Surface Residual Stress Measurement Research Articles

Surface treatment impact on fatigue life at 550 °C of the as-built Inconel 718 manufactured by laser-powder bed fusion

This article compares the impact of surface modification on fatigue life at 550 °C of the Inconel 718 manufactured by laser-powder bed fusion. The surface conditions are as-built, as-machined, shot-peened, oxidized, and combinaisons of those. Surface roughness, microhardness, and residual stress measurements, as well as fractographic analyses are performed to interpret the fatigue results. Fatigue tests show that as-built samples have higher fatigue lifetime than as-machined samples. Moreover, shot peening increases fatigue lifetime. Finally, an oxidation treatment at 550 °C during 41.6 days does not affect fatigue life.

Experimental and numerical study of TGO-induced stresses of plasma-sprayed thermal barrier coating

Experimental and numerical studies were conducted in order to investigate the stress changes of ZrO2-8 wt% Y2O3 (YSZ) thermal barrier coatings (TBC) under isothermal loading. After conducting the isothermal oxidation test at a temperature of 1070 °C, the Thermally Grown Oxide (TGO) growth process and the life of samples were evaluated using scanning electron microscopy (SEM). The X-ray diffraction (XRD) approach was selected for the measurement of surface residual stress and its change in terms of TGO growth. Furthermore, a finite element model based on a real TGO profile, extracted by SEM, was executed, where TGO-induced stress was measured at the TGO/Top Coat (TC) interface direction. Employing the finite element model, following the selection of the appropriate creep strength based on experimental results, the stress evolution and distribution were assessed during time exposure at maximum temperature. It was shown that the lifetime of samples was about 240 h under isothermal loading of 1070℃. By increasing the time exposure, stresses increased and resulted in the formation of a delamination line above peak regions.

Implementation of Gaussian Process Regression to strain data in residual stress measurements by hole drilling

Incremental hole drilling is a prominent tool in the surface and sub-surface residual stress measurements. Generally, strains are measured at drilling steps not corresponding to the stress calculation steps of the integral method. Consequently, strain measurements are fitted using polynomials or splines. However, this process is greatly influenced by user’s experience, affecting the calculated stresses as an additional source of uncertainty. The present work assesses the uncertainty associated with the strain fitting procedure by exploiting Gaussian Process Regression (GPR), a probabilistic machine learning framework which can yield uncertainties on the fit. These uncertainties were propagated to residual stresses by using Monte Carlo simulation. Laser shock peened AA 7050-T7451 specimens were considered as case study.The proposed methodology was corroborated by comparing residual stress results with those obtained exploiting traditional fitting procedures and X-ray diffraction measurements. Finally, the GPR-based approach demonstrated its ability to successfully discriminate different strain signal sources.

Fatigue life improvement of similar and dissimilar aluminum friction stir welds by deep rolling

At the moment, high production costs prevent the friction stir welding (FSW) process from further industrial applications even if comparable high fatigue strength of the joints can be reached. Higher welding speed may reduce the production costs but decrease the fatigue strength of the manufactured FSW joints. A potential solution is a mechanical post weld treatment directly after welding to increase the fatigue strength of the FSW joint again. In this study, hydrostatic deep rolling was applied for fatigue strength improvement of similar and dissimilar FSW joints made of EN AW 5083 and EN AW 6082 alloys manufactured with different welding speed. Additionally, the fatigue strength was directly compared to conventional joints manufactured by metal inert gas (MIG) welding and to base material specimen made of EN AW 5083. The surface state in as-welded condition was characterized by surface roughness and residual stress measurements. Fatigue tests were performed to quantify the fatigue strength of the FSW joints. Similar compressive residual stresses were determined after deep rolling for similar and dissimilar joints. No fatigue life improvement was determined for deep-rolled similar joints made of EN AW 5083. However, in this condition, the fatigue life of the specimen was within the range of the base material. Thus, significant lower fatigue life and a high fatigue life improvement by deep rolling were reached for dissimilar joints. An increase of the welding speed from 300 to 800 mm/min strongly decreased the fatigue strength of dissimilar welded joints in the investigated case.

