Generation Of Compressive Stresses Research Articles

Numerical investigation of mechanical responses and failure features of coal measure strata using combined finite-discrete element method

Investigating the mechanical responses and failure features of the deep coal measure strata (CMS) is essential for designing hydraulic fracturing treatments and preventing geological dynamic hazards in underground engineering. In this paper, a three-dimensional CMS model is established by the combined finite-discrete element method (FDEM). The mechanical properties, stress evolutions and failure features of the CMS with different lithological combinations (sandstone-coal-sandstone (SCS), sandstone-coal-mudstone (SCM) and mudstone-coal-mudstone (MCM)) and thickness ratios are investigated under the uniaxial compressive condition. Results indicate that the SCS combination exhibits the highest uniaxial compressive strength (UCS) and elastic modulus, followed by the SCM and MCM combinations. The mechanical properties of the CMS are predominantly determined by the softer coal layer. The mechanical contrasts in elastic modulus and Poisson’s ratio between adjacent layers cause the generation of additional compressive stress in the softer layer and additional tensile stress in the stiffer layer. Therefore, the softer coal layer exhibits the shear-dominated failure pattern while the stiffer sandstone/mudstone layer displays the tensile-dominated failure pattern in the CMS. Furthermore, as the thickness ratio of the coal layer increases, the UCS, elastic modulus, and interfacial constraint stress decrease in the CMS. Additionally, the UCS and stress–strain relationship in the post-peak stage exhibit the noticeable size effect, whereas the mechanical behaviors in the pre-peak stage and the ultimate fracture patterns show the opposite. The elastic modulus and interfacial constraint stress obtained by the FDEM simulation match well with those calculated by theoretical models. The key findings of this paper can provide fundamental understanding and reliable parameters for multi-scale geomechanical modeling in petroleum and mining engineering.

The enhanced strength and radiation efficiency of alumina reticulated porous ceramics via coal gangue addition

Al2O3 reticulated porous ceramics (ARPC) are widely selected for high-temperature porous media burners due to their excellent thermal stability. However, the relatively low mechanical property and infrared emissivity of ARPC led to a low lifespan and radiation efficiency of the burner, limiting its application in high-temperature porous burners. In this work, a novel strategy was proposed to synergistically improve mechanical and radiation properties of ARPC and recycle of coal gangue. ARPC with three-layered struts, including microporous alumina-mullite coating, alumina skeleton and filling layer, were prepared by polymer replica technique and vacuum infiltration method. The addition of coal gangue could obviously strengthen ARPC, because coal gangue was attributed to form mullite, as well as the generation of residual compressive stress in the coating of three-layered struts. The decomposition of organic compounds within coal gangue enhanced the degree of microporosity on the coating, thereby increasing infrared emissivity by 21.8%, which increased the surface temperature of porous burner from 857.4°C to 905.7°C.

Impact of temperature on residual stress and bonding in diamond-like carbon film: A molecular dynamics study under various deposition conditions

Diamond-like carbon (DLC) films exhibit excellent frictional properties; however, the high residual stress within the film, which causes detachment issues, limits their application in mechanical sliding surfaces. We have previously clarified via molecular dynamics (MD) simulations that the growth of sp3 clusters after the transition from sp2 to sp3 bonds during film formation causes the in-plane compressive stress in DLC films. In this study, we investigated whether this stress generation mechanism holds under various deposition conditions in the MD simulations, wherein incident energy, atomic flux, and substrate temperature were systematically changed. The results of the MD simulations consistently revealed that the compressive stress generation mechanism remains unchanged across different deposition conditions. With increasing incident energy and atomic flux and decreasing substrate temperature, in-plane compressive stress in the film increased owing to increased sp3 content. Conversely, when the incident energy was further increased, both stress and sp3 content decreased. Based on a molecular statistical calculation, it was shown that the stable bonding state changed from sp3 to sp2 at high temperatures. Because the temperature monotonically increased with increasing incident energy, it was concluded that the change in the stable bonding state caused the non-monotonic change in the stress and sp3 content at high temperatures.

