Frequently asked questions
AniForm Core is an implicit solver. Reaching convergence with an implicit solver always obeys the balance between internal and external forces, which is not the driving condition for explicit solvers. Additionally, AniForm is especially tailored to deal with high anisotropy (fibre reinforced composites) and large deformations, which cannot be done properly with standard FEM software you may know. AniForm also considers the material behaviour, which is not possible when dealing with the kinematic draping approach. Years of research on laminate deformations and material behaviour of composites led to a successful modelling approach that is used in this software.
Modelling forming cases
The set up of a basic forming simulation starts by modelling the contact surface of the tooling that will make contact with the blank during forming. Modelling the complete tooling is not always required to obtain a reliable forming prediction. The tooling surfaces require a discretised representation of triangular elements, because the modelling technique is based on finite elements.
Next, a surface representation of the blank/laminate is required. Also here, the surface must be discretised with triangular elements. In AniForm PrePost, material properties and a layup can be assigned to this laminate representation.
You may also involve aided tooling such as blank holders. These can be imported as discretised surfaces as well. Such surfaces can be force or displacement controlled throughout the simulation. Also tensioners/springs can be applied.
Finally, you will need to supply the process conditions. The software needs to know what tooling moves and according to which speed.
In AniForm PrePost, the blank/laminate also needs to be represented by a single layer of triangular elements. When a simulation is invoked, the layer of elements will be duplicated and offset according to the number of plies defined in the layup builder form. The imported mesh representing the laminate may contain mesh sections. An individual layup can be assigned to each section, thereby offering the user the ability to model tailored blanks.
When manually making a model via the AniForm input file (.afi file), one may link to prepared 2D or 2.5D meshes. A layup of meshes will not be created automatically, but the user has full control over the stacking.
Uni-directional plies (UDs) can be described when having its material behaviour under shear, bending, ply-ply slip, and tool-ply slip conditions. At the in-plane level, a single family of fibre directions is added.
Wovens, fabrics, or textiles having two fibre directions also require its material behaviour under shear, bending, ply-ply slip, and tool-ply slip conditions. At the in-plane level, two families of fibre directions are added.
Bi-or tri-axial non crimp fabrics (NCFs) are regarded as two or three uni-directional (UD) plies stacked on top of each other. Each ply is equipped with the material behaviour at the in-plane, out-of-plane, and interface levels. At the in-plane level, one family of fibre directions represents the UD fibres, whereas another family of fibre orientations with a lower stiffness represents the major stitching direction.
Each ply in a laminate can thus be modelled separately, which opens up the possibility that neighbouring plies can slide relatively to each other. When you have a laminate comprising many plies (for example 16, 24, or more plies), you may think of a laminate idealisation procedure. The many plies will then be represented by fewer plies (for example 4, 5, 6, or more), whereas the overall behaviour is close to equivalent with respect to a simulation where you would model all plies. Of course, you will lose a bit more detail, but that is the trade-off when conducting an idealisation.
Regarding the ply deformations, after quite some time of research we have established the following division of deformation mechanisms of a composite ply: in-plane, out-of-plane (bending), and interface mechanisms. This approach is used because one material model cannot be used to describe the total deformation behaviour of a ply at once, which is for example the case when dealing with metals.
Advanced users can model 3D blanks and apply varying fibre orientations over these blanks. This can be done by editing the .afi (AniForm input) file manually.
Material properties and characterisation
- The kinematics that occur in the characterisation test.
- The energy required to deform the specimen under particular experiment conditions.
- The fitter seeks an optimum for the material parameters, which are related to the selected AniForm material models.
The modelled kinematics and the reaction of the material models should eventually predict a material response that is close to the experimentally obtained response.
When you have acquired a commercial AniForm Suite license, AniForm and partners offer solutions to characterise your material under forming conditions. When you have already available experimental data, you can choose to develop your own material model fitting tools. Alternatively, AniForm’s engineering service offers to prepare your experimental data for use in AniForm simulations. More information can be found here.
AniForm PrePost is designed to quickly set-up a simulation. An input file (.afi) is automatically generated when invoking a simulation. The .afi file is read by AniForm Core, which subsequently solves the model. The design of AniForm PrePost is a trade-off between simplicity and functionality.
AniForm Core is a very flexible solver that allows you to perform highly advanced simulations. For example, you can use AniForm PrePost to set up the initial model after which you can manually edit the generated .afi file to convert the basic model into an advanced simulation model.
It must be noted that using the material behaviour at the constant forming temperature already captures 70% of the forming effects that appear in practise. Adding temperature dependency would increase the prediction quality to max 80%, depending on the particular case. In other words, the isothermal approach gives sufficient information about the material’s formability in the majority of the cases we have dealt with.
At the moment we apply our in-house exporter module throughout engineering service projects. The exporter module updates the material properties with the aid of mapping, which processes the new fibre directions as predicted in the simulation. Some export functionality is also in development. Please ask in case more information is required.
Mainly, spring-back/forward can be attributed to three phenomena:
- Chemical effects of the polymer, shrinkage.
- Mechanical stresses induced by the press forming process, which are then frozen-in when the material solidifies. When releasing the part from the tooling, the residual stresses will (partly) be relaxed, resulting in shape distortions.
- Thermal effects due to temperature gradients and differences in thermal expansion coefficients.
The last phenomenon, thermal effects, is the most significant one for the majority of the cases. The standard FEM software usually offers possibilities to conduct a thermal analysis where you need to supply a temperature difference and thermal expansion coefficients in the in- and out-of-plane directions. The differences between these coefficients, the geometry, and the temperature gradient will determine how the geometry will distort.
Processors: two times an Intel Xeon E5-2630 - 2.3 GHz - 6-core
Memory: 32GB (dual channel)
OS: windows server 2012 64 bit
AniForm PrePost uses the video driver functions for optimal performance. It requires the pc where you run AniForm PrePost to satisfy certain video card requirements, which can be found here.