Initial Residual Stress Field Research Articles

Gradient residual stress and fatigue life prediction of induction hardened carbon steel S38C axles: Experiment and simulation

Gradient distribution of triaxial residual stresses to a depth of several millimeters is retained in middle carbon steel S38C axles after high-frequency induction hardening, which has become a critical concern for fatigue structural integrity. To address this, the axial, hoop, and radial gradient residual strains inside the axles were measured for the first time by advanced neutron diffraction. The SIGINI Fortran subroutine was then adopted to reconstruct the global initial residual stress field from the measured data. Experimental and simulation results show that residual stresses of about −520 MPa (axial), −710 MPa (hoop), and −40 MPa (radial) residual stress were retained below the axle surface. Subsequently, the fatigue crack propagation behavior of S38C axles was numerically investigated in the framework of fracture mechanics. The calculated results clearly show that the compressive residual stresses at a depth of 0–3 mm from the axle surface lead to a low crack growth driving force, and that fatigue cracks do not propagate as long as the crack depth is less than 3.7 mm for hollow S38C axles. These results further indicate that the maximum defect size allowed in routine inspections is acceptable from a safety and economic point of view. Accurate measurement and characterization of the global gradient residual stress field through experiments and simulations can provide an important reference for optimizing the mileage intervals of nondestructive testing (NDT) of surface defects in these surface-strengthened railway axles.

Efficient residual stress mitigation in additively manufactured 18Ni300 maraging steel

This study explored an effective post-heat treatment (PHT) strategy for achieving optimal residual stress relief in 18Ni300 maraging steel. The primary focus was to investigate and enhance the mechanisms underlying residual stress relief in distinct PHT processes, including aging and annealing-aging. The investigation considered various parameters, including PHT conditions, component size, and initial residual stress field. Components with cubic geometry were fabricated through laser powder bed fusion (LPBF) and subjected to both aging and annealing-aging processes. The residual stresses were extracted from these specimen variants using the contour method. The simulation involved generating the initial residual stress in the LPBF using the flash heating method and simulating the PHT process using the Arrhenius equation-based creep model. Simulations complemented and elucidated the experimental results. The validated model was employed to comprehensively investigate the intricate interrelationships among the various PHT parameters. The results indicate that the annealing-aging process leads to a substantial reduction in the residual stress compared to the aging process. Furthermore, a process map was established by considering the PHT conditions and component size. This map aids in determining the optimal PHT conditions for achieving the desired levels of stress reduction. Therefore, the present study comprehensively explored the PHT effect and its interrelated parameters, suggesting an effective PHT strategy for residual stress mitigation in LPBF, and particularly a process map that is beneficial for practical applications.

Use of infrared thermography to model the effective stress ratio effect on fatigue crack growth in welded T-joints

Welded joints constitute critical components in the high-cycle fatigue dimensioning of naval structures. In particular, the local geometry of the weld toe as well as the welding process induced residual stress strongly impact the fatigue properties. In that regard, such factors must be considered in a dedicated fatigue life forecast model, identified via an adapted experimental protocol. On the basis of X-ray stress analysis and in-situ crack growth monitoring by thermoelastic stress analysis, the present study introduces a linear elastic fracture mechanics model that considers the initial residual stress field in the vicinity of the weld toe.

An Initial Residual Stress Inference Method by Incorporating Monitoring Data and Mechanism Model

Initial residual stress is the main reason causing machining deformation of the workpiece, which has been deemed as one of the most important aspects of machining quality issues. The inference of the distribution of initial residual stress inside the blank has significant meaning for machining deformation control. Due to the principle error of existing residual stress detection methods, there are still challenges in practical applications. Aiming at the detection problem of the initial residual stress field, an initial residual stress inference method by incorporating monitoring data and mechanism model is proposed in this paper. Monitoring data during machining process is used to represent the macroscopic characterization of the unbalanced residual stress, and the finite element numerical model is used as the mechanism model so as to solve the problem that the analytic mechanism model is difficult to establish; the policy gradient approach is introduced to solve the gradient descent problem of the combination of learning model and mechanism model. Finally, the initial residual stress field is obtained through iterative calculation based on the fusing method of monitoring data and mechanism model. Verification results show that the proposed inference method of initial residual stress field can accurately and effectively reflect the machining deformation in the actual machining process.

