Initial Residual Stress Research Articles

Eigenstrain‐based analysis of why uniformly shot peened aluminium plates bend more in the rolling direction

AbstractShot peen forming is a process widely used to shape aircraft components such as wing skins, yet its fundamental working is still crudely understood. It is understood that a light conventional peen forming treatment applied uniformly over an initially flat plate will induce isotropic in‐plane stretching of the surface layer and will thus lead to a panel curving with identical curvatures in all directions. However, [1] made the startling observation that uniformly peen formed aluminium plates of different aspect ratios all bent along their laminating direction irrespective of the peening direction. This experimental result is counterintuitive because the residual stresses due to the lamination process are 1 order to 2 orders of magnitude smaller than those induced by the shot peen forming treatment. In the present study, we apply the eigenstrain theory to estimate the effect of the different sources of anisotropy on uniformly peen formed aluminium plates. Potential sources of anisotropy included the plastic anisotropy of rolled aluminium, nonequibiaxial initial stresses that redistribute when their equilibrium is disturbed by peening, the geometry of the specimens and externally applied prestress. For the alloy and peening conditions considered, we show that plastic anisotropy had no discernible influence on the resulting shape of the peen formed specimens. Initial residual stresses, on the other hand, caused slightly larger bending loads in the rolling direction of the alloy. Although the magnitude of these loads was approximately 30 times smaller than peening‐induced loads, it was sufficient to overcome the geometric preference for rectangular sheets to bend along their long side and cause all unconstrained specimens to bend along the rolling direction instead. Our analysis highlights the importance of the history of the material that is being peened. Residual stresses already present in the part before peening must be considered to ensure good simulation predictions.

Study on the simplified evaluation method of the remaining load-carrying capacity of a corroded steel I-girder end using FEA

In a steel I-bridge, corrosion often occurs severely at the I-girder end rather than at other parts. Therefore, this work mainly discusses the corrosion at the I-girder end as a bearing problem and shear capacity of the end plate. A finite element model (FEM) of a corroded steel I-girder end is developed considering commonly occurring corrosion at first. For the initial imperfection of a steel I-girder end, the initial deformation and residual stress are considered in the model which are simulated by elastic buckling analysis (eigenvalue analysis) and thermal stress, respectively. To investigate the effect of the corrosion pattern on the strength development, the single-area corrosion (flange, stiffener, web) and multi-area corrosion are included in the case study. Finally, the simulation results from the case study using finite element analysis (FEA) are used to formulate the simplified equations for evaluating the remaining load-carrying capacity of a corroded steel I-girder end with the specified corrosion patterns. Furthermore, for the engineering design and preliminary assessment of a corroded steel I-girder end, the simplified equations for evaluating the remaining yield strength ratio are also provided based on the remaining load-carrying capacity.

Strain stiffening retards growth instability in residually stressed biological tissues

Soft biological tissues often exhibit notable strain stiffening under increasing stretch, and this can have significant effects on tissue growth and morphological development, such as causing symmetry breaking in growing airways and leading to mucosal folding and airway hyperresponsiveness. To investigate the role of strain stiffening and the multifactorial control in growth and remodeling, we consider a growing tubular structure with strain-stiffening effects caused by increased and tightened collagen. In addition, we employ the nonlinear hyperelastic Gent model and initial stress symmetry theory to include the coupling effects of differential growth and initial residual stress. Results show that for strain stiffening that takes place at higher strain (Jm>21), the maximum critical growth ratio matches that obtained using neo-Hookean model calculations. Meanwhile, for biological tissues that exhibit strain stiffening under moderate strain conditions (0.46<Jm<21), the strain-stiffening effect delays significantly the onset of growth instability. When strain stiffening takes place at very low strains (Jm<0.46), stiff biological tissues can prevent growth instability, resulting in a smooth hyperelastic cylindrical tubular structure, and the epithelial tissue remains stable at all growth stages without forming any unstable morphology. Our results suggest that strain stiffening can induce retardation instability during biological growth and remodeling, but airway remodeling can incorporate this effect by increasing wall stiffness and reducing obstruction. This highlights the importance of considering the impact of strain stiffening on biological growth and remodeling, which can inform the development of effective clinical interventions for chronic inflammatory airway diseases.

