Residual Stress Relaxation Research Articles

Deformation behavior and stress relaxation modeling of Cu–3Ti alloy prepared by a novel rolling process with the shortened passes

This study explores the effects of hot rolling with a shortened process on the microstructure and properties of Cu–3Ti alloy compared to the traditional multi-pass forming, and the finite element simulation was used to predict the experimental results. At the same time, the residual stress elimination model was used to provide prediction and reference for the relaxation of residual stress during the alloy annealing process. The residual stress of hot rolled samples through the shortened passes is reduced by an average of about 6.7 % after the appropriate annealing treatment, and accompanied by an obvious increase in the yield and tensile strengths compared to that of the traditional multi-pass forming. At the same time, this hot rolled sample by 90 % reduction with 9 passes contains a 45.7 % proportion of high-angle grain boundaries due to the occurrence of extensive dynamic recrystallization, which remains a good consistence with the higher strength under this condition.

From raw elements to 3D samples: An economical route for Co-Cr-Mo alloy fabrication

This study explores an efficient and feasible method for production of non-commercial Co-Cr-Mo powder feedstock for Laser Powder Bed Fusion (LPBF), employing mechanical alloying (MA) with following thermal treatment. Elemental Co, Cr, and Mo powders were subjected to MA to create a powder nanostructured composite. Initially, the mechanically alloyed powder had a mixed-phase state, with ε-phase as the main one (about 95 wt%) and impurities of Cr2O3. However, an additional 1.5-hour of isothermal annealing at 400 °C successfully reduced the Cr2O3 impurities and caused oxygen content to decrease, indicating a reduction in residual oxygen in the powder. This annealing process activated diffusion and induced relaxation of residual stresses, evidenced by decreased lattice microstrains: 0.8% for the Co-phase, 0.2% for both the Cr-phase and Mo-phase. Powder subjected to the thermal treatment was then utilized in LPBF to create bulk Co-Cr-Mo samples. The formed samples showed a typical LPBF microstructure represented by uniaxial cells and dendrites, with the distance between these structures not exceeding 3 µm. The samples showed homogenous elemental distribution, mixed-phase composition, and oxygen concentration not exceeding 5 wt%. In terms of mechanical properties, the LPBF samples exhibited a microhardness value of 4300 ± 200 MPa, ductility of 0.85, and yield strength of 1.4·103 MPa. Our method yields results comparable to those obtained from commercially spherical powder, indicating that our cost-effective approach does not sacrifice product quality. It represents a substantial cost reduction, approximately 4.5 times lower than that of commercial spherical powder, thereby highlighting the economic efficiency of our powder feedstock preparation for LPBF. Our findings underscore the potential of mechanically alloyed and thermally treated powder in LPBF. This worthwhile and efficient method of preparation powder feedstock breaks ground for more comprehensive facilities in 3D printing, cut down on cost of production with maintaining the high-quality output.

Re-evaluation of fatigue design curves for offshore wind monopile foundations using thick as-welded test specimens

The dominant majority of existing offshore wind turbines are supported using monopile foundations which are fabricated by welding thick steel plates. In the current fatigue design recommendations for welded steel structures, which are commonly used in the design of monopile structures, the initial inverse slope of the S-N curve is fixed to −3. The historic rationales for this assumption are the ease of fatigue life calculations, the effect of long-range residual stresses that were not originally captured in the test data on thin welded specimens, and analogy with fatigue crack growth. While this introduces an unquantified level of conservatism to deterministic fatigue calculations, it was an acceptable (and wise) assumption when the S-N design curves were generated a few decades ago; however, current designs and assessments require more accurate calculations and the need to more confidently quantify the likelihoods attached to these evaluations. In the present study, it is argued that unlike large braced structures (e.g. jackets) in which long-range residual stresses may remain in place as fatigue cracks propagate, in circumferential welds of monopile structures the natural relaxation of residual stresses, in the absence of long-range residual stresses, must be accounted for by performing fatigue tests on thick welded samples. This paper presents a thorough analysis of the fatigue test results obtained from 50 mm thick as-welded samples with representative profiles of residual stresses that exist in monopiles. Basic and advanced statistical approaches were employed in the analysis and higher values of inverse slope were found using the large thickness test data. The results from this study are compared with the recommended fatigue design curves available in international standards. The outcomes from this research draw important observations concerning the need to employ an appropriate value of inverse slope in the design of monopile structures by excluding the conservative approach imposed by a fixed value of −3 which was originally implemented for braced structures with the assumption of large and long-range residual stresses.

