FEMFAT laminates

Fiber-reinforced plastics continue to gain significance in the automotive industry. Due to their outstanding properties, the traditionally mutually opposed objectives of high strength/stiffness and weight savings can be combined even with complex geometries. A positive contribution to the CO2 balance is an added benefit here.

It is not surprising that for correct analysis of continuous fiber-reinforced plastics, new methods had to be implemented in FEMFAT. This process was successfully implemented in FEMFAT 5.1 and led to the new LAMINATES module, that is currently available for ABAQUS.

The most significant characteristics of FEMFAT laminates include:

    • The analysis is element based, meaning that for composite shell elements, it is layer by layer for the top and bottom sides. For other materials, the customary methods are used, meaning that a mixed structure of standard materials and laminates is analyzed in a single computation run.

    • The inclusion of the fiber properties and the matrix requires the definition of a new material class, which has been implemented using the designation “continuous fiber-reinforced plastics.” This includes, among other things, the parameters for the failure curve according to Puck.

    • The failure curve according to Puck for intermediate fiber breakage (the intermediate fiber breakage is determined by the normal stress acting upon the fracture plane and the two shear stresses) is used to limit the Haigh diagram. The stress vector which generally changes over time, is projected in the cutting plane parallel to the fiber direction onto different specified directions. For these, rainflow counting and damage analyses are then performed (see the figure).

    Figure 1: Failure curve according to Puck 

    • The Haigh diagrams and S/N curves are interpolated between tension/compression and shear forces in dependence on the polar angle corresponding to the respective direction (0 degrees represents tension/compression, 90 degrees represents shear). The direction with the greatest damage is considered to be solely responsible for the failure.

    • The analysis method is a version of the “Critical Component in Critical Plane” method which has been adapted for LAMINATES. Thus, both fiber breakage and intermediate fiber breakage in the layer plane as well as normal to it are taken into account.

    • The influences “mean stress,” “statistical influence” and “surface treatment factor” can be taken into account, whereby the latter is applied to all layers.

    • In order to get the considerable calculation effort under control (many layers and damage criteria), calculation filters as well as parallelization are available.

    • The multiplicity of results is displayed in the FEMFAT Visualizer with efficiency and can be saved as an ABAQUS results file in the odb format. When the “Critical” option is used (see the figure), only the decisive results per element are saved in the dma file.

    Figure 2: "Critical" result option

    With this impressive range of functions, FEMFAT laminates makes fatigue analysis with complex load-time curves accessible for continuous fiber-reinforced plastics.

    Ongoing development work foresees enhancements for the interfaces and output options in addition to a refinement of the methods themselves.