Residual Stress Field Research Articles

Microstructure evolution and fatigue properties subjected to optimized coverage areas of laser shock peened TB10 titanium alloy

Laser shock peening (LSP) is an advanced surface modification technology, which can induce the high-strain-rate plastic deformation on the metal surface to improve mechanical properties. In this work, the effects of LSP on the microstructure, microhardness, residual stress and fatigue crack growth (FCG) properties of TB10 titanium alloy were investigated. The results showed that LSP could induce high density dislocations and realize the nanocrystallization on the titanium alloy surface. The magnitudes of compressive residual stress and microhardness increased with increasing LSP energy and impact times. The fatigue test showed that the LSP coverage area significantly affected the FCG performance, especially the fatigue life increased by 40.2 % when the LSP area was perpendicular to the cracks. This was attributed to the fact that the FCG rate of early cracks was significantly reduced by a wider region of the compressive residual stress field perpendicular to the crack direction.

Measurement and simulation of residual stresses in transient liquid phase bonded ferritic steels

Ferritic steel bars—25 mm in diameter—were welded by transient liquid phase bonding (TLPB) using Fe-based amorphous metallic foils as filler material. The resulting residual stress (RS) field shows a low peak magnitude—of 147 MPa—as measured by neutron diffraction. The most distinctive feature of TLPB is the heat input delivered simultaneously at the whole joint that allows much lower cooling rates compared with arc welding (AW). Therefore, the elapsed time between 800 and 500 °C (t 8/5) was particularly long reaching 390 s. As a result, a low RS peak magnitude (147 MPa) was obtained in the as-welded condition. This value is well below the RS peak magnitude obtained with AW which typically attain the yield strength of the base metal (276 MPa). The numerical simulation of RS at the welded bars was performed by a thermal and mechanical analysis. It shows that TLPB produced a large austenized region, low cooling rates and a remarkable t 8/5. Consequently, the large volume in which the heat input is delivered is the driving force to reduce RS peak magnitudes. From the mechanical analysis, it was found that the simulated RS was in good agreement with the measured RS. Therefore, the proposed numerical simulation model can be used to predict RS in TLPB weldments.Graphical abstract

Hybrid post-processing effects of magnetic abrasive finishing and heat treatment on surface integrity and mechanical properties of additively manufactured Inconel 718 superalloys

• The magnetic abrasive finishing and post heat treatment are hybrid to apply for the additively manufactured In718. • The hybrid effects on the surface integrity and mechanical properties are studied comparatively. • Excellent mechanical properties of the additively manufactured In718 are obtained by the proper post-processing sequence. • The influencing mechanism of the hybrid post-processes provides a promising pathway to improve the mechanical properties. Desired microstructure and surface integrity are critical to achieving the high performance of additively manufactured components. In the present work, the hybrid post-processes of magnetic abrasive finishing (MAF) and post-heat treatment (HT) were applied to the additively manufactured Inconel 718 superalloys. Their hybrid effects and influencing mechanism on the surface quality and mechanical properties of the additively manufactured samples have been studied comparatively. The results show that the MAF process effectively reduces the surface roughness by more than an order of magnitude due to the flexibility and geometric consistency of the magnetic particles and abrasives with the finished surfaces. The proper sequence of MAF and HT obtains enhanced mechanical properties for the homogenized-MAF-aged sample with the yield strength of 1147 MPa, the ultimate tensile strength of 1334 MPa, and the elongation of 22.9%, which exceeds the standard wrought material. The surface integrity, compressive residual stress field, and grain refinement induced by the MAF and subsequent aging heat treatment increase the cracking resistance and delay the fracture failure, which significantly benefits the mechanical properties. The MAF process combined with proper post-heat treatment provides an effective pathway to improve the mechanical properties of additively manufactured materials.

