Residual Stress Measurements Research Articles

Fatigue strength of laser-dressed non-load-carrying fillet weld joints made of ultra-high-strength steel

Laser dressing or remelting is one of the post-weld treatment methods that is based on the modification of the local weld toe geometry, and generally, it is similar to TIG or plasma dressing. However, the laser dressing is not covered by common fatigue design guidelines and recommendations of welded structures at the moment. In this research, the effect of laser dressing on the fatigue performance of non-load-carrying fillet-welded cruciform joints made of direct quenched S960 grade steel was studied by means of experimental fatigue testing and geometry and residual stress measurements together with finite element analyses and different statistical calculation processes, such as nominal, structural and effective notch stress methods. In addition, the 4R method was also employed and its applicability for fatigue assessment of laser-dressed weld joints in question was investigated. The experimental results showed the enhancement of the fatigue strength for laser-dressed fillet weld joints compared to as-welded condition and the recommended FAT class for similar TIG dressing case was exceeded. However, the fatigue strength improvement of laser dressing was less compared to the corresponding TIG-dressed joints when applied stress ratio was low. In proportion, the disparity between the improvement effect of laser and TIG dressing methods was reduced with higher applied stress ratio. This can be explained by different residual stress state in the critical weld toe treatment area of laser- and TIG-dressed joints that can be taken into account when using the 4R fatigue assessment method.

Residual stress measurements and weld characterization in wind turbine support structures

Tensile residual stresses are a byproduct of welding processes with a negative impact on fatigue life and structural integrity. In large welded structures, the residual stress field will be superimposed on the service and structural loads, thus potentiating structural limit conditions. Due to the high manufacturing costs involved and harsh environments in which large length welded joints may operate, it is highly desirable to have an overview of the residual stresses present in the structure.In order to gain further insight on the magnitude and nature of welding residual stresses introduced into tubular steel towers, two specimens that were circumferentially welded were obtained from two towers. The specimens were extracted from door cut-outs that permit access inside the towers. The Contour method was employed for measuring the locked-in residual stress, and in order to characterize the welds, a microstructural analysis was performed, and mechanical properties, such as impact toughness and Vickers hardness, were assessed.

Residual stress determination by the layer removal and X-ray diffraction measurement – correction method

Very often a manufacturing process is followed by some surface treatment. Such a process induces residual stress into the manufactured component. Compressive residual stress is desirable for enhancing the fatigue properties of the component. The residual stress is often measured only at the surface, if at all. However, residual stress is equilibrating in the whole component. Therefore, compressive residual stress at the surface induces undesirable tensile stress inside in the component. Knowledge of the residual stress distribution in a body can be very useful in engineering applications. The authors found this knowledge necessary for a proper description of fatigue crack propagation in railway axles described in the original paper [1]. With the onset of modern surface treating technologies, e.g. induction hardening, which can affect the entire cross-section of the component, the residual stress determination is even more critical.The presented paper aims to describe a procedure developed for proper residual stress determination. The procedure can be easily used e.g. in R&D centers, where X-ray diffraction residual stress measurement is already in use. The procedure is suitable for the residual stress determination in sizable cylindrical bodies or components, e.g. railway axles. It uses X-ray diffraction residual stress surface measurement and layer removal by machining. Results experimentally obtained are corrected by a general procedure developed in MATLAB software in order to obtain the original residual stress state in the cylindrical body.•More accurate procedure for a residual stress determination in cylindrical bodies.

Comparison of Non-Destructive Residual Stress Measurement Methods for Additive Manufactured Alsi10mg Alloys

Comparison of Non-Destructive Residual Stress Measurement Methods for Additive Manufactured Alsi10mg Alloys

Plasticity Effects in the Hole-drilling Residual Stress Measurements in Welded Structure

Plasticity Effects in the Hole-drilling Residual Stress Measurements in Welded Structure

Bending analysis and residual stress measurement of thin stainless-steel alloy

LPF (laser peen forming) is a non-contact sheet metal forming technology that uses a laser detonated shockwave to bend, shape, and fix engineering components accurately. To understand the bending behavior of SS-304 steel samples in the form of thin sheets of various thicknesses (0.3, 0.5, and 0.8 mm), laser peening at various power densities (3, 6, and 9 GW /cm2) was used. After the peening process, laser peened samples show convex and concave bending. The bending that occurs as a result of residual stress distribution across the sample's cross section during the laser peening process. As a result, to better understand the role of residual stress in LPF, the residual stress was studied using the usual X-ray based cos approach. The significance of these findings for a possible laser forming process known as laser peen forming are also investigated.

