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A kinetic model of nucleation, propagation and fusion of cracks
172A
PHERIPHERAL SUBJECTS:FRACTURE MECHANICS
943417 End-capping procedure for cored ice samples used in tension tests J. A. Richter-Menge, K. J. Claffey & M. R. Walsh, Journal of Glaciology, 39(133), 1993, pp 698-700. This note describes an end-capping procedure used to prepare cylindrical ice-core samples for direct tension testing. The techniques developed build on work done by Cole and others (1985) and Lee (1986), and include a design modification to the loading train and a more reliable approach to establishing the fresh-water bond used to mount the end caps on the sample. Using these techniques, an 88% success rate was achieved in a recent series of uniaxial tension tests. -Authors
Fracture mechanics 943418 Real-time SEM observations of the microfracturing process in rock during a compression test Y. Zhao, J. Huang & R. Wang, International Journal of Rock Mechanics & Mining Sciences, 30(6), 1993, pp 643 -652. It is now widely accepted that the macroscopic brittle fracture of rocks is a progressive process consisting of the initiation, propagation, proliferation and coalescence of numerous microcracks. To disclose details concerning the transition from the accumulation of microfracturing to the formation of a macroscopic fracture zone and to correlate such a transition with the precursors of various unstable mechanical processes in rocks, such as rock bursts and earthquakes, is undoubtedly of great importance for reducing the hazard caused by those events. In order to obtain a closer view of the details of microfracturing development, the authors have made real-time SEM observations of the failure development of compressed marble plates. By doing so, the whole process of failure development was traced and some new observations were obtained. -from Authors 943419 Fractal damage mechanics of geomateriais T. Chelidze, Terra Nova, 5(5), 1993, pp 421-437. It has become clear that the mechanics of geomaterials should take into consideration the influence of the fracture delocalization process on fundamental fracture characteristics such as the strength, the fractal dimension of a network of cracks, the surface energy of fracture and the elastic properties of fragmented media. The percolation model of fracture treats the destruction process as a sequence of nucleation and coalescence of microcracks caused by their interaction. This approach enables the mathematical description of the entire process of fracturing. Several implications of fractal mechanics for seismology are considered, namely the generalized form of the magnitude-frequency relationship, the phenomenon of anomalous tensosensitivity, the scale-dependence of the elastic moduli of massively faulted rocks and the possibility of apparent seismic boundaries. -from Author 943420 A kinetic model of nucleation, propagation and fusion of cracks Z. Czechowski, Journal of Physics of the Earth, 41(3), 1993, pp 127-137. The fracture of solids is preceded by the appearance of numerous microcracks which can propagate and coalesce. The state of the crack system is descrthed by the size distribution function which satisfies an integro-diffcrential kinetic equation. Under two different assumptions concerning the fusion cross-section, exponential and inversepower solutions are deduced. The exponential form of the crack size distribution is observed in many damage extmriments for.metals but the inverse-power law is typical for more orittle materials like rocks. A second way of analysis of the evolution of crack populations is based on the division of all cracks into n groups, then a set of n nonlinear differential equations is obtained. This model is more handy for numerical calculations and was used for longterm evolution of the crack system. -Author
943421 Analysis of crack propagation in brittle material T. Shioya, JSME International Journal, Series A: Mechanics & Material Engineering, 37(1), 1994, pp 1-6. Analysis of crack propagation in brittle materials is reviewed. Some exam.l~les are introduced in crack path prediction under triaxial stress state, dynamic crack propagation and crack growth in nonhomogeneous materials. -Author 943422 Relationship between stress intensity factors and residual stress distributions evaluated from discontinuous displacements along mixed.mode crack T. Torii, K. Honda & Y. Yoshimura, JSME International Journal, Series A: Mechanics & Material Engineering, 37(1), 1994, pp 14-21. In linear fracture mechanics, stress intensity factors are calculated from stress distributions acting on crack surfaces, with the aid of superposition even for remote applied stresses. A method of evaluating such stress distributions from each mode of discontinuous displacement along a crack with mode