Plastic Strain Distribution Research Articles

Forming of Tailor-Welded Blanks Through Centerline and Offset Laser Welding

The forming of low and high strength tailor welded steel sheets into preferred three-dimensional shapes by a homogeneous distribution of plastic strain is highly difficult. In this work, the formability of tailor-welded blanks prepared using laser welding of low (IS 513 CR2, IS 513 CR3) and high tensile strength (AISI 304) steel sheets were analyzed and the formation of the edge profile and weld line movement was also examined. The results showed a decrease in formability characteristics owing to the increase in Strength Ratio. Scanning Electron Microscope analysis of the fracture locations within unsuccessfully formed tailor welded blanks showed the presence of tempered martensite in the soft zone was found to be the most predominant factor affecting its failure. The formability characteristics of investigated tailored blanks were predicted using finite element simulation and compared with experimented results for validation.

In-situ study of microstructural evolution and strain distribution during single axis tension of 316L stainless steel prepared by selective laser melting

A combined digital image correlation (DIC) and electron backscatter diffraction (EBSD) study of the multi-scale strain distribution and microstructure evolution during tensile loading has been carried on 316L stainless steel samples prepared by selective laser melting (SLM). The initial material shows a range of microstructural heterogeneity. Macro- and micro-DIC results reveal a non-uniform distribution of plastic strain over several length scales, including the formation of localized slip-bands at the intra-grain level. No obvious correlation is found between the heterogeneity in plastic strain and either grain size, grain morphology or the orientation of the cellular structures formed during SLM. Intragranular orientation subdivision and the evolution of local misorientation have also been investigated by EBSD during tension, where a complex pattern of heterogeneity in both microstructure and plastic strain is found.

Failure mechanism of long-distance pipelines having gouged dents

The failure mechanism and ultimate internal pressure of X52 grade steel pipe having gouged dent defects are studied using a full-scale burst experiment and finite element analysis. The accumulation and distribution of the equivalent plastic strain during denting, springback and pressurization process are analyzed. According to the test results and the morphology of the pipeline after burst, a failure criterion is proposed and verified using full-scale tests for two steels with different grades. The influence of defect sizes as well as the interaction of dent and gouge on the failure internal pressure is fully analyzed. Compound defects reduce the pipe strength more than the single defect and the influence degree varies with defect sizes. The findings of this paper are helpful to accurately predict failure internal pressure of pipelines having gouged dent, which is often required for pipeline evaluation and repair work.

Mechanical characteristics of H-shaped shear panel damper fabricated by SPHC steel according to stiffener reinforcement and stud support installation

In this study, cyclic loading tests were conducted at a constant deformation amplitude to investigate the mechanical characteristics of an H-shaped shear panel damper (HSPD)1HSPD: H-shaped shear panel damper; FE: Finite element.1 fabricated with the use of SHPC steel. Two experimental conditions were established according to the test parameters for the damper unit and damper system (damper supported by studs). For each experimental condition, two types of HSPDs were applied according to the presence or absence of stiffener reinforcement. The experimental results of the damper supported by studs showed that the elastoplastic behavior of the damper part was similar to that of the damper unit. Additionally, it was shown that the stud maintained its elastic state until the end of the test. Furthermore, it was found that when the damper was reinforced with a stiffener, the plastic deformation capacity of the damper was improved significantly compared with the case in which the stiffener was not reinforced. The effect of the stiffener reinforcement was also investigated based on nonlinear finite element analysis. The plastic strain distribution of the HSPD panel was evaluated based on the analyzed results that tracked the load–deformation relationship of the experimental results. As a result, the HSPD panel part was segmented by the stiffener and the width-to-thickness ratio was reduced so that the distribution of the plastic deformation values in the panel was relatively stable, and the deformation in the out-of-plane direction could be suppressed. The results showed that the shear buckling for the same shear deformation could be delayed according to the effect of the width-to-thickness ratio of the panel which was reduced by the reinforcement of the stiffener.

