The Challenges to Designing with Composites

Many engineers can picture in their minds how steel will behave in a design; with experience it becomes instinct. But, that isn’t the case with composites as every composite is different.

“With hundreds or even thousands of layers within a single composite it becomes impossible to imagine the behaviour of your design. Now add draping issues and imperfection due to manufacturing and you really have a complicated thought problem,” said Pierre Thieffry, Senior Technical Evangelist at ANSYS, Inc.

Despite the lightweight benefits of composites, its unpredictable properties can make it difficult to formulate a design. This is where simulation an ANSYS’ Composite PrepPost can play a major role.

“The magic is that you need the properties of each ply. Then, ANSYS Composite PrepPost will simulate how each ply, and the composite stack will perform. The simulation will then tell you how the overall part will perform and its mechanical properties down to individual plies,” explained Thieffry.

How ANSYS Composite PrepPost Models the Layered Materials

Composites PrepPost acts as a plug in at the material definition level of ANSYS Mechanical. Therefore, it fits into the typical ANSYS simulation workflow.

“Users use the PrepPost tool to define the material stack of shells and thin layer meshes. This input, can be summarized into a ply book that acts like a recipe for the composite part. When working with shapes with high curvatures, that require draping, the tool will determine the flat-wrap. This is the flat shape of the plies before draping. Finally, the tool will simulate the molding of the part,” said Thieffry.

The PrepPost tool will send the orthotropic and anisotropic material formulations, along with layered shell elements formulations, to the ANSYS FEA solver. Each layer’s material, thickness, orientation, and shape is input into the solver which will determine the material properties. The PrepPost tool can then assess how the part will fail, or use the material properties in a subsequent simulation.

“Analysis of the individual ply failures and ultimate part failures are available determined by the tool’s FEA analysis,” described Thieffry. “The first ply failure is estimated from the deformations, strains, stresses and standard composite failure criteria, we have a comprehensive list of criteria. Ultimate failure, however, requires nonlinear analysis as progressive failures are computed one layer at a time.”

Challenges Simulating Composites

Engineers might find it complicated to incorporate as-manufactured characteristics into the simulation of a part made from composite materials. However, ANSYS’ Composite PrepPost tool is able to assess a part based on its molded shape and manufacturing conditions.

“Shape curvature will have an impact on fiber orientations and draping helps predict these changes. We can also include dependencies of the material based on shear angle and temperature. These materials are often defined as orthotropic, where two main directions are orthogonal, but in reality this orientation may not be a perfect 90 ͦ. Either way, these draping effects can have a significant effect on the composite’s performance.

Other manufacturing conditions that can be complicated to incorporate into the design are non-homogeneity such as voids in the material. “With composite manufacture, these are hard to control,” admitted Thieffry. “After you cure and assemble the material small voids and irregularities can be added to the model. The PrepPost tool allows users to estimate this degradation by estimates, usually at about five to ten percent. This is a highly statistical estimation and depends on the manufacture’s day to day operations. Therefore, determining these values require some testing but implementing them into the simulation isn’t complicated.”

How Do I Simulate a Composite That Isn’t Thin?

Traditionally, the first composites parts were thin, however, this is not always the case anymore. For instance, complex non-thin 3D shapes like turbine blades are being made from composites to reduce the weight of engines. The question then stands: how would you model such a shape in ANSYS?

“One might say, ‘I have a 3D CAD model, let’s try to fit the definition into the shape.’ This would be the wrong way to do it as there is no way to determine how the layers will turn out at the end,” said Thieffry.

So, to work with composites as 3D elements, the model must be set up in an extrusion process. Thieffry explained further, “we basically take the description of the plies from a shell model and extrude them to create a 3D model that has the exact thickness of each ply – either with one element per ply or with multilayer elements.”

He continued, “This sounds simple on paper but it is actually a complex operation, especially for complex shapes such as a turbine blade. However, with ANSYS users can even include the tooling geometries used to remove portions of the 3D model during the part manufacturing. This will add some realism to the simulation.”

One thing is for sure, with ANSYS Composite PrepPost, simulation experts will be able to set up their composites for both thin and 3D models. You still may not be able to imagine the product performance in your head, as you would with steel, but with simulation and composites your part will be just as strong with much less weight.

To learn more about ANSYS’ Composite PrepPost and composite simulation capabilities, follow this link.

Ansys has sponsored this post. They have no editorial input to this post - all opinions are mine. Shawn Wasserman