Numerical Study on Welding Residual Stress Distribution of Corrugated Steel Webs

Residual stresses are an inevitable result of the welded fabrication process of corrugated steel webs (CSWs), resulting in structures with high and unpredictable stress fields, causing unexpected failures. The residual stress field is affected by structural parameters and the welding path of CSWs. This study proposes the welding process simulation method for CSWs with element birth and death technology. The optimization design method of heat source parameters is proposed. The feasibility of the simulation method is verified by comparing the numerical results with the experimental results of relevant literature. As a part of the study, a comparison of residual stress fields upon cooling welded CSWs with bending angles of 30, 45, and 60 degrees is presented. Thereafter, the effect of two types of single-sided welding paths and double-sided welding construction processes on residual stress distribution is discussed. Generally, the study results have shown that the equivalent residual stress near the weld reaches the maximum 344 MPa, which is very close to the Q345 steel yield strength. The size of the bending angle has no major effect on the residual stress distribution pattern, but it influences the residual stress value at the bending position. The residual stress at the bending position increases with the bending angle of CSWs. Different welding paths significantly impact the residual stress of the weld toe, and selecting a reasonable welding path can effectively reduce the residual stress value by 20 to 40 MPa.

Optimization of Residual Stress Field and Improvement of Fatigue Properties of Thin-Walled Pipes by Filling Laser Shock Peening

In the present work, a filling and laser shock peening (LSP) method is put forward and applied to a thin-walled pipe. Specimens were experimentally and numerically investigated to identify the residual stress field and fatigue properties of a pipe with and without LSP treatment. The numerical simulation indicated that the residual compressive stress first increased and subsequently dropped as the laser power density increased, and the extent of influence of the stretching wave, reflected from the lower surface on the unloaded area, increased with the spot diameter, causing surface tensile stress in the unloaded area. By filling the pipe with the guided-wave material, the residual stress field of the pipe that was treated with LSP was optimized, and the influence of the stress wave reflection on the residual stress field was effectively decreased. The surface residual stress of the filled guided wave material was −326 MPa, improving it by 57.6% compared with the pipe not filled with guided wave materials. The fatigue life of the pipe with the filled waveguide material that was treated by LSP was extended by 48.9%, compared with the untreated pipe.

Analysis of the thermal stability of residual stresses induced in Ti-6Al-4 V by high density LSP treatments

Laser Shock Processing (LSP) is increasingly applied as an effective technology for the improvement mechanical and surface properties of metallic materials for different types of components, mostly as a means of enhancement of their fatigue life behavior. As reported in previous contributions, a main effect resulting from the application of the LSP technique consists in the generation of relatively deep compression residual stresses fields allowing an improved mechanical behavior. In this paper, the special case of Ti-6Al-4 V alloy is considered with specific consideration of the microstructural changes and residual stresses fields justifying those macroscopic effects. In particular, the effect of the application of different typical LSP intensities on the microstructure and residual stresses fields introduced in this material and their possible correlation to the associated surface effects are analyzed. Emphasis is placed on the study of the thermal stability of these fields after an aging heat treatment at typical high temperature working conditions. The results show that, according to expectations, a certain level of residual stresses and dislocation density remains after in-work thermal aging. The combination of different material characterization techniques has made possible to obtain a correlated set of results highlighting the complementarity of different scale approaches from surface (X-ray) to bulk (neutrons) methods. A model based on the evolution of immobile dislocation density is proposed to rationalize the distribution of dislocation densities, pointing out that plastic deformation is retained. The results clearly show such material improvement stability under typical heavy-duty conditions, thus endorsing the use of the LSP for Ti-6Al-4 V as a suitable technology for industrial applications in relatively high temperature conditions.

Methods and Equipment for Experimental Evaluation of the Performance of Shell and Hull Structures

Introduction. Pressure-operated thick-walled hull structures are the most common type of high-duty welded structures. When these structures are loaded with internal pressure, a complex biaxial stress field arises in them, which is summed up with the fields of residual welding stresses. Therefore, when selecting a technology for manufacturing critical welded structures, the results obtained during conventional uniaxial tests of samples are insufficient. The variety of factors affecting the performance of structures, and the difficulties of separate assessment of their influence, caused the need to maximize the approximation of experimental conditions to the real working conditions of the structure.Materials and Methods. Testing of full-scale structures has a number of advantages, but they are extremely expensive, and, as a rule, only one, the weakest link, is identified, the bearing capacity of the other structural elements remains unclear. For testing, UDI radiometric installations designed for different sample sizes were used. The presented installations allow testing samples of various shapes, types of welded joints (butt, T-bar), changing the position of welded parts.Results. Without rejecting the results obtained during the testing of full-scale structures in the works of the Bauman Moscow State Technical University, DSTU, NRC «Kurchatov Institute» – CRISM «Prometey», and the authors proposed to conduct the basic scope of the research on individual structural elements that would reflect the characteristic features of loading, manufacturing technology, and operating conditions. The design of the «fitting-sheet» connections was applied, which made it possible to increase the indicators of the failure initiation and propagation to the level of the base metal.Discussion and Conclusions. Schemes of structures for obtaining a biaxial tension or bending field in samples were presented. Samples tested according to the proposed schemes allowed us to draw conclusions about the performance of welded joints under conditions close to the actual operation of the structures under study. The proposed test scheme is used by research laboratories in our country and around the world.

