Mechanism Of Fracture Propagation Research Articles

Fracture Initiation Model of Shale Fracturing Based on Effective Stress Theory of Porous Media

Unconventional oil and gas are important resources of future energy supply, and shale gas is the focus of the development of unconventional resources. Shale is a special kind rock of porous medium, and an orderly structure of beddings aligned in the horizontal direction where causing the strong elastic anisotropy of shale is easy. A new model has been established to calculate the fracture initiation pressure with the consideration of mechanical characteristics of shale and the anisotropic tensile strength when judging rock failure. The fracture initiation model established in this paper accurately reflects the stress anisotropy and matches well with the actual situation in porous media. Through the sensitivity analysis, the results show that σv/σH, Ev/EH, υv/υH, m/s, and A/B have a certain impact on the tangential stress when the circumferential angle changes, and there is a positive relationship between the initiation pressure and the above sensitive factors except for A/B. The results can provide a valuable and effective guidance for the prediction of fracture initiation pressure and fracture propagation mechanism under special stratum conditions of shale.

Analysis of hydraulic fracture initiation and propagation in deep shale formation with high horizontal stress difference

Deeply buried shale formations (vertical depth > 3500 m) that are rich in shale gas are abundant in south China. The primary problems in the exploitation of these formations are the relatively small stimulated reservoir volume (SRV) and low production rates in comparison with their shallower counterparts. These issues are attributed to the high fracturing pressure, limited fracture extension because of sand plugging, significant horizontal stress contrast, and discontinuities produced during hydraulic fracturing. To accurately evaluate and improve the SRV in deep shale formations, the mechanism of fracture propagation must be understood and described. In this regard, a series of large-scale true tri-axial experiments with acoustic emission (AE) monitoring were conducted to characterize the fracture initiation and propagation in a selected deep shale formation. It was found that the difficulty in the complex fracture network formation was because the high stress contrast controls the fracture propagation path to generate large main fractures instead of activating discontinuities. The hydraulic fractures initiated either from open-hole positions or stress concentration locations on a wellbore wall that displayed two types of intersection in terms of crossing and deflection. In general, four types of fracture morphologies were identified: transverse fracture, transverse fracture with bedding planes, natural fracture with bedding planes, and transverse fracture with bedding planes and natural fractures. In addition, the horizontal stress contrast, fluid viscosity, pump rate and fracturing procedure on fracture propagation were evaluated for their effect on the resulting SRV. A low-viscosity fluid can activate discontinuities to form a complex fracture network, whereas a high-viscosity fluid is likely to produce large fractures under a high contrast in the horizontal stresses. On the basis of the above-mentioned analyses, a fracturing procedure applying a periodically varying pump rate and shut-in using a low-viscosity fluid was suggested to be an effective approach to enhance the interaction between the hydraulic fractures and discontinuities as well as to increase the fracture length.

Characteristics of hydraulic fracture surface based on 3D scanning technology.

The surface characteristics of fractured specimens are important in hydraulic fracturing laboratory experiments. In this paper, we present a three-dimensional (3D) scanning device assembled to study these surface characteristics. Cube-shaped rock specimens were produced in the laboratory and subjected to triaxial loading until the specimen split in two in a hydraulic fracturing experiment. Each fractured specimen was placed on a rotating platform and scanned to produce 3D superficial coordinates of the surface of the fractured specimen. The scanned data were processed to produce high-precision digital images of the fractured model, a surface contour map and accurate values of the superficial area and specimen volume. The images produced by processing the 3D scanner data provided detailed information on the morphology of the fractured surface and mechanism of fracture propagation. High-precision 3D mapping of the fractured surfaces is essential for quantitative analysis of fractured specimens. The 3D scanning technology presented here is an important tool for the study of fracture characteristics in hydraulic fracturing experiments.

