Composite Simulation in CAD Improves Analysis Productivity
Shawn Wasserman posted on August 07, 2015 |

Can FEA Help Long-Fibre Composites Break into Commercial Markets?

CAD drawing of the Westward Industries GO-4 vehicle frame. Image courtesy of CIC.

CAD drawing of the Westward Industries GO-4 vehicle frame. Image courtesy of CIC.

Composites have had a long history of light weighting in the automotive sector. Due to their strength, corrosion resistance and low maintenance, composite parts have many advantages over their metallic cousins.

Short-fibre composites have made their way into a variety of interiors. However, the costs and longer production times associated with structural long-fibre composites have limited their use to race cars and high-end vehicles.

Using simulation software like Siemens’ NX Laminate Composites, organizations like the Composites Innovation Centre (CIC) are finding cheaper and faster ways to have long-fibre composites break into the commercial market.

“Initially we worked on smaller, niche applications such as doors on projects like Westward Industries’ GO-4 urban utility vehicle,” said Alastair Komus, principal engineer for ground transportation and design at CIC. “We then expanded the focus of the project to the structural tubs. The vehicle’s power train experienced a lot benefits from a lighter design using fibreglass parts.”

The GO-4 is a street-legal, three-wheeled vehicle often used in compact areas and situations such as parking enforcement, airports, zoos, golf courses and more.

Common Challenges to Simulating Composites in FEA

Mesh of the Westward Industries GO-4 vehicle frame. Image courtesy of CIC.

Mesh of the Westward Industries GO-4 vehicle frame. Image courtesy of CIC.

One of the common challenges when simulating composites are the many variables associated with their orthotropic nature.

“Composites have different properties based on the fibre orientation,” said Komus. “You need to take these factors into consideration when developing your designs.”

In other words, the orientation of each layer in the composite can be at the same, or different, angle than the ones below it. This will create a different performance based on where the loads are applied.

Komus also mentioned that the failure method for composites is very different than that of metallic components. Traditional materials often see linear stress verses strain followed by plastic deformation before breakage. “Composites, however, have more methods of breaking,” explained Komus. “You can see matrix cracking, delamination, fibre breakage and more. This difference in performance can be a challenge to simulate.”

This difference to traditional material breakage is exacerbated by the fact that composites are not usually well understood individually. “We have years and years’ worth of data on metals in various databases. Unfortunately, that doesn’t exist in composites. And when databases do exist, they are often internal intellectual property.”

Komus explained that lack of fatigue and failure analysis for composites is a limitation of the technology. He said there currently isn’t a universally accepted method to assess fatigue, and many failure analyses are based on the breaking of the first ply, which is a conservative estimate due to stiffness redistributions between the remaining plies. The industry is in need of more studies to determine failure and fatigue methods and modes. For now, he said, “many applications that are fatigue dependent use higher safety factors. But the added material can make you lose the advantage of the lighter weight.”

Currently, the replacement of the GO-4 metallic components is in the design and prototyping phase for the rear window panel, seat, floor and dashboard. However, with a better understanding of composites breakage and fatigue, less prototyping would be required. None the less, NX Laminate Composites’ simulation capabilities have been able to curtail some of this prototyping.

The Benefits of Simulation in CAD When Modeling Composites

Simulation displacement results of the Westward Industries GO-4 vehicle frame. Image courtesy of CIC.

Simulation displacement results of the Westward Industries GO-4 vehicle frame. Image courtesy of CIC.

Initially, Komus and his team used separate analysis and CAD software to run a composite simulation.

“It was very time consuming.” said Komus. “With all those variables associated with composites, we had to go back to the CAD and make changes very often. As the simulation and CAD software were separated, we often had to start the simulation from scratch.”

This initial solution was obviously unsustainable. As a result, CIC looked at various software alternatives.

“As NX integrates CAD and simulation, it makes the process much easier. This was a big selling point,” Komus emphasized.

He added that other software didn’t provide the composite analysis capabilities he needed. “They might have orthotropic material definitions but not include composite failure equations,” Komus said. “As NX Laminate Composites integrates all these functions into the CAD, it was very affordable.”

The integration into CAD has increased the speed that Komus and his team can produce simulations and optimizations for Westward Industries. Making small alterations to the GO-4 composites can be quickly assessed by simulation. Komus said, “We can run through multiple simulations and find the best iteration for the design. You can look at the deflection and say, ‘let’s increase the core thickness,’ and test it out very quickly.”

“We have more confidence in our results now,” said Komus. “Depending on the project, we might produce a physical prototype to compare the results to the simulations. Generally, we have very comparable matching to the results of both tests. The times when values did differ, we found that it was due to an overstrained boundary condition. Once we fixed that, the results matched.”

How NX Laminate Composites Works

Strength ratio results of the Westward Industries GO-4 vehicle frame. Image courtesy of CIC.

Strength ratio results of the Westward Industries GO-4 vehicle frame. Image courtesy of CIC.

NX Laminate Composites works by using classical laminate theory. The idea is that, if you define the material, longitudinal, transverse, shear and placement properties for each ply in the composite layup, then you can determine a stiffness matrix of the ply in all directions.

The more detail a user has on the production of the composite, the more accurate the stiffness matrix will be. To that end, detailed information of ply draping will better predict the orientation of the ply due to curvatures.

“Technology to assess draping does exist in NX Laminate Composites, but a more advanced one exists in Siemens’ Fibersim software,” said Komus. Though bi-directional data exchange between NX Laminate Composites and Fibersim is supported it is a separate software.

Once you apply loads and boundary conditions to the laminate, the stiffness matrix can be used to calculate the deformations. This deformation can then be used to calculate the strain and stress for each ply and the whole composite.

Users are able to set up the composite analysis, define each ply and even edit the stiffness matrix manually using the NX Laminate Composites modeller interface.

NX Laminate Composites modeller interface. Image courtesy of Siemens PLM Software.
NX Laminate Composites modeller interface. Image courtesy of Siemens PLM Software.

“Once the plies and loads are defined, then NX will perform the calculations automatically,” said Komus. “If you need to make a correction or change a variable, like a ply’s application angle, then you just make an update in the modeller. It is really quite easy. You can then review the results of the displacement or run an individual post-processing analysis for each ply. This analysis can be based on a safety factor, strain, stress, or deformation.”

NX Laminate Composites is also able to perform automated optimizations for the laminate variables based on the loads and boundary conditions. This optimization can be based on a goal to improve the mass, strength, and modulus, by varying ply angle, materials and thickness. However, this optimization will not take geometry into account. The best option is to determine the loads the composite will experience based on its geometry and full assembly, and then apply that onto the composite block.

“The software can run through multiple iterations to find the best angle position, and layup placement for each ply. For instance, if the part will experience only a longitudinal load then you will want all the fibres in that direction. Additionally, if your composite is bending, then you will want the unidirectional plies  at the outermost region in the layup in order to increase the stiffness of the part. However, the layup order will also be dependent on other factors, such as the desired finishing of the part.

Using NX Laminate Composites’ simulation and optimization technology, CIC was able to reduce the weight of various GO-4 components by 50 percent. Additionally, the software’s analysis capabilities ensured the lighter components didn’t trade-off safety.

“As the design developed, changes were required to the GO-4 CAD model. NX allowed CIC to make these changes to the CAD and FEA model quickly,” said Komus. “This allowed for faster design iterations. It was also necessary to analyse the new components within the entire assembly of the vehicle to better understand load transfer between various parts. NX allowed CIC to integrate the new components within an FEA model of the GO-4 vehicle.”

To find out more about NX Laminate Composites, follow this link.

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

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