Improving stamping tryout using variational analysis, FE simulations and robustness

The tryout process at an automotive sheet metal production plant is outlined and analysed with the aim of augmenting the process to improve robustness through an understanding of variation using Finite Element (FE) analysis and other tools. The result is tryout can produce a more robust stamping process.

Using the solid-shell element to model the roll forming of large radii profiles

Roll forming is an incremental bending process for forming metal sheet, strip or coiled stock. Although Finite Element Analysis (FEA) is a standard tool for metal forming simulation, it is only now being increasingly used for the analysis of the roll forming process. This is because of the excessive computational time due to the long strip length and the multiple numbers of stands that have to be modelled. Typically a single solid element is used through the thickness of the sheet for roll forming simulations. Recent investigations have shown that residual stresses introduced during steel processing may affect the roll forming process and therefore need to be included in roll forming simulations. These residual stresses vary in intensity through the thickness and this cannot be accounted for by using only one solid element through the material thickness, in this work a solid-shell element with an arbitrary number of integration points has been used to simulate the roll forming process. The system modelled is that of roll forming a V-channel with dual phase DP780 sheet steel. In addition, the influence of other modelling parameters, such as friction, on CPU time is further investigated. The numerical results are compared to experimental data and a good correlation has been observed. Additionally the numerical results show that the CPU time is reduced in the model without friction and that considering friction does not have a significant effect on springback prediction in the numerical analysis of the roll forming process.

Stress-based predictions of formability and failure in incremental sheet forming

 This research investigates the deformation mechanism in incremental sheet forming (ISF) with relation to necking and failure. A strain-based forming limit criterion is widely used in sheet-metal forming industry to predict necking. However, this criterion is strictly valid only when the strain path is linear throughout the deformation process. Where the strain path in ISF is often found to be severely nonlinear throughout the deformation history. Therefore, the practice of using a strain-based forming limit criterion often leads to erroneous assessments of formability and failure prediction. On the other hands, stress-based forming limit is insensitive against any changes in the strain path and hence it is used to model the necking and fracture limits. Simulation model is evaluated for a single point incremental forming using AA 6022-T4E32 and checked the accuracy against experiments.

Suppression of necking in incremental sheet forming

Incremental sheet forming enables sheet metal to deform above a conventional strain-based forming limit. The mechanics reason has not been clearly explained yet. In this work, the stress-based forming limit was utilized for through-thickness necking analysis to explain this uncovered question. Stress-based forming limit which has path-independency shows that the stress states in top, middle and bottom surfaces did not exceed the forming limit curve at the same time and each layer has different stress state in terms of their deformation history to suppress necking. It has been found that it is important to consider the gradient stress profile following the deformation history for the proper forming limit analysis of incremental sheet forming. © 2014 Elsevier Ltd. All rights reserved.

Stress based prediction of formability and failure in incremental sheet forming

A strain-based forming limit criterion is widely used in sheet-metal forming industry to predict necking. However, this criterion is usually valid when the strain path is linear throughout the deformation process [1]. Strain path in incremental sheet forming is often found to be severely nonlinear throughout the deformation history. Therefore, the practice of using a strain-based forming limit criterion often leads to erroneous assessments of formability and failure prediction. On the other hands, stress-based forming limit is insensitive against any changes in the strain path and hence it is first used to model the necking limit in incremental sheet forming. The stress-based forming limit is also combined with the fracture limit based on maximum shear stress criterion to show necking and fracture together. A derivation for a general mapping method from strain-based FLC to stress-based FLC using a non-quadratic yield function has been made. Simulation model is evaluated for a single point incremental forming using AA 6022-T43, and checked the accuracy against experiments. By using the path-independent necking and fracture limits, it is able to explain the deformation mechanism successfully in incremental sheet forming. The proposed model has given a good scientific basis for the development of ISF under nonlinear strain path and its usability over conventional sheet forming process as well.

An analytical model for web-warping in variable width flexible roll forming

The development of ultra/advanced high strength steels (U/AHSS) has challenged traditional forming methods due to their higher strength and reduced formability. An alternative method is flexible roll forming, which allows the manufacture of sheet metal of high strength and limited ductility into complex and weight-optimized components. However, one major problem in flexible roll forming is the web-warping defect, which is the deviation in height of the web over the length of the profile. The authors’ previous work developed an analytical model to predict the magnitude of web-warping. That model was purely geometric and neglected the effect of material properties. This work develops an analytical solution for the prediction of web-warping that considers both geometric and material parameters. The model results were validated by comparison with numerical and experimental results. The impact of this new model will be the ability to provide a rapid initial design assessment before an intensive numerical analysis of flexible roll forming is conducted.