Effect of residual stress in gradient-grained metals: Dislocation dynamics simulations

A three-dimensional discrete dislocation dynamics (DDD) method was used to investigate the effect of residual stress on the stress-strain response of gradient nano-grained (GNG) metals. The GNG polycrystalline model was constructed, and different residual stress distributions were introduced in the DDD framework. The simulation shows that the distribution of residual stress has a significant influence on the tensile stress-strain curve. For a GNG sample containing both compressive and tensile residual stresses, the flow stress is firstly lower but ultimately higher than that of the sample without residual stress, which is caused by the combined effects of residual stress and gradient-grained structure. The evolution of dislocation microstructures indicates that the initial tensile residual stress in the sub-surface of the sample can promote the dislocation activation and thus lead to the decrease of overall yield stress. Whereas the compressive residual stress in the topmost surface of the sample can suppress the dislocation activation and multiplication in other regions, resulting in finally higher flow stress in comparison to that of the sample without residual stress. In addition, it is demonstrated that the reduction in the elastic modulus for GNG samples with residual stress observed in some experiments is due to the earlier plastic deformation that occurred in the region with initial tensile residual stress. The present study provides insights into the individual and combined effects of residual stress and gradient-grained structure in GNG metals.

Elastic–plastic analysis of high load ratio fatigue tests on a shot-peened quenched and tempered steel, combining the Chaboche model and the Theory of Critical Distances

This study aimed to predict the uniaxial fatigue strength of shot peened notched specimens made of 42CrMo4 quenched and tempered steel. The Chaboche kinematic hardening rule was employed to model the cyclic plastic behaviour of the tested samples and the evolution of the residual stresses, which were measured with X-ray diffraction and then reproduced through a finite element software with a novel proposed procedure. This latter introduced a distribution of eigenstrains to reproduce the initial residual stresses and a hardening of the material to replicate the residual stresses measured after the loading for a run-out specimen. The Theory of Critical Distances combined with the Smith-Watson and Topper multiaxial fatigue criterion were then employed to predict the fatigue strength resulting in a good level of accuracy.

Investigation of the initial residual stress effects on a work roll maximum in-service stress in hot rolling process by a semi-analytical method

A High magnitude of von Mises stress is considered a source of plastic deformation, fatigue crack initiation, and surface defects. A Detailed finite element model (FEM) was proposed here to estimate the thermo-mechanical stress and residual stress components at an industrial hot rolling work roll during the rolling process. Also, a semi-analytical procedure was developed here to study the effect of the initial residual stress components that remained after the work roll heat treatment process on maximum von Mises stress. The work roll referred to here was an Indefinite Chilled Double Poured (ICDP) cast iron roll that was used as the fifth finishing stand of strips hot rolling. Residual stress components were measured by the ring-core method at two stages, after the heat treatment process as initial components and after the service life of the work roll as final components. Estimated residual stress components by the developed method were compared with final experimental residual stress measurements to verify the FEM. Finally, by using artificial neural network and genetic algorithm, the maximum value of von Mises stress was predicted, and initial residual stress components were optimized in order to minimize the maximum value of attained von Mises stress at the work roll surface, where the most fatigue cracks initiate and grow.

Lateral Torsional Buckling Resistance and Design of High Strength Steel Beams Subjected to Nonuniform Temperature

The mechanical characteristics of high-strength steel (HSS) at high temperatures vary significantly from those of mild steel, so fire design regulations derived from mild steel cannot be transferred to HSS structures. Therefore, this paper investigates the fire resistance of beams made of HSSs, which will facilitate the design and application of HSS structures. Due to the heat dissipation of the concrete floor, steel beams are generally heated from three sides, and the temperature distribution in the steel beam cross-section is nonuniform. For such cases, the fire resistance of HSS beams is assessed using finite element models (FEMs) with nonuniform temperature distribution. A Ramberg–Osgood model was used to predict stress–strain relationships of Q460, Q690, and Q960 HSSs at elevated temperatures, and the model was verified by material tests. In the FEM, the proposed stress–strain relationships of HSSs at elevated temperatures, initial geometric imperfections and residual stress are considered for better accuracy. After verifying the established model against experimental results, the influence of nonuniform temperature distribution, load pattern, steel grade, and slenderness ratio on the resistance of HSS beams is investigated by conducting parametric analyses using the verified FEM. It is found that the critical bending moment of steel beams at elevated temperatures associated with lateral-torsional buckling highly depends on the reduction of elastic modulus of steel. By comparing FEM results with EN 1993-1-2, it is evident that EN 1993-1-2 is unsuitable for assessing the fire resistance of HSS beams. Therefore, the expression specified in EN 1993-1-2 is modified by fitting the finite analysis results to more accurately evaluate the fire resistance of HSS beams considering the nonuniform temperature distribution.