Hydraulic fracturing initiation and near-wellbore nonplanar propagation from horizontal perforated boreholes in tight formation

Tight formations, which are rich in reserves in China, have a high economic development value. Perforated fracturing would need to be used widely to exploit these formations because of reservoir characteristics such as low permeability, low porosity and high density, resulting in high fracture pressure, limited fracture propagation, and complex fracture geometry. To address these issues, the mechanism of fracture propagation under different construction conditions must be understood and described. Therefore, the effect of different perforation parameters (shot density, shot length, shot diameter, shot phase, and shot interval spacing) and different horizontal stress contrasts on fracture geometry, initiation, and propagation from perforated horizontal wells in tight formations is studied based on numerous triaxial hydraulic fracturing simulation experiments. Three types of fracture geometry are observed in these experiments: A single flat fracture can be created by an initiated perforation or more initiated perforations with good connectivity, spiral-shaped fractures are generated by a large perforation diameter or high perforation density, and multiple-parallel fractures are induced by a high perforation phase. Moreover, each fracture type has certain pressure behaviors corresponding to a fracture curve shape. The perforations are likely to initiate when the angle between the perforation and maximum horizontal stress is small, and the initiation position is most likely at the base of the perforation. It is difficult to predict the initiation order of perforations; however, the first initiated perforation can be ensured by the profile of the relief pressure. In addition, a higher horizontal stress contrast value results in more initiated perforations, lower fracture pressure, and shorter breakage time. Ultimately, a large perforation diameter, high perforation density, and perforation phase of 60° should be chosen for fracturing at low fracture pressure and simple fracture geometry in tight formations.

Investigation on the influence of loading-rate on fracture toughness of AHSS grades

The automotive industry is striving for light body-in-white structures while maintaining or improving passenger safety. The aim of this paper is to investigate the influence of the loading rate on the fracture toughness of thin steel sheet metal of three advanced high strength steels. Although steel is a heavy material it plays a significant role for lightweight solutions in car bodies. Three different advanced high strength steel (AHSS) grades, namely dual-phase (DP), quench-partitioning (Q&P) and TRIP-assisted bainitic-ferritic (TBF), are investigated in the present paper. For crash relevant components it is of importance to know the material response under high loading velocities i.e. high strain rates. A standard tensile test system is used for low loading rates, a high-speed tensile testing setup is used to obtain high loading rates. The fracture toughness of the three AHSS grades is evaluated using the methodology of the Essential Work of Fracture (EWF). The tensile specimen used in the present work is the double edge notched tensile (DENT) geometry with a pre-developed crack. High-speed imaging is applied to verify the validity of the evaluation method Essential Work of Fracture at high rates of loading. Results from this work show that knowledge of fracture toughness would improve the understanding of fracture and crack propagation mechanisms for third generation high strength steels used for automotive components.

Experimental study of supercritical CO2 fracturing on initiation pressure and fracture propagation in shale under different triaxial stress conditions

Supercritical carbon dioxide (SC-CO2) fracturing technique used in the development of shale gas has attracted increasing attention in the past decades. Using a self-developed physical simulation system equipped with acoustic emission (AE) and computed tomography (CT) system, the fracture initiation pressure (FIP) and fracture propagation mechanism of shale in the process of SC-CO2 fracturing was invested. The results show that the FIP for the shale without pre-existing fractures obtained from experiment is consistent with the theoretical values under different triaxial stress conditions, indicating that the theoretical calculation for FIP is feasible. The FIP increased gradually with increasing in situ stress for the shale without pre-existing fractures. The pre-existing fractures will affect the FIP and the fracture propagation direction in the shale. AE signals are obviously observed in the fracture initiation and the fracture propagation stages. In addition, the hit, energy, and amplitude are increased stepwise with increasing fracturing time. In the triaxial stress state, the initiation position and the propagation direction of cracks are random in the process of the SC-CO2 fracturing of shale and are along the direction of weak structural planes or pre-existing fractures. The SC-CO2 fracturing technique integrated with AE monitoring and CT scanning can be used to analyse the crack initiation position, the direction and sequence of crack propagation, and the number of the fracturing-generated cracks in shale. This research lays a foundation for SC-CO2 fracturing of shale in the Longmaxi formation in the Sichuan Basin.

Experimental analysis of multiple factors on hydraulic fracturing in coalbed methane reservoirs.

