Stress Depth Research Articles

STRESS INDUCED NITROGEN DIFFUSION IN NITRITED CoCr ALLOY

In the present study the nitrogen transport mechanism in plasma nitrited CoCr alloy at moderate temperature ( 400ºC ) is explained by non-Fickian diffusion model. This mechanism is considered by stress induced diffusion model. The model involves diffusion of nitrogen induced by internal stresses created during nitriding process. The model considers the diffusion of nitrogen in the presence of internal stresses gradient induced by penetrating nitrogen as the next driving force of diffusion after concentration gradient. This model is commonly used for analysis of stainless steel nitriding, however, in this work it is shown that the same nitrogen penetration mechanism takes place in CoCr alloy. For mathematical description of stress induced diffusion process the equation of baro-diffusion is used which involves concentration dependant baro-diffusion concentration. For calculation of stress gradient it is assumed that stress depth profile linearly relates with nitrogen concentration depth profile. The fitting is done using experimental curves of nitrogen depth profiles for medical grade CoCr alloy (ISO 5831-12) nitrited at 400 ºC temperature. The experimental curves are taken from literature. The nitriding duration was 2h, 6h, 20h. Calculated nitrogen depth profiles in CoCr alloy are in good agreement with experimental nitrogen depth profiles. The diffusion coefficient D is found from fitting of experimental data.DOI: http://dx.doi.org/10.5755/j01.ms.21.1.5711

Fatigue performance of medical Ti6Al4V alloy after mechanical surface treatments.

Mechanical surface treatments have a long history in traditional engineering disciplines, such as the automotive or aerospace industries. Today, they are widely applied to metal components to increase the mechanical performance of these. However, their application in the medical field is rather rare. The present study aims to compare the potential of relevant mechanical surface treatments on the high cycle fatigue (R = 0.1 for a maximum of 10 million cycles) performance of a Ti6Al4V standard alloy for orthopedic, spinal, dental and trauma surgical implants: shot peening, deep rolling, ultrasonic shot peening and laser shock peening. Hour-glass shaped Ti6Al4V specimens were treated and analyzed with regard to the material’s microstructure, microhardness, residual stress depth profiles and the mechanical behavior during fatigue testing. All treatments introduced substantial compressive residual stresses and exhibited considerable potential for increasing fatigue performance from 10% to 17.2% after laser shock peening compared to non-treated samples. It is assumed that final mechanical surface treatments may also increase fretting wear resistance in the modular connection of total hip and knee replacements.

Effects of overlapping rate on the uniformities of surface profile of LY2 Al alloy during massive laser shock peening impacts

Massive laser shock peening (LSP) impacts have been a competitive alternative technology to improve fatigue life, corrosion and wear resistance of metallic component which depend strongly on the generated surface profile and plastic deformation in the surface layer. First, a three-dimensional finite element modeling (FEM) model with different overlapping rates has been developed to predict residual stress distribution and surface profile of LY2 Al alloy. Second, residual stress distribution predicted is validated by experimental data. Next, four kinds of overlapping rates, 30%, 50%, 70%, and 90%, are chosen to investigate the uniformity and affected depth of residual stress. Special emphasis is also placed on the effects of overlapping rate on surface profile and plastic deformation of LY2 AL alloy subjected to massive LSP impacts. Finally, the underlying influence mechanism of overlapping rate on surface profile of LY2 Al alloy is also clearly revealed.

Residual stress and microstructure depth gradients in nitrided iron-based alloys revealed by dynamical cross-sectional transmission X-ray microdiffraction