FEM assessment of the local side compression technique efficiency as applicable for notched prismatic specimens

A crack front straightness is one of the test result validity criteria for fatigue precracked static fracture toughness specimens. Actually, the ideally straight crack front cannot be reached due to the presence of residual stress. This is particularly actual for specimens cut out of welded joints containing the residual welding stress (RWS). One of the techniques allowing to lower the RWS effect is a local side compression of specimens. Its efficiency has been proved in physical testing however no quantitative assessments are known in the literature. This work comprises FEM simulation of welding, sampling and side compression processes. The effect of local compression on base metal containing no residual stress is also investigated.It has been found that in the course of local side compression the initial residual stress field caused by welding and specimen making is replaced by another field showing stress gradients more favourable for getting the fatigue crack shape meeting the validity criteria of test results as per approved test methods. The calculation results show that the complete removal of residual stress as in base metal as in welded specimens is not feasible in the range of actual practicable degrees of compression.

Residual Stress Regenerated on Low Plasticity Burnished Inconel 718 Surface After Initial Turning Process

Abstract Low plasticity burnishing (LPB) has been extensively employed in aero-industry to enhance fatigue performance of machined components by introducing compressive residual stress. Effects of various parameters on the residual stress field induced by low plasticity burnishing have been investigated by many researchers. However, initial residual stresses induced by machining are one of the important factors which affect the residual stress regenerated by the LPB process. The present work aims to develop an analytical model which takes into account the initial residual stress and burnishing parameters to predict residual stress field of workpiece material Inconel 718 based on Hertz contact theory and elastic–plastic theory. Initial residual stress fields were produced by turning of Inconel 718 and were measured by using X-ray diffraction technique. Two types of material constitutive models such as the linear hardening model and isotropic–kinematic model were employed to describe the elastic–plastic behavior of workpiece material Inconel 718. An analytical study was performed to analyze the effect of the initial residual stress field and burnishing parameters on residual stress induced by low plastic burnishing. The results of analytical model were verified by conducting the LPB experiments on initial turned Inconel 718. The results showed that the shape and magnitude of the residual stress field obtained with considering the effect of initial residual stress field was in good accordance with experimental measurements.

Weld Magnification Factor Approach in Cruciform Joints Considering Post Welding Cooling Medium and Weld Size.

The objective of this research is to develop an experimental-theoretical analysis about the influence of the cooling medium and the geometry of the welding bead profile in fatigue life and the associated parameters with structural integrity of welded joints. A welded joint with cruciform geometry is considered using SMAW (Shielded Metal ArcWelding), plates in structural steel ASTM A36 HR of 8 mm of thickness, and E6013 electrode input. A three-dimensional computational model of the cruciform joint was created using the finite element method. For this model, the surface undulation of the cord and differentiation in the mechanical properties of the fusion zone were considered, the heat-affected zone (HAZ) and base material, respectively. In addition, an initial residual stress field, which was established experimentally, was considered. The results were a set of analytical expressions for the weld magnification factor Mk. It was found that values for the latter decrease markedly in function of the intensity of the cooling medium used in the post welding cooling phase, mainly due to the effect of the residual compressive stresses. The obtained models of behavior of the weld magnification factor are compared with the results from other researchers with some small differences, mainly due to the inclusion of the cooling effect of the post weld and the variation of the leg of the weld bead. The obtained analytical equations in the present research for Mk can be used in management models of life and structural integrity for this type of welded joint.

Non-linear simulation of coiling accounting for roughness of contacts and multiplicative elastic-plastic behavior

In this paper numerical simulations of coiling (winding of a steel strip on itself) and uncoiling are developed. Initial residual stress field is taken into account as well as roughness of contacts and elastic-plastic behavior at finite strains, considering the Tresca yield function and isotropic hardening. The main output is the residual stress field due to plastic deformations during the process. This enables to quantify additional flatness defects. The presented coiling simulation relies on a modeling strategy that consists in dividing each time step into two sub-steps. Each sub-step can be solved semi-analytically and numerical optimizations enable to obtain a general solution. Thus, reasonable computation times are reached and parametric studies can be performed in order to develop coiling strategies considering the process parameters. Comparisons with previous models from the literature are presented. Moreover, the comparison with a Finite Element simulation presents the same order of magnitude, however, it shows that direct computations using classical FE codes are difficult to perform in terms of computation times and stability if an explicit integration scheme is chosen. Numerical results are also given in order to determine the effect of some parameters such as roughness, yield stress, applied force, strip crown or mandrel’s radius.

