Maximum Compressive Residual Stress Research Articles

Mathematical Model and Verification of Residual Stress Induced by Water Jet Peening

The water jet peening (WJP) technology can induce compressive residual stress (RS) in metal surfaces and, thus, improve the fatigue life of components. In this paper, a mathematical model is proposed for calculating the RS induced by WJP. To validate the proposed mathematical model, experimental and finite element simulation verifications were carried out on Al6061-T6. The distribution of RS along the depth direction, the maximum compressive RS, and the depth of the compressive RS layer were also investigated based on the mathematical model. Results showed that the error of maximum compressive RS between the mathematical model and experiment was within 9% under a jet pressure of 60 MPa, and the error of depth of the compressive RS layer between the mathematical model and experiment was within 13% under a jet diameter of 0.3 mm. Hence, the mathematical model is reliable and accurate. The maximum compressive RS increases with the increase in jet pressure, and the depth of the compressive RS layer approximately linearly increases with the increase in jet diameter.

Parametric Study of Fixtured Vibropeening

Vibropeening is a surface treatment process, which combines the peening effect of introducing residual stress with the polishing effect of reducing surface roughness in one single process step. Vibropeening equipment induces vibrations into the media to impart residual compressive stresses in sub-surface layers, as well as polishing on the surface of the work piece. In addition to process parameters, such as vibration frequency, amplitude, and media mass, which are well known in literature, this paper will focus on the study of two additional parameters: immersion depth and process time. It was found that the lower-middle section of the vibratory trough produced the highest Almen deflection. Different continuous treatment times were also studied to explore the maximum introducible residual compressive stress state, and it was concluded that an optimal time range is required to achieve the best residual stress profile. The study demonstrates that different process parameters can influence the effectiveness of the vibropeening process, and that these can be potentially optimized for higher treatment capability.

Experimental and numerical study of pneumatic needle peening effects on Aluminium Alloy 2024-T3

Pneumatic needle peening process was experimentally calibrated using high speed camera and numerically simulated using previous developed shot peening and peen forming finite element (FE) models. High speed photography was applied to record the images of peening needles and digital image treatment program was developed to calculate needle peening velocities for the FE simulation. A single impact simulation was performed for convergence study and indentation comparison to validate the impact model. Good accordance between the simulated and experimental indentation diameters was obtained. Then multiple needle impact models were applied to simulate the induced stress profiles for different peening conditions. Finally, the induced stress profiles were input into a peen forming FE model to predict the specimen deflections (or arc heights). Similar to Almen intensity definition, saturation point was determined by defining certain peening time, when double peening time, the produced arc height increases equal to 10%. The residual stress profile at saturation time for one of the scenarios was calculated and compared against the X-ray diffraction (XRD) measured results at the same operating pressure. In addition, the residual stress profile calculated from the induced stress profile using analytical equations was compared with XRD measurements. Compared to experimental measurements, both the model simulated and analytical calculated residual stress profiles were able to predict the surface compressive residual stress and maximum compressive residual stress very well. The simulated deflection results for the studied peening pressures were then compared against the deflection measurements using the coordinate measuring machine (CMM). The difference between experimental and predicted results ranges between 0.3% and 13.2% for the four higher peening pressures. However, for the lowest peening pressure, larger error of 50% was obtained, which can be explained as the overestimation of the peening velocity at the lowest peening pressure. In conclusion, the presented methodologies are able to calibrate the peening velocities, to predict the induced stresses and further to calculate sample deflections of thin specimen by pneumatic needle peening.

