[en] Explosive or percussive breakage of rock relies on impulsive rock fracture and both the fracture stress and fracture energy under such conditions are observed to be significantly greater than measured quasi-statically. The implication of rate dependence has been verified in recent tensile fracture studies and in the present report this data has been compiled, significant trends noted, and comparisons made with earlier impulsive fracture criteria

[en] A brief description of fracture definition, classification and fracture transitions from brittle to ductile response were studied. The size effect on stages, i.e., fracture stress and fracture strain and on the other mechanical properties which lead to the transition were also studied. Moreover, the scaling principle, laws were approached for three tests (C.C.P., D.C.B., Hertzin). The characteristic length that was approached by Puttick's theory to be a condition for the fracture transition was scaled for the first time in terms of the scaling factor and the degree of non-linearity. The formulated scaling laws for this length have been generalized to cover the necessary conditions relative to the flaw size. 2 tabs.; 25 figs.; 38 refs

[en] Over 400 papers are presented on various aspects of brittle, ductile and dynamic fracture, fatigue, creep and environmental fracture, computational fracture mechanics, damage mechanics, mixed-mode fracture, fracture of metallic, non-metallic and composite materials, nondestructive examination and fractography. Ten are within INIS scope and are processed separately. (UK)

[en] Based on density logs, the overload of Fazenda Alvorada field located in the state of Bahia (Brazil) presents an anomalous behavior in relation to the same sedimentary package of the Reconcavo Basin. This anomaly is in function of a relative uplift accompanied of a concomitant bearing or not, with the uplift doing with that the gradients of fracture propagation usually present high values. Comparisons among the gradients of fracture propagation, obtained step-rate-tests accomplished in reservoirs different from 7-FAV-83-BA well, and the overload of this field allowed to identify occurrences of vertical and horizontal fractures. The objective of this work is to present as these results were obtained, as well as its importance in the stimulation projects and of secondary recovery. (author)

[en] Highlights: • Fundamental concepts of fracture mechanics are revisited with respect to their applicability in miniaturized experiments • Stress state and notch sharpness in miniaturized samples need to be considered • For size independent fracture toughness values, sample sizes must obey relations to the microstructure and fracture process zone • Different contributions to material toughness can be separated by appropriate experimental design In this review article we consider the crack growth resistance of micrometer and sub-micrometer sized samples from the fracture mechanics point of view. Standard fracture mechanics test procedures were developed for macro-scale samples, and reduction of the specimen dimensions by three to five orders of magnitude has severe consequences. This concerns the interpretation of results obtained by micro- and nano-mechanics, as well as the life time and failure prediction of micro- and nano-devices. We discuss the relevant fracture mechanics length scales and their relation to the material-specific structural lengths in order to conduct rigorous fracture mechanics experiments. To ensure general validity and applicability of evaluation concepts, these scaling considerations are detailed for ideally brittle, semi-brittle and micro ductile crack propagation, subject to both monotonic and cyclic loading. Special attention is devoted to the requirements for determining specimen size for various loading types to measure material characteristic crack propagation resistance at small scales. Finally, we discuss novel possibilities of micron and sub-micron fracture mechanics tests to improve the basic understanding of specific crack propagation processes.

[en] The reliability of equipments and the machine elements are of primary importance in the advanced technology. The equipments or machine elements functioning in hazardous, low temperature or high strength service condition requirements need to be designed as a safe structure against service fracture. The earlier criteria of material selection were based on experience or charpy-notch impact tests. The incompetence of the charpy impact test was realised gradually after catastrophic failures at important installations which led to a thorough study and research. This evolved the new techniques of fracture toughness (Ksub(IC)) testing which are based on natural crack tests simulating the realistic service notch condition. Ksub(IC) properties are in wide use as design parameter as they correlate the defect size with service load. The Ksub(IC) as a tool for estabilshing defect acceptance criteria in non-destructive testing is being adopted progressively. This paper presents comparison of Ksub(IC) with charpy V-notch value (Csub(v)), yield strength and other toughness values along with the description of various toughness tests. The comparisons may be of use where Ksub(IC) testing facilities are not available but advanced design requires the value of Ksub(IC) as a design parameter. (auth.)

[en] In the hydraulic fracturing process, the velocity model of traditional inversion is usually constructed by well logs, seismic data or calibration shots. The variation of pore pressure and fractures in this process has a major influence on the velocity. However, this influence is usually ignored in the velocity estimation model. In this paper, we propose a new microseismic location method that accounts for the influence of hydraulic fracturing on velocity. Firstly, we simulate the 3D hydraulic fracturing process based on mass conservation, the seepage equation and fracture mechanics. Then the equivalent velocity model is constructed using the Coates–Schoenberg method and fracture compliances. The 3D ray-tracing method is applied to forward-model the microseismic data and traditional inversion methods are applied to locate events and analyse the inversion error. Finally, we introduce a new method, called the equivalent velocity inversion method (EVIM), taking into account the influence of the fracturing process. The simulation results show that the hydraulic fracturing process affects the velocity and thus makes the inversion accuracy of the same receiver array vary considerably with the sources in the traditional methods. The new EVIM can effectively reduce the inversion error. (paper)

[en] It is believed that 80% of producing wells deliver below their productive capacities. These wells' fractures have lost their conductivity because of several problems. This conductivity loss is well documented, and much literature considers the original conditions of the well in relation to fracturing projects. However, literature about refracturing a well is scarce, and methods, standards, and specific simulators do not exist. The software used to project a refracture is the same as that used for projecting new fracturing projects. The lack of specific software for refracturing generates doubts about the parameters that should be used for obtaining satisfactory projects. This paper introduces some criteria that should be considered in the design of refracturing projects and offers guidelines for selecting well candidates so that the stimulations can be optimized and the wells' productivity can be increased. (author)

[en] The fracture toughness of materials in which brittle cracks initiate and propagate along grain boundaries (GBs) depends not only on the energy of the GB, but also on its microscopic structure. Exactly how and to what degree brittle GB fracture is affected by the local atomic structure at the crack tip has not yet been studied in detail. Here, we use molecular static simulations to study the atomic-scale fracture behavior of six large-angle tilt GBs in tungsten bicrystals. The fracture toughness depends critically on the propagation direction and on the position of the crack tip within the structural units of the GB. Furthermore, the GB fracture toughness can be significantly larger than for single crystals in the same orientation. These results cannot be explained by the usual thermodynamic approach in continuum-scale fracture mechanics but can be understood by considering the effect of bond trapping of GB cracks

[en] Using variable-capacitance transducer the continuous recording of the free surface rate of an iron sample cooled to 77 K during its splitting-off destruction has been realized. The splitting-off strength and the amplitude of elastic sign at decreased temperatures is determined. A slight effect of cooling on iron strength and rheologic characteristics is pointed out