Hydraulic fracturing can improve the permeability of coalbed methane (CBM) reservoirs effectively, which is of great significance to the commercial production of CBM. However, the efficiency of hydraulic fracturing is affected by multiple factors. The mechanism of fracture initiation, morphology and propagation in CBM reservoirs is not clear and need to be further explored. Hydraulic fracturing experiment is an accurate tool to explore these mechanisms. The quantity of experimental coal rock is large and processing method is complex, so specimen made of similar materials was applied to replace coal rock. The true triaxial hydraulic fracturing experimental apparatus, 3D scanning device for coal rock section were applied to carry out hydraulic fracturing experiment. The results show that the initiation pressure is inversely proportional to the horizontal stress difference (Δσ) and positively related to fracturing fluid injection rate. When vertical stress (σv) is constant, the initiation pressure and fracture width decrease with the increasing of Δσ. Natural fractures can be connected by main fracture when propagates perpendicular to the direction of minimum horizontal stress (σh), then secondary fractures and fracture network form in CBM reservoirs. When two stresses of crustal stress are close and far different from the third one, the fracture morphology and propagation become complex. Influenced by perforations and filtration of fracturing fluid in specimen, fracturing fluid flows to downward easily after comparing horizontal well fracturing with vertical well fracturing. Fracture width increases with the decreasing of elastic modulus, the intensity of fracture is positively related with the elastic modulus of coal rock. The research results can provide theoretical basis and technical support for the efficient development of CBM.

Influence of gravel on the propagation pattern of hydraulic fracture in the glutenite reservoir

The clear mechanism of hydraulic fracture propagation in glutenite reservoirs with high heterogeneity is still not obtained, thus it is difficult to carry out the design of fracturing plan effectively. Based on the characteristics of the glutenite reservoirs, a coupled flow-stress-damage (FSD) model of hydraulic fracture propagation with gravels is established. This model is experimentally verified and the research on the influence of rock physical parameters and gravel property on the hydraulic fracture propagation is conducted. It is shown that as the gravel tensile strength increases, the hydraulic fracture is prone to propagate around the gravel, where the fracture deflection always occurs; as the gravel Young's modulus increases, there is high probability that hydraulic fracture propagates around the gravel, with more obvious fracture deflection; the matrix permeability influences fracture propagating morphology when encountering gravel and total fracture length; the horizontal geostress difference seriously impacts the fracture deflection; as the fracturing fluids injection displacement increases, the fracture is prone to deflect when encountering gravel; the low viscosity fracturing fluids result in the shorter fracture; the larger gravel increases the possibility of fracture deflection; in case of smaller gravel sizes, the increasing gravel content has a big influence on fracture deflection, and the increasing content of large gravel complicates the fracture morphology, resulting in the fine branched fractures; for the well rounded gravel, the fracture propagation around the gravel is prone to occur, and the fracture is not prone to deflect. Compared with the conventional sandstone reservoir, the glutenite reservoirs have higher breakdown and extension pressures, which fluctuate due to the gravel; the larger gravel size results in higher extension pressure. In this paper, a simulation method of hydraulic fracture propagation in the glutenite reservoirs is introduced, and the result provides the theoretical support for prediction of fracture propagation morphology and plan design of hydraulic fracturing in the glutenite reservoirs.

Investigation of fracture propagation characteristics caused by hydraulic fracturing in naturally fractured continental shale

Continental shale from the Ordos Basin, China, is naturally fractured and characterized by silty laminae and interbeds that is different from marine shale. To investigate the hydraulic fracture propagation in this kind of shale, hydraulic fracturing experiments were performed in cylinder continental shale samples under different stress ratio conditions, and propagation characteristics of the visible surface fractures, the main fractures as well as the main fracture surfaces were observed and discussed. The results reveal that the visible induced fractures on the sample surfaces primarily extend along natural fractures, bedding planes and interbeds, and situation of crossing beddings and interbeds can be found, which is different from fractures in marine shale that are mainly vertical to bedding planes with some branches. Besides, the main fractures were observed and measured through electronic microscope. They are wavelike and tortuosity tends to increase as stress ratio decreases. Moreover, three dimensional scanner was employed to study the main fracture surfaces, and Area Ratio and Standard Deviation were used to evaluate the roughness. With the decrease of stress ratio, partial vertical fractures can be found on the main fracture surface, and Area Ratio and Standard Deviation get higher, which indicates that the roughness of the main fracture surfaces becomes higher. Therefore, hydraulic fracture propagation characteristics of continental shale are strongly influenced by the stress and some its own properties, including natural fractures, beddings and interbeds. And the result contributes to understanding the mechanism of hydraulic fracture propagation in continental shale formation.

