Residual Stress Profiles Research Articles

The Welding of Dissimilar Joints with Low Temperature Martensitic Transformation Filler Material

Abstract The welding of dissimilar joints is very common in systems used in oil exploration and production in deep waters. This commonly involves welding low carbon steel pipes with valves, pumps or other pipes, all of them made of alloy steel. Inconel has been used in such welding as filler material; however, this welding is preceded by a process of buttering in the low alloy steel side and a stress relief heat treatment. An alternative technique presented in this paper is the use of a welding filler metal that, during the cooling process, undergoes a martensitic transformation at a relatively low temperature, so that the resulting deformation processing offsets the contraction that occurs as a result of the cooling process. This study evaluates the application of Maraging 250 weld in a dissimilar joint of the oil industry. Tests were performed on joints of ASTM steels A516 grade 70 and AISI 8630M welded with Maraging 250. The residual stress profiles, hardness, microstructure, tensile strength and impact energy absorbed were evaluated. The welded joint with Maraging 250 showed a high intensity of compressive residual stress. The microstructure of different welding regions was analysed and compared.

Evaluation of contact fatigue life of a wind turbine carburized gear considering gradients of mechanical properties

Case hardening processes such as carburizing are extensively applied in heavy-duty gears used in wind turbines, ships, high-speed rails, etc. Contact fatigue failure occurs commonly in engineering practice, thus reduces reliabilities of those machines. Rolling contact fatigue life of a carburized gear is influenced by factors such as the gradients of mechanical properties and profile of initial residual stress. In this regard, the study of contact fatigue life of carburized gears should be conducted with the consideration of those aspects. In this study, a finite element elastic–plastic contact model of a carburized gear is developed which takes the gradients of hardness and initial residual stress into account. Initial residual stress distribution and the hardness profile along the depth are obtained through experimental measurements. The effect of the hardness gradient is reflected by the gradients of yield strength and fatigue parameters. The modified Fatemi–Socie strain-life criterion is used to estimate the rolling contact fatigue life of the heavy-duty carburized gear. Numerical results reveal that according to the Fatemi–Socie fatigue life criterion, rolling contact fatigue failure of the carburized gear will first initiate at subsurface rather than surface. Compared with the un-carburized gear, the rolling contact fatigue lives of the carburized gear under all load conditions are significantly improved. Under heavy load conditions, the carburized layer significantly reduces the fatigue damage mainly due to the benefit to inhibit the accumulation of plasticity. Influence of the residual stress is also investigated. Under the nominal load condition, compared with the residual stress-free case, the existence of the tensile residual stress causes remarkable deterioration of the rolling contact fatigue life while the compressive residual stress with the same magnitude leads to a moderate growth of the rolling contact fatigue life. As the load becomes heavier when plasticity becomes notable, the influence of the initial residual stress on the life is somewhat weakened.

Prediction of micro indention depth of TC4 titanium alloy during laser shock processing

Laser shock peening (LSP) is an effective surface treatment technology to regulate the residual stress and surface profiles of many metallic materials. The micro-indention produced on the metal surface after LSP is an important factor related to the surface profile. In this work, the micro-indention induced by LSP is investigated with theoretically and experimentally, and the residual stress in the micro-indention is also measured. The results show that the micro-indention is induced in the spot center, whose depth is proportional to the laser power density and laser pulse duration. It saturates with the number of impacts beyond 3. The micro-indention transfers materials in the spot center to the surroundings and induces convex deformation at the spot edge. The trend of the residual stress is similar to that of micro-indention depth. The micro-indention and convex deformation increase the peak-to-valley roughness Rz at the edge.

Residual stress prediction based on MTS model during machining of Ti-6Al-4V

The material microstructure attributes are largely ignored in the machining community for the machining mechanics modeling. A physical-based mechanical threshold stress (MTS) model is proposed for the orthogonal turning application of Ti-6Al-4V material. The MTS model takes the material internal state variables, such as dislocation to dislocation interaction and dislocation/interstitial resistance, into the flow stress consideration. The MTS model is embedded into an analytical residual stress prediction model for machining induced residual stress prediction. The experimental data are provided for the model validation. The prediction generally captures the trend of the residual stress profile compared with experiments. The proposed model provides a microstructure insight of the workpiece material in the machining process modeling.

