Fatigue analysis of short fibre reinforced plastics

For an initial design it is useful to use isotropic material models to analyse parts or components made from fibre reinforced plastics. However, as this can lead to high levels of uncertainty, it is recommended to take into consideration the influence of the manufacturing process, and in particular the fibre orientation, on the fatigue analysis. This results in the following process sequence:

  1. Mould-fill simulation 
  2. Mapping the fibre orientation to the FE model
  3. Creating the orthotropic and potentially non-linear material model
  4. FE analysis
  5. Creating the FEMFAT material
  6. FEMFAT analysis, taking the fibre orientation into account

 

In particular, our process uses the following tools:

 

Fig. 1: Workflow fatigue analysis of short fibre reinforced plastics

Injection molding simulation needs to resolve the local fibre orientation especially in notches using fine FE meshes and, as a result, provides local fibre tensors.

The mapping of the results from the injection molding simulation to the FE mesh for the structural analysis is done with a mapping software, in our case Digimat or directly in Moldex3D.

The challenges of creating material data for the structural analysis and the fatigue analysis should be explained in more detail here.

For the mechanical structure simulation as a starting point for the operational strength analysis, we are limiting ourselves to anisotropic, linear material data which is defined in and perpendicular to the fibre orientation. For parallel direction, this is very easy to determine using manufacturers’ datasheets, but for data perpendicular to the fibre orientation the measurement is usually complex and time-consuming. 

For example, based on the results from Moldex3D, an anisotropic material of the type ANISOTROPIC is defined for a linear elastic FE analysis in ABAQUS.

The fact that the measurement results depend on the frequency is another difficulty, as the test frequency must be considerably lower than with metal specimens in order to avoid overheating the specimen. In addition, the material data - depending on the material type - depends on the conditions (moisture).

 

Despite the many challenges, the process described above allows us to make reliable predictions on the fatigue life of short fibre reinforced plastics. Naturally, our valuable experience from long-standing PCCL projects (Polymer Competence Center Leoben) with partners such as BMW, Borealis, EMS, Evonik, Schaeffler and VW naturally plays a decisive role. 

For example PCCL research has shown that material characteristics for intervening orientations can be interpolated with good accuracy.

Depending on the influence parameters to be taken into consideration in FEMFAT, the corresponding values in the material file should be taken into consideration (as is known with metallic materials). In order to take into consideration the non-linearity of the material in the fatigue analysis using the PLAST module, more data needs to be included in the analysis. Here, however, it should be parallel and perpendicular to the fibre orientation, whereby the latter value almost exactly corresponds to the matrix.

For example, the orientation-dependent definition of the Haigh diagram is required to take the mean stress influence into consideration, for which the tensile strength is needed as a minimum configuration. Two other important parameters are mean stress sensitivity M=(SR-1/SR0) -1  and the ratio of alternating strength for tension/compression and bending for the gradient influence.

Investigations have shown that weld lines* cause a further drop in strength, in addition to the fibre orientation. This depends on various factors including temperature, material flow and front angle, and it cannot yet be predicted.

The reduced strength of the weld seam can be activated for the node characteristics via the general surface treatment factor. The weld seam nodes must also be mapped and imported into FEMFAT as a group. A reduction factor of 0.5 has proved useful as a guide value.

Fig. 2: Injection molding simulation with Moldex3D: Shaping of a weld line  

* A weld line is created when, due to an obstacle (for example, drilling), the melt flow splits and rejoins behind the obstacle, or due to multiple sources of the melt. If the temperature of the melt is then too low, the various melt flows can no longer combine sufficiently well.