Combining the finite element method and response surface methodology for optimization of shot peening parameters

In this study, a finite element shot peening model was employed in order to obtain the values of maximum compressive residual stress (σmaxRS), the depth of compressive residual stress field (δ0) and the stress concentration factor (K) in specimens treated by shot peeing process. Based on the finite element simulation results, Response Surface Methodology was conducted to optimize the response variables of σmaxRS, δ0 and K. The effects of three shot peening parameters (shot velocity, shot diameter and coverage ratio) on the values of σmaxRS, δ0 and K were investigated. The influence of the values of σmaxRS, δ0 and K on the fatigue property was discussed via giving different weights to these response variables in the optimization approach. The results showed that the proposed shot peening finite element model could give a reasonable prediction of compressive residual stress field and dimple size. Meanwhile, response surface methodology could be used to optimize these response variables (σmaxRS, δ0 and K) well. The priority should be given to the values of σmaxRS, δ0, and K in sequence when shot peening parameter optimization was conducted within a reasonable shot peening parameters range without microcrack appearing on treated surface. The optimal shot peening parameters were the shot velocity of 88 m/s, the shot diameter of 0.8 mm, and the coverage ratio of 170% for 42CrMo specimens. The fatigue life of 42CrMo specimens treated by shot peening with optimal parameters was improved by 104% in experiments.

On the Evolution of Residual Stresses, Microstructure and Cyclic Performance of High-Manganese Austenitic TWIP-Steel after Deep Rolling

The mechanical properties and the near surface microstructure of the high-manganese twinning-induced plasticity (TWIP) steel X40MnCrAl19-2 have been investigated after deep rolling at high (200 °C), room and cryogenic temperature using different deep rolling forces. Uniaxial tensile tests reveal an increase in yield strength from 400 to 550 due to surface treatment. The fatigue behavior of selected conditions was analyzed and correlated to the prevailing microstructure leading to an increased number of cycles to failure after deep rolling. Deep rolling itself leads to high compressive residual stresses with a stress maximum of about 800 in the subsurface volume characterized by the highest Hertzian pressure and increased hardness up to a distance to the surface of approximately 1 mm with a maximum hardness of 475 0.1. Due to more pronounced plastic deformation, maximum compressive residual stresses are obtained upon high-temperature deep rolling. In contrast, lowest compressive residual stresses prevail after cryogenic deep rolling. Electron backscatter diffraction (EBSD) measurements reveal the development of twins in the near surface area independently of the deep rolling temperature, indicating that the temperature of the high-temperature deep rolling process was too low to prevent twinning. Furthermore, deep rolling at cryogenic temperature leads to a solid–solid phase transformation promoting martensite. This leads to inferior fatigue behavior especially at higher loads caused by premature crack initiation. At relatively low loads, all tested conditions show marginal differences in terms of number of cycles to failure.

Experimental research and analysis on residual stress distribution of circular steel tubes with different processing techniques

To investigate and model the residual stress in 345 and 420 MPa normal strength steel (NSS) as well as 690 MPa high-strength steel (HSS) circular tube sections, an experimental study with a total of 45 specimens was conducted using the hole-drilling and sectioning methods. The parameters altered during the experiment included the processing techniques, diameter-to-thickness ratios, steel strengths and welding locations. More than 3064 original test data samples were used to quantify the distribution characteristics of the residual stresses. The results suggest that the magnitudes and distributions of the residual stresses in the welded circular tubes were significantly affected by the processing techniques, steel strengths and welding locations, while no obvious correlation was identified with the diameter-to-thickness ratios. The maximum compressive residual stress ratio gradually decreased with an increase in the steel strength. However, the variation in the tensile residual stress near the weld seam in the middle section was not affected by the variation in the parameters, and its maximum value was in the range of 0.1 π on both sides of the weld seam. Unlike those of the welded approach, the residual stresses along the circumferential direction of the inner or outer surfaces of the hot-rolled seamless circular steel tube sections were almost identical, which is basically consistent with the ECCS’s theoretical model. On the basis of the test results, a distribution model and its simplified forms for NSS were established, and Yang’s distribution model was further validated. The two distribution models can effectively describe the experimental results.

