Residual Stress (Engineering) Research Papers (original) (raw)

Residual stress has been mapped in the heads of three normal grade and one heat treated (350HT) ex-service railway rails. Transverse, vertical and in-plane shear stresses were deduced using energy dispersive synchrotron X-ray, magnetic permeability and laboratory X-ray measurements. Differences were found between samples both in the distribution of compressive stress introduced at the running surface, and in the locations of the balancing tensile stress below this. The heat treated rail was found to have a thinner layer of surface compression than the normal grade rails, with tensile stress being closer to the active gauge corner. The thinner layer of surface compression in the heat treated rail may reduce the protective benefits for rail integrity that surface compression is thought to confer.

Automobile brake systems transform kinetic energy into heat for the purpose of speed control. During this operation, rapid heating and cooling cycles cause the formation of residual stress in the brake pad backing plate. In this study, the relation between residual stress magnitude and the operation period is investigated to predict the service life of brake pads. Residual stresses are calculated related to longitudinal ultrasonic wave velocity variations which are measured non-destructively using cost effective and accurate immersion ultrasonic technique. Finally, a mathematical model is developed to predict residual stress in brake pad backing plates related to operation period.

- by Fatih Uzun and +1
- •
- Residual Stress (Engineering), Brake System, Ultrasonics

Sputter deposited molybdenum (Mo) thin films are used as back contact layer for Cu (In 1 À x Ga x )(Se 1 À y S y ) 2 based thin film solar cells. Desirable properties of Mo films include chemical and mechanical inertness during the deposition process, high conductivity, appropriate thermal expansion coefficient with contact layers and a low contact resistance with the absorber layer. Mo films were deposited over soda-lime glass substrates using DC-plasma magnetron sputtering technique. A 2 3 full factorial design was made to investigate the effect of applied power, chamber pressure, and substrate temperature on structural, morphological, and electrical properties of the films. All the films were of submicron thickness with growth rates in the range of 34-82 nm/min and either voided columnar or dense growth morphology. Atomic force microscope studies revealed very smooth surface topography with average surface roughness values of upto 17 nm. X-ray diffraction studies indicated, all the films to be monocrystalline with (001) orientation and crystallite size in the range of 4.6-21 nm. The films exhibited varying degrees of compressive or tensile residual stresses when produced at low or high chamber pressure. Low pressure synthesis resulted in film buckling and cracking due to poor interfacial strength as characterized by failure during the tape test. Measurement of electrical resistivity for all the films yielded a minimum v alue of 42 μΩ cm for Mo films deposited at 200 W DC power.

During fabrication of glass lens by precision glass molding (PGM), residual stresses are setup, which adversely affect the optical performance of lens. Residual stresses can be obtained by measuring the residual birefringence. Numerical simulation is used in the industry to optimize the manufacturing process. Material properties of glass, contact conductance and friction coefficient at the glass-mold interface are important parameters needed for simulations. In literature, these values are usually assumed without enough experimental justifications. Here, the viscoelastic thermo-rheological simple (TRS) behavior of glass is experimentally characterized by the four-point bending test. Contact conductance and friction coefficient at P-SK57 TM glass and Pt-Ir coated WC mold interface are experimentally measured. A plano-convex lens of P-SK57 TM glass is fabricated by PGM for two different cooling rates and whole field birefringence of the finished lens is measured by digital photoelas-ticity. The fabrication process is simulated using finite element method. The simulation is validated, for different stages of PGM process, by comparing the load acting on the mold and displacement of the molds. At the end of the process, the birefrin-gence distribution is compared with the experimental data. A novel plotting scheme is developed for computing birefringence from FE simulation for any shape of lens.

- by Tarkes Dora P
- •
- Glass, Residual Stress (Engineering), Photoelasticity

An FEA and experimental investigation on the residual stresses, which occurred in sheet metal test pieces after arc welding process, has been studied in the present study. The test pieces were made of same material and having the same dimensions and welded with speeds of 4 mm/s, 7 mm/s and 10 mm/s by using robotic welding. During welding process, measurement of temperature was done by using thermocouples. After welding process, it is waited for a while for cooling of sheet metal down to room temperature. And residual stress occurred due to heat flux was investigated by using hole drilling method. Simulation modeling for the same arc welding was done by using commercially available software MSC. Marc-Mentat. Von Mises stresses maximum temperature and heat speed values were obtained by 3-dimensional modeling of arc welding simulation. Finally, experimental results were compared with FEM simulation results in this study.

Numerical estimation of residual stress developed in glass products during its production process is necessary for its product quality improvement. Cooling stage of production process is the key stage for development of residual stress in glass products. Heat transfer mechanisms involved during this stage is complex. Hence, in FE simulation of the glass manufacturing process, thermal boundary conditions becomes one of the major unknown parameter that dictates the residual stress level developed in glass. In this paper, a hybrid approach, coupled experimental and numerical results, is used to estimate the unknown contact conductance boundary condition in FE simulation. Further selection of FE modeling approach (2D or 3D) is also demonstrated. Integrated residual birefringence distribution obtained from photoelastic experiments is compared with the same being computed by post-processing FE results until they match for estimation of the thermal boundary conditions.

