Surface Tensile Stress Research Articles

Characteristics of stress distribution within the metal sheet bent by laser shock forming

Laser shock forming(LSF) is an emerging method for metal sheet forming. Besides dimension accuracy, the stress distributions on the surface and within the target material are also important for the actual engineering applications. In this paper, the stress distributions along the sheet depth direction and parallel to the target surface have been made clear through simulations. The deformation mechanisms of two typical deformation modes, convex and concave, were directly verified and further explained by the temporal and spatial residual strain/stress distribution. For the convex deformation, the stress distribution along the thickness direction is roughly divided into three regions: the near surface layer, the intermediate layer and the bottom layer. Among these, both the top and bottom surface layers are mainly subjected to compressive residual stresses. For this case, the stress wave effect does not ‘penetrate’ the sheet material, and a stress gradient is induced along the depth direction of the sheet, which is responsible for final convex deformation. For the concave deformation, the stress distribution along the thickness direction is divided into two regions: the surface tensile stress zone and the lower compressive stress zone. The stress wave effect penetrates the sheet material, directly causing the overall tensile deformation of the sheet material, and finally causes the sheet material to undergo deep plastic deformation. The stress distribution characteristics of the two deformation modes can provide useful references for the practical application.

Dike-induced stresses and displacements in layered volcanic zones

During a volcanic unrest period with dike injection, one of the main scientific tasks is to assess the geometry and the propagation path of the dike and, in particular, the likelihood of the dike reaching the surface to erupt. Currently, the dike path and geometry (including depth and opening/aperture) are both partly determined from geodetic surface data using mostly dislocation models that assume the volcanic zone/volcano to be an elastic half space of uniform mechanical properties. By contrast, field observations of volcanic zones/volcanoes (active and extinct) show that they are composed of numerous layers whose mechanical properties (primarily Young's modulus) vary widely and whose contacts commonly arrest dikes. Here we provide field observations and numerical models on the effects of a typical variation in Young's modulus in an active volcanic zone on the internal and surface stresses and displacements induced by a dike whose tip is arrested at 0.5 km depth below the surface of the volcanic zone. Above the layer or unit hosting the dike are four layers of equal thickness. We vary the Young's modulus or stiffness of the fourth layer (the one adjacent to the layer or unit hosting the dike) from 10 GPa to 0.01 GPa, while all the other layers/units maintain their Young's moduli in the model runs. The results show that as the fourth layer becomes more compliant or soft (0.1–0.01 GPa) dike-induced stresses and displacements (lateral and vertical) above the layer, including those at the surface, become suppressed but the stresses and displacements of the layer/unit hosting the dike increase and their peaks do not coincide in location or magnitude with those of the other layers. Thus, the dike-induced internal deformation of the volcanic zone increases as the fourth layer becomes softer. Also, the tensile-and-shear stress peaks at the surface occur at locations widely different from those of maximum surface uplift. More specifically, for a comparatively stiff fourth layer (1–10 GPa), the surface tensile and shear stresses peak at lateral distances of 0.5–0.7 km from the projection of the dike to the surface. (Essentially no tensile/shear stresses reach the surface when the fourth layer is as soft as 0.1–0.01 GPa, so that there are no stress peaks). By contrast the maximum surface displacements (uplift) peak at lateral distances of 2.8–3.3 km from the dike projection to the surface. If tension fractures or faults – in particular the boundary faults of a graben – are induced by the dike, they should form at the tensile/shear stress peaks and not, as is commonly suggested, at the location of the surface displacement peaks. Our results thus suggest that any dike-induced graben is likely to be of a width about twice the depth to the tip/top of the arrested dike. The results demonstrate that elastic half-space models overestimate the dike-induced surface stresses, and thus the depth to the tip/top of the associated dike. In particular, the models presented here indicate that, for typical dikes little or no dike-induced surface deformation would be expected until the dike tip propagates to depths below the surface of less than a kilometre.

An experimental investigation of surface integrity in selective laser melting of Inconel 625

Inconel 625 is a Ni-based superalloy widely used in nuclear and aerospace applications because of its high strength and corrosion resistance. The current paper investigates selective laser melting of Inconel 625 using a wide range of process parameters: four laser powers, five scan speeds, and three hatch spacings. Cube coupons are produced and their end properties are measured, specifically, relative density, surface roughness, microhardness, and surface residual stresses. Process maps are constructed to correlate processing parameters to the part surface integrity, and the possible underlying causes are highlighted. Experimental results show that highly dense parts can be produced using selective laser melting and low average surface roughness can be obtained in both the scan and hatch directions. Hatch spacing is the most prominent factor to attain relative densities above 99% while high laser powers are key to lower the average surface roughness. Microhardness and microstructure examination are done for a subset of the process parameters and found to not significantly change with laser power. Surface residual stresses are measured on the top surface in the scan and hatch directions and process maps are developed. There is no particular parameter that solely affects surface residual stresses, despite several cases where the increase in laser power reduced the surface residual stresses. 90% of measurements showed surface tensile residual stresses except for a few cases that resulted in surface compressive residual stress.

