Error sources and geometry effects in DCDC-tests

The main advantages of DCDC-specimens are the completely stable crack extension after spontaneous crack initiation due to the decreasing stress intensity factor with increasing crack length and a very high path stability due to the strongly negative T-stress term. In this report, several possible error sources occurring in DCDC-tests are addressed: end effects in short specimens, differently long cracks at both sides of the drill hole, slight offset of the hole and the crack, non-symmetrical loading

Consequences of hydroxyl generation by the silica/water reaction

Water diffusing into silica surfaces gives rise for several effectson diffusion behaviour and mechanical properties. Water added to silica glass increases its specific volume so that the silica expands near the surface. Mechanical boundary conditions give rise for compressive “swelling stresses”. This fact provides a tool for the interpretation of many experimental observations from literature

Consequences of hydroxyl generation by the silica/water reaction - Part II: Global and local Swelling - Part III: Damage and Young\u27s Modulus

Water diffusing into silica surfaces gives rise for several effects on diffusion behaviour and mechanical properties. In a preceding booklet, we focused on diffusion and fiber strengths and deformations which were obtained by water soaking under external loading. In the present booklet we deal with results and interpretations of strength increase in the absence of applied stresses

Effect of negative T-stress on R-curves for Si3N4-ceramics

The main advantages of DCDC-specimens, completely stable crack extension and very high path stability due to the strongly negative Tstress term call for an application in R-curve determination. In this report, it is studied whether the T-stress affects the R-curve of silicon nitride. For this purpose, we selected three Si3N4-ceramics, which showed very flat crack resistance curves in test with edge-notched 4-point bending specimens. These materials were tested with DCDC-specimens. In contrast to the bending tests, the crack resistance decreased clearly. This effect can be understood as a consequence of the T-stress term

Consequences of hydroxyl generation by the silica/water reaction

Water diffusing into silica surfaces gives rise for several effectson diffusion behaviour and mechanical properties. Water added to silica glass increases its specific volume so that the silica expands near the surface. Mechanical boundary conditions give rise for compressive “swelling stresses”. This fact provides a tool for the interpretation of many experimental observations from literature

Investigation of coherent acoustic phonons in terahertz quantum cascade laser structures using femtosecond pump-probe spectroscopy

The dynamics of acoustic vibrations in terahertz quantum cascade laser structures (THz-QCLs) is studied by means of femtosecond pump-probe spectroscopy. The phonon modes are characterized by the folding of the acoustic dispersion into an effective reduced Brillouin zone. An accurate identification of this dispersion allows the sample structure and periodicity to be determined with high precision on the order of 0.1%. By temperature tuning the energy of the electronic levels of the system and performing wavelength dependent measurements, we are able to study the impulsive resonant generation and detection of coherent acoustic phonon modes. These results are supported by simulations of the electronic system that well explain the experimental observations. The effects of interface (IF) roughness on coherent acoustic phonon spectra are clearly observed for equal nominal THz-QCL structures but with different interface qualities. © 2012 American Institute of Physics