Microstructure evolution and residual stress changes of the GH4169 alloy by wet shot peening

GH4169 alloy was shot-peened with processing durations of 3 min and 10 min. The effects of dry and wet shot peening on the microstructure and residual stress were investigated. The results demonstrated that a severely deformed layer manifests on the surface of the shot-peened GH4169 alloy samples. Compared with the dry-peening samples, the surface defects of wet-peening samples obviously decrease. With the shot peening treated for 10 min, the surface microhardness of the dry peening and the wet shot peening increased by 20% and 17% respectively compared with the original surface hardness. The deformation mechanisms of the GH4169 alloy could be attributed to dislocation and deformation twining, as detected by TEM observations. Compressive residual stress was introduced into the surface layer of the shot-peened GH4169 alloy. The maximum surface residual compressive stress (SRS) and maximum residual compressive stress (MRS) were -586 MPa and -795 MPa, respectively. The MRS and SRS were improved with increasing processing duration. The SRS of the wet-treated specimens was slightly higher than that of the dry-treated samples under the same processing parameters. However, the MRS of dry-treated samples was much larger than that of the wet-treated samples due to the higher shot intensity.

Influence of Tempering and Cryogenic Treatment on Retained Austenite and Residual Stresses in Carbonitrided 18CrNiMo7-6 Low Alloy Steel

This work investigated the influence of tempering conditions coupled with cryogenic treatment on thermal stabilization of retained austenite and residual stress distributions in carbonitrided 18CrNiMo76 low alloy steel samples. The carbonitriding conditions were set to enable attaining surface carbon and nitrogen content of 0.87 and 0.34 mass.-percent respectively. After carbonitriding, some of the samples were subjected to varying tempering conditions followed by cryogenic treatment at -120 °C using nitrogen gas. Analysis of both retained austenite and residual stresses was conducted using X-ray diffraction. In the as-quenched state, carbonitrided samples contained 52 mass.-percent. Samples that were directly subjected to the cryogenic treatment after quenching retained only about 20 mass.-percent of austenite. Samples subjected to variant tempering conditions coupled with cryogenic treatment retained at least 30 masses.-percent of austenite. A thermal stabilization of retained austenite which increases with increasing temperature was identified. On tempering at 240°C for 14 hours retained austenite becomes unstable and decomposes to bainite leading to the low initial amount of retained austenite before cryogenic treatment. It can be concluded that the tempering process coupled with cryogenic treatment leads to an increasing hardness, to higher compressive residual stresses as well as to a shift of the location of maximum compressive residual stress toward the surface.

Investigation on the effect of shot peening coverage on the surface integrity

Shot peening is a typical surface strengthening process that is widely applied in engineering practice for it can introduce finer grains in the near-surface layer and generate a compressive residual stress (CRS) layer. The finite element method, together with a Python subscript, is applied to create a multiple-shot model to estimate the evolution of the shot peening coverage. The influences of coverage on residual stress, plastic strain and surface topography are discussed. The predicted results reveal that as the shot peening coverage increases from 100% to 400% under the given processing condition, the maximum CRS and the plastic strain will increase gradually. But there is no evident alteration on the width of CRS layer and plastic strain layer. The variations of each roughness parameter with coverage are different.

Effect of post heat-treatment on the microstructure and mechanical properties of Hastelloy-X structures manufactured by laser based Directed Energy Deposition

This paper reports a systematic investigation of Hastelloy-X (Hast-X) structures built by Laser Directed Energy Deposition in as-built and post heat-treated conditions. The optical microscopy shows microstructures with fine dendrites in as-built condition due to higher cooling rate during deposition, while recrystallized equiaxed grains are observed after post heat-treatment (at 1177 °C) due to recrystallization. X-ray diffraction studies reveal nickel γ-matrix with variation in crystallite size and a peak shift after post heat-treatment, primarily due to change in surface residual stress. Surface topography reveals the reduction in average roughness with post heat-treatment. Further, the maximum compressive residual stress of 350 MPa and maximum tensile residual stress of 252 MPa are observed at the surfaces of post heat-treated and as-built samples, respectively. The average micro-hardness changed from 239 HV1.96N to 208 HV1.96N after post heat-treatment. Single Cycle Ball Indentation studies indicated increase in energy storage capacity by a factor of 1.55 after post heat-treatment. Automated Ball Indentation studies quantify yield strength and ultimate strength as 478 MPa and 765 MPa in as-built samples, while 393 MPa and 630 MPa in post heat-treated samples, respectively. Further, crack propagation studies indicate an improvement in the fatigue life after post heat-treatment, while the specific wear rate increased by a factor of 1.72 with increased delamination in heat-treated samples. Thus, post heat-treatment of Hast-X samples changes the material properties significantly.

