Assessing formability of sheet metals through advanced tensile and LASER speckle analysis

The ability of a metal to resist strain localisation and hence reduction in local thickness, is a most important forming property upon stretching. The uniform strain represents in this regard a critical factor to describe stretching ability - especially when the material under consideration exhibits negative strain rate sensitivity and dynamic strain ageing (DSA). A newly developed Laser Speckle Technique (LST), e.g. see [1], was used in-situ during tensile testing with two extensometers. The applied technique facilitates quantitative information on the propagating plasticity (i.e. the so-called PLC bands) known to take place during deformation where DSA is active. The band velocity (V-band), and the bandwidth (W-band) were monitored upon increasing accumulated strain. The knowledge obtained with the LST was useful for understanding the underlying mechanisms for the formability limit when DSA and negative strain rate sensitivity operate. The goal was to understand the relationship between PLC/DSA phenomena and the formability limit physically manifested as shear band formation. Two principally different alloys were used to discover alloying effects.

Sheared edge quality in the trimming of sheet metals

Examines the sheared edge quality of different automotive sheet metals resulting from the trimming process using experimental tooling and finite element modelling. The significant differences in burr, sliver and edge profile formation have been explained in terms of the strain distribution and tensile properties of the materials.

Prediction of FLD of sheet metals based on crystal plasticity model

 In some advanced sheet metal forming processes such as the incremental forming process, a local fracture strain after necking is very important. In order to accurately predict necking and fracture phenomena, a crystal plasticity model is introduced in the finite element analysis of tensile tests. A tensile specimen is modeled by many grains that have their own crystalline orientation. And each of the grains is discretized by many elements. Using this analysis, necking behavior of a tensile specimen can be predicted without any initial imperfections. A damage model is also implemented to predict sudden drops of load carrying capacity after necking and to reflect the void nucleation and growth of the severely deformed region. From an analysis of the tensile test, the necking behavior is well predicted. Finally, analyses are carried out for various strain paths, and FLDs up to necking and fracture are predicted.

Theoretical approach including friction for determining strain limits in punch stretching of anisotropic sheet metals

A new analytical theory including friction was developed to assess strain limits in punch stretching of anisotropic sheet metals. This new approach takes into consideration the anisotropic behaviour of sheet materials and could explain the mechanical behaviour of a variety of anisotropic sheet materials. The theory explains the sheet metal failure so for the drawing as the stretching region of the forming limit curve, particularly for materials that present the strain-ratio dependence of limit strain ε 1, where dε 1/dρ is not always greater than zero. dε 1/ dρ or dε 1/dε 2 could be equal to or smaller than zero for a range of materials. Therefore, this new theory can explains such experimental observations, besides to assuming that membrane element relations near the pole, for the case of punch stretching are dependent of sheet metal properties as the process history and also suggests that the onset of local necking is controlled by shear. Thus, theoretical results obtained through this new approach are compared with experimental results available in the literature. It is demonstrated the effect of friction on a FLC curve for both regions, drawing and stretching. © 2010 American Institute of Physics.

Experimental investigation of forming limit, void coalescence and crystallographic textures of aluminum alloy 8011 sheet annealed at various temperatures

In this work, a combined forming and fracture limit diagram, fractured void coalescence and texture analysis have been experimentally evaluated for the commercially available aluminum alloy Al 8011 sheet annealed at different temperatures viz. 200 degrees C, 250 degrees C, 300 degrees C and 350 degrees C. The sheets were examined at different annealing temperatures on microstructure, tensile properties, formability and void coalescence. The fractured surfaces of the formed samples were examined using scanning electron microscope (SEM) and these images were correlated with fracture behavior and formability of sheet metals. Formability of Al 8011 was studied and examined at various annealing temperatures using their bulk X-ray crystallographic textures and ODF plots. Forming limit diagrams, void coalescence parameters and crystallographic textures were correlated with normal anisotropy of the sheet metals annealed at different temperatures. (C) 2013 Politechnika Wroclawska. Published by Elsevier Urban & Partner Sp. z o.o. All rights reserved.