Prediction of residual stress profile and optimization of surface conditions induced by laser shock peening process using artificial neural networks

The purpose of this paper is to predict the compressive residual stress profile induced by the laser shock peening process and to optimize the surface conditions on a Ti-6Al-4V titanium super-alloy using the new technology based on the artificial neural networks. This study covers two principal cases: (i) The numerical analysis: the compressive residual stress distribution induced by the laser peening process is simulated based on the finite element method using ABAQUS software (ABAQUS/Explicit code). (ii) The mathematical modeling analysis: the using of the artificial neural network technique has been proposed to predict the residual stress profile induced by the laser shock processing in order to optimize the laser shock peening surface conditions. To train the artificial neural network, we use different numerical experiments measurements as training and test data. The best fitting training data set was obtained with four neurons in the hidden layers. After training and as seen from the mathematical experiments, the calculated residual stress and the damage state are obviously acceptable. The principal goal of this research paper is to predict the surface treatment state induced by the laser shock peening based on the artificial neural networks technique and on the numerical results without making multiple simulations that can take much more time.

A Simple Model for Hardness and Residual Stress Profiles Prediction for Low-Alloy Nitrided Steel, Based on Nitriding-Induced Tempering Effects

AbstractNitriding of low-alloy steels is widely used for gears and bearings in aerospace industry. Some highly stressed surfaces require high nitrided depth, which involves long time/high temperature nitriding treatment. This study focused on identifying process parameter effects on hardness and residual stress profiles in a large range of nitriding time and temperature. We demonstrate that core and case can be considered as two materials, softening of which can be predicted by two tempering laws. In addition, a basic relationship was derived from the nitriding depth and a pseudo-diffusion coefficient, allowing prediction of the hardness profile. Residual stress results show that the diffusion controlled effect can also be used to predict residual stress profile affected depth. Likewise, the tempering controlled effect can be used to predict surface and maximum values of the residual stress profile.

Residual Stress Profile in Tubular Components

Abstract The precise manufacturing of seamless steel pipes requires postprocess straightening treatments that lead to rather complex and not easily predictable residual stress fields. In this work, we combine blind hole drilling (BHD) and X-ray diffraction (XRD) techniques to determine the most relevant component, the circumferential (hoop) stress: BHD is used on the outer surface of the steel pipe and XRD is used on the inside; the results from both techniques are combined with a progressive layer removal procedure to determine the stress profile across the pipe wall.

Multiscale Measurements of Residual Stress in a Low-Alloy Carbon Steel Weld Clad with IN625 Superalloy

Abstract Fatigue fracture is one of the major degradation mechanisms in the low-alloy 4330 carbon steel pumps that are utilized in the hydraulic fracturing process operating under cyclic loading conditions. A weld cladding technology has been developed to improve the ability of these components to resist fatigue crack initiation by cladding them with a secondary material. This process introduces a residual stress profile into the component that can be potentially detrimental for fatigue performance. The cladding technology under examination is a low-alloy 4330 carbon steel substrate weld that is clad with the nickel-chromium–based superalloy IN625 and is investigated herein using several experimental residual stress measurement techniques. Understanding the magnitude and distribution of residual stress in weld clad components is of the utmost importance to accurately assess the performance of the component in service. This study summarizes the results of residual stress measurements that were determined using X-ray diffraction, i.e., hole drilling based on electronic speckle pattern interferometry, deep-hole drilling, and the contour method, to obtain the residual stress distributions from the surface of the weld clad, through the clad layer, and into the substrate material. The results of deep-hole drilling and the contour method show large-scale tensile residual stress in the clad layer and compressive residual stress in the majority of the substrate. However, the X-ray diffraction and hole drilling methods indicate the presence of short-scale compressive residual stress on the surface and near the surface of the clad layer. It was shown that these measurement techniques are complementary in assessing the residual stress profile throughout the entire component.

