Residual Stress Profiles Research Articles

Residual Stress State of X65 Pipeline Girth Welds Before and After Local and Furnace Post Weld Heat Treatment

This research investigated the effects of global (in other words, furnace-based) and local post weld heat treatment (PWHT) on residual stress (RS) relaxation in API 5L X65 pipe girth welds. All pipe spools were fabricated using identical pipeline production procedures for manufacturing multipass narrow gap welds. Nondestructive neutron diffraction (ND) strain scanning was carried out on girth welded pipe spools and strain-free comb samples for the determination of the lattice spacing. All residual stress measurements were carried out at the KOWARI strain scanning instrument at the Australian Nuclear Science and Technology Organization (ANSTO). Residual stresses were measured on two pipe spools in as-welded condition and two pipe spools after local and furnace PWHT. Measurements were conducted through the thickness in the weld material and adjacent parent metal starting from the weld toes. Besides, three line-scans along pipe length were made 3 mm below outer surface, at pipe wall midthickness, and 3 mm above the inner surface. PWHT was carried out for stress relief; one pipe was conventionally heat treated entirely in an enclosed furnace, and the other was locally heated by a flexible ceramic heating pad. Residual stresses measured after PWHT were at exactly the same locations as those in as-welded condition. Residual stress states of the pipe spools in as-welded condition and after PWHT were compared, and the results were presented in full stress maps. Additionally, through-thickness residual stress profiles and the results of one line scan (3 mm below outer surface) were compared with the respective residual stress profiles advised in British Standard BS 7910 “Guide to methods for assessing the acceptability of flaws in metallic structures” and the UK nuclear industry's R6 procedure. The residual stress profiles in as-welded condition were similar. With the given parameters, local PWHT has effectively reduced residual stresses in the pipe spool to such a level that it prompted the thought that local PWHT can be considered a substitute for global PWHT.

Artificial neural network for correction of effects of plasticity in equibiaxial residual stress profiles measured by hole drilling

The hole drilling method is a widely known technique for the determination of non-uniform residual stresses in metallic structures by measuring strain relaxations at the material surface caused through the stress redistribution during drilling of the hole. The integral method is a popular procedure for solving the inverse problem of determining the residual stresses from the measured surface strain. It assumes that the residual stress can be approximated by step-wise constant values, and the material behaves elastically so that the superposition principle can be applied. Required calibration data are obtained from finite element simulations, assuming linear elastic material behavior. That limits the method to the measurement of residual stresses well below the yield strength. There is a lack of research regarding effects caused by residual stresses approaching the yield strength and high through-thickness stress gradients as well as the correction of the resulting errors. However, such high residual stresses are often introduced in various materials by processes such as laser shock peening, for example, to obtain life extension of safety relevant components. The aim of this work is to investigate the limitations of the hole drilling method related to the effects of plasticity and to develop an applicable and efficient method for stress correction, capable of covering a wide range of stress levels. For this reason, an axisymmetric model was used for simulating the hole drilling process in ABAQUS involving plasticity. Afterward, the integral method was applied to the relaxation strain data for determining the equibiaxial stress field. An artificial neural network has been used for solving the inverse problem of stress profile correction. Finally, AA2024-T3 specimens were laser peened and the measured stress fields were corrected by means of the trained network. To quantify the stress overestimation in the hole drilling measurement, an error evaluation has been conducted.

Evolution and empirical modeling of compressive residual stress profile after milling, polishing and shot peening for TC17 alloy

Compressive residual stress is important to improve the fatigue life of components. This paper proposed an empirical model to predict the compressive residual stress profile induced by the integration manufacturing processes (firstly milling, then polishing, finally shot peening). An exponential decay function was used to describe the compressive residual stress profile induced by milling process. Moreover, a sinusoidal decay function was proposed to describe the compressive residual stress profile induced by shot peening process. The integration manufacturing processes model was a deterministic function of the combination of exponential decay function, sinusoidal decay function, and their interaction term. Additionally, an impact coefficient was introduced to describe the influence of polishing process on compressive residual stress profile. The coefficients of the proposed models were related to the input machining parameters. Experiments of TC17 alloy were carried out utilizing response surface methodology and full factorial design to construct these models. Flank wear, tool inclination angle, axial depth of cut, shot peening intensity, and shot peening coverage were selected as five input machining parameters. According to the experimental results obtained, the evolution of compressive residual stress profile after the integration manufacturing processes was investigated, and the proposed models had been developed. The empirical model was validated by two extra experiments and a significantly good prediction was achieved.

