Near-surface Residual Stress Research Articles

Improving the surface durability of wind gears via shot peening and superfinishing

Gearbox power density plays a key role in wind energy competitiveness. As wind turbines have become increasingly larger, gear surface durability and flank fracture load carrying capacity have become key elements in gearbox sizing. While shot peening and superfinishing have been studied independently as potential techniques to improve surface durability, few reported studies have combined both of these processes to increase the gear flank load-carrying capacity. In this work, spot pitting tests were carried out to evaluate the expected load-carrying capacity increase for wind gears through this combination of finishing processes. The results are consistent with previous research's conclusions and highlight the need for further investigations to evaluate potential increased flank fracture risk related to the near-surface residual stress distribution.

Validation of Hole-Drilling Residual Stress Measurements in Workpieces of Various Thickness

BackgroundA recent revision to the ASTM E837 standard for near-surface residual stress measurement by the hole-drilling method describes a new thickness-dependent stress calculation procedure applicable to “thin” and “intermediate” workpieces for which strain versus depth response depends on workpiece thickness. This new calculation procedure differs from that of the prior standard, which applies only to thick workpieces with strain versus depth response independent of thickness.ObjectiveHerein we assess the new calculation procedures by performing hole-drilling residual stress measurements in samples with a range of thickness.MethodsNear-surface residual stress is measured in a thick aluminum plate containing near-surface residual stress from a uniform shot peening treatment, and in samples of different thickness removed from the plate at the peened surface. A finite element (FE) model is used to assess consistency between measured residual stress across the range of sample thickness.ResultsMeasured residual stress varies with sample thickness, with thinner samples exhibiting smaller near-surface compressive stress and a larger gradient of subsurface stress. These trends are consistent with both observed bending (curvature) of the removed samples and the trend in FE-calculated expected residual stress. The measured and expected residual stresses are in good agreement for samples of intermediate thickness, but the agreement decreases with sample thickness. Measured residual stress is invariant with gage circle diameter.ConclusionThe new thickness-dependent stress calculation procedure for hole-drilling provides meaningful improvement compared to thick-workpiece calculations.

Residual Stress Engineering for Wire Drawing of Austenitic Stainless Steel X5CrNi18-10 by Variation in Die Geometries-Effect of Drawing Speed and Process Temperature.

As a result of conventional wire-forming processes, the residual stress distribution in wires is frequently unfavorable for subsequent forming processes such as bending operations. High tensile residual stresses typically occur in the near-surface region of the wires and can limit further application and processability of the semi-finished products. This paper presents an approach for tailoring the residual stress distribution by modifying the forming process, especially with regard to the die geometry and the influence of the drawing velocity as well as the wire temperature. The aim is to mitigate the near-surface tensile residual stresses induced by the drawing process. Preliminary studies have shown that modifications in the forming zone of the dies have a significant impact on the plastic strain and deformation direction, and the approach can be applied to effectively reduce the process-induced near-surface residual stress distributions without affecting the diameter of the product geometry. In this first approach, the process variant using three different drawing die geometries was established for the metastable austenitic stainless steel X5CrNi18-10 (1.4301) using slow (20 mm/s) and fast (2000 mm/s) drawing velocities. The residual stress depth distributions were determined by means of incremental hole drilling. Complementary X-ray stress analysis was carried out to analyze the phase-specific residual stresses since strain-induced martensitic transformations occurred close to the surface as a consequence of the shear deformation and the frictional loading. This paper describes the setup of the drawing tools as well as the results of the experimental tests.

Neutronographic Residual Stress Analysis for Materials With Depth Gradients of the Strain Free Lattice Parameter $${{\varvec{d}}}_{0}$$ for the Example of a Case-Hardened Steel 20MnCr5

AbstractIn the present work, ring-shaped samples made from steel 20MnCr5 were low-pressure carburized (LPC) and subsequently hardened by gas quenching (case-hardened). This results in a near-surface gradient in chemical composition, microstructure- and hardness distribution, as well as a three-dimensional residual stress (RS) distribution, which was investigated by neutron diffraction. Near-surface RSs in the ferrite-/martensite- and austenite phase are additionally determined by X-ray diffraction. It is shown that the chemical gradient has an influence on the chosen $${d}_{0}$$ d 0 strategy and how such a reference sample should be extracted. If near-surface RS values are to be determined by neutron diffraction, the pseudo-strain effect must be taken into account. For this purpose, a suitable approach using the “open source” software SIMRES and STRESSFIT is also presented. By combining neutron and X-ray diffraction data, a complete RS distribution over the whole sample can be obtained.

