Subsurface Stress Research Articles

Research on the rolling contact fatigue life of the bearing considering the friction effect

In this paper, a theoretical model is proposed to analyze the friction effect on the rolling contact fatigue (RCF) life of the angular contact ball bearing (ACBB). Firstly, the quasi-static model without race control hypothesis is introduced to study the load characteristics of the ACBB under different loading conditions. Then, the friction coefficient is quantitatively calculated by analyzing the relation between the gyroscopic torque and the sphere-race contact load. The effects of the axial load, radial load, torque load and rotating speeds on the friction coefficient, the maximum value of the sub-surface von Mises stress (SSVS) and the maximum peak-peak value of the sub-surface shear stress (SSSS) are subsequently investigated. The results show that they can significantly affect the friction coefficient and further influence the stress field beneath the contact surface. Further studies illustrate that the friction coefficient cannot affect the maximum peak-peak value of the SSSS but can change the distribution of the SSVS field. The relative RCF life of the sphere-race contact is calculated and the published studies confirm the accuracy of the calculation.

Internal Shear Stress Distribution and Subsurface Cracks of PPS Thrust Bearings under Rolling Contact Fatigue in Water

In this study, crack propagation in PPS thrust bearings under rolling contact fatigue (RCF) in water was observed, and relation between subsurface crack and internal shear stress parallel to the surface was investigated. It was found the cause of flaking was subsurface crack. They were evaluated in terms of contact stress and friction between their faces. It was discovered that subsurface cracks distributed around shear stress peak, and flaking failure was dominated by subsurface shear stress.

Stress Field Interactions Between Overlapping Shield Volcanoes: Borehole Breakout Evidence From the Island of Hawai'i, USA

AbstractKnowledge of the in situ stress state of the Earth's crust plays a key role in understanding geological processes including plate tectonics, earthquakes, slope failure, and igneous emplacement. In this paper, we determine the in situ stress orientation from the PTA2 borehole on the island of Hawai'i, drilled into a lava flow dominated sequence between Mauna Kea and Mauna Loa. High‐resolution acoustic images were collected from the open hole interval 886 m to 1,567 m. Based on identification of 371 borehole breakouts for a total length of 310 m, the mean orientation of the minimum horizontal principal stress is N106° and remains constant across different volcanic rock fabrics. Changes in borehole breakout shape are linked to the different strength of volcanic facies and intra‐facies. The orientation of the present‐day stress field at Mauna Kea deviates from the plate forces and regional tectonic stress field. We interpret the compressive stress regime at the PTA2 site as resulting from the competing gravitational fields of the large topographic highs of Mauna Kea and Mauna Loa. Our study reveals that the mass accumulation associated with shield volcano growth imparts significant local variations to the subsurface stress state on volcanic islands consisting of overlapping shield volcanoes. The results have significant implications for stress accumulation leading to brittle failure and flank collapse, along with potentially influencing magma accumulation and ascent pathways during volcanic island evolution. This study provides the first insights into the orientation of the present‐day stress field between the major island forming shield volcanoes of Hawai'i.

Pandora's Box-Influence of Contour Parameters on Roughness and Subsurface Residual Stresses in Laser Powder Bed Fusion of Ti-6Al-4V.

The contour scan strategies in laser powder bed fusion (LPBF) of Ti-6Al-4V were studied at the coupon level. These scan strategies determined the surface qualities and subsurface residual stresses. The correlations to these properties were identified for an optimization of the LPBF processing. The surface roughness and the residual stresses in build direction were linked: combining high laser power and high scan velocities with at least two contour lines substantially reduced the surface roughness, expressed by the arithmetic mean height, from values as high as 30 µm to 13 µm, while the residual stresses rose from ~340 to about 800 MPa. At this stress level, manufactured rocket fuel injector components evidenced macroscopic cracking. A scan strategy completing the contour region at 100 W and 1050 mm/s is recommended as a compromise between residual stresses (625 MPa) and surface quality (14.2 µm). The LPBF builds were monitored with an in-line twin-photodiode-based melt pool monitoring (MPM) system, which revealed a correlation between the intensity quotient I2/I1, the surface roughness, and the residual stresses. Thus, this MPM system can provide a predictive estimate of the surface quality of the samples and resulting residual stresses in the material generated during LPBF.