Thermochemical surface treatments such as nitriding result in the formation of complex near-surface gradients of phase fractions, residual stress and microstructure, which influence the structural and functional properties of the material decisively. In this work, a novel cross-sectional synchrotron microdiffraction method is used to characterize such gradients by analyzing cross-sections of nitrided Fe–Al and Fe–V alloy specimens down to a depth of 500μm, with a depth resolution of less than 10μm. The Debye–Scherrer diffraction data collected from the individual sample depths document very different nitride-precipitation mechanisms and resulting stress gradients. In nitrided Fe–Al samples, the delayed precipitation of largely incoherent AlN particles leads initially to the development of internal microcracks, followed by a pronounced increase of compressive stress until plastic deformation sets in, which finally results in the formation of regions with tensile and compressive stress. In contrast, the VN precipitation during nitriding of Fe–V alloys occurs very quickly and generates a desired high compressive stress at the surface of the nitrided part. The tiny and coherent VN precipitates increase the yield strength of the nitrided zone significantly. The evaluated corresponding ferrite-lattice parameter depth profiles can be quantitatively described as the outcome of (competing) effects of solute (Al, V) depletion, (excess) nitrogen dissolution and the emergence of a hydrostatic strain due to elastic accommodation of the precipitate/matrix misfit. The novel technique to expose depth gradients in real space, with micrometer resolution, opens the way to understand the development of microstructure and stress upon (thermochemical) surface treatment.

A general solution for 1D consolidation induced by depth- and time-dependent changes in stress

The paper describes a general analytical solution for obtaining the excess pore pressure in a consolidating layer due to depth and time-dependent changes of total stress, given by a simple product of depth- and time-dependent functions. The solution is validated against three previously published solutions, and the paper ends with analysis of a case history involving settlement and consolidation beneath a periodically loaded silo. Good agreement is observed between the analytical solution and measured settlements, and the transient response is also validated against a finite-element solution.

Prediction of J-Integral dependence to residual stress and crack depth on NACA 0012-34 using FE and ANN

Article history: Received October 6, 2014 Accepted 2 February 2015 Available online 2 February 2015 This paper presents an approach of linking finite element method with artificial neural network to predict J-Integral parameter in desirable airfoil condition. Finite Element (FE) and Artificial Neural Network (ANN) have been employed for the purpose. In other words, a prediction of finite element results has been done using ANN. Ultimately results of two methods have been compared for different cases. Wing fracture is a well-known problem of the planes which depends on various parameters. The J-integral is a vital parameter in evaluations of structure fracture phenomena. On the other hand residual stresses play an influential role in fracture formation. In the current work, effect of residual stresses and crack depth on J-Integral has been investigated in a standard NACA0012-34 airfoil. As will be seen, residual stresses and crack depth influence J-Integral values. It also will be shown that predictions of ANN method are in a good agreement with those obtained by finite element method. © 2015 Growing Science Ltd. All rights reserved.

Strain Profiling of a Ferritic‐Martensitic Stainless Steel Sheet — Comparing Synchrotron with Conventional X‐Ray Diffraction

AbstractTo improve the fatigue resistance of stainless steel sheet, it is a common practice to induce compressive residual stress in the surface through shot‐peening or tumbling. Stress depth profiles obtained by tumbling of thin stainless steel tensile rods were analysed using laboratory and synchrotron X‐Ray Diffraction (XRD). Both the non‐destructive synchrotron and the laboratory XRD etch‐depth profile gave similar results: a residual stress profile decaying over a depth not exceeding 50 µm into the material.

Coexistence of colossal stress and texture gradients in sputter deposited nanocrystalline ultra-thin metal films

This paper demonstrates experimentally that ultra-thin, nanocrystalline films can exhibit coexisting colossal stress and texture depth gradients. Their quantitative determination is possible by X-ray diffraction experiments. Whereas a uniform texture by itself is known to generally cause curvature in so-called sin2ψ plots, it is shown that the combined action of texture and stress gradients provides a separate source of curvature in sin2ψ plots (i.e., even in cases where a uniform texture does not induce such curvature). On this basis, the texture and stress depth profiles of a nanocrystalline, ultra-thin (50 nm) tungsten film could be determined.

