New Microstructural Model for LFTs
To Exploit Maximum Lightweight Potential
The automotive industry uses long fiber-reinforced thermoplastics (LFTs) when manufacturing lightweight structural parts such as bumper brackets or door modules. Accurate simulation methods are needed to exploit their maximum lightweight potential. Therefore, the material’s microstructure needs to be considered. In this respect, however, existing simulation methods currently take into account significant simplifications.
With his new model, Dr. Sascha Fliegener from the Fraunhofer Institute for Mechanics of Materials IWM has made a considerable step towards a realistic representation of the microstructure. Manufacturers of parts and materials can use his model to precisely predict mechanical behavior based on the geometry of the microstructure.
Long fiber-reinforced thermoplastics (LFTs) are well suited for lightweight structural parts: They have a high specific strength and can be mass produced in a cost-efficient manner. In a compounding procedure, an extrudate is formed by mixing the thermoplastic polymer with glass or carbon fibers with a maximum length of up to 10 centimeters. The heated bulk material is pressed between two molds or injected into a closed mold. During this process, the fibers are oriented according to the state of flow within the mold. This results in different regions having vastly different microstructures and quite different material properties which need to be accounted for when structurally simulating the parts.
So far, to perform these simulations, automotive suppliers use mostly classical analytical methods for short fiber composites. Such methods consider significant simplifications for the interaction between the fibers. However, fiber interactions are particularly pronounced for LFTs due to the extended length and high volume fraction of the fibers. One must account for these interactions to precisely predict the material behavior and damage mechanisms within the microstructure. “My microstructural model is an important step towards including these interactions” explains Fliegener, researcher in the Composite Materials Group at Fraunhofer IWM.
Realistic Representation of the Microstructure
Using his new microstructural model, Sascha Fliegener is able to reconstruct the fiber structure at any region of a LFT part based on microstructural information, such as fiber orientation, fiber length and volume fraction. “Now we are able to predict the mechanical response of the material including the complex damage mechanisms under increasing load” Fliegener says. The damage mechanisms at the microscale cannot be observed in experiments. Applying micromechanical simulations helps to visualize how mechanical stresses are transferred from the matrix to the fibers until the critical fiber strength is breached and the material fails.
Based on microstructural data of a specific region within the component, it is possible to generate virtual material samples, which can be used to model different mechanical properties such as elasticity, strength or creep. Fliegener can also perform virtual experiments which allow for the realization of multiaxial stress states as well as the loading of materials in out-of-plane directions. While real experiments are hardly feasibly in these cases, automotive suppliers can use micromechanical simulations to calibrate their material models with better accuracy since they complement the experimental database and thus help to increase the precision of structural simulations of the parts.