High Mean Stress Research Articles

VHCF life of AM1 Ni-based single crystal superalloy after pre-deformation

VHCF properties of the Ni-based single crystal superalloy AM1 with pre-deformation have been investigated at 1000 °C. Pre-deformed material did not present life debit under fully reversed conditions (R = −1). Pre-deformation decreased VHCF lifetime under high mean stress conditions (R = 0.5) with a shift from internal to mode I surface crack initiation. The pre-deformed material has microstructure coarsened bands lying on the former {111} slip planes, facilitating microstructure shearing during VHCF at R = 0.5. Slip traces increased surface roughness, causing a surface recrystallization and initiation of the fatal crack from the surface. Rejuvenation heat treatment after pre-deformation can restore the microstructure and the fatigue life.

Effects of tightening torque on screw stress and formation of implant-abutment microgaps: A finite element analysis

Statement of problemThe mechanical behavior of the conical connection implant with different torque levels requires evaluation. PurposeThe purpose of this finite element analysis study was to investigate the impact of abutment screw torque on the formation of microgaps at the implant-to-abutment interface of a conical connection under oblique loading. This is important because it is thought that bacteria can invade the internal implant space through the abutment-implant microgaps, causing peri-implantitis. Material and methodsThree-dimensional finite element analyses of the conical implant-abutment connection were performed by using screw torques of 20 Ncm and 30 Ncm. Oblique loads from 10 N to 280 N were applied to the prosthesis placed on the implant. The maximum von Mises stress in the abutment screw, the microgap formation process, and the critical load for bridging the internal implant space were evaluated. ResultsThe stresses in the abutment screw under oblique loading had limited sensitivity to the screw torque. However, the residual stress in the screw with a 30-Ncm torque was 35% higher than that with a 20-Ncm torque in the absence of an external load. The area in contact at the implant-to-abutment interface decreased with increasing load for both torque values. The critical load for bridging the internal implant space was 160 N for a screw torque of 20 Ncm and 220 N for a screw torque of 30 Ncm. The maximum gap size was approximately 470 μm with all the loads. ConclusionsIncreasing the screw torque can reduce the formation of microgaps at the implant-to-abutment interface. However, this will result in higher mean stress in the abutment screw, which may reduce its fatigue life and consequently that of the prosthesis. Further research is needed to evaluate the relationship between the abutment screw torque and microleakage in implant-supported restorations.

Rattlers’ involvement for possibly looser critical states under higher mean stress

The critical state of a granular material made of rounded tetrahedral particles is studied through DEM simulations of triaxial compressions. A minimum number of 7,500 particles is first obtained as the representative volume element (RVE) for the present triaxial simulations. Then, the macroscopic critical state line (CSL) is shown to be increasing in the (stress, density) space for low confining pressures. Such an unexpected behaviour is explained by the existence of a significant proportion of rattlers. Considering rattlers as voids indeed reinstates a classically decreasing CSL.

Influence of fluid-structure interaction modelling on the stress and fatigue life evaluation of a gas turbine blade

A computational method of fluid-structure coupling is implemented to predict the fatigue response of a high-pressure turbine blade. Two coupling levels, herein referred to as a “fully coupled” and “decoupled” methods are implemented to investigate the influence of multi-physics interaction on the 3 D stress state and fatigue response of a turbine blade. In the fully-coupled approach, the solutions of the fluid-flow and the solid-domain finite element problem are obtained concurrently, while in the decoupled approach, the independently computed aerodynamic forces are unilaterally transferred as boundary conditions in the subsequent finite element solution. In both cases, a three-dimensional unsteady stator-rotor aerodynamic configuration is modelled to depict a forced-vibration loading of high-cycle failure mode. Also analyzed is the low-cycle phenomenon which arises due to the mean stresses of the rotational load of the rotating turbine wheel. The coupling between the fluid and solid domains (fully-coupled approach) provides a form of damping which reduces the amplitude of fluctuation of the stress history, as opposed to the decoupled case with a resultant higher amplitude stress fluctuation. While the stress amplitude is higher in the decoupled case, the fatigue life-limiting condition is found to be significantly influenced by the higher mean stresses in the fully-coupled method. The differences between the two approaches are further explained considering three key fatigue parameters; mean stress, multiaxiality stress state and the stress ratio factors. The study shows that the influence of the coupling between the fluid and structures domain is an important factor in estimating the fatigue stress history.

