Subsurface Stress Research Articles

Mechanistic driving force for martensite decay in rolling contact fatigue

Martensite decay due to large scale microstructural alterations is the main fatigue process which occurs during extensive rolling contact fatigue in bearing applications. Despite its practical and scientific importance, the full description of the controlling phenomena is still incomplete. Here, we demonstrate that the main mechanistic driving force for martensite decay comes from the positive components of the principal subsurface stress tensor. Such a conclusion is derived using the concept of Continuum Damage Mechanics where the damage rate is controlled by the elastic energy release and plastic strain range. Modeling results are compared with experiments and a good quantitative agreement is obtained. The analysis presented here illuminates the mechanistic reasons on why the fastest material decay occurs at the depth corresponding to the so-called maximum orthogonal shear stress.

A study on the rolling contact fatigue mechanism of ZC-L cast wheel steel

In this paper, the rolling contact fatigue (RCF) failure mechanism of ZC-L cast steel wheels was analyzed by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that obvious fatigue flaking is formed on the macroscopic surface of the wheel tread after RCF failure. The RCF cracks are formed at the maximum shear stress region of the sun-surface of the wheel (1–5 mm below the tread). There are a lot of subsurface white etching microstructure (WEM) and obvious casting defects in the subsurface of the wheel tread. The RCF cracks are mainly formed at the interface between the subsurface WEM and the matrix and at the edge of the casting defects. The subsurface WEM is composed of nano-ferrite grains and residual cementite, and its hardness is about 920HV, which is much higher than that of the matrix structure. During the wear process, the plastic deformation of the subsurface WEM and the matrix is not coordinated, resulting in the formation of RCF cracks. As a hard phase, the hardness of casting defects is much higher than that of matrix. In the process of wear, under the action of subsurface stress, the casting defects can cause obvious plastic deformation in the soft orientation region at the edge of the casting defects. When the plastic deformation reaches the plastic limit, the RCF cracks are formed at the edge of the inclusions due to ratchet failure. The stress at the crack tip is caused by the RCF crack propagation at the edge of inclusions will further lead to the refinement of the microstructure at the edge of the RCF crack. Under the action of the subsurface contact stress, the subsurface WEM and casting defects are the main reasons leading to the RCF failure of the ZC-L casting wheel. Improving the microstructure uniformity and reducing casting defects are the main methods to improve the fatigue property of casting wheels.

Determining Stress in Metallic Conducting Layers of Microelectronics Devices Using High Resolution Electron Backscatter Diffraction and Finite Element Analysis.

Delayed failure due to stress voiding is a concern with some aging microelectronics, as these voids can grow large enough to cause an open circuit. Local measurements of stress in the metallic layers are crucial to understanding and predicting this failure, but such measurements are complicated by the fact that exposing the aluminum conducting lines will relieve most of their stress. In this study, we instead mechanically thin the device substrate and measure distortions on the thinned surface using high resolution electron backscatter diffraction (HREBSD). These measurements are then related to the stresses in the metallic layers through elastic simulations. This study found that in legacy components that had no obvious voids, the stresses were comparable to the theoretical stresses at the time of manufacture (≈300 MPa). Distortion fields in the substrate were also determined around known voids, which may be directly compared to stress voiding models. The technique presented here for stress determination, HREBSD coupled with finite element analysis to infer subsurface stresses, is a valuable tool for assessing failure in layered microelectronics devices.

Subsurface Stresses in Flat-Punch Contact Problems Including Friction

This work analyzes the influence of friction on the surface and subsurface contact stresses generated—in an elastic half-space—by a cylindrical flat-ended punch. For this purpose, the computational framework presented by [J. M. Juliá and L. Rodríguez-Tembleque, Subsurface stress evolution under orthotropic wear and frictional contact conditions, Int. J. Mech. Sci. 234 (2022) 107695] is extended to solve this problem. After the validation of this computational scheme, several numerical studies have allowed us to obtain the following conclusions. (i) The location of the maximum values of the subsurface Von Mises stresses in the [Formula: see text]–[Formula: see text] plane does not depend on the friction coefficient and is located at the contact zone limits — similarly to the normal pressure — (ii) However, the [Formula: see text]-location and the magnitude of the maximum value of the subsurface Von Mises stress is clearly affected by the values of the friction coefficient. (iii) The magnitude of the maximum value of the subsurface Von Mises stress increases and its location can be shifted from the subsurface region to the solid surface, when the friction coefficient increases its value.