HFMI-induced fatigue strength improvement of S355 steel transverse non-load-carrying attachments with lack of fusion in the weld root

The effects of high-frequency mechanical impact (HFMI) treatment and preloading on the fatigue behaviour of the S355 steel transverse non-load-carrying attachments welded with technological defect (‘lack of fusion’) in the weld root are studied. The S355 steel joints were purposely welded with incomplete penetration (a technological defect of 3x3 mm formed over the entire weld root length). As established, this technological defect does not affect the fatigue life since the crack initiation and propagation occur along the weld toe. HFMI leads to a tenfold increase in fatigue life of the joints with technological defects. Preloading the as-welded joints at maximum alternating stress of 260 MPa for 64,000 cycles leads to partial relaxation of tensile residual stresses. HFMI transforms tensile stresses into compressive ones in both non-preloaded and preloaded samples. Compressive stresses of 315 MPa and 100 MPa were respectively registered at distances of 3 mm and 13 mm from the HFMI-formed groove using the hole-drilling method and electron speckle interferometry. Transmission electron microscopy of the weld-heating affected zone reveals that the bimodal microstructure, containing the areas with ultrafine and larger grains, observed after sequentially applied preload and HFMI remains stable during further fatigue cycling and facilitates the enhanced fatigue strength/life.

Local relaxation of residual stress in HFMI-treated high-strength steel welded joints subjected to high-peak loads

This study investigates the effect of high-peak loads on local relaxation of residual stress and fatigue damage in HFMI-treated high-strength steel welded joints. The finite element simulations are applied to consider the combined effect of geometry, material, and residual stress. This simulation model specifically contains the actual treated surface profile with material imperfections. As a result, the material imperfections highly influence the local elastic-plastic behavior under load conditions, including a high tensile and compressive peak load, thus the local relaxation of residual stress and fatigue damage. The results provide further understanding towards a robust modeling approach to the fatigue life estimation of HFMI-treated welds subjected to high-peak loads.

Ensuring the technological strength when welding the shaft with flange of auto-vehicles

In engineering, the use of highly loaded shafts with flanges is mainly intended for additional attaching of various technological equipment. At the same time, the shaft-flange fitting is performed using an annular weld. Next, parts such as gears, pulleys, bushings, etc. are mechanically attached to the flange. The work considers the achievement of the technological strength of the shaft-flange fit when using arc welding methods. It has been established that the emergence of dangerous defects such as weld cracks is possible. The cause for this is a number of structural, technological and metallurgical restrictions. It is shown that it is expedient to apply welding technological schemes that make it possible to minimize areas of structural heterogeneity. It is proposed to use a transition layer made of low-carbon material, which is characterized by high plastic properties. As a result, the creation of a barrier layer in the zone of structural heterogeneity is achieved. The plastic intermediate weld contributes to the partial relaxation of residual stresses of welding and increases the crack resistance of the welded joint as a whole.

Effect of Tensile Loading on the Residual Stress Relaxation Behavior of Induction Hardened SCM440 Steel with a Shallow Hardened Layer

Effect of Tensile Loading on the Residual Stress Relaxation Behavior of Induction Hardened SCM440 Steel with a Shallow Hardened Layer

Overload and variable amplitude load effects on the fatigue strength of welded joints

