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Effect of local elastic softening on crack extension
322A
PROPERTIES:FRACTURE PROCESSES
strength of the rocks in which they form. Field observations on the geometry of fractures constrain theoretical models of fracture initiation and propagation. In this paper veins formed a variety of rock types and tectonic environments were measured at seven field locations to determine a general scaling relationship between the length and opening displacement (aperture). Displacement-length profiles for both singleand multiple-segment veins display centrally located maxima and gently tapered displacement gradients at the tips. This geometry is consistent with elastic-plastic crack growth models. The geometry is used to estimate stresses of formation for the veins. The geometries of muitiple-segment veins are consistent with elastic models for erack-tip stress interactions. (from Authors)
957139 Effect of local elastic softening on crack extension E. Smith, Theoretical & Applied Fracture Mechanics, 22(3), 1995, pp 267-273. Elastic softening materials are brittle materials such that crack extension is associated with a softening zone behind the crack tip, with the material elements within this zone exerting a restraining effect on the crack tip. Crack extension is sometimes characterised in terms of the stress intensity KF, due to the applied loadings, at the front of the softening zone, i.e. the actual crack tip. This paper is concerned with the determination of the maximum load KT value for a general positive geometrical configuration, for the case where the softening zone size is small compared with a solid's characteristic dimension. The resulting expression for KF is compared with the maximum load stress intensity value Kr measured with regard to the initial crack position, i.e. the trailing edge of the softening zone. (Author)
957140 A higher order displacement discontinuity method for analysis of crack problems K. J. Shou & S. L. Crouch, International Journal of Rock Mechanics & Mining Sciences, 32(1), 1995, pp 49-55. A new formulation of a higher order displacement discontinuity method is developed to model two-dimensional elastostatics problems. The new method uses three collocation points for each element, one at the center of the element in question and the others at the centers of the adjacent elements. By incorporating a special treatment for crack tips, a crack with arbitrary geometry can be modeled. The accuracy of the method is demonstrated by example problems. (from Authors)
957141 Difficulties with using continuous fractal theory for discontinuity surfaces A. Den Outer, J. F. Kaashoek & H. R. G. K. Hack, International Journal of Rock Mechanics & Mining Sciences, 32(1), 1995, pp 3-9. Options to characterize roughness of a discontinuity, in terms of a fractal and/or fractal dimension, are investigated mathematically and by simulated roughness profdes. The results show that relations between the JRC roughness profiles and fractal dimension determined by the divider method do not prove that the JRC curves are fractals can easily be inaccurate and can lead to erroneous relations. If roughness profiles are fractals at all, then the divider method is likely not suitable for determining the fractal dimension. (Authors)
957142 Suggested method for determining mode I fracture toughheSS using cracked chevron notched Brazilian disc (CCNBD) specimens R. J. Fowell, J. A. Hudson, C. Xu, J. F. Cben & X. Zhan, International Journal of Rock Mechanics & Mining Sciences, 32(1), 1995, pp 57-64. In 1988, the ISRM Testing Commission recommended two methods aiming to provide a testing method which would yield consistent fracture toughness values. These two methods very soon achieved general acceptance. The introduction of the cracked chevron notched Brazilian disc (CCNBD) specimen into the methods would form a complete set of specimens for a full rock anisotropic fracture toughness investigation since the crack orientations of these three suggested specimen geometries can be easily arranged to be orthogonal to each other if they are machined from the same rock core. Additional advantages are much higher failure loads, fewer restrictions on the testing apparatus, larger tolerance on the specimen machining error, simpler testing procedure and lower scatter of test results. It is suggested that this method is even more suitable for practical purposes. Furthermore, the CCNBD specimen and its origin model the cracked straight through Brazilian disc (CSTBD) are both ideal specimens for pure mode II or mixed mode fracture studies of rock materials. In this version of the document for the CCNBD test method, one level of experiment is presented. (from Authors) 957143 Three4imensional fracture network models from laburutory-scale rock samples L. S. Gertsch, International Journal of Rock Mechanics & Mining Sciences, 32(1), 1995, pp 85-91. The study reported here developed techniques for creating three-dimensional reference models of natural fracture networks from laboratory-scale rock samples. These tools are used to evaluate the accuracy and reliability of fracture mapping methods currently in industrial and research use, as well as newly developed methods. (Author)
957144 Specimen size effect on fracture toughness for a low cement refractory G. B. Palmer & G. Baker, Theoretical & Applied Fracture Mechanics, 22(1), 1995, pp 51-61. The structural size effect is known to affect failure that is controlled by two regions. The first is dominated by a constant stress at failure ductile behaviour, whereas the second follows classical LEFM brittle behaviour. The transition region between represents the greatest interest with respect to size effect in quasi brittle materials. This paper presents experimental data for a low cement refractory prefired to 1200°C (but tested at ambient conditions), showing that specimens do exhibit a size effect, and that typical sizes used in the process industry often lie in the nonlinear transition region. Fracture toughness has been shown to vary as a function of specimen size until the material displays brittle characteristics at which point fracture toughness appears to be a constant material parameter. (Authors)
957145 A statistical model of microcracking of concrete under uniaxlal compression W. K. Yip, F. K. Kong, K. S. Chan & M. K. Lim, Theoretical & Applied Fracture Mechanics, 22(1), 1995, pp 17-27. This paper shows the development of a statistical model for the progressive microcrack growth process within a body of concrete material at monotonic uniaxial loading in compression to ultimate failure. This model is formulated using