Optimization of residual stress field in ultrasonic assisted burnishing process

Surface compressive residual stress and its penetration depth are of important characteristics of burnishing process. The higher values compressive residual stress in the surface of the treated sample can delay the fatigue crack initiation; also, the further penetration depth restricts fatigue crack propagation. However, achieving both of performance at same time needs careful selection of burnishing parameter. In the present study an optimization approach is made to maximize the value of surface compressive residual stress subjected to specified penetration depth. The analytical model of residual stress previously developed by the author is firstly modified with a cubic interval function and then used to find how the ultrasonic assisted burnishing factors i.e. static force, vibration amplitude, ball diameter and material influence the residual stress field distribution. Then the model was then incorporated with particle swarm optimization algorithm to find optimal parameter setting. Results showed that it isn't possible to maximize both the surface compressive residual stress and its penetration depth at same time. Furthermore, to have maximum value of compressive residual stress magnitude at the surface with the penetration depth of 0.5 mm, it is required to use tungsten carbide ball with 3 mm diameter; also, the static load and vibration amplitude should be selected at the values of 120 N and 14 μm. The compressive surface residual stress in optimized condition is about 110 MPa that is compatible well with the 98 MPa obtained by confirmatory experiment. Also, the maximum depth compressive residual stress for optimum condition measured by experiment is 0.7 mm while it is predicted 0.8 mm by analytical approach.

Residual Stress Asymmetry in Thin Sheets of Double-Sided Shot Peened Aluminum

The local mechanical properties and residual stresses of a double-sided shot-peened aluminum alloy AA7050-T7451 sheet were measured by nanoindentation and simulated by the finite element method (FEM). The variation of the pileup area around the indentation due to residual stresses, as well as changes in hardness as a function of cold working, was assessed. Numerical simulations were conducted to investigate the residual stress profile. When the indentation contact area was corrected to account for pileup induced from residual stresses, the hardness increased about 10% over the range of depths at which compressive residual stresses exist, indicating some strain hardening did occur. The maximum compressive residual stress for the second shot-peened surface is on the order of 10% higher than the first surface; this is ascribed to the sequencing during the peening process, in which the first side peening influenced the stress throughout the part thickness for these thin plate structures. The ability to couple FEM simulations and rapid experimental assessments of residual stresses and hardness allows for future prediction of part thickness effects in double-sided peening processes.

Enhancement of rolling contact fatigue performance of ferrite–pearlite steel with laser shock peening

The laser shock peening effect and its influence on rolling contact fatigue of ferrite–pearlite steel were investigated experimentally. Microhardness on the steel surface is increased after laser shock treatment. The depth of surface layer affected by laser shock hardening is above 1 mm. The maximum residual compressive stress generated on the specimen surface is around 1 GPa. The twin-disk test showed that laser shock-treated steel disk presents better rolling contact fatigue resistance than untreated one. Laser shock-induced strain hardening and compressive residual stress alter crack inclination angle and propagation behavior, which results in a longer fatigue life.

