Path independent limiting criteria in sheet metal forming

The Forming Limit Diagram (FLD) is a conventional failure diagram to estimate necking limits of sheet metal for proportional loading conditions. Previous studies reveal that the FLD is not suitable for predicting the influence of nonlinear strain paths. This paper presents methodical comparison among all common available strain path independent strain/stress based limiting criteria. All the strain path independent strain based limiting criteria (Traditional Failure Diagram (TFD), Extended Forming Limit Diagram (XFLD), Extended Stress Ratio Based Forming Limit Diagram (ESRFLD), and Polar Effective Plastic StrainDiagram (PEPSD)) and stress based limiting criteria (Traditional Stress based Failure Diagram (TFSD), Stress Based Forming Limit Diagram (FLSD), Stress Ratio and Stress Based Forming Limit Diagram (SRFLSD), Extended Stress Based Forming Limit Diagram (XFLSD), and Polar Effective Stress Diagram (PESSD)) are approximately path-independent for smaller amount of pre-straining and path dependent for large pre-straining conditions. From advance image correlation technique precisely determination of local strains near necked area is possible today. However direct determination of local stresses near necked area is not possible. Therefore, local stresses and equivalent stress are determined by employing both yield criterion and strain-hardening law. Similarly equivalent strain is calculated by the use of yield criterion. Therefore, the choice of yield criterion has an impact on the results for TFD, XFLD, ESRFLD and PEPSD. However, selections of both yield criterion and strain-hardening law have substantial influence on the results for TFSD, FLSD, SRFLSD, XFLSD and PESSD. The inherent calculation error can be minimized by generation of experimental data for each material and then selection of representable yield criterion and strain-hardening law. Improvement of experimental techniques and generation of rigorous material data bank in various strain paths may help researchers to diagnose and resolve the issue. TFD, TFSD and XFLSD have inherent variables to take care the effect of through thickness stress, however rigorous experimental verification is needed before the field application.

Tailoring a lubricant for use with ZINCANNEAL steel

To be sent to dfm week beg. 9/9/02.

Experimental analysis of friction in forming automotive coated steels

The thesis addressed and answered a number of key issues in the experimental analysis of friction in sheet forming operations. Conventional friction theories were linked with the properties of sheet coatings and the process geometries. Newly derived mathematical models extended the analysis of friction in sheet metal forming applications.

Low temperature formability of differently processed AZ31B magnesium alloy sheets

The research found changes in the performance (formability) of magnesium alloy sheets with a history of different processes. A key outcome found that the steel sheet metal processing of rolling and heat treatment caused a detrimental effect. The material's internal deformation was found to be linked to the poor formability.

Modelling of tooling wear in trim dies for AHSS

Using Advanced High Strength Steels (AHSS) in forming and cutting dies generates higher loads in tools. Tool wear is an issue when applying AHSS. This study investigates the effect of process parameters such as clearance, material properties and punch/die bluntness on contact pressure values and tool wear. Some desirable process parameters that minimise wear have been found.

Formability and forming of advanced steels

The thesis identified how advanced high strength steels perform compared to conventional steels in terms of weight reduction and crash performance for automotive bodies. The novel production method of low pressure tube hydroforming was applied to form these advanced steels to reduce the press tonnage and fluid pressure compared to the conventional high pressure process. In addition analytical models were developed to predict the force and pressure in the low pressure process.

Wear mechanisms in forming of HSS and AHSS

Code : C/64/09

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Symmetric and asymmetric rolling of low carbon steels

 In this study, it is shown that a close to ideal shear texture can be formed throughout the thickness of a rolled sheet. Such rotation of rolling texture not only leads to the enhancement in grain refinement but also the secondary processing as compared to the symmetric rolling.

