Assessment of crack stress thresholds and development of a pre-failure indicator using Digital Image Correlation approach for coal specimen
01 Nov 2021-Vol. 7, Iss: 4
About: The article was published on 2021-11-01. It has received 3 citations till now. The article focuses on the topics: Digital image correlation.
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TL;DR: In this paper , a self-developed magnetic drop hammer impact device was used to measure the change of ultrasonic wave velocity before and after drop-hammer impact, the internal damage of coal samples was quantitatively characterized, and then, the damage factor, damage difference, and crack development direction of briquette samples were analyzed.
Abstract:
To study the response characteristics of coal samples to impact load under different surrounding pressures, the self-developed magnetic drop hammer impact device was used to measure the change of ultrasonic wave velocity before and after drop hammer impact, the internal damage of coal samples was quantitatively characterized, and then, the damage factor, damage difference, and crack development direction of briquette samples were analyzed. Based on the Griffith strength theory, the relationship between stress intensity factor, crack development angle, and surrounding pressure ratio of coal samples with composite cracks was deduced. It was found that a certain surrounding pressure would slow crack expansion and reduce the amount of damage increase. The damage degree of briquette samples caused by impact load under no surrounding pressure is the largest, followed by that at the surrounding pressure ratio of 0 and 2 and that at the surrounding pressure ratio of 1 is the smallest. When the amount of damage difference of the sample is positively correlated with the number of impacts, the crack development is directional; when the amount of damage difference of the sample is nonpositively correlated with the number of impacts, the direction of crack development is random. It is therefore reasonable to predict the direction of crack expansion by the trend in the amount of damage difference. Besides, the difference in impact load between briquette and raw coal samples under different surrounding pressure ratios was discussed, and the fractal dimension of the sample surface under different surrounding pressure ratios was compared and analyzed. It is found that the fractal dimension of raw coal is larger than that of briquette, indicating that the crack development of raw coal is higher after impact; the damage amount of raw coal is larger than that of briquette, indicating that raw coal is easier to be damaged after impact.
1 citations
TL;DR: In this article , the authors derived the stress distribution on the surface of coal-rock by introducing the stress intensity factor into the orifice stress concentration equation; moreover, the internal relationship among the stress wave energy, the blast-induced gas energy and the surface stress of coalrock is described by work-energy principle.
References
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Book•
24 Sep 1993TL;DR: In this article, the authors present a unified continuum, microstructural and atomistic treatment of modern day fracture mechanics from a materials perspective, focusing on the basic elements of bonding and microstructure that govern the intrinsic toughness of ceramics.
Abstract: This is an advanced text for higher degree materials science students and researchers concerned with the strength of highly brittle covalent–ionic solids, principally ceramics. It is a reconstructed and greatly expanded edition of a book first published in 1975. The book presents a unified continuum, microstructural and atomistic treatment of modern day fracture mechanics from a materials perspective. Particular attention is directed to the basic elements of bonding and microstructure that govern the intrinsic toughness of ceramics. These elements hold the key to the future of ceramics as high-technology materials - to make brittle solids strong, we must first understand what makes them weak. The underlying theme of the book is the fundamental Griffith energy-balance concept of crack propagation. The early chapters develop fracture mechanics from the traditional continuum perspective, with attention to linear and nonlinear crack-tip fields, equilibrium and non-equilibrium crack states. It then describes the atomic structure of sharp cracks, the topical subject of crack-microstructure interactions in ceramics, with special focus on the concepts of crack-tip shielding and crack-resistance curves, and finally deals with indentation fracture, flaws, and structural reliability.
3,550 citations
TL;DR: In this paper, the authors measured volume changes of a granite, a marble, and an aplite during deformation in triaxial compression at confining pressure of as much as 8 kb.
Abstract: Volume changes of a granite, a marble, and an aplite were measured during deformation in triaxial compression at confining pressure of as much as 8 kb. Stress-volumetric strain behavior is qualitatively the same for these rocks and a wide variety of other rocks and concrete studied elsewhere. Volume changes are purely elastic at low stress. As the maximum stress becomes one-third to two-thirds the fracture stress at a given pressure, the rocks become dilatant; that is, volume increases relative to elastic changes. The magnitude of the dilatancy, with a few exceptions, ranges from 0.2 to 2.0 times the elastic volume changes that would have occurred were the rock simply elastic. The magnitude of the dilatancy is not markedly affected by pressure, for the range of conditions studied here.
For granite, the stress at which dilatancy was first detected was strongly time dependent; the higher the loading rate the higher the stress. Dilatancy, which represents an increase in porosity, was traced in the granite to open cracks which form parallel with the direction of maximum compression.
1,455 citations
TL;DR: The strength of intact rock is made up of two components: the intrinsic strength, or cohesion; and the frictional strength as discussed by the authors, and it is generally assumed that cohesion and friction are mobilized at the same displacements such that both components can be relied on simultaneously.
1,207 citations
TL;DR: In this article, the initiation and propagation of microfractures and their contribution to material failure in compression are examined, and the early part of the fracture process, the lateral and the axial yield points of the stress-strain curves, are identified with the onset of micro-fracture, respectively at the tensile and the compressive stress concentrations of the elastic flaw boundary.
Abstract: The initiation and propagation of microfractures and their contribution to material failure in compression are examined. The early part of the fracture process, the lateral and the axial yield points of the stress-strain curves, are identified with the onset of microfracture, respectively at the tensile and the compressive stress concentrations of the elastic flaw boundary. Later stages, including the initiation of inclined shear fractures, the mobilization of total resistance and the reduction of strength to the residual level, are discussed in terms of a modified Coulomb model.
344 citations
TL;DR: In this article, the authors investigated the fracture and damage mechanisms of rock induced by the accumulation of microcracks by moment tensor analysis, as well as by the moving point regression technique, both of which were applied to acoustic emission (AE) and strain data obtained from triaxial compression tests.
262 citations