Effect of surface topography and residual stress on the taper connection stability in total hip arthroplasty

In the present work, the influence of the trunnion surface topography and the near-surface residual stresses on the joining process of a taper connection is examined using a replicate of the realistic taper connection as it occurs in conventional hip joint implants. The focus of the work is on the surface of the taper trunnion made of Ti6Al4V ELI and its effect on the connection stability with a CoCrMo counterpart. In this regard, the interrelation between surface topography, residual stresses, the joining behavior and the corrosion behavior under dynamic loading have been systematically investigated. For this purpose, taper trunnions produced by means of three different machining processes were considered, i.e. fine machining, rough machining and a novel furrowing process. These mechanical surface treatments result in different surface topographies and near-surface work hardening and residual stress states. The results show that the primary taper stability is hardly altered by the different types of trunnion surfaces. For all three surface states, the joining/dismantling procedure did not change the residual stress state at the surface. After corrosion testing under dynamic loading, the fine machined taper surface exhibits the highest stability. Moreover, fine machined tapers consolidated during the dynamic corrosion experiment as the ratio between joining and dismantling force increased from 0.49 ± 0.04 to 0.83 ± 0.08. For the furrowed and rough machined taper surfaces, the connection stability showed a tendency towards increase and decrease, respectively, in the course of dynamic corrosion testing. The results indicate that for choosing an optimal taper trunnion surface, the effects of corrosion must be taken into account.

Correlation between pre- and post-treatments of additively manufactured 316L parts and the resulting low cycle fatigue behavior

PurposeThe currently existing restrictions regarding the deployment of additively manufactured components because of poor surface roughness, porosity and residual stresses as well as their influence on the low-cycle fatigue (LCF) strength are addressed in this paper.Design/methodology/approachThis study aims to evaluating the effect of different pre- and post-treatments on the LCF strength of additively manufactured 316L parts. Therefore, 316L specimens manufactured by laser powder bed fusion were examined in their as-built state as well as after grinding, or coating with regard to the surface roughness, residual stresses and LCF strength. To differentiate between topographical effects and residual stress-related phenomena, stress-relieved 316L specimens served as a reference throughout the investigations. To enable an alumina coating of the 316L components, atmospheric plasma spraying was used, and the near-surface residual stresses and the surface roughness are measured and investigated.FindingsThe results have shown that the applied pre- and post-treatments such as stress-relief heat treatment, grinding and alumina coating have each led to an increase in LCF strength of the 316L specimens. In contrast, the non-heat-treated specimens predominantly exhibited coating delamination.Originality/valueTo the best of the authors’ knowledge, this is the first study of the correlation between the LCF behavior of additively manufactured uncoated 316L specimens in comparison with additively manufactured 316L specimens with an alumina coating.

Comparison Between Surface and Near-Surface Residual Stress Measurement Techniques Using a Standard Four-Point-Bend Specimen