Development of the sheared edge in the trimming of steel and light metal sheet, part 1 - experimental observations

Trimming experiments were conducted on sheet metals including two drawing steels, an aluminum alloy and a magnesium alloy, using a specially designed die in a mechanical press. The punch-die clearance was varied and data obtained on the rollover and burr height as a function of the clearance. Samples were also partially trimmed to examine crack initiation, the generation of the fracture surface profile and mechanism of burr formation. The results showed that while the burr height and rollover depth generally increased with increasing clearance for all examined materials, there were differences in the fracture surface profile shape, the burr shape, and the mechanism of burr formation, between the two steels and the two light alloys. The major cause of these differences appeared to be the rate of crack propagation through the sheet material.

Roll forming cryo-rolled nano-structured aluminium sheet

Commercial purity aluminium plate was reduced by rolling under nitrogen in 30 passes from an initial material thickness of 10 mm to a final thickness of 2 mm (80% reduction). Analysis of the microstructure showed that the material produced in this way had an ul-trafine grained microstructure. The sheet was roll formed at room temperature to a V-section using commercial roll forming equipment. Two sets of experiments were per-formed; one with a 15 mm radius in the base of the V and the other with a 5 mm radius. The performance in terms of final shape and springback is compared with the same part shape formed by V-die bending. The mechanical properties of the sheet were determined using the tensile test. It has been found that even if the total tensile elongation is close to zero and bending of the material is very limited, ultra-fine grained and low ductile sheet metals can be roll formed to simple section shapes.

Roll-formability of cryo-rolled ultrafine aluminium sheet

Ultrafine-grain aluminium sheet was produced by rolling at cryogenic (CR) and at room temperature (RTR). Commercial purity aluminium plate was reduced in 30 passes from an initial material thickness of 10 mm to a final thickness of 2 mm (80% reduction). Tensile stress and strength were significantly increased while total elongation was drastically reduced. It was found that despite the low tensile elongation both materials are able to accommodate high localised strains in the neck leading to a high reduction in area. The formability of the material was further investigated in bending operations. A minimum bending radius of 6 mm (CR) and 5 mm (RTR) was found and pure bending tests showed homogeneous forming behaviour for both materials. In V-die bending the cryo-rolled material showed strain localisations across the final radius and kinking of the sample. It has been found that even if the total elongation in tension is close to zero leading to early failure in V-die bending, ultra-fine grained and low ductile sheet metals can be roll formed to simple section shapes with small radii using commercial roll forming equipment.

A non-associated plasticity model with anisotropic and nonlinear kinematic hardening for simulation of sheet metal forming

A material model for more thorough analysis of plastic deformation of sheet materials is presented in this paper. This model considers the following aspects of plastic deformation behavior of sheet materials: (1) the anisotropy in yield stresses and in work hardening by using Hill's 1948 quadratic yield function and non-constant stress ratios which leads to different flow stress hardening in different directions, (2) the anisotropy in plastic strains by using a quadratic plastic potential function and non-associated flow rule, also based on Hill's 1948 model and r-values, and (3) the cyclic hardening phenomena such as the Bauschinger effect, permanent softening and transient behavior for reverse loading by using a coupled nonlinear kinematic hardening model. Plasticity fundamentals of the model were derived in a general framework and the model calibration procedure was presented for the plasticity formulations. Also, a generic numerical stress integration procedure was developed based on backward-Euler method, so-called multi-stage return mapping algorithm. The model was implemented in the framework of the finite element method to evaluate the simulation results of sheet metal forming processes. Different aspects of the model were verified for two sheet metals, namely DP600 steel and AA6022 aluminum alloy. Results show that the new model is able to accurately predict the sheet material behavior for both anisotropic hardening and cyclic hardening conditions. The drawing of channel sections and the subsequent springback were also simulated with this model for different drawbead configurations. Simulation results show that the current non-associated anisotropic hardening model is able to accurately predict the sidewall curl in the drawn channel sections.