Investigation by numerical modeling of the mechano-electrochemical interaction at a dent-corrosion defect on pipelines

Dent-corrosion combination defect causes the curvature change and wall thickness thinning on the pipeline simultaneously, which poses a higher threat to pipeline integrity than a single type of defect. In this work, a finite element (FE) based multi-physical field coupling model was developed to investigate the mechano-electrochemical (M-E) interaction at the dent-corrosion defect on an operating X80 pipeline. Parameter effects, including internal pressure, the diameter-to-thickness ratio of the pipe, and the geometries of the defect, were determined. Results demonstrate that the local equivalent plastic strain and anodic current density are insensitive to practical operating pressure but highly dependent on the diameter-to-thickness ratio and the geometries. A critical dent depth of 4% of the pipe diameter existed, exceeding which the increase of dent depth hardly affects the maximum equivalent plastic strain of the pipeline. The size of the indenter is negatively correlated with the equivalent plastic strain and the anodic current density at the defect center. There is also a critical threshold of 10% pipe diameter, below which the distribution of equivalent plastic strain and anodic current density at the defect is significantly affected. The effect of the circumferential dent is significant, while that of the axial dent is marginal.

Compressive Behavior of a Fully Prefabricated Liftable Connection for Modular Steel Construction

Modular steel construction (MSC) consists of the off-site prefabrication of a fully finished module and the on-site assembling of the module unit. The popularity of MSC is on the rise, attributable to its technical advantages of speed and quality of buildings with repetitive units. Inter-module connection is critical for the overall stability and load-bearing capacity of MSC. An innovative, fully prefabricated liftable connection (FPLC) using standard corner fittings and long stay bolts is proposed in this paper. This paper focuses on the axial compressive behavior and design of FPLC. Five full-scale specimens were tested under axial compression. Local buckling of the column and shear of the long stay bolts were observed during the test. It can be concluded from the test results that the load-bearing capacity may decrease as the number and diameter of the stay bolts increase. A three-dimensional nonlinear finite element model (FEM) was developed and validated against the test results by general purpose finite element software ABAQUS. Furthermore, a parametric study was conducted using the verified FEM to provide a better understanding of the axial compressive behavior of the FPLC. The results of the parametric study indicated that the corner fitting can be up to 15% lighter for columns with thicknesses of 6 mm and 8 mm without substantial reduction of the axial load-bearing capacity of the FPLC. Moreover, the location of the column can be adjusted to achieve a uniform Von Mises stress and equivalent plastic strain (PEEQ) distribution of the connection. The presented research work provides an engineering-practical inter-module connection on its axial compressive behavior, which will provide helpful references for further application of MSC.

Effects of material properties and mismatch on unified constraint parameter

In order to develop unified constraint-based fracture assessment method, it needs to investigate the effects of material properties and mismatches on the unified constraint parameter and develop the parameter solutions for cracked structures. In this work, the effects of material properties (yield strength and hardening exponent) and strength mismatch on the in-plane and out-of-plane unified constraint parameter Ad* of semi-elliptical surface cracks in homogeneous and welded plates have been investigated. The results show that the parameter Ad* does not essentially change with yield strength and hardening exponent of materials for cracks in homogeneous plates under bending and tension loads. The parameter Ad* of weld centerline cracks in welded plates under bending and tension loads is essentially independent on the strength mismatch. These advantages of Ad* are conducive to the development of its solutions for homogeneous and welded structures containing cracks. The effect of weld width on Ad* is related to loading mode, mismatch (undermatch or overmatch) and crack size. The mechanism of the influence of material properties, strength mismatch and weld width on Ad* has been explained by its definition and the crack-tip plastic strain distribution. The development and application of Ad* solutions for homogeneous and welded structures also have been discussed.