Prevalence of temporomandibular joint disorder in the Lebanese population, and its association with depression, anxiety, and stress

BackgroundThe objectives of this study were to study the prevalence of temporomandibular joint disorder (TMD) and its association with anxiety, depression, and stress among the general Lebanese population as well as in a sample of patients recruited from an otolaryngologist clinic.MethodsA cross-sectional study was conducted between September 2018 and December 2019, which enrolled 459 participants from all districts of Lebanon (sample 1) and 37 patients from the otolaryngologist clinic at the Eye and Ear Hospital (sample 2). The temporomandibular disorder screening checklist was used to screen for temporomandibular joint disorder. The Fonseca’s anamnestic index was used to assess for temporomandibular joint disorder related signs and symptoms, as well as for symptoms severity.ResultsThe results showed that 19.7% of the general Lebanese population had TMD, from which 55.9% were female. In contrast, 59.5% of patients in the sample recruited from the clinic were found to have TMD. Higher stress, anxiety, and depression scores were associated with higher temporomandibular disorder severity score (B = 0.23; B = 0.10 and B = 0.10 respectively). Patients in the sample recruited from the clinic had higher mean stress (20.75 vs 11.43), anxiety (12.46 vs 5.78), depression (13.24 vs 6.52), and temporomandibular disorder severity scores (59.5% vs 19.7%) than the general population.ConclusionTemporomandibular joint disorder appears to be associated significantly with depression, anxiety, and stress and remains largely underdiagnosed in the general population.

A Cyclic Plasticity Model with Martensite Transformation for S30408 and Its Finite Element Implementation

The ability of the constitutive model to simulate the ratcheting behavior of metastable austenitic stainless steel S30408 is significant to ensure the safety of the liquefied natural gas (LNG) semi-trailer tanks in the lightweight process of the inner containers. This is because the lightweight inner vessels often encounter cyclic stresses due to the road inertia loads together with high mean stresses due to internal pressures. In this study, we performed cryogenic uniaxial tension experiments and a series of ratcheting experiments to investigate the cyclic plasticity behavior of the metastable austenitic stainless steel S30408. Based on the Ohno-Wang II model, we proposed a new cyclic plasticity constitutive model with martensitic transformation, which relates the content of deformation-induced martensite with isotropic hardening and kinematic hardening. The ratcheting behaviors of S30408 were first simulated by the proposed model with the incremental loading method using MATLAB. The results showed that the model could reasonably predict the ratcheting behavior of S30408, and the evolution law of martensite content could well predict the content of deformation-induced martensite. Under the assumption of the von Mises yield criterion and normal plasticity flow rule, we developed a numerical algorithm of plastic strain with the proposed model to implement the finite element calculation of the model. Internal iteration in the numerical algorithm was implemented with the Euler backward method, which calculated the trial strain for each equilibrium iteration using the consistent tangent matrix. With a user subroutine, the proposed model was programmed into ANSYS for a user - executable version. By simulating the uniaxial ratcheting of a S30408 round bar, we found that the calculated results were in good agreement with the experimental results, which promises further applications in the design of structures, such as LNG semi-trailer tanks.