Sheet forming simulation for AHSS components in the automotive industry

The trend in the automotive industry towards new advanced high strength steels (AHSS), combined with the ongoing reduction in program lead times have increased the need to get tool designs right, first time. Despite the fact that the technology used by sheet metal stamping companies to design and manufacture tooling is advancing steadily, finding optimal process parameters and tool geometries remains a challenge. Consequently, there has been a transition from designs based largely on trial and error techniques and the experience of the stamping engineer, to the increased use of virtual manufacturing and finite element (FE) simulation predictions as an indispensable tool in the design process. This work investigates the accuracy of FE techniques in predicting the forming behavior of AHSS grades, such as TRIP and dual phase, as compared to more commonly used conventional steel grades. Three different methods of simulation, one-step, implicit and explicit techniques, were used to model the forming process for an automotive part. Results were correlated with experimental strain and thickness measurements of manufactured components from the production line.

Development of numerical methodologies for parameter identification and shape optimization in metal forming simulations

Por parte da indústria de estampagem tem-se verificado um interesse crescente em simulações numéricas de processos de conformação de chapa, incluindo também métodos de engenharia inversa. Este facto ocorre principalmente porque as técnicas de tentativa-erro, muito usadas no passado, não são mais competitivas a nível económico. O uso de códigos de simulação é, atualmente, uma prática corrente em ambiente industrial, pois os resultados tipicamente obtidos através de códigos com base no Método dos Elementos Finitos (MEF) são bem aceites pelas comunidades industriais e científicas Na tentativa de obter campos de tensão e de deformação precisos, uma análise eficiente com o MEF necessita de dados de entrada corretos, como geometrias, malhas, leis de comportamento não-lineares, carregamentos, leis de atrito, etc.. Com o objetivo de ultrapassar estas dificuldades podem ser considerados os problemas inversos. No trabalho apresentado, os seguintes problemas inversos, em Mecânica computacional, são apresentados e analisados: (i) problemas de identificação de parâmetros, que se referem à determinação de parâmetros de entrada que serão posteriormente usados em modelos constitutivos nas simulações numéricas e (ii) problemas de definição geométrica inicial de chapas e ferramentas, nos quais o objetivo é determinar a forma inicial de uma chapa ou de uma ferramenta tendo em vista a obtenção de uma determinada geometria após um processo de conformação. São introduzidas e implementadas novas estratégias de otimização, as quais conduzem a parâmetros de modelos constitutivos mais precisos. O objetivo destas estratégias é tirar vantagem das potencialidades de cada algoritmo e melhorar a eficiência geral dos métodos clássicos de otimização, os quais são baseados em processos de apenas um estágio. Algoritmos determinísticos, algoritmos inspirados em processos evolucionários ou mesmo a combinação destes dois são usados nas estratégias propostas. Estratégias de cascata, paralelas e híbridas são apresentadas em detalhe, sendo que as estratégias híbridas consistem na combinação de estratégias em cascata e paralelas. São apresentados e analisados dois métodos distintos para a avaliação da função objetivo em processos de identificação de parâmetros. Os métodos considerados são uma análise com um ponto único ou uma análise com elementos finitos. A avaliação com base num único ponto caracteriza uma quantidade infinitesimal de material sujeito a uma determinada história de deformação. Por outro lado, na análise através de elementos finitos, o modelo constitutivo é implementado e considerado para cada ponto de integração. Problemas inversos são apresentados e descritos, como por exemplo, a definição geométrica de chapas e ferramentas. Considerando o caso da otimização da forma inicial de uma chapa metálica a definição da forma inicial de uma chapa para a conformação de um elemento de cárter é considerado como problema em estudo. Ainda neste âmbito, um estudo sobre a influência da definição geométrica inicial da chapa no processo de otimização é efetuado. Este estudo é realizado considerando a formulação de NURBS na definição da face superior da chapa metálica, face cuja geometria será alterada durante o processo de conformação plástica. No caso dos processos de otimização de ferramentas, um processo de forjamento a dois estágios é apresentado. Com o objetivo de obter um cilindro perfeito após o forjamento, dois métodos distintos são considerados. No primeiro, a forma inicial do cilindro é otimizada e no outro a forma da ferramenta do primeiro estágio de conformação é otimizada. Para parametrizar a superfície livre do cilindro são utilizados diferentes métodos. Para a definição da ferramenta são também utilizados diferentes parametrizações. As estratégias de otimização propostas neste trabalho resolvem eficientemente problemas de otimização para a indústria de conformação metálica.

