V-bending Tests Research Articles

A novel integrated hot forming with in-situ stress relaxation-aging for titanium alloy thin-walled components

The simultaneous achievement of high strength and precision in the fabrication of titanium alloy thin-walled components is a long-standing issue. This work proposes a novel integrated forming process, named Hot Forming with In-situ Stress Relaxation-Aging (short for HF-ISRA) to solve the special issue. In contrast to usual isothermal forming, the forming temperature in the proposed process is raised to the solution treatment temperature. Dies at a lower temperature realize in-situ stress relaxation-aging after hot forming. The role of the dies, in addition to forming, is also achieved post-forming heat treatment. This novel process comprises three main steps: solution heat treatment, rapid forming at solution temperature, and in-situ stress relaxation-aging. An experimental prototype of HF-ISRA was developed for V-bending test, in which a sheet blank was rapidly heated using electric current and the forming die was heated using heating rods. The process window of the proposed HF-ISRA was established based on the V-bending and uniaxial tensile tests of the TA15 titanium alloy. The results showed that compared with cold forming, the springback angle obtained using the optimized HF-ISRA decreased by 97.8 %, from 9°06′ to only 12′. The tensile strength at room temperature and 500 °C was improved by 4.4 % and 10.9 % compared with the as-received material, respectively. The relaxation mechanisms of αs were the precipitation, growth, and then globularization. The relaxation mechanism of αp was dislocation movements. The strength improvement in HF-ISRA was due to the formation of αs and dislocations strengthening. The stress-induced twinning in αs was also a contributor. The proposed novel process provides a new route for the fabrication of titanium alloy thin-walled components with high precision and strength.

Measurement of bending deformation of sheet metal using machine vision

ABSTRACT The aim of this study is to propose a fast and effective method to measure the curvature distribution and bending angle of sheet metal in the bending deformation using machine vision. An open-source computer vision library (OpenCV) was used for image processing. First, the image is calibrated for perspective and lens distortion. After that, the image is subjected to color extraction and binary conversion. The outer geometry of the material was obtained using edge detection. With the proposed method, the bending angle and curvature distribution in the four-point bending and V-bending tests are measured. The proposed machine vision method exhibits errors of 0.55% and 0.12% compared with the 3D scanner measurements for the bending angle in the four-point bend and V-bend tests after springback, respectively. The curvature distribution in bending deformation was also obtained. As an application of the proposed method, the moment – curvature diagram of the material was determined based on the obtained curvature at the center. Additionally, the optimal stroke was calculated for the target bend angle during the V-bending process. The proposed method proved to be an accurate and efficient way to measure bending deformation of sheet metal.

Microstructure and properties of galvannealed coatings at different galvannealed time

Microstructure and properties of galvannealed coatings at different galvannealed time was studied by electron probe micro-analyzer (EPMA), X-ray diffractometer (XRD) and V-bend test. The study showed that δ and Γ1 phase was formed at galvannealed 15 s, and δ, Γ1 and Γ phases were formed at galvannealed 20 s and 25 s. The cracking area of coatings was propagated along Γ1/steel or Γ/steel interface and intersected with the serious cracks, which caused the coatings to be dropped. With the extension of galvannealed time, the areas of cracks and fracture at Γ/steel interface were reached 50% at galvannealed 25 s, and interfacial adhesion of Γ/steel was decreased due to the increase in thickness of Γ phase.

Formation of ultrafine grain and mechanical properties in commercial pure titanium subjected to heavy-reduction thermomechanical processing around β transus temperature