Residual Stresses in Surgical Growing Rods.

The treatment of early onset scoliosis using surgical growing rods suffers from high failure rate. Fatigue resistance can be improved by inducing compressive residual stresses within the near surface region. An in-depth investigation of the residual stresses profile evolution is performed through the sequence of material processing steps followed by surgeons handling operations, in connection to material properties. The final goal is to guide further improvements of growing rod lifetime. Residual stress evaluation was carried out on Ti-6Al-4V rods using digital image correlation applied to microbeam ring-core milling by focused ion beam. This provided experimental stress profiles in shot-peened rods before and after bending and demonstrated that compressive residual stresses are maintained at both concave and convex rod sides. A finite element model using different core and skin conditions was validated by comparison to experiments. The combination of an initial shot peening profile associated with a significant level of backstress was found to primarily control the generation of compressive stresses at the rod surface after bending. Guidelines to promote larger compressive stresses at the surface were formulated based on a parametric analysis. The analysis revealed the first order impact of the initial yield strength, kinematic hardening parameters and intensity of the shot peening operation, while the bending angle and the depth of shot peening stresses were found to be of minor importance. Materials exhibiting large kinematic hardening and low yield strength should be selected in order to induce compressive residual stresses at key fatigue initiation site.

Influence of Induced Local Stress on The Morphology of Porous Anodic Alumina at The Initial Stage of Oxide Growth

A characteristic feature of the disordered pore growth at the initial stage of aluminum anodizing is the development of three large groups of pores: the major pores of larger diameter and two groups of minor pores of smaller diameter. The samples were obtained by the electrochemical oxidation of thin aluminum films (100 nm thick) on SiO2/Si substrates in a 0.3 M oxalic acid at 30 V at 5 °C–40 °C. According to SEM studies, the pore distribution by diameter for the films obtained at 20 and 40 °C has three distinct peaks at ca. 13.5, 17.2, and 20.3 nm. The ratio of the diameter of major pores to the diameter of minor pores of group 1 or group 2 is constant and approximately equal to 1.17 and 1.51, respectively. The generation of local compressive stress influences the development of porous morphology. The distribution of zones with high and low compressive stress levels inside hexagonal cells is shown, and their correlation with the porous morphology is confirmed. The generation of local stress and strains in the anodic alumina layer with a porous, cellular structure is associated with local areas with changes in the geometric properties on its surface.

Generation of compressive residual stress at the root of tube-to-tubesheet welded joints in a heat exchanger

Stress corrosion cracking (SCC) is the main failure mode of tube-to-tubesheet welded joints in a heat exchanger, and weld residual stress can play an important role in the development of these cracks. Local post welding heat treatment (PWHT) is a feasible method to reduce weld residual stress for tube-to-tubesheet welded joints. In this paper, the residual stress evolution and relaxation mechanisms during local PWHT were discussed in depth. The effects of PWHT temperature and hold time on residual stress were clarified. A compressive stress manufacturing method based on local PWHT was proposed. The results show that the stress concentration at the root of tube-to-tubesheet welded joints is the main reason for the SCC of heat exchangers. The residual stresses in the weld and heated affected zone (HAZ) are significantly reduced during local PWHT. The residual stress relaxation of tube-to-tubesheet welded joints can be attributed to the creep strain development and deformation during local PWHT. The extrusion of the tube and tubesheet at cooling stage results in the generation of compressive stress at the root of tube-to-tubesheet welded joints. PWHT temperature has significant effect on the residual stress in the weld and HAZ in comparison with PWHT hold time. The application of the proposed compressive stress manufacturing method realizes the fabrication of compressive stress at the root of tube-to-tubesheet welded joints, and improves the economic benefit by reducing PWHT hold time. This work will provide a good guidance to reduce the SCC risk of heat exchangers.