Digital-image-correlation observation of cyclic plastic strain field during the damage-accumulation mode of fatigue crack propagation under pure cyclic mode II loading for cold-rolled SUS430 steel

Fatigue cracks propagate intermittently by the cyclic initiation and coalescence of voids in front of the fatigue crack tip during fatigue crack propagation in the damage accumulation mode. In this study, digital image correlation is used to measure the evolution of the measured plastic strain distribution near the crack tip with loading and cyclic loading during one cycle. The evolution is estimated based on strain distribution measurements results and mechanical considerations. The strain is set at the unloading point because results indicate that the crack propagated in the region where the experimental shear plastic strain is the maximum in the first quarter cycle (at the first maximum load); the measured plastic strain is almost constant in the region where the crack propagates, and the crack is initiated by a subsequent cyclic strain localization and coalescence with the main crack. This study clarified the damage accumulation mode fatigue crack propagation mechanism with the measured strain behavior, and presents a method to use continuum plastic strain at the crack tip as a mechanical driving force for this mode.

Analysis and mitigation of the bending deformation in girth-welded slender pipes with numerical modelling and experimental measurement

Girth welded pipes are extensively used in the components of energy and chemical industries. The welding deformation modes depend on various influential factors, such as process schemes and geometric dimensions. The objective of this study is to investigate the characteristics and mechanism of the bending deformation generated in the girth welded slender pipes, and to develop mitigation schemes. Firstly, several 304-type stainless steel pipes were girth welded using four different welding schemes, and the 3D shape measuring system was employed to capture the basic welding deformation modes. Numerical models were developed to simulate the corresponding welding processes with experimental verification. And the mechanism of bending deformation was further clarified based on inherent strain theory. The results show that obvious bending was observed in the pipe of continuous full turn welding due to the non-uniform distribution of the axial residual plastic strain on the circumference. Welding of half turn tends to generate a bending from the weld starting position to the finish position besides the bending toward the weld side, which brings about the mutual elimination of the bending deformation with same direction half-turns and the superposition with reverse direction half-turns. Furthermore, a dual-torch scheme was proposed for girth welding of slender pipes and presented a great potential to mitigate the bending deformation with an improved welding efficiency.

Design of a fuse link beam-to-column connection for earthquake resistant moment frames

This paper presents details of numerical investigations on exterior beam-column joint sub-assemblages with a novel fuse link beam-to-column connection. The connection is designed to protect the connected beams and columns of moment frame buildings during strong earthquake shaking by confining the inelasticity within the connection elements. The connection consists of two fuse links and one standard shear tab. Detailed numerical analyses are performed to study the behaviour of the connection including the level of plastic strain distribution around the connection region. First, the results of numerical analyses are validated using experimental results of full-scale experimental investigations on exterior beam-column joint sub-assemblages, which demonstrated the good performance and replaceability of the connection. Then, parametric studies are carried out to identify and investigate the effect of major parameters on the connection behaviour - the key parameters being the geometric parameters of fuse link and the beam depth. Finally, it is shown that the results of the numerical study are in good agreement with those of an analytical model that can be used for proportioning of the fuse link connection at the design stage. Also, the results of the analytical model can be used in global analysis of moment frames using the developed fuse link connection.

Investigation on deformation of DP600 steel sheets in electric-pulse triggered energetic materials forming

Electric-pulse triggered energetic materials forming (ETEF) is a high-speed manufacturing process, which utilizes the chemical energy released by energetic materials (EMs) triggered by underwater wire discharge to plastically shape metals. ETEF is not fully understood, particularly in research on the discharge characteristics of energetic materials triggered by metal wires and the deformation process of metal sheets. The above two problems were investigated in this paper using experimentation and numerical simulation. For the pulse discharge characteristics, the peak values of voltage and current were reduced during the triggering process of energetic materials, and the triggering energy consumption of energetic materials was quantified to be about 200 J. The matching parameters of different capacitor-voltage devices may be insensitive to triggering the energy release of energetic materials. The maximum major strain and thinning rate of the bulged specimen under ETEF conditions were significantly reduced when compared to the quasi-static specimen with the same bulging height, and the specimen’s deformation uniformity and strain distribution were improved. The simulation results showed that the addition of energetic materials significantly improved the plastic strain energy of the blank. The deformation of the blank in ETEF can be divided into two stages: the initial chemical energy action stage and the inertia action stage. The bulging height of sheet metal increased by nearly 301% in inertia action stage, accounting for 80% of the total deformation time, and the effective plastic strain distribution was more uniform.

