Experimental study on the e ect of fracture scale on seismic wave characteristics

In fractured reservoir beds, fracture characteristics affect seismic wave response. Fractured models based on the Hudson＇s fractured medium theory were constructed in our laboratory by a backfilling technique. For the same fracture density, the variations of the velocity and amplitude of the primary wave and shear wave parallel and perpendicular to the fracture were observed by altering the diameter （scale） of the penny-shaped fracture disk. The model test indicated that an increase of fracture scale increased the velocity and amplitude of the primary wave by about 2%. When the shear wave propagated parallel to the fracture, the velocity of the fast shear wave hardly changed, while the velocity of slow shear wave increased by 2.6% with increasing fracture scale. The results indicated that an increase of fracture scale would reduce the degree of anisotropy of the shear wave. The amplitudes of slow shear waves propagating parallel and perpendicular to fractures decreased with increasing fracture scale.

Fracture behavior and self-sharpening mechanisms of polycrystalline cubic boron nitride in grinding based on cohesive element method

Unlike monocrystalline cubic boron nitride(CBN), polycrystalline CBN(PCBN) shows not only higher fracture resistance induced by tool-workpiece interaction but also better selfsharpening capability;therefore, efforts have been devoted to the study of PCBN applications in manufacturing engineering. Most of the studies, however, remain qualitative due to difficulties in experimental observations and theoretical modeling and provide limited in-depth understanding of the self-sharpening behavior/mechanism. To fill this research gap, the present study investigates the self-sharpening process of PCBN abrasives in grinding and analyzes the macro-scale fracture behavior and highly localized micro-scale crack propagation in detail. The widely employed finite element(FE) method, together with the classic Voronoi diagram and cohesive element technique,is used considering the pronounced success of FE applications in polycrystalline material modeling.Grinding trials with careful observation of the PCBN abrasive morphologies are performed to validate the proposed method. The self-sharpening details, including fracture morphology, grinding force, strain energy, and damage dissipation energy, are studied. The effects of maximum grain cut depths(MGCDs) and grinding speeds on the PCBN fracture behavior are discussed, and their optimum ranges for preferable PCBN self-sharpening performance are suggested.