BackgroundDetermination of near-surface residual stresses is challenging for the available measurement techniques due to their limitations. These are often either beyond reach or associated with significant uncertainties.ObjectiveThis study describes a critical comparison between three methods of surface and near-surface residual stress measurements, including x-ray diffraction (XRD) and two incremental central hole-drilling techniques one based on strain-gauge rosette and the other based on electronic speckle pattern interferometry (ESPI).MethodsThese measurements were performed on standard four-point-bend beams of steel loaded to known nominal stresses, according to the ASTM standard. These were to evaluate the sensitivity of different techniques to the variation in the nominal stress, and their associated uncertainties.ResultsThe XRD data showed very good correlations with the surface nominal stress, and with superb repeatability and small uncertainties. The results of the ESPI based hole-drilling technique were also in a good agreement with the XRD data and the expected nominal stress. However, those obtained by the strain gauge rosette based hole-drilling technique were not matching well with the data obtained by the other techniques nor with the nominal stress. This was found to be due to the generation of extensive compressive residual stress during surface preparation for strain gauge installation.ConclusionThe ESPI method is proven to be the most suitable hole-drilling technique for measuring near-surface residual stresses within distances close to the surface that are beyond the penetration depth of x-ray and below the resolution of the strain gauge rosette based hole-drilling method.

Intermethod Comparison and Evaluation of Measured Near Surface Residual Stress in Milled Aluminum

BackgroundWhile near surface residual stress (NSRS) from milling is a driver for distortion in aluminum parts there are few studies that directly compare available techniques for NSRS measurement.ObjectiveWe report application and assessment of four different techniques for evaluating residual stress versus depth in milled aluminum parts.MethodsThe four techniques are: hole-drilling, slotting, cos(α) x-ray diffraction (XRD), and sin2(ψ) XRD, all including incremental material removal to produce a stress versus depth profile. The milled aluminum parts are cut from stress-relieved plate, AA7050-T7451, with a range of table and tool speeds used to mill a large flat surface in several samples. NSRS measurements are made at specified locations on each sample.ResultsResulting data show that NSRS from three techniques are in general agreement: hole-drilling, slotting, and sin2(ψ) XRD. At shallow depths (< 0.03 mm), sin2(ψ) XRD data have the best repeatability (< 15 MPa), but at larger depths (> 0.04 mm) hole-drilling and slotting have the best repeatability (< 10 MPa). NSRS data from cos(α) XRD differ from data provided by other techniques and the data are less repeatable. NSRS data for different milling parameters show that the depth of NSRS increases with feed per tooth and is unaffected by cutting speed.ConclusionHole-drilling, slotting, and sin2(ψ) XRD provided comparable results when assessing milling-induced near surface residual stress in aluminum. Combining a simple distortion test, comprising removal of a 1 mm thick wafer at the milled surface, with a companion stress analysis showed that NSRS data from hole-drilling are most consistent with milling-induced distortion.

Nonlinear surface wave measurements on aged and laser shock surface-treated Inconel 718 superalloy

Because of their ability to probe the material properties at different penetration depths, surface acoustic waves (SAW) are considered for nondestructively characterizing the near-surface properties of a component that has been previously modified. During long-term service, microstructural changes are unavoidable, which in many cases promote the formation of deleterious phases in a material that operates at elevated temperatures impacting the mechanical response of the component. This research investigates the behavior of the nonlinearity parameter (βR) measured using the second harmonic generation of Rayleigh surface waves propagating in the free surface of aged Inconel 718 (IN 718) superalloy specimens with and without laser shock processing (LSP). Also, microstructural and mechanical characterizations were conducted to track first the sole effect of aging time, and secondly, the combined effects of aging + LSP on microhardness, residual stress, and tensile properties of the samples. Our experimental results obtained from aged samples with and without LSP processing indicate that a superposition of different effects of aging and surface treatment in the material contribute to the βR parameter, including microstructure, compressive residual stress, and cold work effects. The experimental results also indicate that it is feasible to observe a perceivable change in the βR parameter of the SAW when the specimen is heat-treated at different temperatures, which has no perceivable effect on the surface roughness. The changes on the βR parameter in aged samples are due to the gradual precipitation of γ′ and γ" hardening phases and the further precipitation of δ-phase, and in aged samples with LSP, electron backscatter diffraction (EBSD) analysis showed that changes are due to microstructural, near-surface residual stresses and texture effects. These results indicate a distinctive trend that shows that the measured βR can be useful in tracking microstructural changes in this material during high-temperature operations.