Using the Q factor to detect closed microfractures

Fractures are ubiquitous within the subsurface and play an important role in the fluid flow, elasticity, and strength of rocks. Because they are essential to geologic systems such as hydrocarbon and geothermal systems, they need to be properly imaged and monitored. We used the spectral-ratio technique to measure the P-wave Q factor ([Formula: see text]) and the S-wave Q factor ([Formula: see text]) of dry and water-saturated crystalline rock samples that were thermally fractured by heating to 250, 450, 650, and 850°C. Increasing the temperature during thermal treatment produces an increased fracture density that is isotropically distributed. The samples were subjected to hydrostatic pressures up to the pressure at which all fractures are closed, and they were axially loaded to 25% of the failure strength. Axial loading of the samples further closes fractures that are oriented perpendicular to the loading direction, but it opens those that are oriented parallel and oblique to the loading direction. Attenuation measurements were made in the frequency range of 0.8 to 1.7 MHz. At the fracture closure pressure, the sample with the highest fracture density showed a reduction in [Formula: see text] and [Formula: see text] when compared to the sample with the lowest fracture density of 84% and 24%, respectively, under dry conditions and by 33% and 5%, respectively, under saturated conditions. The [Formula: see text] and [Formula: see text] increase as the samples were axially loaded. The increase is more pronounced in [Formula: see text] because it is less influenced by the opening of parallel and obliquely oriented fractures. The opening of these fractures mostly affects the propagation of the S-wave and therefore reduces the increase in [Formula: see text]. The results suggest that [Formula: see text] is very sensitive to fracture closure at intermediate and high effective mean stresses. We determine that [Formula: see text] is a useful measure of the fracture density even at high pressures at which fractures might be expected to be fully closed.

PSYCHOLOGICAL IMPACT OF COVID-19 LOCKDOWN (PHASE 2) AMONG INDIAN GENERAL POPULATION: A CROSS-SECTIONAL SURVEY

COVID-19 has caused unprecedented disruption of all spheres of life, including health, financial, and socio behavioral. Given the rampant nature of the pandemic, several nations, including India has instituted stringent public health measures, with one being nationwide lockdown, to mitigate COVID-19 transmission. Previous studies reported increased stress and anxiety levels among general population during phase 1 (complete) lockdown, however, the effect of extended lockdown (phase 2) on mental health outcomes remains the subject of investigation till today. Therefore, this cross-sectional study endeavors to assess the psychological outcomes among general population during phase 2 lockdown. The online questionnaire surveyed 627 individuals from the general population using a non-probability snowball sampling technique. Descriptive statistics, including the frequency distribution, mean and standard deviations were generated. Mean differences across groups were analyzed through independent-samples- t and analysis of variance tests. Consistent with previous studies, our results indicated a higher mean score of anxiety and stress among females compared to males. Young adults aged between 21-39 years had the highest mean stress and anxiety scores compared to other categories. The mean anxiety score increased from retired (M=3.96, SD=4.76, to students (M=7.04, SD=7.11), to unemployed (M=9.0, SD=6.53) occupation groups. The findings of this study highlight the need for designing psychosocial regulatory frameworks and suitable interventions to address the needs of those being mentally traumatized by the pandemic and associated lockdowns. The study also advocates for establishing psychological health monitoring and telepsychiatry systems for identifying and treating mental health problems.

A probabilistic Haigh diagram for notched components considering notch root plasticity due to high mean stresses

In this work, a probabilistic approach to high cycle fatigue failure probability estimation is presented, considering notched components experiencing elasto-plastic material behavior in the notch root due to high mean stresses. A parameterized weakest link model, constituted by a volume integral over the component stress field, is combined with elasto-plastic finite element simulations to account for plasticity induced stress redistributions around notches. In a calibration/prediction methodology, the considered model is applied to fatigue test results of two different aluminum alloys containing tests on eight differently notched sheet specimen types. The results show good agreement between model predictions and test data.

Dynamic Visible Monitoring of Heterogeneous Local Strain Response through an Organic Mechanoresponsive AIE Luminogen.

Local strain concentration is critically important for damage formation of structural components. Therefore, it is of particular interest in developing the structural health monitoring (SHM) method for large-scale, full-field, and on-site monitoring of local strain response in complicated structural components in service. The present work investigated a SHM method based on a pure organic mechanoresponsive luminogen (MRL), 1,1,2,2-tetrakis(4-nitrophenyl)ethane, for heterogeneous local strain concentration. Invisible heterogeneous local strain response in complicated weld joints is transformed into visible fluorescence under monotonic tension and cyclic stress loading. The local strain (<15%) calculated by fluorescence intensity has a good agreement with the results obtained by the conventional digital image correlation method, indicating good measurement accuracy of the calibrated organic MRL method. The heterogeneity of local strain in complicated weld joints increases along with elongation and number of stress cycles. Moreover, the higher mean stress and stress amplitude can induce significantly higher accumulated local strain in the relatively soft fusion zone region. Compared with conventional strain measurement methods, the present organic MRL method opens up new possibilities for large-scale, full-field, and on-site monitoring of local strain concentration and damage in complicated structural components.

