Redistribution Of Residual Stresses Research Articles

Fatigue life prediction of 316L stainless steel weld joint including the role of residual stress and its evolution: Experimental and modelling

Weld residual stresses have a great effect on the fatigue strength of welding structures. As an internal stress, the residual stress is unfounded to be treated as the conventional mechanical load during the fatigue safety assessment due to its relaxation and redistribution under cyclic loading. Thus, establishing an accurate model considering the effects of residual stress is critical for fatigue strength design. Therefore, this paper develops a sequential coupling residual stress analysis and a cyclic viscoplastic constitutive model to predict the residual stress evolution and its role on the fatigue life, which has been validated by an experimental measurement of residual stress and a SEM-based observation fatigue fracture morphology. The results show that the role of residual stress on fatigue life is mainly reflected by the increase of mean stress, and its increased magnitudes depends on the stress redistribution. The redistribution of residual stresses is closely correlated to the fatigue fracture behavior of weld joints. And a fatigue life prediction model of weld joints is proposed based on the experimental data of base metal by further considering the residual stress and its redistribution. Finally, the experimental results verify the proposed model on predicting both the fatigue failure location and lives of weld joints, and the predicted lives are within the 1.5 error band.

Investigation of residual stress in composites produced by explosion welding with ultrasound

The paper presents the results of experimental studies of residual stress in homogeneous steel samples obtained by explosion welding with simultaneous exposure to ultrasound. For experimental determination of residual stress in composites, a method based on sequential removal of thin metal layers from the sample surface and measurement of deformations of the sample from the opposite side using strain gages was chosen. It is shown that the introduction of ultrasound in the process of explosion welding leads to both a redistribution of residual stress and a significant decrease in their maximum values measured in the immediate vicinity of the junction zone of steel samples.

A new in-processes active control method for reducing the residual stresses induced deformation of thin-walled parts

Abstract It is usually deformed severely for thin-walled workpiece when the fixture is removed after machining, which is mainly caused by the redistribution of internal stress and machining induced residual stress (MIRS). Previous works have tried to relax the fixture in-processes to release the residual stresses (i.e. internal stress and MIRS), and perform the following machining operation after re-fasten the workpiece at its totally relaxed state. However, the totally relaxed state is only an equilibrium of residual stresses and the MIRS on the top layer of the workpiece will be changed after the following machining process. The residual stresses of the workpiece become imbalance again and thus will lead to the deformation of final component. In this paper, a novel research work of actively controlling the in-processes deformation of thin-walled part is presented, which is aimed at balancing the internal stress/MIRS and preventing the redistribution of residual stresses after last machining step. Firstly, the variations of internal stress, MIRS and clamping force during machining are analyzed through a simplified in-process model of blade. An equilibrium equation is established by taking the residual stresses as the equivalent loads of clamping point. Next, to achieve the proposed in-processes active control method, a mathematical model of how to actively adjust the fixture between machining operations is established. And then, the finite element method (FEM) is used to calculate the MIRS induced deformation, in which the MIRS is measured through experiment. A prototype of active control fixture for blade part is developed to adjust the in-process deformation. Finally, three groups of machining experiments on a simplified blade with three different treatments on the fixture after the rough machining – no relaxing the fixture at all, totally relaxing the fixture in-process, and actively controlling the deformation in-process– are carried out. The validation demonstrates that the proposed in-process active control method could decrease machining deformation significantly. The maximum deformation reduced to 18.3 % and 42.9 % compare to the no relaxing and totally relaxing the fixture after rough milling.

Study of the Mechanics of Spatial Elastoplastic Strain and Fracture of Weld Joints of Austenite Steels

The results of calculated–experimental and numerical studies of the relative strain intensity factors in the case of semi-elliptical inclined surface cracks in the weld joints of 12Kh18N10T austenite stainless steels with the assumption of redistribution of strain and initial residual stresses under nonlinear boundary loading conditions are presented. A parametric equation for the determination of the relative strain intensity factors in the case of semi-elliptical inclined surface cracks is given. This new parametric equation allows one to evaluate the comparative survivability of the local zones of details and construction in the bulk weld joint upon elastoplastic static and low-cycle loading.