Prediction of critical undeformed chip thickness for ductile mode to brittle transition in the cutting of single-crystal silicon

In order to achieve the ductile mode cutting of silicon and avoid the generation of subsurface crack damage, the undeformed chip thickness in the diamond cutting process must be controlled below the critical value. Therefore, the critical undeformed chip thickness (CUCT) for the ductile mode cutting of silicon is studied in this paper for guiding the selection of cutting parameters in the diamond cutting of silicon. Considering the anisotropy of single-crystal silicon, the prediction method of the CUCT in the ductile cutting of silicon is proposed on the basis of calculating the anisotropic fracture strength of silicon and subsurface stress distribution induced by the cutting force. Moreover, the influence of the anisotropy of single-crystal silicon, cutting tool parameters and friction coefficient in the cutting process on the CUCT are discussed, respectively. The results show that the anisotropic CUCT depends on the cutting surfaces and cutting directions. In addition, the diamond cutting tool with a negative rake angle of −40° is generally suitable for ductile cutting on the silicon (001), (110) and (111) surface when the friction coefficient in the cutting process is in the range of 0.05–0.30. Under this cutting condition, the minimum CUCT on the silicon (001), (110) and (111) surface is in the range of 150 ∼ 250 nm, 160 ∼ 290 nm and 150 ∼ 250 nm, respectively. The lower limit of the minimum CUCT on the three surfaces corresponds to the friction coefficient of 0.30 in the cutting process, and the upper limit corresponds to the friction coefficient of 0.05. Furthermore, a sharp-nosed cutting tool is also beneficial to improve the CUCT in the cutting process.

Subsurface Stress Measurement in GIS Epoxy Composite by Using LCR Waves

Internal stress in basin insulators of gas-insulated metal-enclosed switchgear (GIS) can lead to cracks, which affects the safe operation of these apparatuses. In this research, we proposed a subsurface internal stress measurement method for GIS epoxy composites. This method is based on an ultrasonic longitudinal critically refracted (LCR) wave technique. In this study, some epoxy composite specimens were synthesized with similar materials and manufacturing processes to those of 252 kV GIS basin insulators. An ultrasonic stress measurement system that utilized the LCR wave technique was set-up to investigate the relationship between stress and LCR wave propagation time, as well as to measure the compressive stress of the epoxy specimen within 0–50 MPa. The results show that LCR wave propagation time linearly decreased when stress increased in the subsurface zone and the acoustoelastic coefficient was −4.95. We found the relative errors of stress measurements to be less than 13%.

Predictability and controlling factors of overpressure in the North Alpine Foreland Basin, SE Germany: an interdisciplinary post-drill analysis of the Geretsried GEN-1 deep geothermal well

The North Alpine Foreland Basin in SE Germany is Germany’s most active deep geothermal province. However, in its southern and eastern part the basin is considerably overpressured, which is a significant challenge for drilling deep geothermal wells. In this study, we combine drilling data and velocity-based pore pressure analyses with 3D basin modeling to assess the predictability and controlling factors of overpressure in the sub-regional context (area of 80 km × 50 km) around the Geretsried GEN-1 well, a deep geothermal exploration well in the southern part of the North Alpine Foreland Basin in SE Germany. Drilling data and velocity-based pore pressure analyses indicate overpressure maxima in the Lower Oligocene (Rupelian and Schoeneck Formation) and up to mild overpressure in the Upper Oligocene (Chattian) and Upper Cretaceous, except for the hydrostatically pressured northwestern part of the study area. 3D basin modeling calibrated to four hydrocarbon wells surrounding the Geretsried GEN-1 well demonstrates the dominating role of disequilibrium compaction and low permeability units related to overpressure generation in the North Alpine Foreland Basin. However, secondary overpressure generation mechanisms are likely contributing. Also, the impact of Upper Cretaceous shales, which are eroded in the northwestern part of the study area, on overpressure maintenance is investigated. The calibrated basin model is tested against the drilling history and velocity (VSP) data-based pore pressure estimate of the Geretsried GEN-1 well and reveals that pore pressure prediction is generally possible using 3D basin modeling in the North Alpine Foreland Basin, but should be improved with more detailed analysis of lateral drainage systems and facies variations in the future. The results of the study are of relevance to future well planning and drilling as well as to geomechanical modeling of subsurface stresses and deep geothermal production in the North Alpine Foreland Basin.

