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An experimental approach on propagation and coalescence of pre-existing cracks (fractures) in marble samples under compression is carried out. Two types of newborn cracks are observed: wing (tensile) cracks and secondary (shear) c...
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An experimental approach on propagation and coalescence of pre-existing cracks (fractures) in marble samples under compression is carried out. Two types of newborn cracks are observed: wing (tensile) cracks and secondary (shear) cracks. Both types of cracks initiate from the tips of the fractures and propagate in a stable manner. Wing cracks initiate at an angle with the fractures and tend to propagate towards the direction of the most compressive stress. Secondary cracks, however, initiate in a direction quasi-coplanar to the fractures and also parallel to the wing cracks but in the opposite direction. The orientations and geometries of fractures can decide which kind of wing cracks and secondary cracks will be produced. The interactions between fractures strongly affect the initiation and propagation of wing cracks and secondary cracks, and make examples' failure patterns thoroughly different. The initial fracture angles of wing cracks range among 52degrees-68degrees, which are lower than the results (about 77degrees) of previous experiments on molding materials. In addition, the characters of acoustic emission (AE) of pre-cracked marble samples are also studied. For some rock samples, a secondary peak value phenomenon of AE counts is observed. For the samples with zigzag fractures, the accumulated AE counts at the moment of failure occupy almost half of the summation of whole load processes. This reveals that the failure of materials with zigzag fractures is more unexpected. (C) 2004 Published by Elsevier Ltd.
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The present article investigates the influences of the rock bridge ligament angle, beta, and the confinement on crack coalescence patterns by conducting laboratory and numerical tests on rock-like specimens. Laboratory tests show ...
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The present article investigates the influences of the rock bridge ligament angle, beta, and the confinement on crack coalescence patterns by conducting laboratory and numerical tests on rock-like specimens. Laboratory tests show that no coalescence in the rock bridge occurred for low beta. With an increase of beta, tensile-shear coalescence and tensile coalescences subsequently occurred. In addition, the increase in the confinement first promoted shear coalescence and then restrained crack coalescence for low beta, whereas the tensile coalescence was restrained by the increase in confinement for high beta. The numerical results corroborate the laboratory tests in the coalescence patterns. In addition, the numerical study shows that tensile and shear cracks subsequently initiated near crack tips because of the concentrated tensile and shear stresses, respectively. Regarding the influence of beta on crack coalescence, tensile or shear stress failed to concentrate in rock bridges for low beta. Therefore, the cracks failed to coalesce, whereas with the increase in beta, tensile and shear stress concentrations occurred in the bridge and led to either tensile shear or tensile coalescence. Regarding the influence of confinement on crack coalescence, the increase in confinement restrained the tensile stress concentrations and further hindered tensile crack coalescence in rock bridges for high values of beta. (C) 2019 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
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This work presents a new multiscale technique for the efficient simulation of crack propagation and crack coalescence of macrocracks and microcracks. The fully adaptive multiscale method is able to capture localization effect mesh...
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This work presents a new multiscale technique for the efficient simulation of crack propagation and crack coalescence of macrocracks and microcracks. The fully adaptive multiscale method is able to capture localization effect mesh independently. By modeling macrocracks and microcracks, the extended finite element method offers an accurate solution and captures cracks and their propagation without changing the mesh topology. Propagating and coaliting cracks of different length scales can therefore be easily modeled and updated during the computation process. Hence, the presented method is an efficient and accurate option for modeling cracks of different length scales. This is demonstrated in several numerical examples showing the interaction of microcracks and macrocracks.
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A reverse bending rig has the advantage of relatively cheap construction compared with servo-controlled machines, and its robustness and reliability make it ideally suited to long-term testing programmes. In this paper, the detail...
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A reverse bending rig has the advantage of relatively cheap construction compared with servo-controlled machines, and its robustness and reliability make it ideally suited to long-term testing programmes. In this paper, the details of the mechanical mechanism of a bending rig, the methods of its strain measurement and stress-strain analysis have been presented. A series of tests has been carried out to investigate short crack growth behaviour of AISI type 316 stainless steel under creep-fatigue conditions at 550 deg C. The advantage of this type of test allows a comparison to be made, on one specimen, of the influence of both tensile and compressive hold periods on crack growth behaviour. It has been shown that predominantly intergranular long cracks form on the tensile side and transgranular short cracks on the compressive side and these are a prominent feature between 0.9 percent and 2.5 percent strain range.
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An extensive experimental program has been conducted on pre-cracked specimens of a rock-model material to investigate crack propagation and coalescence from frictional discontinuities. Prismatic gypsum specimens have been prepared...
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An extensive experimental program has been conducted on pre-cracked specimens of a rock-model material to investigate crack propagation and coalescence from frictional discontinuities. Prismatic gypsum specimens have been prepared with three pre-existing closed cracks (flaws). The flaws all have a constant length of 12.7 mm and are parallel to each other. Different geometries are obtained by changing the angle of the flaws with respect to the direction of loading, the spacing, and the continuity of the flaws. In the experiments, three different types of cracks have been observed: wing cracks, coplanar shear, and oblique shear cracks. These are the same types of cracks observed with open flaws. Crack initiation occurs simultaneously at all the tips of the flaws for wing and shear cracks. Mean crack initiation stress is higher for secondary cracks than for wing cracks. The differences however decrease as the flaws are oriented at smaller angles with the direction of loading. The types of coalescence (i.e. the type of cracks and crack pattern that link two flaws) from closed flaws are similar to those from open flaws. However, the type of coalescence observed in a specimen with open flaws is not necessarily produced when using the same geometry but with closed flaws. The most important conclusion reached in this research is that the fracturing processes in open and closed flaws are similar. Friction along the flaws increases the initiation and coalescence stress and favors linkage through shear cracks.
