Function integration using metallic endless fibers


Despite the superior weight-specific mechanical properties of thermoset carbon fiber reinforced plastics (CFRP), the brittle failure behavior under tensile and bending stresses and the resulting low damage tolerance and structural integrity are sometimes an exclusion criterion for these materials. Further application-related exclusion criteria follow from the rather low electrical conductivity compared to metallic lightweight materials. Previous studies have shown that the integration of ductile steel fibers in CFRP enables a significant improvement in damage tolerance and crash performance while simultaneously improving the electrical conductivity. Depending on the proportion of steel fibers, their individual properties, the laminate architecture of such hybrid laminates and the hybridization strategy used, the embedded steel fibers offer alternative load paths due to their high elongation at break and thus enable further load bearing capacity even after failure of the carbon fibers. While previous studies demonstrated the potential of this hybrid material, the current work focuses on understanding and describing the various mechanisms of material behavior before, after and during the failure of carbon fibers.

Based on an extensive experimental investigation, it can be demonstrated that the failure of the carbon fibers triggers a complex load redistribution process. The material behavior after failure of the carbon fibers strongly depends on the local damage and stresses that arise during this process. Furthermore, it can be shown how this damage and stresses can be influenced e.g. by manipulating the laminate stacking sequence.

Based on this investigation, a model is presented which is capable to map the complex load redistribution process after the failure of the carbon fibers which enables a predictive estimation of the material behavior.

In addition, a material model for steel fiber reinforced plastics (SFRP) is implemented for LSDyna©. Combined with a material model for conventional fiber reinforced composites, this enables the consideration of hybrid laminate layups, consisting of SFRP and CFRP layers within design tasks in the FEM.

Important limitation of the predictive abilities of the developed description methods result from the so far incomplete knowledge about the interface properties between the steel fibers and the matrix as well as the behavior of the interface in the moment of failure of the carbon fibers.

M.Sc. Jan Rehra
Mechanical Characterization & Modeling
Leibniz-Institut für Verbundwerkstoffe GmbH
Erwin-Schrödinger-Straße 58
67663 Kaiserslautern
Telephone: +49 631 2017-108

State of strain directly before (1) and after (2) the carbon fiber failure

Comparison of the numerical prediction and the experimental result for a loading-unloading test on steel fi-ber reinforced plastic

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