Thickness Residual Stress Research Articles

Effect of variable-thickness webs on shear buckling performance of welded steel plate girders: Numerical analysis and proposed design rules

The use of welded steel members with varying thickness webs is becoming more popular and is often used as plate girders to accommodate varying loading distributions. Such variable-thickness steel plates are more susceptible to experiencing out-of-plane shear buckling, particularly in the thinner sections. However, no research work has been described investigating the shear buckling strength of welded steel plate girders with variable-thickness webs. This study presents an extensive numerical analysis to examine the shear buckling strength of such steel members with variable-thickness webs. Finite element (FE) models incorporating the material non-linearity and initial geometric imperfections were developed. To validate the modelling approach employed in this investigation, a total of 41 experimental results reported by the authors and other researchers were used. A detailed parametric study was subsequently undertaken, exploring various variables including local geometrical imperfections, aspect ratio, thickness change ratio, residual stress, and yield strength. To assess the accuracy of the current design regulations, the test and parametric study results were compared against design strengths. Upon comparison, the current design regulations as specified in Chinese Code (GB 50017), American Specification (ANSI/AISC 360-10) and Eurocodes (EN 1993-1-5) are over-conservative when computing the strength of such members with variable-thickness webs. Finally, new design rules covering both cases of variable-thickness and constant-thickness webs are proposed. A reliability analysis was conducted, and the values of β were 2.87, 3.00, and 3.04, respectively, indicating that new design methods developed in this study could closely determine the shear buckling strength of such welded steel plate girders with variable-thickness webs.

Fatigue Resistance of an Anodized and Hardanodized 6082 Aluminum Alloy Depending on the Coating Thickness in the High Cycle Regime

In this study, the high cycle fatigue behavior of an anodized 6082 aluminum alloy is investigated. Main focus is on the most relevant influencing factors for crack initiation and propagation under cyclic loading and damage mechanisms considering coating type, thickness, and residual stresses. The bare substrate is compared to anodized and hardanodized specimens with three coating thicknesses, for each coating type, in the range from 20 to 70 μm. Coating hardness and microstructure as well as residual stresses are analyzed. Fatigue and fracture behavior under alternating tension–compression loading is determined. Dependent on the coating thickness, the fatigue strength is reduced by 8%–50% after anodizing and by 50%–62% after hardanodizing. As the coating thickness is equal to the initial crack length from a fracture mechanical point of view, stress intensities at the crack tips are higher for thicker coatings respectively longer initial crack lengths. Therefore, propagation of fatigue‐induced cracks from the coating into the substrate is promoted for a higher coating thickness resulting in premature failure. A significant correlation between the coating thickness and tensile residual stresses induced by both coatings in the subjacent substrate is not found and residual stress influence on the overall fatigue strength is only minor.

Stress simulation of 6-inch SiC single crystal

With the size of SiC boule increasing, the problem of crystal creaking becomes more and more serious. In this work, residual stress in SiC boule is calculated by conducting stress simulation based on COMSOL and STR-VR software. A direct diagram comparing on crystal stress distribution is given. Besides, we provide relevant indications for the complete realization of large size SiC bulk with low stress for growing mainstream 6-inch SiC bulk with competitive thickness (40–50 mm) and low residual stress. And a 6-inch stress-free SiC bulk with improved quality was successfully fabricated based on our theoretical investigations.

ディープホールドリリング法による溶接部残留応力の高精度測定

This paper presents a method to improve accuracy of measuring through–thickness residual stress for welds through deep hole drilling (DHD) technique. The focus of this study is the effect of dimensions of the hollow cylinder, which is manufactured by drilling and trepanning processes, on residual stress measurement at the welds. First, stress concentration–induced plastic region around a reference (drilled) hole was theoretically quantified according to value of existing stress. The plastic region around the hole is not negligible for the welds at where high tensile residual stress exists. In order to reduce the influence of the plastic region, it is considered effective to set a large ratio of trepanning diameter to drilling diameter. Another noteworthy thing is that weld residual stress is present in a relatively narrow region along a weld line. Then, DHD technique was applied to measure through–thickness residual stress at the welds of melt–run welded–plate specimens under conditions of various drilling and trepanning diameters. These DHD residual stress measurement results were compared with those measured by X–ray diffraction and contour methods and calculated by weld thermal elastic–plastic analysis. As the results, it was shown that through–thickness residual stresses measured by DHD technique were obviously influenced by dimensions of the hollow cylinder. Measurement accuracy becomes higher with increasing the ratio of trepanning diameter to drilling diameter. Meanwhile, trepanning diameter should not be larger than width of generation area of high tensile residual stress to be measured at the welds. It was concluded that a method for determining appropriate dimensions of the hollow cylinder to be drilled and trepanned was successfully proposed to accurately assess through–thickness residual stress at the welds through DHD technique.

