Macroscopic Residual Stresses Research Articles

Determination of the macroscopic residual stress: Modeling and experimental evaluation of texture and local grain-interaction

When the texture is present in the sheet, it is extremely difficult to test the residual stress based on the X-ray diffraction method. Based on the two extreme traditional Voigt and Reuss models, this paper proposes two specific corresponding local models (local Voigt and local Reuss) for the measurement of the plane residual stress of cold rolled sheet texture materials. The residual stress test results of CuNiSi specimen using these two local models show that the proposed local models are credible and can accurately characterize the residual stress in the specimen. In addition, based on the X-ray diffraction sin2ψ method of the classic residual stress test, a weighted average residual stress calculation method is proposed for cold-rolled texture materials. The dψ-sin2ψ method was used to analyze the effect of texture on residual stress. The results show that the copper texture C{211}〈111〉 in CuNiSi specimen has the effect of concentrating the residual compressive stress in the region. The stress increased by about 50 MPa. However, the effect of brass texture B{011} 〈211〉 and R texture R{011} 〈211〉 on residual stress was not obvious.

Macroscopic residual stress and properties in different YBa2Cu3O7-x heterogeneous systems

Residual stress and internal strain induced by interface lattice mismatches between YBa2Cu3O7-x thin films and substrates causes variation on microstructures and superconducting properties. In this research, we investigated small misfit misalignments of YBa2Cu3O7-x thin films prepared on (LaAlO3)0.3-(Sr2AlTaO6)0.7 (LSAT), LaAlO3 and SrTiO3 substrates, and their resulting effects on their superconducting properties. Residual stress and internal strain were characterized by an extended small-angle sin 2ψ method and the Williamson-Hall plot method, respectively. The results showed that the relationship of transition temperature Tc had the same variation trend as that of the strain. The existence of compressive stress was confirmed in YBa2Cu3O7-x/LaAlO3 samples while YBa2Cu3O7-x/LSAT and YBa2Cu3O7-x/SrTiO3 samples displayed tensile stress. Moreover, by controlling appropriate macroscopic residual stress and microscopic internal strain would be an indispensable method for tailoring the microstructures and superconducting properties.

On the heat-treatment induced evolution of residual stress and remarkable enhancement of adhesion strength of cold sprayed Ti–6Al–4V coatings

Cold spraying is an extremely viable repair technique for high-value aerospace components made of Ti–6Al–4V alloys. However, significant residual stresses are generated in as-sprayed coatings and it is desirable to relieve such stresses as they could impact coating adherence and subsequent performance. We have used a series of post heat treatments to identify the most suitable one for stress-relieving of cold sprayed Ti–6Al–4V coatings on Ti–6Al–4V substrates. The alleviation of macroscopic residual stresses is accompanied by changes in coating microstructures, which has also been discussed. Post heat treatment particularly at 600 and 950 ​°C results in a significant increase in the interfacial bond strength compared to as-sprayed coating. Reduction in coating porosity and increased fraction of adhered powder particles to the substrate are also attributed to the increase in coating adhesion strength.

Tuning magnetostriction of Fe–Ga alloys via stress engineering

Stress has a strong impact on the magnetization and magnetostriction of magnetic materials due to the magneto-elastic coupling. It offers a clue to understand the 2–5 times enhancement on the magnetostriction of Fe–Ga alloys with rare-earth elements doping. In this article, the stress states of melt-quenched and melt-spun (Fe0.83Ga0.17)100-xTbx (x = 0–0.47) alloys were measured by high energy X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) technique. The macroscopic residual and microscopic stress of different Tb doped alloys were deduced from the shift and broadening of diffraction peaks and the deviation of the identified 1st Fe–Fe and 2nd Ga–Ga bond length. Rietveld refinement confirmed that the dissolved Tb atoms can replace the Fe/Ga site while small-angle neutron scattering (SANS) showed the presence of Tb-rich nanoscale precipitates. The effect of external stress loading and Tb doping on magnetization and magnetostriction of the modified Fe–Ga alloys was compared and discussed. We propose that the magnetization and magnetostriction of Fe–Ga alloys can be controlled through stress engineering by manipulating the magnetic domain structure through chemical doping or mechanical loading.

