Efficient Calibration of Material Cards using Microscale Models

Project »REVIT«: Calibration of Material Cards for FRP Materials Based on Real and Virtual Micro Tensile Tests

In the »REVIT« project, an Fraunhofer ITWM team is working together with the Fraunhofer Institute for Mechanics of Materials IWM to create microscale models – with the help of few micro-tensile tests and CT scans. Based on these, material cards for the mechanical characterization of fiber composites are calibrated. The resulting material model can describe the macroscopic material behavior with high accuracy.

Fiber-reinforced composites (FRP) are being used more and more frequently in areas such as the automotive industry or lightweight construction, as they can be produced quickly and offer a high degree of design freedom. Due to the short process times, fiber-reinforced injection molding and compression molding compounds made of short or long fibers in particular are widely used. Quality assessment is highly complex and requires so-called »Material Cards« which describe the specific material behavior of the FRP. Calibrating these usually requires a large number of expensive laboratory tests, which is why medium-sized companies have so far often had to rely on less precise material cards with low accuracy. In the »REVIT« project, a method was developed to offer small and mid-sized enterprises (SME) a much more efficient and cost-effective solution for this challenge.

The behavior of FRP components under load depends on the orientation of the fibers within the plastic material. Therefore, it is necessary to determine the dependence of the material behavior on the fiber orientation as precisely as possible. This is already possible today, but usually about 250 material tests have to be carried out for this purpose. We – Fraunhofer ITWM experts from the departments »Flow and Material Simulation« and »Image Processing« as well as a team from Fraunhofer IWM – are working on replacing this process by a few physical tests in combination with virtual measurements.

Microscale Models as a Basis for Material Cards

Unlike conventional characterization methods that involve a multitude of macroscopic tests at different stress states and strain rates, we adopt a different approach. Instead, a limited number of micro-scale tensile tests is conducted, with samples having a cross-section of 0.5 × 0.5 mm² and a length of 6 mm. Computed tomography (CT) scans provide complete information about the fiber distribution within the samples.

Based on the micro-scale tensile tests, a micro-scale simulation model is set up using the software »FeelMath« – an analysis software developed at Fraunhofer ITWM for the calculation of effective mechanical and thermal properties of microstructures.

Segmented Micro-Sample Scan
© Fraunhofer ITWM
Segmented micro-sample scan for the development of components made of fiber-reinforced plastic. The glass fibers can be seen in gray, and the matrix material (polypropylene) in green.
Stress-strain curves of exemplary RVE (Representative Volume Elements) simulations with varying fiber orientations
© Fraunhofer ITWM / Fraunhofer IWM
Stress-strain curves of exemplary RVE (Representative Volume Elements) simulations with varying fiber orientations.

The micro model represents the fibers and the matrix as two distinct phases. Initial values of the material parameters can be obtained through DMTA measurements (Dynamic Mechanical Thermal Analysis) on the matrix material. Then, the material parameters are iteratively adjusted to obtain good agreement between simulations and the micro-scale tensile tests of the composite. The real fiber distribution is incorporated into the model since the geometry is taken directly from the segmented CT scan.

Save Costs and Time With Digital Twins

Representative volume elements (RVE) are then generated to conduct virtual tests for the automated creation of a macroscopic material card. In the RVE, the fiber orientation and the fiber volume fraction are varied to observe their influence on the material behavior. Virtual tensile and shear tests are performed at three different strain rates. The mean stress and strain from these virtual tests are used for the automated creation of a material card describing the macroscopic behavior of the composite.

Finally, the macroscopic material model is validated through a series of conventional characterization tests. In particular, the good prediction capabilities of the model are showcased through biaxial pish-through tests. The model accurately characterizes material behavior up to the initiation of failure. Intriguingly, even beyond this critical point, a remarkable agreement is observed between the force-displacement curves obtained from experiment and those predicted by the simulation.

The workload of the characterization method developed in this project is of the order of 20 to 50 traditionally tested samples. With the calibrated micromodel, almost any number of virtual experiments can be carried out cost-effectively. This enables companies to reduce the purely experimental effort by up to 80 percent compared to the traditional methodology.

Comparison of simulation and experiment for a puncture test for method validation.
© Fraunhofer ITWM / Fraunhofer IWM
Comparison of simulation and experiment for a puncture test for method validation.

Our Partners in the Project

Fraunhofer Institute for Mechanics of Materials IWM, Dr. Jörg Lienhard, Dr. Timo Schweiger


Project Duration and Funding

The project is funded as part of the Fraunhofer-internal program for rapid SME-oriented in-house research. It was running for two years from July 2020 to December 2022.