Hydro-mechanical coupled mechanisms of hydraulic fracture propagation in rocks with cemented natural fractures

Natural fractures commonly exist in unconventional reservoirs such as shale and tight gas sandstone, which are mostly cemented (or sealed) with minerals and not able to contribute to reservoir storage or rock permeability. However, during hydraulic fracture stimulation, these cemented natural fractures will be encountered and influence hydraulic fracture geometry greatly and, thereby, gas production. In this work, hydraulic fracture propagation in rock with cemented natural fracture is investigated using our recently developed and validated hydro-mechanical coupled LBM-DEM model. The numerical results show that both the strength ratio (between cemented natural fracture and host rock) and the approach angle (between hydraulic and cemented natural fracture) significantly affect the hydraulic fracture propagation. A larger strength contrast or a smaller approach angle will be more conducive to deflection, which is consistent with experimental observation. For rocks with weakly cemented natural fractures, deflection is mainly caused by shear failure in weakly cemented fracture. However, for rocks with strongly cemented natural fractures, deflection happens accompanying with tensile failure in host rock along the cement wall, which cannot be captured by the previous numerical models where the cemented natural fracture is treated as a bonded interface. In addition, complex fracture network is more easily formed if multiple weakly cemented natural fractures are orthogonal to the hydraulic fracture propagation direction.

Numerical simulation of horizontal well network fracturing in glutenite reservoir

Hydraulic fracturing is an economically effective technology developing the glutenite reservoirs, which have far stronger heterogeneity than the conventional sandstone reservoir. According to the field production experience of Shengli Oilfield, horizontal-well fracturing is more likely to develop a complex fractured network, which improves the stimulated volume of reservoir effectively. But the clear mechanism of horizontal-well hydraulic fracture propagation in the glutenite reservoirs is still not obtained, thus it is difficult to effectively carry out the design of fracturing plan. Based on the characteristics of the glutenite reservoirs, a coupled Flow-Stress-Damage (FSD) model of hydraulic fracture propagation is established. The numerical simulation of fracturing expansion in the horizontal well of the glutenite reservoir is conducted. It is shown that a square mesh-like fracture network is developed near the horizontal well in the reservoir with lower stress difference, in which fracture is more prone to propagate along the direction of the minimum principal stress as well. High fracturing fluids injection displacement and high fracturing fluid viscosity lead to the rise of static pressure of the fracture, which results in the rise of fracture complexity, and greater probability to deflect when encountering gravels. As the perforation density increases, the micro-fractures generated at each perforation gather together faster, and the range of the stimulated reservoir is also relatively large. For reservoirs with high gravel content, the complexity of fracture network and the effect of fracture communication are obviously increased, and the range of fracture deflection is relatively large. In the case of the same gravel distribution, the higher the tensile strength of the gravel, the greater fracture tortuosity and diversion was observed. In this paper, a simulation method of horizontal well fracture network propagation in the reservoirs is introduced, and the result provides the theoretical support for fracture network morphology prediction and plan design of hydraulic fracturing in the glutenite reservoir.

Ductile fracture of advanced pipeline steels: study of stress states and energies in dynamic impact specimens - CVN and DWTT