Finite element simulations of thermal residual stresses in realistic 3D WC-Co microstructures

During cooling of WC-Co cemented carbides from sintering temperatures, large internal residual stresses are obtained due to the difference in thermal expansion of WC and Co. The residual stresses depend on the properties of the microstructure, such as volume fractions and morphology, as well as the thermomechanical properties of the constituents. These residual stresses affect the engineering performance and strength of the hardmetal.In this paper, we present finite element simulations of the cooling of realistic synthetic WC-Co microstructures from sintering temperatures to room temperature. The microstructures are generated in voxel grids using a newly developed tool CCBuilder. Finite element analyses are conducted to obtain effective macroscopic thermomechanical properties and thermal residual stresses of WC-Co microstructures in a free sintered specimen. The analyses are performed within the framework of multi-scale computational homogenization using periodic boundary conditions. In the analyses individual anisotropic elastic and thermal properties of WC-grains are accounted for.An excellent agreement between numerically predicted and experimental results for the macroscopic thermomechanical properties for WC-Co is obtained. In addition, the predicted residual stresses in the microstructure after cooling are in good agreement with experimental data. The influence from different constitutive model assumptions (elasticity, elastoplasticity) for the Co-phase on the residual stresses is studied. Finally, the proposed modelling framework is used to study the influence of WC-grain geometry on the residual stress field and our results show a substantial dependency of the residual stresses on the detailed morphology of the microstructure.In conclusion, we demonstrate the power of the presented modelling framework using realistic synthetic 3D microstructures with periodic boundary conditions together with multi-scale computational homogenization. In the future it may be open up strategies for optimal material design of microstructures of WC-Co cemented carbides.

Effects of welding speed on the multiscale residual stresses in friction stir welded metal matrix composites

The effects of welding speed on the macroscopic and microscopic residual stresses (RSes) in friction stir welded 17 vol.% SiCp/2009Al-T4 composite plates were studied via neutron diffraction and an improved decoupled hierarchical multiscale modeling methods. Measurements showed that the macroscopic and total RSes had the largest variations in the longitudinal direction (LD). Increasing the welding speed led to higher values of measured LD macroscopic and total RSes in the matrix. The welding speed also significantly influenced the distributions and magnitudes of the microscopic RSes. The RSes were predicted via an improved hierarchical multiscale model, which includes a constant coefficient of friction based thermal model. The RSes in the composite plates before friction stir welding (FSW) were computed and then set as the initial states of the FSW process during modeling. This improved decoupled multiscale model provided improved predictions of the temperature and RSes compared with our previous model.

Density and mechanical properties in selective laser melting of Invar 36 and stainless steel 316L

In this study, the process-structure-property relationship for selective laser melting of Invar 36 and stainless steel 316L is discussed. Invar 36 and stainless steel 316L have been used in various industrial applications for their unique properties, especially in the aerospace industry. Invar 36 offers a very low coefficient of thermal expansion while stainless steel 316L offers high corrosion resistance. Since both materials are weldable, but hard to machine, this study is aimed at finding the optimum laser process parameters for producing dense components from both alloys. A full factorial design of experiments was formulated in this paper to study a wide range of process parameters for both materials. The bulk density, tensile mechanical properties, fractography, material composition, and residual stresses of the parts produced were investigated. An optimum process window has been suggested based on experimental work. The induced residual stresses were categorized into two categories: microscopic residual stresses and macroscopic residual stresses. The microscopic residual stresses were measured using X-ray diffraction method and the macroscopic residual stresses were measured using cantilever deflection method and finite element simulations. The paper proposes two laser energy densities for each material: brittle-ductile transition energy density, ET, and critical laser energy density, EC. Below the brittle-ductile transition energy density, the parts exhibited void formation, low density, and brittle fracture. Above the critical energy density, the parts showed vaporization of some alloying elements that have low boiling temperatures. Stable melting ranges were found to occur between these two laser energy densities: 52.1–86.8 J/mm3 for Invar 36 and 62.5–104.2 J/mm3 for stainless steel 316L.