An inverse routine to predict residual stress in sheet material

A method is presented to determine residual stress distribution in sheet material from data collected in a free bending test. It may be used where the residual stress distribution is symmetrical about the mid-surface as it is usually the case for frequently-used sheet metal post-processing techniques such as skin-pass or temper rolling, tension- and roller leveling. An existing inverse technique is used to obtain a residual stress profile and material constants that provide the best fit in a finite element analysis of bending with the experimentally derived moment-curvature relation. The method is verified for bending of a low-carbon stainless steel using measurement of residual stress by X-ray diffraction. The residual stresses were induced in the sheet by cold rolling. The technique described here can be used industrially as a rapid method of investigating residual stresses in incoming sheet. In processes where the deformation is principally one of bending, such as cold roll forming, it is known that residual stresses have an influence on shape defects and springback and the method presented here can be used to determine whether incoming sheet is suitable for further processing and also as a means of obtaining improved material data input for numerical simulation.

Influence of task properties and subjectivity on consistency of triage: a simulation study

Aim. This paper reports a study to determine nurses' levels of agreement using a standard 5-point triage scale and to explore the influence of task properties and subjectivity on decision-making consistency.

Background. Triage scales are used to define time-to-treatment in hospital emergency departments. Studies of the inter-rater reliability of these scales using paper-based simulation methods report varying levels of consistency. Understanding how various components of the decision task and individual perceptions of the case influence agreement is critical to the development of strategies to improve consistency of triage.

Method. Simulations were constructed from naturalistic observation, cue types and frequencies were classified. Data collection was conducted in 2002, and the final response rate was 41·3%. Participants were asked to allocate an urgency code for 12 scenarios using the Australasian Triage Scale, and provide estimates of case complexity, levels of certainty and available information. Data were analysed descriptively, agreement between raters was calculated using kappa. The influence of task properties and participants' subjective estimates of case complexity, levels of certainty and available information on agreement were explored using a general linear model.

Findings. Agreement among raters varied from moderate to poor (κ = 0·18–0·64). Participants' subjective estimates of levels of available information were found to influence consistency of triage by statistically significant amounts (F 5·68; ≤0·01).

Conclusions. Strategies employed to optimize consistency of triage should focus on improving the quality of the simulations that are used. In particular, attention should be paid to the development of interactive simulations that will accommodate individual differences in information-seeking behaviour.

Electron back scattered diffraction (EBSD) characterization of warm rolled and accumulative roll bonding (ARB) processed ferrite

An interstitial free (IF) steel was severely deformed using accumulative roll bonding (ARB) process and warm rolling. The maximum equivalent strains for ARB and warm rolling were 4.8 and 4.0, respectively. The microstructure and micro-texture were studied using optical microscopy and scanning electron microscopy equipped with electron back scattered diffraction (EBSD). The grain size and misorientation obtained by both methods are in the same range. The microstructure in the ARB samples after 6 cycles is homogeneous, although a grain size gradient is observed at the layers close to the surface. The through thickness texture gradient in the ARB samples is different from the warm rolled samples. While a shear texture (⟨110⟩//rolling plane normal direction (ND)) at the surface and rolling texture at the center region is developed in the ARB  samples, the overall texture is weak. The warm rolled samples display a sharp rolling texture through the thickness with increasing the sharpness toward the center. These differences are attributed to the fact that the central region of ARB strip is comprised of material that was once at the surface. The ARB process  can suppress the formation of shear bands which are conventional at warm rolled IF steels. EBSD study on the sample with 6th cycle of ARB following the annealing at 750 ◦C verified a texture gradient through the thickness of the sheet. The shear orientations at the surface and at the quarter thickness layers can be identified even after annealing. The overall weak texture and existence of shear orientations make ARB processed samples unfavorable for sheet metal forming in compare with warm rolled samples.

Low strain processing of LC and ULC steels

Temper rolling and tension levelling are commonly used to manufacture flat rolled steel. Both processes lengthen the steel at strains up to 3% by applying a load and stretching the strip. By latering the balance between the load and the tension the formability of the low carbon and ultra low carbon steel may be optimised.

Modelling of bending-dominated forming processes of metal sheets

Theoretical solutions, finite element models, and experimental techniques are developed for three major sheet metal forming operations: bending (pure bending and cyclic bending), die bending, and deep drawing. These have been applied to two different commercial quality cold-rolled steels, one stainless steel, and one magnesium alloy.

Springback data from stamping and simulating the forming of advanced high strength steel

This dataset is comprised of a spreadsheet of simulation result files, cross-section geometries of stamped parts, strain results of cross-section of stamped parts, simulation data (strain stress displacement energies), and variation data of material properties of a single coil. This data is a collection of both experimental and simulation results from industrial and laboratory stamping of advanced high strength steels (AHSS). The steels that were stamped were a typical high-strength low-alloy (HSLA) steel, a transformation-induced plasticity (TRIP) steel, a super HSLA steel, and a dual phase (DP) steel. The selected part was an automatic Ford Falcon front cross-member component using the Ford Geelong stamping plant. The variation of the material and stamped parts was also collected.