Asymptotic residual stresses in butt-welded joints under fatigue loading

If a weld toe is modelled as a sharp V-notch angle, the stress distribution near the notch tip is singular. Its intensity can then be quantified by means of Notch Stress Intensity Factors (NSIF), which have been proven to be capable of summarising the high-cycle fatigue strength of welded joints having very different global and local geometries. In presence of a singular residual stress field near the weld toe, the local load ratio is modified making the fatigue strength sensitive to residual stresses in the high-cycle regime. However, for an accurate estimation of the fatigue performance of welded joints, it is necessary to consider not only the initial residual stress field but also its variation under load, as both of these may play an important role. In this work the effect of fatigue loading on the asymptotic residual stress redistribution near the weld toe of a butt-welded joint is studied by means of numerical simulations. A model is then proposed to estimate the influence of residual stresses on the fatigue strength of welded joints. Experimental results taken from the literature were found in good agreement with those predicted by the proposed model.

Slip localization and fatigue crack nucleation near a non-metallic inclusion in polycrystalline nickel-based superalloy

Fatigue crack nucleation at a non-metallic agglomerate inclusion has been studied by high spatial resolution Digital Image Correlation (HR-DIC) and high angular resolution Electron Backscatter Diffraction (HR-EBSD). Spatial and temporal characterization and correlation of deformation with underlying microstructures has been performed, with distributions of plastic strain measured from HR-DIC; and residual stress and density of geometrically necessary dislocations (GND) measured from HR-EBSD. Initial residual stress and GND fields, as a consequence of differing thermal expansivities in the metallic and oxide phases, localized around the agglomerate have been quantified using HR-EBSD. The localization of the pre-existing stress and dislocation states appear to lead to early onset of plasticity upon subsequent mechanical loading. Heterogeneous distributions of plastic strain have been observed in the course of the fatigue test by HR-DIC. Crack nucleation via agglomerate/nickel interface decohesion and particle fracture has been demonstrated and this is correlated with the elevation in strain and dislocation density. The measurements of residual stress, strain, and dislocation density provide key information for the mechanisms of fatigue cracking and the development of damage nucleation criteria in these material systems.

FEM and experiment-based initial residual stress field construction for titanium alloy monolithic component blank

An annealing process analysis of titanium alloy monolithic component blank was proposed based on finite element method (FEM), which was employed to construct initial residual stress field. Several key technologies, such as sequentially coupled thermal and mechanical procedure, heat conduction equation, boundary condition and fundamental equations, have been implemented to improve the accuracy of finite element simulation. To verify finite element simulation results, the stress experiment was performed by combining stripping layer method with X-ray diffraction technique. The predicted residual stress values were compared with experimental data and found to be in good agreement, indicating that the initial residual stress field constructed in this paper was right. The research results show that the margin region of blank cools the fastest and next is the surface layer, while the centre region cools the slowest; there is large thermal stress difference between surface layer and centre region along thickness direction, while the small difference along the length and width direction; the stress radiates from the centre region to outside, the centre region being the maximum value, then decreases gradually, until a minimum value is attained at the corner.

Welding Residual Stress Behavior in Tubular Steel Joints under Multiaxial Loading

AbstractThe lack of insight into the initial welding residual stress field and its behavior during the different phases of the fatigue damage namely crack initiation and propagation has led to conservative assumptions in fatigue design codes of welds. Since a significant amount of fatigue failures in welded joints are caused by torsion or combined tension-torsion in machinery components, estimating the potential threat of the inevitable residual stresses to structural integrity seems to be mandatory for the design of the future tubular lightweight welded components and structures. In this paper the residual stresses in cylindrical specimens with bead on tube welds out of S355J2H were determined experimentally by means of X-ray and neutron diffraction. Welding residual stresses as high as almost the yield strength of the base metal were observed in the weld bead. At the weld toes which are potentially fatigue crack initiation sites, however compressive residual stresses were present. After discussing briefly the sources and origins of the residual stress field, its behavior under pure torsional and combined tension-torsion loading will be presented.