Laser Additive Manufacturing using directed energy deposition of Inconel-718 wall structures with tailored characteristics

A process window is developed for fabricating thin walled Inconel 718 (IN718) structures for Laser Additive Manufacturing using Directed Energy Deposition (LAM-DED). In-house developed LAM-DED setup deployed to investigate the effect of process parameters on geometry and quality. The influence of post-heat treatment on microstructure, surface and mechanical properties is also studied. Full factorial experiments are carried out to derive the process window yielding requisite geometry, deposition rate and quality. The optimized process parameters are deployed for thin wall fabrication. The maximum and minimum limits for laser energy per unit length (E) and powder feed rate per unit length (F) are experimentally found to be 210 kJ/m, 105 kJ/m, 12.5 g/m and 4 g/m, respectively for fabricating thin walls of requisite geometry. The fabricated thin walls are observed to be defect free with dendritic microstructure. The microstructural examination of heat-treated samples reveals recrystallized equiaxed grains with reduction in surface tensile residual stress up to 50%, and modified surface topography. The reduction of average hardness by 12%, significant improvement in ductility by 62.5% and improvement in energy storage capacity by 2.4 times are observed during the study.

Surface-tensile-stress induced polishing-voids in cross-point phase-change-memory cells: corrosion mechanism and solution

In the fabrication of cross-point phase-change-memory-cells through atomic-layer-deposition of Ge-doped SbTe (Ge-ST) film followed by chemical-mechanical-polarization (CMP), a remarkable surface tensile stress was generated, originating from the surface stress induced by the pad down force and the surface structure tensile stress in a confined memory-cell structure. It was maximized at the position of the Si3N4-film spacer where the curvature becomes zero. The maximized surface tensile stress produced polishing induced voids via the generation mechanism of the stress corrosion cracking. The generation frequency and nanoscale size of the polishing induced voids rapidly increased at the maximum surface tensile stress during CMP. Then, they slightly decreased and saturated when the surface tensile stress reduced during further CMP. A design of a spacer using a SiO2-film spacer rather a Si3N4-film spacer could prevent the generation of the polishing induced voids, confirming that the generation mechanism of the polishing induced voids is associated with the stress corrosion cracking.

Intrinsic Strain Tuning of Electrocatalyst for Oxygen Reduction Reaction

Surface strain is a powerful strategy to optimize the reactivity of electrocatalysts. Traditionally, surface strain in catalyst overlayer is imposed by external stress through the overlayer-substrate interfaces, which may lead to multiple stability and activity problems. In this presentation, we will introduce a way to generate and tune surface strain by utilizing intrinsic driving forces, i.e. intrinsic surface stress. For Pt(111) surface as an example, the tensile surface stress ( ~4.8 N/m) exerts a pressure on the order of 105 atmospheres on the surface atoms, which provides a strong driving force for surface contraction and reduction of the surface energy. While this surface pressure has little impact on the surface structure of symmetric Pt(111) surface, it can be released by introducing step type surface defects to break the surfaces symmetry constraint, which is accompanied with the contraction of surface lattice. Using well-defined Pt stepped surfaces, we demonstrate that the magnitude of surface strain is inversely proportional to the terrace width. Thus, the atomic-level control of terrace width enables generation and fine tuning of surface strain to optimize catalytic reactivity. Using oxygen reduction as an example, we show that the terrace-width dependent strain not only responses for the improved catalytic activity of a series of Pt-n(111)-(100) step surfaces, but also can be used to design electrocatalysts with simultaneously enhanced specific activity and mass activity.

Surface initiation of rolling contact fatigue at asperities considering slip, shear limit and thermal elastohydrodynamic lubrication

Abstract A numerical investigation was performed, with single axisymmetric asperities passing through lubricated rolling contacts at different slip. Two explanatory and cooperating phenomena were found as to why the damage develops more frequently at negative than positive slip. Metal contact occurred in the inlet, where tractive asperity contacts at negative slip provided a large tensile surface stress outside the contact. As the asperity moved through the contact, sliding supplied it with lubricant and heated the lubricant along the contact. The shear tractions were thus higher near the inlet than the outlet, making them more detrimental for negative than positive slip.