- by Tarkes Dora P
- •
- Glass, Experimental Stress Analysis, Residual Stress (Engineering), Photoelasticity

The Finite Elements Method and the Finite Volume Method numerical control of the welded structures designs, subjected to different loads is more frequently nowadays. The present work proposes the estimation of the welded equivalent loading system based on the residuals deflections caused by the welded processes. For the welded structures modeled by beams, the work develops some exact algorithms based on displacements method and simplex linear transformation algorithms method. For the more complex welded structures, processed using shells and bricks type finite elements, are generated other chain of algorithms based on Greedy methods, for estimation of the optimum welding equivalent loads and implicitly the welding processes parameters.

- by Taus Daniel
- •
- Welding Engineering, Residual Stress (Engineering)

It is very significant to investigate the shot peening mechanism in ensuring a good resistance to fatigue and stress corrosion. This paper reviews the recent advancements in shot peening process. Emphasis is put on the application of numerical simulation techniques and finite element method in residual stress prediction during shot peening process. Different methods related to shot peening modelling and prediction of plastic deformation and surface integrity are reviewed. Some key issues such as algorithms and simulation procedures are discussed.

H13 tool steel with excellent hot working properties is commonly used for manufacturing dies. However, the damage of die surface due to cyclic thermo-mechanical loading is detrimental to the service life. In order to enhance the die life, it has been observed that cladding based repair is superior to welding or thermal spraying repair techniques. In this paper, experimental study of laser cladding of H13 has been carried out. CPM 9V steel powder has been deposited on H13 tool steel plate for repairing the die surface damage using a CW CO 2 laser in conjunction with powder injection system. The effect of laser parameters on clad geometry and clad quality has been investigated. The microstructure of laser cladded samples has been characterized using optical microscope (OM) and scanning electron microscope (SEM). The phases and the residual stresses present in the clad have been determined via X-ray diffraction. The micro-hardness profiles obtained in the clad–substrate system and the hardness change due to cyclic thermal loading have also been characterized. Optical micrographs of the clad microstructure shows existence of vanadium carbide particles embedded in martensite and retained austenite. The hard vanadium carbide particles increase the clad hardness to an average of four times greater than the substrate hardness. It has been observed that compressive residual stresses are generated in clad which is desirable for repair applications as it will impede the crack propagation resulting in enhanced die life.

Titanium and nickel alloys represent a significant metal portion of the aircraft structural and engine components. When these critical structural components in aerospace industry are manufactured with the objective to reach high reliability levels, surface integrity is one of the most relevant parameters used for evaluating the quality of finish machined surfaces. The residual stresses and surface alteration (white etch layer and depth of work hardening) induced by machining of titanium alloys and nickel-based alloys are very critical due to safety and sustainability concerns. This review paper provides an overview of machining induced surface integrity in titanium and nickel alloys. There are many different types of surface integrity problems reported in literature, and among these, residual stresses, white layer and work hardening layers, as well as microstructural alterations can be studied in order to improve surface qualities of end products. Many parameters affect the surface quality of workpieces, and cutting speed, feed rate, depth of cut, tool geometry and preparation, tool wear, and workpiece properties are among the most important ones worth to investigate. Experimental and empirical studies as well as analytical and Finite Element modeling based approaches are offered in order to better understand machining induced surface integrity. In the current state-of-the-art however, a comprehensive and systematic modeling approach based on the process physics and applicable to the industrial processes is still missing. It is concluded that further modeling studies are needed to create predictive physics-based models that is in good agreement with reliable experiments, while explaining the effects of many parameters, for machining of titanium alloys and nickel-based alloys.

- by Durul Ulutan
- •
- Microstructure, Finite Element Methods, Titanium, Wear

Orthopedic implants are used as medical devices that are surgically implemented into the body and designed to replace a missing joint, a bone or to support a damaged structure. Even though orthopedic implants have excellent outcomes such as restoring mobility and increasing the quality of lives, the failure of these implants occasionally takes place. This thesis examines a process called “superfinishing” as an alternative surface modification method for use on orthopedic implants that may yield better patient outcomes. It is believed that superfinishing may improve desired properties such as bending stress, fatigue strength, resistance to bacteria adhesion, wear resistance, chemical inertness, low coefficient of friction, and similar or better adhesion characteristics than stainless steels/titanium. The focus of this thesis is to show improvement in bending strength. In this study, specimens were superfinished in two different types of abrasives: aluminum oxide (Al3O2) and silicon carbide (SiC). The results demonstrated that roughness value of stainless steel rods in Al2O3 powder/walnut shell mix decreased to 0.156μm from 0.221μm and roughness value of stainless steel rods in SC powder/walnut shell mix decreased to 0.100μm from 0.184μm which is 36-44% decrease in roughness value. The experiment indicated that roughness value of these rods becomes stable after 21 minutes of superfinishing process but the maximum average load results were collected after 9 minutes of superfinishing process. This study showed that the longer
iv
superfinishing process does not necessarily increase the bending strength. This study also showed that the bending strength of superfinished Ti-6Al-4V rods was increased by
3.81% with SiC abrasives. In addition, bending strength of superfinished 316L rods were increased by 3.35% in Al3O2 abrasives and by 6.49% in SiC abrasives after 9 minutes of superfinishing process. Residual stresses should be studied as the future work to grasp the effects of superfinishing process.