The purpose of this study was to investigate the flow stress behavior and formation of ultrafine grains (UFGs) by focusing on the effects of strain-induced dynamic transformation (SIDT) and dynamic recrystallization (DRX) under hot forming near the β transus temperature. Heavy-reduction thermomechanical controlled processing (TMCP) of commercial pure (CP) titanium was performed at deformation temperatures ranging from 700 to 1000 °C, a strain rate of 1 s−1, and a 70% height reduction, followed by water cooling immediately after compression. The flow stress at 0.2 strain decreased significantly from 96 to 25 MPa when the deformation temperature increased from 800 to 900 °C. Equiaxed UFGs with a grain size of 2–4 μm formed in this temperature range. The primary mechanisms of grain refinement in the CP titanium under heavy-reduction TMCP were DRX occurring at 800 °C and a combination of DRX and SIDT (β → α) at 900 °C. Of the 700, 900, and 1000 °C TMCP-treated specimens subjected to a 90° V-bending test, the yield force of the 700 °C TMCP-treated specimen was the highest. When the TMCP temperature was increased from 900 to 1000 °C, the yield force decreased significantly, whereas the springback amount increased. The α phase grain refinement achieved by the TMCP at 700 °C increased the force, and the retained β phases within the 1000 °C TMCP-treated specimen increased the springback amount.

Development of double-sided friction stir welding of advanced high strength steel sheets

Double-sided friction stir welding (FSW), performed with a pair of rotation tools, was studied in an attempt to increase the joining speed when applied to welding of automotive advanced high-strength steels (AHSS). The experiments were performed on butt joint of 1.6-mm-thick 1180 MPa grade steel sheets, and sound welds were obtained when the travel speed was no more than 4 m/min, fairly comparable with laser beam welding (LBW), one of the conventional automotive welding processes. It was clarified by experimental and numerical analyses that the enhancement in travel speed was due to the unique material flow interactively developed by a pair of rotation tools. Double-sided FSW was performed at 3 m/min of joining speed on butt joint of dissimilar steel sheets in a combination of 1.5 GPa and 590 MPa grades. The mechanical properties of the welded joints were evaluated by tensile, V-bending and Erichsen tests comparing with LBW, which verified that double-sided FSW exhibited sufficient mechanical properties for the press-forming to produce automotive parts. A prototype automotive part was successfully fabricated using the tailor-welded blank welded with double-sided FSW, which demonstrates its applicability to automotive production.

In-situ investigation on the microstructure evolution of Mg-2Gd alloys during the V-bending tests

In this work, the in-situ observation of the microstructure in the Mg-2Gd (wt%) alloys during V-bending tests was operated. The microstructure and texture evolutions were characterized by the in-situ electron backscatter diffraction (EBSD), scanning electron microscopy (SEM) and the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) technique. The results revealed a unique microcracks nucleation mechanism in the Mg-Gd alloys compared with the traditional Mg alloy (AZ31 alloy). The microcrack was nucleated at the grain boundary for the Mg-2Gd alloy and in the intragranular for the AZ31 alloy during the bending process. This difference mode between the Mg-Gd and the AZ31 alloys was mainly attributed to the more random topology of the grain boundary network, the lower grain boundary cohesion and the enhanced hinder ability for the dislocations due to the segregated Gd atoms in the Mg-Gd alloy. Furthermore, the microcracks that had a large angle θ with the extruded direction (ED) in the Mg-Gd alloys were preferred nucleated since grain boundaries had the larger normal stress during the bending process.

Parameter Identification of Chaboche Model for Aluminum Alloy Sheets Based on Differential Evolution Algorithm

Springback prediction is one of the most challenging in finite element analysis for sheet metal forming processes. The demand requests the development of a kinematic hardening model and parameter identification. This study presents a schematic strategy to identify parameters of Chaboche’s kinematic hardening model based on a differential evolution optimization method. To this goal, several tension-compression (TC) tests were conducted to observe the Bauchinger’s effects and kinematic hardening behaviors of two aluminum alloy sheets: AA6022-T6 and AA7075-T76. A Python code is developed to apply the proposed method in identifying parameters of the kinematic hardening model. The calibrated material models were implemented in Abaqus software to simulate V-bending and U-bending tests for the investigated materials. The predictions for springback amount match well with the experimental measurements, which verifies the effectiveness of the presented identificationstrategy.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium provided the original work is properly cited.