Anomalous Elastic Properties of Attraction-Dominated DNA Self-Assembled 2D Films and the Resultant Dynamic Biodetection Signals of Microbeam Sensors.

The condensation of DNA helices has been regularly found in cell nucleus, bacterial nucleoids, and viral capsids, and during its relevant biodetections the attractive interactions between DNA helices could not be neglected. In this letter, we theoretically characterize the elastic properties of double-stranded DNA (dsDNA) self-assembled 2D films and their multiscale correlations with the dynamic detection signals of DNA-microbeams. The comparison of attraction- and repulsion-dominated DNA films shows that the competition between attractive and repulsive micro-interactions endows dsDNA films in multivalent salt solutions with anomalous elastic properties such as tensile surface stresses and negative moduli; the occurrence of the tensile surface stress for the attraction-dominated DNA self-assembled film reveals the possible physical mechanism of the condensation found in organism. Furthermore, dynamic analyses of a hinged–hinged DNA-microbeam reveal non-monotonous frequency shifts due to attraction- or repulsion-dominated dsDNA adsorptions and dynamic instability occurrence during the detections of repulsion-dominated DNA films. This dynamic instability implies the existence of a sensitive interval of material parameters in which DNA adsorptions will induce a drastic natural frequency shift or a jump of vibration mode even with a tiny variation of the detection conditions. These new insights might provide us some potential guidance to achieve an ultra-highly sensitive biodetection method in the future.

Shot peening effects on residual stresses redistribution of offshore wind monopile multi-pass weldments

In some industrial applications, welding is the only alternative to join different parts. However, the major problem in welded structures is the tensile residual stresses that are inevitably produced during welding process. Surface tensile stresses can threaten the performance of the weldments as they act as an accelerant in fatigue crack initiation and failure. Shot peening is a well-known method which can enhance weldments' fatigue performance and longevity by inducing surface compressive residual stresses in order to eliminate or limit tensile residual stresses.In this study, the effect of shot peening on redistribution of multi-pass welding residual stresses was numerically and experimentally investigated on very large components typical of welded joints used in offshore wind turbine monopiles. In experimental part of the study, the residual stresses were measured using the Incremental Centre Hole Drilling (ICHD) method before and after shot peening. Finite element studies were carried out using 3D welding models and random shot peening analyses. Moreover, extensive finite element analyses were conducted to study the effect of model dimensions and the number of passes on prediction of welding residual stresses. Interesting set of results obtained from both the numerical studies and the ICHD measurements were in good agreements and showed that shot peening can be advantageous even for large components with multi-pass welded joints. Additionally, reducing the number of weld passes in finite element models could considerably lower the computational time without affecting the accuracy of results at surface regions of the models.

The grafting of phthalic acid onto polyurethane copolymer and its impact on the surface hydrophilicity, tensile stress, and shape recovery properties

Phthalic acid (PA) was grafted onto polyurethane (PU) in the PA series due to its rigid aromatic structure and carboxyl groups, but PA was simply blended with PU in the control CPA series. The cross-link density and solution viscosity of the PA series notably increased with increasing PA content, and the same parameters of the CPA series did not increase. The maximum tensile stress of the PA series sharply increased due to the chemical cross-linking of the grafted PA, and the tensile strain at break slowly decreased with increasing PA content. The shape recovery of the PA series at 10 °C rapidly increased upon the grafting of PA; however, that of the CPA series decreased with increasing PA content. The hydrophilicity of PU was enhanced by the grafting of PA based on the water contact angle and water vapor permeation results. The low temperature flexibility of the PA series improved more than that of the CPA series under freezing conditions with increasing PA content due to the chemical cross-linking and steric and electrostatic repulsion between PU moieties. Overall, the grafted PA definitely increased the maximum tensile stress, shape recovery, and low temperature flexibility of PU.

Incorporating Surface Stress in Computations of Heteroepitaxial Quantum Dot Morphology

Surface stress, which is an inherent property of several crystals, is expected to play a role in morphology of nanostructures. Here the authors describe how surface stress effects can be explicitly incorporated in the computation of quantum dot structure and evolution in coherent heteroepitaxial thin films. In the usual continuum formulation for surface evolution, surface stress enters as an additional contribution to the local chemical potential. Additionally, it modifies the film–vacuum interface boundary condition for the set‐up of the elastic problem. The authors show how the resulting evolution can be computed order‐by‐order in the surface slope. The sign of the surface stress with respect to the mismatch affects the stability of the film to surface undulations. For the case of a positive‐mismatched heteroepitaxial system, a tensile surface stress leads to film stabilization and increases the critical thickness for quantum dot formation in Stranski–Krastanov growth.