The effect, on the residual stress measurement accuracy, of the drilling speed of the end-mill during the hole-drilling
measurements was evaluated in Ti6Al4V. In spite of the well-known consideration that the highest achievable speed should
be used during hole drilling, very few experimental works exist analyzing the effects of using lower velocities. Hole-drilling
experiments were performed in this study by measuring the released strain by electronic speckle pattern interferometry. A
known stress state was generated by loading the sample in a four point bending frame up to 50% of the yield strength.
Drilling speed ranging in 5000 ÷ 50000 rpm was investigated by using an electronically controlled mill. The expected stress
field, evaluated by a numerical model in ANSYS®, was compared with the measured one at different drilling speeds.

- by Claudia Barile and +2
- •
- Residual Stress (Engineering), Application of HDM 4, Residual Stress Measurement

An analytical model is developed for prediction of residual stresses in machining. In the thermo-mechanical model of residual stresses both the thermal field of the workpiece and mechanical cutting forces are coupled. The shear energy created in the primary shear zone, the friction energy produced at the rake face-chip contact zone, the heat balance between the chip, tool and workpiece are considered based on the first law of thermodynamics. The temperature distributions on the workpiece, tool and chip are solved by using finite difference method. The calculated workpiece temperature field is used in thermal load calculations. Stresses resulting from thermal and mechanical loading are computed using an analytical elasto-plastic model and a relaxation procedure. The model is verified with experimental measurements of residual stresses on bearing steel 100Cr6 (JIS SUJ2) in the literature. With the analytical model presented here, substantial reduction in computational time is achieved in the predictions of residual stresses.

- by Durul Ulutan
- •
- Machining, Finite-Difference Methods, Residual Stress (Engineering)

Friction Stir Welding (FSW) has become the most important evolving welding technology for the
proposed integral aluminium structures for aerospace fuselage to avoid high cost, excessive
weight. The fatigue performances of FSW parts are superior to riveting. FSW entails a nonconsumable
rotating cylindrical tool, consisting of a shoulder and pin, being pushed in to induce
heat to plasticise the material and then traversed along the abutting surfaces of the two plates to
be welded. Several factors influence the resulting microstructure and mechanical characteristics,
and tool transverse and rotational speed are the most significant.
Characterization of selected pioneering FSW components is presented. AA5083-H111 was
selected because of its superplasticity, formability, weldability, high strength and corrosion
resistance. It has been the preferred material for manufacturing of pressure vessels for aircrafts
and spaceships, and vehicle bodies’ structures.
A CNC milling machine was converted to weld 3mm thick plates of 360mm by 120mm at
combinations of different rotational (400, 500 and 630rpm) and welding speeds (50, 60 and
70mm/min) using a tapered probe and cylindrical shoulder at zero tilt angle. Tensile testing and
bend testing were undertaken. The macrostructure and fractured surfaces were observed using an
optical microscope and an SEM in secondary electron mode.
Stress-strain relationships of the welds were different. Compared to the base material, FSW
samples had lower strain-to-failure results, owing to strain localization in the weld. H1, welded at
400rpm and 50mm/s had the highest UTS, 99.3±0.2%, whereas H2, welded at 400rpm and
70mm/s had the highest yield strength, 97.7±0.3%. Welds performed at 400 rpm were generally
flaw-free, while higher weld speeds produced welds with tunnels, due to the low forging pressure.
In the macrostructure, the weld nugget could be easily distinguished from the TMAZ (Thermo-
Mechanically Affected Zone) on the advancing side, because the grains were finer, being
recrystallized and equiaxed. However, no distinct boundary existed on the retreating side. The
joint-line remnant, which comprises amorphous clusters of alumina, was observed in the nugget,
but had no effect on mechanical properties. There was an increase in hardness in the weld zone,
towards the retreating side, due to grain refinement and better weld consolidation. Conversely,
there was a decrease in hardness in the TMAZ. The weld nugget average hardness of all flaw-free
samples was 79.8±4.6HV0.1, with no definite relationship between weld parameters and hardness
profiles. Tensile specimens fractured in the TMAZ on the advancing side at 45° to the far-field
stress, because of reduced micro-hardness, strain localization and as well as the presence of
tunnels in the flawed samples. Generally, failure occurred in a brittle mode, but areas of local
ductile shear and quasi-cleavage fracture were observed in the SEM.
This research demonstrated that good weld efficiency was obtainable on FSW of AA5083-H111
with a CNC milling machine of sufficient welding power.

- by Prof. Dr. OJ Dada and +1
- •
- Residual Stress (Engineering), Friction Stir Welding, Friction Stir Processing of Al Alloy

Ce livre porte sur la modélisation et la simulation numérique du procédé d'usinage par électroérosion. Une investigation bibliographique exhaustive sur le sujet a révélé une insuffisance des hypothèses adoptées pour approximer la réalité physique du procédé. De ce fait on a commencé par améliorer et intégrer des nouvelles hypothèses traduisant au mieux la réalité physique du procédé afin de produire des modèles assez précis. Par la suite, un code de calcul a été développé sur MATLAB permettant de simuler numériquement les modèles établis. Finalement une confrontation a été faite entre les résultats numériques et les mesures expérimentales afin d'évaluer la validité du modèle à prédire numériquement la rugosité, l'état des contraintes résiduelles, l'épaisseur des couches métallurgiques et du débit d'usinage.

