Initial Tensile Residual Stress Research Articles

Effect of Laser Scanning Speed and Fine Shot Peening on Pore Characteristics, Hardness, and Residual Stress of Ti-6Al-4V Fabricated by Laser Powder Bed Fusion

There is a concern regarding sub-surface pores within laser powder bed fusion of Ti-6Al-4V, which can initiate cracks and reduce mechanical properties, especially after machining for surface finishing. This study investigated the effect of laser scanning speed and fine shot peening on the pore characteristics, hardness, and residual stress of Ti-6Al-4V fabricated by laser powder bed fusion using scanning electron microscopy, X-ray micro-computed tomography, Vickers hardness, and X-ray diffraction. As the laser scanning speed increased, the number of pores and pore size increased, which reduced the hardness of Ti-6Al-4V. Most pores were less than 20 µm in size and randomly distributed. The fine shot peening generated plastic deformation and compressive residual stress on the surface, leading to higher hardness, with similar surface properties at all scanning speeds. The depth of compressive residual stress by fine shot peening varied corresponding to the scanning speeds. Increasing the scanning speed accelerated the rate of conversion between the compressive and tensile residual stresses, and decreased the depth of the maximum hardness by the fine shot peening from initial tensile residual stress within Ti-6Al-4V fabricated by laser powder bed fusion, thus reducing the enhancement achieved by the fine shot peening.

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.

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.

Ex Situ Residual Stress Analysis of Chemical Vapor Deposited Diamond Coated Cutting Tools by Synchrotron X‐Ray Diffraction in Transmission Geometry

When machining difficult‐to‐cut, nonferrous materials, chemical vapor deposited (CVD) diamond–coated cutting tools are applied. The tools’ favorable mechanical property profile is based on the hardness of the coating as well as the adaptability of the substrate. Nevertheless, the reproducibility of machining results and process stability are limited by insufficient coating adhesion. The resulting cutting tool failure is based on coating delamination initiated by crack development. By assessing residual stress as an influence of coating adhesion, an analysis of CVD diamond–coated tools is performed using synchrotron X‐ray diffraction in transmission geometry. Investigation of a nanocrystalline and multilayer morphology on cobalt‐based tungsten carbide (WC‐Co) and a silicon nitride–based ceramic (Si3N4) provides the distribution of the principal in‐plane residual stress tensor component σ22 depending on the coating morphology and substrate material. Contrary to microcrystalline CVD diamond, nanocrystalline layers decrease the compressive residual stress. In addition, the CVD diamond coating deposited on the Si3N4 substrate material tends to induce an overall initial tensile residual stress that leads to increased tool performance compared to WC‐Co‐based coated tools. Variation of the coating morphology as well as the substrate material offers the possibility to extend the current model for residual stress–dependent tool failure.

Laser Welding Deformation Control of STS304 Cylindrical Steel Sheet Structure

Deformation that occurs during girth welding and seam welding of large-diameter STS304 thin plate structure changes the shape of the structure and affects the manufacturing of the next process. In order to control this deformation, the welding deformation generated during girth and seam laser welding was controlled by generating the relationship between the neutral axis and the welding position and initial tensile stress by the tensile force method. The deformation that occurs during girth welding is caused by angular distortion, and the amount of the deformation is affected by the neutral axis and the position of the weld. By changing the position of the welding part, welding deformation by up to 50% has been reduced. Deformation generated during seam welding caused by angular distortion and longitudinal bending deformation created an initial tensile residual stress in the weld, and the deformation was controlled by minimizing the constraint by welding heat stress generated during welding. At this time, when the magnitude of the initial tensile stress due to the external load increased to 50% of the yield stress of the base metal, the final deformation appeared close to zero. Key words: Thermal elastic-plastic analysis, STS304, Laser welding deformation, Initial stress, Elastic pre-straining method

Study on the gear fatigue behavior considering the effect of residual stress based on the continuum damage approach

The fatigue failures of gears have great influence on lifetime and reliabilities of mechanical transmissions. Initial residual stress (IRS) induced in the manufacturing process, together with the applied loading stress, contributes to gear fatigue behavior. Gear fatigue behavior represents a continuous damaging process during a high-cycle period. In the present work, a damage-coupled numerical model considering the effect of IRS is developed to investigate the gear fatigue performance. The deterioration of gear material, the detailed evolution of damage and the stress response during the cyclic loading process are calculated and recorded based on the continuum damage mechanics (CDM) using a user material subroutine (UMAT) in ABAQUS. Both the tooth surface contact fatigue and tooth root bending fatigue of the gear are evaluated, and the influence of IRS on stress response and damage evolution is analyzed. Results show that the risk of surface contact fatigue failure of this gear is much greater than that of the tooth bending fatigue. The initial compressive residual stress has positive effect on contact fatigue performance while the initial tensile residual stress with amplitude of 380 MPa would reduce the gear contact fatigue life by 20%.