Investigation of the asperity point load mechanism for thermal elastohydrodynamic conditions

The rolling contact fatigue damage called pitting or spalling develops more frequently in surfaces with negative than positive slip. Since normal line loads do not cause any tensile surface stresses this investigation considers the effects of small point shaped asperities. Shear traction causes tensile stresses at the trailing edge of asperities entering the contact at negative slip. At positive slip the tensile stresses appear at the leading edge when the asperities exit the contact. It was found that the trailing edge of the asperity breaks through the lubrication film at contact entry. This causes negative slip to be more detrimental than positive slip. At negative slip the location of large frictional shear stresses and tension stresses from normal asperity contact coincide.

Understanding the Residual Stress in DMLS CoCrMo and SS316L using X-ray diffraction

Laser additive manufacturing (LAM) is the next in line revolutionary technology for various turbine applications, as an enabler of new designs and parts and during repair and refurbishment of service returned parts. In LAM process, residual stresses play a key role during component build up as well as while removing the components from the build plate, to ensure distortion free parts, as a standalone or as a hybrid build on an existing part. The present study comprises an experimental determination of residual stresses of two direct metal laser sintered (DMLS) alloys, CoCrMo and SS316L, using the X-ray diffraction sin2 ψ technique. In both cases, the process parameters were optimized to result in dense (> 99.95%) samples. Surface residual stress measurements were carried out on the DMLS samples in the as printed condition and after heat treatment. While SS316L measured a surface tensile stress of 127 MPa, CoCrMo showed a tensile stress of 265 MPa, in the as printed condition. Upon grit blasting, both samples undergo a peening effect and record a compressive stress of 300-400 MPa. Heat treatment of SS316L at 650°C, tends to relieve these stresses and results in zero stress, whereas for CoCrMo there was a systematic variation in the response to heat treatment at different temperatures, correlating with the microstructural evolution. At 1050°C CoCrMo shows compressive stresses because of partial recrystallization and at 1150°C having a completely recrystallized microstructure comprising equiaxed grains of around 40 µm and a completely stress relieved condition.

Understanding the effect of electrolyte on the cycle and structure stability of high areal capacity Si-Al film electrode

High areal capacity silicon-based films are attracting much attention in high energy density lithium-ion batteries (LIBs). However, the enormous volume change of Si, causing film crack and instability of solid electrolyte interphase (SEI) layer, leads to a rapid capacity decay. In this work, the structure evolution and electrochemical performance of Si-Al film (~ 2.5 mAh cm-2) are systematically investigated by galvanostatic discharge/charge test, field-emission scanning electron microscopy (FESEM), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). Particularly, the inner structure is carefully analyzed by the cross-sectional images. In the EC-based and FEC-added electrolytes, the columnar structure is remained. Interestingly, the rigid SEI layer formed in FEC-based electrolyte restrains the surface tensile stress, and the Si-Al film is cracked from the inner part and further rearranged towards compact piling up during cycling. The new formed film displays a weak volume expansion and an improved capacity retention of 85.0% after 100 cycles.

Effects of mechanical polishing treatments on high cycle fatigue behavior of Ti-6Al-2Sn-4Zr-2Mo alloy

In this study, high cycle fatigue (HCF) behavior of Ti-6Al-2Sn-4Zr-2Mo (wt.%) (Ti-6242) alloy sheets subjected to mechanical polishing treatments (MPTs) were investigated at room temperature. The effects of surface roughness and residual stress induced by the MPTs (cold rolling polishing (CRP), sandpaper polishing (SP) and nylon cloth polishing (NCP)) were studied. Results indicate that the fatigue limits of Ti-6242 alloy sheets polished by CRP, SP and NCP were 632.6, 593.0 and 639.0 MPa, respectively. The average surface roughness (Ra) values of the Ti-6242 alloy sheets for CRP, SP and NCP were 0.36, 0.31 and 0.18 μm, respectively. Top surface tensile residual stress (TRS), surface and deep subsurface TRS and top surface compressive residual stress (CRS) were induced by CRP, SP and NCP, respectively. HCF behavior of the Ti-6242 alloy sheets was dominantly influenced by deep subsurface residual stress induced by the MPTs, whereas the effects of surface roughness and top surface residual stress were negligible.