Thermo-mechanical-metallurgical modelling, validation and characterization of 42CrMo4 steel processed by directed energy deposition

In this study, a sequentially coupled thermo-mechanical-metallurgical (TMM) finite element (FE) model for the directed energy deposition (DED) process is developed in ABAQUS/CAE. Due to the large majority of previously published FE models centered around Ti-6Al-4V and Inconel® alloys, this study applies a similar modelling approach for 42CrMo4 steel - a medium carbon alloy chemically equivalent to AISI 4140 steel. In addition to predicting the temperature history, residual stress, substrate distortion and metallurgical phase fractions, this model also incorporates the solid-state transformation strains derived from the thermal history directly into the mechanical analysis. This contribution to strain is known to have a significant effect on the resulting residual stresses for alloys with allotropic phase transformations. Moreover, a pre-processing framework is implemented and validated to enable modelling of complex geometries and laser trajectories. Four experimental case studies were conducted to validate the predictions of the presented model. The case studies consist of three separate thinwall geometries fabricated with varying dwell times and a cube with a more complex laser trajectory. The predicted temperature history, residual stresses, and substrate distortion are compared with in-situ temperature measurements and post-build surface residual stress and distortion profile measurements. The microstructural constituents of each case are also examined by optical and electron microscopy and compared with the simulated phase fractions. While the model predictions generally agree with experimental observations, distortion predictions are more accurate in higher dwell-time prints while phase fraction predictions show the opposite trend. Furthermore, the overestimation of martensite in the case with the longest dwell time may have caused the over prediction of tensile residual stress on the surface.

Attainment of favorable microstructure for residual stress reduction through high-temperature heat treatment on additive manufactured inconel 718 alloy

Mitigation of residual stress in additive manufactured metal parts through post-heat treatment helps enhancement of life during service. The present work aims to investigate the effect of stress-relieving heat treatment (SRHT) at 1065 °C on microstructural changes, residual stress patterns, and mechanical properties of Inconel 718 (IN718) blocks fabricated through use of the laser-based powder bed fusion (L-PBF) process. The X-ray diffraction technique known as cos α method was used to capture the Debye ring for the measurement of surface residual stress. Dissolution of the columnar dendrites along with laves phases in the interdendritic region was seen after SRHT. The resultant microstructure showed partially recrystallized grains along with the formation of annealing twins and strengthening phases such as γ’, γ’’, and tiny metal carbides. The residual stress on the top surface of as-built sample was found to be 77% higher on the corners compared to the center region. The distribution of residual stress was seen as not uniform in the as-built condition. The SRHT sample showed uniform distribution of compressive residual stresses in all the locations. This phenomenon was due to the diffusion of atoms present in the high-stress regions migrated to low-stress regions until attaining their equilibrium. The combination of face centered cubic (FCC) and hexagonal lattices in as-built microstructure led to misfit strain between the grains. After SRHT, microstructure of IN718 experienced relaxation of residual stresses due to the transformation of hexagonal structured laves phase into the body-centered tetragonal (BCT) γ’’and FCC-structured γ’ phases. Furthermore, SRHT significantly alters the mechanical properties of IN718 alloy causing a 23.03% increase in average hardness and a 21.04% increase in tensile strength.

Longitudinal critically refracted (LCR) ultrasonic wave for residual stress measurement

The article deals with the state-of-the-art in the field of Longitudinal critically refracted (LCR) ultrasonic wave, for non-destructive material evaluation. It checks its capability for residual stress identification, and reviews positives and negatives related to its use. Obtained information within the article, are used for the understanding of essence of method and for the evaluation of its use in the engineering practice. The article can be the source of information about the LCR wave measurement technology, which is the part of the complex ultrasonic testing method. For the frequency of using this technology for surface residual stress measurement, it is appropriate to have this information in one whole, which are gathered of the outputs of researches by various authors. The paper is divided in few sections and sub-sections. In the first section, information about LCR wave technique and factors correlated with this method, are provided. The next section writes about residual stresses and the importance of their identification. Next, the principal of residual stresses measurement and basic structure of measurement device, is described. A significant part of study, describes the state so far of theoretical and practical researches within the use of this method, in the technological practice of residual stress identification in surface layers of engineering components. In the conclusion, obtained knowledges are summarised and evaluated. Related positive and negative aspects are included, with a verifying the need of future researches.

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.

Effect of shot peening on notched fatigue performance of powder metallurgy Udimet 720Li superalloy

The tensile and stress-controlled LCF tests of a powder metallurgy (PM) superalloy, Udimet 720Li have been conducted at 650 °C in order to investigate the effect of shot peening (SP) on notched low cycle fatigue (LCF) performance. The measurements of surface tomography, Micro-Vickers hardness, residual stress and failure mechanism analysis were conducted in detail. The results show that there are many regularly shaped machining traces on as-received (AS) surface while lots of highly disorganized dimples are mainly distributed on SP surface. A larger microhardness gradient is produced along the path from SP surface to the bulk of material compared with AS specimen. The maximum compressive residual stress of SP specimen is not more located at surface like AS one but at a depth of approximately 50–90 μm from surface. Compared with the applied stress, the effective stress curve of SP specimen becomes “flat” and the effective stress gradient decreases accordingly. The LCF lifetime of the notched specimen is significantly improved after SP process and the LCF lifetime increment is between 2.05 and 7.66 times. Finally, there are obvious differences on the failure mechanisms, such as fatigue crack initiation, propagation and final fracture between AS and SP specimens.