The development of robust protocols for assessing the structural integrity of gas pipelines is of paramount relevance, since failures can lead to financial and human losses. In this scenario, the material’s ability to slow down the propagation of a running crack (crack arrest) becomes a design requirement. Several empirical models and criteria, calibrated by real pipeline burst tests, have been developed, being the Battele Two Curve Method (BTCM) one example of technique widely employed during decades. With the evolution of steels, there was a significant increase of ductility and toughness, in a way that such semi-empirical models usually based on the energy absorbed in the Charpy impact test (ISO 148-1, ASTM E-23) began to present unsatisfactory predictions. This may be explained by the fact that in current high-ductility and high-toughness materials (e.g.: API-5L X65, X80, X100), the dominant mechanism of fracture propagation is plastic collapse. Consequently, the energies involved in deforming and fracturing a laboratory specimen are remarkably altered and transferability to pipelines by means of the aforementioned models can be lost. Therefore, for a better phenomenological comprehension of the ductile fracture process under such circumstances, this work investigates Charpy and DWTT (ASTM E-436) dynamic tests assessing stress fields and respective energies involved in deformation and fracture. It is of great interest to evaluate the energy associated to steady state ductile fracture and thus try to characterize the energy available to slow down an ongoing fracture. Pipelines are references for the developments and support assumptions and some conclusions. Based on these golas, numerical analyses including damage models (XFEM and GTN) were implemented, including parameters’ calibration and sensitivity analyses. The methodology closely reproduced available experimental results. Besides that, stress fields and energies could be quantified for the studied geometries and such analyses indicated the potential and limitations of Charpy and DWTT specimens to characterize the energies required to describe steady state ductile crack propagation and crack arrestability. Results support further developments related to pipeline integrity assessments.

Experimental Analysis of the Ratio of Similar Materials by Similarity Model Test on Raw Coal

Similarity model test is an effective approach to study the mechanism of hydraulic fracture propagation in coalbed methane reservoirs as well as theoretical analysis and numerical simulation. The efficiency of the similarity model test result is closely related to the selection and ratio of similar materials. Similar material ratio test was conducted to simulate the mechanical parameters of raw coal using orthogonal method and an appropriate similarity model for hydraulic fracturing experiment was developed in this study. Results show that it is suitable to select cement, gypsum as binder and apply pulverized coal as aggregate through the analysis of experimental data. The mechanical parameters of similar materials, including uniaxial compressive strength, elastic modulus, Poisson ratio and firmness coefficient are tested using laboratory tests. The impact of diverse ratios of similar materials on the mechanical parameters is analysed. A proper ratio is selected to make the mechanical parameters of raw coal close to the ones of similar material, in order to meet the demand of the similarity model test based on raw coal. The results can provide theoretical basis and technical support for the selection of similar materials to carry out hydraulic fracturing experiments.

深层油气藏开发中的关键力学问题

The distribution pattern of oil and gas resources in China shows that as the exploration and development degree of oil and gas reservoirs in shallow and middle layers is relatively high, deep and ultra-deep oil and gas resources , whose identified rate are still very low, are of great significance to reserve growth and production stabilization of China. The main characteristics of deep oil and gas reservoir are high temperature, high pressure and high stress (“three high”), which are also the great problems in the process of exploration and development. At present, the research on deep oil and gas reservoirs is limited to hydrocarbon accumulation dynamics. It is urgent to carry out research on key mechanical problems in the development of deep oil and gas reservoirs. In this paper, deep oil and gas resources and development situation are outlined briefly. Based on the characteristics of high temperature, high pressure and high stress in deep formation, mechanisms of multi-physics field coupled flow dynamics in the exploitation of deep oil and gas are reviewed systematically, and it is concluded that rock mechanics at great depth, elasto-plastic artificial fracture propagation mechanism and fully-coupled thermal-hydro-mechanical processes in porous media are key frontier mechanical problems to be solved in the development of deep oil and gas reservoirs. Research approaches are proposed, which promotes the development of energy, especially has significant impact on the effective exploitation of deep oil and gas reservoirs in China.

XFEM modeling of multistage hydraulic fracturing in anisotropic shale formations

The aim of the present study is to help understand hydraulic fracture propagation in shale formations through numerical simulations. The hydraulic fracture propagation regime in shale is analyzed considering the anisotropic nature of the shale rock formation and slick-water fracturing fluid. It is determined that the dominant mechanism of hydraulic fracture propagation is the so-called transitional regime that is characterized by a negligibly small fluid lag region and zero fluid front pressure. For the modeling of the hydraulic fracture evolution over time, we assume orthotropic linear-elastic rock media and that the flow of the fracturing fluid is governed by the Reynold's lubrication equation. For the discretization of the coupled solid-fluid equations within the 2D plane-strain context we use the extended finite element method for the rock media and the finite volume method for the lubrication equation. The problem of the hydraulic fracture evolution over time is modeled as stable quasi-static crack growth where time is the result of upholding the mass conservation principle between the fluid inflow and the crack volume. The Picard iterative approach is used to solve the discrete non-linearly coupled solid-fluid equations. Our model is verified against several analytical solutions. Subsequently, a five-stage hydraulic fracturing problem is simulated to study the interactions between the different fractures. Results show that the on-going hydraulic fractures are attracted by the pre-existing hydraulic fractures as a result of the change of the local stress field relative to the initial in-situ stress field. For the cases considered, fracture deflections are found to be most extensive with decreasing fracture spacing and in-situ stress difference, but insensitive with the increasing the ratio of Young's moduli.