Discrete element modeling of concrete under high stress level: influence of saturation ratio

The discrete element model proposed in this paper addresses the macroscopic behavior of concrete taking into account the presence of free water in pores, thanks to a new interaction law between spherical discrete elements (DE). When concrete structures are subjected to a severe loading, e.g., an impact, material exhibits high triaxial compressive stresses which are highly influenced by the saturation ratio. In this new constitutive model, cracking and compaction are modeled at the interaction level between DEs and free water effects are taken into account by introducing a dependency between the water saturation ratio and the inelastic deformation due to the pore closure. The present numerical model has been implemented in the Yet Another Dynamic Engine code in order to deal with extreme loading situations leading to stress states characterized by a high mean stress level.

Impact of Microstrucutre on Dwell Fatigue in Dual-Phase Titanium Alloys

Dual phase titanium alloys, such as Ti-6242, experience a significant reduction in fatigue lifetime when the peak load is held at each cycle. This type of sustained peak loading, also known as dwell fatigue, mimics the long periods of high mean stress experienced by titanium fan and compressor components during takeoff and cruise. The reduction in fatigue lifetime is known as the dwell debit, and is attributed to the phenomenon of load shedding. Both local microstructure and temperature are known to impact load shedding and thereby the macroscopic response of Ti-6242 when subject to dwell fatigue, but the underlying mechanisms are still under active investigation. This study utilized electron backscatter diffraction (EBSD) and digital image correlation (DIC) to characterize the role of local microstructure and temperature on load shedding during dwell fatigue. EBSD was used to determine local orientation and texture information, and DIC provided information about the heterogeneity of the strain distribution and plastic strain accumulation. Ex-situ tests were performed to investigate the link between the deformation of local microstructures and macroscopic damage. The resultant strain fields and orientation maps were statistically analyzed to provide quantitative insights into the impact of local microstructure on load shedding during dwell fatigue.

Effect of stress ratio and mean stress on high cycle fatigue behavior of the superalloy IN718 at elevated temperatures

The present work shows the effect of mean stress on high cycle fatigue behavior of the superalloy IN718 at 400, 500 and 600 °C, at stress ratios (R) −1, 0.5, and 0.7. Fatigue life is decreased with increase in tensile mean stress. At high mean stress, decrease in fatigue life is associated with multiple crack initiation sites at the specimen surface. Fatigue limit is highly affected by the tensile mean stress and stress ratio, R = 0.7, since the maximum stress approaches near yield stress and causes cyclic ratcheting.

Design and Evaluation of a Deformable Sensor for Interstitial Pore Pressure Measurement in Concrete under Very High Stress Level

Previous studies have shown the strong influence of free water saturation ratio on the triaxial behavior of concrete at high confinement. This influence of the free water is usually attributed to a pore pressure effect. This article presents an experimental method aiming at measuring the pore pressure of free water into concrete samples under very high mean stress. Two types of deformable pressure sensors were designed and tested. The first one works in hydrostatic compression while the second one acts as a flexible membrane. The two sensors give comparable results and show that pore pressure may reach several hundred MPa in saturated concrete sample under a maximum 400 MPa hydrostatic compression. Such levels of pressure may explain the loss of shear strength and increase in volumetric stiffness observed on the macroscopic behavior of concrete due to the presence of free water.

Análisis de tensión de los incisivos centrales superiores tratados endodónticamente con diferentes alturas de férula y tres materiales diferentes de clavija posterior: un análisis de elementos finitos.