Implementation of Gaussian Process Regression to strain data in residual stress measurements by hole drilling

Incremental hole drilling is a prominent tool in the surface and sub-surface residual stress measurements. Generally, strains are measured at drilling steps not corresponding to the stress calculation steps of the integral method. Consequently, strain measurements are fitted using polynomials or splines. However, this process is greatly influenced by user’s experience, affecting the calculated stresses as an additional source of uncertainty. The present work assesses the uncertainty associated with the strain fitting procedure by exploiting Gaussian Process Regression (GPR), a probabilistic machine learning framework which can yield uncertainties on the fit. These uncertainties were propagated to residual stresses by using Monte Carlo simulation. Laser shock peened AA 7050-T7451 specimens were considered as case study.The proposed methodology was corroborated by comparing residual stress results with those obtained exploiting traditional fitting procedures and X-ray diffraction measurements. Finally, the GPR-based approach demonstrated its ability to successfully discriminate different strain signal sources.

The influence of surface topography on mixed plasto-elastohydrodynamic lubrication in point contacts

Surface topography plays an important role in mixed lubrication. However, the influence of the surface topography's morphological characteristics on lubrication performance is still not fully understood. In this article, we employ digital filtering and the Johnson transformation system, and we use a mixed plasto-elastohydrodynamic lubrication model to simulate the lubricated contact between rough spherical surfaces with different morphological characteristic parameters. The influence of skewness, kurtosis, and the wavelength factor, that is, the ratio between the autocorrelation lengths in the transversal and longitudinal directions, on contact pressure, film thickness, and von Mises (subsurface) stress, is examined with the mixed plasto-elastohydrodynamic lubrication model. The results confirm that plastic deformation leads to lower pressure distribution, distributed over a larger contact area than the pure elastohydrodynamic lubrication solution. Moreover, it is found that an increase in the kurtosis may improve the lubrication performance, while the opposite is true for the skewness and the wavelength factor. We believe that the insights obtained by means of the numerical simulations presented herein, would facilitate the design and manufacturing processes of the surfaces used for machine elements operating in the mixed plasto-elastohydrodynamic lubrication regime.

2D and 3D numerical investigation on contact stress behaviour on different combinations of gear pair

Polyamide (PA6) polymer gear tremendously expanded its usage in power transmission applications. PA6 gear were engaged with different combinations of gear pairs (STEEL – PA6, PA6 – STEEL, and PA6 – PA6) were utilized in those applications. Normally gears are subjected to Hertzian contact stress due to their non-conformal contact and ANSYS is one of the widely used Finite Element Analysis (FEA) tool to determine the contact stress behaviour of those gears. Even though numerous 2D and 3D FEA investigation were performed to determine the gear contact stress behaviour, the occurrence of surface and subsurface stresses for different combination of gear pair was not explored especially for PA6 gears. Thus, the work attempted to study the surface and subsurface contact stress behaviour of three combination of gear pairs (STEEL – PA6, PA6 – STEEL, and PA6 – PA6) using 2D (plane stress with thickness condition) and 3D FE analysis (ANSYS APDL). The optimum mesh sizes were selected from the convergence study and further simulations were performed. From the analysis, the stress was observed maximum at the contact pitch point for 2D analysis, whereas, 3D analysis showed the maximum stress at a depth below the contact surface (subsurface stress) for all the combinations of gear pairs. Different trend pattern was obtained between the stress plot of 2D (surface stress) and 3D (subsurface stress) analysis for all the gear pairs. The variation in depth occurrence of maximum stress was observed for Steel-PA6 (0.16 mm depth), PA6-Steel (0.16 mm depth) and PA6-PA6 (0.25 mm depth) gear pairs. And, the Polymer – Polymer (PA6-PA6) gear pair exhibited higher depth occurrence compared to Steel – PA6 and PA6 – Steel gear pairs.