Effect of processing conditions on residual stress in sputtered transition metal nitrides (TiN, ZrN and TaN): Experiments and modeling

Stress evolution during reactive DC magnetron sputtering deposition in TiN, ZrN and TaN films at different growth rates, pressures and relative gas flow rates was measured by a wafer curvature technique. The results indicate that, for a given growth rate, the stress in each material is tensile at higher pressures and becomes more compressive with lower pressures. At lower pressures, the stress tends to become more compressive with increasing growth rate. Increasing the gas flow rate of N2 relative to Ar is observed to enhance compressive stress generation in TaN but has a weaker effect on TiN and ZrN. At lower pressure, the compressive stress magnitude scales with TiN < ZrN < TaN. The stress at different processing conditions was analyzed in terms of a kinetic model that includes the effects of growth kinetics and energetic bombardment. The resulting parameters are compared with calculations from binary collision Monte-Carlo simulation codes.

Structural, electrical and optical properties investigation of nano-sized Sb0.1(SnO2)0.9

Solid state reaction method was used to prepare Sbx(SnO2)1−x (x = 0 and 0.1) with sintering temperatures, 600 °C and 800 °C. The crystallographic properties of undoped and doped SnO2 materials results showed the tetragonal rutile structure of SnO2. The crystallinity was clear and increased with increasing the heat treatment. For the most diffraction peaks the microstrain is negative because do ˂ ds indicating the generation of residual compressive stress in the surface. Rietveld refinement proved that a good fitting parameters Rp, Rwp, and χ 2 makes the derived samples to be in a high quality, especially Sb0.1(SnO2)0.9 sample. Scanning Electron Microscopy (SEM) indicated that a spherical shape of SnO2 with nanoparticles but plates and nanorods shaped of SnO2 were detected for Sb0.1(SnO2)0.9 compound that was sintered at 800 °C. SnO2 has grain size 67 nm and 86 nm at sintering temperatures 600 °C and 800 °C respectively, but after adding Sb the grain size decreases to be ≈44 nm at the same sintering temperatures. The electrical resistivity, ρ, of Sb0.1(SnO2)0.9 behaved as semiconductor-like. The magnetoresistance, MR, results showed that ρ(0.6 tesla) < ρ(0 tesla) at Tsint = 600 °C where ρ (0.6 tesla) > ρ (0 tesla) at Tsint = 800 °C because the crystallinity increased with increasing of the sintering temperatures. The results of seebeck showed that charge carriers are n-tape at Tsint = 600 °C and p-type at Tsint = 800 °C. The optical energy band, E g of Sb0.1(SnO2)0.9 which were 2.49 eV and 3.21 eV at Tsint = 600 °C and 800 °C respectively and the high values of the transmittance make this compound is candidate to work as window layer in solar cell applications. The results of susceptibility denotes that the Sb0.1(SnO2)0.9 compound is a paramagnetic material.

Study on sintering behaviour of ytterbium disilicate and ytterbium monosilicate for environmental barrier coating applications

Yb2Si2O7 (YbDS) and Yb2SiO5 (YbMS) are two promising materials being developed as environmental barrier coatings (EBC) for the protection of Ceramic Matrix Composites (CMC) applied to gas-turbine engines operating in high-temperature corrosive environments. In this work, sintering behaviours of YbDS, YbMS, and YbDS/YbMS composites compacts were investigated. The effect of the thermal ageing at 1350 °C on microstructural characteristics as well as crack healing and elastic properties were examined. It was found that YbDS had a lower critical sintering temperature and higher grain growth rate than that of YbMS due to lower activation energy. The sintering behaviour of the YbDS/YbMS composites showed that the addition of YbMS retarded the grain growth rates and contributed to the stabilisation of the elastic properties. In addition to this, composite containing 22 wt %YbMS displayed a crack healing behaviour during high-temperature exposure, which was attributed to the generation of compressive stress, consequently accelerated diffusion in the YbDS matrix.

