When rock salt is subjected to discontinuous fatigue, time intervals (periods during which the stress remains constant) can accelerate plastic deformation and reduce the fatigue life due to the ...internal residual stress generated between various defects and the host material. However, the time interval effect on rock salt in a three-dimensional stress state has rarely been investigated despite its great significance in the use of underground salt caverns as storages. In this research, a series of triaxial discontinuous cyclic compression tests were conducted on salt and the residual stress was reproduced through numerical simulations. Experimental results show that the interval effect decreases with the increase of the confining pressure. The change in plasticity caused by time intervals is small and constant when the stress level (ratio of the maximum stress to strength) is below “60%” but then increases as a linear function of the stress level. During simulations, it was observed that the greater the difference in the elasticity modulus between the host material and impurities, the larger the residual stress. The moderate effect of time interval under confinement is the result of the fact that a higher confining pressure strengthens the host material, reduces the volume of impurities and thus leads to a smaller residual stress. In addition, when the stress level is below a certain threshold (which is shown to be at 0.8–0.85 of the critical yield stress), the residual stress is relatively low in magnitude and area of influence. Above that threshold, the residual stress grows rapidly. This is the reason why the interval effect displays a distinct behavior before and after dilatancy point.
Understanding the influence of process conditions and coating architecture on the microstructure and residual stress state of multi-layered coatings is essential for the development of novel ...thermally and mechanically stable coatings and requires advanced depth resolving characterization techniques. In this work, an arc-evaporated multi-layered coating, consisting of alternating Al70Cr30N and Al90Cr10N sublayers with an individual layer thickness between 120 nm and 380 nm, was investigated. The as-deposited state of the multi-layered coating and the state after vacuum annealing at 1000°C for 30 min was studied along its cross-section by synchrotron X-ray nano-diffraction using a beam with a diameter of 50 nm. The results revealed sublayers with alternating cubic and hexagonal phase, causing repeated interruption of the grain growth at the interfaces. The in-plane residual stress depth distribution across the coating thickness could be tuned in a wide range between pronounced compressive and slight tensile stress by combining the effects of the coating architecture and the modulated incident particle energy controlled by the substrate bias voltage ranging from −30 V to −250 V. This resulted in an oscillatory stress profile fluctuating between −2 GPa and −4.5 GPa or pronounced stress gradients with values between −4 GPa and 0.5 GPa. Finally, the decomposition routes of the metastable cubic Al70Cr30N phase could be controlled by the Al90Cr10N sublayers which acted as nucleation sites and governed the texture of the decomposition products as Cr2N. The results demonstrate that the cross sectional combinatorial approach, relying on a sophisticated multi-layer architecture combining various materials synthesized under tailored conditions, allowed for resolving structural variations and stress profiles in the individual layers within the complex architecture and pioneers the path for knowledge-based development of multi-layered coatings with predefined microstructure and a dedicated stress design.
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As a result of conventional wire-forming processes, the residual stress distribution in wires is frequently unfavorable for subsequent forming processes such as bending operations. High tensile ...residual stresses typically occur in the near-surface region of the wires and can limit further application and processability of the semi-finished products. This paper presents an approach for tailoring the residual stress distribution by modifying the forming process, especially with regard to the die geometry and the influence of the drawing velocity as well as the wire temperature. The aim is to mitigate the near-surface tensile residual stresses induced by the drawing process. Preliminary studies have shown that modifications in the forming zone of the dies have a significant impact on the plastic strain and deformation direction, and the approach can be applied to effectively reduce the process-induced near-surface residual stress distributions without affecting the diameter of the product geometry. In this first approach, the process variant using three different drawing die geometries was established for the metastable austenitic stainless steel X5CrNi18-10 (1.4301) using slow (20 mm/s) and fast (2000 mm/s) drawing velocities. The residual stress depth distributions were determined by means of incremental hole drilling. Complementary X-ray stress analysis was carried out to analyze the phase-specific residual stresses since strain-induced martensitic transformations occurred close to the surface as a consequence of the shear deformation and the frictional loading. This paper describes the setup of the drawing tools as well as the results of the experimental tests.
Combined shot peening and laser shock peening (CSP) can introduce high surface compressive residual stress (CRS) and deep CRS layer, and reduce the high surface roughness caused by shot peening. The ...relationship between CSP sequences and surface integrity, including CRS and surface roughness, is far from satisfying the actual demand. In this paper, we construct a finite element model considering initial surface profile features. The experimental results agree with the simulated results. Based on the simulation results, axial and longitudinal stress wave transfer effects on surface CRS and depth CRS are investigated. Compared with shot peening (SP) and laser shock peening (LSP) alone, CSP treatment can increase CRS depth and CRS magnitude by 1060 μm and 50 ∼ 80Mpa, respectively. In addition, the roughness following CSP treatment is lower than that observed after SP treatment alone, and the roughness varies depending on the sequence of CSP treatment. Specifically, the roughness values for SP, SP + LSP, and LSP + SP are 0.938 ± 0.031 μm, 0.982 ± 0.11 μm, and 1.168 ± 0.093 μm, respectively. The surface morphology, hardness, and simulation results are combined to reveal the reasons for the reduction of surface roughness after CSP treatment. The changes of peaks and valleys after CSP treatment compared with single SP treatment were studied, and the variation trend of surface roughness was further explained.