Investigation of thickness and welding residual stress effects on fatigue crack growth

This paper presents a numerical investigation of the effects of thickness and welding residual stress (WRS) on fatigue crack growth (FCG) for centrally cracked steel specimens. Various thicknesses and different welding heat inputs were used to study their influence on WRS distribution, crack driving force, and fatigue life. The effect of mean stress (σm) on the crack driving force and crack growth was also discussed. The results showed that thickness significantly influences the crack driving force and fatigue life when WRS is considered. A large crack driving force was obtained at and near the welded surface compared to that generated at the back surface, forming a variable distribution along the crack tip with increasing thickness. FCG rate at the welded surface will be faster than that at the back surface for the thicker specimens. The longest fatigue life with remarkable retardation was obtained under the smallest σm when WRS is dominant. The retardation effect disappeared with increasing σm. The mechanism controlling crack growth behavior due to the change in σm and thickness was also discussed.

Thermoforming process effects on structural performance of carbon fiber reinforced thermoplastic composite parts through a manufacturing to response pathway

Thermoforming process of thermoplastic-based continuous CFRP's offer a major advantage in reducing cycle times for large-scale productions, but it can also have a significant impact on the structural performance of the parts by inducing undesirable effects. This necessitates the development of an optimal manufacturing process that minimizes the introduction of undesirable factors in the structure and thereby achieves the targeted mechanical performance. This can be done by first establishing a relationship between the manufacturing process and mechanical performance and successively optimizing it to achieve the desired targets. The current study focuses on the former part, where a manufacturing-to-response (MTR) pathway is established for a continuous fiber-reinforced thermoplastic composite hat structure. The MTR pathway incorporates the thermoforming process-induced effects while determining the mechanical performance and principally comprises of material characterization, finite element simulations , and experimental validation. The composite material system selected for this study is AS4/Nylon-6 (PA6) with a woven layup. At first, the thermoforming simulations are performed above the melt temperature of PA6 using an anisotropic hyperelastic material model, and the process-induced effects such as thickness variation, fiber orientations, and residual stresses are captured from the analysis. Residual stresses developed in the formed structure during quench cooling from the elevated temperature are predicted by the implementation of classical laminate theory (CLT). These results are then mapped onto a duplicate part meshed suitably for mechanical performance analysis. A quasi-static 3-point bend test and a dynamic impact test are carried out and the results are compared with experimental tests. Experimental results from thermoforming, bending and dynamic impact trials show good agreement with the simulation results for the hat structure under consideration. Further, the static and dynamic performance is evaluated for the thermoformed structure and the effects of the thermoforming process are compared numerically, for the cases with and without the inclusion of process effects.

Effect of residual stress in longitudinal and transverse weld on global stability of box section steel column

In order to study the influence of welding residual stress on the global stability of box section steel column, the simulation of the side effects from the welding process of Q345 box section steel column by ABAQUS thermal-structure coupling analysis was conducted. A temperature-displacement explicit analysis step was established, and the residual stress caused by the temperature difference of the welding specimen was simulated by applying the body heat flux directly to the solid element within the same thickness range of the weld seam and defining the load amplitude; The calculated welding residual stress was taken as the initial defect of the structure and the global stable bearing capacity of the box section steel column was solved by the arc length method. Considering the influence of thickness and welding residual stress on the global stability of box section steel column, the calculation results of 12 finite element models were compared. The results show that under the same body heat flux, the residual stress of steel columns with different thicknesses during welding is different, and the maximum value can reach 791MPa; Under the condition of the same residual stress, the global stability bearing capacity of the box section steel column will also change irregularity with the change of thickness, and the global stability bearing capacity can be reduced by 3.07% compared with that no residual stress.