Modeling and Prediction of Residual Stresses in Additive Layer Manufacturing by Microplasma Transferred Arc Process Using Finite Element Simulation

Prediction of residual stresses induced by any additive layer manufacturing process greatly helps in preventing thermal cracking and distortion formed in the substrate and deposition material. This paper presents the development of a model for the prediction of residual stresses using three-dimensional finite element simulation (3D-FES) and their experimental validation in a single-track and double-track deposition of Ti-6Al-4V powder on AISI 4130 substrate by the microplasma transferred arc (µ-PTA) powder deposition process. It involved 3D-FES of the temperature distribution and thermal cycles that were validated experimentally using three K-type thermocouples mounted along the deposition direction. Temperature distribution, thermal cycles, and residual stresses are predicted in terms of the µ-PTA process parameters and temperature-dependent properties of substrate and deposition materials. Influence of a number of deposition tracks on the residual stresses is also studied. Results reveal that (i) tensile residual stress is higher at the bonding between the deposition and substrate and attains a minimum value at the midpoint of a deposition track; (ii) maximum tensile residual stress occurs in the substrate material at its interface with deposition track. This primarily causes distortion and thermal cracks; (iii) maximum compressive residual stress occurs approximately at mid-height of the substrate material; and (iv) deposition of a subsequent track relieves tensile residual stress induced by the previously deposited track.

Surface modification of AA7075 T651 by laser shock peening to improve the wear characteristics

The Enhancement of surface properties of various materials by laser shock peening (LSP) to prolong the service life of the industrial components gained significant importance for the past decade. Though several investigations are made on the different aluminum alloys through LSP route, very limited studies are reported on AA 7075 T651 aluminum alloy, which is extensively used in upper wing cover and fuselages of aerospace industries. In the present investigation, a Q-switched Nd:YAG laser having different pulse energies of 200, 300 and 400 mJ with black paint and water as the ablative layer and confinement medium respectively are used to study the variations in the mechanical properties. A deformed layer thickness of about 500 μm with a peak hardness of 236 Hv at the surface is obtained with pulse energy of 400 mJ. Further, the maximum compressive residual stress induced in the said specimen is 317 MPa and a wear rate of 1.18 × 10−6 g m−1 is measured. The microstructural studies performed using HR-TEM, revealed the enhancement in the properties are attributed to the grain refinement and dislocation strengthening effected by the LSP process.

Peripheral milling-induced residual stress and its effect on tensile–tensile fatigue life of aeronautic titanium alloy Ti–6Al–4V

ABSTRACTThe special application environment puts forward the higher requirement of reliability of parts made from titanium alloy Ti–6Al–4V, which is closely related to the machining-induced residual stress. For the fact of the non-linear distribution of residual stress beneath the machined surface, distribution of peripheral milling-induced residual stress and its effect on fatigue performance of titanium alloy Ti–6Al–4V are still confusing. In the present study, residual stress profile induced by peripheral milling of Ti–6Al–4V is first studied. And then, energy criteria are proposed to characterise the whole state of the residual stress field. Finally, the effects of residual stress profile and surface energy on tensile–tensile fatigue performance of titanium alloy Ti–6Al–4V are discussed. The conclusions were drawn that the variation trend of surface residual stress (σr,Sur), maximum compressive residual stress (σC,ax), location (hr0) and response depth (hry) of residual stress profile with cutting parameters showed a similar pattern for both measure directions those parallel (σ1) and perpendicular (σ3) to the cutting direction. Cutting speed and feed rate have a main effect on surface residual stress, and the depth of cut has little effect on all the four key factors of residual stress profile. With the increase of cutting speed and feed rate, machining-induced surface energy tends to become larger. But increasing the depth of cut caused the strain energy stored in unit time to decrease. Furthermore, the effect of depth of cut on surface energy was weakened when the value of cutting depth becomes larger. Both the surface compressive residual stress and the maximum compressive residual stress are beneficial for prolonging the fatigue life, while large value of machining-induced surface energy leads to a decrease of fatigue life. Analysis of variance result shows that maximum residual compressive stress has a greater impact on fatigue life than other residual stress factors.