Residual Stress Measurement of Quenching Induction Hardened Axle Shaft by Contour Method and NumericalAnalysis of Residual Stress Relaxation behavior Induced by Bending Correction Process

Residual Stress Measurement of Quenching Induction Hardened Axle Shaft by Contour Method and NumericalAnalysis of Residual Stress Relaxation behavior Induced by Bending Correction Process

Application of vortex air cooling in automated angle grinder: effects on surface integrity – exploratory study

Framatome has developed an automatic grinding bench to reproduce manual dry grinding applied in workshop. This surface finish operation generally leads to tensile residual stress in welding materials. The current study proposes to investigate the effect of vortex air cooling as an innovative option for the lowering of surface effects induced by dry grinding.Grinding assisted by vortex cooling was tested on AISI 304L and Alloy 690 (UNS N06690). The effect of cooling assistance on microstructural properties of these materials was evaluated using microhardness, XRD residual stress measurements, roughness, SEM & EBSD characterizations. The results suggest that vortex cooling has a lower effect on surface finish parameters of stainless steel 304L than those of alloy 690.

Peen forming and stress peen forming of rectangular 2024–T3 aluminium sheets: Curvatures, natural curvatures and residual stresses

AbstractAluminium skins on the lower wings of most commercial aircraft are shaped using shot peen forming. This process, which involves bombarding the skins with hard shot, uses nonuniform plastic flow to induce curvatures—in the same way that differential expansion makes metal bilayers bend when heated. Here, we investigate experimentally how constraining conditions affect the final shape of peen formed parts. We report peen forming experiments for 4.9‐mm‐thick rectangular 2024–T3 aluminium sheets of different aspect ratios uniformly shot peened on one face with a low intensity saturation treatment. Some specimens were free to deform during peening while others were elastically prestressed in a four‐point bending jig. For each aspect ratio and prestress condition, residual stresses were measured near the peened surface with the hole drilling method. Additional residual stress profiles were also obtained with the slitting method. The residual stress measurements show that the progressive deformation of unconstrained specimens had the same effect as an externally applied prestress. For the peening conditions investigated, this progressive deformation caused unconstrained strips to exhibit curvatures 33% larger than identical strips held flat during peening. Furthermore, we found that the relative importance of material anisotropy and geometric effects did determine the bending direction of unconstrained specimens.

Experimental and numerical studies of laser-welded slender stainless steel I-section columns

Stainless steel has extensive applications in the construction industry because of its attractive appearance, unique corrosion resistance, durability, and superior mechanical properties. Laser-welding is a manufacturing process that is suitable for connecting various steel and stainless steel. High precision and low heat input characteristics make it have smaller heat-affected zones than the traditional welding process, resulting in lower thermal distortions and residual stress to give it increased load-carrying capacity. The present paper focuses on investigating the structural response of laser-welded slender stainless steel I-section columns to address the lack of test data. Nine stub columns tests and ten long columns tests were analysed for slender I-sections of austenitic laser-welded stainless steel of two stainless steel alloys (austenitic EN 1.4301 and EN 1.4307). Additionally, the material tensile coupon tests, residual stress measurements, and initial geometric imperfection measurements were performed. The observed results verified a representative residual stress pattern. The finite element model was validated based on the test results and parametric analysis was carried out on laser-welded slender stainless steel I-section axially compressed members. Further, the study evaluated the cross-sectional and column buckling resistances for European code EN 1993-1-4, American Design Guide AISC 27, and Continuous Strength Method. Interestingly, the continuous strength method reflected significantly better buckling behaviour of slender I-sections. Although this method gives more accurate results than the code predictions, slight conservatism predictions were still found for slender I-section columns with room for improvement.

Multi-Scale Modeling of Residual Stresses Evolution in Laser Powder Bed Fusion of Inconel 625

Laser powder bed fusion exhibits many advantages for manufacturing complex geometries from hard to machine alloys such as IN625. However, a major drawback is the formation of high tensile residual stresses, and the complex relationship between the process parameters and the residual stresses has not been fully investigated. The current study presents multi-scale models to examine the variation of process parameters on melt pool dimensions, cyclic temperature evolutions, cooling rate, and cyclic stress generation and how they affect the stress end state. In addition, the effect of the same energy density, which is often overlooked, on the generated residual stresses is investigated. Multi-level validation is performed based on melt pool dimensions, temperature measurements with a two-color pyrometer, and finally, in-depth residual stress measurement. The results show that scan speed has the strongest effect on residual stresses, followed by laser power and hatch spacing. The results are explained in light of the non-linear temperature evolution, temperature gradient, and cooling rate during laser exposure, cooling time, and the rate during recoating time.