Design and application of experimental methods for steel sheet shearing

Shearing is the process where sheet metal is mechanically cut between two tools. Various shearing technologies are commonly used in the sheet metal industry, for example, in cut to length lines, slitting lines, end cropping etc. Shearing has speed and cost advantages over competing cutting methods like laser and plasma cutting, but involves large forces on the equipment and large strains in the sheet material. The constant development of sheet metals toward higher strength and formability leads to increased forces on the shearing equipment and tools. Shearing of new sheet materials imply new suitable shearing parameters. Investigations of the shearing parameters through live tests in the production are expensive and separate experiments are time consuming and requires specialized equipment. Studies involving a large number of parameters and coupled effects are therefore preferably performed by finite element based simulations. Accurate experimental data is still a prerequisite to validate such simulations. There is, however, a shortage of accurate experimental data to validate such simulations. In industrial shearing processes, measured forces are always larger than the actual forces acting on the sheet, due to friction losses. Shearing also generates a force that attempts to separate the two tools with changed shearing conditions through increased clearance between the tools as result. Tool clearance is also the most common shearing parameter to adjust, depending on material grade and sheet thickness, to moderate the required force and to control the final sheared edge geometry. In this work, an experimental procedure that provides a stable tool clearance together with accurate measurements of tool forces and tool displacements, was designed, built and evaluated. Important shearing parameters and demands on the experimental set-up were identified in a sensitivity analysis performed with finite element simulations under the assumption of plane strain. With respect to large tool clearance stability and accurate force measurements, a symmetric experiment with two simultaneous shears and internal balancing of forces attempting to separate the tools was constructed. Steel sheets of different strength levels were sheared using the above mentioned experimental set-up, with various tool clearances, sheet clamping and rake angles. Results showed that tool penetration before fracture decreased with increased material strength. When one side of the sheet was left unclamped and free to move, the required shearing force decreased but instead the force attempting to separate the two tools increased. Further, the maximum shearing force decreased and the rollover increased with increased tool clearance. Digital image correlation was applied to measure strains on the sheet surface. The obtained strain fields, together with a material model, were used to compute the stress state in the sheet. A comparison, up to crack initiation, of these experimental results with corresponding results from finite element simulations in three dimensions and at a plane strain approximation showed that effective strains on the surface are representative also for the bulk material. A simple model was successfully applied to calculate the tool forces in shearing with angled tools from forces measured with parallel tools. These results suggest that, with respect to tool forces, a plane strain approximation is valid also at angled tools, at least for small rake angles. In general terms, this study provide a stable symmetric experimental set-up with internal balancing of lateral forces, for accurate measurements of tool forces, tool displacements, and sheet deformations, to study the effects of important shearing parameters. The results give further insight to the strain and stress conditions at crack initiation during shearing, and can also be used to validate models of the shearing process.

A finite element simulation of micro-mechanical frictional behaviour in metal forming

Friction is a critical factor for sheet metal forming (SMF). The Coulomb friction model is usually used in most finite element (FE) simulation for SMF. However, friction is a function of the local contact deformation conditions, such as local pressure, roughness and relative velocity. Frictional behaviour between contact surfaces can be based on three cases: boundary, hydrodynamic and mixed lubrication. In our microscopic friction model based on the finite element method (FEM), the case of dry contact between sheet and tool has been considered. In the view of microscopic geometry, roughness depends upon amplitude and wavelength of surface asperities of sheet and tool. The mean pressure applied on the surface differs from the pressure over the actual contact area. The effect of roughness (microscopic geometric condition) and relative speed of contact surfaces on friction coefficient was examined in the FE model for the microscopic friction behaviour. The analysis was performed using an explicit FE formulation. In this study, it was found that the roughness of deformable sheet decreases during sliding and the coefficient of friction increases with increasing roughness of contact surfaces. Also, the coefficient of friction increases with the increase of relative velocity and adhesive friction coefficient between contact surfaces.