Simulation on Residual Stress of Shot Peening Based on a Symmetrical Cell Model

The symmetrical cell model is widely used to study the residual stress induced by shot peening. However, the correlation between the predicted residual stresses and the shot peening coverage, which is a big challenge for the researchers of the symmetrical cell model, is still not established. Based on the dynamic stresses and the residual stresses outputted from the symmetrical cell model, the residual stresses corresponding to full coverage are evaluated by normal distribution analysis. The predicted nodal dynamic stresses with respect to four corner points indicate that the equi-biaxial stress state exists only for the first shot impact. Along with the increase of shot number, the interactions of multiple shot impacts make the fluctuation of the nodal dynamic stresses about an almost identical value more and more obvious. The mean values and standard deviations of the residual stresses gradually tend to be stable with the increase of the number of shot peening series. The mean values at each corner point are almost the same after the third peening series, which means that an equi-biaxial stress state corresponding to the full coverage of shot peening is achieved. Therefore, the mean values of the nodal residual stresses with respect to a specific transverse cross-section below the peened surface can be used to correlate the measured data by X-ray. The predicted residual stress profile agrees with the experimental results very well under 200% peening coverage. An effective correlation method is proposed for the nodal residual stresses predicted by the symmetrical cell model and the shot peening coverage.

A Study on Structural Integrity Assessment of Pipeline using Weight Function Solution

There are many Industry Code and Standard (ICS) for Structural Integrity Assessment (SIA) on welded structure with defect. The general ICSs, such as R6, BS 7910 and API 579-1/ASME FFS-1, provide equations to determine the upper bound residual stress profiles based on collections from many literatures. However, these residual stress profiles used in the SIA cause the conservative design for welded structures. In this study, the structural integrity assessment for girth weld in pipeline has been conducted based on fracture mechanics. In addition, thermo-elastic plastic FE analysis was performed for evaluating the residual stress of girth weld in pipeline. The weight function solution is used to determine the stress intensity factor using the residual stress profile obtained by the FE analysis. This approach can account for redistribution and relaxation of residual stress as the defects grow. In order to the evaluate quantitative comparison between BS 7910 and weight function solution, structural integrity assessment determining allowable crack size on cracked pipe was performed with failure assessment diagram. Key words: Structural Integrity Assessment, Welding residual stress, Weight function solution, FE analysis, Stress intensity factor

Numerical investigation of formation mechanism of welding residual stress in P92 steel multi-pass joints

Three plate joints, namely, single-pass weld, double-pass weld, and four-pass weld were fabricated using P92 steel. The hole-drilling strain-gauge method and X-ray diffraction technique were employed to measure the welding residual stresses. Optical microscope and Vickers hardness tester were used to characterize the micro-structure and hardness of these joints, respectively. A thermal-metallurgical-mechanical finite element method (FEM) based on SYSWELD software was developed to simulate welding residual stress in P92 steel joints. Using the developed computational approach, the welding residual stress profiles in single-pass, double-pass and four-pass welded joints were calculated. The effectiveness of the developed computational approach was verified by the experimental measurements. The longitudinal stress vs. temperature evolution in double-pass welded joint was also examined. Numerical experiments of Satoh test were performed to study the influence of solid-state phase transformation, peak temperature and re-heating on the evolution of residual stresses. The solid-state phase transformation has a significant influence on the formation of welding residual stress, and the peak temperature of final thermal cycle can determine the magnitude of final residual stress.