Overview and prospect of fracture propagation and conductivity characteristics in deep shale gas wells

There are tremendous shale gas resources in China, however, many of them can’t achieve economical production due to a lower effective stimulated reservoir volume (ESRV). Up to now, very few researches have been conducting on the mechanisms of fracture propagation and conductivity, which are the two main controlling factors of the effective development of shale gas play. As we all know that with the increasing depth of shale gas wells, many parameters such as in-situ stress, stress difference between minimal horizontal stress and maximal one, rock mechanics, etc. are all changed a simultaneously, which results in different fracture propagation and conductivity mechanisms. Therefore, firstly, a comprehensive analysis on fracture propagation and its conductivity characteristics in shallow wells were conducted. The mechanism of single fracture propagation and multiple fractures’ propagation simultaneously in open hole wells were summarized in detail. Besides, the effect of casing completion and fracturing treatment parameters on single fracture initiation and propagation was analyzed, and the corresponding results for multiple fractures may take references to that in open hole wells. Based on above studies, the mechanism of conductivity of single fracture and complex one in the condition of multiple affecting factors were analyzed too. After that, the proppant transport pattern within the complex fracture and its transportation law in divergent fracture were simulated, which may be used to optimize treatment parameters of hydraulically fracturing and achieve the object of expected conductivity and maximal ESRV. Finally, according to the formation geologic characteristics of deep shale gas play, the future research tendency was indicated, such as the mechanism of fracture propagation in the mode of well pad with multiple horizontal wells and multiple clusters, and in the condition of mixed natural fractures pattern with both horizontal interface and high angle fractures, the mechanism of high conductivity of network fracture formed by rock self-supported, which has great significance for developing deep shale gas play economically and effectively.

Investigation of the Effect of Cemented Fractures on Fracturing Network Propagation in Model Block with Discrete Orthogonal Fractures

Researchers have recently realized that the natural fractures in shale reservoirs are often cemented or sealed with various minerals. However, the influence of cement characteristics of natural fracture on fracturing network propagation is still not well understood. In this work, laboratory-scaled experiments are proposed to prepare model blocks with discrete orthogonal fractures network with different strength of natural fracture, in order to reveal the influence of cemented natural fractures on the interactions between hydraulic fractures and natural fractures. A series of true triaxial hydraulic fracturing experiments were conducted to investigate the mechanism of hydraulic fracture initiation and propagation in model blocks with natural fractures of different cement strength. The results present different responses of interactions between hydraulic and natural fractures, which can be reflected on the pump pressure profiles and block failure morphology. For model blocks with fluctuated pump pressure curves, the communication degree of hydraulic and natural fractures is good, which is confirmed by a proposed new index of “P-SRV.” The most significant finding is that too high and too low strength properties of cemented natural fracture are adverse to generate complex fracturing network. This work can help us better understand how cemented natural fractures affect the fracturing network propagation subsurface and give us reference to develop more accurate hydraulic fracturing models.