Effect of residual stress redistribution and weld reinforcement geometry on fatigue crack growth of butt welded joints

Redistribution of residual stress during crack growth in butt welded joint of Al5083-H111 was studied. Furthermore, the effect of post weld heat treatment and removing the weld reinforcements on fatigue crack growth rate was investigated. Three dimensional simulations of welding process and fatigue crack growth were conducted. It was shown that removing the weld reinforcements causes an increase in the residual stress. A correlation between the fatigue crack growth rate data of welded joints can be obtained with that of the parent metal by considering the weld geometry and redistribution of residual stresses in calculation of stress intensity factor.

Energy principle and material removal sequence optimization method in machining of aircraft monolithic parts

In the machining process of aircraft monolithic parts, the initial residual stress redistribution and structural stiffness evolution often lead to unexpected distortions. On the other hand, the stress redistribution and stiffness reduction during the machining process depend on the material removal sequence. The essence of the stress redistribution is releasing the initial elastic strain energy. In the present study, the influence of the material removal sequence on the energy release is studied. Moreover, a novel optimization method is proposed for the material removal sequence. In order to evaluate the performance of the proposed method, the mechanism of the machining distortion is firstly analyzed based on the energy principle. Then a calculative model for the machining distortion of long beam parts is established accordingly. Moreover, an energy parameter related to the bending distortion and the procedure of the material removal sequence optimization is defined. Finally, the bending distortion analysis and material removal sequence optimization are performed on a long beam with a Z-shaped cross-section. Furthermore, simulation and experiments are carried out. The obtained results indicate that the optimized sequence results in a low distortion fluctuation and decreases the bending distortion.

Application of laser interferometry to the choice of processing modes by the criterion of the residual stress level

Optimization of the modes of technological processing aimed at reduction or redistribution of residual stresses requires the use of modern and reliable methods of control. The method of laser interferometry leads to minor damages which are considered acceptable or can be easily removed. It is possible to use the method in industrial conditions of the workshops. Unlike physical methods, which have restrictions imposed on the classes and characteristics of materials in terms of structure, magnetic properties, and hardness, the method of laser interferometry exhibits a universal character. The method allows data saving in a digital format on speckle interferograms, thus providing a possibility of the traceability of measurement stages, expert comparison and reliable reporting. Laser interferometry provides determination of the absolute values of stresses with the error of the yield point measurement below 10%. The method can be successfully used to control the stress state in the production of critical welded structures from aluminum alloys, which are subject to stringent requirements for dimensional stability, accuracy and reliability. The technology of manufacturing the above welded structures usually includes vibration treatment, combined with the arc welding process. This technology, unlike heat treatment, is rather efficient, environmentally friendly, and low energy consumption process. Optimization of the technology plays a key role in the industrial implementation and can significantly reduce the economic costs of subsequent machining. The method of laser interferometry provides effective determination of the optimal technological mode by the parameter of residual post-welding stresses, as well as determination of the degree of the reduction of those stresses compared to the case without concomitant vibration treatment.

Experimental and numerical study of mooring chain residual stresses and implications for fatigue life

Residual stresses in large mooring chains were measured for the first time in this study. Two measurement techniques were employed (neutron diffraction and hole drilling). Elastic-plastic finite element simulation of the proof loading was conducted, and the computed residual stresses were compared to the experimental measurements. Further, the cyclic plasticity of the material was taken into account to investigate residual stress redistribution caused by introduction of corrosion pits and cyclic service loads. A critical damage parameter was employed to estimate fatigue crack initiation life of corroded mooring chains subjected to various service load levels considering the effect of residual stresses.

