We had an interesting and lively Q&A session during the Simulation Metal Forming Processes - Material Characterisation Webinar and we wanted to share with you the questions that we didn't have the time to address during the event.
Q. Many software companies provide material data with their software – can we rely on this for simulation?
TD:For initial feasibility, costing, etc studies it may be OK but for detailed analysis I would recommend obtaining data specific to the material being used for the process
SKH:Material properties can vary according to the process route used to manufacture the material. It is certainly better to obtain data specific to the material being used particularly when the simulation result is intended to be used for the manufacture of physical tooling
Q. Validating a simulation model is important. How would you go about validating a simulation model?
TD:Close the loop (review simulations against production – this can be difficult as designs evolve), use standard tooling or refer to published benchmarks (we hope to collate a list of benchmarks as part of MAN WG)
SKH: Agreed. The need for validation cannot be overstated
Q. Is there any simulation available for rotary forging or orbital forming?
TD:Of the codes I have used I know that Simufact can do this but I am sure there are others
Q. How can the forming limit curves be used in the simulation software?
TD:The FLC is generally used as a post-processing step, to review the results of the simulation after solving; in some cases it can be used as a failure criteria (causing element deletion) during the solve
SKH: The FLC is critical for the prediction of splits, particularly because the failure of sheet material depends on the strain path taken. It is insufficient to use tensile data
Q. Which softwares you used for forging simulation and casting simulation?
TD:For forging I have used Simufact but there are many others; I have not done casting simulation but there are several commercial codes (some codes are mentioned in the Why Do Manufacturing Simulation book)
Q. Given the increase of popularity of sheet metal forming processes on microscale, what factors should be considered in order to take into account size effects ? to what degree is it correct to extrapolate and assume that a material model obtained from full-scale testing data would also describe deformation on microscale?
TD:I have no experience with this – though I think true microscale simulation will require a different material modelling approach addressing the multi-phase crystalline structure of the metal directly
SKH: By and large, a material model at the ‘engineering’ scale is not applicable at smaller scales. This is because they are based on continuum assumptions. Generally, you carry out microscale simulations to understand/predict the heterogeneous strain distribution within a material’s microstructure. The nature of the distribution will depend on the phases present, the grain size, its distribution etc. This type of heterogeneous behaviour is not well explained by material models used on the engineering scale. What some people do is to start from crystal plasticity models with the aims of predict large scale engineering behaviour. These adopt homogenisation rules to average the microstructural heterogeneity to the engineering scale.
Q. How do you do transformation from tensile to thermomechancial properties? Could you recommend literature related to this topics please?
TD:You would need to do the tensile/compressive/biaxial tests across a range of temperatures – I have known of work using the Gleeble machine or else in customised testing rigs where the loading cell was contained in an oven.I don’t know of any specific literature on this.
SKH:I am not familiar with any direct transformation without the additional testing mentioned by Trevor
Q. ISO 10303-235 Engineering properties and materials information can represent any property derived by any method in a computer processable form that is independent from propriety software.It could be used both for the input data from testing the material
TD:I have responded directly to the author of this comment.
Q. Follow up question;Do you consider that ISO 10303-235 would be helpful in the simulation technology?
Q. Is it possible to estimate the effect of cold working on strength and ductility of material using simulation?
TD:formerly we would do this crudely with shifted stress-strain data but now there are many options to use the data generated in the process simulation and map it to other CAE models
Q. Do you involve the Additive Manufacturing process, specifically the selective laser melting in you working group?
TD:please refer to the AM Focus Group for more info
Q. Can you explain how to optimize calculation durations with explicit solvers for slow forming processes? (Standard time steps are very small in explicit solvers...)
TD:consider using an implicit solver but if that’s not possible then certain solvers offer scaling options to allow the process to be accelerated without affecting rate-sensitive material properties, or else have different mass scaling methods (quasi-static where all elements have their densities adjusted or selective mass scaling where a partial implicit method is used to solve for stiffness)
SKH: I would certainly try an implicit solver. They are much better at handling large models nowadays
Q. Detailed about sheet metal forming process like incremental forming, Flow Forming and Spinning process
TD:(this may be related to the previous question?) simulating long duration processes such as this may require use of localised element adaptivity (fission and fusion) to keep the model size and hence run time within acceptable limits.Also MPI based parallel processing can speed up the run.I have some experience with incremental forming – it can be done in a practical manner.
Q. Due to the variability in material coefficients, the forming simulation results will also vary which will affect my further durability or crash simulation. How to take decision what material coefficient values to use in the simulation?