Neutron diffraction residual stress determinations in titanium aluminide component fabricated using the twin wire-arc additive manufacturing

In recent years, twin wire-arc additive manufacturing (T-WAAM) technique has been considered as a promising method of fabricating and shaping titanium aluminide components with high efficiency and low cost. However, excessive thermal input of the non-consumable tungsten electrode arc deposition induces significant residual stresses in the buildup component, thus accurate measurement of residual stresses is necessary for T-WAAM buildup part quality assessment. In the present research, non-destructive neutron diffraction residual stresses measurements on as-fabricated and heat-treated T-WAAM produced Ti48Al titanium aluminide components are performed. To exclude the influence of initial large T-WAAM residual stresses on d 0 hkl sample alignment, different d 0 hkl sample dimensions are designed: one is normal thin slice and the other is the meshed sample. According to the obtained results, the conducted post-production heat treatment has partially released initial residual stresses. Also, the meshed design of d 0 hkl sample shows better measurement accuracy than the simple sliced d 0 hkl sample. In addition, T-WAAM fabricated titanium aluminide wall component performs featured tensile-compressive alternating residual stress distribution induced by the layer-by-layer arc deposition.

Influence of LPBF-Surface Characteristics on Fatigue Properties of Scalmalloy®

Laser powder bed fusion (LPBF) has indisputable advantages when designing new components with complex geometries due to toolless manufacturing and the ability to manufacture components with undercuts. However, fatigue properties rely heavily on the surface condition. In this work, in-process surface parameters (three differing contour parameter sets) and post-process surface treatments, namely turning and shot peening, are varied to investigate the influence of each treatment on the resulting fatigue properties of LPBF-manufactured specimens of the aluminium–magnesium–scandium alloy Scalmalloy®. Therefore, metallographic analysis and surface roughness measurements, as well as residual stress measurements, computer tomography measurements, SEM-analyses, tensile and fatigue tests, along with fracture surface analysis, were performed. Despite the fact that newly developed in-process contour parameters are able to reduce the surface roughness significantly, only a minor improvement in fatigue properties could be observed: Crack initiation is caused by sharp, microscopic notches at the surface in combination with high tensile residual stresses at the surface, which are present on all in-process contour parameter specimens. Specimens using contour parameters with high line energy show keyhole pores localized in the subsurface area, which have no effect on crack initiation. Contours with low line energy have a slightly positive effect on fatigue strength because less pores can be found at the surface and subsurface area, which even more greatly promotes an early crack initiation. The post-process parameter sets, turning and shot peening, both improve fatigue behaviour significantly: Turned specimens show lowest surface roughness, while, for shot peened specimens, the tensile residual stresses of the surface radially shifted from the surface towards the centre of the specimens, which counteracts the crack initiation at the surface.

Predicting part deformation based on deformation force data using Physics-informed Latent Variable Model

Part deformation prediction and control is a crucial issue for obtaining tight dimensional accuracy so as to ensure product quality with high performance, and deformation prediction is the fundamental of the deformation control. However, existing machining deformation prediction methods are based on the prediction or measurement of residual stress and suffering from two challenges: (i) the measurement accuracy of residual stress field is limited by physical principle and (ii) low prediction in accuracy. In order to address these issues, this paper presents a method for predicting part machining deformation based on deformation force using the proposed Physics-informed Latent Variable Model involved physics knowledge. Deformation force is introduced to represent the inner unbalanced residual stress state of the workpiece, and it is a much easier and more accurate signal compared with residual stress. Machining deformation is predicted by fusing the data-driven method and the prior knowledge of deformation mechanical relationship by taking advantage of the latent variable. The proposed method was verified both in simulation and actual machining environment, and accurate machining deformation prediction has been achieved. The proposed method can be readily extended to the prediction problems involved with difficult-to-measure physical quantities.

Investigation of microstructure and mechanical properties evolution in 7050 aluminum alloy and 316L stainless steel treated by laser shock peening

Laser shock peening (LSP) has been widely applied to enhance the mechanical properties of metallic materials by modifying their sub-surface microstructures. However, controversies still exist on whether grain refinement can be obtained after the LSP process. To investigate the effect caused by LSP in metallic materials, a 7050 aluminum alloy and 316L stainless steel, which are typically high and low stacking fault energy (SFE) materials, respectively, were selected for this study. The microstructure modified by different LSP cycles and energy densities in both materials was illustrated by Electron Backscatter Diffraction (EBSD). The result shows that no grain refinement was observed regardless of the laser cycles and energy density. The most evident change was the increase of dislocation density, and higher dislocation density was observed with the increase of LSP cycles and energy density. The hardness and residual stress measurements around the LSPed areas show that LSP can effectively introduce a plastic deformation layer ranging from 600~1300 μm. It was revealed from the tensile tests that the yield strength of both materials was improved after the LSP process with the scarification of their elongations. Moreover, a potential method to calculate the dynamic yield stress of metallic materials was put forward with the help of the LSP process.