A critical comparison between XRD and FIB residual stress measurement techniques in thin films

Residual stress has a significant effect on the performance of thin films, in terms of adhesion, hardness, wear and fatigue resistance. Thus, when assessing innovative coatings or new deposition technologies, it is important to perform residual stress evaluation by means of a suitable and reliable technique.X-ray diffraction (XRD) is one of the commonly used techniques, because it is non-destructive, surface sensitive and phase selective. However, it is subject to certain limitations: X-ray diffraction allows stress evaluation (i.e. its indirect deduction from the measured diffraction profile) only in case of crystalline materials, and the results may be subject to aberrations in the presence of texture or stress gradients often occurring in thin films.Recently, a new class of methods for residual stress evaluation has been proposed, based on incremental focused ion beam (fib) milling, combined with high-resolution in situ scanning electron microscopy (SEM) imaging and full field strain analysis by digital image correlation (DIC).The aim of the present paper is to explore in some detail the significance of the stress values obtained for the same coating by X-ray diffraction and focused ion beam milling, and to demonstrate that the analysis of residual stress depth gradients is possible by using FIB-DIC techniques. Finally, a comparative assessment of the weaknesses and strengths of the two techniques will be carried out. For this purpose, a chromium nitride highly textured thin film sample was chosen. The residual stress state evaluation by the two methods was carried out for the coating deposited by the cathodic arc evaporation (CAE-PVD) technique.Although many significant differences were identified between the X-ray diffraction and focused ion beam milling methods, careful consideration of the gauge volumes and weighting demonstrated that satisfactory agreement was obtained. The analysis highlighted the importance of the issues related to (a) probe-to-sample interaction volume (gauge volume), (b) film texture, and (c) the elastic anisotropy. All these factors must be taken into account in order to enable valid comparisons to be drawn.

Effects of depth of cut on the redistribution of residual stress and distortion during the milling of thin-walled part

The distortion of thin-walled part is significantly affected by the residual stress generated after the material is cut away. During this process, the relations between the redistributed residual stress and the distortion are quite complex; as a result the control of distortion of thin-walled part always is being considered tough issues. In this study, an approach of optimizing the profile and magnitude of residual stress is proposed by analyzing the effects of depth of cut on the redistribution of residual stress. Experiments and simulations are conducted to compare the cutting forces, temperature and residual stress. The results show that, in the roughing, by selection of a subsequent depth exceeding prior depth of maximum compressive residual stress, the material which contains the main machined residual stress can be removed in favor of the subsequent machining. Furthermore, in the finishing, different depths of cut are utilized in different cutting stages, resulting in smaller magnitude of maximum machined residual stress and depth of maximum compressive residual stress. In addition, to verify the approach, an aviation thin-walled part is used as the experimental object. The results demonstrate that the magnitude of distortion and residual stress can be decreased and optimized efficiently by controlling and optimizing the depth of cut in the roughing and finishing.

Experimental and Finite Element Simulation Study of Thermal Relaxation of Residual Stresses in Laser Shock Peened IN718 SPF Superalloy

An integrated experimental and modeling/simulation approach was developed to investigate and secure a quantified knowledge of the impact of high temperature exposures on the stability of residual stresses in a laser shock peened (LSP) high temperature aero-engine alloy, IN718 SPF (super-plastically formed). Single dimple LSP and overlap LSP treatments were carried out utilizing a Nd:Glass laser (λ = 1.052 μm), and subsequent heat treatments on the LSP-treated coupons were conducted at different temperatures between 550 and 700 °C. A 3-D nonlinear finite element (FE) computational model and the rate-dependent Johnson-Cook material model were calibrated using the experimental results of residual stress from the single dimple LSP and thermal relaxation treatments, and were further extended to the overlap LSP treatment case. Both experimental and FE simulations show that: a) a high level of compressive residual stress (~700 MPa at surface) and residual stress depth (~0.4–0.6 mm) were achieved following LSP, and b) the overlap LSP treatment gave higher residual stress and greater depth. The magnitudes of the initial residual stress (and plastic strain), heating temperature and exposure time were identified as the key parameters controlling the thermal relaxation behavior. The stress relaxation mainly occurs initially before 20 min exposure and the extent of relaxation increases with an increase in temperature and a higher magnitude of the initial as-peened residual stress. In addition, in regions deeper than ~300 μm or after initial thermal exposure where the residual stress was lower than ~300 MPa, stress relaxation was found to be negligible. Kinetic analysis of the experimental thermal relaxation data based on Zener-Wert-Avrami model gave an activation enthalpy of 2.87 to 3.77 eV, which is near that reported in the literatures for volume and/or substitutional solute diffusion in Nickel. These results suggest that thermal relaxation of the LSP-induced residual stress occurs by a creep-like mechanism involving recovery, rearrangement and annihilation of dislocations by climb.