Effect of Surface Layer on the Fatigue Strength of Selectively Laser Melted 17-4 PH Steel

The effect of surface layer properties on the fatigue strength of selectively laser melted 17-4 PH stainless steel flat specimens is presented within this work. X-ray surface residual stress measurements in loading direction show that the residual stresses are negligible if a final heat-treatment is applied, whereas values of only about 3% of the nominal yield strength of the base material are evaluated. The presented additively manufactured surfaces highlight irregular surface topographies, at which an area-based assessment of the surface roughness parameters should be applied instead of a line-based evaluation using defined profiles which may lead to an underestimation. Fatigue tests under cantilever bending at a load stress ratio of R = −1 demonstrate a fundamental impact of the surface layer on the fatigue strength especially within the high-cycle fatigue region. A comparison to the test results of machined specimens reveals a significant increase in the fatigue strength by 29% at ten million load-cycles due to the machining process. Finally, a fracture surface analysis highlights that not only the surface topography itself, but also the surface layer is of great importance in terms of fatigue. Thereby, surface-near imperfections or lack of fusion of the contour layer to the bulk material can act as origin for fatigue crack initiation, which illustrates the necessity to consider the surface layer properties in the fatigue design of additively manufactured 17-4 PH steel structures.

Effect of cutting process on the stress corrosion susceptibility of AISI 304L stainless steel

AbstractDuring manufacturing of a component, cutting, turning, grinding, and milling operations are inevitable and these operations induce surface residual stresses. In this study, it is shown that, depending on the process employed for cutting, residual stresses generated at the cut surfaces can vary widely and they can, in turn, make the cut surfaces of austenitic stainless steel (SS) prone to stress corrosion cracking (SCC). An austenitic SS 304L plate was cut using three different procesess: bandsaw cutting, cutting using the cut‐off wheel, and shearing. Surface residual stress measurement using the X‐ray diffraction (XRD) technique is carried out close to the cutting edges and on the cross‐section. SCC susceptibility studies were carried out as per ASTM G36 in 45% boiling magnesium chloride solution. Optical microscopic examination showed the presence of cracks, and confocal microscopy was used to measure the depth of cracks. The study confirmed that high tensile residual stresses present in the cut surfaces produced by cut‐off wheel and shear cutting make the surfaces susceptible to SCC while the surfaces produced by bandsaw cutting are resistant to SCC. Hence, it is shown that there is a definite risk of SCC for product forms of austenitic SS with cut surfaces produced using cutting processes that generate high tensile residual stresses stored for a long period of time in a susceptible environment.

Investigating the correlation between magnetic Barkhausen noise emission and the fatigue life of shot-peened AISI 4140 steel

The performance of engineering components under conditions of fatigue can be improved by shot peening. The location and magnitude of the highest residual compressive stress, the depth of the compression zone and the stability of the residual stress state during cyclic loading are critical parameters for maximum utilisation of the advantageous influence of shot peening on fatigue performance. For accurate assessment of fatigue lifetime, residual stresses should be considered because they are algebraically summed with applied stresses. Therefore, the development of a non-destructive testing (NDT) method that has the ability to rapidly monitor surface residual stresses has industrial importance when verifying the achievement of the design goals of shot peening. The utilisation potential of the magnetic Barkhausen noise (MBN) technique in shot peening of ferromagnetic steels includes automated sorting of the components and measurement of surface residual stress. This study aims to establish the correlation between MBN emission and the fatigue life of shot-peened AISI 4140 steel.