Different load spectra and individual load peaks might substantially relax high residual stresses as well as induce compressive residual stresses in welded components and, consequently, affect the fatigue performance of these joints. Consideration of peak loads and resulting relaxation of residual stresses in fatigue analyses can substantially enhance the accuracy of life prediction. The aim of the current study is to experimentally investigate the fatigue strength of welded joints subjected to different levels of overloads and variable amplitude (VA) loads and to develop local fatigue assessment method to account for the relaxation of residual stresses via a mean stress correction using the 4R method. The 4R method applies a local stress ratio for the mean stress correction considering material strength, residual stresses, applied stress ratio of external loading and local weld geometry in elastic–plastic material behaviour. Fatigue tests were carried out on fillet-welded longitudinal gusset joints made of S700 high-strength steels under applied stress ratio R = 0–0.1. A mild strength steel (S355) and ultra-high-strength steel (S1100) were selected as reference steel grades for the fatigue testing to study the material strength effects. Numerical analyses were conducted to evaluate the fatigue notch factors using the effective notch stress concept with the reference radius of rref = 1.0 mm and theory of critical distance (TCD) using the point method. The experimental results indicated that a substantial improvement in the fatigue strength capacity can be claimed in the joints subjected to tensile overloads, particularly in the studied S700 and S1100 steels. The higher-level overload (0.8fy), corresponding to the nominal cross-sectional area, improved the mean fatigue strength of the welded joints manufactured of high-strength S700 steel by approximately 60%, while the lower overload (0.6fy) improved the mean fatigue strength by 20%. In addition, a use of equivalent nominal stresses for the joints subjected to VA loads resulted in conservative assessments when employing S–N curves obtained for the CA loading. The 4R method, via the local mean stress correction for individual cycles, provided higher accuracy for the fatigue assessments.

Thermal relaxation of compressive residual stresses in surface gradient nanostructure of TC11 titanium alloy

The thermal relaxation behaviour of compressive residual stress (CRS) in the gradient nanostructure of TC11 titanium alloy treated with ultrasonic surface rolling process (USRP) was investigated by X-ray residual stress test, transmission electron microscope (TEM) and high-resolution TEM (HRTEM). The Zener-Wert-Avrami (ZWA) model was successfully used to simulate the CRS relaxation behaviour in nanocrystalline and non-nanocrystalline regions at different temperatures. The results show that the relaxation of CRS is thorough and fast at higher temperature (650 ºC). There are two distinct relaxation stages at lower temperatures, and the relaxation rate in the first stage is much faster, dominated by the annihilation of lattice defects through short-range diffusion. The relaxation in the second stage is slow and dominated by dislocation rearrangement. In contrast to the situation at low temperatures, the nanocrystals appear to be a negative factor in stabilizing the CRS at high temperatures, as the unstable sub-grain boundaries act as sources of stress relaxation and the multiple grain boundaries act as high-speed diffusion channels and accelerate the annihilation of dislocations.

Pair interaction of intersecting dilatation and disclination defects

An elastic interaction of the intersecting dilatation and disclination defects located in an infinite linear isotropic media is investigated. The eigenstrain approach is employed to obtain the analytical expressions describing the pair interaction between intersecting dilatational lines and intersecting wedge disclinations. It is demonstrated that the interaction energy strongly depends on the intersection angle between the defects. The energy reaches the maximum value if the defect lines are coincided while the energy reaches the minimum value if the defect lines are orthogonal. Besides, it is shown that interaction energy of intersecting wedge disclinations strongly depends on the elastic properties of the media: the less thePoisson ratio, the less the energy. The obtained analytical results seem to be applicable for the theoretical analysis of the residual stress relaxation mechanisms in heterostructures with pentagonal symmetry such as icosahedral particles

Estimation of residual stress relaxation in low alloy steel with different hardness during fatigue by in situ X-ray measurement

In situ X-ray stress measurements were conducted to investigate the relaxation of compressive residual stress induced by fine particle peening during fatigue. To quantitatively assess the stability of the compressive residual stress, a concept of relaxation threshold stress was proposed. The relaxation threshold stress, which indicates the stability of the compressive residual stress, showed a direct correlation with specimen hardness. If the combined value of the compressive residual stress and compressive applied stress surpass the relaxation threshold stress, then the compressive residual stress relaxes. The relaxation threshold stress can be used to estimate the relaxation that occurs during fatigue.

Verification of stress transfer mechanism in fatigue crack growth under overload

Cyclic plastic finite element modeling was carried out to investigate effect of overload on fatigue crack growth under blunting and no-blunting mechanisms at different overload ratios. The stress transfer mechanism was verified and was found more obvious in the case of crack blunting though its effect was time-limited. The level of crack closure varied periodically due to gradual relaxation of compressive residual stress during crack propagation. Both the overload plastic zone and the overload affected zone were linearly correlated with overload ratio, and an overload ratio value of 1.1 was the best choice for studying overload effect.