INVESTIGATION OF SURFACE MACROSCOPIC RESIDUAL STRESSES OF CUTTING CERAMICS AFTER MECHANICAL MACHINING

Currently, the extensive research in the field of cutting ceramics is conducted and there are efforts to replace cemented carbides by these materials. However, the problem is to improve the production of ideal compact samples. Therefore, the influence of mechanical machining technologies on the values of macroscopic residual stresses was investigated by X-ray diffraction. The resulting values were discussed depending on machining parameters and surface structure of the studied samples.

Separating macro- (Type I) and micro- (Type II+III) residual stresses by ring-core FIB-DIC milling and eigenstrain modelling of a plastically bent titanium alloy bar

A novel approach to separating macroscopic (Type I) from microscopic (Type II + III) residual stress is presented, based on Focused Ion Beam – Digital Image Correlation (FIB-DIC) ring-core stress evaluation and eigenstrain modelling. This approach was applied to study the residual stresses for a titanium alloy bar following plastic four-point bending. It was found that electrochemical polishing is a surface preparation technique that is very well suited for FIB-DIC ring-core measurements, in the sense that it removes the influence of prior sample grinding and polishing, leads to a stress profile that satisfies force and moment equilibrium, and thus enables the evaluation of absolute values of total residual stress. The obtained relief strain profile across the bar width is asymmetric, highlighting the difference in the alloy's response to tension and compression. Total experimental residual stress values were calculated using (i) the assumption of material elastic isotropy, with an average Young's modulus, and (ii) under the assumption of elastic anisotropy, taking into account the crystallographic orientation of each investigated grain. Based on the measured relief strain values, the eigenstrain distribution in the bar was reconstructed and used to obtain the macroscopic (Type I) residual stress profile. The differences in the residual stress between the eigenstrain reconstruction values and the individual experimental results were ascribed to the local microscopic (Type II + III) residual stresses. This conclusion was substantiated by revealing the correlation between the residual stress values in individual grains in the elastic zone and their respective Young's moduli in the loading direction, as well as the correlation between the residual stress values in grains located in the plastic zone and their respective Schmid factors for basal slip.

Mechanical response of stainless steel subjected to biaxial load path changes: Cruciform experiments and multi-scale modeling

We propose a multi-scale modeling approach that can simulate the microstructural and mechanical behavior of metal/alloy parts with complex geometries subjected to multi-axial load path changes. The model is used to understand the biaxial load path change behavior of 316L stainless steel cruciform samples. At the macroscale, a finite element approach is used to simulate the cruciform geometry and numerically predict the gauge stresses, which are difficult to obtain analytically. At each material point in the finite element mesh, the anisotropic viscoplastic self-consistent model is used to simulate the role of texture evolution on the mechanical response. At the single crystal level, a dislocation density based hardening law that appropriately captures the role of multi-axial load path changes on slip activity is used. The combined approach is experimentally validated using cruciform samples subjected to uniaxial load and unload followed by different biaxial reloads in the angular range [27°,90°]. Polycrystalline yield surfaces before and after load path changes are generated using the full-field elasto-viscoplastic fast Fourier transform model to study the influence of the deformation history and reloading direction on the mechanical response, including the Bauschinger effect, of these cruciform samples. Results reveal that the Bauschinger effect is strongly dependent on the first loading direction and strain, intergranular and macroscopic residual stresses after first load, and the reloading angle. The microstructural origins of the mechanical response are discussed.