In the recent decades, multi-pass sheet metal spinning proved to be a critical process for many industrial components such as jet engines, large size pressurized tanks and vessels, but residual stresses and springback in these spun parts remain an alarming problem. This study is intended to provide wider insight into the controlling parameters of these features. Two finite element (FE) simulations were conducted based on explicit code for the spinning process and implicit code for springback and residual stresses prediction. The numerical results were verified and found to be well correlated with the experimental data. Optimum combination between small springback and safety against residual stresses was obtained using low feed ratio and friction coefficient with large number of spinning passes. The common issue is low loading rate. Better control of the final spinning pass can add value to the mechanical and dimensional characteristics of the spun component.

Films and coatings have become widely used in structures and components to protect the underlying material from mechanical degradation, corrosion, oxidation and high
temperatures or improve surface properties. Residual stress generated in these multi-layer systems is one of the main causes of coating delamination and eventual failure. Systematic
measurement and monitoring of the residual stresses are a vital basis for integrity evaluation and remaining life prediction. Raman spectroscopy has been recognised as one of
the most important approaches to measure the stress in films and coatings. This review considers the measurement of stresses in films and coatings using Raman spectroscopy. It
addresses the following questions: what is Raman spectroscopy, why is stress important for films and coatings, how is strain/stress derived from Raman spectra and what confidence do we have in this technique and the limitations. To elucidate specific issues related to the application of the Raman technique for stress measurement, despite the wide
range of coatings available, important films and coating are chosen as representative examples.

Titanium and nickel alloys represent a significant metal portion of the aircraft structural and engine components. When these critical structural components in aerospace industry are manufactured with the objective to reach high levels of quality and reliability, machining induced surface integrity becomes one of the most relevant parameters for product life cycle. The residual stresses induced by machining of titanium and nickel based alloys are very critical due to safety and sustainability concerns. This paper presents experimental investigations and finite element simulations on turning of Ti-6Al-4V titanium alloy and IN-100 nickel based alloy with uncoated and TiAlN coated tools. Face turning of Ti-6Al-4V and IN-100 using uncoated and TiAlN coated carbide tools are conducted; and residual stresses are measured in radial and tangential directions using X-ray diffraction with Cu-K radiation. 3-D Finite Element (FE) modelling is utilized to predict forces and machining induced stress fields. The feasibility and limitations of predicting machining induced residual stresses by using elasto-viscoplastic finite element simulations and temperature-dependent flow softening constitutive material modeling are investigated. A friction determination method is proposed to identify friction coefficients in presence of tool edge radius and flank wear. The predicted stress fields are compared against measured residual stresses. Effect of friction on the predicted stress profiles is also investigated. The results are found useful in predicting machining induced surface integrity that is critical to determine the fatigue life of nickel and titanium alloy components.

- by Durul Ulutan
- •
- Finite Element Methods, Titanium, Residual Stress (Engineering), Ti64

This is a short history of the UK disability charity, The Cambodia Trust (founded Adderbury, Oxon, November 1989), which peace negotiator, Stan Windass, former diplomat, John Pedler and Peter Carey co-founded in November1989 in Oxfordshire and which was active in the establishment of rehabilitation centres (clinics) and training Schools for P&O students in Sri Lanka and Southeast Asia (Cambodia, Indonesia, the Philippines, Myanmar, and Timor-Leste) from 1989-2014, when it was renamed Exceed. An article which appeared on the Trust's work entitled 'Transforming Nations' in O&PEdge, which looks in particular at the Trust's Timor-Leste initiative (establishment of the national rehabilitation centre in Becora, Dili, March 2005) in February 2009 by Morgan Stanfield, is also included.

Surface integrity of machined components has a critical impact on their performance. Magnesium alloys are lightweight materials used in the transportation industry and are also emerging as a potential material for biodegradable medical implants. Surface integrity factors, such as grain size, crystallographic orientation and residual stress, have been proved to remarkably influence the functional performance of magnesium alloys, including corrosion/wear resistance and fatigue life. In this study, the influence of dry and cryogenic machining (liquid nitrogen was sprayed on the machined surface during machining) using different cutting edge radius tools on surface integrity was investigated. Compared with the initial material, cryogenic machining when using a large edge radius tool led to enhanced surface integrity in terms of the following: (1) improved surface finish; (2) significant grain refinement from 12 mm to 31 nm in the featureless surface layer; (3) large intensity of (0002) basal plane on the machined surface; (4) 10 times larger compressive areas in residual stress profiles; these changes should notably improve the functional performance of machined AZ31B Mg alloy. In addition to the frequently reported benefits on tool life, this study suggests that cryogenic machining may also enhance the surface integrity of the workpiece and improve the performance of machined components.

- by Pu Zhengwen
- •
- Machining, Residual Stress (Engineering), Corrosion, Biomedical

It is shown that the conventional ‘sheet by sheet’ analysis of residual stress measurement by the deep hole drilling method is inaccurate when in-plane stress gradients exist. The influence of the interfacial tractions is analysed by the distributed dislocation technique. Finite element modelling is used to provide data for an example residual stress field with radial variation.

A set of Ni-based single crystal superalloy aero-blade sections with electron beam physical vapour deposited thermal barrier coating (EBPVD-TBC) were used to investigate the role of substrate curvature on the microstructure and the residual stresses in the coating system. The residual stresses generated in the thermal barrier coating (TBC) and the thermally grown oxide (TGO) have been evaluated non-destructively by Raman spectroscopy (RS) and photo-stimulated luminescence piezo-spectroscopy (PLPS), respectively. In addition, residual stresses were measured in free-standing individual columns extracted from the TBCs for further understanding of the large Raman shift towards the tension side in the ceramic layer. For the TGO, three areas with varied substrate curvature were mapped by PLPS. These residual stresses are analysed as a function of the thermal exposure and the substrate curvature. Discussion focuses on the implications on how to correctly evaluate the residual stresses in EBPVD-TBCs based on Raman shifts and subsequently the role of substrate curvature on the degradation of the microstructure and evolution of residual stresses in this type of TBCs.

