Development of underwater housings for the deep sea made of ultra-high performance concrete (UHPC) and long-term testing at the Arctic Sea
The aim of this research is the design and analysis of underwater housings made of ultra-high performance concrete (UHPC) within the Helmholtz Alliance “ROBEX” (robotic exploration of extreme environments) [1]. Due to the extremely corrosive maritime environment, mainly expensive materials such as titanium, ceramics, special stainless steel and aluminum are used so far for the construction of temporary and permanent research stations to guarantee long-term deployment. In this project the suitability of high-strength concrete with compressive strength up to 210 MPa for underwater pressure housings and constructions is examined to provide a low cost and durable alternative especially for large housings in future.
Material Behavior of Ultra-High-Strength Concrete under Multiaxial Stress States
To evaluate new fields of application of concrete structures in any processes where complex stress states occur, and to optimize concrete structure geometries regarding such applications, the material behavior of concrete under multiaxial loading has to be known. With the aim of determining the material behavior of an ultra-high-strength concrete (UHSC), 35 multiaxial stress states with primarily one tensile stress component are examined and the measured stress-strain curves are shown. Using the test results, the arbitrary parameters of a damage-based material law concerning the single-stress ratios are determined. To describe the material behavior for load-induced isotropic and orthotropic damage, an approximation of the calculated arbitrary parameters is deduced. These enable determining the material behavior of the investigated UHSC for any stress ratios. The prediction of the described material law is compared and discussed regarding the measured stress-strain curves and the maximum strength values from the tests.
Microstructure of the solder-casting zone in bridges of dental gold alloys.
The composition and structure were studied in the cast and in the solder of bridges which had failed clinically. Both materials were gold alloys but with different contents of Pt, Ag and Zn. The metallographic investigation revealed defects mostly in the solder, situated in the subsurface layer. SEM studied of the fracture surface revealed large porosites and a structure of dense parallel lines--striations--, indicating that the material had failed from fatigue. The materials in the fractured bridges were identified by micro-probe measurements. It was stated that cast material and solder material used together, were of different composition.