Stress Development Research Articles

Comprehensive Understanding of TGO Morphology Effect on the Thermal Barrier Coatings Failure Under Free Edges

Abstract The growth stresses induced by the thermally grown oxide (TGO) will be amplified at the free-edge site, making the free-edge site a weak part of the thermal barrier coatings (TBCs). In this study, the TBCs failure behavior is investigated based on different TGO morphologies under free edges. The thermomechanical model is established by creating straight lines and simplified sinusoidal curves, respectively. Dynamic TGO growth is realized by the secondary development of the subroutine. The cohesive element is inserted at the TC/TGO interface to simulate the delamination. The stress evolution near different TGO morphologies under the influence of the free edge are examined. In addition, the interfacial cracking behavior near the free edge is also explored. The results show that the appearance of the free edge will deteriorate the stress condition in the nearby area, change the preferred cracking area, and induce the earlier failure behavior. The straight line morphology has the most “friendly” stress distribution. The sinusoidal curves have peaks and valleys, and different areas of the TGO shape are different under the influence of the free edge, but all of them have the effect of stress “convergence”. These results can provide significant guidance to develop the next-generation advanced TBCs.

Excellent high temperature mechanical properties of Fe2Ni2CrAlx (x= 0.9 to 1.3) multi-principal elements alloys

The high temperature mechanical properties of a family of Fe2Ni2CrAlx (x=0.9, 1.1, 1.2, 1.3 in molar ratio) multi-principal element alloys (MPEAs) were determined. The initial microstructure and grain size of Fe2Ni2CrAlx MPEAs were characterized. Effect of Al on the density, melting point and mechanical properties at elevated temperature were investigated. Fe2Ni2CrAl1.3 MPEA exhibit the lowest density of 6.78 g/cm3 and highest melting point of (1337.7 ∼ 1392.6 °C). The as-cast Fe2Ni2CrAl1.3 MPEA showed an excellent combination of strength and plasticity at elevated temperatures. Fe2Ni2CrAl1.3 MPEA showed high strength of 2119 MPa and 1722 MPa at 200 °C and 400 °C, respectively. The Fe2Ni2CrAlx alloys exhibited exceptional high-temperature plasticity, no fracture occurred even at a compressive strain above 70%. Recrystallization softening appeared at above 600 °C, the transformation of coarse equiaxed grains into subgrains were captured during plastic deformation. Significantly, the specific yield strength of the Fe2Ni2CrAl1.3 MPEA was comparable with traditional Ti based alloys and Ni-based superalloy. The fracture mechanism of Fe2Ni2CrAlx MPEAs was a combination of cleavage and slip separation. Deformation behavior and work hardening ability at elevated temperature were discussed. Finally, finite element method (FEM) was performed to capture the evolution of stress and strain under high-temperature compression.

The development of grain resolved stress fields around notch tips in soft-textured zirconium polycrystals: A three-dimensional synchrotron X-ray diffraction study

Texture, microstructure, and local grain neighbourhood contribute to the development of localized stresses in polycrystals. For hexagonal close-packed materials, crystal's elastic and plastic anisotropy can also be a major contributing factor, yet there is a paucity of experimental studies focusing on the extent of contribution of such parameters on the magnitude of localized stresses at microscales. This study focuses on addressing this knowledge gap by deforming double-edge-notched soft-textured α-zirconium specimens in-situ, while measuring grain scale tensorial stresses using high energy synchrotron X-ray diffraction. The specimens were subjected to cyclic loads to study the evolution of stresses in the vicinity of both shallow and deep notches. The soft-texture of the specimens is such that there are no c-axes of grains aligned along the macroscopic loading direction thereby inhibiting deformation twinning. The “as-measured” microstructures and notch geometries were imported into a crystal plasticity finite element model for further analysis. Results show that despite the absence of c-axes of grains aligned along loading direction, the developed stresses were substantially influenced by crystallographic orientations. Stress drop was observed near the onset of plasticity with further loading and the orientation and position effects were highlighted. A plastic deformation mechanism was revealed where, upon specimen loading, the mechanical constraints enforced during grain-grain interactions led to hardening. Accordingly, a parameter was devised to quantify the grain level hardening arising from this mechanism. It was shown that grain-scale stress concentration factors vary significantly before the onset of plasticity, but they settle in the plastic zone and with the progression of cycles.