- by Dong Liu and +1
- •
- Aerospace Engineering, Thin Films and Coatings, Raman Spectroscopy, Residual Stress (Engineering)

The objective of this work is to develop 3D models and techniques for predicting the temperature field, distortions and residual stresses induced by a butt welding process. Finite element model of a plate subjected to double ellipsoidal moving heat source has been developed to simulate the welding process. The obtained transient temperature distributions have been applied as thermal loads in mechanical analyses to predict the structural responses. Temperature distribution, residual distortion, and shrinkages of the plate have been calculated and discussed in detail after validating the proposed numerical approach. The longitudinal stresses in different sections have been discussed and verified with theoretical prediction. As the continuously work of the previous study on the effects of weld parameters on the thermal response, their effects on the structural response of butt-welded plates are included as well.

- by Baiqiao Chen
- •
- Mechanical Engineering, Residual Stress (Engineering), Welding

We have examined the influence of mechanical surface finishing on the development of residual stresses, and on the subsequent formation of stress corrosion cracks, in 316Ti austenitic stainless steel after exposure to boiling magnesium chloride. The surface residual stresses of as-received plate, prior to machining, were found to be biaxial and compressive. However, abrasive grinding produced significant compressive stresses in the machining direction but much lower perpendicular stresses. On the other hand, milling produced high biaxial tensile stresses (approaching the ultimate tensile strength, UTS, of the material), which were found to be relatively insensitive to cut depth but to vary as a function of feed rate. On the milled surfaces a distinctive pattern of stress corrosion cracking was evident with longer primary cracks nucleating along the milling direction and secondary, shorter, cracks nucleating perpendicularly. As the surface tensile stress was lower perpendicular to the milling direction, we postulate that the nuclea- tion of primary cracks parallel to machining must be driven by the surface profile after machining (and associated micro-stresses) as much as by the macroscopic residual stresses.

Vertical cylindrical vessel-chambers as a part of coal — drying plants, whose purpose is to collect wastewater, are supported at 3 points in upper dish head area and are made of fine-grained Mn steel plates, joined by welding. Significant thinning and leaking in upper dish head area of the vessel occurred due to original design provoking an intensive abrasion, cracking and rupture. After reconstruction, in the upper zone of cylindrical shell, two new joints were made in sity by manual arc welding, with subsequent, local post-weld heat treatment. However, cracks appeared firstly in new welded zones in radial and axial joints, and then in zones of openings. Later, the similar failure features began to appear in the area of original welded joints. All of these cracks were repaired by properly specified technology. Unfortunately, after some period of exploitation the initiation of new cracks was observed, at first by the visual inspection. This problem was detected in the repaired areas in all (16) vertical cylindrical vessels. Cracks have propagated in different directions with various penetration depths, up to 3 mm. It is interesting to note that crack appeared in the HAZ of vertical joints, while in the area of radial welded joints the cracks were randomly distributed in a larger zone. For cracks up to 3 mm, deep grinding was applied whereas for greater crack depths repair welding with local post-weld heat treatment was used. Since welding with subsequent heat treatment could only be used twice to repair the welded joints [1, 2], it is clear that in the areas repaired several times (up to 7) proper mechanical characteristics of the material is rather difficult to maintain.

- by Milos Djukic and +1
- •
- Materials Science, Finite Element Methods, Metallurgy, Welding Technology

A B S T R A C T Ultrasonic assisted ball burnishing process is a newly developed alternative of conventional ball burnishing process that enhances the surface properties of engineering materials through imposing both the static and dynamic loadings. The process needs careful selection of design parameters to improve the performance measures such as surface roughness and hardness. In the present study an experimental investigation was carried out to analyze effect of ultrasonic vibration amplitude, feed rate and static force on surface roughness and hardness of aluminum 6061-T6 alloy. Here, number of 20 experiments was carried out through face centered central composite design and values of surface roughness and hardness after each experiment were measured. Hereafter, response surface methodology (RSM) was utilized to correlate empirical relationship between process parameters and responses. Further, numerical simulation of process using ABAQUS software has been carried out to study states of residual stress and plastic equivalent strain under different processing condition. Results indicated that in order to achieve maximum hardness and minimum surface roughness simultaneously, ultrasonic vibration amplitude of 8 µm, feed rate of 1000 mm/min and static force of 38 N, should be selected. The obtained optimal results were then experimentally verified and the prediction errors for both the responses were lower than 10%, implying rigidity of proposed methodology in finding the optimum results. The results obtained by FE simulation showed that the maximum value and effective depth of compressive residual stress in ultrasonic assisted process is significantly higher than that of conventional burnishing.