Numeric simulation of steel twin disc system under rolling-sliding contact

Various mechanical parts come into high-load rolling and sliding contact at the contact surface. Even with technological advancements, mechanical failures still occur. Rolling-sliding mechanical contact issues are the primary cause of over 90 % of surface and subsurface metallic failures and they are only becoming worse. Using discretised continuum 2D finite element methods (FEM), this research investigates the parametric contact effect of a steel twin disc system subjected to rolling-sliding contact at varied surface friction and comprehensive load condition. The equivalent von Mises stress distribution, contact pressure distribution and shift position of maximum subsurface stress on the contour region are all influenced by changes in compressive load and coefficient of friction, according to a numerical surface-to-surface contact simulation performed with Abaqus at mean Hertzian pressure. The maximal equivalent stress, for a given load, reaches a peak in the subsurface and moves farther away from the surface when the coefficient of friction decreases and comes close to the contact surface when the coefficient of friction increases. Consistency is shown by the analytical and numerical results.

Sub-Surface Analysis of Grinding Burns with Barkhausen Noise Measurements

Barkhausen noise (BN) measurements are commonly used for surface characterisation. However, often there is also a need to verify the sub-surface region because detrimental tensile stresses may be present after different manufacturing steps. Especially in a grinding burn, the surface stress may be compressive, but it changes quickly into tensile stress below the surface. The aim of this study was to find out whether regular surface-sensitive BN measurement is also sensitive to the stresses below the surface caused by grinding burns. More specifically, the aim was to study the relationship between BN features and sub-surface stresses and to identify a model that estimates sub-surface stresses. Real samples were collected from an actual process. The samples were cylindrical samples manufactured from commercial alloyed AISI/SAE L6 steel that was through-hardened prior to grinding. Barkhausen noise measurements were carried out for 42 grinding burn locations followed by X-ray diffraction-based residual stress surface measurements and residual stress depth profiles. Depth information was obtained through step-by-step electrolytic removal of thin layers. The stress profiles were pre-processed through interpolation and averaged stress was calculated as a function of depth below the surface. Correlation analysis was carried out to evaluate the relationships between BN features and stress at different depths and among BN features. The main outcome of the analysis was that BN measurement is dominated by the sub-surface tensile stresses rather than the compressive stress at the surface. It was also noticed that BN features form two groups, corresponding to average Barkhausen activity and magnetising field strength leading to maximum Barkhausen activity. Models for stress at different steps were identified systematically. The performance of the models for sub-surface stresses was reasonable with R2 values of around 0.85 and root mean squared error (RMSE) values of around 95 MPa. Based on the results, it is concluded that BN measurement provides information about sub-surface stresses and that stress can be evaluated through straightforward modelling, allowing fast detection of grinding burns.

Subsurface Lateral Flows Buffer Riparian Water Stress Against Snow Drought

AbstractIn the Sierra Nevada, CA, the Mediterranean climate exposes montane forests to water stress during the summer drought. Normally, spring snowmelt alleviates summer water stress, especially in riparian ecosystems that receive subsurface lateral inputs from groundwater. However, snow drought could potentially eliminate these beneficial effects. This research investigates how subsurface lateral redistribution mediates hillslope‐scale vegetation responses to snow drought. We apply a spatially‐distributed ecohydrologic model (Regional Hydro‐Ecologic Simulation System (RHESSys)) to a snow‐dominated watershed in the Sierra Nevada. We incorporate observed sap flow data along a hillslope to estimate relative differences in water stress for upslope and riparian sites, and constrain RHESSys drainage parameter uncertainty. Our model results show that subsurface lateral inputs buffer riparian water stress against snow drought. For all drought types, both upslope and riparian sites experience substantial losses of net primary productivity (NPP), and on average upslope sites are more adversely affected (upslope loss of NPP = 45% vs. riparian = 28%). Dry droughts from a lack of rain or snow induce substantial loss of biomass for upslope and riparian trees because of low annual precipitation, but a small snowpack retained during cold, dry years slightly alleviates these effects and delays water stress by 2 weeks in riparian areas and 3 weeks upslope. While riparian forests are often buffered against drought stress, our study also shows that riparian forests can suffer from water stress. We found 21% of drought scenarios where riparian trees experience greater water stress than upslope trees that were mostly associated with wet and warm snow droughts.

