Effects Of Surface Residual Stress Research Articles

Correlation of Lack of Fusion Pores with Stress Corrosion Cracking Susceptibility of L-PBF 316L: Effect of Surface Residual Stresses.

Stress corrosion cracking (SCC) of laser powder bed fusion-fabricated 316L was studied under the variation in energy input density to emulate the existence of distinctive types of defects. Various electrochemical polarization measurements were performed in as-received polished and ground states, to elucidate the effect of defect type on corrosion and SCC behaviour in marine solution. The results revealed severe localized corrosion attack and SCC initiation for specimens with a lack of fusion pores (LOF). Moreover, the morphology of SCC was different, highlighting a more dominant effect of selective dissolution of the subgrain matrix for gas porosities and a more pronounced effect of brittle fracture at laser track boundaries for the specimens with LOF pores.

Surface Bending Resistance in Architected Nanoporous Metallic Materials

AbstractFinite element method (FEM) is considered as a powerful tool for predicting the mechanical behavior of complex structures. However, the commercially available numerical packages based on FEM are mainly limited to the evaluation of multiphysical properties at the continuum scale and are unable to accurately evaluate the response of nanomaterials since the dominant surface effects in nanoscale analysis are overlooked. In this study, our introduced numerical methodology not only incorporates the effects of surface residual stress and surface tensile stiffness based on the Gurtin–Murdoch surface elasticity but also takes into account the bending stiffness of nanosurfaces in the numerical analysis. The computational results reveal that the stress concentration in nanoporous metallic materials is affected by the void geometry and is enhanced by the surface bending stiffness. In addition, the effect of void geometrical parameters on the elastic properties of nanoporous metallic metamaterials with negative Poisson's ratio is studied and the mechanism of surface tensile/bending stiffness is revealed in detail. The results show that the surface bending stiffness increases the effective Young's modulus of nanoarchitected metallic materials with negative Poisson's ratio and randomly distributed nanopores.

On the exceptional stress corrosion cracking susceptibility of selective laser melted 316L stainless steel under the individual effect of surface residual stresses

SCC and electrochemical behaviour of selective laser melted 316L stainless steel was studied under the combined effect of surface residual stresses (RS) and energy input variation. A significant correlation was found between columnar subgrains width and pore size with the electrochemical response and SCC susceptibility. There existed a critical pore size affecting local corrosion behaviour which decreased with an increase in RS magnitude. The critical RS needed for SCC occurrence was lower compared to wrought counterpart and the SCC mechanism was hypothesized as the synergistic effect of exceptional selective corrosion attack at subgrains and mechanical rupture of subgrain boundaries.

Simulation and experimental study on effect of surface residual stresses in dry orthogonal micro turning sample

ABSTRACT Turning is a drastically favoured machining process for producing cylindrical components with required dimensions. Prediction of chip formation is essential when damages caused by chips are into consideration during machining. A high amount of residual stresses are generated in the machined component due to a large number of forces and temperature distribution during machining. The scope of this paper is to investigate the percentage of residual stresses induced in a component during the dry micro turning process. Hence, in this work, Inconel 718, A2024-T351, and AISI D2 (62HRC) are selected as a workpiece materials and WC as tool material, simultaneously comparison studies of chip morphology have been carried out. Also, cutting speed and power consumption were investigated experimentally by opting varied tool rake angles (positive, negative, and zero) and a depth of cut. Residual stresses, nose wear, and chip morphology were found experimentally while machining Inconel 718. From the simulation and experimental results obtained, one can adopt a suitable tool rake angle to minimise power consumption during the micro turning process which leads to a sustainable machining process.

Effect of surface residual stress and surface layer stiffness on mechanical properties of nanowires

Effect of surface residual stress and surface layer stiffness on mechanical properties of nanowires

Effect of innovative finishing operations on the tribological performance of steel 27MnCr5

Transmission shafts used in the automotive sector must have a good tribological performance, and therefore an enhanced surface integrity. This paper aims to study the effect of eco-friendly innovative finishing operations (belt finishing, cryogenic grinding and dry grinding) on the surface integrity and tribological performance of steel grade 27MnCr5, and compare to the behavior of components produced by conventional wet grinding process. For that purpose, a total of seven finishing conditions were analysed: wet grinding as reference, two dry grinding conditions, two cryogenic grinding conditions and two hard turning+belt finishing conditions. The surface integrity (roughness, residual stresses, hardness and microstructural defects) of samples was assessed. Finally, the step-loading test method was used to determine the scuffing resistance of the samples. Tested innovative finishing operations led to higher scuffing resistance than conventional wet grinding. Results demonstrate that higher surface hardness and roughness leads to higher scuffing resistance, while the effect of surface residual stresses is not significant.

