AN EXPERIMENTAL STUDY OF FRETTING BY MEANS OF X-RAY DIFFRACTION

This paper introduces a new method of investigation in the field of fretting. This method employs X-ray diffraction for the characterization of surface layer damage through residual stresses and work-hardening by tribologjcal action. The effect of hardness and material residual stresses upon the state of the surface have been studied. The results showed that fretting, being a surface phenomenon, created work-hardening of soft material or work-softening of hard material in the near-surface layers. When the initial residual stresses were high, fretting could not modify them because they had already reached the saturation limit. A Pole figure shows that fretting modified the texture of the specimen surface.     

Effects of material non-linearity on the residual stresses in a dendritic silicon crystal ribbon

Thermal stresses developed in a dendritic silicon crystal ribbon have been shown to cause plastic deformation and residual stresses in the ribbon. This paper presents an implementation of a numerical model proposed for thermo-elasto-plastic behaviour of a material. The model has been used to study the effects of plasticity of silicon on the residual stresses. The material properties required to implement this model are all assumed, and the response of the material to the variations in these assumed parameters of the constitutive law and in the finite element mesh is investigated. The steady state growth process is observed to be periodic with non-zero residual stresses. Numerical difficulties are also encountered in the computer solution process, resulting in sharp jumps and large oscillations in the stress responses.     

Stochastic fatigue damage accumulation in a T-welded joint accounting for the residual stress fields

The residual stresses that occur as a result of nonhomogeneous heating and cooling during welding may have a significant effect on the accumulation of fatigue damage in a welded joint. The problem is complicated not because of the complex spatial distribution of the residual stress fields, but because those fields typically change under an applied load. The present study considers the effect of residual stresses on fatigue damage accumulation in a welded joint subjected to stochastic loading.The influence of residual stresses on stochastic fatigue damage accumulation is accounted for by a simple approach based on an elastic–perfectly-plastic material model and the Gerber correction factor. The model assumes that the residual stress remaining at the critical location depends on the largest nominal stress ever endured by a welded joint. The model predicts that the residual stresses during stochastic loading randomly decay to zero. The effect of material yielding is additionally investigated by considering an elastic–plastic material model with linear kinematic hardening. The residual stresses in this case are computed through Monte Carlo simulations. It is demonstrated that the effect of material hardening is to reduce the rate of residual stress decay and thus to accelerate the rate of fatigue damage accumulation.     

Shakedown limit of rail surfaces including material hardening and thermal stresses

Sliding friction between railway wheels and rails results in elevated contact temperatures and gives rise to severe thermal stresses at the wheel and rail surfaces. The thermal stresses have to be superimposed on the mechanical contact stresses. Due to the distribution of stresses, the rail surface is generally subjected to higher stresses than the wheel surface. The elastic limit is reduced and yield begins at lower mechanical loads. During the first cycles of plastic deformation, the material hardens and residual stresses build up. The residual stresses provide the structure to shake down to pure elastic behaviour in subsequent load cycles up to a shakedown limit. The kind of hardening observed for rail steel has a considerable influence on the shakedown limit. The shakedown limit is dropped to lower mechanical loads due to the thermal stresses in the rail surface as well. This might cause structural changes in the rail material and rail damage.     

Numerical simulation of plasticity induced fatigue crack opening and closure for autofrettaged intersecting holes

The autofrettage of intersecting holes leads to extremely high compressive residual stress fields. These stresses in combination with the plastic deformations decelerate fatigue cracks initiated at the hole intersection notch. Simulations of plasticity induced crack closure of such cracks are presented based on the strip yield and a finite element model. The strip yield model has been extended to allow for an input of residual stresses coming from elsewhere, e.g. from a finite element calculation or measurements. The calculations are applied for constant as well as variable amplitude loading. The numerical expense of the finite element based modelling for variable amplitude loading is still too high if millions of cycles have to be considered. Therefore, a new approximation method is proposed introducing compensatory load sequences. Simulation results are compared to experimentally determined results showing good agreement. However, the accuracy of crack initiation life estimates has turned out to provide a high potential for further improvement.     