Fatigue deformation behavior and fracture mechanism of high fatigue-resistant laser directed energy deposition fabricated titanium alloy: Effect of multi-scale microstructure

The application of the laser directed energy deposition (LDED) titanium (Ti) alloys has been severely impeded by their poor low cycle fatigue (LCF) properties in aerospace industry. Herein, we propose to improve the LCF properties of LDED-ed Ti6Al4V alloys via microstructure adjustment. A solid solution aging 850AA treatment is made to regulate the microstructure into a multi-scale composition dominated by discontinuous αGB and coarsened αP+(αS+β) phases. The LCF properties of LDED-ed Ti6Al4V with a multi-scale microstructure even outperform the wrought Ti6Al4V at high strain amplitudes. Furthermore, this multi-scale microstructure has special local plastic deformation behavior and dislocation activities under cyclic stress loading, which strongly affects the internal stress evolution and crack propagation. The fatigue softening behavior is attributed to the combined effect of back stress and friction stress. Fatigue cracks are deflected by the α/β boundary when the angle (φ) between the long axis of α lamellae and the crack direction is less than 35°. The transgranular fracture occurs when φ is close to 90° (70°–90°). Meanwhile, the fatigue crack usually propagates along the basal or prismatic planes with high Schmid factors. These findings provide novel insights into the microstructure design of LDED-ed Ti alloys with high fatigue damage tolerance.

Microscale Normal Compression Behaviors of Clay Aggregates: A Coarse-Grained Molecular Dynamics Study.

Given the limitations of micromechanical experiments and molecular dynamics simulations, the normal compression process of clay aggregates was simulated under different vertical pressures (P), numbers of particles, loading methods, and environments by a Gay-Berne potential model. On the basis of the variations of particle orientation and the distribution of stacks, the evolution of deformation and stresses was elucidated. The results showed that the effects of the pressure level and loading environment on the deformation were significant. In the range of 0.1-10 MPa, the changes in the void ratio were essentially the evolution of the distribution of stacks determined by attractive short-range van der Waals interactions. The deformation under constant pressure was larger than that under step loading. Because the interactions between clay particles were mainly controlled by mechanical force when in the range of 40-100 MPa, the void ratios under various loading conditions were consistent. It was also found that changes in three-dimensional stresses during compression were dependent on those of the distribution of stacks. In the vacuum environment, owing to the lateral movement of interlocked small stacks, the horizontal stress decreased. The lateral pressure coefficients (k) were greater in an atmospheric environment because the anisotropic particle orientation was relatively less obvious. In the range of 10-100 MPa, when the loading path became longer, k was similar in vacuum but became smaller in an atmosphere. If the initial loading pressure was increased, the number of large stacks sharply increased and the anisotropy was significant in a vacuum environment, which was less prone to lateral expansion. In contrast, more consistent particle arrangements were maintained in an atmosphere. This work will be conducive to explaining experimental observations of long-term ripening.

A modified constitutive model for whole-life thermal-mechanical fatigue incorporating dynamic strain aging in 316LN stainless steel

A comprehensive analysis of experimental data is presented for 316LN stainless steel subjected to isothermal and thermal-mechanical fatigue loading conditions within the temperature range of 350–600 °C. The study aims to provide a thorough understanding of the cyclic behavior through meticulous data integration, supplementation, and the use of stress decomposition methods combined with a classical damage evolution definition. The modeling approach employed in this study is based on the classical AF-OW-Kang model, with the incorporation of the Arrhenius term in the plastic flow rate equation. Furthermore, to consider the effect of dynamic strain aging at varying temperatures, temperature-dependent terms are introduced into the equations that govern the evolution of isotropic stress and backstress, resulting in enhanced accuracy in describing cyclic hardening behavior. Additionally, a modified damage evolution equation is utilized, along with equations for isotropic stress and backstress evolution, to address cyclic softening. Simulation results confirm the effectiveness of the modified model in capturing the cyclic hardening/softening behavior of 316LN stainless steel throughout the whole-life time, under both isothermal and thermal-mechanical fatigue loading conditions.