Effect of a large population of seeded alumina inclusions on crack initiation and small crack fatigue crack growth in Udimet 720 nickel-base disk superalloy

Crack initiation, crack coalescence and small crack growth behavior were monitored for over 400 seeded inclusions during interrupted low cycle fatigue testing conducted on the P/M Udimet 720 nickel disk alloy at 650 °C. Two types of seeded alumina inclusions with average sizes of 54 µm and 122 µm were used in the study performed at varying loading conditions resulting in LCF lives ranging from 2,000 cycles to over 1,000,000 cycles. The fatigue behavior was sub-categorized into four groups. Visual maps detailing inclusion size/cycle history were developed. The effect of surface residual stresses on the fatigue life was also investigated.

Surface Effects on Effective Young’s Modulus of Nanoporous Structures

Nanoporous materials and structures have attracted widespread attention due to their excellent mechanical properties. Based on the surface elasticity, the effective Young’s moduli are derived for four typical nanoporous structures with periodic unit cells. When the cross-sectional size reduces to nanoscale, the effective Young’s modulus is revealed to be strongly size-dependent. Both the effects of residual surface stress and effective-surface Young’s modulus are examined. The results indicate that negative effective Young’s modulus can be achieved when the residual surface stress is less than zero. The influences of the cross-sectional shape on the relationship between the overall deformation and applied loads are examined. The relative density also plays an important role to the mechanical characteristics not only at macroscales, but also at nanoscales.

On the surface elastic-based shear buckling characteristics of functionally graded composite skew nanoplates

Abstract The prime objective of the present investigation is to predict the shear buckling characteristics of skew nanoplates made of a functionally graded material (FGM) in the presence of surface stress effect. For this purpose, the Gurtin-Murdoch surface theory of elasticity is applied to the higher-order shear deformation plate theory within the framework of the oblique coordinate system. Different types of the homogenization scheme including Reuss model, Voigt model, Mori-Tanaka model, and Hashin-Shtrikman bounds model are taken into consideration in order to extract the effective mechanical properties of FGM skew nanoplates. The Ritz method using Gram-Schmidt shape functions is utilized to obtain the surface elastic-based shear buckling loads of FGM skew nanoplates. It is indicated that by increasing the value of the index associated with the material property gradient, the significance of the surface stress type of size effect on the shear buckling behavior of a FGM skew nanoplate improves. Moreover, by changing the boundary conditions from simply supported ones to clamped ones, the influence of the skew angle on the surface elastic-based shear buckling load of a FGM skew nanoplate increases. Also, it is illustrated that by increasing the width to thickness ratio of a skew nanoplate, the free surface area increases which results in to enhance the effect of surface residual stress on its shear buckling characteristics.

Effect of residual surface stress on parametrically excited nonlinear dynamics and instability of double-walled nanobeams: an analytical study

A class of nonlinear Mathieu–Hill equation is established to determine the bifurcations and the regions of nonlinear dynamic instability of a short double-walled nanobeam, while the emphasis is placed on investigating the effect of residual surface stress on instability. To achieve this goal, first, a short double-walled nanobeam is modeled and embedded on a viscoelastic foundation and subjected to an axial parametric force. Second, based on the nonlocal elasticity and nonlinear von Karman beam theories, the nonlinear governing equation of motion is derived. Finally, Galerkin technique and multiple time scales method are used to solve the equation. Numerical examples are treated which show various discontinuous bifurcations. Also, infinitely stable and unstable solutions are addressed.

Effect of residual surface stress on parametrically excited nonlinear dynamics and instability of viscoelastic piezoelectric nanoelectromechanical resonators

The present study mainly investigates the effect of the residual surface stress and the applied electric voltage on the nonlinear dynamic instability of the viscoelastic piezoelectric nanoresonators under parametric excitation. In fact, great attention is given to the influence of the residual surface stress on the nonlinear instability of the system. Numerical examples are treated which show various bifurcations. By means of a bifurcation analysis, it is shown that the instability of the system can be significantly affected by considering the residual surface effect. The results also show that a discontinuous bifurcation is always accompanied by a jump. Finally, stable and unstable regions in dynamic instability of viscoelastic piezoelectric nanoplates are addressed.

Nonlinear dynamics and stability of viscoelastic nanoplates considering residual surface stress and surface elasticity effects: a parametric excitation analysis

The present study mainly investigates surface effect on nonlinear dynamic instability of viscoelastic nanoplates under parametric excitation. In fact, great attention is given to the influence of residual surface stress on nonlinear dynamic behavior of the system. To achieve this goal, the governing equation of motion is derived by modeling a nanoplate embedded on a visco-Pasternak foundation and then, applying surface effect relations, nonlocal elasticity and nonlinear von Karman theories and Hamilton’s principle, respectively. Galerkin technique and multiple time scales method are also used to solve the equation. A class of nonlinear Mathieu–Hill equation is established to determine the bifurcations and the regions of nonlinear dynamic instability. The numerical results are performed, while the emphasis is placed on investigating the effect of residual surface stress, visco-Pasternak foundation coefficients, and parametric excitation. It is shown how residual surface stress leads to high values of amplitude response. Finally, stable and unstable regions in dynamic instability of viscoelastic nanoplates are addressed.