Ultrasonically generated pulsed water jet peening of austenitic stainless-steel surfaces

In this study, the effects of pulsating water jets were investigated as a surface treatment process using circular and flat nozzles by considering the integrity of a stainless steel (AISI 304) surface. The local energy distribution was controlled by changing the traverse speed and the pulsating water jet (PWJ) effects were assessed in terms of the residual stress and strengthening effect. The strengthening effect of the process was evaluated by measuring the micro-hardness of the treated surface and by studying the impact of the treatment on the surface based on micro-structural analyses using scanning electron microscope (SEM). The residual stress of the subjected area was evaluated using X-ray diffraction technique. Based on the results from the studied samples, it was found that the initial tensile residual stress was relieved and converted to a compressive residual stress. An increase in the hardness of the treated samples was also observed as compared to the untreated samples up to certain depth along the cross-section of the treated region. The micro-structural examination of the samples revealed the plastic deformation that occurred during the treatment process. Additionally, the acoustic emission (AE) generated during the impact was used as an online monitoring tool for observing the behaviour of the elicited signals under different parametric conditions, and as a control mechanism for obtaining better results. The experimental results show that the pulsating water jet constitutes a new potential technology for surface treatment processes.

Numerical Study of Laser Shock Peening Effects on Alloy 600 Nozzles With Initial Residual Stresses

This paper investigates the effect of initial residual stress and prestrain on residual stresses due to laser shock peening for Alloy 600 using numerical simulation. For simulation, the strain rate dependent Johnson–Cook hardening model with a Mie–Grüneisen equation of state is used. Simulation results are compared with published experimental data, showing good agreement. It is found that the laser shock peening (LSP) process is more effective for higher initial tensile residual stress and for larger initial prestrain in terms of compressive stress at the near surface. However, the effective depth decreases with increasing initial tensile residual stress and initial prestrain.

Evaluation of thermal residual stresses in laser drilled alumina ceramics using Micro-Raman spectroscopy and COMSOL Multiphysics

Abstract This paper presents evaluation of thermal residual stresses in the heat affected zone of laser drilled alumina ceramic by using Micro-Raman spectroscopy. The residual stresses were evaluated for the holes corresponding to the optimal parameters of laser power, scanning speed, frequency and hole diameter. Three such cases were considered for the study. Residual stresses were obtained as a function of the Raman shifts. The nature and magnitude of the residual stresses were indicative of the extent of damage caused in the heat affected zone. In cases where the initial tensile residual stresses exceeded the tensile strength of alumina, cracks were initiated. Laser drilling with higher laser power and lower scanning speed induced initially high compressive and cyclic thermal stresses, causing greater damage to the hole. Transient thermal analysis was performed using COMSOL Multiphysics to predict residual thermal stresses and to validate the micro-Raman results. Scanning Electron Microscopy was used to confirm the damage caused in the heat affected zone.

On Welding Residual Stresses Near Fatigue Crack Tips

Residual stresses in welded joints are supposed to change during fatigue loading. Residual stresses can be, depending on the loading conditions, relaxed as well as build up and they can be affected by fatigue crack growth. Furthermore residual stresses are supposed to affect the fatigue strength of welded steel structures. However, tensile residual stresses decrease and compressive residual stresses increase the fatigue strength. This residual stress effect on the fatigue strength is mostly explained with different crack propagation rates.The presented work is on welded longitudinal stiffeners, made of unalloyed fine grained carbon steel, with different fatigue crack lengths in which residual stresses have been determined. The residual stress measurement was conducted using Neutron diffraction. The results of the diffraction measurement show that the initial tensile residual stresses near the weld toe are, caused by phase transformation at low temperatures, below the base materials yield strength. Tensile residual stresses in the order of the yield strength can be found below the materials surface. These areas of high residual stresses are moving with the propagating crack tip through the specimen during fatigue loading.

Effect of impact energy on residual stresses induced by hammer peening of 304L plates

Residual stresses induced by welding may reach the yield strength of the material and decrease the life of the components by premature cracking. The hammer peening process has been used to improve the fatigue resistance in welded components, especially at the weld toe. It is expected that the powerful hammer peening process produces a deeper compressive layer than other currently used processes and thus a better improvement in fatigue life for heavy welds. To better understand residual stress relaxation by hammer peening, this study evaluates the effect of each hammer blow on the residual stresses field under the treated surfaces. An impact machine was used for hammering and generating unitary impacts at the same location. 304L steel plates were tested under 10-impact with various energy levels. Both initial stress free plates and plates with residual stresses were tested. Strain gages were used for recording strains at the surface in the near field of the impact, whereas in-depth residual stresses at 8 mm from the impact center were measured using the hole-drilling technique. The micro-hardness profiles were also measured to evaluate the hardening affected zone of the hammer peening. It was found that in initial stress free plates, hammer peening introduces compressive residual stresses. In plates with initial tensile residual stresses, hammer peening either released them or turned them into compression. It was found that increasing energy improves the hammer peening efficiency and that the first three or four hammer blows were the most effective. It was also found that the extent of the hammer-peening induced compression was around 1 mm. Microscopic observations of samples from the tested plates did not reveal cracks on the treated surfaces.