Effects of Laser Shock Peening over Minimally Detectable Partial Through-Thickness Surface Cracks

Abstract Laser shock peening (LSP) has evolved as a viable alternative to other surface treatments (shot peening, burnishing, etc.) that induce beneficial residual stress in structural components. While fatigue life improvements have been recognized by the aerospace industry, in-service components may have surface flaws with sizes below current inspection limits. A concern is that the application of LSP over an existing crack may cause unintended detrimental consequences. To address this concern, two different LSP processes were applied over a 0.25 in. (6.35 mm) partial through-thickness surface fatigue crack in 7075-T651 aluminum. The resulting residual stresses were measured using combinations of included X-ray diffraction with layer removal, incremental center-hole drilling, and neutron diffraction. Both LSP processes resulted in a through-thickness residual stress profile in which compressive stresses at either surface were balanced by tensile stresses in a band centered in the mid-thickness of the specimen. Peened and unpeened (baseline) specimens were then tested under constant-amplitude cyclic loading to assess the fatigue response. The average fatigue life of the baseline specimens was about 49,000 cycles. The peened specimens (both treatments) survived to runout, exhibiting no crack growth when examined with an optical microscope even when the crack tip was either very close to or inside the region of tensile residual stress.

Visualisation of deformation gradients in structural steel by macroscopic magnetic domain distribution imaging (Bitter technique)

AbstractWhile classically used to visualise the magnetic microstructure of functional materials (e.g., for magnetic applications), in this study, the Bitter technique was applied for the first time to visualise macroscopic deformation gradients in a polycrystalline low‐carbon steel. Spherical indentation was chosen to produce a multiaxial elastic–plastic deformation state. After removing the residual imprint, the Bitter technique was applied, and macroscopic contrast differences were captured in optical microscopy. To verify this novel characterisation technique, characteristic “hemispherical” deformation zones evolving during indentation were identified using an analytical model from the field of contact mechanics. In addition, near‐surface residual stresses were determined experimentally using synchrotron radiation diffraction. It is established that the magnetic domain distribution contrast provides deformation‐related information: regions of different domain wall densities correspond to different “hemispherical” deformation zones (i.e., to hydrostatic core, plastic zone and elastic zone, respectively). Moreover, the transitions between these three zones correlate with characteristic features of the residual stress profiles (sign changes in the radial and local extrema in the hoop stress). These results indicate the potential of magnetic domain distribution imaging: visualising macroscopic deformation gradients in fine‐grained ferromagnetic material with a significantly improved spatial resolution as compared to integral, mean value‐based measurement methods.

Inversion procedure for dual-mode electromagnetic nondestructive characterization of shot-peened IN718

The authors have recently developed a new technique for nondestructive Hall coefficient measurement based on inductive sensing of the Hall-Corbino current produced by injecting high-frequency alternating current into the component under test. This paper investigates the feasibility of separating the competing effects of near-surface residual stress and cold work in shot-peened fully hardened IN718 alloy in high-frequency dual-mode Hall impedance and eddy current conductivity measurements. For this purpose, an inversion procedure is proposed based on iterative application of a simplistic analytical approximation of the sought depth profiles. The inversion procedure was adapted to Hall coefficient depth profiling by accounting for the secondary eddy currents produced by the Hall-Corbino current at high frequencies. In order to test the accuracy of the proposed inversion procedure, representative Hall coefficient and electric conductivity distributions were selected based on typical residual stress and cold work depth profiles obtained by destructive XRD measurements. These distributions were calculated using gauge factors measured in previous studies that investigated the influence of applied elastic and plastic strain on the Hall coefficient and electric conductivity of fully hardened IN718. Numerical forward simulations were conducted to estimate the Hall impedance and apparent eddy current conductivity spectra produced by the selected elastic and plastic strain distributions. These spectra were then used as simulated “measured” data to test the inversion procedure. It was found, that the competing residual stress and cold work contributions of shot peening can be separated using dual-mode high-frequency inspection. The sub-surface peak compressive stress and half-peak depth of the residual stress profile were reproduced with less than ±50–100 MPa and ±5–10 μm errors, respectively, that is considered quite sufficient in subsurface residual stress profiling for life prediction purposes.

