Macroscopic Residual Stresses Research Articles

An efficient viscoelastic multiscale model for investigation on the cure‐induced microscopic residual stresses of composite laminates

AbstractThe cure‐induced residual stresses have a significant influence on the deformation and mechanical properties of composites. Moreover, the cure‐induced residual stresses exhibit multi‐level and multiscale distributions due to the complexity of micro‐structure of composites and anisotropic characteristics. In this paper, a computationally efficient multiscale procedure is proposed to predict the microscopic residual stresses induced by cure temperature, cure shrinkage and macroscopic residual stresses, which is based to the linear viscoelastic theory combined with the micro‐mechanics method. The instantaneous microscopic residual stresses are expressed as a linear combination of the instantaneous temperature increment, degree of cure increment and macroscopic strain increment at all times of the cure process until to the current time. The proposed method is verified in comparison with the published results in literature and the microscale FEA at different length scales, respectively. The results show that the proposed method is about a thousand times faster than directly using FEA in the microscale RVE model once the influence coefficient matrices are determined. At length, the microscale stresses of all nodes of the entire macroscale model are examined by using the proposed method. It is found that the thermal‐chemical‐mechanical loads lead to serious microscopic stresses for the matrix and interphase constituents, especially for those due to the macroscopic residual stresses.Highlights Microscale residual stress is written as a linear combination of macroscale loads. Applying macroscale loads on microscale FEA model to obtain influence matrices. The presented model avoids repeatedly FEA microscopic to improve the efficiency. The online time required for the proposed model is much less than microscale FEA. The microscale stress increases the risk of matrix cracking and interface bonding.

Resist Thermal Shock Through Viscoelastic Interface Encapsulation in Perovskite Solar Cells

Enhancing the lifetime of perovskite solar cells (PSCs) is one of the essential challenges for their industrialization. Although the external encapsulation protects the perovskite device from the erosion of moisture and oxygen under various harsh conditions. However, the perovskite devices still undergo static and dynamic thermal stress during thermal and thermal cycling aging, respectively, resulting in irreversible damage to the morphology, component, and phase of stacked materials. Herein, the viscoelastic polymer polyvinyl butyral (PVB) material is designed onto the surface of perovskite films to form flexible interface encapsulation. After PVB interface encapsulation, the surface modulus of perovskite films decreases by nearly 50%, and the interface stress range under the dynamic temperature field (−40 to 85 °C) drops from −42.5 to 64.8 MPa to −14.8 to 5.0 MPa. Besides, PVB forms chemical interactions with FA+ cations and Pb2+, and the macroscopic residual stress is regulated and defects are reduced of the PVB encapsulated perovskite film. As a result, the optimized device's efficiency increases from 22.21% to 23.11%. Additionally, after 1500 h of thermal treatment (85 °C), 1000 h of damp heat test (85 °C & 85% RH), and 250 cycles of thermal cycling test (−40 to 85 °C), the devices maintain 92.6%, 85.8%, and 96.1% of their initial efficiencies, respectively.

Residual Stress Measurement Using X-ray Diffraction in Friction Stir-Welded Dissimilar Titanium Alloys.

Surface residual stresses in welded specimens significantly influence properties such as fatigue resistance, fracture toughness, and the superplasticity of joints. In this study, we employed friction stir welding, a well-established joining method, to weld dissimilar titanium alloys. By combining two distinct titanium alloys, we aimed to harness their unique properties when subjected to cyclic loading, impact, or superplastic forming processes. Utilizing X-ray diffraction, macroscopic surface stresses were assessed in dissimilar titanium alloys (Ti-6242 standard grain (SG) and Ti-54M) welded via friction stir welding, assuming a linear lattice distortion. The study accounted for misalignment, significant distortion, and grain refinement in the stir zone. Macroscopic surface residual stresses were quantified on the weld surface and at a depth of 1.5 mm beneath it within a square cross-section (1 × 1 mm2) by oscillating the specimen in the (X-Y) direction. The sin2φ method, implemented through the LEPTOS® (v7.8) software, was employed for residual stress measurement. The analysis of the results was conducted with respect to different rotation and traverse speeds. It was noted that at the center (CEN) of the weld, commonly referred to as the weld nugget, approximately 50 MPa of tensile stress was observed under the lowest values of both tool rotation speed and traverse speed. Tensile residual stresses were evident at the boundaries and within the stir zone. No discernible pattern was observed at the specified locations. Notably, the resultant values of residual stress, influenced by rotation and traverse speeds, exhibited asymmetry.

