Relaxation Of Compressive Residual Stress Research Articles

An analysis of thermal autofrettage process with heat treatment

In this work, a finite element method (FEM) analysis of a thermal autofrettage process coupled with heat treatment is carried out. The objective is to strengthen a cylindrical pressure vessel with compressive stresses on the outer as well as inner walls. In the proposed method, after thermal autofrettage, the outer wall of the cylinder is heated above the lower critical temperature to cause austenization of the cylinder in its vicinity. When the cylinder is quenched to room temperature, it retains beneficial compressive residual stresses at both the inner and outer walls. The analysis considers temperature and microstructure dependent material properties of AISI 1080 steel. Phase transformation kinetics is incorporated through user defined subroutine UMATHT in commercial FEM package ABAQUS®. The simulation results indicate that heat treatment after thermal autofrettage converts tensile residual stresses at the outer wall to compressive residual stresses. On the other hand, it does cause a slight relaxation of compressive residual stresses at the inner wall. Nevertheless, with the ability to retain significant amount of compressive residual stresses at both the walls, the process can mitigate the problems of stress-corrosion cracking and fatigue failure due to tensile residual stresses at the surface.

Microstructure‐property relation in alumina ceramics during post‐annealing process after laser shock processing

AbstractLaser shock processing (LSP) is a new surface engineering approach to introduce significant compressive residual stress into ceramics to improve their mechanical properties. However, LSP of ceramics may induce microcracks, which limit the further improvement of mechanical properties of ceramics. In this research, the effect of a post‐LSP annealing process on α‐Al2O3 ceramics was investigated. The annealing treatment can cause thermal relaxation of compressive residual stress generated by LSP while still maintain the positive attribute of LSP. The compressive residual stress was stabilized after annealing after 10 hours at 1100‐1300°C. The healing of microcracks in α‐Al2O3 ceramics was observed during the post‐LSP annealing process, which is caused by diffusion bonding mechanisms and accompanied by dislocation and void formation. The combination of the stabilized compressive residual stress and microcrack healing can improve the cracking resistance of α‐Al2O3 ceramics to mechanical impact on the surface by 69%.

Relationship between Compressive Residual Stress Relaxation and Microhardness Reduction after Cyclic Loads on Shotpeened ASTM A516 Grade 70 Steel

Shot peening process is a cold performed function to enhance the mechanical properties which is widely used in many industries. This process introduces compressive residual stress which was proven to increase the fatigue life, geometry stability and corrosion resistance. However, the benefit of the residual stress is still unstable due to the relaxation during the operation. This paper will study on the trend of the relaxation of residual stress against cyclic loading as well as the change in the hardness. The material used in this study is carbon steel ASTM A516/ SA 516 Grade 70. Shot peening process with steel shots was applied to the samples to introduce compressive residual stress in the samples. Cyclic load was applied to samples after shot peening process with low load of 52Mpa (20% of Yield Strength) and high load of 208Mpa (80% of Yield Strength). The measurement of residual stress using X-Ray diffraction and hardness test was done on the samples to study the trend of the relaxation of residual stress and the change in hardness values. The result shows that more relaxation of residual stress occurs if the applied cyclic load is higher. The change of hardness trend is found non-sequenced in this study due to random coverage of shot peening.

Annealing effect on the mechanical properties of ultrafine WC–Co materials

In dry machining, the tool-chip interface temperature is a critical factor affecting the engineering performance of WC–Co tools. As the temperature variation of the cutting tool in dry machining is similar to that in annealing process, the effect of tool-chip interface temperature on the mechanical properties of cutting tool materials in dry machining can be studied by means of annealing experiment. In this paper, a series of annealing experiments of ultrafine-grained WC–Co materials were conducted, and the effects of the annealing temperature and holding time on the mechanical properties of WC–Co materials, including flexural strength, hardness and fracture toughness, were investigated. The results show that under the experimental conditions, the annealing temperature dominates the mechanical properties of WC–Co materials. The flexural strength and fracture toughness of the ultrafine-grained WC–Co materials decrease in different extent with increasing of annealing temperature, whereas the hardness does not change obviously. The measurements of the microstructures and residual stresses revealed that the deterioration of mechanical properties of WC–Co materials after annealing can be attributed to the relaxation of compressive residual stress.

