moment of strain localization requires taking into account the actual size of the loaded pipe and the calculation of true stresses. The article proposes the implementation of the method of predicting the limit values of true stresses that appear in the pipe at different ratios of internal pressure and axial tension. The physical and mechanical properties of the material, the type of stress state and the change in the actual dimensions of the loaded element are taken into account.

Probing the in-plane liquid-like behavior of liquid crystal elastomers

When isotropic solids are unequally stretched in two orthogonal directions, the true stress (force per actual cross-sectional area) in the larger strain direction is typically higher than that in the smaller one. We show that thiol-acrylate liquid crystal elastomers with polydomain texture exhibit an unusual tendency: The true stresses in the two directions are always identical and governed only by the area change in the loading plane, independently of the combination of imposed strains in the two directions. This feature proves a previously unidentified state of matter that can vary its shape freely with no extra mechanical energy like liquids when deformed in the plane. The theory and simulation that explain the unique behavior are also provided. The in-plane liquid-like behavior opens doors for manifold applications, including wrinkle-free membranes and adaptable materials.

Applied problems of strength and plasticity

It has been established that for extrapolation of the functional dependence of true stresses for tensile conditions of cylindrical samples of metal materials in the region of large deformations, a power dependence is better suited, where the approximation parameters are uniquely determined by the standard mechanical characteristics of materials. A method is proposed for calculating the ultimate deformations during stretching of a sample under the simultaneous action of hydrostatic pressure. It is shown that the calculation results are in satisfactory agreement with the known experimental data obtained by P. Bridgman and J. Bell.

Prediction of the Boundary States for Thin-Walled Axisymmetric Shells Under Internal Pressure and Tension Loads

A method for calculating the ultimate true stresses arising in the walls of shells of revolution in the area of uniform plastic deformation is developed in the research. In order to derive the stability loss for the plastic deformation process the criterion of maximum load is taken as the basis, simple differential equations were solved. It has been shown analytically that the level of the boundary true stresses is much lower when the values of the principal stress ratios approach to 2 or 1/2 compared to the adjacent stress states.

Numerical prediction of the strength of a thin-walled pipe loaded with internal pressure and axial tension taking into account its actual dimensions

If a thin-walled pipe loaded with internal pressure and tension allows the appearance of plastic strains takes place, then the uniform plastic stability loss with the emergence of a local plastic deformation zone is considered the limit state, the corresponding stresses are considered as the limit ones. Correct prediction of the stress-strain state at the moment of strain localization requires taking into account the actual size of the loaded pipe and the calculation of true stresses. The article proposes the implementation of the method of predicting the limit values of true stresses that appear in the pipe at different ratios of internal pressure and axial tension. The physical and mechanical properties of the material, the type of stress state and the change in the actual dimensions of the loaded pipe are taken into account. For two grades of steels (carbon steel 45 and alloy steel 10MnН2MoV), an increase in the calculated strength threshold is shown with an insignificant additional load of a pipe loaded with pressure and axial tension. Analysis of the results showed that it is possible to establish a balance between the actual geometry of the element and the load, which will solve the problem of finding the optimal ratio of «weight-strength», important for practical applications in aircraft, rocket and mechanical engineering. The proposed method for finding the limiting values of actual stresses makes it possible to calculate a realistic safety factor and make improved engineering solutions at the design and operation stages of structural elements; to increase the efficiency and safety of using pipeline and shell-type saving systems.

A Strain-Compensated Constitutive Model for Describing the Hot Compressive Deformation Behaviors of an Aged Inconel 718 Superalloy

The hot deformation behaviors of an aged Inconel 718 superalloy are investigated by isothermal compression experiments at four strain rates and five deformation temperatures on a Gleeble-3500 thermo-mechanical simulator. The experimental results show that the true stresses are obviously affected by strain rate and deformation temperature. The true stress increases rapidly at the beginning of hot compressive deformation, which ascribes to the intense work hardening. The true stresses at high deformation temperatures are lower than those at lower deformation temperatures. The dynamic softening induced by DRX is weak at a relatively low deformation temperature. A strain-compensated Arrhenius-type constitutive equation linked with true stress, strain rate and deformation temperature is developed for the studied superalloy. The material constants (\alpha , n, Q and A) in the developed model are expressed as the functions of true strain. The flow stresses calculated by the developed constitutive equation are nicely consistent with the experimental ones, which confirms that the developed constitutive equation can accurately describe the hot deformation behaviors of the studied superalloy.

Assessing delamination initiation of angle-ply laminates from fiber and matrix properties

As a laminate is essentially bonded together from laminae by the matrix, any delamination of it must be initiated from a matrix failure. This article predicts the initiation of a laminate delamination through analyzing the matrix failure. A three-dimensional finite element approach is used to calculate the stress field of an angle-plied laminate under a uniaxial tension. The stresses in each lamina of the laminate are averaged with respect to the lamina thickness to eliminate a weak singularity near a free edge. Homogenized stresses in the matrix are then obtained through bridging model, which are further converted into true values by virtue of stress concentration factors of the matrix in the composite. Influence of any interface crack between the fiber and the matrix on the true stresses has been taken into account. A criterion for detecting the delamination initiation is proposed, which only involves the true stresses and the original strength data of the matrix. To assess the efficiency, comparisons between the predictions and available experiment data are made for a number of angle-plied laminates. Reasonable correlation has been found.

A Mathematical Analysis of Scale Similarity

Scale similarity is found in many natural phenomena in the universe, from fluid dynamics to astrophysics. In large eddy simulations of turbulent flows, some sub-grid scale (SGS) models are based on scale similarity. The earliest scale similarity SGS model was developed by Bardina et al., which produced SGS stresses with good correlation to the true stresses. In the present study, we perform a mathematical analysis of scale similarity. The analysis has revealed that the ratio of the resolved stress to the SGS stress isγ2, whereγis the ratio of the second filter width to the first filter width, under the assumption of small filter width. The implications of this analysis are discussed in the context of large eddy simulation.

True and Nominal Stress during Low Cycle Fatigue Tests of Metals

In this paper results of P91 cast steel after static and fatigue tests were presented. During the tests longitudinal and transverse strains of the specimen were measured. Basing on the results a comparative analysis of nominal stresses σn,true stresses σrz, nominal strain energy ΔWpl (n) and true strain energy ΔWpl (n) was carried out. It was stated that differences between determined paramters rise with increasing strain.