Stiffness Boost Thanks to Composite Simulation

Composite Stiffness: From Single Ply to Laminate

The fiber-reinforced plastics used in bicycle frame construction fundamentally consist of a composite of epoxy resin as the matrix and carbon fibers for the decisive stiffness and strength. These composite materials exhibit orthotropic material behavior, meaning they possess significantly higher mechanical properties in the fiber direction compared to directions perpendicular to the fiber orientation. To optimally utilize this characteristic and to ensure maximum stiffness, unidirectional (UD) layers are used, which orient the fibers exclusively in one direction.

In various areas of the frame, the orientation of UD layers can be flexibly adjusted to the respective loads. A 0° layer along the tube axis strengthens the bending stiffness and supports the laminate under tensile and compressive stresses. An orientation at 45° to the main direction counteracts shear stresses, while a 90° layer improves resistance against the collapse of the cross-section. Depending on the stress, the orientations of the layers and their number in the frame sections can vary, creating a flexible layer structure that significantly influences the mechanical properties.

The stiffness of a planar multi-layer composite can still be manually calculated. The stiffness of individual layers is transferred based on the fiber orientation into a global coordinate system. Afterward, using classical laminate theory, the so-called ABD matrix can be determined to describe the elastic behavior of the laminate. It consists of three submatrices, where the A matrix represents the in-plane stiffness. The D entries describe the resistance to bending or the plate stiffness, and the coupling stiffness matrix B provides information on whether there are couplings between in-plane and bending properties, such as shear as a result of bending, within the laminate.

How do I define a composite in Ansys?

The manual derivation of the stiffness characteristics of a laminate and their subsequent implementation via APDL into an FE calculation is, in principle, feasible – but honestly, for complex models like a bicycle frame, this would be an incredibly tedious endeavor. A far more efficient and user-friendly solution is offered by the software add-on Ansys Composite PrePost (ACP), integrated into the Workbench. This tool not only determines the mechanical properties of the laminate but also allows for the analysis of manufacturing-relevant effects, such as draping effects on highly curved surfaces.

The workflow begins with the creation of a shell mesh using the proven Ansys Mechanical features. Furthermore, the grouping of surfaces for possible individual layer definitions can be carried out using "Named Selections". Within ACP itself, all laminate-specific definitions are established. The range of possibilities extends from the definition of a "Fabric", which represents the used textile, through "Rosettes", which provide a solution if the fibers must follow a complex guideline, to "Oriented Selection Sets" for defining different laminate zones. Finally, the layers themselves are defined and structured under "Modeling Plies".

After defining the laminate and checking the manufacturing parameters in ACP, as shown in the video, a straightforward connection with a simulation block is made through simple drag-and-drop within the Workbench. In addition to a conventional static calculation in Ansys Mechanical, the coupling also enables, among other things, a transfer to an explicit analysis in LS-DYNA. The connection of ACP with a simulation block results in the transfer of the laminate properties to the shell mesh of the analysis. After defining the simulation boundary conditions, the simulation model can be analyzed in the usual manner.

Ideal Layer Definition and Further Insights

Using the integrated parameter set in the Workbench, the 27 design variants can be quickly generated, and the desired deformation values can be calculated in under 20 minutes. The evaluation is based on the diagram shown below. Only 5 design variants are located in the upper left quadrant (highlighted in green) and thus meet the requirement of 90 Nm/° head tube stiffness at a maximum weight of 1000 g. The variant DP12 shows the best ratio of stiffness to weight, with 97.58 Nm/° at only 908 g. In the parameter set, this variant's layer configuration is shown as [2x0°/2x+45°/90°/2x-45°/2x0°], totaling 9 layers.

The success of the layer configuration becomes evident when compared with an equivalent aluminum frame: The latter would have over 30% more weight at the same stiffness! The designs DP5 and DP7 from the diagram show additional remarkable insights. Despite having the same or even higher number of layers – as is the case with DP7 – they exhibit lower head tube stiffness. These comparisons underline the importance of fiber orientation in relation to the load. While some of these aspects might be intuitively understood with experience, simulation with the help of Ansys ACP opens up an endless expansion of possibilities with minimal time investment.

Is variant DP12 the layer configuration to proceed with into manufacturing? Short and concise answer: No. The preliminary design provides a very good indication, but it must be considered that it was limited to one load case and ignored other criteria such as bottom bracket or rear triangle stiffness. A symmetry of the laminate of 〖["2x0°/+45°/-45°/" ("90°" ) ̅]〗_S would additionally bring an improvement. The same layer configuration across the entire frame represents another idealization. Ideally, the individual parts of the frame would have different layer configurations. These and other possible improvements can be included in the simulation.