Correlation Between the Microstructural Defects and Residual Stress in a Single Crystal Nickel-Based Superalloy During Different Creep Stages

The present work attempts to reveal the correlation between the microstructural defects and residual stress in the single crystal nickel-based superalloy, both of which play the significant role on properties and performance. Neutron diffraction was employed to investigate the microstructural defects and residual stresses in a single crystal (SC) nickel-based superalloy, which was subjected to creeping under 220 MPa and 1000 °C for different times. The measured superlattice and fundamental lattice reflections confirm that the mismatch and tetragonal distortions with c/a > 1 exist in the SC superalloy. At the initially unstrained state, there exists the angular distortion between γ and γ’ phases with small triaxial compressive stresses, ensuring the structural stability of the superalloy. After creeping, the tetragonal distortion for the γ phase is larger than that for the γ’ phase. With increasing the creeping time, the mismatch between γ and γ’ phases increases to the maximum, then decreases gradually and finally remains unchanged. The macroscopic residual stress shows a similar behavior with the mismatch, indicating the correlation between them. Based on the model of shear and dislocations, the evolution of microstructural defects and residual stress are reasonably explained. The effect of shear is dominant at the primary creep stage, which greatly enlarges the mismatch and the residual stress. The dislocations weaken the effect of shear for the further creep stage, resulting in the decrease of the mismatch and relaxation of the residual stress. Those findings add some helpful understanding into the microstructure-performance relationship in the SC nickel-based superalloy, which might provide the insight to materials design and applications.

Evaluation of Residual Stresses in PVD Coatings by Means of Strip Substrate Length Variation and Curvature Method of Plate Substrate

The aim of the study was to determine macroscopic residual stresses in Physical Vapor Deposits (PVD) coatings through measurement of the length variation of the strip substrates coated on both sides. The length change of the strip was reduced to the deflection of the middle cross-section of the elastic element and was recorded by four strain gauges. For validating the obtained results, the conventional curvature method was used. As an application, residual stresses in hard AlCrN PVD coatings were investigated. The coatings were nanolayered to achieve better coating toughness for blanking and punching applications. The steel strips and steel plates with two thicknesses were used as the substrate. The values of the compressive residual stresses, determined by both methods for the investigated coatings, were very high (3.3 -3.6 GPa) independent of coating thickness and practically equal within the measurement uncertainty of the method. Good agreement between the experimental results obtained with both methods suggests that the presented method, strip length variation, is applicable for determination of residual stresses in coatings. Compressive stresses in coatings are desirable as they strengthen the coating.

A multi-scale analysis of the residual stresses developed in a single-phase alloy cylinder after quenching

Residual stresses, RSs, both at the macroscopic and microscopic scales, are developed during component manufacture. Knowing the magnitude of these stresses is crucial in structural design optimization. While determining the macroscopic residual stress, M-RS, by diffraction methods is well known, the calculation of the microscopic residual stress, m-RS, which varies from grain to grain in single-phase alloys is still a pending task. In this work, a multi-scale analysis to calculate both types of stresses in a single-phase alloy has been conducted for the first time using synchrotron radiation diffraction data and a “composite material” approach. This analysis, together with the results derived from a finite element model, FEM, demonstrates the strong influence of the severity of thermal-mechanical treatments on the generation of a grain orientation dependent m-RS and, contrary to other kinds of m-RS (such as in metal matrix composites, MMCs), its irreversible nature. It is also seen that, while it is possible to generate an “appropriate” M-RS for a given application (e.g., a compressive surface stress), a strong and detrimental m-RS field may arise simultaneously. This can be the origin of local inter-granular cracking during service of components.

An analysis of macro- and micro-scale residual stresses of Type I, II and III using FIB-DIC micro-ring-core milling and crystal plasticity FE modelling

Mechanical failure frequently initiates at the grain level, at which intra-granular stresses are of paramount importance. Under cyclic loading conditions regions within grains that experience high values of tensile residual stress are more prone to damage processes that lead to the formation of slip bands, defects, micro-voids and fissures that induce crack nucleation and propagation. For these reasons, the knowledge and understanding of residual stress across the scales (Types I, II and III) is crucial for improving the accuracy of mechanical failure prediction.The present study was carried out with the purpose of revealing the presence and nature of inter- and intra-granular residual stresses (known as Type II and III) that were present in an Aluminium alloy sample as consequence of plastic deformation. To this end, a well-defined macroscopic residual stress field was introduced in a miniature four-point bent beam. Following sample microstructure mapping by EBSD, the evaluation of Type II & III residual stress at the grain level was conducted using FIB-DIC micro-ring-core method. The combination of two calibrated models at different length scales enabled the simulation of stress across the scales, from the continuum large scale down to the crystal level (CP-FEM). As the outcome of this multi-scale modelling, the RS simulation predictions at all scales (Type I, II & III) were obtained and compared with the experimental results using a statistical approach. A key significance of the finding was that the Standard Deviation of the local residual stress values (95% confidence interval half width) amounted to as much as 2/3 of the macroscopic Type I value. This highlights the importance of including the information about Type II+III stresses in predictive design for structural integrity and the avoidance of failure.Error propagation due to measurement uncertainty was accounted in the analysis.By considering Schmid factor at locations of residual stress measurement, a modest correlation was found with Type II & III residual stresses.