Study of distortion on milled thin-wall aluminum parts influenced by initial residual stress and toolpath strategy

Monolithic aluminum alloy parts are highly required in the aeronautical industry, but they show significant geometrical distortion after the machining process. This work investigated the distortion attributed by the initial residual stress of the raw material and the machining-induced residual stress during the milling process, as well as exploring the effects of the machining toolpath strategy. Single-/multi-pocket parts were milled from 7050-T7451 aluminum blocks with different initial residual stresses, and an element deletion method was developed for the numerical study to simulate different sequences of material removal. It was revealed that the toolpath parallel to the long side of the block caused more distortion on the side surfaces of the final part. The value of distortion was positively correlated to the magnitude of the initial residual stress of raw material. The simulation results indicated that the distortion attributed by machining-induced residual stress accounted for about 15% of the final distortion. The finding promotes the design optimization of machining monolithic parts by minimizing distortion, thereby benefitting the application of large monolithic parts in industry.

Effect of Deposition Temperature on Long-Term Residual Stress Evolution of Au Films.

To increase the residual stress stability of Au films while reducing the residual stress level, the effect of deposition temperature on long-term residual stress evolution of Au films under different conditions were studied. Au films with a thickness of 360 nm were deposited using e-beam evaporation on fused silica under different temperatures. Observations and comparisons were made of the microstructures of Au films deposited under different temperatures. Results showed that by increasing the deposition temperature, a more compact microstructure of Au film was obtained, which was manifested in increased grain size and reduced grain-boundary voids. After deposition, a combined process consisting of natural placement and 80 °C thermal holding was conducted on the Au films, and the residual stresses were monitored using the curvature-based technique. Results showed that the initial tensile residual stress of the as-deposited film decreased with the deposition temperature. The Au films with higher deposition temperatures showed better residual stress stability, maintaining low stress levels in the subsequent long-term combination of natural placement and thermal holding. The mechanism was discussed based on the differences in microstructure. Comparisons were made between post-deposition annealing and increased deposition temperature.

Microplastic deformation activating residual stress relief for Al alloy

Thermal-vibratory technique has been demonstrated to effectively and rapidly reduce residual stress and improve mechanical properties of materials. But the underlying residual stress relief mechanism remains unclear, limiting the development and application of the technique. In this work, the relationship between residual stress and microstructure evolution during thermal-vibratory process in 7085 Al alloy is systematically investigated, using a combination of experiments, molecular dynamic (MD) simulations, and artificial neural network (ANN). Experimental results show that increasing the thermal-vibratory temperature improves the residual stress relief rate and triggers the uniform distribution of dislocations owing to temperature-enhanced dislocations activity. At the atomic scale, the MD simulation also observed an increasing tendency in residual stress relief rate with increasing temperature. Moreover, MD simulations further reveal that the high shear strain around grain boundaries triggers dislocation motion, accompanied with the decrease of residual stress in the grain interior. That means the residual stress relief is controlled by the dislocation activation around grain boundaries, and the residual stress is relieved only when the material undergoes plastic deformation. Based on the ANN, a map of the residual stress relief rate versus the initial residual stress, thermal-vibratory amplitude, and temperature is constructed, to guide the optimal thermal-vibratory process. These results provide fundamental knowledge and method to determine the residual stress after thermal vibratory treatment, and can further realize the precise control of shape and properties of materials during manufacturing.

The inclusion and role of micro mechanical residual stress on deformation of stainless steel type 316L at grain level

Validating crystal plasticity models requires careful consideration of all aspects. The initial conditions are important when comparing a model and experiment, as the initial residual stresses within the material can be significant but are often overlooked due to experimental limitations. Therefore, their inclusion has the potential to improve model predictions. This work explores the efficacy of using type-III residual elastic stresses, measured using high resolution electron backscatter diffraction (HR-EBSD),as pre-test stress distributions in 316L stainless steel. Two methods of processing the stresses collected (direct and using a least squares solver) were used. A existing method was used to incorporate the experimental measurements as initial residual stresses; The model results were compared with each other, a model containing no residual stress and the experiment. The least squares method helped remove extreme stresses calculated by the cross-correlation method at points with high mean angular error. The modelled stress distributions from both the direct and least squares model did not fully match the experimentally observed stresses but similarities were seen within some grains.After loading, little difference was seen between the models, with and without the inclusion of residual stress, implying a negligible effect on the deformation response of the material. However, the predicted stress distribution did not match with the experimentally measured stress distribution after deformation, suggesting further physical effects must be accounted for in crystal plasticity models.