Surface Layer Modification by Cryogenic Burnishing of Al 7050-T7451 Alloy With Near Ultra-Fine Grained Structure

Abstract Burnishing has been increasingly utilized to improve the surface integrity of manufactured components. The generation of surface and subsurface layers with ultrafine grains, attributed to severe plastic deformation (SPD) and dynamic recrystallization (DRX), leads to improved surface integrity characteristics including surface and subsurface hardness and reduction in surface roughness. Additionally, due to the generation of compressive stresses within the refined layers, increase in fatigue life and improved wear and corrosion resistance can be achieved. In this study, we apply cryogenic burnishing on Al 7050-T7451 discs and compare the surface integrity characteristics with dry conventional burnishing. A special roller burnishing tool with flexible rotating roller head was designed and used to perform the cryogenic burnishing experiments using liquid nitrogen as the coolant. The results show that cryogenic burnishing can increase the surface hardness by an average of 20–30% within a layer depth of 200 µm compared with only 5–10% increase using dry conventional burnishing. Refined layers with nano grain structure were also generated. During cryogenic burnishing, the tangential burnishing forces were higher than those of dry conventional burnishing due to rapid cooling and work hardening of the material.

Modeling the superposition of residual stresses induced by cutting force and heat during the milling of thin-walled parts

Previous studies on machining-induced residual stress mainly concentrated on qualitative analyses of mechanical load and thermal load. Nevertheless, it remains a major challenge to quantify the influence of cutting force and heat on residual stress generation due to the lack of superposition mechanism. A newly empirical model of milling residual stress superposition was constructed to distinguish the effect of milling force from the effect of milling heat. The influence proportions of milling force and heat on the residual stress generation have been determined. Firstly, the residual stresses induced by the milling force, heat and force-heat coupling load were simulated independently, and the simulation models were verified by experiments. The simulation results presented that the milling force caused both residual compressive stress and residual tensile stress on the machined surface, while milling heat induced mainly residual tensile stress. Then, a mathematical model was put forward to quantify the superposition relationship between the force-induced residual stress and the heat-induced residual stress presented in the simulations. It has been found that the influence of milling force accounts for 53% to 78% in the generation of the surface residual stress when the force-to-heat ratio (RFT) varies from 0.09 to 0.39. Finally, with a constant material removal rate, the conclusion can be drawn that the parameter combination with a higher force-to-heat ratio is more beneficial for the generation of residual compressive stresses on the machined surface, which has been confirmed by the case experiments. Therefore, quantitative analysis of residual stress generation and superposition turns to be possible by using the proposed modeling.

Thermal Effects on Surface and Subsurface Modifications in Laser-Combined Deep Rolling

Knowledge of the relationships between thermomechanical process loads and the resulting modifications in the surface layer enables targeted adjustments of the required surface integrity independent of the manufacturing process. In various processes with thermomechanical impact, thermal and mechanical loads act simultaneously and affect each other. Thus, the effects on the modifications are interdependent. To gain a better understanding of the interactions of the two loads, it is necessary to vary thermal and mechanical loads independently. A new process of laser-combined deep rolling can fulfil exactly this requirement. The presented findings demonstrate that thermal loads can support the generation of residual compressive stresses to a certain extent. If the thermal loads are increased further, this has a negative effect on the surface layer and the residual stresses are shifted in the direction of tension. The results show the optimum range of thermal loads to further increase the compressive residual stresses in the surface layer and allow to gain a better understanding of the interactions between thermal and mechanical loads.

Stress measurement in the iron oxide scale formed on pure Fe during isothermal transformation by in situ high-temperature X-ray diffraction

The stress development in the iron oxide scale formed on pure Fe during isothermal oxidation at 700 °C followed by isothermal transformation at 500 or 380 °C was measured by in situ high-temperature X-ray diffraction with the sin2ψ method. The eutectoid transformation resulted in compressive stress generation in the Fe3O4 and Fe in the eutectoid structure. This compressive stress was relaxed during the isothermal heat treatment after the eutectoid reaction. The stress generation was ascribed primarily to volume changes associated with the oxidation and/or reduction of the iron oxide at the interfaces, Fe3O4 precipitation, and the eutectoid reaction.