Flaws and cracks stability in strengthened glass by residual stress

ABSTRACTStrengthened glass is widely used in structural glazing applications in the building and transportation sectors. Surface flaws and cracks are the main glass strength limiting factors. In this theoretical study both thermally (TSG) and chemically (CSG) strengthened glass are considered. A general approach is proposed, based on the calculation of the stress intensity factor (KI) resulting from the superposition of externally applied stresses, both mechanical and thermal, and residual stress introduced in the strengthening processes. In order to represent a severe design condition, an externally applied stress with a maximum tensile component of 50 MPa is introduced. The analysis is performed for soda‐lime glass for immediate breakage condition by comparing the stress intensity factor with the critical factor (KIC) and for time delayed breakage activated by subcritical crack growth by comparing the stress intensity factor with the corresponding threshold value (KIth). The classical parabolic residual stress distribution is employed for thermally strengthened glass while a new approach is proposed for the evaluation of the residual stress profile of chemically strengthened glass. A crack growth stability analysis based on the concept of apparent fracture toughness is performed. The residual stress profiles analysed for thermally and chemically strengthened glass are the typical ones resulting from common technological processes. The crack growth stability for the analysed stress profiles do not match the criteria to be defined as flaws insensitive.

Investigation on thermal relaxation of residual stresses induced in deep cold rolling of Ti–6Al–4V alloy

In the present study, a finite element model has been developed to simulate the deep-cold rolling process on Ti-6Al-4V specimens and the following short term exposure of the treated components to elevated temperature. The developed model can be effectively used to predict the residual stress profiles induced by the process at room temperature and the following residual stress relaxation at the elevated temperature. In the present study, the thermal relaxation stage is performed using a viscoplastic model which couples the creep and plasticity deformation mechanisms to predict the state of residual stresses at the elevated temperature. For this purpose, a new set of hyperbolic creep law coefficients are identified in order to describe the primary creep at 450 °C. The accuracy of the developed finite element model to predict residual stresses is validated by comparison with the experimental data available in the literature. It has been shown that the finite element predictions correlate well with experimental results with error generally less than 10%. The developed model can be effectively utilized for parametric studies to understand the effect of different process parameters on the induced residual stresses without performing expensive experimental tests.

A numerical investigation on the local mechanical behavior of a 316-L part during and after an EDM basic electrical discharge

This study proposes a novel numerical approach to elucidate the mechanical behavior of the EDMed layer during an electrical discharge and enhance the numerical prediction of the EDM-induced residual stresses and work hardening, through advances at the levels of models, loads, and boundary conditions. In this work, a single-pulse discharge was simulated using finite element method carried out in ABAQUS/Explicit code. A fully coupled thermomechanical consistent model was developed based on a hydrodynamic Gruneisen-type behavior for the hydrostatic part of the stress, coupled with a Johnson-Cook plasticity model that takes into account a strain-rate-dependent stress in the range of a shockwave condition. A time-dependent heat source and pressure pulse are concurrently applied on the workpiece-loaded boundary. Numerical results highlighted relevant findings, especially the pre-eminence of the uniform distribution of the heat flux to predict the in-depth residual stress profile and the evident effect of the plasma-induced pressure on the work hardening and less on the residual stresses.