Influence of laser shock peening on the residual stresses in additively manufactured 316L by Laser Powder Bed Fusion: A combined experimental–numerical study

Detrimental subsurface tensile residual stresses occur in laser powder bed fusion (LPBF) due to significant temperature gradients during the process. Besides heat treatments, laser shock peening (LSP) is a promising technology for tailoring residual stress profiles of additively manufactured components. A multi step process simulation is applied aiming at predicting the residual stress state after applying LSP to a cuboid shaped specimen manufactured by LPBF in two different building directions as well as comparing it with a post-build heat treatment. The validity of the numerical simulation is evaluated based on comparisons of residual stresses determined by incremental hole drilling technique within different stages of the multi step process: in the as-build condition, after subsequent heat treatment as well as after applying LSP to the as-build and heat treated specimens, showing overall a good experimental-numerical agreement throughout each of the process stages. Applying a heat treatment to the as-build LPBF sample at 700 °C for 6 h showed not to be effective in eliminating the surface tensile stress entirely, reducing the tensile residual stresses by 40%. However, the application of LSP on LPBF components showed promising results: LSP was able even to convert the detrimental near surface tensile residual stresses in the LPBF component into compressive residual stresses next to the surface, which is known to be beneficial for the fatigue performance.

Explicit Analytic Solutions for the Subsurface Stress Field in Single Plane Contacts of Elastically Similar Truncated Cylinders or Wedges

As has been pointed out recently, a possible solution strategy to the wear–fatigue dilemma in fretting, operating on the level of contact mechanics and profile geometries, can be the introduction of “soft” sharp edges to the contact profiles, for example, by truncating an originally smooth profile. In that regard, analysis of possible mechanical failure of a structure, due to the contact interaction, requires the knowledge of the full subsurface stress state resulting from the contact loading. In the present manuscript, a closed-form exact solution for the subsurface stress state is given for the frictional contact of elastically similar truncated cylinders or wedges, within the framework of the half-plane approximation and a local-global Amontons–Coulomb friction law. Moreover, a fast and robust semi-analytical method, based on the appropriate superposition of solutions for parabolic contact, is proposed for the determination of the subsurface stress fields in frictional plane contacts with more complex profile geometries, and compared with the exact solution. Based on the analytical solution, periodic tangential loading of a truncated cylinder is considered in detail, and important scalar characteristics of the stress state, like the von-Mises equivalent stress, maximum shear stress, and the largest principal stress, are determined. Positive (i.e., tensile) principal stresses only exist in the vicinity of the contact edge, away from the pressure singularity at the edge of the profile, and away from the maxima of the von-Mises equivalent stress, or the maximum shear stress. Therefore, the fretting contact should not be prone to fatigue crack initiation.

Correlation Between Cutting Force and Residual Stress in Dry End-Milling of Inconel HX

Residual stress in the end-milled subsurface can significantly affect the fatigue performance of end-milled material. In the end-milling process, the generation of residual stress is extremely complex, which is closely related to the spindle speed, feed per tooth and cutting force. Thus, it is crucial to elucidate the influence of spindle speed and feed per tooth on cutting force and residual stress, also the correlation between cutting force and residual stress in terms of spindle speed and feed per tooth. According to this, dry end-milling of Inconel HX was performed by climb-milling using Kennametal KYS40 solid ceramic end-mill. From this experimental test, cutting force and residual stress showed a U-shaped relationship with the increase of spindle speed, while cutting force and residual stress showed a linear relationship with the increase of feed per tooth. Furthermore, for low cutting force and residual stress, the focus should be on choosing the optimum combination of spindle speed (21,400 to 24,100 rpm) and feed per tooth (0.014 to 0.016 mm/tooth).