Residual Stress Relaxation Behaviors and Fatigue Strength of Micro-Shot Peened EA4T Axle Steel under Constant and Variable Loading

Residual Stress Relaxation Behaviors and Fatigue Strength of Micro-Shot Peened EA4T Axle Steel under Constant and Variable Loading

Fatigue crack growth prediction for shot-peened steel considering residual stress relaxation

The compressive residual stress field (CRSF) relaxes during the fatigue crack growth (FCG) process, and the pattern of this relaxation remains unclear. Residual stress (RS) relaxation leads to a change in residual stress intensity factor (RSIF), which significantly affects the fatigue crack growth rate (FCGR) and thus affects the assessment of fatigue crack growth rate. To this end, this work designed a series of experiments to explore the FCG behaviour in compressive residual stress fields (CRSFs) by considering RS relaxation. Firstly, shot peening(SP) treatments were conducted on the specimens. Then, residual stress fields (RSFs) at different crack propagation stages were measured to analyse the relationship between surface RS and inner RSF distribution. The derived weight function and the real-time RSF are combined to calculate the RSIF with/without considering RS relaxation. Finally, the effect of RS relaxation on the accuracy of FCGR assessment was investigated. The results show that the positive effect of CRS on the fatigue performance of the peened specimens is primarily embodied in the early stage of the FCG process. With the continuous relaxation of the CRSF, the difference in FCGR between the peened and unpeened specimens gradually decreases and tends to disappear. When the applied SIFs are large enough, the CRS would completely relax, and the inhibiting effect of CRSF on FCGR would fail. The proposed method in this work can effectively predict the FCGR of the peened specimen when considering the RS relaxation. This work could provide some meaningful insights for the assessment method of FCGR prediction in CRSF induced by surface strengthening treatment.

Hot salt corrosion behavior of Ti–6Al–4V alloy treated with different surface deformation strengthening processes

In this study, the effects of shot peening (SP) and ultrasonic surface rolling process (USRP) on the high-temperature oxidation and hot salt corrosion (HSC) behavior of the Ti–6Al–4V alloy were systematically investigated. The results showed that the high-temperature oxidation resistance of this titanium alloy can be significantly improved by the above two treatments. However, the hot salt corrosion resistance of Ti–6Al–4V alloy cannot be improved by SP due to surface roughness increasement, the surface defects and the complete relaxation of compressive residual stress (CRS) at high temperature. On the contrary, the hot salt corrosion resistance of USRP treated specimen was remarkably improved because USRP treatment significantly reduced the surface roughness and introduced deep CRS field with good thermal stability. In addition, the refined surface microstructure promoted the formation of a protective oxide film, which was also beneficial for inhibiting hot salt corrosion.

Impact of fast heating and cooling on the mechanical behaviour of alumina-magnesia refractory castables

The alumina-magnesia castables is one kind of monolithic refractories for steel ladle, whose thermo-mechanical properties vary with temperature during the lining pre-heating process and metallurgical service. The understanding of their variation tendency and sensibility to temperature fluctuation is important for the improvement of high-efficiency pre-heating strategy and real-time service performance. In this study, the influence of heat treatment temperature and heating/cooling rate on the thermo-mechanical properties and fracture behaviour of alumina-magnesia castables is investigated using uniaxial compressive tests equipped with an induction heating system and Brazilian test combined with digital image correlation and acoustic emission. The brittleness and mechanical strength of alumina-magnesia castables increase with heat treatment temperature due to the better sintering and formation of calcium hexaluminate and spinel phases. Without full relaxation of residual stresses, the stiffness increases while the strength decreases after fast heating of 200 °C/min. Meanwhile. the same castables after different thermal treatments show disparate mechanical responses to cooling rates.

