Brittle deformation

By nature, brittle deformation is discontinuous. It is often studied through mechanical tests, both in laboratories and outdoors, in mines and quarries. Brittle deformation also concerns civil engineering (road maintenance, strength of retaining structures such as bridges, dams, galleries etc.) and is well integrated with investigations in rock mechanics. Hydraulic fracturing is extensively used in the geothermal sector, for oil or gas production enhancement, or recovery of shale gas. Along with in-situ stress measurements, it has expanded the interest of geologists within the domain of rock mechanics. A solid knowledge of the mechanisms governing rock failure is necessary to understand the processes operating at the origin of earthquakes and volcanic eruptions, as well as the genesis of ore vein deposits. Beyond the elastic threshold of mechanical tests, rock failure takes place after development of a certain amount of non-elastic deformation. The fact that a progressive transition exists between ductile and brittle deformation suggests that these two behaviours are not mutually exclusive. Indeed, the study of the brittle-ductile transition paves the way to new concepts that enrich our understanding of the mechanisms of failure, in turn allowing practical applications. In this chapter, a presentation of the relationships between fracture orientation and principal stress directions will be followed by an examination of the macroscopic and microscopic aspects of brittle deformation.

Small strain shear modulus and damping ratio of two unsaturated lateritic sandy clays

In this study, resonant column tests carried out to investigate the influence of suction on the shear modulus and damping ratio of two compacted lateritic sandy clays from Ghana (GL) and Nigeria (NL) are reported. Each type of soils was tested under two confining pressures and at three suctions. The microstructure of the soils was also studied through a scanning electron microscope. It is found that the effects of suction on maximum shear modulus (G<u>_</u>0<u></u>) are about 10% larger for GL than NL, mainly due to the existence of smaller aggregates in GL. Moreover, an increase in suction from 0 to 300 kPa for both soils resulted in a lower elastic threshold shear strain, different from the behaviour of other soils reported in the literature. The uniqueness of lateritic soils is likely attributed to their high sesquioxide content and much larger aggregates, which shrink upon an increase in suction. Drying of specimens from 0 to 300 kPa resulted in an increase of about 22% and 100% in initial damping ratio (D₀) for GL and NL, respectively. The difference in D₀ for GL and NL and is attributed to larger aggregation of NL because of its higher iron sesquioxide content, leading to more cladding

A case of AndraStent® fracture in a patient with aortic coarctation: a review of the literature

Percutaneous treatment of aortic coarctation is based on angioplasty and/or stenting of the isthmus. We report a case of a 28-year-old girl suffering from aortic coarctation syndrome (coarctation + ventricular septal defect + bicuspid aorta). She underwent coarctectomy with subclavian flap and pulmonary bandage followed by ventricular septal defect closure and bandage removal in her first year of life. When she was 27 years old, a follow-up echocardiography detected an isthmic pressure gradient and a demodulated Doppler in abdominal aorta. A cardiac catheterisation confirmed the diagnosis of aortic re-coarctation. An AndraStent® XL 48 mm was implanted with a resolution of the isthmic gradient. One year later, because of the reappearance of demodulated Doppler in abdominal aorta, a chest X-ray was performed, which showed a stent third-grade fracture. The fracture was corrected by positioning a covered stent cheatham platinum 45 mm through the fragments. The rarest complication after stenting procedures is the fracture of the device with an incidence between 0.01% and 0.08%. Pressure overload beyond the elastic threshold of the material and the pulsatile tension exerted by the blood flow on the walls of the stent are the main mechanisms at the base of the fracture, together with the compliance of the tissue. A vessel that underwent multiple surgical rearrangements could interfere with and complicate the physiopathology at the basis of the fracture. In conclusion, stenting is a safe technique to treat aortic coarctation; stent fracture is a rare event, and different anatomical and haemodynamic factors are related to this complication.

Formulation of a Yield Surface for Sand Based on the Elastic Threshold Strain

The present study proposes a rigorous expression of a yield function for sand based on the linear elastic threshold strain concept and empirical expression for the maximum shear modulus. The new yield function was calibrated for Toyoura sand. The calibration results show that the proposed yield surfaces are nonlinear curves that depend on the void ratio of sand in the p′-q plane, whereas the linear lines have been adopted in the bounding surface modeling of sand. This study also found that elliptic yield surfaces are the best fitted with the proposed yield surface and they can be used as alternatives to the proposed yield surface under the undrained shearing where the void ratio (or density) of sand is fixed.

OPTIMUM RELIABILITY BASED DESIGN CRITERIA FOR ELASTIC STRUCTURES SUBJECT TO RANDOM DYNAMIC LOADS

In this work a new method for the optimal design of generic elastic structures, subject to random dynamic loads, is proposed. Elastic structures are described as deterministic multi-degree of freedom systems in which the structural failure probability, referred to as a first crossing passage problem, is minimized. The proposed method is here applied to optimize the shape of a vertical column with an extra-mass located at the free top end, subject to a base acceleration modeled as a Gaussian, stationary, filtered stochastic process. The elastic threshold crossing probabilities are determined in a finite number of column sections and the objective function is assumed to be a measure of these probabilities. Finally, this measure is minimized under a constant weight constraint (or even under more general conditions).