Numerical and experimental investigations on shot-peened high-strength steel by means of hole drilling, X-ray, synchrotron and neutron diffraction analysis

Abstract Numerous experimental works have been devoted to studying the influence of different shot peening parameters on the surface material conditions in different metals in the last decades. Most of the research work has been focused on experimental determination of the residual stress and its depth profile by means of X-ray diffraction and corresponding electro-polishing of the surface layers or byhole drilling method. This state of knowledge has led to the development of phenomenological models to describe the surface material conditions qualitatively. Since there are quite a number of parameters, which could influence the residual stress field after shot peening, covering the whole possible process parameters combinations with the purpose of experimentally determining the residual stress profiles could be difficult. A deeper insight into the residual stress states after shot peening could be possible on the basis of sound physical principles by means of numerical approaches. In this study, shot peening of high strength steel S690QL has been modeled and simulated. The results have been compared with the residual stress depth profiles determined by X-ray, synchrotron, neutron diffraction and hole drilling method.

Prediction of residual stresses in girth welded pipes using an artificial neural network approach

Management of operating nuclear power plants greatly relies on structural integrity assessments for safety critical pressure vessels and piping components. In the present work, residual stress profiles of girth welded austenitic stainless steel pipes are characterised using an artificial neural network approach. The network has been trained using residual stress data acquired from experimental measurements found in literature. The neural network predictions are validated using experimental measurements undertaken using neutron diffraction and the contour method. The approach can be used to predict through-wall distribution of residual stresses over a wide range of pipe geometries and welding parameters thereby finding potential applications in structural integrity assessment of austenitic stainless steel girth welds.

Influence of customized cutting edge geometries on the workpiece residual stress in hard turning

Depending on the intensity of mechanical and thermal loads during hard turning, compressive and/or tensile residual stress can be obtained. However, only compressive residual stress contributes to avoid crack initiation and propagation and increase fatigue life. In order to induce compressive residual stress in the workpiece surface and subsurface, cutting edge geometry is one of the most important influence factors. Taking this into account, the influence of new customized cutting edge geometries on the parameters of a hook-shaped residual stress profile (typical of a hard turning process) is investigated and possible causes for the encountered phenomena are explained. It was found that edge geometries, which provide an increase in contact length between tool and workpiece, lead to higher compressive residual stress in the subsurface and deeper affected zones.

Surface integrity and fatigue behaviour of electric discharged machined and milled austenitic stainless steel

Machining of austenitic stainless steels can result in different surface integrities and different machining process parameters will have a great impact on the component fatigue life. Understanding how machining processes affect the cyclic behaviour and microstructure are of outmost importance in order to improve existing and new life estimation models. Milling and electrical discharge machining (EDM) have been used to manufacture rectangular four-point bend fatigue test samples; subjected to high cycle fatigue. Before fatigue testing, surface integrity characterisation of the two surface conditions was conducted using scanning electron microscopy, surface roughness, residual stress profiles, and hardness profiles. Differences in cyclic behaviour were observed between the two surface conditions by the fatigue testing. The milled samples exhibited a fatigue limit. EDM samples did not show the same behaviour due to ratcheting. Recrystallized nano sized grains were identified at the severely plastically deformed surface of the milled samples. Large amounts of bent mechanical twins were observed ~5μm below the surface. Grain shearing and subsequent grain rotation from milling bent the mechanical twins. EDM samples showed much less plastic deformation at the surface. Surface tensile residual stresses of ~500MPa and ~200MPa for the milled and EDM samples respectively were measured.

Improvement of microstructure and mechanical properties in Wire + Arc Additively Manufactured Ti-6Al-4V with Machine Hammer Peening

Inter-pass rolling is a cold-working technique to improve microstructure and associated mechanicalproperties in Ti-6Al-4V wire + arc additive manufacturing (WAAM) [1]. Initially developed to eliminate residualstress and distortion in WAAM, rolling equipment is cumbersome and the application is limited to simplefeatures and geometries. To exploit the potential design freedom of AM processes, this study investigatesthe potential of novel machine hammer peening (MHP) tools to complement or replace rolling as inter-passcold working technique, as these tools can be manipulated easily with a greater degree of freedom. The coldworking behaviour of the tool was assessed using EBSD plastic strain maps, as well as hardness and residualstress profiles. After the tool characterisation, inter-pass peening was applied to Ti-6Al-4V WAAM walls. Thesuccessfully achieved grain refinement has met the expectations.