Compressive residual stress relaxation in hardened steel during cyclic and static load

The benefits of applied compressive residual stress on fatigue properties of materials is a well-known phenomenon, but not well described in all respects. The fatigue life and the fatigue limit could be improved by targeted created compressive residual stress in the surface layers therefore, diversified surface compressing methods are developed and used in the engineering industry. The relaxation of the compressive residual stress state during a cyclic and static load is determinative for the life time of a componenet. Compressive stress relaxation was experimentally determined during the cyclic and static load. The compressive residual stress was induced by shot penning on the surface of stainless steel, micro alloyed high strength steel and hardened steel specimens. The residual stress state was investigated nondestructively by X-ray diffraction method then these specimens were load. After a certain number of cycles the fatigue load was stopped and the residual stress state was recorded again and again until fracture. To investigate the relaxation process during static load a four-point bending bench was used. The compressive residual stress relaxation was correlated to the applied fatigue stress level, the cycle number the quality of alloys.

Residual Stress Evaluation Induced by Fatigue Crack in Shot Peened, Notched Specimens

The advantages of applied compressive residual stress on fatigue properties of materials is a well-described topic, but the residual stress state of machine elements can change during application, therefore it is necessary to describe how these changes are related to the operational parameters. So the monitoring of residual stress relaxation gives more details to improve the lifetime. The surface residual stress state evolution of hardened (quenched and tempered), shot peened, 2 side notched quadratic fatigue specimens made of 42CrMO4 steel was investigated nondestructively by X-ray diffraction method. Residual stress state was monitored on the surface with 1 mm spatial resolution in 5 mm environment of the notch. The compressive residual stress relaxation associated the notching and the fatigue test and the reversal of sign of the residual stress (from compression to tension) associated the crack propagation were experimentally showed.

Low cycle fatigue life prediction in shot‐peened components of different geometries—part I: residual stress relaxation

AbstractIn this study, the residual stress relaxation behaviour occurring during low‐cycle fatigue in shot‐peened specimens with either a flat or a notched geometry has been studied. A representative low‐pressure steam turbine material, FV448, was used. The residual stress and strain hardening profiles caused by shot peening were measured experimentally and were then incorporated into a finite element model. By allowing for both effects of shot peening, the residual stress relaxation behaviour was successfully simulated using this model and correlated well with the experimental data. Although more modelling work may be required to simulate the interaction between shot peening effects and external loads in a range of notched geometries, the model predictions are consistent with the specimens tested in the current study. The novelty of this study lies in the development of such a modelling approach which can be used to effectively simulate the complex interaction between shot peening effects and external loads in notched regions. Compared with the un‐notched geometry, the notched geometry was found to be more effective in retaining the improvement in fatigue life resulting from shot peening, by restricting the compressive residual stress relaxation during fatigue loading.

Thermo-mechanical relaxation of compressive residual stresses induced by shot peening

Experimental and theoretical studies have shown that the compressive residual stresses induced by shot peening, have been identified, generally, as the principal factor for fatigue strength improvement. However, it is worth noticing, that the initial induced compressive shot peening residual stresses may be redistributed and relaxed under mechanical, thermal or thermo-mechanical cyclic loading. It is important to consider the relaxation phenomenon in the prediction of the fatigue behaviour of shot-peened parts. This paper presents a numerical approach to evaluate the mechanical, thermal and thermo-mechanical relaxation of the compressive residual stresses induced by shot peening.

The effect of compressive fatigue loads on fatigue strength of non-load carrying specimens subjected to ultrasonic impact treatment

This paper provides description and results from an investigation aimed to determine whether loading histories specific to marine and offshore structures cause shakedown of compressive residual stress produced by high-frequency mechanical impact (HFMI) treatment and therefore affect fatigue resistance of welded joints improved by these methods. Fatigue tests were carried out to investigate the influence of compressive fatigue loads on fatigue strength of non-load carrying specimens with transverse attachments subjected to ultrasonic impact treatment (UIT). It was found that significant relaxation of compressive residual stresses is achieved under application of compressive cycles in fatigue testing. It was also observed that not only the magnitude of the applied compressive stress/mean stress affects relaxation of the compressive residual stress but also the number of applied compressive cycles. This relaxation may reduce the beneficial effect of HFMI treatment under application of load histories specific to marine structures.

Fatigue strength improvement of out-of-plane welded joints of steel girder under variable amplitude loading

This study investigates improvement effects of fatigue strength by the postweld treatments, which are hammer peening treatment and additional welding with low-temperature transformation welding material. High cycle fatigue tests with constant and variable amplitude loadings are performed on out-of-plane gusset-welded joints of girder specimens. Test results indicate that this treatment can improve fatigue life under both the constant and variable amplitude loadings. The damage accumulations (D) of as-welded and cleaning obtained by Miner’s rule with the mean curve of constant amplitude loading are conservative (D > 1.0). However, since the beneficial effects of the compressive residual stress on fatigue life should decrease due to the relaxation of compressive residual stress, the D in specimens prepared by cleaning plus pneumatic peening, cleaning plus electric peening, and additional welding with low-temperature transformation welding material are extremely nonconservative (D ≈ 0.2–0.6). The modified Miner’s rule based on the results of fatigue test with constant maximum stress, which is the same as the maximum stress in the variable amplitude loading, is appropriate to the improved welded joints.