Coupled effect of prestrain and intermediate annealing on mechanical properties and microstructural evolution of GH5188 superalloy

The forming of sheet metal parts with complex structures often requires multiple processing. The Co-based superalloy GH5188 sheet is taken as the research object for two-stage uniaxial tensile experiments including intermediate heat treatment, where the prestrain is 10%, 20% and 30%, respectively, in the first stage. The results indicate that the macroscopic residual stress increases with increasing prestrain, which is manifested as the increase of the initial yield strength, the deterioration of ductility and work hardening ability. The increase in dislocation density in the plastic deformation process is the decisive factor. After annealing, the number of deformed grains greatly reduces, and the grain size is refined, resulting in a corresponding reduction in dislocation density. Intermediate heat treatment can eliminate dislocation stacking and stress concentration in the alloy and reduce the influence of internal stress generated during work hardening. Furthermore, the intermediate annealing can decrease the initial yield stress and increase the elongation of the predeformed sheet, while retaining desirable work hardening ability and tensile strength, ultimately enhancing the subsequent work performance.

ANALYSIS OF ADDITIVELY MANUFACTURED AlSi10Mg ALLOY USING X-RAY DIFFRACTION

This research deals with the study of additively manufactured AlSi10Mg alloy samples by X-ray diffraction. The effect of sample orientation during 3D printing by Selective Laser Melting on the values of macroscopic residual stresses and microstructural parameters and their possible inhomogeneities of the cylindrical sample was investigated. Phase analysis identified the intermetallic phase Mg2Si in addition to the major phases (solid solutions of Al and Si). Tensile residual stresses weredetected in the surface layer for all samples. Inhomogeneities of the investigated parameters of all samples were observed along the circumference of the shank. These effects may be related to the direction of the layers relative to the axis of the sample. These inhomogeneities form so-called microstructural notches or weak spots that are prone to the formation of fatigue cracks, which have a major influence on the fatigue life.

Measurements of stresses in β‐(Y,Yb)2Si2O7 environmental barrier coating layers using Raman spectroscopy

AbstractResidual stress measurements in environmental barrier coatings (EBCs) can help in understanding coating durability and potential damage mechanisms. We demonstrate the capability of Raman spectroscopy to measure average residual stresses in two different β‐RE2Si2O7 (RE = rare earth or Y3+, denoted as REDS) EBCs with a nominal (Yb,Y)2Si2O7 composition, focusing on macroscopic residual stresses acting at macroscopic length scales. Uniaxial compression measurements of dense β‐(Yb,Y)2Si2O7 ceramics are used for calibration curves for applied stress versus Raman peak shift. We use these calibration curves to evaluate the macroscopic thermoelastic residual stress in the (Yb,Y)2Si2O7 layers from coefficient of thermal expansion (CTE) mismatch of two different (Yb,Y)2Si2O7‐based EBCs on SiC‐based substrates. Microscopic stress contributions to Raman peak shifts are subtracted using reference free‐standing coatings and analytic estimates for inclusion stresses when (Yb,Y)2Si2O7 coatings have a high, variable volume fraction of (Yb,Y)2SiO5. The macroscopic thermal residual stress in these (Yb,Y)2Si2O7 EBCs is compressive, as expected from the smaller CTE of β‐REDS versus SiC. These thermal residual stresses can vary between different YbYDS EBC coatings by more than a factor of three with a measured layer residual stress of approximately –180 MPa (compressive) for a high modulus, low CTE coating, and our measured values generally agree with calculated estimates.

Prediction of microscopic residual stresses using genetic programming

Metallurgical manufacturing processes commonly used in the industry (rolling, extrusion, shaping, machining, etc.) usually cause residual stress development which can remain after thermal heat treatments. These stresses can be detrimental for the in-service performance of structural components, which makes their study and understanding important. Residual stress variations are usually determined at a macroscopic scale (commonly, using diffraction methods). However, stress variations at the microscopic scale of the individual crystallites (grains), are also relevant. Contrary to the macroscopic residual stresses, microscopic residual stresses are difficult to quantify using conventional procedures. We propose to use machine learning to find equations that describe microscopic residual stresses. Concretely, we show that we are able to learn equations to reproduce the diffraction profiles from microstructural characteristics using genetic programming. We evaluate the learned equations using real neutron diffraction peaks as a reference, obtaining accurate results for the most frequent grain orientations with runtimes of a few minutes.

