Dynamic crack bifurcation in PMMA

An investigation of the branching characteristics of small PMMA single edge notched tensile (SENT) specimens is presented. The influence of notch depth and specimen thickness was examined and it was found that branching only occurred for thicker specimens and very short notch depths. The location at which successful branching occurred was very consistent for a given notch depth. Subsequently, however, a statistical variation of branching patterns was observed.A series of simulations was then performed to provide further insight into these tests and in particular to examine the evolution of the fracture process region ahead of the running crack. A finite volume/cohesive zone formulation was used to model micro-crack nucleation and dynamic interaction in the process zone. The cohesive strength and fracture resistance were estimated from unnotched tensile tests and the application of LEFM to the notch test data. Even though a very simple criterion was used to govern the insertion and subsequent behaviour of the cohesive surfaces in the model, many of the experimental observations were reproduced, including high frequency oscillations in crack velocity, the substantial increase in the fracture surface area due to the formation of subsurface micro-cracks, and the location at which successful branching took place.     

Robotic Assembly System in Semiconductor Industry

A sensor-based robotic production system for assembly of rectifier bridges is described. Assembly tasks in semiconductor industry require a robot with good positional accuracy having the capability of high velocities and accelerations in all degrees of freedom. An important part of the semiconductor production is to measure the diode properties. The trajectory of the manipulator end-effector is determined by the results from these measurements. A vaccum gripper with an air flow sensor is also used in this particular task. The robotic language DARL was used to control the assembly process.     

Temperaturverteilung und Wirkungsgrad einer trockenen quadratischen Rippe

Zusammenfassung Berechnet wird die Temperaturverteilung und der Wirkungsgrad einer auf ein Rohr aufgesetzten quadratischen Rippe. Hierbei ist noch besondere Aufmerksamkeit der zweidimensionalen W?rmeleitung in der Rippe und der konvektiven W?rmeübertragung in der umstr?menden Luft gewidmet. Das aufgestellte Differentialgleichungssystem für die Beschreibung der Temperaturverteilung an der Rippe wird numerisch mittels eines Differenzenverfahrens gel?st. Die Berechnungsergebnisse der Temperaturverteilung werden in dreidimensionalen Diagrammen qualitativ dargestellt. Eine Mehrzahl von Resultaten weist auf, da? die W?rmeleitung in der Rippe wirklich zweidimensional ist. Der Vergleich zwischen den vorliegenden und den nach dem eindimensionalen Modell berechneten Ergebnissen zeigt bestimmte, auch erwartete Unterschiede. Diese sind vor allem mit den Verschiedenheiten der Annahmen des ein- und zweidimensionalen Modells verbunden.     

Preparation of highly porous hydroxyapatite from cuttlefish bone

Hydroxyapatite structures for tissue engineering applications have been produced by hydrothermal (HT) treatment of aragonite in the form of cuttlefish bone at 200°C. Aragonite (CaCO₃) monoliths were completely transformed into hydroxyapatite after 48 h of HT treatment. The substitution of CO₃ ²⁻ groups predominantly into the PO₄ ³⁻ sites of the Ca₁₀(PO₄)₆(OH)₂ structure was suggested by FT-IR spectroscopy and Rietveld structure refinement. The intensity of the ν₃PO₄ ³⁻ bands increase, while the intensity of the ν₂CO₃ ²⁻ bands decrease with the duration of HT treatment resulting in the formation of carbonate incorporating hydroxyapatite. The SEM micrographs have shown that the interconnected hollow structure with pillars connecting parallel lamellae in cuttlefish bone is maintained after conversion. Specific surface area (S _BET) and total pore volume increased and mean pore size decreased by HT treatment.