Residual Stress Evolution during Fatigue Test of a Shoot Peened Steel Sample

Directly-generated compressive residual stress has become a widely used surface modification technique. Intentional residual stress in solid materials can be produced by different surface compression methods. It is well known how residual stress field is influenced by the parameters of inducing technology, but during the operation of a surface compressed machine element the problem of compressive residual stress relaxation can occur. Only few articles address this phenomenon. The literature on residual stress decrease caused by fatigue load is scarce and the kinetics of this mechanism is not yet investigated. The present work reports on the examination of the effect of the fatigue load on compressive residual stress strengthened specimens. Investigations have been conducted at three stress levels. Residual stress data have been measured before fatigue load and measurements have been repeated after defined fatigue cycles.

Improvement Effects of Fatigue Strength by Burr Grinding and Hammer Peening Under Variable Amplitude Loading

This study investigates improvement effects of fatigue strength by burr grinding and hammer peening treatments. High-cycle fatigue tests with constant and variable amplitude loadings are performed on out-of-plane gusset plate specimens. Test results indicate that the reduction of stress concentration by the burr grinding greatly improves the fatigue life under both the constant and variable amplitude loadings. For the variable amplitude loading, the damage accumulation D obtained by Miner’s rule with the mean curve of constant amplitude loading is conservative (D > 1.5). However, under the variable amplitude loading, since the beneficial effects of the peening treatment on the fatigue life decrease due to the relaxation of compressive residual stress, the damage accumulations in specimens prepared by cleaning and subsequent hammer peening are extremely non-conservative (D ≈ 0.2). In addition, the stress patterns do not affect the fatigue life significantly.

Relaxation Behaviour of Laser Peening Residual Stress due to Mechanical Loading on Aluminum Alloy

The relaxation behavior of residual stress induced by laser peening during mechanical loading was investigated on an aluminum alloy A2024. The residual stress relaxation process was measured by x-ray diffraction method and analyzed by a finite element method (FEM). The plastic deformation behavior was evaluated from the x-ray diffraction peak width and the FEM. The surface residual stress relaxation under tensile loading occurred when the plastic-deformation started at the inside of material where the balancing tensile residual stress existed. Under the compressive loading, the surface residual stress relaxation started due to the plastic deformation beneath the surface where the maximum compressive residual stress existed. The plastic deformation at the inside of material caused the redistribution of the residual stress and resulted in the relaxation of the surface residual stress. For both tensile and compressive loading, the surface compressive residual stress relaxation occurred before the total stress ( = (residual stress) + (applied stress)) at the surface reached the yield condition. All discussions in this study based on the mechanical deformation behaviour of the material. Therefore, the conclusions of this study are thought to be able to apply to the behavior of the residual stress under the mechanical loading on metallic materials treated by any mechanical surface treatment.

Mechanical properties and residual stresses changing on 00Cr12 alloy by nanoseconds laser shock processing at high temperatures

Effects of laser shock processing (LSP) on 00Cr12 alloy's mechanical properties at high temperatures were analyzed by investigating the changing of the residual stress and tensile properties. The results showed that the compressive residual stress produced by LSP, which makes the 00Cr12 alloy's yield strength and tensile strength increase, still played an important role in the high temperature tensile test. The tensile fracture showed that the break position and expand direction were changed because of the residual compressive stress existing, and the residual compressive stress was greatly released with the increasing temperature. The results demonstrated that the cycle, stress amplitude and temperature-dependent relaxation of compressive residual stresses were more pronounced than the decrease of near-surface work strengthening.

Residual stress relaxation in shot peened high strength low alloy steel and its implications for hydrogen assisted cracking

The relaxation of compressive residual stress in shot peened and laser peened 300M high strength low alloy steel in response to applied loads has been investigated and for the shot peened specimens the implication for resistance to hydrogen assisted cracking assessed. The near surface residual stress was measured predominantly by X-ray diffraction (first 10 μm) but supplemented by measurements using high resolution, fine increment hole drilling to yield depth profiling. Relaxation of residual stress was most significant under compressive loading (single load cycle) with a loss of ∼70% of the peak residual stress when the applied stress was at the compressive 0·2% proof stress. Constant load tests in 3·5%NaCl were conducted on cadmium or SermeTel coated specimens at open circuit and with uncoated specimens at an applied potential corresponding to the minimum value of the protection potential associated with these coatings. For all peened specimens, stress relaxation was induced by a prior applied stress, usually close to the compressive 0·2% proof stress. Most tests were conducted with notched specimens with a target notch depth about 50–60 m m, close to the peak residual stress on shot peened specimens. Despite the comparative severity of the tests, with applied tensile stresses up to 95% of the tensile 0·2% proof stress and the maximum expected residual stress relaxation, none of the shot peened specimens failed. This is reassuring from an engineering perspective. However, notched specimens did fail below the tensile 0·2% proof stress under slow strain rate conditions and the performance under corrosion fatigue conditions remains to be established.