I and H components has been proposed. The calculation method, restricted to constant stress fields, has been developed for use in nonuniform stress fields for application to fatigue cracks with residual stresses in the wake of the propagation. Using this method, local stress distributions which control the crack deformation and stress intensity factors for the fatigue crack are predicted under a mixed-model stress condition, and effects of residual stresses on mode II crack deformation are discussed. -from Authors 943423 A theory of caustics for mixed-mode fast-running cracks (higher-order theory) T. Nishioka, R. Murakami, Y. Ohishi & N. Maeda, JSME International Journal, Series A: Mechanics & Material Engineering, 37(1 ), 1994, pp 22-30. A higher-order theory of caustics of mixed-mode running cracks is developed by using the general asymptotic solutions for a dynamically propagating crack tip. The analytical expressions are obtained for the Jacobian equation that determines the initial curve, and for the image equations on a screen. With the use of the higher-order coefficients determined by the finite-element simulations of actual dynamic fracture experiments, the effects of the higherorder terms on the caustic curve are investigated on the basis of the present theory. It was found that the r lrz stress field plays an important role in the formation of caustic patterns. A higher-order theory of caustics for stationary cracks is also derived in this paper. -Authors 943424 Mode II and mode I l l stress singularities and intensities at a crack tip terminating on a transversely isotropic-orthotropic bimaterial interface J. Wang & B. L. Karihaloo, Proceedings - Royal Society of London, A, 444(1922), 1994, pp 447-460. This paper analises the case when the tip of a crack terminates on the transversely isotropic-orthotropic bimaterial interface and the nature of the singularity at the crack tip depends on the elastic properties of both media. The analysis is performed for both inplane (mode II) and out-of-plane (mode HI) shear loading. General solutions are obtained for the crack tip stress singularities and corresponding stress intensity factors, together with the influence of the elastic properties and geometry of the media upon the stress field. These solutions are specialized to the limiting case when the crack terminates on the interface between dissimilar isotropic media in order to demonstrate consistency with published results. -from Authors
PHERIPHERAL SUBJECTS:FRACTURE MECHANICS
943417 End-capping procedure for cored ice samples used in tension tests J. A. Richter-Menge, K. J. Claffey & M. R. Walsh, Journal of Glaciology, 39(133), 1993, pp 698-700. This note describes an end-capping procedure used to prepare cylindrical ice-core samples for direct tension testing. The techniques developed build on work done by Cole and others (1985) and Lee (1986), and include a design modification to the loading train and a more reliable approach to establishing the fresh-water bond used to mount the end caps on the sample. Using these techniques, an 88% success rate was achieved in a recent series of uniaxial tension tests. -Authors
Fracture mechanics 943418 Real-time SEM observations of the microfracturing process in rock during a compression test Y. Zhao, J. Huang & R. Wang, International Journal of Rock Mechanics & Mining Sciences, 30(6), 1993, pp 643 -652. It is now widely accepted that the macroscopic brittle fracture of rocks is a progressive process consisting of the initiation, propagation, proliferation and coalescence of numerous microcracks. To disclose details concerning the transition from the accumulation of microfracturing to the formation of a macroscopic fracture zone and to correlate such a transition with the precursors of various unstable mechanical processes in rocks, such as rock bursts and earthquakes, is undoubtedly of great importance for reducing the hazard caused by those events. In order to obtain a closer view of the details of microfracturing development, the authors have made real-time SEM observations of the failure development of compressed marble plates. By doing so, the whole process of failure development was traced and some new observations were obtained. -from Authors 943419 Fractal damage mechanics of geomateriais T. Chelidze, Terra Nova, 5(5), 1993, pp 421-437. It has become clear that the mechanics of geomaterials should take into consideration the influence of the fracture delocalization process on fundamental fracture characteristics such as the strength, the fractal dimension of a network of cracks, the surface energy of fracture and the elastic properties of fragmented media. The percolation model of fracture treats the destruction process as a sequence of nucleation and coalescence of microcracks caused by their interaction. This