Predictive Modeling of Laser Shock Peening Induced Near-Surface Residual Stress in Alumina

Laser shock peening (LSP) is a non-contact surface treatment method that has been experimentally found to help increase fracture toughness, induce near-surface compressive residual stress and increase hardness in ceramic materials. Numerous experiments, with associated costs and challenges, are needed to identify the application-specific LSP parameters. Physics-based computational models provide a less expensive and more flexible alternative to performing the requisite experiments, yet the trade-off between computational cost and accuracy of different models needs to be considered. In this work, LSP treatment finite element simulations are executed using a calibrated Drucker-Prager (DP) plasticity model combined with a Mie-Grüneisen equation of state as well as Johnson-Cook rate dependence and a damage initiation criterion. Unlike existing Johnson-Holmquist (JH) methods, the calibrated DP constitutive model does not require damage model parameters to be continually updated with deformation, resulting in lower computational time. Near-surface compressive residual stress, predicted by the model, is compared to measurements reported in recent experimental studies. At the lowest pulse energy tested (1 J) an approximate 5% difference exists between the simulated RS and that measured by Raman spectroscopy in the experiments. This difference appears to increase with greater pulse energies. A parametric comparison of computational time for differences in laser pulse energy, constitutive models, and materials is also performed. Results reveal that the demonstrated model at the lowest pulse energy on alumina is up to 37.5% faster, and when applied to silicon carbide, is up to 15.7% faster in comparison to existing JH methods.

Inhomogeneous Strains and Dislocation Evolution in Surface-Treated Ni-Base Superalloys

Surface treatment processes such as shot-peening create near-surface residual stresses in the component, which retard the initiation and growth of fatigue cracks. These processes, however, also induce considerable amount of strain hardening and alter the sub-surface dislocation structure of the material, which can impact its mechanical and electrochemical properties. Although these effects are critical for part performance, they have not so far been studied for surface-treated Ni-base superalloys, which are widely used for load-bearing applications. To address this, we have developed an X-ray diffraction (XRD) framework based on Convolutional Multiple Whole Profile (CMWP) fitting approach that can be used to quantify dislocation densities in shot-peened Inconel 718 components. The method exploits physics-based mechanisms for size and strain broadening to model these effects and uses dislocation contrast factors to account for deformation-induced crystallographic anisotropy in the diffraction profile. Our results suggest that shot-peening leads to one order of magnitude increase in the dislocation density of Inconel 718 alloys, which is supported by direct evidence from transmission electron micrographs. The approach developed in this work can be extended to other types of Ni-base superalloys, enabling a simple non-destructive method to calculate dislocation densities in this commercially important class of materials.

Circumferential Rosette Design for Extended Depth Hole-Drilling Residual Stress Measurements

The radial design for strain gauge rosettes used for hole-drilling residual stress measurements was established over 50 years ago and has remained almost unchanged since then. The design was developed to promote strain sensitivity, but was made without regard for ability to identify interior stresses. Consequently, only near-surface residual stresses can be identified. The aim of this study is to investigate rosette geometric features that may be refined to enable a significant increase in ability to identify interior stresses without significant loss of strain sensitivity. A finite element analysis was undertaken to model the influence of various geometrical factors on the depth response of a strain gauge rosette. It was found that that circumferentially oriented strain gauges allow residual stress measurements to be made to twice the depth of traditional radial strain gauges, with only minor loss in strain sensitivity. A new circumferential strain gauge rosette design based on the presented analysis is proposed for use where extended depth measurements are desired.

Rapid in-line residual stress analysis from a portable two-dimensional X-ray diffractometer