Surface layer nanocrystallization of carbon steels subjected to severe shot peening: Analysis and optimization

Severe shot peening (SSP) process is widely used for surface nanocrysallization of a bulk material that demonstrates excellent mechanical properties compared with its coarse-grained equivalents. In this study, a plastically deformed surface was produced with nanostructured grains on different materials of AISI 1045, 1050, and 1060 carbon steels by means of SSP. Shot peening was applied with a wide range of Almen intensities and coverages. Optical microscopy, scanning electron microscopy, field emission scanning electron microscopy, high resolution transmission electron microscope observations, and X-ray diffraction analysis were employed to analyze the mechanism of grain refinement experimentally as well as the surface roughness and residual stress measurements. Afterwards, different shot peening treatments were used to develop a novel alternative approach based on artificial neural network (ANN) for modelling as well as parametric and sensitivity analysis of grain refinement and surface roughness. The experimental results were utilized to implement the ANN. The modelling results indicated that the neural network-based approach can be used to effectively analyze nanocrystallization and roughness variations of the shot peened carbon steels.

Altering the wetting properties of orthopaedic titanium alloy (Ti–6Al–7Nb) using laser shock peening

This work focuses on exploiting the effects of laser shock peening (LSP) to control the wetting characteristics of bio-material surfaces integrated with surface characteristics such as surface energy, macro and nano-topography. In particular, the effects of laser energy and beam footprint overlap of LSP were explored on Ti–6Al–7Nb alloy, quantified by using the measurement of dynamic contact angle, followed by determination of the surface-free energy and the work of adhesion. Surface modification by LSP was conducted at laser energy of 3 J, 5 J, 7 J, & overlap of 33%, 50%, 67% at 3 mm laser spot diameter. An incremental hole drilling method was employed for near to surface residual stress measurement. The results showed that compressive residual stress of between −42MPa and −516 MPa were formed on the sub-surface of LSPned Ti–6Al–7Nb. The results showed that surface roughness, surface-free energy and work of adhesion were proportional to laser energy, contact angle, however, was found to be inversely proportional to laser energy at consistent overlap. Additionally, surface-free energy and work of adhesion are proportional to overlap, but surface roughness and contact angle have a negative correlation with overlap. The correlation between laser energy and contact angle can be explained by Wenzel's theory while the relationship between overlap and contact angle is described by Cassie-Baxter model. This investigation on effects of LSP on the wetting characteristics not only addresses the required parameters for cell response on LSP modified titanium alloys, but also identifies that a metallic material strengthening process such as laser shock peening can also modify the wettability of a solid metallic surface as well as benefit the mechanical properties of metallic implants.

Surface integrity and residual stress analysis of pulsed water jet peened stainless steel surfaces

Enhancement of the life of the engineering components is essential from the perspective of its performance, therefore, it is necessary to eliminate the negative effects that attenuates the life of these components. Numerous surface treatment methods are used at present out of which water jet peening process have been stated to overcome the shortcoming reported in other surface treatment methods like surface defects caused by the embedment of particles during shot peening, thermal effects caused by laser shock peening. It has been reported in studies that the technological modification of water jet technology, ultrasonically generated pulsed water jet (PWJ) can be used for the peening application but still the detailed study of parametric variations for the peening process is not clearly explained. The present work aims at reporting the effect of variation of parameters: jet pressure (40 MPa to 100 MPa), traverse speed of the nozzle (5 mm/s to 25 mm/s) and standoff distance (15 mm to 31 mm) during the ultrasonically generated PWJ peening process. The peened surface was quantified in terms of surface residual stress measurements, micro hardness and surface roughness measurements. The results indicate that the variations in the parameters shows significant changes in the surface residual stress enhancement and strengthening process. The lower pressure (40 MPa), lower traverse speed (5 mm/s) and higher standoff distance (31 mm) showed the maximum improvement in the surface residual stress (up to 540 MPa) and micro-hardness (up to 570 HV). The impact of the repeated pulses causes plastic deformation on the surface and sub-surface layers which results into the change in the microstructures of the affected area. The microstructural examination through field emission scanning electron microscopyshowed variations in these deformations under different parametric conditions. This study can give better understanding for the proper selection of the peening parameters for practical applications using ultrasonically generated pulsed water jet technology.

Understanding the Residual Stress in DMLS CoCrMo and SS316L using X-ray diffraction

Laser additive manufacturing (LAM) is the next in line revolutionary technology for various turbine applications, as an enabler of new designs and parts and during repair and refurbishment of service returned parts. In LAM process, residual stresses play a key role during component build up as well as while removing the components from the build plate, to ensure distortion free parts, as a standalone or as a hybrid build on an existing part. The present study comprises an experimental determination of residual stresses of two direct metal laser sintered (DMLS) alloys, CoCrMo and SS316L, using the X-ray diffraction sin2 ψ technique. In both cases, the process parameters were optimized to result in dense (> 99.95%) samples. Surface residual stress measurements were carried out on the DMLS samples in the as printed condition and after heat treatment. While SS316L measured a surface tensile stress of 127 MPa, CoCrMo showed a tensile stress of 265 MPa, in the as printed condition. Upon grit blasting, both samples undergo a peening effect and record a compressive stress of 300-400 MPa. Heat treatment of SS316L at 650°C, tends to relieve these stresses and results in zero stress, whereas for CoCrMo there was a systematic variation in the response to heat treatment at different temperatures, correlating with the microstructural evolution. At 1050°C CoCrMo shows compressive stresses because of partial recrystallization and at 1150°C having a completely recrystallized microstructure comprising equiaxed grains of around 40 µm and a completely stress relieved condition.