Redistribution of residual stresses in girth weld of a pipe of strength class K60 after ultrasonic impact treatment

When applying technological methods of post-treatment of welded joints, it is necessary to control residual stress fields. The degree of redistribution of residual stresses mainly depends on the strength characteristics of steel, the geometric dimensions of the joint, the initial stress state, and treatment modes. The paper studies the redistribution of residual stresses in the ring joint of a pipe with a diameter of 530 mm made of steel (0.10%C-2%Mn-1%V-1%Nb-1%Al) of strength class K60. The pipes are connected by arc welding. Residual stresses were measured by the X-ray method based on measurement of microstrain of the crystal lattice of the material under its influence. Ultrasonic impact treatment (UIT) was done using a special technological complex. It consisted of an ultrasonic generator UZGT 0.5/27 and an ultrasonic impact tool BOMBUS. It is shown that after arc welding tensile residual stresses in the girth weld are formed in the inner side of the pipe. It was revealed that the UIT of the girth weld at a speed of 0.06 m/min from the inner side of the pipe wall contribute to the redistribution of residual welding stresses up to -100... 200 MPa, which provides the formation of compression stresses on the inner surface of the pipe. It’s shown that ultrasonic impact treatment reduces the level of tensile residual stresses and transforms them into compressive residual stresses. This can have a positive effect on the fatigue strength of the pipe girth weld.

Effect of Load Ratio on the Mechanical Field around Crack Front of Dissimilar Metal Weld Joints

Stress corrosion cracking (SCC) of the dissimilar metal weld (DMW) joints in nuclear power equipment has been paid more attention in recent years. Besides the geometric size, load level, multiaxial stress state and mechanical heterogeneity, the relaxation and redistribution of residual stress make it very difficult to directly study the influence of residual stresses on the actual welded components. To study the effect of load ratio on residual stress around the crack front in the weld zone of DMW joints, the distributions of residual stress and stress triaxiality under different load ratios are simulated and discussed. The analyses show that the stress becomes larger with the increasing load ratio. The stress is tensile stress near the crack front. While the stress becomes compressive stress when it is far away from the crack front. The external load rotio has great effect on the stress around the circumferential direction at the crack front. The higher load ratio are more likely to cause the crack propagation. In the region of crack angle θ=80-120 °, the load ratio has little effect on the stress triaxiality.

Distortion mitigation in additive manufacturing of AlSi10Mg by multilayer laser peening

Distortion in metal additive manufacturing remains an industry barrier due to the inadequate thermal management that gives rise to residual stresses. Mitigation strategies primarily involve developing a print recipe based on the build material; however, variability among machine platforms, build strategies, and raw material stock inhibit development of universal print recipes that eliminate distortion. There is a need for a universal manufacturing technology that enables distortion free additive manufacturing. A solution to provide distortion free parts that are machine platform and build strategy independent is coupling additive manufacturing with multilayer peening. Peening enables favorable redistribution of residual stresses that drive distortion using mechanical means rather than thermal management. The cumulative nature of residual stress during layer-by-layer peening and the compounding effect on distortion is poorly understood. Thus, the research objective of this work was to measure residual stress and distortion after multilayer laser peening on AlSi10Mg printed by laser powder bed fusion on the Matsuura Lumex Avance-25. Residual stress was measured for several interlayer peening frequencies. An optimum condition was identified and evaluated for distortion mitigation. To measure deflection, bridge samples based on NIST AMB2018-01 were printed and interlayer peened. Results indicated that laser peening reduced distortion 45% over an as-printed sample.

Effect of ultrasonic impact treatment on redistribution of residual stresses in dissimilar thickness girth welds

Effect of repeated ultrasonic impact treatment on redistribution of residual stresses in a dissimilar thickness girth butt weld of pipes with a diameter of 530 mm of low alloyed (0.9%C-2%Mn-1%Si) steel was studied. Residual stresses were measured by the X-ray method based on measurement of microdeformation of a crystal lattice of a material under its influence. The research results showed that tensile residual stresses in the girth weld were formed at the inner side of the pipe after welding. Ultrasonic impact treatment of the weld at the inner side caused complete transformation of residual stresses from tensile to compressive ones when speed was in the range from 0.06 to 0.12 m/min. However, reduce in speed of treatment down to 0.04 m/min gave a negative effect. The values of compressive residual stresses in heat affected zone on the side of the thinner pipe were decreased.