Static analysis of flexoelectric nanobeams incorporating surface effects using element free Galerkin method

The present study addresses the static bending analysis of a dielectric nanobeam subjected to external electro-mechanical loads, considering the flexoelectric coupling, surface elasticity (SE) and surface residual stresses (SRS). In the current model, surface energies corresponding to the surface elastic strains and a traction jump across surfaces, based on the generalized Youngs-Laplace model, have been included. The numerical analysis is carried out by deriving the element free Galerkin (EFG) model for the dielectric beam based on a moving least squares (MLS) interpolation of the field variables. The effect of SE and SRS over the flexoelectric behaviour of the dielectric nanobeam are thoroughly examined here. The contrary effects of SE over the converse and direct flexoelectric behaviour is noted here. The effect of the SRS is observed to be dependent on the electrical field gradients. The present study reveals the significance of the SE and SRS for analysis involving flexoelectric behaviour in nano-sensors and actuators.

Effect of machining‐induced surface integrity on the corrosion behavior of Al–Li alloy 2A97 in sodium chloride solution

Al–Li alloy 2A97 is one promising alloy that could be widely used in the aerospace field to achieve weight lighting. Due to the addition of lithium element, its anticorrosion attracts widespread interest. In this study, the milled surfaces and ground‐polished surfaces of alloy 2A97 are characterized and compared from three aspects of surface integrity, namely surface roughness, residual stresses, and work‐hardening. After potentiodynamic polarization measurements that were conducted on the machined samples with different surfaces in 3.5 wt% sodium chloride solution, the corrosion parameters regarding corrosion thermodynamics and kinetics are analyzed. Experimental results reveal that the surface micro‐hardness of the milled surfaces is greater than that of the ground‐polished surfaces because of the plastic deformation induced by milling process. Work‐hardening is beneficial while increasing surface roughness is detrimental to anticorrosion of alloy 2A97, and the effect of surface residual stresses is insignificant. Moreover, the corrosion parameters and surface corrosion morphology indicate that the corrosion resistance of Al–Li alloy 2A97 underwent milling process is superior than that obtained by grinding and subsequently polishing, which could further promote the application of mechanical milling Al–Li alloy process in aerospace industry.

Effect of surface elasticity on extensional and torsional stiffnesses of isotropic circular nanorods

We present a continuum formulation to obtain the effects of surface residual stress and surface elastic constants on extensional and torsional stiffnesses of isotropic circular nanorods. Analytical expressions of axial force, twisting moment, and extensional and torsional stiffnesses are obtained. Unlike the case of rectangular nanorods, we show that the stiffnesses of circular nanorods also depend on surface residual stress components. This is attributed to non-zero surface curvature inherent in circular nanorods. We further normalize these expressions and analyze their asymptotic limits in the limit of the nanorod’s radius approaching both zero and infinity, corresponding to surface-dominated and bulk-dominated regimes, respectively. Finally, we use the recently proposed helical Cauchy–Born rule and perform molecular statics calculations to obtain axial force, twisting moment, and stiffnesses of the tungsten nanorod. The tungsten material is selected since its bulk crystal exhibits isotropy in the stress-free state. The results from molecular statics calculations are shown to match the derived continuum formulas accurately.

The study of the relationship between life limiting factor and stress level for FGH96

FGH96 is a Chinese made powder metallurgy nickel based superalloy. This reserach aims to investigate the effect of stress on the microstructure of the worst fatigue life at 600°C. The specimens were first polished by abrasive paper and then electropolished to elimate the effect of surface residual stress. The fatigue tests were conducted at 600°C, and the loading ratio was 0.05 at the frequency of 5Hz. Tests were conducted at 900, 1000, 1100, 1200MPa respectively. The fatigue life under the same condition were compared and the fractography of specimens were examined under SEM to further identify the life-limiting factors of the material. The initiation mode transferred from facet initiated to inclusion initiated from lower stress level to higher stress level. Meanwhile, the initiation position transferred from internal to surface with the increase of stress level. This means at higher stress level, fatigue life is mainly limited by inclusion at surface while at lower stress level the limit is controlled by internal facet.