Effect of machining-induced residual stress on springback of creep age formed AA2050 plates with asymmetric creep-ageing behaviour

Abstract Effect of machining-induced residual stresses on springback of creep age formed (CAFed) AA2050 plates with asymmetric creep-ageing behaviour has been investigated experimentally and numerically in this study. A finite element (FE) model integrating unified asymmetric creep-ageing constitutive model for CAF of AA2050 has been developed through a commercial FE software-PAM-STAMP and a simplified machining-induced residual stress field for aluminium alloy plates has been proposed and implemented into the FE model. Different levels of residual stresses have been considered in the FE model and numerical results have been compared with corresponding CAF experiments and surface residual stress measurements. Results show that the springback prediction of the FE models has been significantly improved when the residual stress effect was considered. The best prediction can be achieved with a certain residual stress level for each plate thickness and the level increases with decreasing thickness of the plates produced by machining, which agrees well with the surface residual stress measurement results. For CAF of AA2050 with asymmetric creep-ageing behaviour, different springback behaviour has been observed with different residual stress levels: introduction of residual stress fields can either decrease or increase formed deflections of CAFed AA2050 plates, depending on the relative levels of residual stresses and the stresses caused by applied external forces. When the residual stress is large, subsurface of the plate may exceed the elastic limit after applying external force, leading to a significant decrease of springback in the CAFed plates.

Influence of Microstructure on Shot Peening Effects of Ti-6Al-4V Alloy

Ti-6Al-4V alloys with equiaxed, martensitic and lamellar microstructures, respectively, were shot peened. Microstructure observation, phase analysis and surface residual stress measurement were conducted to investigate the microstructure influence on shot peening effects. The experimental results indicate that after shot peening microstructure of surface layer became fine and homogeneous for the as-received alloy which showed equiaxial microstructure. Cracks were found in the surface layer for the sample with acicular martensitic microstructure. And the sample with coarse lamellar structure transformed to equiaxed microstructure in the surface layer. Surface residual stress showed obvious difference. The samples with equiaxed microstructure achieved a compressive residual stress of 660 MPa, the ones with acicular martensitic microstructure achieved 366 MPa, and the ones with coarse lamellar microstructure showed a residual stress only 125 MPa.

An Arrhenius equation-based model to predict the residual stress relief of post weld heat treatment of Ti-6Al-4V plate

Creep models are generally utilized for predicting the residual stress relief during post weld heat treatment (PWHT). However, a comprehensive model considering the temperature effect is still lacking. In the present work, a new temperature dependent Arrhenius equation-based model has been developed to predict the residual stress relief of PWHT of Ti-6Al-4V. The model was validated by the surface residual stress measurement of the Ti-6Al-4V plate before and after PWHT using X-ray Diffraction (XRD). The comparative study of modelling results and measurements indicates that the temperature dependent Arrhenius equation-based model developed in this work is accurate and effective for predicting the residual stress relief of PWHT of Ti-6Al-4V. Besides, the creep induced strain increment takes the dominant role in residual stress relief during the PWHT process of Ti-6Al-4V.

Change of Residual Stress due Surface Conditions of Al7075-T6

Important goal of Structural Health Monitoring is detection of disintegration process in material in early stage of operation. One possibility is by monitoring the influence of surface degradation process on change of residual stresses during fatigue loading. Results of this investigation are important in analysis of condition and possibility for a Fiber Bragg Grating sensor installation for continuous strain measurements on the surface. The initial condition of the material surface is a consequence of sheet Al7075-T6 plate manufacturing procedure, but during the high cycling fatigue loading, the surface degradation with micro-grain separation occurred. Quantitative measurement of this process consist SEM surface analysis and residual stress measurement in some stage of fatigue loading. It was found the material surface is affected by fatigue loading level and loading ratio. Electropolished and no-electropolished surface have been observed and analyzed. Results show that the range of residual stress change is the similar after same number of fatigue cycles.