RESUMEN
Why are strike slip faults not perfectly linear but made of successive segments? Are they reminiscences of the fracture of an initially sound crust by the bottom-up propagation of a crack subjected to mode III loading? The plausibility of this newly proposed scenario will be investigated here through model experiments and some theoretical explanations in the framework of brittle fracture.This article is part of the theme issue 'Statistical physics of fracture and earthquakes'.
RESUMEN
To unravel how the microstructure affects the fracture surface roughness in heterogeneous brittle solids like rocks or ceramics, we characterized the roughness statistics of postmortem fracture surfaces in homemade materials of adjustable microstructure length scale and porosity, obtained by sintering monodisperse polystyrene beads. Beyond the characteristic size of disorder, the roughness profiles are found to exhibit self-affine scaling features evolving with porosity. Starting from a null value and increasing the porosity, we quantitatively modify the self-affine scaling properties from anisotropic (at low porosity) to isotropic (for porosity >10%).
RESUMEN
In an effort to investigate the link between failure mechanisms and the geometry of fractures of compacted grains materials, a detailed statistical analysis of the surfaces of fractured Fontainebleau sandstones has been achieved. The roughness of samples of different widths W is shown to be self-affine with an exponent zeta=0.46+/-0.05 over a range of length scales ranging from the grain size d up to an upper cutoff length xi approximately =0.15 W. This low zeta value is in agreement with measurements on other sandstones and on sintered materials. The probability distributions pi delta z(delta h) of the variations of height over different distances delta z>d can be collapsed onto a single Gaussian distribution with a suitable normalization and do not display multiscaling features. The roughness amplitude, as characterized by the height-height correlation over fixed distances delta z, does not depend on the sample width, implying that no anomalous scaling of the type reported for other materials is present. It is suggested, in agreement with recent theoretical work, to explain these results by the occurrence of brittle fracture (instead of damage failure in materials displaying a higher value of zeta approximately =0.8 ).
RESUMEN
The propagation of fractures during the drying of a colloidal silica suspension confined in a vertical microtube is investigated. During the drying process, the particle concentration increases until gel formation. In the gelled region, the tensile stresses increase and lead to the formation of two vertical perpendicular cracks propagating in the drying direction, dividing the tube into four equivalent regions. Throughout the drying process, these two fractures do not propagate at the same velocity. The top crack inhibits the propagation of the crack that is left behind, and the slow propagation of a fracture is followed by the rapid propagation of the other.
RESUMEN
Retinoids are potent therapuetic agents that have been found to be effective in a variety of skin diseases. They are of benefit in various forms of severe psoriasis. With severe plaque psoriasis, they are used most effectively in combination with other forms of therapy, such as phototherapy. With generalized pustular psoriasis, they are effective monotherapy and are frequently helpful for the control of exfoliative psoriasis. A variety of new retinoid analogs have been studied in clinical investigations. This article discusses important aspects of the use of etretinate, isotretinoin, acitretin, and arotinoid ethyl ester in the treatment of severe forms of psoriasis.