Measurement of Through–Thickness Residual Stress in Thick–Section Weldment by Advanced Deep Hole Drilling Technique

Measurement of Through–Thickness Residual Stress in Thick–Section Weldment by Advanced Deep Hole Drilling Technique

Residual stresses in the aluminium alloy 2014A subject to PAG quenching and vibratory stress relief

Generic hollow pump bodies made from aluminium alloy 2014A forgings have been solution heat treated and quenched into either cold water or polyalkylene glycol (PAG) solutions. An industry standard PAG type synthetic quenchant was evaluated, and the influence of two concentrations of the PAG solution on residual stress in the pump bodies was characterised, using neutron and X-ray diffraction. These residual stresses were then compared to those resulting after a pump in the as quenched condition was subject to a widely known but controversial commercial vibratory stress relieving procedure. The use of neutron diffraction allowed the through thickness residual stresses to be evaluated in the pumps. Results demonstrate that PAG quenching is highly effective at significantly reducing residual stress when compared to cold water quenching, but the impact is non uniform and cannot be simply quantified. In this investigation, we prove the effect of vibratory stress relief is negligible, and much less than that achieved by PAG quenching.

The influence of ultrasonic vibration on parts properties during incremental sheet forming

The integration of ultrasonic vibration into sheet forming process can significantly reduce the forming force and bring benefits including the enhancement of surface quality, the enhancement of formability and the reduction of spring-back. However, the influencing mechanisms of the high-frequency vibration on parts properties during the incremental sheet forming (ISF) process are not well known, preventing a more efficient forming system. This paper comprehensively investigates the effects of different process parameters (vibration amplitude, step-down size, rotation speed and forming angle) on the micro-hardness, minimum thickness, forming limit and residual stress of the formed parts. First, a series of truncated pyramids were formed with an experimental platform designed for the ultrasonic-assisted incremental sheet forming. Then, micro-hardness tests, minimum thickness measurements and residual stress tests were performed for the formed parts. The results showed that the surface micro-hardness of the formed part was reduced since the vibration stress induced by the ultrasonic vibration within the material which eliminated the original internal stress. The superimposed ultrasonic vibration can effectively uniform the residual stress and thickness distribution, and improve the forming limit in the case of the small deformation rate. In addition, through the tensile fracture analysis of the formed part, it is shown that the elongation of material is improved and the elastic modulus and hardening index are decreased. The findings of the present work lay the foundation for a better integration of the ultrasonic vibration system into the incremental sheet forming process.

Novel residual stress measurement technique to evaluate through thickness residual stress fields

In this present study, a novel approach of measuring through-thickness residual stresses based on the general strain relaxation methodologies has been described. This new method is an amalgam of the conventional deep hole drilling technique and the contour method with a reduced degree of damage to the component made during measurements. The annular deformation of a reference hole made at the measurement location has been interpreted as the displacement boundary conditions in a two-dimensional finite element model. In this regard, a thick, partially plastic four-point bent bar specimen was used to validate the proposed method. Through-thickness axial residual stresses induced in the bar were measured using the proposed technique and were observed to be in a reasonably close match with the theoretical and DHD results.

On mechanical properties of NiTi SMA wires prestrained by cold rolling

This study investigates mechanical properties and recovery stress of NiTi shape memory alloy (SMA) wires prestrained by different degrees of cold rolling. Using differential scanning calorimetry (DSC), the phase transformation temperature in cold rolled wires are compared with as received one. Monotonic and hysteretic tensile tests are conducted to determine the stress-strain curves of each cold rolled wire. Thickness recovery ratio, recovery and residual stress of constrained wires are measured during temperature variations. It is found that cold rolling can reduce detwinning process in tensile behavior and finally eliminate it with more cold work. Moreover, the optimal degree of cold rolling is obtained in terms of maximum induced recovery stress, safety of storage and minimum morphological changes and costs. Based on the experimental data, the degree of cold rolling can be categorized into three ranges of slight, moderate and extensive.