Prediction of welding distortion in butt joint of thin plates

This paper investigates distortions and residual stresses induced in butt joint of thin plates using Metal Inert Gas welding. A moving distributed heat source model based on Goldak’s double-ellipsoid heat flux distribution is implemented in Finite Element (FE) simulation of the welding process. Thermo-elastic–plastic FE methods are applied to modelling thermal and mechanical behaviour of the welded plate during the welding process. Prediction of temperature variations, fusion zone and heat affected zone as well as longitudinal and transverse shrinkage, angular distortion, and residual stress is obtained. FE analysis results of welding distortions are compared with existing experimental and empirical predictions. The welding speed and plate thickness are shown to have considerable effects on welding distortions and residual stresses.     

A new method of calibrating strain release coefficients in measuring welding stresses

The measured values of welding residual stresses often exceed material yield strength (σ_y) or tensile strength (σ_t), in measuring welding residual stresses in the welded joints of low carbon steel or stainless steel by drilling a blind hole. It is quite evident that measured values do not represent reality. Local plastic strain caused by residual stress concentrations round the blind hole has a main effect on the measuring error of welding residual stresses. This article presents a new method of calibrating strain coefficients A and B for eliminating influence of local plastic strain. In calibrating strain release coefficients A and B, calibrating stresses applied on the specimen were increased so that material round the blind hole produced local plastic deformation, ie coefficients A and B contained a component of the plastic strain. Numerial values of coefficients A and B were classified into four grades. Every grade coefficient contained a different quantity of the plastic strain for offseting the influence of the plastic strain round the blind hole. Test results have proved that by adopting these coefficients more satisfactory values for residual stresses can be obtained in practical engineering. The measuring average error in stainless steel (1Cr18Ni9Ti) reduces from 109% to 1.52%. The measuring average error in low carbon steel reduces from 54.69% to 3.21%.     

Fatigue behaviour of an autofrettaged high-pressure vessel for the food industry

The autofrettage technique is commonly used to produce compressive tangential residual stresses near the bore of high-pressure vessels. These compression stresses improve the fatigue life of the vessel during the loading–unloading high-pressure cycles. The present paper presents the fatigue design of an autofrettaged thick-walled vessel for the food industry, working at an internal pressure of 500 MPa. A finite element analysis has been performed in order to obtain the residual stresses after the autofrettage at an internal pressure of 925 MPa. The material of the vessel was a 15-5PH stainless steel hardened by precipitation, which shows a strong Bauschinger effect. For FE simulations, the material has been modelled considering an elastic–perfectly plastic behaviour for the loading phase and a Ramberg–Osgood behaviour for the unloading phase, with its coefficients depending on the previous equivalent plastic strain reached during the loading process. The simulation procedure is explained in detail. Finally, the fatigue life of the vessel was obtained using the residual stresses obtained in the previous simulations stage.     

Dugdale plastic zone of a penny-shaped crack in a piezoelectric material under axisymmetric loading

The Dugdale plastic zone ahead of a penny-shaped crack in a piezoelectric material, subjected to electric and axisymmetric mechanical loadings, is evaluated analytically. Hankel transform is employed to reduce the mixed boundary-value problem of the penny-shaped crack to dual integral equations, which are solved exactly under the assumption of electrically permeable crack face conditions. A closed-form solution to the mixed boundary-value problem is obtained to predict the relationship between the length of the plastic zone and the applied loading. The stress distribution in and outside of the yield zone has been derived analytically, and the crack opening displacement has been investigated. The electric displacement has a constant value in the strip yield zone. The current Dugdale crack model leads to non-singular stress and electric fields near the crack front, and it is observed that the material properties affect the crack opening displacement.     

Plastic buckling with residual stresses—1. Postbifurcation behaviour

The influence of residual stresses on the bifurcation load and initial postbifurcation behaviour of elastic—plastic structures is studied. The applied load is supposed to be proportional to a monotonically increasing parameter and the 'perfect' version of the structure is assumed to display an 'abrupt' bifurcation, which takes place when the material is in the plastic range. A strain-hardening flow theory of plasticity for solids characterized by a smooth yield surface is employed as constitutive relation. Symmetric self-equilibrated residual stress distributions are onsidered and the analysis is based on Hill's incremental approach to bifurcation and on its extension into the initial postbifurcation range as developed by Hutchinson. The proposed method consists of an investigation of the effect of residual stresses on the lowest possible bifurcation load and on the first two terms of an asymptotically exact postbifurcation expansion. It is also possible to obtain estimates of the maximum load supported by structures containing residual stresses. An illustration of the theory is provided by two problems, namely a continuous spring model and an axially compressed cylindrical column. The analytical results are compared with fully numerical solutions.     

