Pedro Teixeira

On the Determination of Flow Stress Using Bulge Test and Mechanical Measurement

The standard uniaxial tensile test is a widely accepted method to obtain relevant properties of sheet metal materials. These fundamental parameters can be used in numerical modeling of sheet forming operations to predict and assess formability and failure analysis. However the range of strain obtained from tensile test is limited and therefore if one will need further information on material behavior, extrapolation of tensile data is performed. The bulge test is an alternative to obtain ranges of deformation higher than tensile test, thus being possible to obtain non-extrapolated data for material behavior. Several methods may be used to obtain stress-strain data from bulge test, but a common concept is behind them, which needs the measurement of bulge pressure, curvature of bulge specimen, its thickness at the pole and the application of membrane theory. Concerning such measurements, optical methods are being used recently but classical mechanical methods are still an alternative with its own strengths. This paper presents the use and development of a mechanical measuring system to be incorporated in a hydraulic bulge test for flow curve determination, which permits real-time data acquisition under controlled strain rates up to high levels of plastic deformation. Numerical simulations of bulge test using FEM are performed and a sensitivity analysis is done for some influencing variables used in measurements, thus giving some directions in the design and use of the experimental mechanical system. Also, first experimental results are presented, showing an efficient testing procedure method for real time data acquisition with a stable evaluation of the flow curve.

On the formability of hole-flanging by incremental sheet forming

Hole-flanging by incremental sheet forming is developing as an innovative metal forming technology for flexible small batch-production of cylindrical or conical flanges in blanks with pre-cut holes. The process is seen as an alternative to hole-flanging by conventional press-working due to significant cost savings through the replacement of complex press-tools by simple dieless forming apparatuses and to widespread belief that the limiting forming ratio of hole-flanging by incremental sheet forming is always higher than that of hole-flanging by conventional press-working. This article focuses on the aforementioned assumption and investigates the influence of material failure by necking and fracture on the limiting forming ratio. The experimental work is performed in a CNC machining center and a hydraulic press equipped with apparatuses for multi-stage single point incremental forming and conventional press-tooling, and the strain loading paths resulting from each forming process are determined by circle grid analysis. Results in Aluminium AA1050-H111 and Titanium (grade 2) blanks demonstrate that contrary to what has often been said in the literature there are process operating conditions leading to higher limiting forming ratio of hole-flanging by conventional press-working than by incremental sheet forming due to closeness of the forming limit curve and fracture forming limit line in the principal strain space.

Application of Life Cycle Engineering for the Comparison of Biodegradable Polymers Injection Moulding Performance

The use of biodegradable and compostable polymers (BDP) has a rising concern derived from its particular characteristics. Currently, various BDPs are combined to improve technical performance, to open up new applications or to reduce costs. In this paper a Life-Cycle-Engineering model is developed to compare the economical, environmental and technical dimensions of performance for 4 different types of BDPs when processed through injection moulding technology. The proposed model allows for comprehensive alternative comparison, supporting informed material selection decisions in a product-design context. The use of a ternary decision space supports materials comparison and the identification of their ‘‘best alternative domains’’.

Determination of Flow Curve Using Bulge Test and Calibration of Damage for Ito-Goya Models

The standard uniaxial tensile test is the widely accepted method to obtain relevant properties of mechanical characterization of sheet metal materials. However the range of strain obtained from tensile test is limited. The bulge test is an alternative to obtain ranges of deformation, higher than tensile test, thus permitting a better characterization for material behaviour. This paper presents a sensitivity analysis for some influencing variables used in bulge measurements, thus giving some guidelines for the evaluation of the stress-strain curve from experimental results using a developed experimental mechanical system. Additionally, using bulge test up to fracture shall give material information regarding damage, which in turn may be used to evaluate and calibrate damage models. A methodology is presented to be used for evaluation and calibration of Ito-Goya damage model of damage prediction.

Prediction on Localized Necking in Sheet Metal Forming: Finite Element Simulation and Plastic Instability in Complex Industrial Strain Paths

The use of Finite Element Simulation allows accurate predictions of stress and strain distributions in complex stamped parts. The onset of necking is strongly dependent on the strain paths imposed to the parts and therefore the prediction of localized necking can be a difficult task. Numerical models of plastic instability have been used to predict such behavior and recent and more accurate constitutive models have been applied in these calculations. In many manufacturing areas such as automotive, aerospace, building, packaging and electronic industries, the optimization of sheet metal processes, through the use of numerical simulations, has become a key factor to a continuously increasing requirement for time and cost efficiency, for quality improvement and materials saving. This paper makes an analysis of the evolution of strain gradients in stamped parts. The combination of Finite Element Analysis with a Plastic Instability Model, developed to predict localized necking under complex strain paths, shows that it is possible to predict failure with precision. Several constitutive laws are used and comparisons are made with experiments in stamped benchmark parts. Considering non linear strain paths, as detected in stamped parts, more accurate failure predictions are achieved. The work described in this paper shows the need to include a post processor analysis of failure, capable of predicting the behavior of the material under non linear strain paths. Taking this phenomenon into account, it is shown that it is possible to increase the accuracy of the onset of localized necking prediction.

