Crystals growing in confined spaces can generate stress and are a major cause of damage in porous materials. The present paper is an experimental study of the isothermal hydration of MgSO 4 • H 2 O, kieserite, in porous glass filters with different in situ techniques, i.e., X-ray diffraction under controlled conditions of temperature and humidity, scanning electron microscopy after cryofixation of samples, and electronic speckle pattern interferometry. Based on the phase diagram of MgSO 4 + H 2 O derived from a careful evaluation of the available thermodynamic data, all experiments were carried out under appropriate conditions in a controlled environment. Hydration of MgSO 4 • H 2 O and formation of MgSO 4 • 6H 2 O (hexahydrite) is a true solid state reaction below the deliquescence humidity of kieserite. This reaction, however, is kinetically hindered. Above the deliquescence humidity, the reaction proceeds via a two stage reaction pathway involving the dissolution of kieserite and the subsequent crystallization of hexahydrite from a highly supersaturated solution. The hydration of kieserite in confined spaces generates substantial stresses, resulting in considerable deformations of the glass filters that were used as porous substrate. The deformation measurements confirm that the deliquescence-recrystallization pathway is more efficient than the direct solid state reaction in generating stress. Finally, theoretical considerations confirm that the stress generated during the hydration is sufficient to damage nearly every building material.

The intensity and phase reconstructed from digital in-line holograms by the convolution approach are analyzed. Distortions of particle images depending on their position in the plane transverse to the optical axis are identified. For this purpose, the object fields of numerically simulated particle holograms as well as of experimental data are reconstructed. The results of three-dimensional correlations of numerical and experimental data are superior when the numerically generated reference volumes are adapted to the transverse locations of the particle. Thus, proof is given that the characteristics of a particle image change distinctly with the transverse position of the particle and that the numerical model successfully simulates these changes. Hence, this knowledge can be integrated in future particle position detection algorithms.

A commonly applied method to reduce the cost of wind energy, is alleviating the periodic loads on turbine blades using Individual Pitch Control (IPC). In this paper, a data-driven IPC methodology called Subspace Predictive Repetitive Control (SPRC) is employed. The effectiveness of SPRC will be demonstrated on a scaled 2-bladed wind turbine. An open-jet wind tunnel with an innovative active grid is employed to generate reproducible turbulent wind conditions. A significant load reduction with limited actuator duty is achieved even under these high turbulent conditions. Furthermore, it will be demonstrated that SPRC is able to adapt to changing operating conditions.

In order to investigate deterioration processes in the paint layers on the famous 2000-yr-old Chinese Terracotta warriors, a low coherence ESPI system was designed. In this modified set-up a short-coherent superluminescence diode instead of a laser is used. By changing the path length of one of the interfering beams it is possible to select a region limited in depth where deformations are measured even if it is below the surface. Results on an artificial test object and on original terracotta fragments demonstrate the limits and the capabilities ofthe new method.

No abstract