Numerical and Experimental Analysis of Titanium Sheet Forming for Medical Instrument Parts.

The paper analyses the forming of the surgical instrument handles made of Grade 2 titanium sheets. Sheet metal forming is a technology ensuring high strength and light weight of products. Replacing stainless steels with titanium further reduces instrument weight and additionally provides the required resistance to corrosive environments typical for surgeries. The low instrument weight is important to prevent fatigue of surgeons and allow them to maintain high operational accuracy during long term surgeries. The numerical analysis of the technological process was performed in order to adapt it to forming tool handles using titanium sheets instead of steel sheets. The numerical calculations were experimentally verified. It was found that, in the case of titanium handles, it is necessary to use a blank holder in the first forming operation to eliminate sheet wrinkling in the flange area. The shape and dimensional accuracy of the drawn part after trimming were high enough and the 4th forming operation became unnecessary. Moreover, the process modification included lubrication using rapeseed oil with the addition of boric acid, which effectively prevents the galling of titanium on the working surfaces of the steel tools and ensures a more uniform distribution of plastic strains in the drawn part.

A novel process for manufacturing large-diameter thin-walled metal ring: Double-roll pendulum hot rotary forging technology

Large-scale thin-walled metal ring parts are the key main structural parts in new energy sources and aerospace equipment. How to form reliable large-diameter thin-walled metal rings has become the focus of attention of researchers. This paper proposes a double-roll pendulum hot rotary forging (DRPHRF) process for forming such components and demonstrates the feasibility of forming rings with a diameter of 4 m and a thickness of 0.1 m (axial) using this process. The forming process of DRPHRF is analyzed and designed, and the critical instability and forming conditions are obtained. The finite element model of the ring DRPHRF was established in the DEFORM-3D software using the scale-down process with a ratio of 10:1, and the accuracy of the model was verified through experiments. Based on the reliable finite element model, the deformation behavior of the ring DRPHRF is analyzed. The results show that each part of the ring is subjected to periodic stress, which reduces the residual stress. The uniform circumferential equivalent plastic strain (PEEQ) distribution and the characteristics that the axial PEEQ becomes uniform as the thickness decreases are conducive to forming a thin-walled ring with uniform performance. There is less heat loss during the forming process, which is beneficial to reduce the stress during forming.

Analysis of Strain Inhomogeneity in Extruded Al 6061-T6 Processed by ECAE

This paper presents an analysis of the test and simulation of the equal channel angular extrusion (ECAE) of a commercial Al 6061-T6 alloy previously extruded. Special emphasis is given to the analysis and comparison of the simulated values and distribution of equivalent plastic strain with those calculated with analytical models across the height of the middle section of the deformed billet. The results reveal the limitations of the analytical models when the effects of the inner die corner and curvature of the outer wall on the material response during the test are considered for the ECAE device used in this work. Specifically, in the simulations performed in this work, the plastic deformation zone, far from being uniform, extends along with the height of the billet where, in particular, near the inner wall over the inlet and outlet channels, even in the central region of the billet, the equivalent plastic strain is not homogeneous or discontinuous, varying mainly due to the effect of the curvature of the outer channel.