Dans cette étude cinq critères de fatigue
multiaxial ont été étudiés. L’état nitruré est évalué et
comparé avec l’état non traité. Les modifications induites
par le traitement de nitruration ionique, les contraintes
résiduelles de compression et la dureté superficielle, ont
été prises en considération. Les critères utilisés donnent
une bonne aptitude prédictive des seuils d’endurance,
l’indice de validité ne dépasse pas les 5 % pour la majorité
des critères étudiés .
Mots clés : fatigue polycyclique, nitruration, contraintes
résiduelles, durcissement superficiel

- by BECHOUEL Rafik
- •
- Residual Stress (Engineering), Fatigue

The effect of passive water cooling in laser forming of thin sheets made of AISI 304 stainless steel is experimentally investigated. Indeed, since each laser scan can produce only small bending angles, multiple laser scans are required to produce a given deformation with a significant increase of production time due to cooling between consecutive scans. Therefore, passive water cooling is tested to verify its influence on minimum time between consecutive scans (cooling time), bending angle, and surface quality. A parametric approach is involved in the investigation and main process parameters are changed among the experiments by varying laser scanning speed, laser beam power, sheet thickness, and cooling media among several levels. It was discovered that the employment of passive water cooling in laser forming of thin sheets would be beneficial since the capability to dramatically reduce the cooling time and oxidation of both irradiated and cooled surfaces. In addition, the bending angle is only marginally affected by employment of water cooling. The effect of water cooling on stress and deformations are discussed by developing a numerical model based on finite element model.

- by Alfonso Paoletti and +3
- •
- Laser, Sheet Metal Forming, Residual Stress (Engineering), Laser Forming

Nanogrinding of a fine-grained WC-Co composite was developed to achieve an optical quality surface without further polishing. Direct planar grinding was conducted with a CNC grinding machine using a metal-bond diamond wheel of grit size of 15 μm, under the nanogrinding conditions selected. The ground planar surfaces were examined using laser and optical interferometry, atomic force microscopy, and scanning electron microscopy to measure flatness, surface roughness, and surface integrity as a function of grinding conditions. Damage-free, planar mirror surfaces with a flatness (peak-to-valley, PV) at the submicron scale and surface roughness <5 nm Ra were obtained.

An ultrafine-grained surface layer was produced on Mg-Al-Zn alloy by a new severe plastic deformation process, cryogenic burnishing. The total burnishing-influenced zone was found to be over 3.4 mm thick. A large increase in hardness from 0.86 to 1.35 GPa was obtained near the topmost surface, where grains were refined from 12 lm down to 263 nm. The corrosion resistance was significantly enhanced, which may due to the combined effects of grain refinement and strong basal texture.

- by Pu Zhengwen
- •
- Machining, Residual Stress (Engineering), Corrosion, Biomedical

In this work, the effect of deep rolling on bending
fretting fatigue resistance of Al7075 is investigated. Rotary
bending fretting fatigue tests are conducted on a Moore rotary
bending apparatus. The effects of various parameters of deep
rolling such as rolling ball diameter, rolling depth, rotational
speed of specimen, feed rate, and rolling pass number are
investigated by experimental and numerical simulations. The
residual stress distribution due to rolling is determined using
Abaqus finite element code. In order to consider the real material
characteristics such as Bauschinger effect and cyclic
kinematic hardening behavior, the Chaboche nonlinear kinematic
hardening model is used in the numerical simulations.
Fractography of specimens after failure is accomplished using
optical microscopy.

This study analyzes acoustic emission (AE) signals during the intergranular corrosion (IGC) process of 316L stainless steel welded joints under different welding currents in boiling nitric acid. IGC generates several AE signals with high AE activity. The AE technique could hardly distinguish IGC in stainless steel welded joints with different welding heat inputs. However, AE signals can effectively distinguish IGC characteristics in different corrosion stages. The IGC resistance of a heat-affected zone is lower than that of a weld zone. The initiation and rapid corrosion stages can be distinguished using AE results and microstructural analysis. Moreover, energy count rate and amplitude are considered to be ideal parameters for characterizing different IGC processes. Two types of signals are detected in the rapid corrosion stage. It can be concluded that grain boundary corrosion and grain separation are the AE sources of type 1 and type 2, respectively.

- by Mengyu Chai
- •
- Residual Stress (Engineering), Corrosion, Acoustic Emission, Fatigue crack growth

Hole drilling (HDM) is the most widespread approach for measuring residual stress profile and it is, at the moment, the only technique to be ruled by a standard. Many improvements were introduced along the years and recently research papers investigated the feasibility to use optical systems to replace strain gage rosette as measurement tool of the strain field generated by the drilled hole. Generally speaking, the accuracy of the measurement is influenced by the proper choice of the experimental and analysis parameters. In this paper the effects of the choice of a proper analysis area, to be intended as the portion of the image captured by the CCD camera whose data are used to calculate the strain values, are shown. Furthermore the influence of the drilling rotation speed of the cutter used for drilling the hole is investigated. Experimental tests were performed on a Ti6Al4V specimen loaded in a four point bending frame up to 50% of the yield strength. Drilling speeds ranging between 5000 ÷ 50000 rpm were investigated by using an electronically controlled mill. Results obtained at different speeds and for different locations of the analysis area were compared with the theoretical expected value and with results from a numerical model implemented in ANSYS®.