Mechanical behavior and interfacial damage of carbon steel-stainless steel corrosion resisted-alloy (CRA) cladded plate: Hybrid analysis based on experiment and finite element modeling

The deformation behavior of a hot roll-bonded stainless steel/carbon steel (STS/CS) clad plate is investigated by focusing on the mechanical properties-microstructure relationship along the plate thickness and clad interface. In detail, the mechanical responses of the STS/CS clad plate and its constituents are evaluated using uniaxial tension, interfacial debonding tests (in normal and shear modes), and various microstructure characterization techniques, which clarify the interfacial heterogeneity of the STS/CS clad plate. The results of experimental analysis present the validity for applying a hybrid numerical–experimental identification procedure for the cohesive zone model (CZM) in the simulation of heterogeneous STS/CS interface using finite element (FE) method. Especially, the mixed-mode bilinear CZM coupled with the inhomogeneously distributed elastic-plastic properties over thickness can be optimized to accurately predict the measured load-displacement curves and springback in the two debonding tests and the V-bending test. Finally, the interfacial failure characteristics of the STS/CS clad are further presented through the comparison between the FE method and fractography analysis at the clad interface. The combined experimental and numerical study reveal interesting aspects beyond commonly reported interfacial behavior. These include: (1) The failure along the STS/CS clad interface is controlled by both “material factors” such as the carbon migration from the interface and its resultant differential strengthening through the thickness, and “process factors” that includes the gradient of strength and temperature across the clad and rolling tools. (2) The major deformation mode is variable according to locations and the mixed normal-shear mode is dominant at tool contacts for initiating the failure during the V-bending test. (3) The interfacial failure mechanism of cladding featured not only in a confined deformation zone where contact directly with tools, but also the exterior area where springback after unloading caused a drastic change of stress state. These abrupt interfacial failures could be clarified via stress state and local damage analysis of the well-reproduced FE model of cladding material.

Characterization of kinematic and distortional hardening by cyclic twin-bridge shear tests for sheet metal with inverse engineering approach

This study characterizes the plastic behavior under cyclic loading for an aluminum alloy of 5182-O and advanced high strength steel of QP1180. Twin-bridge shear specimens were loaded cyclically to evaluate the reverse shear loading behavior of the tested materials with different pre-strain from small (0.05 major strain) to large (0.15). The plastic behavior under cyclic loading is modelled by kinematic and distortional hardening models under the non-associated flow rule (NAFR) by anisotropic Drucker (Aniso-Drucker) yield function. An additional shear constraint proposed by (Abedini et al., 2018) is imposed on the plastic potential of NAFR. The Zang and HAH20 models are selected as kinematic and distortional hardening models respectively. They are calibrated inversely by the reverse torque-torsion angle curves from experiments. Finally, V-bending tests with the pre-strained strips were carried out and simulated by the calibrated hardening model to evaluate its predicting accuracy. The results show that twin-bridge shear test combined with the recommended inverse engineering approach is implementable to overcome the drawback of the simple shear test due to inhomogeneous deformation in large strain. The constitutive models are critical for the inverse calibration. It is found that the calibration accuracy under NAFR is higher than the associated flow rule (AFR) especially for large strain conditions. Also, the additional shear constraint imposed on the plastic potential of NAFR is found to be an effective approach to enforce the simulated shear stress at zero hydrostatic pressure. As for hardening model, the HAH model has more flexibility compared to the kinematic hardening model benefiting from its distortional hardening form. Also, parameters related to permanent softening in the new HAH20 model need to be evaluated carefully at the large deformations.

Springback Reduction of Ultra-High-Strength Martensitic Steel Sheet by Electrically Single-Pulsed Current.

This paper investigates the reduction of springback by an electrically single-pulsed current for an ultra-high-strength martensitic steel sheet, MART1470 1.2t. In order to evaluate the springback reduction by the electric current, V-bending tests were performed with various parameter-sets (current density and pulse duration). The amount of springback reduction was then calculated from the measured bent-angle of tested specimens. Experimental results show the springback is reduced with the increase in the current density, the pulse duration, and the electric energy density. In order to clarify thermal and athermal portions in the effect of electric current on the springback reduction, two ratios of force and isothermal flow stress were calculated based on bending theory. From the comparison of the ratios, it is noted that the athermal portion mainly contributes to the force relaxation, so the springback amount decreases. The athermal portion significantly increases as the electric energy density increases. Microstructures and micro-Vickers hardness were observed to confirm the applicability of the single-pulsed current to forming processes in practice. The springback reduction can be achieved up to 37.5% without severe changes in material properties when the electric energy density increases up to 281.3 mJ/mm3. Achievable reduction is 85.4% for the electric energy density of 500 mJ/mm3, but properties remarkably change.