Role of Brittle Behaviour of Soft Calcarenites Under Low Confinement: Laboratory Observations and Numerical Investigation

The strength decay that occurs in the post-peak stage, under low confinement stress, represents a key factor of the stress–strain behaviour of rocks. However, for soft rocks this issue is generally underestimated or even neglected in the solution of boundary value problems, as for example those concerning the stability of underground cavities or rocky cliffs. In these cases, the constitutive models frequently used in limit equilibrium analyses or more sophisticated numerical calculations are, respectively, rigid-plastic or elastic–perfectly plastic. In particular, most of commercial continuum-based numerical codes propose a variety of constitutive models, including elasticity, elasto-plasticity, strain-softening and elasto-viscoplasticity, which are not exhaustive in simulating the progressive failure mechanisms affecting brittle rock materials, these being characterized by material detachment and crack opening and propagation. As a consequence, a numerical coupling with mechanical joint propagation is needed to cope with fracture mechanics. Therefore, continuum-based applications that treat the simulation of the failure processes of intact rock masses at low stress levels may need the adoption of numerical techniques capable of implementing fracture mechanics and rock brittleness concepts, as it is shown in this paper. This work is aimed at highlighting, for some applications of rock mechanics, the essential role of post-peak brittleness of soft rocks by means of the application of a hybrid finite–discrete element method. This method allows for a proper simulation of the brittle rock behaviour and the related mechanism of fracture propagation. In particular, the paper presents two ideal problems, represented by a shallow underground cave and a vertical cliff, for which the evolution of the stability conditions is investigated by comparing the solutions obtained implementing different brittle material responses with those resulting from the assumption of perfectly plastic behaviour. To this purpose, a series of petrophysical and mechanical tests were conducted on samples of soft calcarenite belonging to the Calcarenite di Gravina Fm. (Apulia, Southern Italy), focusing specific attention on the post-peak behaviour of the material under three types of loading (compression, indirect tension and shear). Typical geometrical features representative of real rock engineering problems observed in Southern Italy were assumed in the problems examined. The numerical results indicate the impact of soft rock brittleness in the assessment of stability and highlight the need for the adoption of innovative numerical techniques to analyse these types of problems properly.

Numerical modeling of shallow magma intrusions with finite element method

A numerical approach for simulation of magma intrusion process, considering the couplings of the stress distribution, the viscous fluid flow of magma, and the fracturing of host rock, has been developed to investigate the mechanisms of fracture initiation and propagation in host rock during magma intrusion without pre-placing a set of fractures. The study focused on the dike intrusions filled with injected viscous magma in shallow sediments. A series of numerical modellings were carried out to simulate the process of magma intrusion in host rocks, with particular attention on the magma propagation processes and the formation of intrusion shapes. The model materials were Mohr-Coulomb materials with tension failure and shear failure. The scenarios of both stochastically heterogeneous host rocks and layered host rocks were analyzed. The injected magma formed intrusions shapes of (a) dyke, (b) sill, (c) cup-shaped intrusion, (d) saucer-shaped intrusion. The numerical results were in agreement with the experimental and field observed results, which confirmed the adequacy and the power of the numerical approach.

Trimming and Sheared Edge Stretchability of Light Weight Sheet Metal Blanks

Abstract In recent years, implementation of aluminum alloys, advanced high strength steels (AHSS) and ultra high strength steels (UHSS) is quickly increasing in automotive components due to their superior structural performance and vehicle weight reduction capabilities. However, these materials are often sensitive to sheared edge cracking if stretching along the sheared edge occurs in such processes as drawing of panels with blanked windows, stretch flanging and stretch hemming of edges of the panel. This study is dedicated to development of experimental techniques necessary to account for all these factors as well as reporting the experimental results and general trends. Analysis of hole punching process indicated that uniformity of the cutting clearance is rather difficult to keep consistent, especially for UHSS material where cutting forces are substantially higher than for mild steels or aluminum alloys, and stiffness of the tool starts playing critical role. Therefore, majority of experimental studies were performed as tensile tests of samples sheared along straight line in a dedicated trim tool where special measures were taken to achieve consistency of the die clearance. Experimental results on sheared edge stretchability of Aluminum alloy 6111-T4 and UHSS steel DP980 are reported. Mechanism of fracture propagation in trimming and hole punching processes is discussed in conjunction with sheared edge stretchability. Propagation of fracture from the upper cutting edge to the lower cutting edge of the trim die is the mechanism which enables higher stretchability of the sheared edge. Mechanism of fracture leading to crack propagation to the free surface of the blank usually results in formation of burrs and lower sheared edge stretchability. Rather unique mechanism of fracture observed for trimming of UHSS DP980 steel leads to burr breaking at the final stage of the shearing process.