MECHANICAL BEHAVIOR OF NOVEL ORGANO-SANDWICH COMPONENTS

Abstract

Lightweight design is a common philosophy which enables engineers to keep the mechanical performance and functionality of a structure while reducing its weight. Continuous Fibre reinforced plastics (FRP) have the highest specific mechanical properties and are predestined for lightweight structural applications. The restricted industrial processability of continuous fibre reinforced plastics is a cost driving factor which reduces the scope of application actually to some premium components for example in aviation or automotive industry. Especially FRP based on thermoplastic matrices offer an attractive alternative due to their advantages in cost and in production. Compared to thermoset FRP, thermoplastic composites can be processed with short cycle times and a high reproducibility. Furthermore the weldability of thermoplastics leads to further process relevant advantages like thermoform ability and a high recycle ability. These positive features give reasons for the increasing demand on continuous fibre reinforced thermoplastics. Therefore there is also a growing development and improvement on thermoplastic semi-finished products like organic sheets and unidirectional (UD) tapes. A novel and innovative semi-finished product system, called organo-sandwich, was developed by EconCore (Leuven, Belgium). Organo-sandwich materials consist of a sandwich structure with a thermoplastic honeycomb core and organic sheets as cover layers. The good meltability of the thermoplastic materials enables the production in a continuous fully automatic production process. Therefore this novel material combines a continuous production of the thermoplastic honeycomb core with an inline lamination of the organic sheets. The honeycomb core is manufactured in the patented ThermHex process by extruding a thermoplastic film with subsequent vacuum rotation thermoforming and folding to a honeycomb structure. This continuous and fully automatic procedure makes a production of semi-finished sandwich structures with a high profitability possible. Furthermore the sandwich design and FRP design enables a significant enhancement of the specific bending stiffness compared to conventional organic sheets and composites. Thus higher bending stiffness makes the organo-sandwich suitable for flat or thin component areas which are especially sensitive to stability failure like buckling or kinking. In this work these semi-finished organo-sandwich components are characterized respective to their mechanical as well as thermal properties. The main focus of the whole research project is the integration of this novel material group into the process chain of lightweight structures for automotive applications. The influence of manufacturing parameters as well geometrical parameters (e.g. core height) to the mechanical properties will be analysed in detail. Due to their meltability organo-sandwich components are suitable for the industrial thermoforming processes. The semi-finished components will be heated up by infrared radiators and formed in a mould by a hot stamp. It is also possible to melt the honeycomb core down to a thin ply of thermoplastic. This will support the forming process especially at complex shaped component areas and will enable the integration of load application elements. Furthermore, an accurate temperature control during the heating process is also significant to the forming process. It must be assured, that only the areas which have to be formed are heated up to thermoforming temperature. On the one hand the thermoplastic matrix of the top layers has to be melted to ensure interlaminar sliding of the layers. On the other hand the honeycomb core has to remain far away from its melting temperature to keep its compressive strength and avoid collapsing. Therefore the heating up behaviour of the organo-sandwich has to be determined to link thermal and mechanical characterization. The specific mechanical properties like tensile strength, compressive strength and shear strength will be determined depending on different process relevant temperatures by experimental testing. Additional the influence of different cell widths and core heights as well as different layups are examined in a specific designed test bench.

Full text

FRAUNHOFER INSTITUTE FOR MICROSTRUCTURE OF MATERIALS AND SYSTEMS IMWS
MECHANICAL BEHAVIOR OF
NOVEL ORGANO-SANDWICH COMPONENTS
A. Geyer, Th. Gläßer, J. Pflug, R. Schlimper and M. Zscheyge
Fraunhofer Institute for Microstructure of Materials and Systems IMWS
Walter-Huelse-Strasse 1 | 06120 Halle (Saale) | Germany
Telephone +49 (0) 345 5589-438 | anne.geyer@imws.fraunhofer.de
Raising the honeycomb
core density leads to a
doubling of compressive
strength. Keeping the
density equal and varying
cell size and core height
leads to a different
compressive strength as
well as a different slope
of the stress-strain curves
and with that to different
compressive moduli.
The authors wish to thank for the financial support for this research project
which was funded by the EU in terms EFRE program via the “Investionsbank
Sachsen-Anhalt” under grant no. 1604/00019.
Making thermoplastic Organo-Sandwiches advantageous for automotive
applications, they have to be processed easily in short cycle times, shape
complex near net geometries and enable an efficient load application.
INTENTION PROCESS CHAIN
THERMOFORMING CHARACTERISATION MECHANICAL CHARACTERISATION
ACKNOWLEDGEMENT
Hybrid Injection Molding
Comparing sandwich parameter like density, cell size and core height,
compression tests according to ASTM C365M were performed.
Following the process chain, Organo-Sandwich sheets are heated up to
processing temperature with medium waved infrared radiation (IR).
Afterwards they are automated inserted into the injection molding cavity.
With closing the injection mold the thermoforming step is performed. In the
following injection molding cycle the Organo-Sandwich is functionalized.
Heating Inserting Thermoforming Injection Molding
In a thermal analysis an ideal heating profile for the Organo-Sandwich was
determined. At an representative volume element the core temperature is
analyzed while heating the top layers up to 165°C processing temperature.
Ideal temperature profile Overheated honeycomb core
For an optimal thermoforming the heat input has to be regulated precisely to
heat up the top layers to the melting temperature of the thermoplastic
matrix material. Simultaneously the core temperature has to stay below this
melting point to prevent failure due to the thermoforming pressure.
Ideal formed Organo-Sandwich Collapsed honeycomb core
The Organo-Sandwiches
were also tested at a
temperature of 80°C to
examine the thermo-
formability of the core.
Comparing the results, it
can be seen that an
elevated core temperature
leads to a significant loss
in both compressive
strength and compressive
modulus. Therefore the
core temperature has to
be considered at a
thermoforming profile.