Application of ‘cosα’ method using portable 2DXRD instruments has attracted considerable attention for rapid, in-line residual stress measurements. However, the validity of this technique has often been questioned, particularly under field conditions, because the mechanisms of how critical measurement parameters such as sample-to-detector distance and X-ray incidence angle affect the accuracy of measured residual stress is not adequately understood. To address this issue, near-surface residual stress in a shot-peened AISI 4140 alloy coupon was determined using ‘cosα’ method for a wide range of sample-to-detector distances and incidence angles – this data was then compared with the data from conventional ‘sin2ψ’ technique. Results showed that there exists a window of incidence angles for which the values of residual stress from 2DXRD are reliable. However, outside of this window, data loses accuracy due to beam defocusing and errors in determining its orientation with respect to diffracting crystallites. Unlike incidence angle, the impact of sample-to-detector distance on the accuracy of residual stress was found to be less significant, as it only changed the size of the Debye ring and did not affect its distortion. Results also showed that increase in beam size can reduce the exposure time required for data collection, but it comes at the cost of sacrificing the intrinsic resolution and signal-to-noise ratio (SNR) of the diffraction peak. Insights from the experimental data presented in this work can establish proper measurement protocols to enable accurate and rapid residual stress analysis using portable 2DXRD instruments and improve the current practice of quality control in surface treatment industry.

Strain analysis in narrow boreholes by means of microscopic digital image correlation

Residual stresses play a decisive role in stress analyses, as they can exert positive or negative influence on the durability of parts. A widely-used method to measure residual stresses experimentally is the hole drilling method. With this method, however, only near-surface residual stresses can be determined. This disadvantage is to be eliminated by the newly developed method of staggered machining. The underlying procedure is described in this paper. In addition, a manufacturing process for production fine speckle patterns, which are required for the Digital Image Correlation (DIC) is explained. The principle of staggered machining is shown by means of preliminary tests and their evaluation.

Non-destructive detection of machining-induced white layers in ferromagnetic alloys

Abstract Machining-induced white layers are an undesirable surface integrity feature which, due to their physical properties, can have a direct effect on the in-service performance of aero-engine components. Typically, destructive methods such as cross-sectional microscopy are used during inspection to identify white layers. This is costly, both in terms of parts sacrificed and time-consumed. A non-destructive evaluation method could speed-up inspection and allow all parts to be inspected before entering service as well as throughout the component life cycle. The present work covers the quantitative characterization of machining-induced white layers in super chrome molybdenum vanadium steel through destructive methods in addition to Barkhausen noise non-destructive testing of the same surfaces. White layers formed by machining with severely worn inserts were measured to be up to 50% harder than the bulk material, possess nano-scale grains and can have an associated compressive residual stress state of up to -1800 MPa. Barkhausen noise testing was used to show that surfaces with a white layer formed by SPD could be detected by measuring shifts in the peak frequency of the Barkhausen noise signal, caused by the compressive near-surface residual stress state associated with the formation of white layers of this type.

Modified Mechanical Surface Treatment for Optimized Fatigue Performance of Martensitic Stainless Steel AISI 420

Mechanical surface treatments, deep rolling or shot peening can enhance the fatigue performance of metallic materials because near-surface compressive residual stresses and work hardening states can inhibit or retard crack initiation and propagation. Thus, the stability of compressive residual stresses and work hardening states during fatigue is very important. However, generated compressive residual stresses and work hardening layers can be relaxed because of dislocation movements, rearrangements and annihilations. The strain ageing concept is one of the ways to pin or obstruct dislocations by solute atoms (called the Cottrell cloud) as a consequence of high stability of near-surface residual stresses and work hardening states. In this research, the conventional deep rolling and its modifications, high temperature deep rolling and deep rolling followed by appropriate annealing were performed on martensitic stainless steel AISI 420. Fatigue tests were performed using a rotary bending fatigue machine to evaluate fatigue performance as well as the strain ageing effects. It was found that the conventional deep rolling could enhance the fatigue performance through generated compressive residual stresses and work hardening layers at the surface, and near-surface regions. The modified deep rolled (high temperature deep rolling and deep rolling followed by annealing) conditions exhibited a greater fatigue lifetime and performance than those of the conventional deep rolling, although some residual stress and work hardening relaxations were observed after the modified deep rolling processes.