Isolating the effect of residual stresses on coating wear by a mechanical stress relaxation technique

Residual stresses play an essential role for the tribological performance of sputtered coatings since compressive residual stresses yield positive effects on wear resistance. As deposition induced residual stresses are always related to the coating microstructure, a combination of different effects is typically assessed if coatings are validated by tribological testing. Therefore, the real effect of residual stresses on the tribological performance of coatings is not entirely clear. The study aims to examine this issue by isolating the residual stress effect from the microstructural impact by mechanical relaxation of the coating after the deposition process. For this purpose, MoS2 coated substrates are mechanically pre-stretched during the coating process and relaxed again afterwards in order to reduce the coating's macroscopic residual stresses. Other samples, which are not pre-stretched but coated inside the same process with identical distance to the target, serve as a reference material. The proper function of the relaxation technique is demonstrated by applying a recently developed method for residual stress measurement, namely the “Focused Ion Beam – Digital Image Correlation” method. Both samples, the relaxed and as-deposited one, were subjected to a tribological pin-on-disk test. The results show that at high compressive residual stress levels, only mild wear at the upper surface inside the contact area occurs. In contrast, at low compressive residual stress levels, wear results from an abrasive material removal inside the contact track but most significantly from additional coating detachments outside the contact area. The observations can be related, firstly, to a superimposition of harmful bending stresses and beneficial compressive residual stresses beneath the contact area and, secondly, to a stress relaxation effect taking place only inside the tribological contact zone. In summary, owing to the developed stress relaxation technique, a microstructurally independent effect of residual stresses on wear resistance of sputtered MoS2-coatings has been proved.

Evaluation of Residual Stresses in PVD Coatings by Means of the Curvature Method of Plate

Physical Vapor Deposition (PVD) coatings are primarily designed for metal cutting tools operating in extreme machining and blanking conditions. Residual stresses arising during coating deposition exert an important effect on the service life of the coating through influencing mechanical and tribological properties and adhesion. To determine macroscopic residual stresses, the conventional curvature method was used. As an application, residual stresses in four aluminum based PVD hard coatings, i.e. AlTiN, AlTiSiN, AlCrN, and AlCrSiN, were investigated in the presence of the Ti adhesion layer. Nickel steel plates and steel plates were used as the substrate. Residual stresses were compressive and high (3.0-7.5 GPa) in all coatings. Compressive stresses in coatings are desirable in cohesive tool damage as they strengthen the coating. The values of residual stresses were not significantly dependent on the angle of plate placement (parallel (0°), inclined (45°) and perpendicular (90°)) in relation to the PVD cathode in the deposition chamber. The magnitude of residual stresses is influenced by intrinsic strain at layer growth rather than by thermal stress.

Enhanced multiscale modeling of macroscopic and microscopic residual stresses evolution during multi-thermo-mechanical processes

After several decades, it is still difficult to predict “macroscopic and microscopic (M-m)” residual stresses (RSes) in metal matrix composites (MMCs) after welding. In this work, an enhanced multiscale model is developed to predict the evolution of M-m RSes in MMCs during several thermo-mechanical processes including welding. This multiscale model is capable of handling non-zero initial M-m RSes and integrates the temperature history dependent constitutive model (THDCM) at both macroscale and microscale. Meanwhile, thermal source model of friction stir welding (FSW) is integrated. The extension to other welding thermal source is straightforward. This multiscale model is used to study the generation, inheritance, and evolution of M-m RSes in a SiC/Al composite during quenching, FSW and post-welding heat treatment (PWHT). The effects of initial M-m RSes and material constitutive models on the prediction of M-m RSes are systematically assessed. It is found that using the THDCM and taking into account the initial RSes, this multiscale model shows the best predictions of RSes in the FSW joint of MMCs. The predictions agree with the neutron diffraction measurements reasonably well. It is found that the reduction of RSes during PWHT is mainly caused by the stress relaxation during the solution treatment stage.