Surface Integrity and Corrosion Performance of Biomedical Magnesium-calcium Alloy Processed by Hybrid Dry Cutting-finish Burnishing

Biodegradable magnesium-calcium (MgCa) alloy is a very attractive orthopedic biomaterial compared to permanent metallic alloys. However, the critical issue is that MgCa alloy corrodes too fast in the human organism. Compared to dry cutting, the synergistic dry cutting-finish burnishing can significantly improve corrosion performance of MgCa0.8 (wt %) alloy by producing a superior surface integrity including good surface finish, high compressive hook-shaped residual stress profile, extended strain hardening in subsurface, and little change of grain size. The measured polarization curves, surface micrographs, and element distributions of the corroded surfaces by burnishing show an increasing and uniform corrosion resistance to simulated body fluid.

Fatigue Resistance Study of Quenched and Tempered High-Strength Steel Submitted to Low Intensity Shot Peening Treatments with Different Types of Shots

The aim of this research is to study the fatigue life enhancement produced in quenched and tempered AISI4340 steel with a tensile strength of 2000 MPa after being submitted to shot peening surface treatments. These treatments generate compressive residual stress fields in a superficial layer of the material at the same time as inducing some kind of damage on the surface. Different kind of projectiles were chosen to perform the treatment (ceramic and steel shots), studying the way these affected the fatigue life of the specimens. The surface topography of the samples was analysed using a roughness tester and by means of scanning electron microscopy (SEM). The compressive residual stress profile induced by these treatments was measured using X-ray diffraction (XRD) plus electro-polishing. The fatigue behaviour of the treated samples was subsequently studied by means of 4-point rotating bending tests and their fracture surfaces were analysed using SEM. The best fatigue performance was obtained after shot peening with ceramic beads under 8A Almen intensity. The main difference in relation to the treatment performed under the same intensity but using steel cut wire shots was the much lower surface damage induced by the impacts with the ceramic shots compared with the cut wire projectiles, which in turn is justified by the greater geometric perfection and hardness of the former. Furthermore, fatigue specimens shot peened with ceramic beads under 8A intensity always gave rise to internal fatigue crack initiation, which took place outside the zone subjected to residual compressive stresses. Moreover, fatigue initiation was always linked to the presence of hard and rigid alumina inclusions, which acted as microstructural stress concentrators.

Optimization of Process Parameters of Shot Peening Using ABQUS

This paper involves an elastoplastic numerical approach to investigate effects of process parameters of Shot Peening process. The process produces compressive residual stress at the surface of treated components. It can prevent the propagation of fatigue cracks which results in an extension of fatigue life. The magnitude and depth of the compressive residual stress profile directly related to the effectiveness of the retardation of the fatigue cracks propagation. Surface material response on the development of subsurface residual stress on hardened 4340 steel surface using cast steel shots of 50 mm diameter at 67 m/s impact speeds is analysed. Simulations were carried out using ABAQUS software and it was used to model the jobs, pre-processing and post processing. The modelling involved building parts, defining materials and section, applying boundary conditions and pre-defined fields and meshing. As a result, for effective shot peening the shot direction should be kept as normal to the surface as possible by rotating the nozzle when a curved surface is to be shot peened.

Numerical and Empirical Modelling of Machining-induced Residual Stresses in Ball end Milling of Inconel 718

Machining induced residual stresses can be crucial for the performance and life of the end product. In this paper, a 3D numerical model and an empirical model for residual stresses prediction were built to evaluate the near-surface residual stresses in Inconel 718 under ball end milling conditions. A series of ball end milling experiments of Inconel 718 have been conducted and X-ray diffraction measurements have been utilized to obtain the test residual stress profiles. The comparison results reveal that the numerical model can give an efficient estimation of the residual stress state beneath the machined surface, but the accuracy of the simulation model still needs to be improved. On the basis of the measured residual stresses, an exponentially decaying cosine function was fitted using a particle swarm optimization method. For the purpose of predicting machining induced residual stresses with different cutting parameters, a prediction model based on cutting parameters was developed. A good correlation was observed between the experimental and the predicted results derived from the empirical model.