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A new approach to the description of short crack growth - the P-a curve - has been tested using extensive data on submillimetre cracks in a grey cast iron. It is shown that this approach, in which the probability of growth is plot...
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A new approach to the description of short crack growth - the P-a curve - has been tested using extensive data on submillimetre cracks in a grey cast iron. It is shown that this approach, in which the probability of growth is plotted as a function of crack length, is valid in that it produces a plot that is consistent from specimen to specimen and is a function of both crack length and stress level. The growth probability, P_g, can be defined systematically in terms of the average amount of crack growth, Δc_(mean) within a given interval of cycles, ΔN; this leads to the concept that P_g characterizes growth at a particular rate: Δc_(mean)/ΔN. Predictions were made of the endurance, N_f, using a numerical model that simulates the growth of a large number of cracks. Crack coalescence was found to be a critical feature, both of the model and of the experimental findings. The great majority of crack growth was found to occur by coalescence; a simple model of coalescence, based on fracture mechanics, was successfully incorporated into the predictions.
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An investigation has been carried out on short crack growth behaviour of AISI type 316 stainless steel under creep-fatigue conditions at 550℃ for high strain ranges of 0.9-2.5% and 60 min hold-time using a high temperature revers...
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An investigation has been carried out on short crack growth behaviour of AISI type 316 stainless steel under creep-fatigue conditions at 550℃ for high strain ranges of 0.9-2.5% and 60 min hold-time using a high temperature reverse bending rig. The analysis revealed the dominant failure characteristics to be the individual initiation and growth behaviour of many minor cracks in Stage Ⅰ, and their subsequent coalescence in Stage Ⅱ. Increasing the strain range increases the number of minor cracks and promotes the process of minor crack coalescence. Predominantly intergranular long cracks are found to form under tensile stresses and transgranular short cracks under compressive stresses.
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The envelope method is considered for analyzing multiple coplanar surface cracks used in ASME and GB/T19624 for the safety assessment of equipment with cracks. Strain energy density factors at the crack front are calculated using ...
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The envelope method is considered for analyzing multiple coplanar surface cracks used in ASME and GB/T19624 for the safety assessment of equipment with cracks. Strain energy density factors at the crack front are calculated using finite element method. Crack propagation direction is investigated by comparing the strain energy density factors at different points on the crack front. It is found that if the elliptical surface crack is flat, the crack would preferably propagate along depth direction. In this case, the finally coalesced crack is much larger in size and the envelope method is not appropriate.
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This paper studies experimentally the coalescence mechanism between two parallel three-dimensional (3-D) pre-existing surface cracks in granite specimen under uniaxial compression. The bridge angles between the two pre-existing cr...
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This paper studies experimentally the coalescence mechanism between two parallel three-dimensional (3-D) pre-existing surface cracks in granite specimen under uniaxial compression. The bridge angles between the two pre-existing cracks vary from 0° to 135°. The digital speckle correlation method (DSCM) is used to analyze the captured images producing strain fields during the cracking process, and thus reveals the mode of cracking (either tensile, shear or their combination). Petal crack that initiates from internal crack fronts of the pre-existing surface cracks plays an important role in crack initiation, propagation and coalescence. Microcracks underneath the specimen surface appear as white patches on the surface, which always appear preceding the appearance of surface macrocracks (either wing or anti-wing cracks). Cracks, which grow in a direction opposite to that of wing cracks, were consistently observed before the appearance of wing cracks; and these cracks are therefore called "anti-wing cracks". In addition, DSCM results suggest that wing crack and anti-wing crack are tensile during initiation but may subsequently turn into mixed mode. The coalescence between the two 3-D pre-existing surface cracks takes place both on the specimen surface (through wing, anti-wing cracks or secondary cracks) and inside the specimen (through internal petal cracks). Depending on the bridge angle β between the two pre-existing cracks, there can be no crack coalescence, coalescence in forms of tensile cracks (tensile mode), coalescence by mixed mode cracks (mixed mode), and finally coalescence in forms of a secondary crack jointing an initiated wing or anti-wing crack, emanating from one tip, to the other tip (secondary crack mode).
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A methodology is presented for fracture and fatigue crack growth simulations of multiple cracks. Computed stress intensity factors and a crack growth law are used to predict incremented positions of crack front nodes. The procedur...
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A methodology is presented for fracture and fatigue crack growth simulations of multiple cracks. Computed stress intensity factors and a crack growth law are used to predict incremented positions of crack front nodes. The procedure is repeated until failure criterion is reached. The cases presented in this study include structures containing two or more cracks which propagate, coalesce or grow as planar and non-planar cracks. Comparisons of results with literature data shows excellent agreement. Therefore, it is concluded that the presented procedure can be used to accurately to assess fatigue crack propagation behavior of structures with multiple three-dimensional cracks. (C) 2015 Elsevier Ltd. All rights reserved.
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