Residual stress and fracture toughness of thick 8YSZ-Al2O3 composite coatings via a modified Vickers indentation method

The aim of current study is to explore a modified Vickers indentation method for characterization of the residual stress and fracture toughness of thick 8YSZ-Al2O3 composite coatings. According to the results, the through thickness residual stress of coating with thickness of 870 μm increased from −114.7 MPa to −368.3 MPa as the Al2O3 content varied from 10 wt% to 40 wt%, which was higher than that with thickness of 590 μm. It was revealed that the increase of Al2O3 content and coating thickness resulted in accumulation of the compressive residual stress in these thick composite coatings. Simultaneously, the fracture toughness of coatings with different thicknesses also exhibited an increasing tendency with the increase of Al2O3 content.

Experimental and Numerical Investigation on Global Buckling of Q690 High Strength Steel Tubes under Axial Compression

Due to the excellent bearing capacity, high-strength steel (yield stress ≥ 460 MPa) is gradually utilized in the design of super high-rise buildings, long-span bridges and other important structures to reduce section size and self-weight. In the present study, the axial compression test and numerical simulation were carried out to investigate the global buckling of high strength steel tubes. Eighteen specimens of Q690 high strength circular steel welded tubes were tested under axial compression to obtain the ultimate bearing capacity and buckling mode. An effective finite element numerical model incorporating the residual stress and geometric imperfection was established to analyse the influence of slenderness ratio, diameter thickness ratio and residual stress on the global stability coefficient. According to the calculation results, the column curve of Q690 circular steel tube was obtained and compared with three different codes. Finally, a formula to calculate the global stability coefficient of Q690 welded circular steel tubes was suggested.

Significance of material constitutive model and forming parameters on the crashworthiness performance of capped cylindrical tubular structures

An accuracy of crushing performance indicators is critical to evaluate in finite element crushing simulations particularly for the press-formed capped tubular energy absorbing structures. It is essential to select the appropriate material constitutive model and to incorporate the forming parameters into the finite element crushing model as a vital input. Hence in the present article, the influence of various material constitutive models and forming (multi-stage deep drawing) parameters on the axial crashworthiness characteristics of thin-walled capped cylindrical tubes were investigated numerically. Both forming and crushing simulations were executed by nonlinear finite element LS-DYNA® code. The forming parameters such as thickness distribution, residual stress, and effective plastic strain were mapped to a finite element crushing model of the tube. The numerical predictions of the thickness distribution and final deformed profiles of the capped cylindrical tubes are correlated with the experiments. The results revealed that the forming parameters have a substantial effect on the crushing performance of the deep drawn capped cylindrical tubes. As a result of these analyses, the thickness and strain predictions strengthens the tube and significantly influenced the crushing performance indicators such as initial peak crushing force, mean crushing force, and the energy absorbing capacity.

Effect of stand-off distance in thermally sprayed copper coating

ABSTRACTThe present work demonstrates a two-step method to deposit copper over ABS plastic using electric arc thermal spray means. The energy dispersive spectroscopy confirmed the deposition of copper metal. The adhesive strength demonstrates an increasing trend as the stand-off distance increases from 11 to 17 cm, and decreases as the stand-off distance further increases till 23 cm. For surface roughness, porosity and coating thickness, an increasing trend was noticeable as the stand-off distance increases from 11 to 15 cm, and then assumes a decreasing trend till 23 cm of stand-off distance. The surface residual stress demonstrates decreasing course as the stand-off distance increases from 11 to 23 cm. Different factors engaged during the deposition of molten metal droplet on the ABS substrate such as thermal softening, oxide content, particle velocity and cooling rate dictated the characteristic outcome such as adhesive strength, roughness, porosity coating thickness and surface residual stress.