Surface smoothening and strengthening combined effect of mechanical hammer peening on inconel 718 superalloy

ABSTRACT In this study, the surface strengthening and smoothening combined effect of the mechanical hammer peening (MHP) treatment is demonstrated. On a IN718 nickel-based superalloy workpiece surface, the MHP treatment introduced a subsurface residual stress field featured with a maximum stress around −1.4 GPa and a penetration depth exceeding 1 mm, and an increase of the micro-hardness from 286.43 HV to 426.27 HV. Simultaneously, it produced a significantly smoothened surface (Ra 27 nm) compared with the untreated sample (Ra 555 nm). Moreover, applying an electric pulse of 100 A would further reduce the surface roughness, without deteriorating the surface strengthening effect. The surface strengthening effect of MHP treatment mainly originates from the increase of dislocation density, grain misorientation and refinement, which are caused by impacts of the punching head; while the surface smoothening effect is the result of precisely controlled and regularly aligned punching indents. Utilizing MHP in the manufacturing process chain of aircraft parts could combine the surface strengthening and smoothening processes into one single process, which substantially improves both the production efficiency and enhances the surface finish. Especially, the working principle of MHP makes it suitable for arbitrary or complicated surfaces, that is, aeroengine blades.

Soil moisture-vegetation interaction from near-global in-situ soil moisture measurements

Although the interactions between soil moisture (SM) and vegetation dynamics have been extensively investigated, most of previous findings are derived from satellite-observed and/or model-simulated SM data, which inevitably include multiple sources of error. With the effort of many field workers and researchers in in-situ SM measurement and SM data integration, it is now possible to obtain the integrated in-situ SM dataset in the global range. Here we used the in-situ SM dataset of the International Soil Moisture Network to analyze the anomaly correlation between SM and leaf area index (LAI). We found that positive (negative) correlations exist between SM (LAI) and temporally lagged LAI (SM). The peak correlation and lagging time to reach it (often less than 3 months) depends on climate, land cover and rooting depths. The high SM-LAI anomaly correlation prevails in water-limited regions, e.g. dryland, where plant physiology has strong sensitivity to subsurface water stress. Dynamics of vegetation with deeper maximum rooting depths are not always correlated with SM in deeper soil layers, and vegetation dynamics with shallower maximum rooting depth may strongly correlate with SM in deeper soil layers. Overall, we highlight the potential of the global in-situ SM observation network to analyze the interactions between SM and vegetation dynamics.

Angular Contact Ball Bearing Modeling with Different Types of Coatings

The purpose of this study is to determine the stress distribution of uncoated and coated ball bearings by using finite element analysis. The coatings used in this study are titanium nitride (TiN), titanium carbide (TiC) and chromium nitride (CrN) with a thickness of 5 microns. A contact analysis has been performed on ball bearings to compare the performance between coated and uncoated ball bearings. Boundary loads of 5000 N is used for contact analysis. This study tries to establish a simple, two-dimensional expression for the elastic deformation with the inner ring and ball bearing as a angular or curvature model in terms of the geometry of the coating contact surfaces. The coating of ball and raceway surfaces is a requisite but difficult factor to be determined during design, so it is desirable for engineering to understand the effect of surface coating on the motion of ball and subsurface stresses in bearing. For contact analysis, the maximum contact pressure and maximum stress on the coating, inner ring and ball bearing have been used for comparison. The results of this study show that, among other coatings, TiC provides the best protection for the ball of the ball bearing. This is because the low Poisson's ratio of TiC in other coated ball bearings helps reduce the stress on the ball bearing, even though TiC has the lowest Young's modulus in the coating. When a lower boundary load is applied, high COF will also cause an abnormal increase in the maximum stress on the contact surface between the coated or uncoated ball and the inner ring of the ball bearing.