Cracking mechanism of CMAS-corroded thermal barrier coatings based on a coupled thermo-chemo-mechanically phase-field fracture model

A thermo-chemo-mechanically coupled theoretical framework is proposed to describe the calcium-magnesium-alumina-silicate (CMAS) corrosion process during the cooling process. A phase-field fracture model is developed to investigate the effect of cooling temperature and CMAS concentration on the degree of corrosion reaction, the stress evolution and the crack initiation and propagation. σ11 concentrates in the region beneath the overlay of CMAS and σ22 appears at the interface between top ceramic coating (TC) and bond coating (BC). The higher stress concentration of σ11 and σ22 contribute to the formation of both vertical and transverse cracks. Transverse cracks first emerge at the interface between TC and BC in the edge region, followed by the formation of vertical cracks in the CMAS-coated region. Vertical cracks propagate to the interface and deflect into transverse cracks. The transverse cracks at the interface further propagate and merge, ultimately leading to the coating delamination. The higher initial cooling temperature and CMAS concentration contribute to the accelerated development of vertical cracking and the increase of the quantity and length of transverse and vertical cracks. The model provides a significant advantage in predicting the failure of TBCs during the cooling stage of CMAS corrosion.

Gradient residual stress evolution and its influence on fatigue life under combined carburising heat treatment and ultrasonic surface rolling process

This study combined carburising heat treatment (CHT) with the ultrasonic surface rolling process (USRP) to improve the rotary bending fatigue properties of 18CrNiMo7-6 alloy steel. Based on the experimentally obtained fatigue limit (σ-1), the evolution of gradient compressive residual stress (CRS) of CHT- and CHT + USRP-treated specimens was analysed below the fatigue limit stress level. Subsequently, the evolution mechanism of gradient CRS of the two reinforced specimens was discussed and analysed from the opposing effects of austenite transformation and dislocation density change on the evolution of CRS. The results demonstrated that the selective bending fatigue performance of the alloy was evidently improved after the introduction of CRS; however, the change in fatigue limit was not evident. The durability under other loads (such as cyclic tension/compression and torsion loads) needs further study. The gradient CRS evolution of CHT specimens was mainly caused by the large residual austenite transformation and small dislocation density change. Therefore, the volume expansion caused by residual austenite transformation gradually increased the gradient compression residual stress of CHT specimens with an increased number of cycles. Contrarily, the degree residual stress evolution of CHT + USRP specimens was more affected by the dislocation density, and the gradient compression residual stress gradually decreased with an increased number of cycles. Finally, the residual stress evolution rate of the two reinforced specimens was divided into two stages: fast and slow.

New high sensitivity approach for a quantitative visualization of the continuous whole-field stress evolution in digital photoelasticity

Digital photoelasticity is a widely used technique for experimental stress analysis on mechanical models subjected to loading conditions, due to its ability to reveal the stress information through color fringe patterns and consequently demodulate it computationally to achieve a quantitative evaluation. In this process, some limitations persist, such as, expertise dependence, experimental repeatability, sensitivity to load increments, and weak performance under experimental conditions, especially in time-dependent scenarios. In response to these weaknesses, the method “Quantitative Visualization of the Continuous Whole-Field Stress Evolution” introduced the ability to perform a complete demodulation of the fringe patterns generated in time-dependent applications, achieving high performance compared to conventional photoelasticity methods. This paper proposes a new highly sensitive approach for a quantitative visualization of the continuous whole-field stress evolution in digital photoelasticity by incorporating an inverse signal process and a multi-stage strategy. The algorithm aims to ensure the performance under high fringe order conditions while exhibiting high sensitivity in cases where the maximum stress obtained is less than one fringe order. The results presented validate its effectiveness in demodulating full-field stress distributions, especially in scenarios where the loading condition induces fringe orders less than one, thus contributing to the broader applicability of digital photoelasticity.

