Simultaneous Stress Relaxation Research Articles

THE PROBLEMAIICS OF ENSURING THE RELIABILITY OF THE SIDE SURFACE OF THE PISTONS OF BOOSTED ICES

Prospective directions and trends in the development of special-purpose engines are analyzed. An increase in swept volume power density of engines leads to a critical raise in thermal and mechanical stress in the engine structural elements and to a corresponding reduction in its resource. Pistons are among the elements of boosted engines that are most prone to failure. Among the characteristic critical zones of the piston design, in which losses of parametric and physical reliability were observed, a separate critical zone on the loaded and unloaded side of the piston skirt near the piston pin bores is singled out. An analysis of publications was carried out and it was shown that the occurrence of the phenomenon of wear or seizures of the side surface of the piston skirt, which is observed for the boosted designs in the specified zone, is not explained exhaustively in the existing system of scientific views. Understanding the causes of the described phenomenon of loss of reliability is necessary for diagnosing and developing measures to prevent it at the design stage. The paper puts forward a hypothesis that explains the occurrence or non-occurrence of the loss of reliability in the process of deformation of the local zone of the side surface of the piston skirt during uneven expansion of the material in presence of creep and simultaneous stress relaxation with possible strengthening of the material by the property of creep limit. The possible cases of deformation of a certain local zone of the side surface of the piston in an elastic setting and in the presence of creep and stress relaxation processes are considered. It is shown that the process of creep deformation in the studied zone can end before the occurrence of reliability loss even for the materials that are unable to be strengthened by the property of creep limit or could be strengthened only insignificantly. The general ways of increasing the reliability of the side surface of the piston are outlined, related to ensuring the level of local stresses being below the creep limit, ensuring the completion of the creep process before the clearance reaches the critical values, as well as achieving strengthening of the material by the property of creep limit, for materials capable of it, before the occurrence of creep in operation.

Deformation of a Ti-Nb alloy containing α"-martensite and omega phases

Microscopic deformation of a Ti-17at. %Nb alloy with high fractions of α″-martensite and ω phases was studied by in-situ synchrotron X-ray diffraction. Textures, phase fractions, individual lattice strains, and peak intensities during deformation were studied. It is found that, to accommodate the external tensile strain, some of the α″ and ω grains were first transformed to the β-phase, which then continuously transformed to the α″-phase with chosen variants that effectively accommodate the deformation strain. A strong (010)α″ fiber texture was formed at the expense of the (001)α″ and (111)α″ fiber textures. Above 400 MPa applied stress, (110)α″ deformation twinning was triggered with a simultaneous stress relaxation in the (110)α″ family and a slight increase in its texture strength in the loading direction.

Modelling of Precipitation Kinetics with Simultanous Stress Relaxation

AbstractMisfit strain connected with the formation of large internal stresses may drastically influence the precipitation kinetics. A new model for the simultaneous precipitate growth and misfit stress relaxation in binary systems has been developed based on the idea that the internal stresses are relaxed by generation or annihilation of vacancies at the matrix/precipitate interface. The model is applied to the Fe-C system by considering the growth of the cementite precipitate in the ferritic matrix. The influence of the stress relaxation on the precipitate growth kinetics is demonstrated.

Residual Stresses in Amorphous Polymers

AbstractThe origin of internal stresses in a polymeric component, subjected to an arbitrary cooling, will be described. The differential volume relaxation arising as a result of the different thermal history suffered by each body point was considered as the primary source of stresses build up. A numerical routine was developed accounting for the simultaneous stress relaxation processes and implemented within an Ansys® environment. The volume relaxation kinetics was modelled by using the four‐parameter TNM (Tool‐Narayanaswamy‐Mohynian) phenomenological theory using the PVT (Pressure–Volume‐Temperature) data obtained on a polystyrene sample. The numerical algorithm translates the specific volume theoretical predictions at each body point as applied non‐mechanical loads acting on the component. Thermorheological complexity emerged due to coupling of volume and stress relaxation phenomena A linear dependence of Poisson's ratio on the tensile relaxation modulus was postulated. A quantitative predictions of time dependent stress distribution was realized for two specific test cases.