Evaluation of the Residual Stress and Microstructure of Extruded and Shot Peened Aluminum Alloy 6082

Aluminum-magnesium-silicon alloys have been widely used as extruded products due to its mechanical strength and high ductility. The effects induced by shot peening has been extensively used in materials that have potential for structural applications. In this context, the purpose of this study was to evaluate the residual stress induced by shot peening of extruded aluminum alloy 6082. Initially, the effect of heat treatments such as solution treatment and ageing of this alloy was studied. The residual stress measurements were carried out using x-ray diffraction. The microstructure of the alloy was studied by optical microscopy. The crystallographic texture was determined using x-ray diffraction and back-scattered electron diffraction. The heat treatment sequence that resulted in the highest hardness of Al alloy 6082 was solution treatment at 560oC for 30 min, followed by ageing at 185oC for 5 h. The residual stress in compression of the extruded alloy’s surface increased by 87.38%, from-66.6 to-124.8 MPa, caused by shot peening. The residual stress profile indicated an increase in its value up to a depth of 86 μm, beyond which the values obtained were unreliable. The extruded section revealed accentuated crystallographic texture in the (111) plane parallel to the cross-section and in the (200) and (220) planes oriented preferentially in the longitudinal direction to extrusion and perpendicular to the (111) plane.

Effect of vibratory peening on the sub-surface layer of aerospace materials Ti-6Al-4V and E-16NiCrMo13

Shot peening (SP) is a widely used fatigue life improvement process consisting of impacting a metallic component with high-velocity shot. The plastic deformation induces a layer of compressive residual stress and cold work which delays crack initiation and crack propagation. However, shot peeing also increases surface roughness, which acts as stress concentrator and decreases fatigue life. Vibratory finishing (VF) is a mass finishing process in which components are placed in an oscillating bowl or tub containing polishing media. The media flow around the component polishes its surface and decreases its roughness. Vibratory peening (VP) is a modified vibratory finishing process that induces compressive residual stresses comparable to those induced by shot peening and which produces a much better surface finish. In this paper, the effects of vibratory peening on surface roughness, on residual stresses, and on hardness are investigated. Two aerospace materials are assessed, titanium alloy Ti-6Al-4V and carburized steel E-16NiCrMo13. The process parameters studied are frequency, media mass and processing time. A study of Almen intensity as a function of machine parameters is carried out in order to determine the machines parameters leading to the target Almen intensities of 0.12 mmA, 0.18 mmA and 0.25 mmA. The influences of media mass and frequency on peening intensity are examined. The study results show that the VP process can deliver the targeted intensities consistently. Roughness measurements show that for both the materials studied, VP produces good surface roughness with a consistent average roughness value in the range of 0.3–0.4 μm. The residual stress profiles produced by VP are comparable to those produced by conventional SP for identical Almen intensity. Compared to conventional SP, VP is found to lead to higher maximum compressive values for steel and unchanged values for titanium, with greater depth of the compressive region (+126% for steel). Hardness measurements show that VP has limited influence on a layer with depth of 70 μm. In general, the VP process is capable of consistently producing a beneficial layer of compressive residual stress together with a good surface finish, which is an appealing avenue to improve the fatigue properties of aerospace components.

Fatigue and Fracture Behavior of Induction-Hardened and Superimposed Mechanically Post-treated Steel Surface Layers

This paper focuses on the effect of induction hardening and superimposed stroke peening, as mechanical post-treatment, on the fatigue and fracture behavior of gas engine 50CrMo4 steel crankshafts. Comprehensive analysis in regard to local microstructure, hardness and residual stress profile in depth is performed to examine the local characteristics of the investigated crankshafts and the representative notched round specimens. An evaluation of the equivalent von Mises residual stress condition in depth demonstrates an increase by over 70% of the average compressive residual stress state due to the superimposed stroke peening compared to the induction-hardened condition. Rotating bending fatigue tests reveal that the additional stroke-peening process elevates the high-cycle fatigue strength of the induction-hardened surface layer by 20%, which validates the beneficial effect of the enhanced compressive residual stress state. An extensive fracture surface analysis presents the position of crack initiation for each investigated manufacturing condition indicating that the failure origin preferably starts at the surface for higher load levels. On the contrary, the crack initiation shifts beneath the hardened layer in case of lower load levels leading to subsurface failure origin.