Effect of synergistic cavitation erosion-corrosion on cavitation damage of CoCrFeNiMn high entropy alloy layer by laser cladding

The CoCrFeNiMn high entropy alloy (HEAs) clad layer was prepared on the surface of 304 stainless steel using coaxial powder feeding laser cladding. The effects of laser energy density on the macroscopic residual stress, crystallographic characteristics, microhardness, corrosion resistance and cavitation erosion (CE) resistance of HEAs clad layer were studied. With the decrease of laser energy density from 45 J/mm2 to 36 J/mm2, pore defects and abnormal coarsening of microstructure in HEAs cladding layer can be effectively avoided, the average grain size can be refined to 74.3 ± 1.7 μm, the average Schmidt factor is 0.43, which also shows the highest dynamic passivation tendency and corrosion resistance in artificial seawater. After 10 h-CE, the carrier density ND in the passivation film of HEAs clad layer can be as low as 3.24 ± 0.08 × 1022 cm−3, and the lowest mean depth erosion rates (MDER) is 0.68 ± 0.07 μm/h, which is much lower than that of 304 stainless steel substrate (4.23 ± 0.13 μm/h). The excellent CE resistance of the HEAs clad layer is attributed to the fact that its grains can effectively resist the plastic deformation and the surface passivation film can maintain a relatively high degree of integrity and continuous level after long aging CE, which has a considerable application prospect in marine engineering.

Finite element analysis of the surface finishing of additively manufactured 316L stainless steel by ball burnishing

As additive manufacturing of metal components is becoming increasingly significant due to high levels of design freedom yet features insufficient surface quality, finishing processes need to be further developed. As an alternative to machining, investigating the ball burnishing of powder bed fusion (PBF) workpieces provides opportunities to advance fundamental research. Studying this method's capabilities, finite element analyses (FEA) were conducted based on a flow stress model parameterized with Split-Hopkinson Pressure Bar (SHPB) tests. The roughness smoothing and a macroscopic approach mapping residual stress distributions was modelled and validated. Improvements in surface integrity were confirmed in both the FEA and experimentally.

Determination of the through-thickness residual stress in thick duplex stainless steel welded plate by wavelength-dependent neutron diffraction method

In this study, a wavelength-dependent neutron diffraction method used to measure the residual stress in thick duplex stainless steel (DSS) welded plate was proposed, which was validated by contour method. By choosing appropriate wavelength, the penetration capability of neutron was improved. The neutron with appropriate wavelengths was then applied to investigate the through-thickness volume fraction, macroscopic and phase-specific residual stress in a 55 mm thick DSS welded plate. The results showed that a wavelength at 2.395 Å had more superiorities than the conventionally-used wavelength at 1.593 Å in the measurement of longitudinal and transverse stresses of a thick DSS plate. In general, high tensile stresses were generated in austenite while high compressive stresses were found in ferrite. For elastic mismatch residual stresses, their maximum values in austenite and minimum values in ferrite were located near the mid-thickness region. Reasons for the formation and characteristics of the phase-specific stresses were also discussed. Besides, large tensile macroscopic stresses were found near the surface whilst significant compressive stresses were found to generate in the mid-thickness of weld metal.

The effect of residual stresses and dislocations on microwave dielectric loss in rutile-related (Ti0.6Zr0.4)0.8(Zn1/3Nb2/3)0.2O2

Strain and stress play an important role in functional materials, but they are often neglected in electroceramics. Here we report the stress effect on microwave dielectric loss in rutile-related (Ti 0.6 Zr 0.4 ) 0.8 (Zn 1/3 Nb 2/3 ) 0.2 O 2 solid solutions. The macroscopic, microscopic and nano-scale residual stresses have been systematically investigated, including their relationships. The average Raman spectra have demonstrated considerable macro-residual stresses, especially for the heavily cracked sample. The high-resolution Raman images reveal the inhomogeneity of residual stress distribution at the grain-level, which is reflected by the shifts of Raman-active modes. The high-resolution transmission electron microscope (HRTEM) images and geometric phase analysis (GPA) analysis show the aggregation of dislocations and resulting distortion of crystal. It can be found that the dielectric loss depends strongly on residual stresses, that samples with greater residual stress exhibit much higher dielectric loss because the residual stress increases the vibration anharmonicity of the lattice.