Welding Residual Stresses in Tubular Joints

In spite of an increased awareness of welding residual stress threat to structural integrity, the extent of its influence on fatigue especially under multiaxial loading is still unclear and is a matter of debate. One important reason for this lack of clarities is that the determination of the initial welding residual stress field in welded structures even at the fatigue crack initiation sites is difficult and requires complementary instruments. Since the fatigue crack initiation in sound welds almost always occurs on the surface, the determination of surface residual stresses could increase the awareness of the extent of their threat to the structural safety. In this paper the development of residual stresses in different TIG-welded tubular specimens out of S355J2H and S690QL steel is studied and compared. The mechanisms of the development of residual stresses based on heat input and cooling rate are discussed. The welding parameters and thus heat inputs are varied and the mechanisms leading to different residual stress states are investigated. X-ray method was used for residual stress state characterization.

Welding residual stress behavior under mechanical loading

For an accurate estimation of the fatigue performance of welded joints, not only the initial residual stress field but also its variation under load is decisive. The portion of residual stress which stays stable can shift the load stress range much the same way as the mean stresses do in fatigue loading. The relaxation of residual stresses during fatigue loading however reduces this hazard to the structural health. That is, before considering the influence of residual stresses in fatigue, the effect of fatigue on residual stresses should be understood. In this work, the influence of the quasi-static and cyclic loading on the relaxation of welding residual stresses in small specimens and large components out of different steels namely S235JRG2, S355J2G3, P460NL, S690QL, S960QL, and S1100QL has been investigated experimentally. X-ray diffraction analysis as a reliable method has been used for the determination of residual stress profiles in surface layers. For residual stress analysis in deeper layers, synchrotron and neutron diffraction techniques were applied as complementary methods. A clear recognition of the difference between initial welding residual stresses in small- and large-scale specimens was observed. Tensile residual stresses as high as the yield strength could appear in large-scale welded specimens. That was not the case for small-scale welds due to the low grade of restraint. Nevertheless, in both types of samples, the highest residual stresses were obtained in the weld centerline. At the weld toe which is critical regarding fatigue crack initiation, the magnitude of the residual stresses is lower. Beside of that, it was observed that in the case of relaxation, the first load cycles especially in components out of low-strength steels are decisive. The influence of loading conditions and local mechanical properties on the relaxation of welding residual stresses was investigated based on principles of solid mechanics. The von Mises failure criterion was able to describe the relaxation behavior.

Finite Element Simulation and Analysis of Initial Residual Stress Field for Titanium Alloy Monolithic Component Blank

In this paper, the initial residual stress field for titanium alloy monolithic component blank is simulated and analyzed by applying a sequentially coupled thermal and mechanical procedure based on finite element method. The control equation of heat conduction of annealing process is studied, and the boundary condition and fundamental equations of simulation are given. The research results show that the initial residual stress field for titanium alloy monolithic component blank radiates from the center region to outside, the center region being the maximum value, then decreases gradually, until a minimum value is attained at the corner.

Finite element validation of the deep hole drilling method for measuring residual stresses

The influence of free surfaces, local geometry and plasticity on the residual stress analysis by the deep-hole drilling (DHD) method is addressed in this paper. A finite element (FE) analysis using the commercial software ABAQUS was used to model a number of selected specimens that had been subjected to thermal process conditions that create initial residual stress fields. The essential steps of the DHD method were then simulated at a selected number of locations through the specimens. Two sets of analyses were conducted; one set assumed only that the process of simulating the DHD method was entirely elastic. The second set assumed that the DHD process was elastic–plastic. In the majority of the test cases the conventional DHD method is shown to provide reconstructed in-plane residual stresses that agreed well with the initial residual stresses and did not suffer from expected near free-surface or geometric effects. The results for the out-of-plane residual stresses were however dependent on the assumed state of stress and the interpretation of the axial distortions. The reconstructed out-of-plane residual stresses were shown to have limited agreement with the initial stresses. For the elastic–plastic study, results from a ring welded cylinder were successfully mapped on to a DHD model. A new incremental technique was shown to reconstruct the initial in-plane residual stresses with reasonable accuracy with only limited success for the out-of-plane stress.