Surface Integrity Analisys in the Hard Turning of Cemented Steel AISI 4317

This work was based on the surface integrity analysis of hardened steel, AISI 4317 case carburized, quenched and tempered with 58 HRC, obtained by a turning operation executed with CBN (cubic boron nitride) tool, varying three basic cutting parameters (cutting speed, feed rate and cutting depth). The surface integrity characterization was conducted analyzing the behavior of the surface roughness Ra, residual stresses and with layer presence after turning. Also, the cutting forces were measured. The experimental planning used was a central composite design. The results and data were statistically treated by the Statistica software, enabling the generation of a mathematical model, relating the dependent variables with the independent variables. The roughness values Ra obtained after the experiments ranged from 0.31 to 2.8 µm, providing an indication that is possible to replace grinding process by hard turning using CBN tools that could reduce machining time and costs. The hard turning process generated compressive residual stresses profiles and white layer formation, from 1.1 to 5.1 µm, on the surface of the samples. The penetration force showed the highest values for the turning forces measurements. The lowest values for the cutting parameters represented the optimized surface integrity of the AISI 4317 steel.

Raman Strain Sensing and Interfacial Stress Transfer of Hierarchical CNT-Coated Carbon Fibers

Hierarchical reinforcements of carbon fibers (CFs) coated with carbon nanotubes (CNTs) were produced via two different methodologies, namely chemical vapor deposition (CVD) and wet chemical treatment. These advanced smart structural materials are possible candidates for inherent strain sensing and improved interfacial properties when incorporated in the volume of structural fiber-reinforced polymer composites. The morphology and wetting properties of the hierarchical CFs were evaluated via SEM and contact angle measurements. Enhanced values in the strain-induced Raman sensing ability of the hierarchical reinforcement produced via CVD in comparison with the uncoated fibers were measured at single fiber level. The residual stress transfer profiles of both the hierarchical and the uncoated carbon fibers were also evaluated using the Raman spectroscopy in order to evaluate the phase interaction in the hierarchical structure. Compressive residual stresses in the order of 1.5 GPa were found to act on the CF surfaces, while tensile residual stresses were measured for both the CNT-coated systems. The effects of the preparation process of the CNT coatings on the residual stress profiles were also prominent, with the CVD-grown coatings resulting in higher tensile residual stresses, 1.76 GPa instead of 1.22 GPa for the coatings produced via wet chemical treatment. The change in the residual stress profiles established the interaction between the CFs and the CNTs via an anchoring mechanism which significantly altered the stress field at the vicinity of the interface.

Modeling of cavitation peening: Jet, bubble growth and collapse, micro-jet and residual stresses

Surface treatment methods like shot peening are used to introduce compressive residual stresses in metals and alloys. The presence of compressive stresses prevent the initiation and growth of cracks and hence improve the fatigue life of mechanical parts. Cavitation peening is a similar surface treatment process which acts by the generation of cavitating bubbles in front of the workpiece surface. The modeling of this process is challenging because of the complexity of cavitation phenomenon. The main difficulty is the prediction of the mechanical loading at the surface of the material. One of the key process parameters is the pressure within the reservoir where cavitation is generated. A strategy based on the modeling of a single cavitating bubble is proposed. Spherical and aspherical collapse of bubbles close to a solid surface are studied first analytically and then numerically. These two sources of mechanical loading are compared and it is shown that the pressure pulse impact due to the collapse of a spherical bubble near a wall is much greater than the one resulting from a micro-jet impact. A comparison with experimental data is made and it is found that the magnitude of the residual stresses within the treated material produced by the collapse of a single bubble matches. However the impact of one pressure wave is not sufficient to represent the entire process because of the low compressive depth generated. Finally, a macroscopic cavitation model has been derived by associating the numerical simulation of the cavitating jet ahead of the nozzle and the localization of the cavitation zone. A good overall agreement is found between the residual stress profile calculated with the proposed model and the experimental data.