Statement of problem : fracture of endodontically treated teeth is reduced by the use of a post with ferrule, but the effect of different ferrule configurations and dowel materials is not clear. Purpose: to evaluate the effect of ferrules with different configurations and heights on the stress of endodontically treated teeth restored with three different post and dowel materials. Materials and Methods: fifteen models of maxillary central incisors restored with porcelain fused to metal crowns were obtained using pro engineer software. the models were divided into three groups, each consisting of five models with ferrule heights of 0mm, 2mm, 4mm, 2mm with oblique fracture, 4mm with oblique fracture, the models under group GFR were restored with fiberglass reinforced post (GFR) and composite core build-up, group NiCr with a custom cast post metal alloy (NiCr), and group Zr with zirconia post (Zr) and composite core build-up. an oblique load of 100N and 150N at an angle of 135 degrees was applied to the palatal surface of the tooth, a vertical load of 100N and 150N at an angle of 90 degrees was applied to the incisal tip of the tooth. The maximum principal stress and the von mises stress was calculated for the remaining tooth structure and post apex using the finite element analysis (FEA) software. Results: the maximum von misses stress was observed in the apex of the post (p&lt;0.05). Group Zr showed the highest mean stress (6.39Mpa) followed by group NiCr (5.65Mpa). There was a significant difference between post and between NiCr and Zr post for 2mm and 4mm ferrule height, while for 0mm ferrule there was a significant difference between the GFR and NiCr groups (p&lt;0.05). Under oblique load, the maximum mean stress was observed in remaining tooth structures while for vertical load, it was observed at the apex of the post. Regarding ferrule heights, there were significant differences between 0mm-2mm, and 0mm-4mm uniform ferrule in post apex in the case of NiCr posts (p&lt;0.05). Absence of ferrule resulted in higher stress for the NiCr group. Conclusion: higher loads that led to fracture were observed only at the apex of the post. Zirconia posts (group Zr) had higher fracture loads, whereas absence of ferrule resulted in higher fracture load with custom cast posts (group NiCr). Fracture thresholds were high on the remaining tooth structure for all the dowel systems especially for composite core build up irrespective of ferrule height and configuration. Clinical implications: appropriate selection of post and dowel materials in different configurations of ferrule heights ensures clinical success.

Experimental Investigation of Coupled Geomechanical, Acoustic, and Permeability Characterization of Berea Sandstone Using a Novel True Triaxial Assembly

Technological advancement of laboratory testing methods for investigating comprehensive geomechanical and transport properties of reservoir rocks is of great importance to the development of comprehensive geomechanical models that can improve operational risk assessment and production optimization. The in situ stress state can largely change due to drilling, completion, fracturing, and production, depending on the initial complexity of the geological setting, intrinsic rock anisotropy, and the degrees of artificial geomechanical and hydraulic disturbances. The versatility of a geomechanical model is therefore assured by reflecting the effects of stress magnitude and anisotropy on the considered properties. The purpose of this study was to investigate the effects of intermediate principal stress on the constitutive and flow behavior of Berea sandstone under true triaxial stress state. We present results of experiments with independent variation of intermediate principal stress under an array of octahedral normal and shear stresses, performed in dry and water-saturated cylindrical Berea sandstone core samples using a novel true triaxial testing apparatus. Data analysis indicated presence of microstructural behavior suggestive of opening and closing of microfractures, and revealed the effect of intermediate principal stress on deformation, wave velocities, and permeability. Vertical permeability displayed reduction to a smaller extent under increased vertical stress than under increased radial stress while exhibiting a steady decline under higher mean stress.

Fatigue strength assessment of duplex and super-duplex stainless steels by 4R method

Duplex and super-duplex stainless steels are increasingly used in applications where good fatigue strength is demanded in addition to corrosion resistance. In this research work, the fatigue strength of duplex and super-duplex steels was investigated experimentally, using standard fatigue strength assessment methods, and theoretically, using conventional design methods and a novel effective notch stress-based procedure, the 4R method. The experimental tests included testing of welded joints with and without post-weld treatment. The experimental results were compared with the 4R method. The test results indicated good fatigue strength properties for both materials in the as-welded (ASW) condition, and post-weld treatment by high frequency impact (HFMI)-treatment improved the fatigue resistance at low stress ratios. No improvement, however, was found in the case of high mean stress of the applied load. The results obtained by the theoretical investigation agreed quite well with the experimental results.