The influence of coverage for high frequency mechanical impact treatment of different steel grades

Abstract The High Frequency Mechanical Impact (HFMI) process is a comparably new, manually-driven machine hammer peening (MHP) process applied to increase the fatigue performance of metallic components. The HFMI-process is characterized by a large number of impacts, also when the tool was used with a high feed rate. This leads to a fast saturation and highly plastic deformation at the surface layer. Experimental investigations of the surface topography, near-surface residual stresses, and work hardening were performed on HFMI-treated, flat specimens made of S355J2, S690QL and S960QL steel grade to investigate the influence of the process parameters on the surface conditions. The residual stress state, experimentally determined by neutron diffraction, was supplemented by the numerical simulation of the HFMI-process. For this, an elasto-viscoplastic material model was implemented and calibrated with experimental test data. As a unified parameter for the process quality, a representative coverage value, defined as the number of impacts per reference surface area, was calculated for different sets of process parameters. This coverage value was also used to compare the numerical simulation and the experimental investigations by neutron diffraction techniques of the HFMI-induced residual stress field. It is shown, that high coverage has a strong effect on the depth of indentation and in the work hardening effect for mild and medium steel grade. Only slight changes of the process-induced residual stresses could be observed with an increasing coverage.

Finite element analysis of laser shock peening induced near-surface deformation in engineering metals

This paper investigates near-surface residual stress and equivalent plastic strain distributions using finite element method for further understanding of laser shock peening induced plastic deformation. The results indicate that residual stress and equivalent plastic strain at the topmost surface are sensitive to mesh size while they can be well predicted in the subsurface layer. Increasing laser spot size or overlapping rate deepens the compressive residual stress layer while reducing laser spot size leads to uniformly distributed residual stress and equivalent plastic strain. A minimum overlapping rate of 62.5% is necessary to create fully covered compressive residual stress on the target surface using round laser spot. Higher strain hardening exponent results in higher maximum compressive stress and equivalent plastic strain, deeper compressive residual stress and plastic deformation. In addition, higher elastic modulus results in lower equivalent plastic strain and shallower plastic deformation depth.

Impact of supercritical carbon dioxide cooling with Minimum Quantity Lubrication on tool wear and surface integrity in the milling of AISI 304L stainless steel

In this study, the effect of supercritical carbon dioxide cooling with Minimum Quantity Lubrication (scCO2+MQL) on tool wear and surface integrity of AISI 304 L austenitic stainless steel in milling was investigated. A series of machining experiments based on a Design of Experiments (DoE) was carried out at various combinations of cutting parameters to investigate the effect of cutting speed and feed rate on tool wear, near-surface residual stresses, surface roughness and microhardness. The results were compared with the experimental results obtained from milling with flood coolant. A significant improvement in tool life was observed in milling with the scCO2+MQL using multilayer coated tungsten carbide inserts. The tool life in terms of cutting time increased by ∼324%, in comparison to a baseline flood coolant. Further, a decrease in surface roughness value (Ra) by about 30%, from 1.09 µm for flood coolant to 0.78 µm after face milling with scCO2+MQL was seen. Additionally, the Ra value slightly increased after machining, for both cooling methods with the increase of cutting speed of ∼19%. The observed changes in Ra value were discussed in terms of a built-up-edge (BUE) formation. There were no apparent differences in surface microhardness between both cooling methods. However, the surface microhardness increased with feed rate after milling with both scCO2+MQL and flood coolant due to the increased strain hardening. Also, there was no significant difference in residual stresses after milling, neither with scCO2+MQL nor the flood coolant. The surface residual stress values obtained in the transverse and longitudinal directions were consistent with a predictive model with errors of around 3–8%.

Investigations on residual stress generation in full-forward-extrusion

Within cold-forming processes, the influence of forming induced residual stresses is a critical issue regarding the life-time behaviour of the formed components. However, a targeted use of forming-induced residual stresses is also capable to improve the components properties. The rotation-symmetrical full-forward extrusion is used as a reference process to investigate the generation of residual stresses within manufacturing. A novel and generally applicable computational evaluation procedure is introduced, which achieves to compute and visualize the evolution of the residual stress state during the entire forming operation. Simulative results are compared with their experimental counterparts. The near-surface residual stresses in the formed component are evaluated in axial and tangential direction by X-ray diffraction. Indentation tests are used to determine the micro-hardness inside the components and optical instruments quantify geometrical differences.