Stress effects in sol-gel derived ferroelectric thin films

Residual stress development during processing of sol-gel derived ferroelectric thin films influences electromechanical properties and performance. The present work investigates the effects of stress on field-induced polarization switching in ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) (52/48) thin films. Film response is measured as a function of externally applied mechanical stress using a double-beam laser-Doppler heterodyne interferometer. This apparatus successfully eliminates any displacement contribution from flexural vibration of the substrate and enables measurement of the strain-electric field hysteresis loops as a function of applied stress. The field-induced strain in the PZT film increases with increasing compressive stress, while the opposite trend is observed for applied tensile stress. The dependence of electromechanical response on the external stress is attributed to the initial tensile residual stress state in the film. Tensile stress creates an in-plane clamping effect on the domains in the film, hindering polarization switching. The application of a compressive stress reduces tensile residual stress in the film and the constraint on the domains, leading to higher field-induced strains. Applied tensile stress amplifies the clamping effect, leading to lower field-induced strains.

Linear elastic fracture mechanics (LEFM) analysis of the effect of residual stress on fatigue crack propagation rate

In this research, both residual and applied stresses are converted to stress intensity factors independently and combined using the superposition principle. The fatigue crack propagation rates are predicted. Experiments using two different loading modes, constant applied stress intensity factor (SIF) range, and constant applied load modes are done for samples with and without initial tensile residual stresses. The samples with initial tensile residual stresses exhibit accelerations of the crack propagation rates. The results show that the weight function method combined with the three-component model provides a good prediction of fatigue crack propagation rates in tensile residual stress fields.

X-ray fractography on fatigue fracture surface formed by crack propagation in initial tensile residual stress field : Akita, K., Inoue, T., Takahashi, S., Misawa, H. and Kodama, S. Journal of the Society of Materials Science, Japan (1996) 45, 746–751

X-ray fractography on fatigue fracture surface formed by crack propagation in initial tensile residual stress field : Akita, K., Inoue, T., Takahashi, S., Misawa, H. and Kodama, S. Journal of the Society of Materials Science, Japan (1996) 45, 746–751

ｘ線材料強度 初期引張残留応力場をき裂が進展した疲労破面におけるｘ線フラクトグラフィ

Fatigue tests were performed using CCT specimens having initial tensile residual stress caused by welding. The residual stress distribution on the fatigue fracture surface was measured by X-ray diffraction method. The monotonic plastic zone depth corresponds to the Kmax, true which is the sum of the Kmax related to the applied maximum stress and the Kres caused by the initial residual stress. So, if the initial residual stress distribution is known, the Kmax or the applied maximum stress can be estimated from the monotonic plastic zone depth. The residual stress on the fatigue fracture surface corresponds to the sum of the tensile residual stress related to the Kmax, true and the compressive residual stress related to the ΔKeff. So, even if the initial residual stress distribution is unknown, the ΔKeff or the stress amplitude can be estimated from the residual stress on the fatigue fracture surface and the monotonic plastic zone depth.

圧縮残留応力場から伝播する疲労き裂の開口挙動に関する解析的検討

Initial residual stress due to welding was produced by thermo-elasto-plastic analysis based on F.E.M. calculation, thereafter cyclic-elasto-plastic analysis for fatigue crack propagation was carried out. A crack was propagated by 0.2 mm per one load cycle from compressive residual stress field to tensile stress field (stress ratio was 0.0). Crack opening and closing behavior was investigated in relation to re-dis-tribution of residual stress and change of plastic region.Main results obtained are summarized as follows:1. When the crack tip was in the stationary compressive residual stress field, as crack planes were kept in contact, the re-distribution of residual stress did not occur so much at unloading state. The crack opening was occurred at the crack root then moved to the crack tip, and this was also recognized in base metal analysis.2. As the crack approached the initial tensile residual stress field, the residual stress re-distributed and the crack-opened-region remained near the crack tip even at unloading. The crack opening points were moved both to crack tip side and crack root side, and the crack tip opening occurred earlier than the full crack opening.3. It was clarified that crack opening stress level would be able to be determined by strain gages glued just behind the crack tip, and using superposition model, the stress level could be estimated from the compressive residual stress distribution on the crack plane.