Investigation on the influence of riveting of adhesive bonded steel sheets on springback and delamination

In the present study, the influence of riveting of adhesive bonded steels sheets on springback and delamination is investigated. The adhesive bonded sheets are made of deep drawing quality (DDQ) steel sheets of 0.6 mm thickness and bonded by using two-part epoxy adhesive. In riveting of adhesive bonded sheets, the blind rivet size (3 mm, 4 mm, and 5 mm), location (15 mm, 30 mm, and 45 mm from the center), and a number of rivets (2, 4, and 6) are varied. In addition, the influence of adhesive properties by varying hardener to resin (H/R) ratio on springback and delamination of adhesive bonded steel sheets is also discussed. The V-bending test was carried out to monitor deformation pattern and delamination behaviour, and to evaluate the springback of adhesive bonded sheets. The results show that there is a significant influence on the H/R ratio of adhesive, size, location, and a number of riveting of adhesive bonded sheets on springback and delamination behavior. The delamination and springback of adhesive bonded sheets are significantly controlled with an increase in the size of the rivet, the number of rivets, and a reasonable distance of riveting from the loading location. The present study helps us select suitable adhesive properties and riveting conditions to control the springback and delamination of adhesive bonded steel sheets in real-time applications.

Simplified measurement of the strain to fracture for plane strain tension: On the use of 2D DIC for dual hole plane strain tension mini Nakazima specimens with dihedral punch

A plane-strain-tension stress state leads to a minimum in ductility in most micro-mechanical and phenomenological fracture models as well as Forming Limit Curves. Hence, this stress state plays a crucial role in many applications and a reliable measurement of the strain to fracture under plane strain tension is of particular importance when calibrating modern fracture initiation models. Recently, a new experimental technique has been proposed for measuring the strain to fracture for sheet metal after proportional loading under plane strain conditions. The basic configuration of the novel setup includes a dihedral punch applying out-of-plane loading onto a Nakazima-type disc-shaped specimen with two symmetric circular cut-outs. 3D Digital Image Correlation (DIC) is used to measure the surface strains of the specimen up to fracture. In contrast to the widely used V-Bending test, the maximum obtainable strain is not limited when using this set up, and fracture will always initiate after proportional loading in a plane strain tension stress state. In the present study, a major simplification of the testing methodology is proposed, reducing from 3D DIC to a simple 2D DIC method for measuring the fracture strain. Comparisons are presented on three metals, a 1.5 mm thick DP600 steel, a 0.8 mm thick DP450 steel and a 1.2 mm thick AA2024 aluminum alloy.

Manufacturing and performance of 3D printed plastic tools for air bending applications

In the sheet metal forming industry, rapid tools, especially those made of polymeric materials, are increasingly used for small to medium volume productions, to compress the times and costs of tooling. The present work is aimed at evaluating the performance of 3D printed tools for sheet metal V-die air bending process. Samples made of polycarbonate and polylactide were characterized through compression tests, using different printing strategies, to verify the effect of the printing parameters on the strength of the materials. FEA simulations of air bending were performed in order to predict the state of stress and strain of the plastic tools. Polymeric dies were then produced and used for repeated V-bending tests of metal sheets. The endurance and performance of the 3D printed tools were evaluated by analysing the changes in their surface and the repeatability of the bending angle after springback. The results show how the polymeric materials and their printing parameters influence the performance of the polymeric tools. Among the various tested configurations, the PLA dies, printed with a printing pattern of 45-90-45 degrees exhibit the best performance.