Thermal Relaxation Features of Residual Stresses Arising upon Laser Shock Processing of Heat-Resistant Materials

In this paper, we study the thermal relaxation features of compressive residual stresses generated during laser-shock wave processing in high-alloyed heat-resistant Iron GH2036 alloy. A finite element modeling of thermal relaxation of the generated compressive residual stresses was performed. The features of the thermal effects on the redistribution of the compressive residual stresses in the temperature range from 200 to 650°C are studied. Based on comparative analysis, the results of the finite element modeling correlated well with the experimental data known in the literature.

Predicting fatigue crack propagation in residual stress field due to welding by meshless local Petrov-Galerkin method

This paper introduces a novel procedure to develop the MLPG method for investigating residual stress effect on the Fatigue Crack Propagation (FCP) rate. A new formulation is introduced based on thermoelastic-plastic equation for this method to numerical analysis of the residual stress due to welding. The most important part of numerical analysis by MLPG method is to determine the residual stress redistribution due to crack growth and calculation of the Stress Intensity Factor (SIF) in residual stress field. A good agreement is seen between the outputs of MLPG method to the Hole-Drilling Strain-Gage method results. The standard weight function is developed without increasing the computational time for simulation of the displacement and stress around the crack. The Superposition principle is employed to consider the residual stress effect on the SIF and cycle ratio. Finally, the Walker’s FCP equation is modified to take into account the simultaneous effects of cyclic loading and residual stress. It was discovered that the results obtained from the purposed method is in a good agreement with FCP experimental results. Therefore, it can be concluded a new approach is developed to analyze the calculation of SIF in the residual stress field and its effect on the FCP rate.

FE analysis of the residual stresses for the laser welded T-joint of Al-Li alloy under service loads

This paper attempts to reveal the evolution of welding residual stresses (WRS) in skin-stringer structure of Al-Li alloy exposed to service loads. 3D thermal-structural finite element (FE) formulation of the dual laser-beam bilateral synchronous welding process is first derived, and its accuracy is verified by experimental tests to acquire accurate WRS. FE numerical analyses incorporated with the elastic-plastic constitutive model, taking the WRS, strains distribution and total equivalent plastic strains as an initial condition, are next carried out to explore the residual stresses redistribution of the T-joint under service loads. The results unveil that longitudinal stress relaxation is pronounced (especially at the weld) in the skin after the end of the loads applying. However, the longitudinal stress in the middle of the stringer is not sensitive to the service loads. The extended high tensile stress concentration zone inside the joint in case 2 and case 4 may be the potential failure location after the progressive loading of the skin-stringer welding structure. Additionally, the weld toe, which is the obvious stress sudden change zone in the service process, is a non-negligible position for the improved failure estimation of the aerospace skin-stringer structure.

Residual stress in laser cladded rail

Abstract To improve the fatigue life of components subject to loads with high surface strain gradients, it is possible to coat them with an alloy of higher durability. The present study focuses on the effect of cladding high value track components, made of a standard rail steel UIC 900A/grade 260, with a layer of a premium martensitic stainless steel to reduce wear and fatigue. The laser cladding process inevitably generates residual stresses in the clad and parent metal, which could be detrimental to the integrity of the component. Therefore, measurements to determine the residual stress state of cladded rail were performed using semi-destructive centre-hole and deep hole drilling and non-destructive neutron diffraction techniques. Subsequently, the effects of cycling loading and wear, representative of typical service loads, on the redistribution of the residual stress field were investigated. It was observed that laser cladding causes a triaxial compressive residual stress field in the clad and near the interface and a tensile stress field in the parent material. The stress field is shown to change when the first cycle of load is applied but reaches a steady state after only 10 cycles: After the 10th cycle there is no evidence that the clad continues accumulating strain which could indicate that there is low risk of ratcheting. Wear effect on residual stress redistribution was found to be local on the surface of the specimen only.