A method to model crystalline anisotropy in contact using semi-analytical method

In this paper, the contact problem between a cylinder and a half-space with a crystalline anisotropic behavior is solved. The model is based on semi-analytical methods to solve a three dimensional contact problem. A numerical technique based on a Voronoi tessellation is implemented using the Eshelby's equivalent inclusion method to account for the effect of the material microstructure on the contact pressure distribution and subsurface stresses. Fast Fourier Transforms (3D and 2D) are used to reduce the computation cost of the simulations. An application of this method to compute the scatter in fatigue life of rolling element bearings is also presented. Three different critical stresses are used in the Lundberg-Palmgren equation and results are compared in term of Weibull plot slope.

Influence of surface integrity on fatigue life of bearing rings finished by precision hard turning and grinding

Machined surface integrity of bearing rings is often the main concern for the bearing manufacturers as it affects their fatigue life. The present study investigates the surface integrity of AISI 52100 bearing rings finished by precision hard turning and grinding and its influence on fatigue life. A twin-disk machine is used to evaluate the RCF life. As main results, precision hard turning can achieve low surface roughness 0.1 μm Ra with respect to the required 0.2 μm Ra obtained by grinding. Both processes introduce microstructural changes. At subsurface, precision hard turning induces subsurface compressive and maximum residual stresses at 10–50 μm depth, corresponding to the transition zone formed after a thin white layer (<1 μm); however, grinding induces tensile residual stresses from 15 μm depth, corresponding to the bulk material. The residual stresses measured during RCF test (after running-in process and after spalling) of ground ring specimens exhibit peak compressive value in subsurface from 140 μm depth. The ring specimens machined by precision hard turning have fatigue life four times higher than those machined by grinding. This enhancement of RCF life is due to the low roughness reached and also to the subsurface compressive residual stresses at shallow depth before RCF test.

Effects of 3D anisotropic heterogeneous subsurface topology on film thickness, pressure, and subsurface stresses in an elasto-hydrodynamically lubricated point contact

Bearing steel on a sufficiently small scale is strongly heterogeneous and anisotropic. To enable evaluation of the criticality of particular aspects of the microstructure, in this paper an EHL model is solved by the developed multigrid algorithm for a full 3D elastic domain containing varying anisotropic heterogeneous material. Pressure fluctuations and local stress concentrations occur mostly near the boundaries of grains that have large orientation differences. As a consequence, the crystallographic microstructure may have a significant effect on rolling contact fatigue life unless grains are very small relative to the Hertzian contact. However, to the contrary, the influence of crystallographic microstructure on the film thickness distribution under the considered steady state conditions is very small.

A lubricated wear model for determining wear surface geometry on journal-bearing surfaces

In this study, the friction fatigue mechanism is employed to describe the quantitative information concerning characterization of the steady-state wear process in journal bearings, which combined with the load sharing notion in mixed lubrication. For this approach, the asperity stress cycle number is estimated by combining the statistical dynamic contact model with the mixed elasto-hydrodynamic lubrication (EHL) simulation model. The maximum shear stress theory is applied together with the subsurface stress distribution of contact asperities to predict the average volume of wear fragment for asperity. Quantitative analysis of steady-state wear profiles for the bearing reveals that the maximum wear depth appears at both ends of the bearing. With the increase of running time, the wear gradually extends to the bearing centre, and the wear rate rapidly decreases to a certain value and then tends to be stable. With this lubricated wear model, the influences of surface roughness, external load, and shaft speed are also discussed. The proposed wear simulation method is based on the interdisciplinary theory, which includes the description of surface micro-morphology (Gaussian rough surfaces), the friction fatigue theory for explaining the generation of wear debris, and the coupling effect of wear and lubrication that is expressed by considering the wear surface geometry in oil film thickness equation. Additionally, the simulation results and available experiment data are compared to demonstrate the fatigue wear modeling scheme.

