Papers by Keyword: Microfracture

Abstract: In this paper a cohesive formulation is proposed for modelling intergranular and transgranular damage and microcracking evolution in brittle polycrystalline materials. The model uses a multi region boundary element approach combined with a dual boundary element formulation. Polycrystalline microstructures are created through a Voronoi tessellation algorithm. Each crystal has an elastic orthotropic behaviour and specific material orientation. Transgranular surfaces are inserted as the simulation evolves and only in those grains that experience stress levels high enough for the nucleation of a new potential crack. Damage evolution along (inter-or trans-granular) interfaces is then modelled using cohesive traction separation laws and, upon failure, frictional contact analysis is introduced to model separation, stick or slip. Moreover some physical consideration based on cohesive energies were made, in order to guarantee the cohesive model in consideration was appropriate for the purpose of this work. Finally numerical simulations have been performed to demonstrate the validity of the proposed formulation in comparison with experimental observations and literature results.

Abstract: The stress-dependent permeability of ultra-low permeability rocks has a significant impact on the seepage. A number of experimental studies on the variation of permeability with stress change in low permeability reservoir have been done, but few are involved in ultra-low permeability cores. In this study, first, we fabricated the shear-stress microfracture cores and then the stress sensitive experiments were conducted. Second, a fracture deformation model was introduced in order to depict the stress sensitivity features of fracture media theoretically. Finally, a comparison of the theoretical curves with the experimental data had been done. The results show that the fabricated microfracture cores can be used in the permeability stress sensitive experiments. The permeability stress sensitive hysteresis of the microfracture cores is not obvious and the recovery degree of stress sensitive hysteresis is high in the unloading cycle for the grains were compacted in the process of shearing. The theoretical curves have the desired behavior which is consistent with the experimental data well. It indicates that the method of fabricating microfractures can similarly reveal the stress sensitivity features of the real microfracture ultralow permeability reservoir. This study do great benefit to reveal the stress sensitivity features of ultra-low permeability reservoir.

Abstract: This paper presents a fractal analysis of the self-sharpening phenomenon of the grain cutting edges in cBN grinding. To clarify the self-sharpening mechanism due to the micro fracture of the cutting edges, the changes in three-dimensional profile of the cutting edges in the grinding process have been measured using a scanning electron microscope with four electron probes and evaluated on the basis of the fractal analysis. The fractal dimension for surface profile of the cutting edge formed by the micro fracture is higher than that of the cutting edge formed by the ductile attritious wear. Therefore, the complicated changes in shape of the cutting edge due to the self-sharpening can be evaluated quantitatively using the fractal dimension.

Abstract: In this paper, intergranular microfracture evolution in polycrystalline brittle materials is simulated using a cohesive grain boundary integral formulation. A linear cohesive law is used for modelling multiple microcracking initiation and propagation under mixed mode failure conditions, encountering the stochastic e=ects of the grain location, morphology and orientation. Furthermore, in cases where crack surfaces come into contact, slide or separate, fully frictional contact analysis is performed.

Abstract: An AE measurement system that uses a laser interferometer has been developed to evaluate microfractures at elevated temperatures in various materials. This technique has the advantage that it can estimate the temperature where microfractures were generated. The results give useful information to control process conditions. AE during sintering ceramics and thermal spying of ceramic powder on metal substrates were successfully detected by this laser AE measurement system. Effect of process conditions on damage process was discussed from the detected AE behaviour during in-process monitoring.

Abstract: The material removal mechanism of ultrasonic machining sintered Nd-Fe-B magnetic materials was studied theoretically. The relation between critical load and central crack is given. In order to assure the material removal mode on material surface is brittle micro-fracture, the acting force of a single abrasive particle working on workpiece surface should be higher than the critical load. Experimental results show that there should be an optimal static load and an optimal abrasive size in certain ultrasonic machining system. The research results are helpful to guide practical production.