Effects of microstructure and inherent stress on residual stress induced during powder bed fusion with roller burnishing

A hybrid metal-additive manufacturing (AM) process that combines laser-based powder bed fusion (PBF-LB) with interlayer burnishing is investigated using a comprehensive modeling framework to provide new insights into how the inhomogeneous microstructure and residual stress from the laser powder bed fusion process affect the induced residual stress field that evolves during interlayer burnishing. Researchers have recently studied changes in microstructure resulting from similar hybrid metal-additive processes, however, it was only hypothesized that the resulting microstructure has some influence on the induced residual stress. In addition, researchers have numerically investigated the influence of burnishing/rolling process parameters on induced stress but neglected the effects of microstructure, thereby making homogeneous, isotropic assumptions. Such practice inhibits the prediction of microstructure-driven anisotropy that can exist in the inhomogeneous fused layer. This paper parametrically examines the influence of microstructure modeling, inherent residual stress mapping, and environment temperature on the induced residual stress during the hybrid metal-additive process. The demonstrated modeling framework incorporates inherent residual stresses that emerge from the laser powder bed fusion process, as well as the predicted microstructure, in a subsequent burnishing simulation to elucidate their individual and combined influences on the burnishing-induced residual stress. Findings reveal that modeling an inhomogeneous PBF-LB microstructure introduces an anisotropic distribution of plastic strain and residual stress along the burnished surface; a periodicity in planar stress components along the treated surface coincides with the PBF-LB scan lines. Effects of inherent residual stress on the burnishing-induced residual stress is less significant, but nonetheless observable. Elevated temperatures not only reduce the magnitude of compressive residual stress induced but also result in less variation of residual stress component magnitudes predicted along scan lines and hatch spaces. The presented framework offers new insights into the decoupled influences of microstructure and PBF-LB residual stress on burnishing-induced stresses that are not distinguishable via experimental techniques. However, trends in averaged residual stress through the depth of the specimen, as well as surface hardness magnitudes after burnishing show good agreements, respectively, with X-ray diffraction and microindentation measurements documented in the literature.

Thermomechanical properties of cold-sprayed copper coatings from differently fabricated powders

Cold spraying is a material coating deposition process that accelerates powders to high velocities, thereby causing them to undergo intensive plastic deformation upon impact in solid states at temperatures far below their melting point. The critical powder velocity is the most influential factor determining the bonding strength of cold spray coatings and strongly depends on the thermomechanical interactions between the sprayed powder and the substrate material. In this study, three commercially available Cu feedstocks fabricated through electrolysis (EP), gas-assisted water atomization (WA), and inert gas atomization (GA) were characterized and used to produce cold spray coatings. The kernel average misorientation (KAM) data from electron backscatter diffraction (EBSD) analysis were used to determine the plastic strain distribution of the Cu powders and coatings through local crystallographic misorientation. The percentages of the local misorientation within grains with angles higher than 1.5° in the EP (3.0%) and GA (2.8%) powders were similar, whereas that in the WA powder (0.6%) was the smallest. According to the EBSD data, more dynamic recrystallization and, in turn, more plastic strain release occurred in the as-sprayed EP and GA coatings than in the WA coating. The tendency of recrystallization of the twin grains in the as-sprayed GA coating and its influence on the ductility of the coating are discussed in relation to the GA powder's bonding mechanisms. The static recrystallization in the postannealing treatment (at 190 °C for 1 h) of the EP and GA coatings being more prominent than in that of the WA coating supports differences in dynamic recrystallization among the cold-sprayed coatings determined through EBSD analysis. The thermal conductivity of cold-sprayed Cu coatings is influenced by recrystallization and strain release.

Phase-field framework with constraints and its applications to ductile fracture in polycrystals and fatigue

Modeling of ductile fracture in polycrystalline structures is challenging, since it requires integrated modeling of cracks, crystal plasticity, and grains. Here we extend the typical phase-field framework to the situations with constraints on the order parameters, and formulate two types of phase-field models on ductile fracture. The Type-I model incorporates three sets of order parameters, which describe the distributions of cracks, plastic strain, and grains, respectively. Crystal plasticity is employed within grain interiors accommodated by J2 plasticity at grain boundaries. The applications of the Type-I model to single crystals and bicrystals demonstrate the influences of grain orientations and grain boundaries on crack growth. In the Type-II model, J2 plasticity is assumed for the whole system and grain structures are neglected. Taking advantage of the efficiency of the fast Fourier transform, our Type-II model is employed to study low cycle fatigue. Crack closure and striation-like patterning of plastic strain are observed in the simulations. Crack growth rate is analyzed as a function of the J-integral, and the simulated fatigue life as a function of plastic strain agrees with the Coffin–Manson relation without a priori assumption.

