Detailed modelling of delamination buckling of thin films under global tension.

Tensile specimens of metal films on compliant substrates are widely used for determining interfacial properties. These properties are identified by the comparison of experimentally observed delamination buckling and a mathematical model which contains the interface properties as parameters. The current two-dimensional models for delamination buckling are not able to capture the complex stress and deformation states arising in the considered uniaxial tension test in a satisfying way. Therefore, three-dimensional models are developed in a multi-scale approach. It is shown that, for the considered uniaxial tension test, the buckling and associated delamination process are initiated and driven by interfacial shear in addition to compressive stresses in the film. The proposed model is able to reproduce all important experimentally observed phenomena, like cracking stress of the film, film strip curvature and formation of triangular buckles. Combined with experimental data, the developed computational model is found to be effective in determining interface strength properties.

3D-FEM and histomorphology of mandibular reconstruction with the titanium functionally dynamic bridging plate.

Biomechanical investigation of the mandible is difficult due to the complex geometrical structure. A three-dimensional finite element model of the mandible and masticatory muscles was produced with approximately 23,000 hexahedral elements. On this model, mesial and distal portions of the jaw were resected and bridged with a buccal and/or caudally positioned bridging plate. The plate was fixed caudal or buccal to the mandible. The defect was left as it was or reconstructed with an exactly fitting transplant defined as bone. The jaw was loaded at a predefined point. The changes in stresses and deformations of bone, the transplant and the bridging plate were analysed. In the caudally positioned bridging plate, finite element analysis showed lesser stresses around the fixation screws of the bridging plate. During reconstruction of the lateral defect, the buccal (ramus)-caudal (corpus) position of the bridging plate showed fewer stresses and deformations than purely buccal positioning. The caudal position of the bridging plate has biomechanical advantages and facilitates fixation of the plate, and fixation of a bone graft on the jaw stumps. Histomorphological investigations, 12 weeks and 7 years after reconstruction, show partial osseous integration or transformation of autologous iliac crest transplants.

Studying the deformation of arachnid slit sensilla by a fracture mechanical approach.

Slit sensilla are sensory organs which measure strains in the exoskeleton of arachnids. They occur as isolated slits, in loose groups and in close parallel arrangements known as lyriform organs or compound slit sensilla. The deformations of the slits' faces induced by far-field strains acting on groups of slits are studied using Kachanov's analytical approximations for the opening displacements of cracks, a method developed within the framework of fracture mechanics. The accuracy of the approach is assessed by comparisons with results obtained by finite element analysis. The limits of its applicability to slit sensilla are found to be reached when the lateral spacing between interacting slits is less than half their length, i.e., the method is suitable for studying single slits and loose groups but not lyriform organs. The influence of a number of geometrical parameters of slit sensilla on the deformation patterns of the faces of parallel slits in generic arrangements is studied, viz., spacing between slits, longitudinal shifts between slits, and slit length. The results are presented as opening distances along the length of the cracks and in terms of normalized diagrams that relate the opening distances at mid-length of the slits to the geometrical parameters. In addition, Kachanov's method is used to find a set of slit lengths that give rise to prescribed opening distances.

Arthropod touch reception: stimulus transformation and finite element model of spider tactile hairs.

Striving towards an in depth understanding of stimulus transformation in arthropod tactile hairs, we studied the mechanical events associated with tactile stimulation. A finite element model was developed taking a tarsal tactile hair of the spider Cupiennius salei as an example. Considering hair diameter, wall thickness, and curvature, the hair is subdivided into six regions each with its specific mechanical properties. When the hair is touched from above with a flat surface oriented parallel to the tarsus the point of stimulus contact moves towards the hair base with increasing load and hair deflection. Thereby the effective lever arm is reduced protecting the hair against breaking near its base. At the same time the mechanical working range of the hair increases implying higher mechanical sensitivity for small deflections (about 5x10(-5) N/degrees) than for large deflections (about 1x10(-4) N/degrees). The major stresses within the hair shaft are axial stresses due to bending. The position of stress maxima moves along the shaft with the movement of the stimulus contact point. Remarkably, the amplitude of this maximum (about 1x10(5) N/m2) hardly changes with increasing loading force due to the way the hair shaft is deflected by the stimulus.