Effect of Residual Stresses on Wheel Fatigue Life and Experimental Validation

Steel wheels, consisting of rims and spokes, are important load-bearing parts of vehicles, and the fatigue fractures’ life estimation accuracy directly determines the stability and safety when applied in the transportation industry. The most common form of failure is fatigue fracture. The strong elastic–plastic deformation during the rim roll-forming process generates a residual stress field in its surface layer, which changes the actual stress distribution in the rim when it is loaded and thus affects the fatigue life of the steel wheel. In this paper, ABAQUS software was used to establish a rim-rolling simulation model to obtain the residual stress field distribution after forming and compare it with the actual residual stress on the formed surface of the rim to verify the reliability of the model. On the basis of this model establishment, the service hazard areas and maximum stresses of steel wheels with or without superimposed residual stress fields were calculated separately, and their fatigue lives were predicted separately using the local stress-strain method. The simulation results show that the maximum stress of the rim before and after the superimposed residual stress occurs in the area of the bottom of the groove, which is consistent with the actual failure location. However, the maximum stress after superposition increased from 120.7 MPa to 332.9 MPa, and the corresponding calculated life decreased from 158,340,000 to 459,500 cycles, which is closer to the actual test results. The results of the study can provide a theoretical basis for the lightweight design and process improvement of automotive steel wheels.

Numerical Study on Laser Shock Peening of Pure Al Correlating with Laser Shock Wave.

Laser shock peening (LSP) is an innovative and promising surface strengthening technique of metallic materials. The LSP-induced plastic deformation, the compressive residual stresses and the microstructure evolution are essentially attributed to the laser plasma-induced shock wave. A three-dimensional finite element model in conjunction with the dislocation density-based constitutive model was developed to simulate the LSP of pure Al correlating with the LSP-induced shock wave, and the predicted in-depth residual stresses are in reasonable agreement with the experiment results. The LSP-induced shock wave associated with the laser spot diameter of 8.0 mm propagates in the form of the plane wave, and attenuates exponentially. At the same time, the propagation and attenuation of the LSP-induced shock wave associated with the laser spot diameter of 0.8 mm are in the form of the spherical wave. The reflection of the LSP-induced shock wave at the bottom surface of the target model increases the plastic deformation of the target bottom, resulting in the increase of dislocation density and the decrease of dislocation cell size accordingly. Reducing the target thickness can significantly increase the reflection times of the LSP-induced shock wave at the bottom and top surfaces of the target model, which is considered to be conductive to the generation of the compressive residual stress field and grain refinement.

Evaluation of Residual Stress Fields in Friction Stir Welded Zone Based on the Plastic Strain Increment and Mises Yield Criterion

Evaluation of Residual Stress Fields in Friction Stir Welded Zone Based on the Plastic Strain Increment and Mises Yield Criterion

Surface integrity of gear shot peening considering complex geometric conditions: A sequential coupled DEM-FEM method

Shot peening is widely used in gear surface strengthening. Due to the complex 3D geometry of the gear target, the collision of shots occurs in the tooth space, resulting in the different shot dynamic and nonuniform features of surface integrity along the tooth profile. The existing models cannot accurately predict the surface integrity of gear shot peening, which hinders the process improvement. In this paper, a sequential coupled DEM-FEM method is proposed. The DEM model is used to represent the collision between shots and the final velocity distribution. Besides, with the determined shot kinematics, the FEM simulation for the gear shot peening process and its time optimization strategy are presented. The proposed method is validated by the gear shot peening tests. Based on the proposed method, the influence of gear geometry on shot impacts and uniformity of tooth surface integrity (surface roughness, surface topography, and compressive residual stress field) is investigated. This study provides an effective tool for optimizing gear shot peening technology and could be extended to other industrial parts with complex shapes.