Friction Coefficient Effect on Stress-Strain Distribution of Ceramic Coated Aeroengine Substrate Using Finite Element Analysis

This study presents the investigation of friction coefficient effect on stress-strain distribution of ceramic coated aeroengine specific material substrate using explicit finite element method. Half-cylinder-on-flat contact configuration subjected to normal and tangential loading is examined. Elastic ceramic coated elastic-plastic Ti-6Al-4V and Super CMV substrates are assigned to study the influence of different friction coefficient on contact pressure, von Mises stress, tangential stress and equivalent plastic strain distribution. The outcome of present research is quite revealing that stress-strain distribution response is remarkable for sliding step which experiencing significant traction compared to normal loading step. Higher possibilities of coated substrate equivalent plastic strain (plastic deformation) are registered under higher friction coefficient cases due to substantial resistance to overcome relative to tangential motion of contacting bodies.

Evolution of stress and strain in 2219 aluminum alloy ring during roll-bending process

The residual stress of large aluminum alloy ring after heat treatment will cause serious deformation in subsequent processing. The conventional methods for reduction of residual stress (such as stepwise cold pressing and bulging) have little effect in the residual stress reduction for large-scale ring component and will induce inhomogeneous stress distribution. In this paper, roll bending process is adopted to reduce the quenched residual stress of 2219 aluminum alloy super-large ring. The numerical model of roll bending process was established, and the evolution and distribution of stress and strain after roll bending were studied. The influence of roll winding number on the uniformity of stress and strain was analyzed. The results show that the arch-shaped quenched residual stress of the ring changes to N-shaped distribution from inside to outside after roll bending process. The value of the residual stress reduces from ± 180 MPa in quenched state to the value within ± 50 MPa in roll bended state. With the increase of roll winding number, the stress uniformity is improved, but the stress reduction amplitude is basically the same. By analyzing the elastic–plastic strain distribution characteristics and strain springback law of the ring after roll bending, the formation mechanism of N-shaped residual stress distribution after roll bending is revealed.

An Improvement in Constrained Studded Pressing for Producing Ultra-Fine-Grained Copper Sheet

In this study, constrained studded pressing (CSP) was modified to produce ultra-fine-grained copper sheets and is called modified-CSP. In modified-CSP, due to the selection of asymmetric semicircle studs, the maximum groove depth can increase up to three times the sheet thickness. In the CSP method, the groove depth is selected to be equal to the sheet thickness. To investigate the effective plastic strain, a finite element model (FEM) of modified-CSP was established. For this purpose, DEFORM-3D commercial software was used. A simulation showed that the modified-CSP process was capable of using higher strain, about 0.8 in each pass than in CSP. Copper sheets were deformed up to 10 passes by modified-CSP. The microstructure of the produced samples was analyzed. The results show that the grain size decreases in the first pass. In addition, with increasing plastic strain, the structure of the twins bands was observed. Mechanical properties, including tensile properties and Vickers microhardness, of the samples during the process were investigated. The strain inhomogeneity factor (SIF) and the hardness inhomogeneity factor (HIF) were used to quantitatively express the uniformity of distribution of effective plastic strain and Vickers microhardness, respectively. Compared with CSP and constrained groove pressing (CGP), the results showed that although the ultimate tensile strength (UTS) has significantly increased, the ductility values have remained almost constant. Moreover, in modified-CSP, the load-die stroke diagram increases almost evenly due to the removal of the stud interface area and progressive engagement. Therefore, modified-CSP copper sheets showed superior tensile properties such as good toughness.