Stress control in thick AlN/c-Al2O3 templates grown by plasma-assisted molecular beam epitaxy

Stress evolution was studied in up to ~3 µm thick AlN templates, comprising ∼65 nm thick AlN nucleation layers (NLs) and thick buffer layers (BLs), grown using different growth modes and conditions by plasma-assisted molecular beam epitaxy (PA MBE) on c-Al2O3. Growth of both the NL and BL in a standard PA MBE mode at N-rich conditions (at flux ratio Al/N2* ∼ 0.5) led to instant generation of a relatively high tensile stress (∼1.5 GPa) which is maintained throughout the entire growth. On the contrary, NLs, grown using a migration-enhanced epitaxy (MEE), demonstrated a transition from the initial compressive stress to stress-free growth, which is usually observed in the Volmer–Weber films. Further growth of thick AlN BLs on the MEE–NLs at various Al/N2* ratios revealed a wide variety of stress evolution processes. The BL growth by using metal-modulated epitaxy (MME) at Me-rich conditions with Al/N2* ∼ 1.33 led to a gradual decrease in the initial compressive stress in the 2D AlN layers, whereas standard PA MBE growth of 3D BL at N-rich conditions (Al/N2* ∼ 0.92) exhibited a fast transition from the initial compressive stress to tensile stress. Moreover, we succeeded in achieving the quasi-stress-free growth of a 3.1 µm thick AlN BL using the MME growth mode at the optimum flux ratio Al/N2* = 1.05. These results were compared with the results of other authors and explained using a kinetic approach to description of stress evolution during PA MBE of AlN/c-Al2O3 templates, taking into account several simultaneously acting competitive mechanisms of continuous generation of tensile and compressive stresses.

Stabilization of complex orthorhombic o-Cr3C2 thin films under high energetic growth conditions: Experiments and calculations

We report on the deposition and characterization of hard and wear resistant orthorhombic Cr3C2 thin films. Films are deposited using a novel linear filtered cathodic arc (LFCA) at high energetic conditions and relatively low substrate temperatures (<200 °C). The deposited films have amorphous (a-Cr3C2), nanocrystalline (nc-Cr3C2) and orthorhombic (o-Cr3C2) structure by carefully tuning the growth process via the ion current density and substrate bias voltage. The o-Cr3C2 films presented a moderate stress (−5.3 GPa), low coefficient of friction, low nanowear and high hardness values (33.4 GPa) exceeding in almost 12 GPa the reported hardness values for bulk Cr3C2 (21.5 GPa). Here we show that the generation and relief of compressive stresses in the Cr3C2 films can be controlled through the energy delivered during the growth. The effect of stresses generated by ion bombardment on the mechanical properties of the films was comprehensively studied utilizing ab-initio calculations and compared with experimental results. The hardness increment with compressive stress cannot be assigned only to changes in the electronic structure, but also due to specific microstructural features and chemical bonding of the films. The possibility to deposit the o-Cr3C2 protective films at relatively low temperature on temperature-sensitive substrates shows its potential for industrial applications.

Thermo-mechanical-metallurgical modeling and validation for ferritic steel weldments

This work explained the effects of solid-state phase transformation (SSPT) on welding residual stress (WRS) evolution in SA508 Gr.3 C1.1(SA508) ferritic steel single-pass and multi-pass weldments. A thermo-mechanical-metallurgical (TMM) model incorporating thermo-elastic-plastic stains and SSPT-related strains was presented. The SSPT kinetics and transformation-induced plasticity (TRIP) models of SA508 steel were established by employing user-defined subroutines developed for ABAQUS. The simulated WRS distribution in the single-pass weldment was compared with previously published neutron diffraction measurement data, and a good agreement was achieved and thus validating the developed TMM model. The microstructural and WRS distribution characteristics of the multi-pass weldment were further investigated based on the validated TMM model. Numerical simulations illustrated how the transformation and heat transfer/cumulative behavior associated with different welding process conditions resulted in the generation of triaxial compressive and tensile stresses in the upper and underlying weld beads at the multi-pass welded joint, respectively. Moreover, an additional simulation model with SSPT but without TRIP was developed to reveal the potential effects of TRIP on WRS distribution. This study numerically provided valuable understanding of microstructure and WRS evolution in air cooling bainitic steel weldments under multiple thermal cycles and constituted a fundamental basis for developing new welding materials to minimize the formation of deleterious tensile WRS by simultaneously combining bainitic and martensitic transformations.