Evolution of Material Properties and Residual Stress with Increasing Number of Passes in Aluminium Structure Printed via Additive Friction Stir Deposition.

Additive friction stir deposition (AFSD) is an emerging solid-state additive manufacturing process with a high deposition rate. Being a non-fusion additive manufacturing (AM) process, it significantly eliminates problems related to melting such as cracking or high residual stresses. Therefore, it is possible to process reactive materials or high-strength alloys with high susceptibility to cracking. Although the residual stresses are lower in this process than with the other AM processes, depending on the deposition path, geometry, and boundary conditions, residual stresses may lead to undesired deformations and deteriorate the dimensional accuracy. Thermal cycling during layer deposition, which also depends on the geometry of the manufactured component, is expected to affect mechanical properties. To this day, the influence of the deposit geometry on the residual stresses and mechanical properties is not well understood, which presents a barrier for industry uptake of this process for large-scale part manufacturing. In this study, a stepped structure with 4, 7, and 10 passes manufactured via AFSD is used to investigate changes in microstructure, residual stress, and mechanical property as a function of the number of passes. The microstructure and defects are assessed using scanning electron microscopy and electron backscatter diffraction. Hardness maps for each step are created. The residual stress distributions at the centreline of each step are acquired via non-destructive neutron diffraction. The valuable insights presented here are essential for the successful utilisation of AFSD in industrial applications.

3D printing with stabilized earth: Material development and effect of carbon sequestration on engineering performance

This research examines the feasibility of using excavated soil (clay content of 44 %) as 25 % and 50 % replacement of natural sand in 3D printable formulations. A combination of ordinary Portland cement (OPC) and fly ash (FA) are used as binders for stabilization. The development of engineering properties subject to two curing conditions – normal curing and CO2 curing (5 % concentration for 5 h) are investigated. Experimental findings suggest that a combination of FA and clay (mainly kaolinite) in the used soil reduces plastic viscosity and improves flow retention, thus enhancing the extrusion quality and stability. The flocculation of clay at rest accelerates the evolution of static yield stress after extrusion, imparting 18–30 % better thixotropy. As a result, OPC-FA-soil mixes could be stacked to a height of 1.20 m compared to 0.48–0.54 m for OPC-sand and OPC-FA-sand mixes. Carbon sequestration via CO2 curing leads to an increase in wet compressive strength of OPC-25 % soil and OPC-FA-25 % soil by 29–38 % and 26–47 % respectively at 1-day age, which is ascribed to the densification effect of mineral carbonates. This is also reflected in the reduction in water sorptivity through the inter-layer zones and total shrinkage by 20 % and 16 % respectively. Due to reduced shrinkage and capillary sorption, inter-layer bond strengths in CO2-cured OPC-25 % soil are also improved by 20–50 % than the normally cured counterpart. In summary, the research demonstrates a feasible pathway to develop carbon-sequestering earth-based materials (CO2 uptake of 9–11 wt% of OPC) for additive manufacturing of masonry building and infrastructural members.

Study on Mechanical Characteristics and Durability of Prestressed Anchorage Structure of Rock and Soil under Fatigue-Corrosion Coupling Action