Microstructural Evolution, Mechanical Properties and Thermal Stability of Gradient Structured Pure Nickel

The microstructural evolution of pure nickel treated by deep rolling (DR) technique with different indent depths was investigated by means of optical microscopy and transmission electron microscopy. The surface roughness, hardness and residual stress distribution along the depth from surface were measured. Moreover, the DR-treated sample was annealed at temperatures from 300 to 700 °C for 2 h. The results reveal that dislocation movements are the fundamental mechanisms of gradient grain refinement during the DR process. With increasing indent depth of the DR, the gradient microhardness on the cross section of sample significantly increases, the maximum compressive residual stress decreases, and the affecting region of residual stress increases. The results of thermal stability depict that the microstructure can be stable as temperature up to 300 °C, and the abnormal grain growth and annealing twins are observed at 600 °C.

Application of design of experiments for laser shock peening process optimization

Laser shock peening—a very promising life enhancement technique—has demonstrated great success regarding the improvement of fatigue behavior via deep compressive residual stresses. However, the prediction and adaption of residual stress fields on basis of the laser peening parameters are still not comprehensively established. The aim of the current work is to investigate the effects of the laser pulse energy, the number of treatment overlaps as well as the laser spot size on the resulting residual stress distribution, characterized by following quantities: the residual stress close to the surface, the maximum compressive residual stress, and the integral compressive stress area over the specimen depth. For a systematic investigation of all main and interaction-based process parameter effects, and a subsequent parameter optimization, the general full factorial design is employed. The results show that laser shock peening with different process parameter combinations, inducing residual stresses with comparable integral stress area, can lead to a minimum fatigue life extension of approx. 100,000 cycles, representing a minimum fatigue life of 250% of the base material. The experimental scatter in the number of cycles to failure follows the Weibull distribution which qualitatively correlates with the standard deviation of the integral stress area.

Numerical and Experimental Studies on Subscale Behaviors of Ultrasonic Surface Peening

Displacement-controlled and force-controlled surface-striking conditions were applied for finite element method simulations of the single path ultrasonic nanocrystal surface modification (UNSM) surface peening process. The single path UNSM treatment was done on a 4 mm-thick 6061 T6 Al sheet, and the surface topology and through-thickness residual stresses were measured and compared to the simulation results. Both displacement- and force-controlled simulations predicted maximum compressive residual stresses near the surface and subsurface for the stresses along the strike progression direction and transverse direction, respectively. The experimentally measured through-thickness residual stress profile also showed the maximum compressive residual stress near the subsurface. Both simulation conditions showed a topological pile-up on the surface in front of the leading strike. However, only the force-controlled simulation showed the pile-up breakdown at a critical number of strikes followed by the redistribution of subsurface strain fields, and lead to topological undulations along the edge and valley of the UNSM path, which were confirmed to be consistent with the surface topology measured from the experiment. It was concluded that the actual UNSM strike is close to the force-controlled surface-striking condition.

Multifunction cavitation technology to improve the surface function of Al – Cu alloy

Multifunction cavitation (MFC) combines ultrasonic cavitation technology and water jet peening (WJP) technology. In this processing technique, it has been reported that the residual stress necessary for improving the fatigue strength on the pure Al surface is large, whereas the amount of surface scraping by this processing is small. Moreover, improvement of the corrosion resistance of the surface of Al–Cu alloys has been reported. In this study, to improve the efficiency and accuracy of MFC technology, the dependence of the surface condition, residual stress, and surface roughness on the distance from the water jet nozzle (standoff distance) to the specimen surface were evaluated. When Al–Cu alloy specimens were processed by MFC with a swirl flow nozzle (SFN–MFC), WJP with an SFN (SFN–WJP), and conventional WJP, erosion at the center of the specimens occurred when the standoff distance was 45 mm and was reduced as the standoff distance was increased. The maximum compressive residual stress was reached at standoff distances of 55, 65, and 45 mm for SFN–MFC, SFN–WJP, and WJP, respectively. The residual stress was found to be correlated with voids formed by cavitation processing. In SFN–MFC, at standoff distances of 55 and 60 mm, the cavitation bubbles were in the high-pressure and high-temperature regimes, respectively. These results demonstrate that high-temperature and high-pressure regimes exist in MFC, unlike in conventional WJP technology, and the temperature and pressure in the cavitation bubbles depend on the standoff distance.