Comparison of different approaches to model fatigue for additively manufactured specimens considering production-related characteristics

Additive manufacturing processes such as selective laser melting (SLM) gain more and more importance and are already applied in various industries ranging e.g. from aerospace to automotive. However, in order to further spread their distribution among the common manufacturing technologies, reliable material data is required. While static material data for various materials is available in literature, fatigue data is comparably scarce.The properties of a specimen vary depending on the building direction and the building parameters. Furthermore, due to the manufacturing process, the parts show especially high surface roughness, isolated defects at overall low porosity and significant macroscopic residual stresses in the unprocessed state. As it is not always possible to alter these features, it seems advisable to incorporate them in a fatigue model for SLM specimens, especially as these are the decisive features regarding fatigue strength.Estimation of the fatigue limit is carried out via the defect stress gradient (DSG) approach and a Weibull distribution. Both approaches are capable to include the aforementioned features and are therefore used and compared for the fatigue strength calculations of 316L produced via SLM.

Failure analysis of cracking in S30408 weld joint between cylinder and flange of pure steam sterilization pot

The sterilization pot is widely used in the production and processing of food. The cracking failure of S30408 cylinder-flange weld joint of sterilization pot was analyzed by through the analysis of the composition, macroscopic fracture morphology, metallographic morphology, residual stress, microhardness and finite element modelling. The reasons for cracking and corresponding preventive methods were put forward. The cracking in inner surface of flange is induced by the stress corrosion cracking and accelerated by fatigue. The corrosion resistance deterioration of materials and the existence of dissolved oxygen and chloride ions are the main reasons to induce cracking. The cyclic tensile stress in weld joint accelerated crack propagation in inner surface. In order to avoid crack of sterilization pot, the reduction method on welding residual stresses should be performed and the contents of chloride ion and dissolved oxygen in the medium should be strictly monitored.

Comparison of the microstructure, mechanical properties and distortion of stainless steel 316 L fabricated by micro and conventional laser powder bed fusion

Micro laser powder bed fusion (μLPBF) technology offers great benefits to industries as it enables fabrication of complicated metallic components with greater accuracy and minimum feature size as small as 50 µm. Employing finer laser beam and smaller metal powder in μLPBF leads to many variations from the conventional LPBF (cLPBF) in terms of microstructure, mechanical properties and distortion, which have not yet been well understood. This work provides a comparative study of the μLPBF and cLPBF of the well-known material, stainless steel 316 L based on the surface quality, crystal structure, solidification microstructure, tensile properties and distortion of as-printed parts, and their sensitivities to μLPBF process parameters are also studied. Results show that lower surface roughness (Ra= 3.4 µm for top surfaces) is obtained after μLPBF. Stronger < 110 > texture along building direction is developed in the μLPBFed samples, accompanied with smaller grain size, higher density of low-angle grain boundary (LAGB) and geometrically necessary dislocation (GND). μLPBF creates a cellular microstructure with smaller cell size and cell wall thickness compared with cLPBF. The yield strength of μLPBFed samples is marginally lower than cLPBFed ones, which is dominated by the difference of compositional microsegregation in the cellular structures. Both cLPBFed and μLPBFed samples show a strong anisotropy in terms of yield strength, ductility and deformation behavior. The distortion measurement of the printed cantilever design suggests a lower level of macroscopic residual stresses in the μLPBFed samples due to the smaller molten pool and more thermal cycles. Moreover, the microstructure, mechanical properties and distortion of μLPBFed samples remain at the same level with variation of laser power and scanning speed. Overall, better surface finish, finer microstructure, more desirable mechanical properties and smaller part distortion can be obtained by μLPBF.

Synchrotron x-ray diffraction and crystal plasticity modeling study of martensitic transformation, texture development, and stress partitioning in deep-drawn TRIP steels

The micromechanics of the formability of a lean duplex TRIP steel was investigated using Synchrotron X-Ray Diffraction (S-XRD) measurements and Crystal Plasticity Finite Element Modeling (CPFEM). Specifically, the effect of the ferrite phase on the reduction of stress concentrations in the martensite phase and the influence of the austenite texture on the distribution of the martensite phase in the deep-drawn duplex TRIP steel were studied in comparison to a TRIP steel case. A series of deep-drawing processes were carried out to examine the sheet formability at ambient temperature, followed by S-XRD evaluations of the phase fraction, texture, and the residual-stress distributions in the deep-drawn cups. The macroscopic residual stress and its partitioning among constituent phases were studied using both S-XRD and CPFEM. In the deep-drawn TRIP steel, large tensile hoop residual stresses concentrated in the strain-induced α’ martensite phase, correlating well with the cracking phenomenon observed. Furthermore, the initial austenite texture influenced the martensite transformation kinetics during the deep-drawing process, resulting in a heterogeneous distribution of the martensite phase fractions around the circumference of the deep-drawn cups, which, in turn, caused an orientation-dependent cracking behavior. In the deep-drawn duplex TRIP steel, the tensile hoop residual stresses in the α’ martensite phase were significantly reduced due to a favorable load partitioning to the ferrite phase, resulting in a better formability.

