NewsDate: 19-04-2018 by: Tiffany Won
The Difficult Task of Using FEA to Simulate CFRP
Finite element analysis (FEA) allows us to simulate various forces acting on virtual components and informs us
NewsDate: 19-04-2018 by: Tiffany Won
The Difficult Task of Using FEA to Simulate CFRP
Finite element analysis (FEA) allows us to simulate various forces acting on virtual components and informs us of design decisions before we even cut a single piece of physical material. This can save a lot of time and money. However, FEA does some jobs better than others. For example, basic scenarios, such as a steel beam undergoing bending, are easier to simulate than a carbon fiber reinforced plastic (CFRP) beam undergoing the same bending force. The reason? A piece of steel is homogenous in terms of its composition, whereas a CFRP beam is composed of laminated layers and has fibers running at angles to each other. It exhibits anisotropic behavior, porosity and friction between the layers—factors that make behavior difficult to predict.
Solvay is a CFRP product manufacturer that has a range of products including Evolite, a thermoplastic CFRP, and SolvaLite, a thermoset CFRP. There are fundamental differences in how these materials behave, so there is no one-rule-fits-all simulation or manufacturing solution suitable for both types of product. Much research is needed in both the simulation and manufacture of these products.
CFRP car door. (Image courtesy of Solvay.)
Thankfully, material scientists, manufacturing engineers and chemists alike work toward increasing our knowledge of CFRP manufacturing simulation. They do so not just from a mechanical perspective, but for every stage of manufacture and beyond, including simulation of crash tests. They are achieving this with the help of various simulation software solutions developed by Troy Mich. based Altair Engineering, Inc. (Altair).
“There are three areas in the process that we are aiming to simulate,” said Fabio Bressan, Solvay virtual engineering manager. “We hope to simulate the material at nano and microscale levels, the tooling at macro levels, and also simulate the trim curing process. Curing simulation is a Multiphysics problem and is time, temperature and viscosity dependent.”
The dynamic relationship between factors affecting curing is one of the main issues that differentiates thermoplastic and thermoset CFRP behaviors. More on that later.
CFRP Manufacture 101
Before we delve into the actual details of software used, let’s take a brief look at CFRP products and how they are manufactured. In order, the sequence goes like this:
1) Prepreg material cutting and assembly
2) Preforming and draping
3) Application of pressure and heated curing
The first step involves the cutting of prepreg material and assembly (layering) to produce thicker sheets. A prepreg material, as opposed to a dry material, consists of carbon fiber filaments or sheets with the plastic resin already impregnated into the filaments.
The next phase involves draping the sheets onto a mold base to provide the geometry of the near-net shaped preform component. This draping phase can cause issues if not performed correctly, and this is one step that can be simulated by using Altair’s HyperWorks simulation software suite. If the prepreg material is not draped correctly, the fiber direction can be altered, resulting in a non-optimum part. This step produces a 3D part with the matrix and fibers consolidated for the next stage.
(Image courtesy of Altair and CEDREM.)
The preform is then moved to a second mold where it is compressed and another resin is injected into the part. The timing of this step is critical. Pressure must be applied at the correct time because it is a thermoset plastic matrix, and as a result the viscosity and phase of the plastic are irreversible. Once that sweet-spot is missed, it can result in sub-standard parts.
Demolding is the final step. When the final part has finished curing it can be removed from the mold and is ready for inspection. Simulation can also assist here by determining the best location for the mold ejector pins.
The bulk of the simulation work takes place within the Altair HyperWorks environment.The various stages of manufacture can be simulated with use of plugins and fairly elaborate coding. The robustness of the results is fully dependent on the quality of the inputs.
“If you put garbage in, you get garbage out,” Bressan said.
In terms of inputs, the entire process of simulating CFRP begins with the creation of what are referred to as “material cards.” The material cards are basically virtual material for the types of prepreg material configurations and contain information related to directional stiffness and strength for each material, as well as other mechanical data such as modulus of elasticity, Poisson’s ratio, shear modulus and so on.
The material cards are developed by first performing mechanical tests on physical samples while simultaneously modelling the mesoscale behaviour in a finite element environment.
The results of these parallel tests/simulations are combined to create the actual material cards, which are written for various commercial solvers including Altair’s RADIOSS solver for nonlinear dynamic analysis.
Next up, the cards are tested to see if they fit up to the physical experiments by running simulations on the card models while performing another set of physical tests. If the experimental/numerical results correlate, then it is a good card and can be used for future design and simulation work.
“Material cards are not simple ASTM data tab