Applying electron backscattering diffraction to macroscopic residual stress characterisation in a dissimilar weld

Dissimilar metal welds are complicated in nature because of the complex microstructure characteristics in the weld fusion zone. It is often necessary to know the phase distribution in a dissimilar metal weld especially at the interface such as fusion zone and heat affected zone to be able to predict the behaviour of the joint and its fitness for service. In this paper, a dissimilar metal weld made between ferritic/martensitic modified 9Cr-1Mo steel (P91) and austenitic AISI 316LN stainless steel using autogenous electron beam (EB) welding was analysed. The weld fusion zone has a local segregation of bcc and fcc phases. The EBSD technique was applied to determine the volume fractions of each of these phases in the weld fusion zone. This information was incorporated into the analysis of neutron diffraction data from the weld zone, and the macro-scale residual stresses were calculated from phase-specific stresses arising from the welding process. The results indicate that the overall macroscopic residual stress distribution in the weld centre is predominantly compressive in nature driven by the solid-state phase transformation of the weld pool during rapid cooling, with tensile peaks pushed adjacent to the heat affected zone (HAZ)/Parent boundaries on both sides of the fusion zone.

Evaluation of curing residual stresses in three-phase thin composite laminates considering micro-scale effects

Generally, in thin laminates, residual stresses are studied in two scales; micro mechanics and macro-mechanics. So far, no study has included both the micro- and macro-scale analysis for predicting the thermal residual stresses of composite laminates. In other words, no literature model is proper for both the micro and macro scale at the same time. This paper presents a procedure to integrate micro and macro thermal residual stresses in composite laminates. The combination of micromechanics-based analytical results and hole-drilling calibration factors leads to predict the macro thermal residual stresses of a unidirectional laminate. Classical lamination theory (CLT) is then employed to predict macroscopic thermal residual stress of each layer in laminated composites (with different orientations). Comparison of longitudinal, transverse and in-plane shear stresses between the proposed approach and published experimental results is made for glass/epoxy laminates in cross-ply and quasi-isotropic configurations. Available experimental data from incremental hole-drilling method show that the present combination procedure yields more precise residual stresses predictions in comparison with the state that the CLT is used only. In other words, the presented procedure which adds the micro residual scale results to the CLT, gives rise to a better prediction of residual stress fields. Consequently, ignoring the micro-mechanical effects may result in some uncertainties in thermal residual stress evaluation of thin composite laminates. Furthermore, while the CLT is not able to take into account the in-plane shear residual stresses for the cross-ply composites, the present procedure can provide an estimation of shear residual stress field.

Influencia de las tensiones residuales sobre el comportamiento frente al corte del gres porcelánico esmaltado

This study was undertaken to examine the influence of residual stresses on porcelain tile behaviour under cutting. To do so, two samples of glazed industrial porcelain tiles that exhibited different behaviour under cutting were selected. Using these industrial tiles, cutting tests were performed and the macroscopic residual stresses were determined by the strain relaxation incremental slotting method. The influence of the cooling rate on the arising residual stresses and their effect on tile cutting were also studied.For the porcelain tile with appropriate cutting behaviour, the residual stress profile in the body was symmetrical and could be fitted by just using the second- degree Legendre polynomial. This was the expected behaviour for homogenously cooled ceramic materials (same cooling rate at the proper surface as at the rib). For pieces with inappropriate cutting behaviour, it was necessary to use more terms of the series, which suggested that cooling had not been homogeneous. With regard to the influence of cooling, the temperature range in which residual stresses were generated was determined and it was verified that pieces with a greater level of stresses exhibited worse cutting behaviour.