Numerical investigation of fracture onset in sheet metal forming

Sheet metal forming processes involve finite inelastic strains that are mainly restricted by the occurrence of strain localization and instability due to necking. Therefore the ability to predict the formability limit is of paramount importance in order to optimize the process, examine the influence of each parameter on the necking occurrence and consequently to improve the press performance. Forming limit diagrams, associated with finite element simulations are currently performed by powerful commercial codes. Nevertheless, when complex strain paths are involved these predictions may fail to give the right answer.

Geometrías de Referencia Experimental en Procesos de Conformación Plástica de Chapas y su Modelación Numérica

Este articulo presenta geometrias de referencia experimentales para ser usadas en procesos de estampado de chapas metalicas. Cada geometria de referencia pretende modelar un comportamiento tipico de piezas de chapa metalica. Se han comparado los resultados de este trabajo con otros presentados en la literatura. Tambien se discuten los aspectos relacionados con la reproducibilidad y fiabilidad de esos resultados. El resbalamiento sobre el punzon y la velocidad de la prensa son dos variables que tienen un efecto importante en esta reproducibilidad. Finalmente se presentan simulaciones numericas de una de las geometrias de referencia presentadas usando elementos finitos. Los resultados se comparan con valores experimentales y se muestra que hay una buena correlacion entre ellos.

Numerical Modeling of Electromagnetic Tube Expansion and Formability Assessment

In the present paper, an EMF numerical model has been developed following an uncoupled approach, being the Lorentz forces acting on the workpiece estimated by solving Maxwells equations and then transferred to solve the mechanical problem. For formability analysis, a fracture indicator based on the linear forming limit diagram was applied through the use of a post-processing tool developed by the authors. To illustrate the applicability of the implemented code in the fracture prediction, an example of electromagnetic tube expansion is presented. The corresponding numerical simulation is performed and its results are compared with experimental obtained from literature for a selected material.

Prediction of Forming Limits Based on a Coupled Approach Between Anisotropic Damage and Necking Models

This article presents an integrated approach for localized necking and failure prediction in sheet metal forming processes, based on the coupling between the anisotropic damage evolution law proposed by Lemaitre [1] and the modified maximum force criterion (MMFC) proposed by Hora et al. [2]. To illustrate the essential features of the coupled approach, an aluminum alloy has been selected and numerical predictions have been compared with experimental forming limits, obtained for both linear strain paths evolutions. Numerical results show that the introduction of the softening behavior, caused by the increase of damage, into the necking criterion can play a significant role in triggering local necking, providing an improved prediction of the necking occurrence together with the capability of performing calculations of limit strains when they are governed by fracture rather than by local necking.

Failure Analysis of Metallic Materials in Sheet Metal Forming using Finite Element Method

The optimisation of sheet metal processes by using numerical simulations has become a key factor to a continuously increasing requirement for time and cost efficiency, for quality improvement and materials saving, in many manufacturing areas such as automotive, aerospace, building, packaging and electronic industries. The introduction of new materials brought new challenges to sheet metal forming processes. The behaviour observed with conventional steels may not be applied when using high-strength steels or aluminium alloys. Numerical codes need to model correctly the material and different constitutive equations must be considered to describe with greater accuracy its behaviour. This enhancement of material description may provide a better prediction of the forming limits, enabling an assessment of the influence of each forming parameter on the necking occurrence and the improvement of press performance. This paper presents two numerical approaches for failure prediction in sheet metal forming operations: one is the implementation of the Lemaitre’s ductile damage model in the Abaqus/Explicit code in accordance with the theory of Continuum Damage Mechanics and the other is the traditional use of FLDs, usually employed as an analysis of the finite element solution in which the necking phenomenon is carried out in the framework of Marciniak-Kuczinsky (M-K) analysis coupled with the conventional theory of plasticity. The previous strategies and corresponding results are compared with two experimental failure cases, in order to test and validate each of these strategies.