•A three-dimensional finite element model with real initial surface roughness is constructed.•The axial and longitudinal stress wave transfer on surface CRS and depth CRS are investigated.•The variation mechanism of surface roughness after CSP treatment is revealed.•The changes of peak and valley after CSP treatment compared with single SP treatment are investigated.
High frequency mechanical impact treatment is observed to increase the fatigue strength of welded joints. This technique induces compressive residual stresses, increases the local hardness, and ...reduces the stress concentration by modifying the weld toe radius. The goal of this study was to investigate residual stresses induced by ultrasonic impact treatment in S355, S700MC, and S960 grades steel experimentally and numerically. Plate specimens were manufactured and treated with different treatment intensities i.e. vibration amplitudes of the Sonotrode. The indentation depths were measured by the aid of a laser scanner and residual stresses using X-ray diffraction technique. The effect of steel grade and treatment intensity on the induced compressive residual stress state was firstly studied experimentally. In addition, displacement controlled simulations were carried out to estimate the local residual stress condition considering the effect of different material models. Both the numerically estimated and experimentally measured residual stresses were qualitatively in good agreement. Residual stress state in S355 and S700MC can be estimated well using combined strain rate dependent material model. No significant effect of the treatment intensity is observed on the indentation depth and residual stress state for S355 grade steel. The indentation depth decreases with the increase in the yield strength of the steel.
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•Compressive residual stress state induced in S355, S700MC, and S960 grade steel.•Numerical models are developed to estimate the residual stress state.•The effect of UIT intensity on the induced residual stress state has been studied.
EDDIDAT is a MATLAB‐based graphical user interface for the convenient and versatile analysis of energy‐dispersive diffraction data obtained at laboratory and synchrotron sources. The main focus of ...EDDIDAT up to now has been on the analysis of residual stresses, but it can also be used to prepare measurement data for subsequent phase analysis or analysis of preferred orientation. The program provides access to the depth‐resolved analysis of residual stresses at different levels of approximation. Furthermore, the graphic representation of the results also serves for the consideration of microstructural and texture‐related properties. The included material database allows for the quick analysis of the most common materials and is easily extendable. The plots and results produced with EDDIDAT can be exported to graphics and text files. EDDIDAT is designed to analyze diffraction data from various energy‐dispersive X‐ray sources. Hence it is possible to add new sources and implement the device‐specific properties into EDDIDAT. The program is freely available to academic users.
EDDIDAT is a program that provides a graphical user interface for the evaluation of energy‐dispersive X‐ray diffraction data with the focus on depth‐resolved residual stress analysis.
Cold spray (CS) is a solid state deposition presented in the literature to produce coatings for various applications, e.g., corrosion-resistant 316L stainless steel, Ti light alloy parts repairing, ...and hydrophobic Cu coatings. CS sprays particles under high velocity, impacting onto a prepared surface or substrate, and the powders bond by a severe and fast plastic deformation, consolidating the coating layer by layer. The number of layers depends on the designed coating thickness since each powder and CS parameters results in a different layer thickness. This work evaluates three feedstock powders (Cu, Ti, and 316L) for CS processing, and the characteristics and properties of the CS-ed coatings, studying the effects of the thickness coatings on their microstructure, porosity, hardness, adhesion to the substrate, and residual stress generated. The images of microstructures were obtained by optical microscopy and SEM, and the near-to-surface residual stress was measured by X-ray diffraction (XRD). Increasing the coating thickness produced a substrate deformation due to the compressive residual stress dominating the coating, which is numerically obtained and proven by XRD. Besides that, increasing the coating thickness slight increased the deposition efficiency for Cu ant Ti, but went in the opposite way for 316L; reduced the coating adhesion to the substrate; and did not altered significantly the hardness.
•For CS-ed Cu, Ti, and 316L coatings on carbon steel, the thickness did not alter the residual stress;•The substrate material and properties alter the CS Cu, Ti, and 316L coating deposition efficiency on carbon steel.
Electrical discharge machining (EDM) is considered as an efficient alternative to conventional material removal concepts that allows for much higher material removal rates. However, EDM generates ...unwanted features such as re-cast layer (RCL), tensile residual stresses and a rough surface. In order to recover the surface integrity, different post processes has been compared: high-pressure water jet (HPWJ), grit blasting (GB) and shot peening (SP). Surface integrity has been evaluated regarding microstructure, residual stresses, chemical content and surface roughness. The results showed that a combination of two post processes is required in order to restore an EDM processed surface of discontinuous islands of RCL. HPWJ was superior for removing RCL closely followed by grit blasting. However, grit blasting showed embedded grit blasting abrasive into the surface. Regarding surface roughness, it was shown that both grit blasting and HPWJ caused a roughening of the surface topography while shot peening generates a comparably smoother surface. All three post processes showed compressive residual stresses in the surface where shot peening generated the highest amplitude and penetration depths. However, the microstructure close to the surface revealed that shot peening had generated cracks parallel to the surface. The results strongly state how important it is to evaluate the surface at each of the different subsequent process steps in order to avoid initiation of cracks.
Additional to the conventional requirements for the component quality, the residual stress state of the sub-surface is becoming increasingly important as it affects the fatigue and wear behaviour of ...the component. First results indicate that micromilling allows the induction of high residual compressive stresses to increase the component's service life. Therefore, a specific design of the micromilling process with regard to the process parameters (feed per tooth and depth of cut), the use of cooling lubricant as well as the tool's cutting edge aimed for the controlled induction of residual compressive stresses in a hardened high-speed steel is presented.
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