approach enables the mathematical description of the entire process of fracturing. Several implications of fractal mechanics for seismology are considered, namely the generalized form of the magnitude-frequency relationship, the phenomenon of anomalous tensosensitivity, the scale-dependence of the elastic moduli of massively faulted rocks and the possibility of apparent seismic boundaries. -from Author 943420 A kinetic model of nucleation, propagation and fusion of cracks Z. Czechowski, Journal of Physics of the Earth, 41(3), 1993, pp 127-137. The fracture of solids is preceded by the appearance of numerous microcracks which can propagate and coalesce. The state of the crack system is descrthed by the size distribution function which satisfies an integro-diffcrential kinetic equation. Under two different assumptions concerning the fusion cross-section, exponential and inversepower solutions are deduced. The exponential form of the crack size distribution is observed in many damage extmriments for.metals but the inverse-power law is typical for more orittle materials like rocks. A second way of analysis of the evolution of crack populations is based on the division of all cracks into n groups, then a set of n nonlinear differential equations is obtained. This model is more handy for numerical calculations and was used for longterm evolution of the crack system. -Author
943421 Analysis of crack propagation in brittle material T. Shioya, JSME International Journal, Series A: Mechanics & Material Engineering, 37(1), 1994, pp 1-6. Analysis of crack propagation in brittle materials is reviewed. Some exam.l~les are introduced in crack path prediction under triaxial stress state, dynamic crack propagation and crack growth in nonhomogeneous materials. -Author 943422 Relationship between stress intensity factors and residual stress distributions evaluated from discontinuous displacements along mixed.mode crack T. Torii, K. Honda & Y. Yoshimura, JSME International Journal, Series A: Mechanics & Material Engineering, 37(1), 1994, pp 14-21. In linear fracture mechanics, stress intensity factors are calculated from stress distributions acting on crack surfaces, with the aid of superposition even for remote applied stresses. A method of evaluating such stress distributions from each mode of discontinuous displacement along a crack with mode I and H components has been proposed. The calculation method, restricted to constant stress fields, has been developed for use in nonuniform stress fields for application to fatigue cracks with residual stresses in the wake of the propagation. Using this method, local stress distributions which control the crack deformation and stress intensity factors for the fatigue crack are predicted under a mixed-model stress condition, and effects of residual stresses on mode II crack deformation are discussed. -from Authors 943423 A theory of caustics for mixed-mode fast-running cracks (higher-order theory) T. Nishioka, R. Murakami, Y. Ohishi & N. Maeda, JSME International Journal, Series A: Mechanics & Material Engineering, 37(1 ), 1994, pp 22-30. A higher-order theory of caustics of mixed-mode running cracks is developed by using the general asymptotic solutions for a dynamically propagating crack tip. The analytical expressions are obtained for the Jacobian equation that determines the initial curve, and for the image equations on a screen. With the use of the higher-order coefficients determined by the finite-element simulations of actual dynamic fracture experiments, the effects of the higherorder terms on the caustic curve are investigated on the basis of the present theory. It was found that the r lrz stress field plays an important role in the formation of caustic patterns. A higher-order theory of caustics for stationary cracks is also derived in this paper. -Authors 943424 Mode II and mode I l l stress singularities and intensities at a crack tip terminating on a transversely isotropic-orthotropic bimaterial interface J. Wang & B. L. Karihaloo, Proceedings - Royal Society of London, A, 444(1922), 1994, pp 447-460. This paper analises the case when the tip of a crack terminates on the transversely isotropic-orthotropic bimaterial interface and the nature of the singularity at the crack tip depends on the elastic properties of both media. The analysis is performed for both inplane (mode II) and out-of-plane (mode HI) shear loading. General solutions are obtained for the crack tip stress singularities and corresponding stress intensity factors, together with the influence of the elastic properties and geometry of the media upon the stress field. These solutions are specialized to the limiting case when the crack terminates on the interface between dissimilar isotropic media in order to demonstrate consistency with published results. -from Authors