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Despite the availability of surgical and non‐surgical techniques, the repair of articular cartilage lesions remains a current clinical problem. Especially for the treatment of large osteochondral cartilage defects, the replacement of the subchondral bone plate is a crucial step for optimal cartilage repair. However, no artificial implant material can yet fully restore the properties of the subchondral bone plate. For optimal cartilage tissue repair, mechanical stability for the first two months is essential. Subsequently, a rapid degradation process of the implant material would allow optimal supply of nutrition to the regenerating tissue. To this end, we investigated whether the implantation of an open porous degradable scaffold made of a magnesium alloy (AZ91) could serve as a sufficient temporary replacement of the subchondral bone plate. The results show that this alloy degrades too rapid in vivo to allow sufficient cartilage repair above the scaffold. However, the surrounding cartilage tissue was not negatively affected by the rapid degradation process and new bone formation was observed at the rim of the degrading implant. In conclusion, magnesium scaffolds degrade in vivo but the initial high corrosion rate must be reduced to allow the formation of an appropriate cartilage tissue. Future research will therefore be directed to optimized alloys and additional coating with magnesium fluorides or calcium phosphates like hydroxyapatite.

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SummaryThe deformation characteristics of the metal matrix composites Ag/Ni and Al/Al 2 O 3 are studied at microstructural level by a scanning electron microscope-based grating method and finite element (FE) simulation. The measured strain was found to localize in narrow bands in the ductile matrix of both composites. In the case of the Al/Al 2 O 3 composite, the bands are preferentially initiated in Al regions adjacent to the interface of large Al 2 O 3 particles, leading to local strain maxima. The band positions found in the Ag/Ni composite are also affected by the less deformable Ni phase, but strain localization first occurs by sliding of single Ag grains sometimes located away from the Ni phase. Using a FE model of real phase geometry and measured border displacements as boundary conditions, the simulation agrees reasonably with the experiment. The differences in the case of the Al/Al 2 O 3 composite are due to particle cracks and voids at the particle/matrix interface. This effect was found in the experiment but not considered in modelling. For the Ag/Ni composite the band positions agree fairly well. However, the level and gradient of strain is clearly different as the crystallographic orientation of the Ag grains was not accounted for in modelling.

Microstructural changes like micro deformation and damaging due to tensile load precede the macroscopical failure of a component. In order to contribute to the understanding of such processes, the microstructure of tensile test specimens was imaged by microtomography in the course of deformation. The specimens consist of particle reinforced metal matrix composites (the MMCs Cobalt/Diamond and Al/TiN) manufactured on a powder metallurgical route. Tomograms of a volume in the gauge length of the specimens were reconstructed from the projection data acquired at different deformation stages. Both polychromatic radiation of a microfocus X-ray tube and monochromatic synchrotron radiation were used for projection data acquisition.With the help of 3D data processing 3D surface nets were extracted from the tomograms which indicate the particle/matrix interface. These nets which are composed of triangles were afterwards optimized with respect to the shape of the triangles. Using the triangles as seeds a 3D FE-mesh without gaps consisting of tetrahedra was generated. 3D FE-simulations were carried out utilizing both arbitrary and realistic boundary constraints. Realistic conditions were derived from an iterative matching procedure of tomograms. The effect of finite element type (tetrahedron or hexahedron) on the simulated distribution of stresses was analyzed. The appearance and development of plastic zones in the metal matrix depending on externally applied displacements were studied in the simulations. The calculated peak stresses are compared with the loci of cracks found in the tomograms.

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