- by Claudia Barile and +1
- •
- Residual Stress (Engineering), Speckle interferometry

Selection of a proper material for fabricating an
industrial product through welding process is not only
one of the initial necessities but also plays an important
parameter in achieving a safe design. At the same time,
researchers have shown that part of the residual stresses
in weldments could be generated under the influences of
joints material. Thus, values of the residual stresses that
produced due to variation of the characteristics of
different materials can be a basic criterion for reaching
this goal. In a similar condition, each case in
comparison with the others that begets less residual
stresses can be selected. On the other hand, prediction
of the behavior of the residual stresses is complex and
time-consuming in experimental methods and even in
numerical models. Therefore, in this work, entropy
generation methodology is presented as an efficient tool
to estimate the distribution of residual stresses,
qualitatively. In this approach, the behavior of the
induced residual stresses caused by the variation of
different parameters such as weldment material is
examined based on the behavior of generated entropy.
To verify the application of the proposed method, a 3D
model is employed to study the similar behavior of
residual stresses and entropy generation for three
different steels that are in frequent use. Furthermore, the
metallurgy analysis of the steels is performed in welding
processes and the results are compared with the residual
stresses and entropy generation ones. Based on the
outcome of the present work, one can find that the
behavior of entropy generation and residual stresses
with respect to different weldment materials are very
similar. Also, the entropy generation and residual
stresses results are comparable and in-line with the
metallurgy analysis ones. Finally, in the proposed
methodology, studying the induced residual stresses
without trying to obtain residual stresses directly which
used to be very complicated and time-consuming is
possible as well.

- by amir fallahi and +1
- •
- Materials Science, Welding Engineering, Residual Stress (Engineering), Welding Metallurgy

Measurement of residual stresses in glass with high transmittance in visible range and relatively low stress optic coefficient (‘C’ ≈ 0 TPa-1) using digital photoelasticity is vital in precision glass molding industries. Quantification of residual stresses in them necessitates the evaluation of photoelastic constant of glass. Photoelastic calibration methodologies of glass with low C using beam as the specimen needs a very high external load, sometimes beyond fracture point, for formation of significant and measurable retardations. In this paper a methodology of photoelastic calibration of glasses with low C using the solution of Flamant’s problem is demonstrated for a Schott glass, SF-57™ which has high transmittance and low C. Phase Shifting Technique (PST) is used to determine the retardations for a point load acting on a free surface. An automatic polariscope is used here to employ PST. The loading axis is precisely detected using Sobel edge detection algorithm available in MATLAB. Along the loading axis, retardation at multiple pointsis extracted for various loads from the whole field retardation data. The slope of retardation vs. load graph for various points along the load axis is used in solution of Flamant’s problem to evaluate C.

In the present work surface and bulk residual stresses generated in partially crystallized Li 2 O AE 2SiO 2 glass-ceramics are analyzed after different heat treatments. The phase specific residual stresses in the crystalline Li 2 Si 2 O 5-phase are evaluated for the first time in the nearsurface zone and the bulk of the samples using both medium and high energy synchrotron radiation. The results reveal that in the crystals within the bulk of the samples micro residual stresses generated by the thermal anisotropy of the isolated individual crystallites depend on the crystallographic direction. In contrast, the residual stress state in the near-surface zone is isotropic due to the superposition of thermal residual stresses in and around the crystals of the near-surface area. Residual stress calculations using a modified Selsing's model yield a good estimate of the anisotropic residual stresses in the bulk crystallites, whereas the isotropic residual stress state in the crystallized surface layer can be described by an elastic stress model for thin films.

The results are presented concerning simulations of the distribution of thermal residual stresses in a ceramic matrix particulate-reinforced composite in the SiC-TiB2 system. The stresses arise during cooling of the material after sintering due to differences in thermal expansion and elastic properties of the component phases, and belong to the most important factors for increasing fracture toughness of ceramic composites. A computational model was built on the basis of the real microstructure of the SiC-TiB2 composite. The material properties of component phases used in calculations included their temperature dependences. A temperature difference caused shrinkage and residual stress was adopted by means of the analysis of the sintering curves. The simulations were performed by using the finite element method. The results of simulations were compared with the calculated values of residual stresses, basing on analytical models and experimental data. The comparison shows good agreement. This allows an elaborated model to be used to solve more complex problems concerning fracture analysis of ceramic matrix composites.

In this work the numerical simulation of the Additive Manufacturing (AM) process is addressed. The coupled thermo-mechanical framework used to solve the balance equations, as well as the constitutive laws to describe the material behavior in the entire temperature range are presented. The numerical model has been calibrated through the experimental campaign carried out at the State Key Laboratory of Solidificati on Processing (SKLSP) where a Laser Solid Forming (LSF) machine is operated. This machine makes use of the blown powder technique to perform the Metal Deposition (MD) process in a layer-by-layer manner. Both the software and the machine read the same scanning sequence given through Common Layer Interface (CLI) format, that is, the sequence of polylines and hatching to cover the entire section of the component for each layer. The power absorption coefficient and the Heat Transfer Coefficients (HTC) for both heat convection and heat radiation laws have been calibrated to capture the temperature evolution at the different locations where the thermocouples have been placed. The response of the thermo-visco-elastic-visco-plastic constitutive model has been calibrated by comparing the distortion of the supporting plate at different locations monitored during the full duration of the manufacturing process. Remarkable agreement between experimental and numerical results is shown.