This paper presents a comprehensive approach encompassing indoor experiments, theoretical analysis, and numerical simulations to investigate the durability of prestressed anchorage structures subjected to fatigue loads and corrosion. The study addresses the critical issue of gradual aging and damage caused by cumulative loads and corrosion, which ultimately leads to a decrement in structural durability. Through a rigorous analysis of the effects of fatigue load and corrosion on the performance of steel bars, numerical simulations were conducted to elucidate the failure mechanisms and variation patterns within the internal anchoring section. After subjecting steel bars to fatigue and corrosion tests for a defined duration, they were systematically categorized and exposed to varying fatigue tensile cycles in diverse acidic and alkaline environments. Employing the PFC2D program, a numerical model of the prestressed anchorage structure under the coupled effects of fatigue load, corrosion, and fatigue load was developed. This model allowed for a comparative analysis of the evolution of shear stress, axial stress, and displacement fields at the bolt-grout interface under two distinct conditions. The findings reveal the microscopic mechanisms underlying bond degradation at the bolt-grout interface under the synergistic impact of fatigue load and corrosion. The proposed methodology and experimental results demonstrate that geotechnical anchoring technology can effectively reinforce up to 70% of geotechnical structures, significantly reducing soil loss by approximately 80%. This research provides valuable insights into the durability of prestressed anchorage structures, paving the way for future improvements and optimizations.

Exploratory Studies of Mechanical Properties, Residual Stress, and Grain Evolution at the Local Regions near Pores in Additively Manufactured Metals.

Directed energy deposition (DED) is gaining widespread acceptance in various industrial applications since its unique manufacturing features allow the DED to print metallic parts with very complex geometries. However, DED inevitably generates a lot of internal pores which can limit the widespread applications of the DED technique. The current studies on DED porosity are mostly focused on analyzing pores' bulk-scale influences on mechanical properties and performances. Since DED pores have a micro-scale existence, with dimensions ranging from a few microns to several hundred microns, it is fundamental to explore the pores' influences on the micro-scale, including local mechanical properties, residual stress, and grains near pores. However, this important research direction has been neglected. The objective of this work is to fill the above gap in DED porosity research and acquire a fundamental understanding of the role of porosity on a microscopic scale. The authors used nanoindentation approaches to investigate internal pores' effects on mechanical properties and residual stress in local regions surrounding the pores. In addition, the grains near pores were observed through EBSD, and simulated with the Kinetic Monte Carlo model. The research findings can be provided for DED researchers and industrial practitioners as technical guidance. Most importantly, the research results can work as a good reference for tracing the source of bulk-scale mechanical performances and properties of DED parts with internal pores.

Shear-induced phase behavior of bidisperse jammed suspensions of soft particles

Particle dynamics simulations are used to determine the shear-induced microstructure and rheology of jammed suspensions of soft particles. These suspensions, known as soft particle glasses (SPGs), have an amorphous structure at rest but transform into ordered phases in strong shear flow when the particle size distribution is relatively monodisperse. Here, a series of bidisperse SPGs with different particle radii and number density ratios are considered, and their shear-induced phase diagrams are correlated with the macroscopic rheology at different shear rates and volume fractions. These shear-induced phase diagrams reveal that a combination of these parameters can lead to the emergence of various microstructures such as amorphous, layered, crystals, and in some cases, coexistence of amorphous and ordered phases. The evolution of the shear stress is correlated with the change in the microstructure and is a shear-activated process. Stress shows pseudo-steady behavior during an induction period before the final microstructural change leading to the formation of ordered structures. The outcomes provide a promising method to control the phase behavior of soft suspensions and build new self-assembled microstructures.

Mechanistic Analysis of the Stress and Displacement Evolutions of Asphalt Mixtures Based on Finite Element Simulation

Mechanistic Analysis of the Stress and Displacement Evolutions of Asphalt Mixtures Based on Finite Element Simulation

Influence of friction and back stresses evolution on cyclic softening of laser powder bed fusion Ti-6Al-4V ELI alloy