Polycrystal Plasticity Modeling of Cyclic Residual Stress Relaxation in Shot Peened Martensitic Gear Steel

Using a three-dimensional crystal plasticity model for cyclic deformation of lath martensitic steel, a simplified scheme is adopted to simulate the effects of shot peening on inducing initial compressive residual stresses. The model is utilized to investigate the subsequent cyclic relaxation of compressive residual stresses in shot peened lath martensitic gear steel in the high cycle fatigue (HCF) regime. A strategy is identified to model both shot peening and cyclic loading processes for polycrystalline ensembles. The relaxation of residual stress field during cyclic bending is analyzed for strain ratios Rε=0 and −1 for multiple realizations of polycrystalline microstructure. Cyclic microplasticity in favorably oriented martensite grains is the primary driver for the relaxation of residual stresses in HCF. For the case of Rε=−1, the cyclic plasticity occurs throughout the microstructure (macroplasticity) during the first loading cycle, resulting in substantial relaxation of compressive residual stresses at the surface and certain subsurface depths. The initial magnitude of residual stress is observed to influence the degree (percentage) of relaxation. Describing the differential intergranular yielding is necessary to capture the experimentally observed residual stress relaxation trends.

Evolution of residual stresses in a stress-free titanium alloy subjected to fretting fatigue

This study investigated the complete history of residual stress evolution in a stress-free titanium alloy, Ti–6Al–4 V under fretting fatigue loading condition. Compressive residual stress developed in the contact region due to the local plastic deformation between contacting bodies. The compressive residual stress then increased initially with increasing number of fretting fatigue cycles reaching to a maximum value, and then it decreased (or relaxed) with further cycling. This relaxation of compressive residual stress was due to the delamination and detachment of flake-like (wear sheet) material in the fretted region.

Effects of fretting fatigue on the residual stress of shot peened Ti–6Al–4V samples

X-ray diffraction residual stress measurement has been utilized as nondestructive tool for the characterization of fretting fatigue damage in shot peened samples of Ti–6Al–4V. Prior to fretting fatigue damage, compressive residual stresses were found to be uniform over the entire face of the sample and independent of the measurement direction. After fretting fatigue, inside and in the vicinity of the fretting damage zone large relaxation of compressive residual stress was observed. An anisotropic residual stress distribution has been observed in the fretting fatigue damaged region. Residual stress measurements in interrupted fretting fatigue experiments showed that the relaxation of residual stress increases as the number of fretting fatigue cycles increase. The results are discussed in the light of their importance in establishing X-ray diffraction residual stress measurement technique as a nondestructive tool to characterize fretting fatigue damage.

Measurement of residual stress in DLC films prepared by plasma-based ion implantation and deposition

A hybrid process of plasma-based ion implantation and deposition (PBIID) was used to achieve relaxation of compressive residual stress in diamond-like carbon (DLC) films. The residual stress in each film was determined from the curvature of quartz glass plate using Stoney's equation. The compressive residual stress in the DLC film prepared using acetylene gas was about 0.46 GPa without the negative pulsed voltage for ion implantation. The residual stress decreased considerably with the increase of the negative pulsed voltage during deposition and reduced to about 0.16 GPa at the negative pulsed voltage of −20 kV. The residual compressive stress in the DLC films prepared using toluene gas was reduced to about several MPa at the region of negative pulsed voltage of −5 to −7 kV. These reduction of stress might be due to the thermal spike effect by ion implantation. However, at the negative pulsed voltage exceeding about −10 kV, the reduction of residual stress was not observed because of generation of glow discharge by the negative pulsed voltage. In either case, the observed residual stress of the DLC film was almost two orders less than that of conventional DLC films.

High temperature fatigue behavior and residual stress stability of laser-shock peened and deep rolled austenitic steel AISI 304

In this paper, we investigate how laser-shock peening and deep rolling affect the cyclic deformation and S/N-behavior of austenitic stainless steel AISI 304 at elevated temperatures (up to 600 °C). The results demonstrate that laser shock peening can produce similar amounts of lifetime enhancements as deep rolling. The cycle, stress amplitude and temperature-dependent relaxation of compressive residual stresses is more pronounced than the decrease of near-surface work hardening.