Surface residual stresses in additive/subtractive manufacturing and electrochemical corrosion

Hybrid manufacturing approaches with additive and subtractive manufacturing is a promising technology for many sectors from personal care goods to aerospace. When it comes to the residual stresses, there are hardly about the additive manufacturing (AM). Although some researches have been conducted for the hybrid laser additive manufacturing and subtractive manufacturing (SM) processes, only the machining parameters, material properties, and surface quality were analyzed; there were a few experiments about the residual stresses. Taking this into account, this paper aims to analyze the surface residual stresses in hybrid manufacturing approaches. Additive machining process, electrochemical corrosion, and surface residual stresses testing were described in detail. The main results of the research would be references for the surface residual stresses when 316L stainless steel specimens are manufactured with laser metal deposition and milling after AM immediately. The oxidation is much serious in additive manufacturing, and the surface tensile stresses are quite small. Both the electrochemical corrosion and milling at high temperature will affect the distribution of the surface residual stresses. The residual tensile stresses are distributed symmetrically after milling process while they are asymmetric after electrochemical corrosion.

Effects of Cutting Edge Microgeometry on Residual Stress in Orthogonal Cutting of Inconel 718 by FEM.

Service performance of components such as fatigue life are dramatically influenced by the machined surface and subsurface residual stresses. This paper aims at achieving a better understanding of the influence of cutting edge microgeometry on machined surface residual stresses during orthogonal dry cutting of Inconel 718. Numerical and experimental investigations have been conducted in this research. The cutting edge microgeometry factors of average cutting edge radius , form-factor K, and chamfer were investigated. An increasing trend for the magnitudes of both tensile and compressive residual stresses was observed by using larger or introducing a chamfer on the cutting edges. The ploughing depth has been predicted based on the stagnation zone. The increase of ploughing depth means that more material was ironed on the workpiece subsurface, which resulted in an increase in the compressive residual stress. The thermal loads were leading factors that affected the surface tensile residual stress. For the unsymmetrical honed cutting edge with K = 2, the friction between tool and workpiece and tensile residual stress tended to be high, while for the unsymmetrical honed cutting edge with K = 0.5, the high ploughing depth led to a higher compressive residual stress. This paper provides guidance for regulating machine-induced residual stress by edge preparation.

Alleviating surface tensile stress in e-beam treated tool steels by cryogenic treatment

Electron beam (e-beam) treatment of tool steel surfaces has been available for several decades as an approach to create hard surface layers on tool steels. The e-beam process produces ultrafine grain sizes through ultra-rapid cooling of the melt layer, which helps with improving wear resistance to increase the service life of tools. However, its implementation in many applications has been limited by the accompanying residual tensile stresses in the surface layer, which is undesirable and may lead to premature fracture. Here we report the utilization of cryogenic freezing after e-beam treatment to reduce the residual tensile stress. The e-beamed specimen contained high levels of retained austenite in the surface layer. The cryogenic treatment converted the retained austenite into martensite, and the corresponding volume expansion reduced the peak tensile residual stress by 28%, which makes it a promising method to expand the applications of e-beamed tool steel.

Sag Bending Control in the Production of Automobile Windshields

Random inspection quality control of laminated windshields during sag molding was performed. The deviations from the technical specifications of the surface tensile stresses at the edge of the glass were revealed. Adaptive models describing the dependence of the tensile stresses at the glass edge on the sag bending temperature were constructed. An algorithm was developed for controlling the sag bending temperature. It is shown by means of simulation modeling that the magnitude of the surface tensile stresses in the produced articles can be stabilized.

Structural impact of creep in tungsten monoblock divertor target at 20 MW/m2

In order to increase erosion lifetime of the divertor target, in the 2nd design phase of R&D work package ‘Divertor’ for European DEMO, armor thickness of tungsten monoblock divertor target is increased from 5 mm to 8 mm. By increasing armor thickness, surface temperature increases nearly linearly, which makes effect of creep no longer negligible at slow transients of 20 MW/m2. In this work, structural impact of creep in tungsten monoblock divertor target is for the first time quantitatively analyzed with the aid of finite element method. The numerical simulations have revealed that creep results in an increase of inelastic strain accumulation. With increasing armor thickness, tensile surface stress along x-axis (the longer edge at the plasma-facing surface of tungsten monoblock) reduces, while surface stress along z-axis (axial direction of the cooling tube) changes from tensile to compressive. Creep will accelerate this change. With increasing grain size, creep strain accumulation at loading surface increases due to higher creep rates, while plastic strain accumulation decreases. Creep can mitigate the risk of deep cracking by reducing the driving force for crack opening, and has a positive impact for preventing the contact between the upper parts of neighboring monoblocks in high heat flux tests.