Identification of Strain Hardening Phenomena in Sheet Metal at Large Plastic Strains

AbstractA new experimental/numerical method to identify post-necking strain hardening phenomena in ductile sheet metal is presented. The identification of the post-necking strain hardening behaviour is based on the minimization of the external and the internal work in the necking zone during a tensile test. The proposed method takes the material state and the shape of the whole deforming tensile specimen into account. The post-necking hardening behaviour of a cold rolled interstitial-free steel sheet is identified. A hardening law which enables disentangling pre –and post-necking strain hardening behaviour is presented. The method is experimentally validated using an independent material test. For that purpose, the uniaxial tube expansion test is conducted to obtain uniaxial strain hardening behaviour beyond the point of maximum uniform strain in a tensile test. Finally, the presented method is compared with a hydraulic bulge test

On solving inconsistencies between different types of hardening models

This paper presents the results of a numerical study in which different hardening models were compared. More specifically the following three hardening models were considered: isotropic hardening, combined isotropic-kinematic hardening and combined isotropic-kinematic-distortional hardening. Furthermore, all three models were used in combination with the same Hill 1948 yield surface. The isotropic hardening model was used to simulate tensile tests in different directions and a torsion test. It was then tried to reproduce the outcome of these simulations with the more advanced hardenings models. Therefore, the advanced models were calibrated based on one of the tests simulated with the isotropic hardening model. It was expected that different hardening models would predict the same behaviour for monotonous strain paths, but this is not the case for the models considered in this study. This seems to be an inconsistency complicating the calibration of more advanced constitutive models. In this paper it is shown that this inconsistency can be solved by scaling some parameters in the evolution equations of the kinematic and distortional hardening model with a ratio of two equivalent stresses

Experimental and Numerical Study of Clinched Connections (Experimentele en numerieke studie van clinchverbindingen)

Manufacturing thin-walled structures and products inevitably involves the selection of suited joining techniques. The ongoing search for new, sustainable and innovative lightweight materials puts high demands on joining skills. In order to integrate these new materials in structures, products or vehicles, appropriate joining techniques have to be available. In particular for the joining of lightweight metallic sheets, which can be coated and/or dissimilar, alternative joining techniques have emerged in recent years. Amongst those, clinching -or press-joining- can assemble sheet metal parts by solely relying on local plastic deformation of the combining sheets. Unlike traditional joining techniques, clinching does not use additional material inserts such as a rivet or a bolt, and, as a consequence, the mechanical strength of such a connection highly depends on the final geometry after forming. This work focuses on the so called single stroke round clinch process with a closed die. In addition, the quasi-static mechanical response of joints produced by this technology is studied. This thesis can be divided in three major parts:1. Since clinch forming involves severe plastic deformation, the first part embarks on the identification of plastic material properties at large plastic strains. Two alternative sheet metal material tests are presented. The purpose of these tests is to reveal the plastic material behaviour hidden in the post-necking regime of thin metal sheet.2. In the second part, the complex metal flow during forming is studied with the aid of finite element techniques. Since friction plays an important role in this forming process, a strategy to identify the elements of the tribological system in clinch forming is presented. The predicted final geometry of the joint after forming is systematically compared with experiments.3. The third part concerns the mechanical response of clinched joints to quasi-static loading conditions. To be specific, the capability of finite element techniques to reproduce the experimental results obtained with pull-out tests, single lap shear tests and multi-axial loading of clinched joints is investigated. In addition, analytical models to estimate the shear and pull-out strength of a clinched connection are presented.status: publishe

Mechanical Behaviour of Clinched Joints in Configurations

Clinch joining or clinching is a mechanical joining technique for sheet material. In this paper, the mechanical behaviour of multiple clinched joints under mixed-mode loads (peel, shear and pull-out) is investigated using a modified Arcan test. The experimental results are compared with a proposed equivalent model for clinched joints to validate if the model can reproduce the deformation behaviour up to maximum force. The theoretical maximum resistance force of the configurations are then compared to the experimental maximum resistances to investigate the influence of interaction effects on the maximum strength of the configuration. This study is part of a global design strategy for clinched joints in large structures

Identification of Strain Hardening Phenomena in Sheet Metal at Large Plastic Strains