Механизмы возникновения напряжений в тонких пленках и покрытиях

The paper considers current conceptions of generation of mechanical stresses in epitaxial, polycrystalline and amorphous films during their growth and under different external actions. The mechanism of stress generation in geteroepitaxial films due to misfit in crystal lattices of the film and substrate is described. The relation between arising of the misfit stress in heterostructures and variation of their growth mode is shown. The mechanisms of generation of compressive and tensile stresses in polycrystalline films caused by nucleation and coalescence of islands at the beginning of their growth are considered. Different aspects of evolution of intrinsic stresses in continuous films are discussed in dependence of their deposition conditions, chemical composition, microstructure and mechanical properties. Special attention is given to consideration of generation mechanisms of intrinsic stresses in thin films concerned with formation of pint defects, incorporation of impurities and phase transformations during deposition. Factors leading to arising extrinsic stresses in thin films during their storage and operation are described in details.

Modeling birefringence in SiO2 glass fiber using surface stress relaxation

AbstractThe retardance of silica glass fibers was evaluated using photoelastic techniques. Here, surface birefringence in glass fibers is shown to be a consequence of surface stress relaxation for as‐received fibers drawn from Suprasil II. The surface features of the birefringent fibers were compared to a model of the residual axial stress profile resulting from a diffusion‐controlled surface stress relaxation. Additionally, a uniform birefringence in the fiber equivalent to a constant tensile stress was recognized and attributed to structural anisotropy produced during fiber drawing. The contribution of structural anisotropy to the observed birefringence remained constant as the surface features were successively etched away. Surface compressive stress generation was also observed, as retardance corresponding to a surface compressive stress was found to increase with applied tensile stress during short heat treatments. Significant features of the retardance profile in as‐received silica glass fibers, with a thin surface compressive stress layer and compensating interior tensile stress, agreed with the residual stress profiles predicted by the surface stress relaxation model after correcting for this observed structural anisotropy.

Effect of brass composition and phases on stress corrosion mitigation by laser shock peening

While laser shock peening (LSP) is meant to mitigate stress corrosion cracking (SCC) by inducing compressive residual stress, it is also known to produce dense dislocation tangles in the treated surface. The impact of such microstructure changes on SCC resistance is material-dependent and not well-understood. In this study single-phase brass 260 and two-phase brass 280 with different zinc content were LSP treated and subjected to SCC. Besides the competing effects of compressive stress and dislocation generation, dezincification was identified as a contributor to SCC specifically in brasses. Reduction of dezincification from LSP was evaluated using energy dispersive spectroscopy (EDS).

Mechanical stress in the inner bark of 15 tropical tree species and the relationship with anatomical structure

Recent studies have shown that the inner bark is implicated in the postural control of inclined tree stems through the interaction between wood radial growth and tangential expansion of a trellis fiber network in bark. Assessing the taxonomic extent of this mechanism requires a screening of the diversity in bark anatomy and mechanical stress. The mechanical state of bark was measured in 15 tropical tree species from various botanical families on vertical mature trees, and related to the anatomical structure of the bark. Significant tensile or compressive longitudinal stresses were observed in the stems of most species. Tensile longitudinal stress was observed in various botanical families and was always associated with fibers arranged in a trellis-like structure and strong dilatation of rays. The highest tensile stress was recorded in species with gelatinous fibers forming a treillis. Compressive stress was typically associated with a large amount of sclereids in the bark, supporting the differentiation of sclereids as a potential origin of the generation of longitudinal compressive stresses in bark. In species exhibiting both a fibrous trellis structure and a significant amount of sclereids, the sign of longitudinal stress may depend on the balance between these two mechanisms.