- by Miguel Cervera and +3
- •
- Computational Mechanics, Finite Element Methods, Computational Modelling, Finite Element Analysis (Engineering)

In precision manufacturing processes surface integrity is of the utmost importance for the performance and life-cycle of the final products. An important aspect of surface integrity is associated with residual stresses induced in the workpiece during machining. According to the relevant literature, tool rake angle plays an important role on the features of residual stresses, regarding their magnitude and distribution within the workpiece. In this paper, numerical investigations with the use of the finite elements method are presented that allow the evaluation of the influence of the tool rake angle on residual stresses for the case of hard turning of stainless steel. The investigation is performed in a wide range of positive and negative rake angles. Numerical results verify the dominant role of tool rake angle on the residual stresses. The proposed models can be used for the a priori evaluation of the characteristics of compressive stresses that are considered favorable for the produced components.

- by Angelos P Markopoulos
- •
- Residual Stress (Engineering), Hard Turning

After machining nickel-based superalloys, tensile surface residual stresses can cause end-product issues such as fatigue failure. Modeling the residual stress profile is currently tedious and inaccurate. This study introduces a new method of understanding the residual stress profile in terms of quantifiable key measures: peak tensile stress at the surface, magnitude and depth of peak compressive stress, and depth at which residual stress becomes near-zero. Experiments in turning IN-100 and milling GTD-111 have been conducted and subsequent X-ray Diffraction measurements have been utilized to obtain residual stress profiles. Using a sinusoidal decay function fitted to measured residual stress profiles, these four key profile measures are extracted and then the effects of process parameters such as cutting speed, feed, cutting edge radius, and tool coating on these measures are investigated.

- by Durul Ulutan
- •
- Modeling, Machining, Residual Stress (Engineering), Superalloys

This work describes the local-global strategy proposed for the computation of residual stresses in Friction Stir Welding (FSW) processes. A coupling strategy between the analysis of the process zone nearby the pin-tool (local level analysis) and the simulation carried out for the entire structure to be welded (global level analysis) is implemented to accurately predict the temperature histories and, thereby, the residual stresses in FSW. As a first step, the local problem solves the material stirring as well as the heat generation induced by the pin and shoulder rotation at the heat affected zone. The Arbitrary Lagrangian Eulerian (ALE) formulation is adopted to deal with the rotation of complex pin shapes. A thermo-rigid-viscoplastic constitutive law is employed to characterize the viscous flow of the material, driven by the high strain rates induced by the FSW process. A mixed temperature-velocity-pressure finite element technology is used to deal with the isochoric nature of the the strains. The output of this local analysis is the heat generated either by plastic dissipation or by friction and it is used as the power input for the welding analysis at structural (global) level. The global problem is tackled within the Lagrangian framework together with a thermo-elasto-viscoplastic constitutive model. Also in this case the mixed temperature-displacement

- by Miguel Cervera and +1
- •
- Finite Element Methods, Residual Stress (Engineering), Friction Stir Welding

How to cool the weldments is the one of the important
input parameters to control the welding process and
achieve a safe design. Generally, several coolants can be
used for this aim. However, air and water that have been
studied in this paper not only have too many practical
applications but also will be examined as representative
coolants. Analysis of the convection coefficients is the
best method to investigate the effect of these coolants
on induced residual stresses behavior. To fulfill this, in
present work, entropy generation has been introduced as
a tool to predict the distribution of residuals tresses
qualitatively. To check the application of the proposed
method, a 3D numerical model is employed to study the
similar behavior of residual stresses and entropy
generation due to variation of different convection
coefficients. Furthermore, the effect of convection
coefficients on residual stresses and entropy generation
has been investigated for both free and forced
convections. Based on the present method's results, one
can find that the behavior of entropy generation and
residual stresses distribution with respect to different
convection coefficients are very similar. Finally, the
application of entropy generation minimization is
checked in looking for the range of a satisfactory design
in welding.

- by amir fallahi
- •
- Convection, Welding Engineering, Residual Stress (Engineering), Entropy generation

Residual stresses are among the most important factors affecting the properties and service lifetime of materials and components. In the cold spray coating process there are two contradictory factors that influence the final residual stress state of the coated material; the impact of the high velocity micron-size particles induces compressive residual stresses, whereas the gas temperature can have an opposing annealing effect on the induced stresses. These two simultaneous phenomena can in turn change the residual stress profile, thus complicate the assessment of the final residual stress state.In this paper the residual stress evolution during cold spray coating process has been studied through experimental measurements and numerical simulations performed on several series of samples coated using different spray process parameters. A detailed finite element (FE) analysis of the process has been developed to calculate the stresses induced through impacts and then the annealing effect has been taken into account through an analytical model. The results of the experiments and numerical–analytical approach confirm the considerable effect of annealing on the eventual stress distribution in the coated samples.

- by Durul Ulutan
- •
- Machining, Finite-Difference Methods, Residual Stress (Engineering)
- by Adnene TLILI
- •
- Residual Stress (Engineering), Matlab, Comsol Multi Physics, Matlab Programming

The laser power density and pulse length directly affect the recoil pressure impact intensity, duration and spot size in the laser shock processing (LSP) of substrate materials. Optimization of these laser beam parameters can thus result in higher productivity together with improved material properties. Process parameters including substrate material properties, type of overlay, number of laser pulses per shot and heat transfer conditions at the interfaces are also considered to be important role players. Unfortunately, experimental investigation of all of these factors is limited due to cost and complexity. Published literature addressing the effect of these process parameters is therefore rather sketchy.

- by Khaled Al-athel and +2
- •
- Computational Mechanics, Finite Element Methods, Residual Stress (Engineering), Residual Stress