© 2014 The Authors. Published by Elsevier Ltd. A new experimental/numerical method to identify post-necking strain hardening phenomena in ductile sheet metal is presented. The identification of the post-necking strain hardening behaviour is based on the minimization of the external and the internal work in the necking zone during a tensile test. The proposed method takes the material state and the shape of the whole deforming tensile specimen into account. The post-necking hardening behaviour of a cold rolled interstitial-free steel sheet is identified. A hardening law which enables disentangling pre -and post-necking strain hardening behaviour is presented. The method is experimentally validated using an independent material test. For that purpose, the uniaxial tube expansion test is conducted to obtain uniaxial strain hardening behaviour beyond the point of maximum uniform strain in a tensile test. Finally, the presented method is compared with a hydraulic bulge test.publisher: Elsevier articletitle: Identification of Strain Hardening Phenomena in Sheet Metal at Large Plastic Strains journaltitle: Procedia Engineering articlelink: http://dx.doi.org/10.1016/j.proeng.2014.10.112 content_type: article copyright: Copyright © 2014 The Authors. Published by Elsevier Ltd.status: publishe

Modelling strategy for clinched joints in assemblies

Clinching is a mechanical joining technique which involves severe local plastic deformation of two or more metal sheet parts resulting in a permanent mechanical interlock. Today, it is a reliable joining technique used in automotive, HVAC and general steel constructions whilst still gaining interest. As it is not computationally feasible to include detailed sub models of these type of joints in FE simulations of real-life clinched assemblies, this paper proposes a methodology to represent these connections with simplified elements. In order to calibrate the parameters governing the equivalent model, a simple shear lap and pullout test is used. This methodology is applied to clinched configurations and validated using a modified Arcan test in which both shear and pull-out loads are considered.status: publishe

Forming Simulation Considering the Differential Work Hardening Behavior of a cold rolled interstitial-free steel Sheet

The multiaxial plastic deformation behavior of a cold rolled interstitial-free steel sheet with a thickness of 0.65 mm was measured using a servo-controlled multiaxial tube expansion testing machine for the range of strain from initial yield to fracture. Tubular specimens were fabricated from the sheet sample by roller bending and laser welding. Many linear stress paths in the first quadrant of stress space were applied to the tubular specimens to measure the contours of plastic work in stress space up to an equivalent plastic strain of 0.289 along with the directions of plastic strain rates. The test material exhibited differential hardening (DWH). A material modeling method for reproducing the DWH in a finite element simulation has been developed. Hydraulic bulge forming simulation results based on the DWH model had a closer agreement with the experimental results than those calculated using the isotropic hardening models with selected yield functions. © 2014 Trans Tech Publications, Switzerland.status: publishe

Equivalent modelling strategy for a clinched joint using a simple calibration method

Clinching is a mechanical joining technique that involves severe local plastic deformation of two or more metal sheet parts resulting in a permanent mechanical interlock. Today, it is a reliable joining technique used in heating, ventilation and air conditioning (HVAC), automotive and general steel constructions whilst still gaining interest. As it is not computationally feasible to include detailed sub models of these type of joints in FE simulations of clinched assemblies during the design stage, this paper proposes a simple methodology to represent these connections with simplified elements. The key point of the method is the use of uncoupled plastic behaviour to model the joint plastic properties. In order to calibrate the parameters governing the equivalent model, a simple shear lap and pull-out reference test of a single clinched joint was used. The presented methodology is validated using a modified Arcan test of a single joint, which enables to exert a combination of shear and pull-out loads. Finally, a peel test is conducted to study the influence of bending moments on the behaviour of the joint.publisher: Elsevier articletitle: Equivalent modelling strategy for a clinched joint using a simple calibration method journaltitle: Thin-Walled Structures articlelink: http://dx.doi.org/10.1016/j.tws.2016.12.002 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved.status: publishe

Application of digital image correlation for the study of strain concentrations due to change in geometry in weld beads: parent interface of structural steel welds

© 2016 Institute of Materials, Minerals and Mining. The paper aims to characterise the strain concentration of welded junctions subjected to elastic loads using digital image correlation (DIC). When measuring such small strains and heterogeneous strains fields with DIC, a compromise between noise and accuracy arises where a suitable choice of the image processing settings becomes crucial to obtain accurate results. An extensive study of the influence of the different processing parameters was carried out to ensure the best trade-off between accuracy and noise. A practical application was later performed with the found optimal settings; welded specimens were subjected to uniaxial and biaxial loads to assess possible influence of the strain state on the strain concentration. Results show a great variability of the strain concentration along the weld bead base, evidencing a significant influence of local geometrical characteristics. On the other hand, the strain state influence was found to be moderate, or almost inexistent for lower strain concentrations.peerreview_statement: The publishing and review policy for this title is described in its Aims & Scope. aims_and_scope_url: http://www.tandfonline.com/action/journalInformation?show=aimsScope&journalCode=ystw20status: publishe