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SGL Technologies GmbH supplies the German Company Medi Bayreuth with carbon fibre struts for prosthetics, the reason for using carbon fibre in prosthetics being the high rigidity and light-weight, which for this application means a better quality of life for the user.
The requirement from Medi Bayreuth was to define a range of struts, all with the same geometry, but with four different bending and torsional stiffnesses. To achieve this, SGL Technologies GmbH developed an algorithm for determining the laminate for the different strut properties.
The algorithm found laminate solutions for all four struts and the struts were measured as regards to bending and torsional stiffness. The laminate-build-up consisted of SGL’s standard commercial prepregs, in both fabric and unidirectional forms, and each of the four struts fulfilled the requirements as intended by Medi Bayreuth.
The struts were then tested for bending strength. All the struts fullfilled the strength requirements, although one of struts types showed a peculiar breaking characteristic – a distinctive failure in one particular layer, not far from the top ply. The layer was a 90° UD-Layer, which served as off-axis reinforcement.
SGL Technologies decided to use the Plate Module of Composite Oracle to further scrutinize and investigate the reason for the unusual failure. All the modules on Composite Oracle have a facility to determine the maximum load(s) and stresses of different load cases regardless of layer composition, so a random bending load was applied to the strut. The graphical display of the results showed immediately that the stress and strain level in the layer in question was overloaded due to tensile failure perpendicular to the fibre direction, as evaluated according to Puck’s failure criteria.
Composite Oracle can be used to determine not only atypical failure characteristics, but also more normal failure types, in three different modules – a plate module, a sandwich module and a filament winding module. Composite Oracle is free for its users and supported by sponsors.
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