Numerical study on residual stress redistribution of shot-peened aluminum 7075-T6 under fretting loading

This study attempts to analyze the residual stress redistribution under fretting loading by using FEM. The FE analysis model used in this study considers the variation of surface yield strength, the evolution of contact surface profile and the variable friction coefficient. The initial residual stress of shot-peened specimen is measured by X-ray diffraction method. Nano-indentation experiment is conducted to measure the yield strength of material surface layer. Pin-on-plate wear experiment is used to measure the wear coefficient of shot-peened aluminum 7075-T6. A fretting experiment is designed to measure the friction coefficient and verify the FE analysis model. The results show that the relaxation of residual stress is more likely to occur near the trailing edge of contact area, and the specimen with higher shot peening intensity undergoes the larger residual stress relaxation. In addition, the residual stress near the contact center region increases with the increase of loading cycles due to the fretting wear.

Modeling thermal and mechanical cancellation of residual stress from hybrid additive manufacturing by laser peening

Additive manufacturing (AM) of metals often results in parts with unfavorable mechanical properties. Laser peening (LP) is a high strain rate mechanical surface treatment that hammers a workpiece and induces favorable mechanical properties. Peening strain hardens a surface and imparts compressive residual stresses improving the mechanical properties of a material. This work investigates the role of LP on layer-by-layer processing of 3D printed metals using finite element analysis. The objective is to understand temporal and spatial residual stress development after thermal and mechanical cancellation caused by cyclically coupling printing and peening. Results indicate layer peening frequency is a critical process parameter affecting residual stress redistribution and highly interdependent on the heat generated by the printing process. Optimum hybrid process conditions were found to exists that favorably enhance mechanical properties. With this study, hybrid-AM has ushered in the next evolutionary step in AM and has the potential to profoundly change the way high value metal goods are manufactured.

Stress states and crack behavior in plasma sprayed TBCs based on a novel lamellar structure model with real interface morphology

Abstract To ascertain the crack growth behavior and coalescence mechanism in thermal barrier coatings (TBCs) is beneficial for understanding the failure of TBCs and proposing the probable optimization methods. In this work, a novel lamellar structure model with real interface morphology is developed to explore the crack growth behavior and the failure mechanism of TBCs during thermal cycling. Three typical defects which include pore, inter-splat crack, and intra-splat are incorporated in the model. To simulate the oxidation process of the bond coat (BC) realistically, The oxidation growth process is simulated via changing the BC properties to thermally grown oxide (TGO) properties layer by layer. The effects of the lateral growth strain distribution through TGO thickness on the stress states are executed. Moreover, the influences of BC creep on the crack growth and coating lifetime are further elaborated. The results show that the larger the lateral growth strain gradient, the smaller the residual tensile stress. The irregular interface morphology results in the redistribution of residual stresses. Although the pores and cracks can alleviate the tensile stress near the valley, large stress concentration will occur near them. At the early phase of thermal cycling, the cracks grow steadily. After more cycles, the cracks propagate rapidly and merge with others. The simulated delamination path is in agreement with the experiment results. Not only does BC creep change the crack coalescence mechanism, it also decreases the thermal cyclic lifetime of TBCs. The coating optimization method proposed in this study provides another option for developing advanced TBCs with longer lifetime.

Shot peening effects on residual stresses redistribution of offshore wind monopile multi-pass weldments

In some industrial applications, welding is the only alternative to join different parts. However, the major problem in welded structures is the tensile residual stresses that are inevitably produced during welding process. Surface tensile stresses can threaten the performance of the weldments as they act as an accelerant in fatigue crack initiation and failure. Shot peening is a well-known method which can enhance weldments' fatigue performance and longevity by inducing surface compressive residual stresses in order to eliminate or limit tensile residual stresses.In this study, the effect of shot peening on redistribution of multi-pass welding residual stresses was numerically and experimentally investigated on very large components typical of welded joints used in offshore wind turbine monopiles. In experimental part of the study, the residual stresses were measured using the Incremental Centre Hole Drilling (ICHD) method before and after shot peening. Finite element studies were carried out using 3D welding models and random shot peening analyses. Moreover, extensive finite element analyses were conducted to study the effect of model dimensions and the number of passes on prediction of welding residual stresses. Interesting set of results obtained from both the numerical studies and the ICHD measurements were in good agreements and showed that shot peening can be advantageous even for large components with multi-pass welded joints. Additionally, reducing the number of weld passes in finite element models could considerably lower the computational time without affecting the accuracy of results at surface regions of the models.