Influence of Bauschinger effect on springback and residual stresses in plane strain pure bending

A finite deformation elastic plastic analysis of plane strain pure bending of a wide sheet is presented. The general closed-form solution is proposed for a kinematic hardening law assuming that the material is incompressible. The stage of unloading is included in the analysis to investigate the influence of the Bauschinger effect, elastic properties of the material and process parameters on the distribution of residual stresses and springback. A simple example is provided to illustrate the procedure for finding the solution and some quantitative features of the process.     

Influences of grain size and precracking load on the critical stress intensity factor of mild steel

The effects of grain size and precracking load on the critical stress intensity factor are studied. A plane stress model of elastic-plastic stress distribution which includes the strain hardening effects is used. The effects of residual stresses and strain hardening due to fatigue load are calculated by choosing plastic zone size as fracture criterion. Experimental results are obtained to demonstrate the reliability of theoretical calculations.     

Thermo-elastic-plastic porous material undergoing thermal loading

A model for predicting elastic–plastic stresses within a surface-heated porous structure has been developed. The relevant phenomena for the moisture, pressure, temperature and displacement fields in thermo-elastic-plastic porous material are analysed. Considering mass and energy transfer processes, a set of governing differential equations is presented. The solution of the problem has been obtained with a finite difference scheme. The results demonstrate the influence of the evaporation mechanism on pressure and thermal stresses within the porous material.     

Residual stresses arising in materials deformed by bending in surface-active media

The distribution of residual stresses in rectangular beams after loading by clear bending in inactive, surface-plasticizing and surface-hardening media was investigated. It was found that the residual stresses in a more plastic structural component are typically opposite in sign to active load stresses, whereas in a harder component they are coincident with the latter. The influence of residual stresses on the point defect distribution in the samples after deformation is discussed.     

Stresses produced on a rough irregular half-space by a moving load

The stresses developed in a body due to a moving load causing fracture are an interesting problem of mechanics having its application toward the stability of a medium. This paper is concerned with the stresses developed in an irregular isotropic half-space due to a normal moving load at a rough free surface. The irregularity has been taken in the form of a parabola and a rectangular irregularity has also been discussed as a special case of parabolic irregularity. Closed-form expressions for the normal and shear stresses have been obtained. The effect of friction, irregularity factor, and maximum amplitude of irregularity has been discussed for both stresses.     

Experimental and analytical study of residual stresses relaxation in nitrided 42CrMo4 parts

Initial compressive residual stresses, induced by ionic nitriding treatments, relax during component operating life and it is important to consider the relaxation phenomenon in the design of the component. The paper presents three analytical models (Seungho Han et al., Zaroog and Kodama) to estimate the level of residual stress after cyclic loading. The initial and final residual stresses has been measured after each loading cycle for many load magnitudes using x‐ray diffraction. It was found that about 40 %–50 % of the maximum residual stresses are relaxed during the first cycle. The reduction in the residual stress, are dependent on the applied load because of the plastic deformation caused by the superposition between the nitrided residual stress and the applied load. It was found that the estimated average values by analytical models are comparable with the results of experimental studies after cyclic loading.     

Development of a method of accounting for the effect of stress concentration and residual stresses on the cyclic longevity of steel 10GN2MFA. Report 1. Analysis of material's stress—Strain state

The stress-strain state induced in a material due to a variable load and residual stresses is calculated for the elastoplastic case. The need is demonstrated and a method proposed for the accounting of residual stresses in calculations involving the cyclic longevity of structural components. The resultant relationship makes it possible to account for the material's plastic properties under static and cyclic loading.Translated from Problemy Prochnosti, No. 8, pp. 3–13, August, 1993.     

Impact of the non-homogenous temperature distribution and the coatings process modeling on the thermal barrier coatings system

The present study deals with a numerical investigation of the residual stresses arising during the plasma-sprayed coatings process and their effects on the final stress state of the thermal barrier coatings system (TBCs) during service. A new thermo-mechanical finite element model (FEM) has been designed to function using a non-homogenous temperature distribution. Several phenomena are taken into account in the model such as: residual stresses generated during the spraying of coatings, morphology of the top-coat/bond-coat interface, oxidation at the top-coat/bond-coat interface, thermal mismatch of the material components, plastic deformation of the bond-coat and creep of all layers during thermal cycling. These phenomena induce local stresses in the TBCs that are responsible of micro-crack propagation during cooling and thermal cycling, specifically near the ceramic/metal interface.