On One Approach in Fracture Mechanics of Composites with Parallel Cracks Under the Action of Initial (Residual) Stresses

On One Approach in Fracture Mechanics of Composites with Parallel Cracks Under the Action of Initial (Residual) Stresses

Fire resistance of low alloy Q420d high-strength steel column under high-temperature creep.

The high-temperature mechanical and creep characteristics of Q420D steel are investigated in this essay. To determine the steel's high-temperature yield strength, the high-temperature tensile test of Q420D steel was first performed. In the temperature range of 400°C-800°C, the high-temperature creep test under various pressures was conducted, and the creep strain curve over time was produced. Finite element analysis and comparison were done to examine the impact of creep strain on the Q420D steel column's bearing capacity under high-temperature conditions. The findings demonstrated that: Using Abaqus, a finite element fire resistance analysis of a Q420D steel column, was conducted while taking initial geometrical flaws, residual stress, and creep effect into account. As a result, the critical temperature of a Q420D steel column under various load ratios was determined. The largest deviation from the critical temperature in the standard GB51249-2017 was 2.9% when the creep effect was taken into account under the load ratio R = 0.3. The highest reduction in fire resistance limit time under low load ratio conditions, taking into account the creeping impact of Q420D steel columns, is 35%. The findings demonstrate that the high-temperature creep energy greatly lowers the steel column's fire resistance.

Biomimetic vascular tissue engineering by decellularized scaffold and concurrent cyclic tensile and shear stresses

Decellularization by chemical approaches has harmful effects on extracellular matrix (ECM) proteins, and damages lots of functional peptides and biomolecules present in the ultrastructure. In this study, we employed a combination of chemical and physical decellularization methods to overcome these disadvantages. The induced osmotic pressure by hypertonic/hypotonic solutions dissociated and removed most of cellular membranes significantly without any detergent or chemical agent. In total, 0.025% trypsin solution was found adequate to remove the remaining debrides, and ultimately 1% Triton X-100 was utilized for final cleansing. In addition, conducting all the decellularization processes at 4 °C yielded an ECM with least damages in the ultrastructure which could be inferred by close mechanical strength and swelling ratio to the native vessel, and high quality and quantity of cell attachment, migration and proliferation which were examined by optical microscopy and scanning electron microscopy (SEM) of the histology samples. Moreover, the obtained biological scaffold (BS) had no cytotoxicity according to the MTT assay, and this scaffold is storable at −20 °C. Employing bioreactor for concurrent cyclic tensile and shear stresses improved the cell migration into pores of the BS and made the cells and the scaffold compact in analogous to native tissue. As opening angle test showed by decellularizing of the blood vessel, the residual stress dropped significantly which revealed the role of cells in the amount of induced stress in the structure. However, intact and healthy ECM explicitly recovered upon recellularization and beat the initial residual stress of the native tissue. The tensile test of the blood vessels in longitudinal and radial directions revealed orthotropic behavior which can be explained by collagen fibers direction in the ECM. Furthermore, by the three regions of the stress–strain curve can be elucidated the roles of cells, elastin and collagen fibers in mechanical behavior of the vascular tissues.Graphical

Overall stability behavior and design method of Q460 high strength steel welded I-section overhanging beams

To investigate the overall stability behavior of Q460 high strength steel (HSS) welded I-section overhanging beams and develop the corresponding design method of codified specifications, nine overhanging beams with various top flange strengthened section forms and overhanging span length ratios were tested under concentrated loads. The initial geometric imperfections and residual stresses were measured in advance. Test results indicate that the overall stability capacity of the specimens decreases with the increase of overhanging span length ratio and top flange strengthened has a greater effect on the overall stability capacity of overhanging beams than overhanging span length ratio when the dominative stability segment of specimens is located at the simply supported span. Finite element analysis (FEA) models verified by test results were established to examine the effects of load form, load ratio, section form, height-to-width ratio, simply supported span length and overhanging span length ratio on the overall stability capacity of Q460 HSS welded I-section overhanging beams. On the basis of the eigenvalue buckling analysis results and the formulas for the elastic moment of simply supported beam stipulated in Design code of steel structures (GB50017-2017), the calculation formulas of elastic critical moment for overhanging beams were obtained by regression. Furthermore, the calculation formulas of ultimate bending moment for Q460 HSS welded I-section overhanging beams were proposed based on the obtained formulas for elastic critical moment of such beams, test results and nonlinear buckling analysis results.