Modeling of a nanoscale flexoelectric energy harvester with surface effects

This work presents the modeling of a beam energy harvester scavenging energy from ambient vibration based on the phenomenon of flexoelectricity. By considering surface elasticity, residual surface stress, surface piezoelectricity and bulk flexoelectricity, a modified Euler-Bernoulli beam model for the energy harvester is developed. After deriving the requisite energy expressions, the extended Hamilton's principle and the assumed-modes method are employed to obtain the discrete electromechanical Euler-Lagrange's equations. Then, the expressions of the steady-state electromechanical responses are given for harmonic base excitation. Numerical simulations are conducted to show the output voltage and the output power of the flexoelectric energy harvesters with different materials and sizes. Particular emphasis is given to the surface effects on the performance of the energy harvesters. It is found that the surface effects are sensitive to the beam geometries and the surface material constants, and the effect of residual surface stress is more significant than that of the surface elasticity and the surface piezoelectricity. The axial deformation of the beam is also considered in the model to account for the electromechanical coupling due to piezoelectricity, and results indicate that piezoelectricity will diminish the output electrical quantities for the case investigated. This work could lead to the development of flexoelectric energy harvesters that can make the micro- and nanoscale sensor systems autonomous.

Nonlinear free vibration of a cantilever nanobeam with surface effects: Semi-analytical solutions

Nanobeams and nanowires are widely used as building blocks in the rapid development of Nano/Micro-electro-mechanical system (N/MEMS), micro-sensors, energy harvesting and storage devices, etc., and their vibration behaviors have aroused great concerns in both pure science and engineering applications. In this study, we investigate the nonlinear free vibration of a nanobeam considering its surface effects, including the surface elasticity and residual surface stress. Firstly, a mechanics model on the transverse vibration of a cantilever nanobeam is developed according to the Hamilton's principle. In use of the Galerkin and complex normal form methods, the approximate analytical solution of the nonlinear equation is obtained, which has been corroborated by the numerical simulation. Next, the effects of residual surface stress on the nonlinear dynamic behaviors of the system are examined both theoretically and numerically, which indicate that surface effects have great impact on the quasi-periodic and chaotic motions of the system. The present work can provide a theoretical basis for the precise design of nanowires or nanofibers in atomic force microscopy, generators and nano-sensors in electronic devices.

Size-dependent effect on biaxial and shear nonlinear buckling analysis of nonlocal isotropic and orthotropic micro-plate based on surface stress and modified couple stress theories using differential quadrature method

The size-dependent effect on the biaxial and shear nonlinear buckling analysis of an isotropic and orthotropic micro-plate based on the surface stress, the modified couple stress theory (MCST), and the nonlocal elasticity theories using the differential quadrature method (DQM) is presented. Main advantages of the MCST over the classical theory (CT) are the inclusion of the asymmetric couple stress tensor and the consideration of only one material length scale parameter. Based on the nonlinear von Karman assumption, the governing equations of equilibrium for the micro-classical plate considering midplane displacements are derived based on the minimum principle of potential energy. Using the DQM, the biaxial and shear critical buckling loads of the micro-plate for various boundary conditions are obtained. Accuracy of the obtained results is validated by comparing the solutions with those reported in the literature. A parametric study is conducted to show the effects of the aspect ratio, the side-to-thickness ratio, Eringen’s nonlocal parameter, the material length scale parameter, Young’s modulus of the surface layer, the surface residual stress, the polymer matrix coefficients, and various boundary conditions on the dimensionless uniaxial, biaxial, and shear critical buckling loads. The results indicate that the critical buckling loads are strongly sensitive to Eringen’s nonlocal parameter, the material length scale parameter, and the surface residual stress effects, while the effect of Young’s modulus of the surface layer on the critical buckling load is negligible. Also, considering the size dependent effect causes the increase in the stiffness of the orthotropic micro-plate. The results show that the critical biaxial buckling load increases with an increase in G12/E2 and vice versa for E1/E2. It is shown that the nonlinear biaxial buckling ratio decreases as the aspect ratio increases and vice versa for the buckling amplitude. Because of the most lightweight micro-composite materials with high strength/weight and stiffness/weight ratios, it is anticipated that the results of the present work are useful in experimental characterization of the mechanical properties of micro-composite plates in the aircraft industry and other engineering applications.

Preparation of Samples used in Fatigue Testing of Aircraft Materials

Abstract The application of a new type of alloy requires the evaluation of its properties, which is typically achieved with destructive methods. For this purpose, among others, static and fatigue mechanical tests are performed. Tests are performed on standardized samples in a way which reflects the level of stress occurring in real elements. These tests should limit random errors associated with sample preparation. For this reason the proper preparation of samples is crucial, not only in terms of their geometric dimensions but also in terms of the residual stress level. A sample preparation process was developed, involving checking samples’ surface for cracks, scratches, roughness, and the state of stress. The measurements are performed with nondestructive methods so as not to affect the proceeding research. In this study, the residual stress and features of a mechanically prepared surface were characterized. The specimens were subjected to various surface finishes mainly, lathe turning and grinding surface conditions. The effects of residual surface stress (measured by XRD) were studied after machining and polishing.