Laser Powder Bed Fusion (LPBF) of Ti-6Al-4V ELI enables the fabrication of complex and individualized load-bearing patient-specific implants (PSI). These PSI are subjected to cyclic loading, which necessitates their fatigue performance evaluation. In this work, the fatigue properties of LPBF Ti-6Al-4V ELI in heat-treated conditions were characterized by studying the low cycle fatigue (LCF) behavior and underlying deformation mechanism. The fully reversed strain-controlled fatigue tests were performed at various strain amplitudes at room temperature. The ratio of fatigue life of conventional wrought alloy and LPBF Ti-6Al-4V ELI at the lowest (0.8 %) and the highest (1.8 %) strain amplitudes were found to be approximately 2.7 and 4.5, respectively. Further, the softening response during fatigue loading was correlated to the variation in the back and friction stresses. The TEM observations revealed that the dislocation pileup along α/β interfaces caused an increase in dislocation density, which drives the increase in back stress. However, the ease of slip transmission at high strain amplitude (1.8 %) reduced the heterogeneity between α and β phases, causing a decrease in the back stress. The TEM observations further suggested that the dislocation annihilation and subsequent rise in dislocation-free zones in α-phase reduced the friction stress at all strain amplitudes, which resulted in cyclic softening of LPBF Ti-6Al-4V ELI.

A direct analytical methodology for the assessment of ductile fracture in metals based on multiaxial tests

AbstractThe prediction accuracy of ductile damage models is subject to a sound calibration strategy, which normally involves the execution of complex multiaxial tests and requires dedicated facilities. In addition, finite element (FE) analysis is mandatory to retrieve the stress and strain states at the critical point, which cannot be directly measured from experiments. To overcome this complexity, a minimal set of simple multiaxial tests is selected, and an analytical‐numerical approach is proposed to evaluate, without resorting to FE, both the stress evolution with plastic deformation and the fracture strain, under any different loading condition of each test. This is achieved from the sole knowledge of the material bilinear stress–strain relation and of the applied test displacement at fracture. The obtained results are compared with a traditional testing and calibration methodology, and the robustness of the approach is proved on a 17‐4PH steel, an X65 steel, and a Ti6Al4V alloy.

Effect of stress evolution in destressed borehole on gas desorption in deep coal seam

Hydraulic flushing is a crucial technology for enhancing deep coal seam gas extraction, while the gas desorption characteristic is a critical factor impacting efficient gas extraction. This paper first analyzed stress variation around borehole during flushing and subsequently conducted experimental research to explore the effect of stress evolution on gas desorption. The results indicated that stress variation had a relatively insignificant effect on gas desorption during the early stage. When stress variation peaked in coal samples, instability and damage occurred, leading to a surge in the gas desorption rate. The ultrasonic wave velocity and dynamic elastic modulus of coal under different stress paths exhibited significant changes. With an increase in desorption time, the coal dynamic elastic modulus decreased, and the damage coefficient gradually increased. When comparing gas desorption behavior during constant stress and stress relief, Pingdingshan Mine (PDS) and Jiaozishan Mine (JZS) coal samples exhibited desorption amounts 2.85 and 1.41 times higher after stress relief, respectively. Based on the influence of stress relief on gas desorption rate and structure, a theoretical matrix scale change model was deduced. The mechanism controlling the rate of gas desorption by stress relief was illustrated. These research results offer theoretical guidance for implementing stress relief techniques to optimize coal seam gas extraction in practical field applications.

Rate-Dependent Evolution of Microstructure and Stress in Silicon Films Deposited by Electron Beam Evaporation

Rate-Dependent Evolution of Microstructure and Stress in Silicon Films Deposited by Electron Beam Evaporation

Conforming mesh modeling of multi-physics effect on residual stress in multi-layer powder bed fusion process

The current research aims to predict the residual stress accumulation and evolution in the powder bed fusion processed multi-layer thin wall structures through a conforming mesh modeling approach. It involves the discrete element method (DEM) interfaced with the volume of fluid (VOF) method using computational fluid dynamics (CFD) coupled with the finite element method (FEM). The conforming mesh approach developed in the research predicts multi-physics, its induced porosity, and the cumulative effect on the residual stress in the powder bed fusion processed Ti-6Al-4V thin wall structures. The results of the residual stress in the multi-layered component from this method were further quantitatively compared with the non-conforming finite element method. The results show the conforming mesh approach was not only effective in capturing the layer geometry, and defects induced during the printing, but also predicted the residual stress in the region of the defect more accurately than the non-conforming mesh methods.