Influence of tartaric‐sulfuric acid anodic film on four‐point bending fatigue behavior of AA 7050‐T7451 samples

AbstractTartaric–sulfuric acid (TSA) anodizing is an electrochemical process used for generating an anodic layer on the surface of aluminum alloys for aerospace applications. It must provide corrosion resistance and ensure no detrimental effect on the required fatigue properties for structural applications. This work aimed at analyzing the properties of the anodic layer and evaluating its influence on the fatigue behavior of the AA 7050‐T7451. The film properties were analyzed using optical and scanning electron microscopies. The surface residual stress (SRS) field in the as‐machined and anodized conditions was evaluated by the X‐ray diffraction technique.The results showed that TSA induces an appreciable reduction in the fatigue life, especially at lower values of stress amplitude, due to the presence of defects at the interface between the substrate and the anodic film. No correlation was observed between the presence of the anodic layer and the development of a tensile SRS field.

Heat dissipation and fatigue crack kinetic features of titanium alloy Grade 2 after laser shock peening

The work is devoted to experimental investigation of the laser shock peening (LSP) effect on fatigue crack propagation rate and heat dissipation at the crack tip in specimens made of titanium alloy Grade 2 with a stress concentrator. It is shown that the LSP can leads both to positive and negative effect on fatigue lifetime. The effective processing scheme, which includes stress concentrator zone, was proposed. This type of treatment forms an optimal residual stress field, which slows down the crack initiation and propagation processes. The effective LSP processing scheme reduces the value of the stress intensity factor and, as a consequence, effects on an intensity of plastic deformation at the crack tip. This effect can be visualised by measurement of heat flux from the crack tip area. Both heat flux from the crack tip and crack rate are less in the LSP processed specimens. Structural investigations of LSP treated material near fatigue crack path have shown that structural defects (twins) that appear on the surface of the material as a result of LSP do not have a significant effect on the fatigue crack propagation, and the configuration of the residual stresses field created by LSP plays a decisive role.

Stress and Strain State Analysis of Crack Front in Dissimilar Metal Welded Joints with Dual Field of Mechanical Heterogeneity and Residual Stress

The micro-mechanical state at the crack front is one of the key factors affecting the stress corrosion cracking (SCC) growth behavior. The mechanical heterogeneity and residual stress in the dissimilar metal welded joint (DMWJ) induce the micro-mechanical state at the crack front to become more complex. The sandwich model and dual-field model of the DMWJ with inner surface axial crack were established in this study. The stress and strain states at the crack front with different crack locations and lengths under the interaction of the mechanical property and the residual stress were investigated. The results show that a more accurate evaluation of stress and strain states can be obtained when using the dual-field model to describe the material mechanical property and residual stress of the DMWJ. The sandwich model overestimates the crack driving force including the stress and strain at the crack front. The tensile stress in the middle of shallow cracks is smaller than that at both ends, while the tensile stress in the middle of deep crack is larger than that at both ends. The variation trend of the tensile stress and normal strain at the crack apex is basically the same as that of the residual stress with the crack depth. However, there is almost no normal plastic strain in the initial stage of crack propagation due to the small residual stress in the initial stage.

Exploration of Key Process Parameters and Properties of 40Cr Steel in Ultrasonic Surface Rolling Process

Ultrasonic surface rolling (USRP) is an effective process to improve a workpiece’s fatigue property, in which ultrasonic vibration and static force are applied on the workpiece’s surface. In order to clarify the ultrasonic rolling strengthening mechanism of critical components and optimize the USRP parameters, a theoretical model of ultrasonic rolling was established. Based on the stress-strain curve of 40Cr steel, the USRP parameters were formulated. The compressive residual stress field of single point impact was analyzed by finite element simulation, and the simulation results were validated by conducting an experimental research. In addition, the changes in the surface properties of specimens under different USRP parameters were studied experimentally. The results show that with the increase in depth, the compressive residual stress on the external surface increases firstly and then decreases, and the maximum compressive residual stress is −338 MPa. As the amplitude is 12 μm and the frequency is 20 kHz, the static force of 600 N is optimal for the ultrasonic rolling of 40Cr steel. This study could provide a guide for the key parameters’ selection in USRP.

Analysis of the Polymer Two-Layer Protective Coating Impact on Panda-Type Optical Fiber under Bending.