Finite Element Validation of the over-Coring Deep-Hole Drilling Technique

The main objective of the present study is to validate a simple over-coring deep-hole drilling (oDHD) residual stress measurement technique by utilising finite element simulations of the technique. A number of three dimensional (3D) finite element analyses (FEA) were carried out to explore the influence of material removal and the cutting sequence during the deep-hole drilling (DHD) residual stress measurement process on the initial residual stress field. Two models were considered in the study. First, the residual stress field predicted in a rapid spray water quenched solid cylinder was used as the initial stress field for the DHD FE model. The DHD reconstructed residual stresses were compared with the initial FE predicted stresses. Different cutting sequences and different dimensions were systematically simulated before arriving at an optimum solution for the oDHD technique. The oDHD technique significantly improved the spatial resolution and was applied in a second model consisting of a 40mm thick butt-welded pipe. The DHD reconstructed residual stresses compared very well with the initial FE predicted weld residual stress thereby validating the oDHD technique.

Relaxation and Stability of Welding Residual Stresses in High Strength Steel under Mechanical Loading

AbstractThe relaxation of near surface welding residual stresses in S1100QL under static and cyclic mechanical loadings was studied. By increasing static loads the residual stress relaxes continuously. Under cyclic loading the residual stresses behave depending on the initial status, local yield strength and maximum applied load. If the von Mises stress as a function of residual stress, load and mean stresses, exceeds the local yield strength, plastic deformation and thus relaxation occurs. The first load cycle is of importance in which a considerable portion of the initial residual stress field relaxes. Afterwards regardless of whether in high or low cycle fatigue conditions, the relaxation rates were the same and comparatively smaller than the first relaxation. When no relaxation in the first load cycle occurred, the number of cycles did not change the residual stress. The specimens were loaded until failure or a maximum of 2x106 numbers of cycles.

Mechanisms of Residual Stress Relaxation and Redistribution in Welded High-Strength Steel Specimens under Mechanical Loading

In this article, the relaxation of surface and near-surface welding residual stress fields in S690QL under static and cyclic loadings has been studied. The influence of local mechanical properties in the weld metal, heat-affected zone and in the base metal upon relaxation and redistribution of residual stress were considered. Welded specimens were loaded statically and cyclically. Under static loading, the variation of the residual stress field under tension and compression was investigated and it was observed that, by increasing the load, residual stress relaxes continuously. However, the relaxation under compression sets in with delay, since the tensile residual stresses should be first overcome. Under cyclic loading, with or without mean stress, the behaviour of the residual stresses depends on the initial residual stress, local yield strength and maximum applied load. If the von Mises stress, which is a function of residual stress, load and mean stress, exceeds the local yield strength, plastic deformation and thus a relaxation occurs. In the case of relaxation, the first load cycle is of importance, in which a considerable amount of the initial residual stress field is relaxed. After the first relaxation, no significant variation of the residual stress field in the fatigue crack free phase could be observed. When no residual stress relaxation in the first load cycle occurred, the number of load cycles did not change the residual stress field. The specimens were loaded until failure or maximum 2×106 cycles.

On the residual stress field induced by a scratching round abrasive grain

Most of superfinishing processes are based on the action of abrasive grains scratching the surface of a workpiece. These processes are known to improve the surface texture but they also modify the surface integrity, and thus the durability of engineering parts. In this paper, the residual stress field induced by the scratch of a single round abrasive grain is investigated by means of an implicit finite element analysis. First, it is shown that the residual stress profile is always modified over a depth of two or three times the contact radius, whatever the mechanical and physical properties. Second, key-parameters having a high influence on the tensile and compressive zone have been identified. They are the contact severity, the friction coefficient and the workpiece hardness. Third, it appears that the residual stress profile is not really affected by the presence of initial homogeneous residual stress field in the material. These results are then used to explain the experimental residual stress fields induced by a belt finishing process and a honing process on a AISI52100 hardened steel. Finally the possibility to design appropriate abrasive tools to improve both surface texture and surface integrity of manufactured parts is discussed.