Numerical evaluation of structural relaxation-induced stresses in amorphous polymers

Abstract An attempt to calculate the internal stresses in a cylindrically shaped polystyrene component, subjected to an arbitrary cooling rate, will be described. The differential volume relaxation arising as a result of the different thermal history suffered by each body point was considered as the primary source of stresses build-up. A numerical routine was developed accounting for the simultaneous stress relaxation processes assuming thermorheological simplicity. The volume relaxation kinetics were modelled by using the four-parameter TNM (Tool–Narayanaswamy–Mohynian) phenomenological theory, with optimum estimated values of the four model parameters, ( A , Δ h , x , β ), obtained, once and for all, fitting the pressure–volume–temperature (PVT) liquid dilatometer measurements. The numerical routine was implemented within an Ansys environment. The algorithm translates the predictions of the TNM model for the volume at each body point, as applied non-mechanical loads, that acting on the component generates a complex stress state possibly affecting the integrity of the system. Stress relaxation effects were modelled by a weighted exponential series. Specific boundary and initial conditions were also defined for the considered test case.

Stress Effects and Non-Linearities in Dwfusional Mixing of Multilayers

ABSTRACTIn classical treatments of diffusional mixing of multilayers it is usually supposed that i) the intrinsic diffiision coefficients, Di, are independent of concentration, and they are equal (the Kirkendall shift is zero) [1]; ii) the stress effects are discussed in the framework of the CahnHilliard--theory [1,2]; iii) the deviation from the continuum description can be analysed for a sinusoidal concentration distribution in the discrete Fick equations. These limitations are discussed here and numerical simulations are used to show the effects of the strong concentration dependence of Di. The role of stresses are discussed in the framework of the treatment given by Stephenson [3], which contains the effect of simultaneous stress relaxation (Kirkendall-shift) as well. For the correct description of the Kirkendall-effect a generalised analytical expression for the gradient energy term is also given. The consequences of above effects on the decay of the intensity of the first small angle Bragg peak, due to the concentration modulation, is illustrated.

Influence of internal stresses on crystallization of intergranular phases in ceramics

Crystallization of the secondary phase in structural ceramics may lead to improved high temperature properties. Internal stresses due to both volume changes upon crystallization and cooling can result in microcrack formation or crack bridging and crack deflection processes affecting mechanical properties. Models for crystallization and stress evolution are developed which treat (i) the equilibrium degree of crystallization for non-relaxed volume mismatch stresses involving the influence of elastic constants, supercooling, volume change and amount of secondary phases on the crystallized volume fraction, (ii) the kinetics of the degree of crystallization leading to the equilibrium state, (iii) possible relaxation mechanisms of stresses involving (a) viscous flow of the intergranular phase and (b) solution-reprecipitation of primary grains and (iv) the crystallization kinetics controlled by simultaneous stress relaxation.

Simultaneous Stress Relaxation in Tension and Creep in Torsion of 2618 Aluminum at Elevated Temperature

Experiments are reported for stress relaxation and simultaneous stress relaxation and creep with proportional and nonproportional loading and unloading. Results were compared with predictions of a viscous-viscoelastic theory, and modifications, and strain hardening. Predictions were calculated from results of combined constant-stress tension and torsion creep and recovery tests only, which were reported previously. Results showed that a modified viscous-viscoelastic theory predicted all the observed features and predicted the creep and relaxation rates reasonably well.

Thermal stresses in fibre reinforced composites

A theoretical analysis is made of the dimensional changes, stresses and plastic flow accompanying an allotropic transformation in one component of a fibre composite, taking into account the simultaneous stress relaxation which occurs. The theory predicts the transformation dimensional changes observed experimentally under various temperature cycling conditions and provides an estimate of the stresses. Extending the theory to include the effect of a variation in temperature, several categories of coefficients of thermal expansion are obtained, none of which correspond to the rule of mixtures. The predicted transition from one coefficient of expansion to another during continuous cooling is confirmed experimentally and the resulting dimensional changes verified.

Behavior of Nonlinear Viscoelastic Material Under Simultaneous Stress Relaxation in Tension and Creep in Torsion

An experimental and analytical investigation is presented for simultaneous stress relaxation in tension and creep in torsion of polyurethane in the nonlinear range of stresses. The method employed a multiple integral approach with an assumed product form of kernel function to describe creep behavior. The required constants were determined from pure creep experiments on polyurethane. The tension stress and shearing strain versus time for simultaneous stress relaxation and creep were computed from results of these pure creep tests alone, and the results were compared with experiments on the same polyurethane under simultaneous stress relaxation and creep. In the method of analysis, direct inversion of the equation for creep was used as a first approximation for relaxation. Means for obtaining successive approximations and for accounting for cross effects are described. The second approximation was found to be adequate to describe the observed behavior very satisfactorily.