Solidification microstructure and residual stress correlations in direct energy deposited type 316L stainless steel

Localised fluctuation in residual stress distribution for direct energy deposited 316L stainless steel thin plate was revealed through high-energy synchrotron X-ray diffraction. The macroscopic residual stress levels are changed steadily across the article according to the characteristic cooling rates, as implied by the measured dendrite arm lengths. Localised fluctuations of residual stress in the length scale, between 500 and 600 µm, coincides well with the morphological variations of the solidification microstructure that formed during direct energy deposition. Computations indicate a greater propensity for related mesoscopic thermal gradient fluctuations along the solidification front of the moving melt pool. Novel perspectives on residual stress across multiple length-scales and its relevance to the formation of solidification microstructure in metal additive manufacturing is analysed by considering the experimental results.

Residual stresses in hot bulk formed parts: microscopic stress analysis for austenite-to-martensite phase transformation

In this contribution, the two-scale analysis of residual stress states in a hot bulk formed part with subsequent cooling in the framework of the hbox {FE}^2-method is presented. The induction of specific residual stress states in order to improve a component’s properties is an area of current research. In general, residual stresses can be induced inside a component in different ways, e.g., quenching, phase transformation in hot forming processes or dislocation movements. It is widely known that different types of residual stresses can be characterized based on the scale the type acts on. In addition to the macroscopic residual stress analysis, in which residual stresses of first type are considered, this contribution specifically analyzes the microscopic residual stress evolution as a consequence of the cooling of the component.

Residual strain–stress in manganese steel railway crossing determined by synchrotron and laboratory X-rays

The residual stress within the nose of a worn manganese steel railway crossing was investigated. The macroscopic residual stress at the running surface was determined in areas with and without visual faults by laboratory X-rays. To get information about the penetration depth of the residual stress, synchrotron X-ray diffraction was used to map the distribution of local (within grain) residual strains below the running surface. The laboratory X-ray measurements revealed significant compressive stress on the rail running surface. The synchrotron measurements documented that local strains exist through the entire depth investigated. Relations between residual stress, plastic deformation, and crack formation are analysed, leading to a discussion on potentials/limitations of the two experimental methods for investigations of residual stress in large engineering components.

Further insights on the stress equilibrium method to investigate macroscopic residual stress fields: Case of aluminum alloys cylinders

The stress equilibrium, SE, method has been shown to be a very powerful, reliable tool for calculating the un-stressed lattice parameter, a0, as a critical requirement for the determination of the residual stress, RS, by diffraction techniques. The method can be used in cases where others, such as the powder or comb ones, are limited (e.g., in age-hardenable aluminum alloys components, where a0 can be affected). Owing to its potential, this method can address complex cases that, however, have not yet been sufficiently exploited. In particular, it makes it possible to delve into the effect of microstructural factors (e.g., texture variations) on the macroscopic residual stress, M-RS. Here, the M-RS obtained after quenching and slow cooling of cylindrical samples of aluminum alloys has been investigated. Data obtained from neutron and synchrotron radiation diffraction in AA5083 and AA2014 alloy samples (of different size), respectively, have been used. Specifically, the development of a texture gradient, a very common consequence derived from conventional manufacturing processes of components, has been taken into account. The known fact that a parabolic profile of the M-RS is obtained in cylindrical samples is used to generalize the SE method to determine a0 and to strengthen its quality and reliability.

Internal stress in titanium G4 before and after metal forming in increased temperature and shot-peening determinated by sin2ψ method

The article presents the assessment of the macroscopic residual stresses in titanium, grade G4, determined by the sin2ψ X-ray method. The values of stresses measured in the source material and in samples of sub/microstructure obtained after plastic processing in the Gleeble plastometer at the temperature 875 K, optimized strain ε and strain velocity έ were compared. After shot peening, regardless of the structure diversity, the roughness of the polished surfaces of the samples sometimes increased even about 10 times. Small differences occurred in hardness, higher in the more plastic sub/microstructure, which was also reflected in the magnitude of internal stresses.