Machinability of inconel 718 during turning: Cutting force model considering tool wear, influence on surface integrity

Abstract Machining accuracy can be compromised by elastic workpiece deformation and subsurface residual stress introduction during cutting. In order to anticipate the impact of cutting forces and surface integrity evolutions on finished surface and its geometrical errors, it is necessary to better understand the influence of cutting conditions and tool wear. In this study, machinability of Inconel 718 using a round carbide tool in finish turning conditions is assessed. Cutting forces evolution during tool life are analysed and accompanied by advanced investigations of cutting phenomena. An original mechanistic cutting force model is developed, identified and tested. It includes the effect of tool wear over time in its local formulation. This model allows predicting cutting forces evolution along tool pass for a wide range of finishing cutting conditions. Furthermore, a thorough analysis of residual stress profiles at different tool wear levels is led. It features quantitative results for fresh and worn tools. A study on the influence of cutting parameters and tool wear on residual stress profiles in the machining affected zone is highlighted.

Microstructure, microhardness and residual stress of laser additive manufactured CoCrFeMnNi high-entropy alloy subjected to laser shock peening

Abstract Laser shock peening (LSP) is a surface modification technology to synthesize nanostructures on the surface layer of materials, thereby improving mechanical performances. In this work, a laser additive manufactured CoCrFeMnNi high-entropy alloy (HEA) is processed using LSP. The microstructure evolution during LSP is investigated via electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) observations. The micro-hardness distributions on cross-sections and subsurface residual stress states as functions of the laser energy are also determined. Results indicate that the LSP results in a thick hardened layer with high microhardness, and transforming the tensile stress in the subsurface into compressive stress. The microstructure apparently refines because of the formation of nanoscale grains on the surface after LSP. A novel grain refinement mechanism under ultra-high plastic strain is proposed. Surface strengthening modes are proposed to describe the relationship between strengthening behaviors and microstructure characteristics, thereby revealing the strengthening mechanism.

Offset dependence of overburden time‐shifts from ultrasonic data

ABSTRACTDepletion or injection into a reservoir implies stress changes and strains in the reservoir and its surroundings. This may lead to measurable time‐shifts for seismic waves propagating in the subsurface. To better understand the offset dependence of time‐shifts in the overburden, we have systematically quantified the time‐shifts of three different overburden shales in controlled laboratory tests. These experiments may be viewed as an analogue to the time‐shifts recorded from seismic field surveys. For a range of different stress paths, defined as the ratio between the horizontal and the vertical stress changes, the changes of the P‐wave velocities in different directions were measured such that the offset dependence of time‐shifts for different stress paths could be studied. The time‐shifts are stress path dependent, which is particularly pronounced at large offsets. For all stress paths, the time‐shifts exhibit a linearly decreasing trend with increasing offset, that is, a negative offset gradient. At zero offset, for which the ray path is normal to the bedding, the time‐shifts are similar for all investigated stress paths. The isotropic stress path is associated with the smallest offset gradient of the time‐shifts. In contrast, the constant‐mean‐stress path shows the largest gradient with a flip in the polarity of the time‐shifts for the largest offsets. The separate contributions from the strain and velocity changes to the time‐shifts were also quantified. The time‐shifts for the isotropic stress path are dominated by the contribution from velocity changes at all offsets. In contrast, the strain contributes significantly to the time‐shifts at small offsets for the constant‐mean‐stress path. This shows that the offset dependence in pre‐stack seismic data may be a key to understand the changes of subsurface stresses, pore pressure and strain upon depletion or injection. To utilize this knowledge from laboratory experiments, calibrated rock physics models and correlations are needed to constrain the seismic time‐shifts and to obtain an adequately updated geological model reflecting the true anisotropic nature of the subsurface. This may have important implications for improved recovery and safety, particularly in mature fields.