TD:See the slides on optimisation and robustness in the presentation; where it is practical we can define the material data input as a distribution rather than a single value and then run with a “monte carlo” method to randomise the input and determine either the most influential parameters to control or else a mean result within the point cloud of possible results.However, this requires a lot of computation; ultimately we may have to rely on safety factors to avoid outliers.
SKH: Some codes allow for a user-defined Design-of-Experiment to optimise a process. The danger is that these methods can allow you to define ‘impossible’ materials. For example, in sheet metals, a material with high work-hardening usually (not always!) has higher elongation. However, these codes can allow the analysts to optimise processes with materials with high work-hardening and high elongation, even though these do not usually exist.
Q. Do you think DOE/Optimization is not utilized enough during forming simulation study? And do you think we should encourage optimization more in simulation study with different bead force, thickness for getting optimum thinning and FLD?
TD:yes (see also above) – we would like to do more, we have all the tools to do this but rarely have the time – faster solvers (e.g., one-step or inverse solvers) may help and we have used an auto blankholder force method to home in on the optimum.Other process variables such as bead forces can also be optimised in this way.Thickness is usually specified in the design so may not be open for us to modify.
SKH: I think time is usually the obstacle so this type of simulation is restricted to critical/important components
Q. Technological Guide for sheet metal forming simulation process
TD:thanks, we will consider this; it will need to be non-code specific.The training that I referred to does cover some of this – please contact NAFEMS with more details of your requirements
Q. How to find spring back effect on stamping simulation?
TD:after the forming process simulation we routinely run a springback calculation (using an implicit solution); accuracy depends on considering how the part will be supported/clamped including possibly the effect of gravity. We are getting good results and use the data to morph the tool to compensate for springback and create revised tooling surfaces – we have had good success with this
Q. How to validate results obtained from stamping/forging simulation
TD:this was answered previously – dedicated test tooling (cross-die or similar) can be used, also refer to published benchmarks (e.g., NUMISHEET for stamping)
Q. Is the sheet forming section in the technology guide applicable to spring forming?
TD:sorry, I am not sure what is meant by “spring forming”?You can download the guide free of charge – if you have further questions let us know
Q. Detailed about sheet metal forming simulation process like Incremental Forming, Flow Forming and Spinning Process
TD:see previous answer; in terms of Material Characterisation these processes are similar to sheet metal pressing
Q. How account multi directional plastic deformation in stamping simulation ?
TD:use the correct material models and element formulations; sheet metal pressing usually relies on a shell element with no through thickness compression but if this is a feature of your process then look at thick shells or shells with a compressive stress option
SKH: Does this question refer to changing strain paths during stamping? If so, these material models are under investigation in academia. Examples of these include failure models such as the forming limit stress curves. Yield models are still under investigation.
Q. Technological Guide for Sheet metal Forming Simulation Process
TD:repeat of an earlier question?
Q. What are the material data needs to be included for simulation and to what extend we can trust the experimental tested material data?
TD:please have a read of the Technology Guide and then get back to us if you have further questions; if we can’t trust the test data then we are going to struggle!Model validation is key as noted in the presentation
Q. Looks like everything is 2D (shell type model) and there were no 3D properties listed as needed for measurement. Is this the standard for all metal forming simulations? What happens when radii approach thickness values?
TD:bulk forming processes are 3D using solid elements and full 3D material models – please have a look at the Technology Guide for more details.
SKH: Be sure to select material models such as yield models that reflect the 3D state of stress in your process
Q. Can you include plastic anisotropy in metalforming
TD:yes many models include this I believe
SKH: Anisotropy is usually included through yield models (for stamping).
Q. Is there any simulation availabe for measuring strian rates in the formed parts
TD:Strain rate should be available in the output data from most simulations; rate-sensitivity can and often must be included in process simulation
Q. What is your opinion on the new strategy for calibration of material parameters, the virtual fields method?
TD:sorry, I am not very familiar with this, I will look into it further.Perhaps it is similar to the “single element” method - we have used this with an optimisation tool to characterise material model parameters from test data (when those parameters are not directly available)
SKH: I am not familiar with it either
Q. To characterise Flow Stress material properties for Aluminium for example, which test is better, Tensile, Compression or Tensile
TD:it depends on the process – are the main deformation modes tensile or compressive?
SKH: I agree with Trevor. The one exception I can think of is using tensile data for compression models if you are prepared to assume that the tensile response is the same as the compressive response. In general, this is not true and will add an error to your prediction.