Buckling and shape control of prestressable trusses using optimum number of actuators

This paper describes a method to control the nodal displacement of prestressable truss structures within the desired domains. At the same time, the stress in all members is unleashed to take any value between the allowable tensile stress and critical buckling stress. The shape and stresses are controlled by actuating the most active members. The technique considers the members’ initial crookedness, residual stresses, and slenderness ratio (S). Furthermore, the method is premeditated so that the members with an S between 200 and 300 can carry only tension before and after adjustment (i.e., the maximum compressive stress for the members with an S between 200 and 300 is zero). In addition, the derived equations are coupled with an optimization function that relies on five optimization algorithms (interior-point, trust-region-reflective, Sequential quadratic programming (SQP), SQP-legacy, and active-set). The algorithms identify and then exclude inactive actuators in the subsequent iterations. The technique is applied to several examples, and its results are compared with a quoted method in the literature.

Numerical Analysis of Low-Cycle Fatigue Using the Direct Cyclic Method Considering Laser Welding Residual Stress

The sequential-coupled thermo-mechanical model and direct cyclic technique are employed to investigate laser welding and low-cycle fatigue residual stress, respectively. The effects of residual stress on fatigue properties and the relaxation behaviour are analyzed. The simulation results highlight the strong dependence of laser welding residual stress on constitutive models and how low-cycle fatigue residual stress is influenced by the initial laser welding residual stress. Furthermore, the simulation indicates that residual stress redistributes and relaxes in the weld and heat-affected zone after low-cycle fatigue. To validate the accuracy of the simulation, the neutron diffraction experiment is carried out, and the experimental data are consistent with the simulation results.

Formation of residual stresses during reaming of holes

A new approach to estimating the initial residual stresses during cutting is considered, which involves determining the stress-strain state of the surface layer by numerically searching for the equilibrium of shells surrounding a certain spherical core with a volume equal to the volume of the metal pressed into each point of the machined surface. Keywords: reaming, tension, blade, radius of point, shell, core. kondrat.leonid@yandex.ru

Stress-Induced Deformation of Thin Copper Substrate in Double-Sided Lapping

Double-sided lapping is an precision machining method capable of obtaining high-precision surface. However, during the lapping process of thin pure copper substrate, the workpiece will be warped due to the influence of residual stress, including the machining stress and initial residual stress, which will deteriorate the flatness of the workpiece and ultimately affect the performance of components. In this study, finite element method (FEM) was adopted to study the effect of residual stress-related on the deformation of pure copper substrate during double-sided lapping. Considering the initial residual stress of the workpiece, the stress caused by the lapping and their distribution characteristics, a prediction model was proposed for simulating workpiece machining deformation in lapping process by measuring the material removal rate of the upper and lower surfaces of the workpiece under the corresponding parameters. The results showed that the primary cause of the warping deformation of the workpiece in the double-sided lapping is the redistribution of initial residual stress caused by uneven material removal on the both surfaces. The finite element simulation results were in good agreement with the experimental results.

Residual stress stability of HFMI-treated transverse attachments under variable amplitude loading with the P(1/3) and the linear spectrum

The potential of HFMI treatment to increase fatigue life under service loading remains in debate. However, some recent studies show that even under variable amplitude loading (VAL), fatigue strength is increased compared to untreated welds. Discussions generally focus on the stability of initial compressive residual stresses, which may be reduced during VAL due to high peak stresses. In this context, the potential of HFMI treatment is often only attributed to residual stress stability. This study presents further results on the effect of VAL with a P(1/3) and a linear load spectrum on the residual stress stability of HFMI-treated transverse stiffeners (TS) made of mild steel (S355) and high-strength steel (S700M). The impact of random, High-Low and Low-High loading sequences on the fatigue strength as well as on the residual stress behaviour of HFMI-treated joints will be discussed.