Effect of copper concentrations on microstructure, residual stress and corrosion behavior of Ni100−x-Cux alloy films processed by magnetron co-sputtering

Nickel based alloys have gained importance due to their very good mechanical properties as well as their electrochemical behavior in different environmental conditions. In this regard, thin films of nanocrystalline Ni-Cu alloys have been processed by RF / DC magnetron co-sputtering in Ar gas environment for different copper concentration (10 %, 21 %, 28 % and 39 %). Grain size determination, phase identification and residual stress measurement have been done using an X-ray diffraction technique. A compressive residual stress has been observed and the value of the compressive stress increases with an increase in the Cu concentration in the Ni100−x-Cux alloy films. The electrochemical behavior of thin films of the Ni100−x-Cux alloy has been investigated with the help of potentiodynamic polarization experiments which revealed susceptibility towards pitting corrosion. The rate of corrosion decreases from 0.92 ×10−2 to 0.26 ×10−2 mm / year with the increase in Cu concentration from 10 % to 39 %. The microstructure of thin films of the Ni100−x-Cux alloy has been investigated using a field emission scanning electron microscope (FESEM). The FESEM micrograph shows the formation of pits, and the size of pits decreases with increasing concentration of doped Cu. Experimental results also reveal that the increase in corrosion resistance can correlate with the developing residual stress in the films.

Measurement of Residual Stress of AlN Thin Films Deposited by Two-Facing-Targets (TFT) Sputtering System

Measurement of Residual Stress of AlN Thin Films Deposited by Two-Facing-Targets (TFT) Sputtering System

Technology development for in-situ measurement of residual stress in arc welded joints of MDN 250 by portable Cosα X-ray diffraction method

Residual stresses are inherent stresses that exist in engineering components even though no external load is applied. They are caused by the non-uniform volumetric shift of the metallic component during manufacturing processes. Welding is a key manufacturing technique that has a substantial impact on the economy since it is required for the production of a diverse variety of products used in the engineering sector. The residual stress primarily affects the stability, durability and performance of the welded joints. Hence its determination is of utmost importance. X-ray diffraction (XRD) is the most commonly used method for residual stress analysis. There are mainly two approaches for measuring residual stress using XRD; one is the sin2ψ method and the other is the cosα method. The residual stress measurements using the cosα method are handy, quick and convenient compared to the sin2ψ method. This method is well suited for welded joints, as it provides flexibility for testing immediately after the welding operation. Apart from residual stress measurements, the cosα method also gives valuable insights in the form of Debye-Scherrer (DS) rings and full width at half maximum. The present study focuses on the development of a novel technique that not only enables residual stress measurement but also provides a quantitative estimation of hardness and qualitative estimation of grain size without performing metallurgical or mechanical characterization. The material used for the present study is an arc-welded joint of MDN 250 grade maraging steel. The residual stress results show a compressive profile throughout the weldment, with a maximum value of compressive residual stress of 428 MPa at the fusion zone.

A novel residual stress model for welded I-sections

Residual stresses resulting from the fabrication of welded steel beams have a significant influence on the stability behaviour of steel members. To consider them in advanced numerical simulations, the models according to ECCS publication no. 33 or prEN1993–1-14 are commonly used. While the ECCS model was developed for steel grades S235 and S355, this model was also adapted in prEN1993–1-14 for the application to high-strength steels and scaled with the yield strength of the material. However, numerous investigations show that these models do not adequately represent existing residual stresses.This paper presents experimental studies of residual stresses conducted at the University of Coimbra and Ruhr-Universität Bochum. Based on these measurements, the influencing parameters on residual stresses were identified and subsequently a novel model for the representation of residual stresses was developed. The model was validated with an extensive data set from residual stress measurements from the last 50 years considering steel grades S235 to S890. Finally, numerical stability analyses were executed to investigate the effects of different residual stress approaches on the load-bearing behaviour of steel columns and beams.

Biaxial residual stress measurement by indentation energy difference method: Theoretical and experimental study

In this paper, an energy difference method is proposed to estimate biaxial stress components based on indentation technique. The indentation energy is chosen as an analytical parameter, which can be calculated directly from the load-depth curve. The presence of residual stress in a material causes the change of indentation energy. The mapping relationship between uniaxial stress and indentation energy difference was established by finite element analysis. Meanwhile, the Knoop tip was applied since the indentation response varies according to the indenter orientation. Using this feature, a biaxial stress state can be transformed into two equivalent uniaxial stress states by indentation energy conversion. Finally, the biaxial residual stress components can be solved by the energy difference functions of two orthogonal indentations. A portable indentation instrument was developed to implement this approach, which is suitable for in-site measurement. The obtained results show good agreements with the artificial stresses applied by cross-shape sample experiment.