PROPERTIES:FRACTURE PROCESSES
strength of the rocks in which they form. Field observations on the geometry of fractures constrain theoretical models of fracture initiation and propagation. In this paper veins formed a variety of rock types and tectonic environments were measured at seven field locations to determine a general scaling relationship between the length and opening displacement (aperture). Displacement-length profiles for both singleand multiple-segment veins display centrally located maxima and gently tapered displacement gradients at the tips. This geometry is consistent with elastic-plastic crack growth models. The geometry is used to estimate stresses of formation for the veins. The geometries of muitiple-segment veins are consistent with elastic models for erack-tip stress interactions. (from Authors)
957139 Effect of local elastic softening on crack extension E. Smith, Theoretical & Applied Fracture Mechanics, 22(3), 1995, pp 267-273. Elastic softening materials are brittle materials such that crack extension is associated with a softening zone behind the crack tip, with the material elements within this zone exerting a restraining effect on the crack tip. Crack extension is sometimes characterised in terms of the stress intensity KF, due to the applied loadings, at the front of the softening zone, i.e. the actual crack tip. This paper is concerned with the determination of the maximum load KT value for a general positive geometrical configuration, for the case where the softening zone size is small compared with a solid's characteristic dimension. The resulting expression for KF is compared with the maximum load stress intensity value Kr measured with regard to the initial crack position, i.e. the trailing edge of the softening zone. (Author)
957140 A higher order displacement discontinuity method for analysis of crack problems K. J. Shou & S. L. Crouch, International Journal of Rock Mechanics & Mining Sciences, 32(1), 1995, pp 49-55. A new formulation of a higher order displacement discontinuity method is developed to model two-dimensional elastostatics problems. The new method uses three collocation points for each element, one at the center of the element in question and the others at the centers of the adjacent elements. By incorporating a special treatment for crack tips, a crack with arbitrary geometry can be modeled. The accuracy of the method is demonstrated by example problems. (from Authors)
957141 Difficulties with using continuous fractal theory for discontinuity surfaces A. Den Outer, J. F. Kaashoek & H. R. G. K. Hack, International Journal of Rock Mechanics & Mining Sciences, 32(1), 1995, pp 3-9. Options to characterize roughness of a discontinuity, in terms of a fractal and/or fractal dimension, are investigated mathematically and by simulated roughness profdes. The results show that relations between the JRC roughness profiles and fractal dimension determined by the divider method do not prove that the JRC curves are fractals can easily be inaccurate and can lead to erroneous relations. If roughness profiles are fractals at all, then the divider method is likely not suitable for determining the fractal dimension. (Authors)
957142 Suggested method for determining mode I fracture toughheSS using cracked chevron notched Brazilian disc (CCNBD) specimens R. J. Fowell, J. A. Hudson, C. Xu, J. F. Cben & X. Zhan, International Journal of Rock Mechanics & Mining Sciences, 32(1), 1995, pp 57-64. In 1988, the ISRM Testing Commission recommended two methods aiming to provide a testing method which would yield consistent fracture toughness values. These two methods very soon achieved general acceptance. The introduction of the cracked chevron notched Brazilian disc (CCNBD) specimen into the methods would form a complete set of specimens for a full rock anisotropic fracture toughness investigation since the crack orientations of these three suggested specimen geometries can be easily arranged to be orthogonal to each other if they are machined from the same rock core. Additional advantages are much higher failure loads, fewer restrictions on the testing apparatus, larger tolerance on the specimen machining error, simpler testing procedure and lower scatter of test results. It is suggested that this method is even more suitable for practical purposes. Furthermore, the CCNBD specimen and its origin model the cracked straight through Brazilian disc (CSTBD) are both ideal specimens for pure mode II or mixed mode fracture studies of rock materials. In this version of the document for the CCNBD test method, one level of experiment is presented. (from Authors) 957143 Three4imensional fracture network models from laburutory-scale rock samples L. S. Gertsch, International Journal of Rock Mechanics & Mining Sciences, 32(1), 1995, pp 85-91. The study reported here developed techniques for creating three-dimensional reference models of natural fracture networks from laboratory-scale rock samples. These tools are used to evaluate the accuracy and reliability of fracture mapping methods currently in industrial and research use, as well as newly developed methods. (Author)
957144 Specimen size effect on fracture toughness for a low cement refractory G. B. Palmer & G. Baker, Theoretical & Applied Fracture Mechanics, 22(1), 1995, pp 51-61. The structural size effect is known to affect failure that is controlled by two regions. The first is dominated by a constant stress at failure ductile behaviour, whereas the second follows classical LEFM brittle behaviour. The transition region between represents the greatest interest with respect to size effect in quasi brittle materials. This paper presents experimental data for a low cement refractory prefired to 1200°C (but tested at ambient conditions), showing that specimens do exhibit a size effect, and that typical sizes used in the process industry often lie in the nonlinear transition region. Fracture toughness has been shown to vary as a function of specimen size until the material displays brittle characteristics at which point fracture toughness appears to be a constant material parameter. (Authors)
957145 A statistical model of microcracking of concrete under uniaxlal compression W. K. Yip, F. K. Kong, K. S. Chan & M. K. Lim, Theoretical & Applied Fracture Mechanics, 22(1), 1995, pp 17-27. This paper shows the development of a statistical model for the progressive microcrack growth process within a body of concrete material at monotonic uniaxial loading in compression to ultimate failure. This model is formulated using