An Enhanced Analytical Model of Residual Stress for Peripheral Milling

Efficient prediction of the residual stress of peripheral milling is a critical issue in academic research and industrial practice. An enhanced analytical method is developed based on inclusion theory, taking account of the sequential effect of varying thermal-mechanical load due to periodic instantaneous chip thickness. Provided with accurate analysis of the deformation in metal cutting, rigorous solution is available in principle for arbitrary two-dimensional residual stress field in complex cutting process, which is difficult to tackle by traditional relaxation procedure. Combined with mechanistic model of cutting force and approximate incremental analysis of inelastic contact, two-dimensional residual stress profile in peripheral milling is achieved efficiently by the proposed method for the first time. The predictions are verified experimentally by the X-ray diffraction measurements on the flat specimens of AISI4340 steel.

Simulation of shot peening process

Shot Peening process is widely used to impose compressive residual stresses in fatigue critical surfaces and its intensity estimated empirically by inserting Almen strips near the intended target and measuring their post-peening curvature. It becomes difficult to calculate the intended residual stress profile in the target from the strip curvature as it depends on the dynamic response of the target material. This work presents an elasto-plastic numerical approach, pseudo-dynamic explicit model is validated by single-ball dropping experiment from literature, and applied to the design of parameters for multi-shot peeing of hardened steel surface.

熱応力解析理論に基づくショットピーニング面の残留応力深さ分布推定

In the shot peening process, the residual stress on the shot-peened surface and its distribution under the surface are important evaluation items for the high value-added surface creation. In this paper, it was proposed that the residual stress distribution under the shot-peened surface for a thin plate was non-destructively estimated from the measured value of residual stress on the surface and that of the arc height, by applying the theory of linear thermal stress analysis for a thin plate. The analyzed residual stress profile showed good agreement with that of the experimental results obtained from the surface removal by electro-polishing. From the results of analysis, it was found that the depth of compressive residual stress (the depth which changes from the compressive stress to the tensile stress), the residual stress at the depth of the inherent strain and the residual stress on the opposite surface to the shot-peened surface characterizing the profile of residual stress distribution had a relationship with the ratio of a thickness of plate to an inherent strain depth.

Analysis of residual stress profiles in plastic materials using the hole drilling method – Influence factors and practical aspects

The current work introduces a procedure to locally measure residual stresses in polymers. For this purpose, the hole drilling method was adapted to the prevailing boundary conditions. Particular attention is given to the analysis of the measured strain signal which is time dependent and strongly influenced by local temperature fluctuations. The different sources of heat prevailing during the measurement and their impact are outlined. By thorough analysis of the viscoelastic and thermal deformation occurring during and after drilling the material, a procedure is proposed to measure the correct strain relaxation from which residual stresses can be calculated. Using quenched polycarbonate samples with known residual stress profiles, the potential of the proposed approach was demonstrated and is discussed.

Microstructure and mechanical properties of cold sprayed 6061 Al in As-sprayed and heat treated condition

Microstructural and mechanical properties of cold sprayed 6061 aluminum deposits on 6061-T6 aluminum alloy substrates are investigated under various heat treatment conditions, i.e. as-deposited, stress relieved and T6. The local mechanical property variation in the as-deposited material are explored using nanoindentation technique, and correlated with microstructural characterization conducted via electron back-scattered diffraction. It is found that the prior particle boundaries have ~0.4GPa higher hardness than particle interiors, which is attributed to grain refinement in these regions promoted by local dynamic recrystallization. Also, the bulk-scale mechanical properties of the deposits are evaluated by microtensile testing in various post-heat treatment conditions and compared to those of conventionally processed 6061-T6 aluminum. The as-deposited material showed markedly higher ultimate strength (~460MPa) and lower ductility (~3%) compared to conventionally processed material and this is attributed to significant cold working during the cold spray deposition process and associated grain boundary strengthening and dislocation strengthening mechanisms. Heat treated specimens showed a slight improvement in both ultimate strength and ductility compared to the as-deposited condition. These improvements are attributed to an improvement in metallurgical bonding at prior particle boundaries and a modest increase in the density of strengthening precipitates. Fractography of the specimens revealed that the heat treatment also changes the fracture characteristics of the cold sprayed 6061 aluminum deposit. The residual stress profiles and bond strength of the deposits are also studied using x-ray diffraction, tensile pull-off and three lug shear testing, respectively.