Investigation into the distribution of residual stresses in pressed-braked thin-walled steel lipped channel sections using the 3D-FEM technique

Cold formed steel sections are normally produced by cold work manufacturing processes. The amount of cold work to form the sections may induce residual stresses in the section, especially in the area of bending. Studies by previous researchers of the effects of local buckling on the failure mechanics of thin-walled compression members have shown that ultimate failure will occur when the yielding has reached most of the middle surface in the corner region of the sections. Hence, these cold work processes may have significant effects on the section behaviour and load-bearing capacity. Most of the studies have investigated the effect of residual stresses raised from roll-forming operation and its influence on steel section behaviour. However, press-braking has not received much attention. Therefore, a 3D finite element simulation has been employed to simulate this forming process. This study investigated the magnitude and distribution of residual stresses along the length of the corner region and through thickness residual stress variations induced by the press-braking forming process. The study concluded that residual stresses are not linear longitudinally (along the corner region). Maximum residual stresses exist near the middle surface of the plate. The neutral surface contains a combination of compressive and tensile residual stresses. The neutral axis is shifted from mid-surface by 7.5% of the plate thickness due to bending. The comparison of the 3D-FE results with the existence of 2D-FE results illustrates that the 3D-FE results show a variation in the transverse and longitudinal residual stresses along the plate length.

In-situ observations of the fracture and adhesion of Cu/Nb multilayers on polyimide substrates

Cu/Nb nanoscale metallic multilayers have been extensively investigated to understand how their mechanical behavior is influenced by the individual layer thickness. The general observed trend is that the yield stress of the multilayer increases with decreasing layer thickness. Important mechanical behaviors that have not been studied in-depth are the fracture of these multilayers and adhesion energy between the multilayer films and their substrate. Here, the influences of the layer thickness, layer order, and initial residual stresses of Cu/Nb multilayers on polyimide were examined using in-situ x-ray diffraction and confocal laser scanning microscopy under tensile loading. With these techniques, it was possible to calculate the stresses developing in the individual materials and measure buckles that could be used to evaluate the interfacial adhesion. Layer thickness, deposition order, and the initial residual stresses were not shown to influence the initial fracture strains of the Cu/Nb multilayer systems under tensile loading conditions. However, the adhesion energy between the multilayer and substrate was affected by the layer deposition order and by the initial residual stresses.

Fatigue Life Prediction for Transverse Crack Initiation of CFRP Cross-Ply and Quasi-Isotropic Laminates.

Carbon fiber reinforced plastic (CFRP) laminates are used as main structural members in many applications. Transverse cracks that form in 90° layers of CFRP laminates are mostly initial damage in the case where tensile loading is vertically applied to the 90° layers of CFRP laminates, and they are the origin of more serious damage of delamination and fiber breakage. It is thus important to predict quantitatively the transverse crack initiation of CFRP laminates subjected to cyclic loading to ensure the long-term reliability of the laminates. The initiation and multiplication behaviors of transverse cracks strongly depend on the laminate configuration, thickness, and thermal residual stress. Therefore, a model based on the Walker model was proposed to predict transverse crack initiation in CFRP cross-ply and quasi-isotropic laminates under cyclic loading in the present study. The usefulness of the proposed model was verified with 10 different CFRP laminates formed from four different prepregs with epoxy resin matrices. The analysis results were in good agreement with experimental results. The fatigue life was expressed with three constants, which related to the fatigue strength reduction, the normalized fatigue strength at N = 1 cycle, and the contribution of stress amplitude to the fatigue life, and they are independent of the laminate configuration.

Evolution of White Layer during Wire-Cutting and Comparison of Several Methods to Improve Surface Integrity for Fineblanking Tools

Powder metallurgical high speed steel (such as S390) has superior mechanical properties and been used as fineblanking tools. The electrical discharge machining has been widely used for cutting fine blanking tools which are made of especially hard tool steels. Whereas, its thermal nature causes great concerns regarding surface integrity, which matters a lot to tool life. In the present study, the evolution of surface integrity of the S390 with multi-cutting is comprehensively compared. The result shows that the surface roughness, white layer thickness and surface residual stress decrease with the increase of cutting pass. Additionally, the effectiveness to remove white layer on HSS S390 by manual and towed polishing and electrolytic polishing are compared. At last, a device of abrasive water jet polishing is designated to remove the white layer resulted from wire-cutting.