Subsurface stress-field analysis of generating ground helical gear based on finite line-contact mixed elastohydrodynamic lubrication

Tooth surface roughness and lubrication status have significant influence on the contact performance and fatigue life of helical gear pair. Yet, despite the development in elastohydrodynamic lubrication-based contact analysis and solution of subsurface stress field, researches in subsurface stress field of helical gears considering both lubrication and surface roughness are not quite comprehensive. In this study, three-dimensional surface roughness of generating ground gear is measured, a finite line-contact mixed elastohydrodynamic lubrication model is established to perform the contact analysis, and, on this basis, the influence of tooth surface roughness on the subsurface stress field is studied. Results show that compared with the smooth surface, the overall level of subsurface stress is raised; maximum stress values and plastic zones occur in the close vicinity of tooth surface, which adds to the risk of surface failure; within sections in the valley regions of roughness, locations of maximum stresses are generally similar to the smooth surface situation, i.e. in relatively deep zones, while within sections in the peak regions, the majority of locations with maximum stresses shift much closer to the surface; contact pressure and stress status see only mild undulation between different sections distributed along the contact line, but intense changes between sections distributed along the entraining direction.

Friction of stable gradient nano-grained metals

In this paper, a novel strategy for lowering the coefficients of friction (COFs) of metals during dry sliding is introduced by producing a gradient nano-grained (GNG) surface layer without changing chemical compositions or phase constitution. Surface morphology variation, subsurface structural evolution and stress field analysis unfold that sliding-induced strain localization and microstructural instabilities are significantly mitigated through dislocation activity and grain boundary activities in the GNG surface layers. In addition, design guidelines for GNG materials with unprecedented low COFs are discussed based on interlinking tribological stress, surface deformation mechanisms and friction behavior.

Study on logarithmic crowning of cylindrical roller profile considering angular misalignment

The angular misalignment of cylindrical rollers causes the stress edge effect at roller ends to be aggravated, which would affect bearing capacity and fatigue life of bearings. Therefore, based on the roller angular misalignment, the geometric interference model of the cylindrical contact pair was established in this paper. Thereafter, two types of logarithmic crowning models were theoretically deduced, in which design redundancy was considered through special treatment of pre-pressure. Taking the aero-engine main shaft bearing as an example, contact characteristics of the cylindrical roller with these two logarithmic profiles were studied by the DC-FFT method and the conjugate gradient method (CGM). The results show that two profiles can effectively eliminate the stress edge effect, improve contact pressure distribution and subsurface stress field of the roller in misalignment state, and at the same time ensure fine contact characteristics in alignment state. The research can provide a theoretical basis and reference for finite line contact pairs under angular misalignment.

A Coarse Grained Model for Viscoelastic Solids in Discrete Multiphysics Simulations

Viscoelastic bonds intended for Discrete Multiphysics (DMP) models are developed to allow the study of viscoelastic particles with arbitrary shape and mechanical inhomogeneity that are relevant to the pharmaceutical sector and that have not been addressed by the Discrete Element Method (DEM). The model is applied to encapsulate particles with a soft outer shell due, for example, to the partial ingress of moisture. This was validated by the simulation of spherical homogeneous linear elastic and viscoelastic particles. The method is based on forming a particle from an assembly of beads connected by springs or springs and dashpots that allow the sub-surface stress fields to be computed, and hence an accurate description of the gross deformation. It is computationally more expensive than DEM, but could be used to define more effective interaction laws.

Introduction to special section: Characterization of the subsurface stress, deformation, and pore pressure

Introduction to special section: Characterization of the subsurface stress, deformation, and pore pressure

Prognozowanie trwałości zmęczeniowej promieniowego łożyska walcowego poddanego złożonemu obciążeniu z wykorzystaniem MES

The article presents the results of studies on the impact of a combined load of a radial cylindrical roller bearing for its predicted fatigue life. The distributions of maximum equivalent subsurface stresses and their depths, necessary during calculations of fatigue life, were determined using the finite element method, using the basic package of the ANSYS program. The calculations took into account the geometrical parameters of the bearing, including radial clearance and the shape of the rolling elements generators. The calculation results showed that the axial load of the radial cylindrical roller bearing and the tilt of the rollers associated with its operation reduces fatigue life. The obtained results were compared with the results of calculations according to the SKF catalogue method, obtaining satisfactory compliance.