The deformation and fracture behavior of 316L SS fabricated by SLM under mini V-bending test

In the present study, mini V-bending testing was conducted on 316L stainless steel specimens fabricated by selective laser melting (SLM). In order to analyze the microstructure evolution during the V-bending test, FE-SEM and EBSD techniques were performed along the building (BD) and scanning directions (SD). Microstructure analysis revealed that the as-fabricated SLM specimens contained pores in the surface and subsurface regions, which affected the deformation and fracture behavior during the V-bending process. The effect that these microstructural heterogeneities exerted on the deformation behavior of the SLM specimens is discussed herein with respect to the evolution of the Kernel average misorientation (KAM), grain boundaries (GBs), and Σ3 twin boundaries (TBs). Two-level finite element (FE) simulations were performed to explain the deformation and fracture behaviors during the V-bending process. The first-level FE simulation was done to understand the macroscopic deformation of the specimen during the V-bending process. During the second-level FE simulation, representative volume elements (RVEs) were considered in discussing the evolution of equivalent plastic strain and stress triaxiality at regions near the pores. High equivalent plastic strain and stress triaxiality states at regions near the pores in the surface and subsurface of the specimens made a considerable contribution to the nucleation and propagation of cracks during the V-bending process.

An Infrared Local-Heat-Assisted Cold Stamping Process for Martensitic Steel and Application to an Auto Part

The automotive industry has tried to employ ultra-high-strength steel (UHSS), which has a higher strength with a thinner thickness. However, because of its low formability, there is a limit to the use of UHSS in industrial applications. Even though the hot-press-forming method can resolve the formability problem, elevated-temperature conditions lead to side effects—heat transfer and productivity issues. This work presents the concept of an infrared local-heat-assisted cold stamping process. Before the forming process, local areas, where the formability problem occurs, are locally heated by the gathering of infrared rays and cooled to room temperature before delivery. Since the heat treatment is completed by the material supplier, the stamping companies can conduct cold stamping without new investments or the productivity issue. In this work, a heat-assisted cold V-bending test was conducted with a martensitic (MS) 1.5 GPa steel, the CR1470M steel provided by POSCO. The heating effects on the microstructure, hardness, and local ductility were also observed. Finally, a commercial door impact beam was successfully manufactured with the present method. In this application, only a targeted small area was heated. The results show that the present method can improve the formability and springback problems of MS steel in the stamping process.

Microstructure effects on the material behaviour of magnesium sheet in bending dominated forming

Abstract Predicting springback in the bending of magnesium sheet is difficult because it has high mechanical anisotropy leading to significant differences in the material behaviour in tension and compression. The tension/compression yield mismatch in magnesium is affected by the twinning behaviour which depends on the grain size and the direction of loading in regard to the crystal (C-axis) orientation. In this study the C-axis orientation of magnesium sheet was controlled by cutting sheets from a hot rolled block. Combined with heat treatment to produce different conditions for grain size and material strength this enabled for the first time to separately analyse the effect of material strength, grain size and C-axis orientation on the springback of AZ31 sheet metal. Pure and V-bending tests were performed and combined with advanced Digital Image Correlation (DIC) to experimentally determine the neutral layer shift for different levels of outer fibre bending strain before and after springback. Our study shows that springback in AZ31 reduces with increasing grain size but that it does not change with the C-axis orientation despite a reversal in the neutral layer position. The variation of springback shows more correlation with the bending yield strength measured directly from the pure bend test rather than the tensile yield strength and strongly correlates with the change in neutral layer shift in relation to grain size. Based on the experimental results analytical equations were developed to account for the effect of grain size on springback in magnesium. The model assumes the fully plastic, plane strain bending of strip and takes into account the effect of the tension/compression yield mismatch on the neutral layer position to determine the resulting bending moment. The developed equations provide, for the first time, a simple and clear mathematical explanation for the effect of material anisotropy (Yield strength mismatch) on the bending behaviour and springback of magnesium. If tuned with experiments, the model enables the estimation of springback as a function of the microstructure (grain size) in industrial practice.

New Methodologies for Fracture Detection of Automotive Steels in Tight Radius Bending: Application to the VDA 238–100 V-Bend Test