Investigation on relaxation properties of maize kernels based on the multicomponent structure

The damage to maize (Zea mays) during processing is closely related to its viscoelastic properties, but the viscoelastic properties of the components of the maize kernel are unknown. This work aimed to measure the stress relaxation of components, evaluate the relaxation behavior, and observe the microstructure for interpreting relaxation properties. It was found that the stress relaxation curves of the components shifted downward with increasing moisture content. The two-element Maxwell model could successfully express the stress relaxation curves. There was a decreasing trend in the model parameters of the components as moisture content increased. The differences in the degree of the elastic behavior would result in a difference in the residual stress percentage and relaxation rate, which could be better interpreted by the microstructure analysis. The results could help optimize maize processing strategies to maintain product quality.

Effect of Alternated Multiaxial Stress State on Fatigue Strength and Relaxation of Residual Stress in Welded S355 Steel with Nonlinear and Combined Hardening

This work aims to analyse the effect of the alternated multiaxial stress state on the relaxation mechanisms of residual stresses and the integrity of welded S355 steel, presenting a nonlinear and combined hardening behaviour. The first part of this work proposes a model to predict the residual stresses introduced by the thermal effect of the welding process. The stresses resulting from the thermal cycle were integrated into a second model in which the material was subjected to a cyclic load to analyse the relaxation process. To ensure a good accuracy of the relaxation results, the kinematic and isotropic hardening were combined in a nonlinear model. In the last part of this paper, a multiaxial fatigue strength analysis was carried out, taking into account the effect of residual stresses and the relaxation process. The results show a considerable relaxation of the residual stresses if they are subjected to low load levels applied under specific conditions. The relaxed residual stresses can have a considerable effect on fatigue strength. Also, the importance of the consideration of the relaxation process on the accuracy of the fatigue strength results is illustrated.

Improving the high cycle fatigue property of Ti6Al4V ELI alloy by optimizing the surface integrity through electric pulse combined with ultrasonic surface rolling process

To improve the surface integrity and high cycle fatigue property of Ti6Al4V ELI alloy, the electric pulse has been introduced into the ultrasonic surface rolling process (USRP), which is called electric pulse-assisted ultrasonic surface rolling process (EUSRP). With the help of “electroplasticity” of the electric pulse, the thickness of the surface gradient deformation layer was about three times of the USRP specimens by adjusting the pulse current level. However, the surface hardness decreases due to the continuous effect of the pulse current and the “skin effect” during treatment. It is worth noting that the higher the applied pulse current, the more severe the softening. This paradox causes the fatigue performance of EUSRP specimens lower than that of USRP specimens. To break this paradox, the EUSRP treatment is followed by a USRP treatment. The EUSRP-2 (with a pulse current of 200 A) +USRP specimens exhibit excellent surface hardness, a gradient deformation layer thickness of about 400 µm, low surface roughness and high compressive residual compressive stress. Besides, the hardening mechanisms of the different surface strengthening specimens have been quantitatively analyzed in combination with microstructure analysis. The fatigue life of Ti6Al4V ELI alloy can be improved by about 25 times at 780 MPa using the EUSRP-2+USRP treatment, the main reason for the highest fatigue life is the deepest surface gradient layer and the deepest crack initiation site. The fatigue limit of the EUSRP-2+USRP specimens is not the highest because too much surface hardening causes compressive residual stress relaxation during cycling and the beneficial effect of compressive residual stress is eliminated.

On the residual stress relaxation in Inconel 718 superalloys at high temperature by real-time neutron diffraction

Residual stresses in nickel-based superalloy engineering structure develop during various manufacturing stages, which can result in component distortion or reduction of performance reliability during high temperature service. Experimental quantification of residual stress built-up and its relaxation at high temperatures provides the time-dependent evolution and model validation for residual stress predictions. Via in situ time-of-flight neutron diffraction, a real-time study of residual stress relaxation is carried out on Inconel 718 (IN 718) blocks with different initial residual states after oil or water quenching and annealing processes. The residual stresses are largely relieved after 14 ∼ 20 h at 718 °C, and multi-stage kinetics of relaxation are observed. The staged feature is correlated to the competing stress relaxation mechanisms in the atomic scale that are clued from the evolution of neutron diffraction data. Although with large initial discrepancy in samples due to different thermal treatments, the final residual stresses tend to be within small variations after the relaxation.