Thin film stress and microstructure analysis by energy-dispersive diffraction

The most important advantage of energy dispersive (ED) diffraction compared with angle dispersive methods is that the former provides complete diffraction patterns in fixed but arbitrarily selectable scattering directions. Furthermore, in experiments that are carried out in reflection geometry, the different photon energies E(hkl) of the diffraction lines in an ED diffraction pattern can be taken as an additional parameter to analyze depth gradients of structural properties in the materials near surface region. For data evaluation advantageous use can be made of whole pattern methods such as the Rietveld method, which allows for line profile analysis to study size and strain broadening [1] or for the refinement of models that describe the residual stress depth distribution [2]. Concerning polycrystalline thin films, the features of ED diffraction mentioned above can be applied to study residual stresses, texture and the microstructure either in ex-situ experiments or in-situ to monitor, for example, the chemical reaction pathway during film growth [3]. The main objective of this talk is to demonstrate that (contrary to a widespread opinion) high energy synchrotron radiation and thin film analysis may fit together. The corresponding experiments were performed on the materials science beamline EDDI at BESSY II which is one of the very few instruments worldwide that is especially dedicated to ED diffraction. On the basis of selected examples it will be shown that specially tailored experimental setups allow for residual stress depth profiling even in thin films and multilayer coatings as well as for fast in situ studies of film stress and microstructure evolution during film growth.

Impact of Shot-Peening on a Single Crystal Nickel-Based Superalloy

Shot-peening is used to improve the lifetime of mechanical components through the introduction of compressive residual stresses (RS) in a surface layer. In this study, we investigate the impact of such a pre-stressing treatment on a single crystal nickel-based superalloy for high pressure turbine blades of engine aircrafts. In addition to conventional metallographic tools used to characterize the alloy microstructure and the zone affected by shot-peening, X-ray measurements have been performed in order to determine residual stress depth profiles.

Residual Stresses in Multi-Pass Butt-Welded Tubular Joints

In this study, the mechanics of the residual stress development in multi-pass butt-welded joints of ferritic-pearlitic steel pipes are investigated. Therefore, the effects of the welding process and its parameters, as well as the impact of the preheating temperature on the residual stress state are studied by determining the temperature profile during welding and performing X-ray diffraction measurements on the pipes inner and outer surfaces. Additionally, residual stress depth distributions are determined using the incremental hole-drilling method. The residual stress distributions can be explained by phase transformations and thermal shrinkage, the latter being the governing mechanism in the tubular geometry of the welds.

Incremental Hole-Drilling Method <i>Vs.</i> Thin Components: A Simple Correction Approach

The incremental hole-drilling method is a widely used technique to determine residual stress depth profiles in technical components. Its application is limited in respect to the components geometry, for instance the components thickness. In this paper, a direct correction of the measured strain relaxations is proposed to consider the impact of deviant geometries, here the component thickness, on the residual stress evaluation that moreover, allows the application of commercially available evaluation software. The herein proposed approach is based on finite element simulation of the incremental hole drilling. The simulated strain relaxations for thin metal sheets are evaluated with an algorithm as used in commercially available evaluation software (i) for uncorrected data as well as (ii) for strain data corrected by the proposed correction procedure. It is shown that the correction approach leads to a significant improvement of the measurement accuracy. Further, by means of the approach residual stress depth profiles in thin metal sheets can be as usual determined using commercial evaluation software for the incremental hole-drilling method regardless of the algorithm used, i.e. differential or integral.