Analysis of injection-induced shear slip and fracture propagation in geothermal reservoir stimulation

Permeability enhancement via shear slip is commonly accepted as the main stimulation mechanism. However, the mechanism of permeability increase appears to have been understood to be limited to shear dilation and perceived to exclude the propagation in tensile and shear mode of the natural fractures that experience slip. This has led to the claims of the discovery of a new stimulation mechanism, namely, stimulation via wing crack propagation. The root cause of the misconceptions is likely the inability to model natural fracture propagation and coalescence. However, natural fracture propagation in general and wing cracks in particular are to be viewed as an integral part of the shear slip stimulation mechanism because shear slip increases the stress-intensity at the fracture tips, potentially leading to fracture propagation. In an effort to better illustrate the underlying mechanisms in the geothermal reservoir stimulation process, a displacement discontinuity (DD) model is developed and used to simulate secondary crack propagation associated with natural fracture slip. The model uses Mohr-Coulomb joint (contact) elements and rigorously accounts for fracture propagation. The model is applied to explore the conditions conducive to shear and tensile mode fracture propagation. When natural fractures undergo shear slip due to direct and indirect water injection, out-of-plane wing (tensile) cracks form and propagate at injection pressures below the minimum in-situ stress level and turn toward the maximum in-situ stress direction as they grow longer. It was found in our results that the injection pressure is stabilized at approximately the minimum in-situ stress level. The secondary cracks form as wing cracks and/or shear cracks. The predominance of wing cracks and their lengths and propagation paths were found to be controlled by the relative value of differential and mean stresses, natural fracture length, as well as rock and natural fracture frictional properties. In deeper geothermal reservoir with relatively low differential stress conditions and/or high mean stress levels, the shear crack propagation could play a major role in fracture network formation and permeability enhancement.

Very High Cycle Fatigue of Ni-Based Single-Crystal Superalloys at High Temperature

Very high cycle fatigue (VHCF) properties at high temperature of Ni-based single-crystal (SX) superalloys and of a directionally solidified (DS) superalloy have been investigated at 20 kHz and a temperature of 1000 °C. Under fully reversed conditions (R = − 1), no noticeable difference in VHCF lifetimes between all investigated alloys has been observed. Internal casting pores size is the main VHCF lifetime-controlling factor whatever the chemical composition of the alloys. Other types of microstructural defects (eutectics, carbides), if present, may act as stress concentration sites when the number of cycles exceed 109 cycles or when porosity is absent by applying a prior hot isostatic pressing treatment. For longer tests (> 30 hours), oxidation also controls the main crack initiation sites leading to a mode I crack initiation from oxidized layer. Under such conditions, alloy’s resistance to oxidation has a prominent role in controlling the VHCF. When creep damage is present at high ratios (R ≥ 0.8), creep resistance of SX/DS alloys governs VHCF lifetime. Under such high mean stress conditions, SX alloys developed to retard the initiation and creep propagation of mode I micro-cracks from pores have better VHCF lifetimes.

Scale Effects of Footings on Geocell Reinforced Sand Using Large-Scale Tests

The scale effect on bearing capacity of shallow footings supported by unreinforced granular soils has been evaluated extensively. However, the subject has not been addressed for shallow footings on geocell-reinforced granular soils. In this study, load-settlement characteristic of large square footings is investigated by performing large-scale loading tests on unreinforced and geocell-reinforced granular soils. The effects of footing width (B), soil relative density of soil (Dr), and reinforcement depth (u) have been investigated. The test results show that the scale effects exist in geocell-reinforced soils, like unreinforced soils, and the behavior of small-scale models of footings cannot be directly related to the behavior of full-scale footings due to the difference between initial conditions of tests and the initial state of mean stresses in the soil beneath the footings having different dimensions. Large footings create higher mean stresses in the soil, resulting in low soil friction angle and initial conditions of the test approach to the critical state lines. The results of tests indicate that model experiments should be conducted on low-density soil for better prediction of the behavior of full-scale footings, otherwise, the predicted behavior of full-scale footings does not seem conservative.