Reinforced plastic parts can give CAE analysts a headache if they try to account for localized changes in fiber orientation. Engineers at BASF Corp. have now made that important task easier. They recently developed a proprietary method for predicting fiber orientation through the thickness of reinforced parts and then passing that detailed orientation information into commercial structural analysis codes--currently Abaqus or LS-DYNA.
The resulting structural analysis can then take advantage of the detailed orientation information as it predicts warp shape and the mechanical behavior of these anisotropic parts. The method also includes data transfer tools that make it easy to perform non-linear simulations of reinforced parts--such as those featuring large deflections or high strain rates.
Proprietary software called FIBER lies at the heart of the new method. Jim McGuire, a senior CAE engineer with BASF, explains that the software builds on results of a standard Moldflow simulation, which has long offered fiber-orientation functionality of its own. FIBER, however, refines the Moldflow results with BASF’s own anisotropic material model.
This model accounts for non-uniform material properties caused by localized differences in fiber orientation through the part thickness. According to Stefan Glaser, BASF’s manager of structural part applications engineering, this localized orientation can result from a number of sources, including fountain flow at the gates or a stretching of the melt at the flow front.
In the past, by contrast, the structural analysis of reinforced plastic parts based their calculations on the assumption that parts exhibit isotropic properties. To account for the fact the parts are really anisotropic, CAE analysts typically reduce the stiffness values of the part to about 75 percent of the stiffness expected from an isotropic part. Glaser says this approach yields stiffness predictions that are way off the mark, as much as 25 percent high or low depending on the loading of the fiber and geometry of the part. This inaccuracy, in turn, forces engineers to apply larger safety factors to their reinforced parts they design. Glaser has even seen cases where the analysis resulted in beefed-up structural parts that weigh as much as 30 percent more than they should.
Mechanical Behavior of Anisotropic Layered Shells
Once FIBER calculates the orientation effects at different layers of the part--with the user specifying the number of layers--it creates an input deck for the structural analysis code. Glaser notes that the data transfer is “purely geometrical,” so that the data can be applied to a verity of meshes. Thus while FIBER doesn’t couple filling and structural analysis in a single analysis environment, it does map the mesh from filling software onto mesh used by the structural analysis software and vice versa. “That’s a pretty slick feature,” says McGuire.
Perhaps the biggest downside to FIBER right now is its availability. BASF currently uses the software as in internal applications engineering tool, and the company’s engineers will happily analyze their customers’ or potential customers’ structural plastics applications. The company does not, however, intend to market the software. For more information, visit BASF's Plastics Portal.