Mats Larsson

Bio: Mats Larsson is an academic researcher from Saab Automobile AB. The author has contributed to research in topics: Necking & Forming processes. The author has an hindex of 4, co-authored 12 publications receiving 68 citations.

The definition of incipient necking and its impact on experimentally or theoretically determined forming limit curves

Abstract: In the current paper various methods for Forming Limit Curve (FLC)evaluation are presented and discussed. The capabilities of modern optical systems for strain measurement are demonstrated. A method for limit strain evaluation, based on the onset of instability, is described in detail. The method can be applied for the evaluation of experimental strains, as well as of strains from Finite Element (FE) simulations. Three different approaches have been applied for the determination of FLCs for a DP600 steel. The method used for limit strain evaluation is shown to have a considerable influence on the level of the resulting FLC. Also results from FE-simulations of the same tests are presented. The importance of always questioning how a certain FLC has been evaluated is emphasized.

On The Prediction Of Plastic Instability In Metal Sheets

Abstract: The current report presents some results from a study on the prediction of necking failure in ductile metal sheets In particular methods for creating Forming Limit Curves (FLCs) are discussed in the present report Three groups of methods are treated: Experimental methods, Theoretical/analytical methods, and the Finite Element Method (FEM) The various methods are applied to two different materials: An aluminum alloy and a high strength steel These materials do both exhibit a distinct necking behavior before fracture, and they do both exhibit only a small strain rate dependence As can be expected, the resulting FLCs from the various experimental, theoretical, and numerical methods show a substantial scatter The reasons for these deviating results are analyzed, and some conclusions are drawn regarding the applicability of the different methods

a Comprehenisve Analysis of Benchmark 4: Pre-Strain Effect on Springback of 2d Draw Bending

Abstract: In order to be able to form high strength steels with low ductility, multi-step forming processes are becoming more common. Benchmark 4 of the NUMISHEET 2011 conference is an attempt to imitate such a process. A DP780 steel sheet with 1.4 mm thickness is considered. In order to understand the pre-strain effect on subsequent forming and springback, a 2D draw-bending is considered. Two cases are studied: one without pre-strain and one with 8% pre-stretching. The draw-bending model is identical to the "U-bend" problem of the NUMISHEET'93 conference. The purpose of the benchmark problem is to evaluate the capability of modern FE-methods to simulate the forming and springback of these kinds of problems. The authors of this article have previously made exhaustive studies on material modeling in applications to sheet metal forming and springback problems, [1],[2],[3]. Models for kinematic hardening, anisotropic yield conditions, and elastic stiffness reduction have been investigated. Also procedures for material characterization have been studied. The material model that mainly has been used in the current study is based on the Banabic BBC2005 yield criterion, and a modified version of the Yoshida-Uemori model for cyclic hardening. This model, like a number of other models, has been implemented as User Subroutines in LS-DYNA. The effects of various aspects of material modeling will be demonstrated in connection to the current benchmark problems. The provided material data for the current benchmark problem are not complete in all respects. In order to be able to perform the current simulations, the authors have been forced to introduce a few additional assumptions. The effects of these assumptions will also be discussed.

Advances in anisotropy and formability

Abstract: This paper presents synthetically the most recent models for description of the anisotropic plastic behavior. The first section gives an overview of the classical models. Further, the discussion is focused on the anisotropic formulations developed on the basis of the theories of linear transformations and tensor representations, respectively. Those models are applied to different types of materials: body centered, faced centered and hexagonal-close packed metals. A brief review of the experimental methods used for characterizing and modeling the anisotropic plastic behavior of metallic sheets and tubes under biaxial loading is presented together with the models and methods developed for predicting and establishing the limit strains. The capabilities of some commercial programs specially designed for the computation of forming limit curves (FLC) are also analyzed.

Investigation and comparative analysis of plastic instability criteria: Application to forming limit diagrams

Abstract: The prediction of forming limit diagrams (FLDs) is of significant interest to the sheet metal forming industry. Although a large variety of plastic instability criteria have been developed during the previous decades, there is still a lack of comparison of their respective theoretical bases. The aim of this paper is to present the theoretical formulations of a representative selection of diffuse necking and strain localization criteria based on the maximum force principle, the Marciniak–Kuczynski method and the bifurcation approach. The theoretical foundations and underlying assumptions for each criterion are specified prior to their application to several materials to determine the associated FLDs. The capability of the criteria to predict the formability of thin metal sheets is discussed and a classification of some of the criteria is attempted according to their order of occurrence in terms of the localization prediction.

Thermo-mechanical sheet metal forming of aero engine components in Ti-6Al-4V – PART 1: Material characterisation

Abstract: Ti-6Al-4V is one of the most frequently used titanium alloy in aerospace applications such as for load carrying engine structures, due to their high strength to weight ratio in combination with favourable creep resistance at moderate operating temperatures. In the virtual development process of designing suitable thermo-mechanical forming processes for titanium sheet metal components in aero engine applications numerical finite element (FE) simulations are desirable to perform. The benefit is related to the ability of securing forming concepts with respect to shape deviation, thinning and strain localisation. The reliability of the numerical simulations depends on both models and methods used as well as on the accuracy and applicability of the material input data. The material model and related property data need to be consistent with the conditions of the material in the studied thermo-mechanical forming process. In the present work a set of material tests are performed on Ti-6Al-4V at temperatures ranging from room temperature up to 560°C. The purpose is to study the mechanical properties of the specific batch of alloy but foremost to identify necessary material model requirements and generate experimental reference data for model calibration in order to perform FE-analyses of sheet metal forming at elevated temperatures in Ti-6Al-4V.