Machining-induced residual stress analysis and multi-objective optimization for milling process of Mg–Li alloy

Machining-induced residual stress in an Mg–Li alloy can lead to a significant deformation of its machined thin-walled parts. To solve this issue, an integrated method that combined the grey relational analysis (GRA), back propagation (BP) neural network, and non-dominated sorting genetic algorithm-III (NSGA-III) was proposed in this study aimed to analyze the dominant mechanism of residual stress and achieve a super-objective cooperative optimization of residual stress in the LA103Z Mg–Li alloy milling process. A typical spoon-shaped distribution of surface tensile stress and subsurface compressive stress was presented in the Mg–Li alloy owing to the coupling interaction between the cutting force and temperature. The increase in the cutting temperature owing to an increase in the cutting speed was proven to be the overarching factor that induced residual stress by the correlation analysis of cutting parameters, force–heat of cut and residual stress. The BP prediction models of the residual stress were established, and the predicted values were in good agreement with the test values, with a maximum error of 5.07 %. The NSGA-III optimization algorithm was used to search for the lowest values of in-plane tensile and compressive stresses (σtx, σty, σcx and σcy). The optimized cutting speed, feed, and cutting depth were determined to be 61 m/min, 0.068 mm/z, and 0.58 mm, respectively. The maximum predictive error of four residual stresses was experimentally verified to be 15.94 %, which proved the feasibility of the multi-objective optimization approach. The residual stress level of the optimized scheme evidently decreased compared with that of the current milling scheme, with the maximum reduction of 28.37 %. Therefore, the propsoed approach can effectively reduce the risk of warping and distortion of machined Mg–Li alloy parts.

A comprehensive review of fatigue behavior of laser shock peened metallic materials

Laser shock peening (LSP) is widely used in manufacturing industry due to its contribution to the improvement of mechanical performance for metallic components. In practical applications, LSP processing that introduce compressive residual stress and finer grain in surface and subsurface, which affected the fatigue behaviors of metallic materials. Appropriate design of the processing parameters can significantly improve the fatigue performance. The fatigue of peened metallic materials is thus a topic of research and practical interest. In this review, a brief introduction on LSP and its applications is first provided. Then experimental studies on fatigue behaviors of LSP metallic materials are summarized with an emphasis on the effects of residual stress and microstructure on the fatigue improvements and fatigue failure mechanisms. Finally, recent progress in simulation studies on fatigue and fracture of LSP metallic materials is presented. The knowledge derived from this paper will provide insights and guidance to the applications of LSP.

XFEM rolling contact fatigue crack propagation in railways considering creepages on contact shear stresses

In this work the influence of considering wheel-rail contact creepages on fatigue crack growth rates due to Rolling Contact Fatigue(RCF) is studied. For this purpose, the FASTSIM algorithm, which considers the moving complex pressure distribution with slipping and adhesion zones of the wheel-rail contact patch, has been implemented in Abaqus using FORTRAN code subroutines. The developed methodology has been validated with a 60E1 rail profile model which uses XFEM, by comparing the obtained Stress Intensity Factors(SIFs) and sub-surface shear stresses with numerical results available in the literature. Finally, the RCF crack propagation analysis of a 60E1 rail profile with different contact conditions has been performed using the XFEM. The obtained results justify the necessity of considering contact creepages on contact shear stresses for crack growth analysis.

Performance Analysis of 7075 Aluminum Alloy Strengthened by Cavitation Water Jet Peening at Different Scanning Speeds

Shot peening technologies can improve the performance of materials and extend the service life of parts. Cavitation water jet peening (CWJP) is a new shot peening technology that uses the shear effect of a high-speed jet in a submerged environment to produce a cluster of bubbles, thereby achieving a significant increase in impact performance. In this paper, CWJP was used to strengthen the surface of the 7075 aluminum alloy (Al7075), and the effects of CWJP with different scanning speeds on its microstructure and mechanical properties were investigated. The morphology evolution revealed that the plastic deformation of the sample surface became more serious and the surface roughness increased with the decrease in scanning speed. The distribution of residual stress and microhardness along the depth direction of the Al7075 was measured before and after the CWJP impact, and it was found that CWJP causes work hardening on the surface of the Al7075 and can convert the subsurface tensile stress of the Al7075 into compressive stress. The mechanism of grain refinement of the Al7075 during plastic deformation was systematically revealed. Therefore, CWJP can effectively improve the surface properties of the 7075 aluminum alloy.