The article discusses the effects of thermal-force on the Panda-type optical fiber. The studies used a wide temperature range. The research used two thermal cycles with exposures to temperatures of 23, 60 and −60 °C. The field of residual stresses in the fiber formed during the drawing process was determined and applied. Panda was considered taking into account a two-layer viscoelastic polymer coating under conditions of tension winding on an aluminum coil in the framework of a contact problem. The paper investigated three variants of coil radius to analyze the effect of bending on fiber behavior. The effect of the coating thickness ratio on the system deformation and optical characteristics was analyzed. Qualitative and quantitative patterns of the effect of temperature, bending, thickness of individual polymer coating layers and relaxation transitions of their materials on the Panda optical fiber deformation and optical characteristics were established. Assessment of approaches to the calculation of optical characteristics (values of the refractive indices and fiber birefringence) are given in the framework of the study. The patterns of deformation and optical behavior of the Panda-type fiber with a protective coating, taking into account the nonlinear behavior of the system materials, were original results.

A phase field approach to fracture for hyperelastic and visco-hyperelastic materials applied to pre-stressed cylindrical structures

In this investigation, the mechanical modeling of nonlinear visco-hyperelastic residually stressed materials obtained from an invariant-based constitutive energy framework is coupled with the phase field approach to fracture. The main target regards the extension of the phase field method to simulate pre-stressed cylindrical structures subjected to monotonic axial pulling load upon failure. This formulation is incorporated into a numerical procedure using the Finite Element Method (FEM), in particular, it is implemented in the commercial FE package ABAQUS as a user subroutine UMAT. Results suggest the dependence of the mechanical behavior and the crack pattern of these structures on not only viscous parameters like the relaxation time and the displacement rate, but also on the strength of the residual stress field, which in turn depends on geometrical characteristics of the cylindrical structure such as the radius or the length. A range of solutions related to crack propagation is shown for different cylindrical structures, from azimuthal crack propagation to axial one. The proposed framework aims to provide an extended application for the already-defined visco-hyperelastic formulation by the inclusion of residual stresses.

Global stiffness and residual stresses in spinal fixator systems: A validated finite element study on the interconnection mechanism

Posterior spinal fixation systems are the gold standard to treat different column disorders using rods and screws. The proper connection between them is guaranteed by the Interconnection Mechanism (IM), consisting of different metallic subcomponents held together through the application of tightening torque. The response of the fixation system is defined by its overall stiffness, which in turn is governed by the local residual stress field arising during tightening. Although literature computational models for studying spinal fixation are becoming increasingly anatomically complex, most studies disregard completely the realistic modeling of the IM, namely choosing elastic-plastic material models and proper contact interactions. In this frame, the present study aims at increasing awareness in the field of spinal fixation modeling by investigating the mechanical response of the IM in terms of overall stiffness and local residual stresses. Once validated through dedicated experiments, the results of the proposed model have been compared with the current literature, highlighting the key role of the IM in the reliable modeling of spinal fixation.

Shape dynamics of a red blood cell in Poiseuille flow

In this study, we used a three-dimensional biconcave capsule with a residual stress field to numerically simulate the dynamics of a single red blood cell (RBC) in both confined and unconfined Poiseuille flow. We confirm that taking the stress-free configuration of the RBC to be an oblate near-sphere leads to results which are most consistent with the experiments in both shear flow and Poiseuille flow settings.

Fatigue life enhancement of foreign object impacted railway axle EA4T steel with surface shot peening

This study investigates the effects of surface reinforcement and foreign object damage (FOD) on the fatigue property of the EA4T alloy steel used for high-speed railway axles. The results of characterization of its microstructure revealed the evolution of the shot-peening-reinforced layers and the craters induced by the impact. Moreover, the results of reconstruction of the residual stress field at the crater, obtained by combining the Johnson–Cook model with the temperature field difference setting method, showed the development of residual tensile stress at the rim of the impact crater and residual compressive stress at its bottom. The fatigue test results demonstrated that surface reinforcing can effectively enhance the fatigue resistance of FOD induced specimens. Probabilistic fatigue SN curves constructed by an improved backward statistical inference method can be used to derive reliable fatigue properties. The fatigue-induced fracture showed that the combination of competing effects of stress concentration, residual stress, micro damage and microstructure were the main fatigue failure mechanisms of axles subject to foreign object damage. The curves of the crack growth rate predicted by the iLAPS model indicated that the service life of the damaged axle can be adequately calculated based on its static tensile properties.