The VDA 238–100 tight radius V-bend test can be used to efficiently characterize the bendability and fracture limits of sheet metals in severe plane strain bending. Material performance in plane strain bending is critical for the selection of advanced high strength steels for energy absorbing structural components. The detection of failure based upon a reduction in the punch force can lead to erroneous predictions of failure for ductile or thin gage alloys in the VDA 238–100 test. New failure criteria were proposed and evaluated across a range of automotive steels. Four detection methods in the V-bend test were evaluated based upon the load drop, bending moment, novel stress metric and the strain rate for seven steels with strength levels from 270 to 1500 MPa. The appropriate failure threshold was identified from visual inspection of the surface during bending. The vertical punch force will decrease as a consequence of the mechanics in the V-bend test at intermediate bend angles even without fracture. The novel stress-based metric accounts for sheet thinning and could successfully identify “false positives” and punch lift-off when considering the strain-rate evolution. Failure detection using the VDA load threshold method may significantly under-report the bend performance of alloys with intermediate-to-high bendability or thin gauges. The proposed stress-based metric can reliably detect fracture for bend angles in excess of 160° and be readily calculated using the existing data. The VDA load threshold for failure can work well for materials that exhibit significant cracking.

Modeling differential permanent softening under strain-path changes in sheet metals using a modified distortional hardening model

In this study, a modified distortional hardening model is proposed based on the homogeneous yield-function based anisotropic hardening (HAH). An improvement is made to capture differential permanent softening (DPS) under strain-path changes in sheet metals, whereas the other anisotropic hardening features such as the Bauschinger effect, transient behavior, and cross hardening phenomenon are retained as they are in the original HAH model.The modified HAH (M-HAH) model is implemented in a commercial finite element software using the user-defined material subroutine, and validated well by reproducing the flow stress behaviors after differently programmed strain-path changes for dual-phase steel, extra deep drawing quality steel, and ferritic stainless steel. Additionally, the M-HAH can simulate the DPS in two-step tension tests, which cannot be accurately modeled by the original HAH. Finally, the V-bending springback tests with pre-tensioned specimens are evaluated with different modeling schemes including the classical isotropic hardening, kinematic hardening and distortional hardening models.

Evaluation of the VDA 238–100 Tight Radius Bend Test for Plane Strain Fracture Characterization of Automotive Sheet Metals

Accurate characterization of the fracture limit in plane strain tension of automotive sheet metals is critical for the design and crash performance of structural components. Plane strain bending using the VDA 238–100 V-bend test has potential for proportional fracture characterization by avoiding a tensile instability. The VDA 238–100 V-bend test was evaluated using DIC strain measurement to characterize the plane strain fracture limit under proportional plane stress loading and to evaluate the effect of the VDA pre-straining methodology for ductile alloys upon the material response. The load-based failure criterion of the V-bend test was evaluated with DIC to monitor the development of surface cracking. The influence of the non-linear strain path imposed by the pre-straining procedure for ductile materials was then evaluated for three automotive alloys: an advanced high strength dual phase steel, DP1180, a rare-earth magnesium, ZEK100, and an AA5182 aluminum. A fracture criterion based on the load threshold was reasonable for the three alloys considered. Pre-straining in uniaxial tension prior to plane strain bending affected each alloy differently. The DP1180 was not affected by the non-linear strain path whereas the cumulative equivalent strain for the AA5182 and ZEK100 increased by strains of 0.07 and 0.05 strain, respectively. The non-linear strain path within the VDA pre-straining methodology creates ambiguity in comparing the fracture limits of different materials. The plane strain fracture limit for proportional loading can be readily obtained in the V-bend test with DIC strain measurement.

Fracture toughness measurements to understand local ductility of advanced high strength steels

The determination of the material parameters that best predict the local ductility of high strength sheet materials has become the focus of active research. Even though several correlations have been proposed, they can sometimes be not accurate enough and discussion is still open on this topic. This paper investigates the suitability of different fracture toughness measurements for local ductility prediction in multiple advanced high strength steels (AHSS). Fracture toughness is characterized by means of essential work of fracture and Khan tear tests. The results show that the essential work of fracture, we, correlates well with different local formability (HER, critical bending angle from V-bending tests and local strain at fracture from uniaxial tensile tests) and crash resistance parameters (energy absorbed in axial impact tests). It confirms that fracture toughness, measured in the frame of fracture mechanics, is a relevant material property to rationalize cracking issues associated to the local ductility of AHSS. On the other hand, it is also shown that Khan tear tests, which are conventionally used to evaluate the fracture resistance of thin metal sheets, can overestimate crack propagation resistance and offer a poor prediction ability for local formability and crash performance.