Effect of Different Manufacturing Process Steps on the Final Residual Stress Depth Profile along the Process Chain of Autofrettaged Thick-Walled-Cylinders

Selectively induced compressive residual stress depth profiles are gaining increasing importance as design tool for internally pressurized components. Hydraulic Autofrettage (AF) is a well-known manufacturing process to induce pronounced compressive residual stresses. However, AF does not stand alone in the technical process chain. In this paper, results from neutron diffraction experiments performed on thick-walled cylinders are presented and compared to finite-element simulations with Abaqus/CAE. The impact on the final residual stress depth profile after pre-machining, Autofrettage and post-machining is discussed.

Residual Stress Measurement of Shot-Peened Steel Rings by Barkhausen Noise, ESPI Hole-Drilling and X-Ray Diffraction

Shot-Peening is used to generate surface-near compressive residual stresses in final shape parts, usually to improve their fatigue properties. The success of the process can be checked in different ways. XRD and Hole-Drilling determine quantitative stress depth profiles in specific locations. Barkhausen noise measurements evaluate a near-surface zone in real-time and can cover large areas quickly. It is a fully nondestructive method. This study compares ESPI Hole-Drilling, which is a very fast technique, and XRD, which is very precise but slower, to Barkhausen Noise results for the case of three steel rings shot-peened with different intensities.

Combining XRD with Hole-Drilling Method in Residual Stress Gradient Analysis of Laser Hardened C45 Steel

LLocalized laser hardening of steel surfaces performed with optimal parameters not only hardens the surface layer, but is also capable of generating advantageous hook-like shape of residual stress depth distribution. Upon combining two most common methods of XRD and hole-drilling, the residual stresses up to the depth of 3 mm were analysed for two widths of high-powered diode laser beam. Narrower laser beam generates wider area with compressive stresses and the diffraction technique as compared with hole-drilling method always indicates higher values of compressive residual stress maxima.

Residual stress redistribution in shot peened samples subject to mechanical loading

Shot peening is a well-established surface treatment process that imparts large compressive residual stresses onto the surface and at shallow depths to retard initiation and growth of fatigue cracks. The plastic deformation developed during the surface treatment sets up a constraint that retains compressive stresses on the surface balanced by tensile residual stresses in the interior. However, component service histories that produce subsequent plastic deformation may redistribute these residual stresses. In most engineering components, this additional plastic deformation is localized to stress concentration sites such as holes, notches, and fillets. In the case of gross plastic deformation where the entire cross section experiences material yielding the residual stress profile may redistribute, resulting in tensile stresses on the outside surface balanced by compression in the interior. This paper describes a series of experiments combined with models to explain the redistribution in residual stress depth profiles subject to applied stresses producing gross plastic strains in shot peened laboratory specimens. The initial room temperature residual stress and plastic strain profiles provide initial conditions for predictions. Model predictions correlate well with experimental results on shot peened dogbone specimens subject to single cycle and fatigue loading conditions at elevated temperature. Experiments on shot peened notched specimens do not exhibit the same stress redistribution even for larger applied stresses.

Equi-penetration grazing incidence X-ray diffraction method: Stress depth profiling of ground silicon nitride

In this work a new modification of the grazing incidence X-ray diffraction method for residual stress determination is presented. This equi-penetration grazing incidence X-ray diffraction method is especially suitable for the precise determination of the residual stress depth profile in materials. It originates from a sin2ψ approach based on the determination of the lattice spacing of various selected diffraction planes. Additionally, for each measurement the condition ∂τ/∂ψ=0 is here fulfilled and one dataset corresponds strictly to a specific mean penetration depth τ independent of the tilt angle ψ. This can be achieved by the individual adjustment of the incidence angle for each measured diffraction maximum. In the actual work, the influence of different surface finishes on the stress depth profile of commercial silicon nitride ceramic samples is investigated. The ground specimen displays an almost biaxial compressive stress parallel and near to the specimen surface of 3GPa, which decreases almost up to zero in a depth of 5μm. After the polishing process, the specimens show a strongly reduced compressive stress maximum at the surface of 1GPa, which diminishes similarly with increasing depth. The orientation between the stress component and the grinding direction is of minor influence. Furthermore, the influence of a possible residual stress component perpendicular to the sample surface is discussed.