Experimental uniaxial compression loading tests and scanning electron microscope (SEM) tests are carried out on rock-like specimens containing single pre-existing cracks to study the mechanical properties and microscopic damage evolution. The present study has distinguished tensile or shear cracks based on different SEM observations on a micro scale. Specifically, six typical micro patterns are defined according to their geometry shapes, namely, flocculent, flaw, circle, flow, layered, and broken circle pattern. These micro patterns display distinct characteristics on structure surfaces, boundary lines, and the distribution of grain debris. Moreover, the microscopic damage of both tensile and shear cracks is quantitatively studied using the image post-processing technique. The damage evolution, which associates the macroscopic cracking processes, has been investigated. It is indicated that the microcracks develop from the pre-existing cracks prior to the initiation of any macroscopic observable cracks, and the damage is not rapidly accumulated after the initiation of both tensile and shear cracks.
Natural rock contains discontinuities, including fractures, pores, and other defects, which govern the fracturing behaviors of the rock masses under loading. Numerous theoretical, experimental, and numerical studies have been carried out to study mechanical properties of jointed rocks or other rock-like materials (Griffith, 1921; Brace and Bombolakis, 1963; Horii and Nematnasser, 1985; Bobet and Einstein, 1998; Wong and Einstein, 2009a; 2009b; 2009c; Park and Bobet, 2010; Zhang and Wong, 2012; 2013; Gonçalves da Silva and Einstein, 2013; Haeri et al., 2014; Yang et al., 2017; Zhao et al., 2018). In these researches, tensile and shear cracks are always be regarded as two basic crack types and fundamental of the rock mechanic. (Cheng and Wong, 2018). Bombolakis (1963) firstly observed the propagation of tensile wing cracks from straight cracks under uniaxial compression, which consists well with the Griffith theory. Lajtai (1974) carried out uniaxial compression loading tests on plaster of Paris, the results consist of five crack types, including both tensile and shear cracks. Petit and Barquins (1988) observed that shear zones develop extensively in addition to the occurrence of tensile wing cracks. Using scanning electron microscope (SEM), Sagong and Bobet (2003) investigated tensile and shear cracks in gypsum specimens on a micro scale. Li et al. (2005) conducted experimental tests on marble specimens, and they discovered two cracking phenomena: wing cracks and secondary quasi-coplanar cracks. Although the mechanical properties of these two cracks were not clearly identified by the authors, it is accepted that wing cracks are tensile cracks and secondary cracks are shear cracks. Wong and Einstein (2009a) systematically characterized the tensile/shear cracks which emanate from a single pre-existing crack. Seven different crack types (including three tensile types, three shear types, and one mixed type) were identified based on geometry and propagation mechanism. Subsequently, they studied the orientation of microcracking zones of the wing cracks (Wong and Einstein, 2009c). As a summary, the previous studies focused on the differences between tensile cracks and shear cracks in three main aspects (Cheng and Wong, 2018): First, the tensile/compressive stress concentration phenomenon around the pre-crack tips; Second, the initiation direction and propagation trajectories of observable cracks; Third, the microscopic observation of the crack surfaces.
It is well known that rock failure modes are complex and difficult to quantify or predict. No straightforward mathematical or numerical analysis model can ascertain the nature of fracture development in rocks. Consequently, investigations of failure modes in the laboratory appear to be the only viable option that could provide useful information concerning rock failure, which is of great concern in any rock engineering environment. In line with the author’s involvement in several experimental studies in relation to this issue, this paper presents salient points relating to fracture patterns in granite, schist, and sandstone under uniaxial compression and indirect tensile tests, with reference to their respective strengths. Additionally, the influence of fine drill-holes (a circular channel across the diameter in the middle of the core in the case of uniaxial specimens, and a circular hole aligned along the core axis in case of indirect tensile test specimens) on the failure strength and failure mode of sandstone was explored.
Granite, schist, and sandstone from different parts of India (geologically belonging to Malanjkhand Granitoid, Singhbhum Shear Zone, and Barakar Formation, respectively) were investigated. It was observed that there is an apparent relationship between fracture patterns and corresponding strengths for all three rock types. This can be broadly explained in terms of damage evolution within the rocks under both compression and indirect tension. While exploring the influence of fine drill-holes on the compressive strength of the sandstone, it was found that such fine holes/channels do not play any defining role. This could be attributed to an overriding effect of inherent cracks/pores of a brittle porous rock like the sandstone investigated. Nevertheless, these fine drillholes/ channels seemed to have an influence on the failure patterns of the sandstone specimens. In the indirect tensile tests, it was found that the ring tensile test produces more consistent results than the Brazilian test. It was also observed that the ring tensile strength provides a better measure of the applied energy than the Brazilian tensile strength.
This paper is devoted to micro-scale fracture testing of Arctic steel, by use of focused ion beam machined notched cantilevers. Bainite packets of a weld-simulated course grained heat-affected zone (CGHAZ) was the targeted microstructural aspect, with reference tests performed in pure iron. Micro-scale fracture testing has been developing in the last decade. The main objectives of micro-scale fracture tests are to obtain relevant toughness values for materials used at this scale, and to evaluate the fracture toughness of local microstructural aspects. The latter is the focus of this paper. Several models, including multiple barrier models, require specific material property inputs that are not obtainable through traditional testing at larger scale. Hence, micro-mechanical fracture has been applied to quantify these properties. Linear-elastic and elastic-plastic fracture mechanics parameters are presented and compared, with respect to testing material and temperature. Additionally, a new analytical tool is utilized to determine the criticality of a growing crack in terms of determining the energy required for further crack growth following initiation of stable crack growth.
The industrial activity in the Arctic is rapidly increasing, where accidents may cause severe ecological ramifications. Rough climate conditions and temperatures as low as -60°C require materials with specialized mechanical properties. The materials must display sufficient fracture and wear resistance at low temperatures, while avoiding excessive maintenance and maintaining lifetime integrity. In order to overcome these challenges, small-scale fracture mechanisms and properties must be understood.
BCC structures typically exhibit a rapid transition from ductile to brittle fracture, due to reduced mobility of screw dislocations and a reduced number of available slip systems, as the temperature is lowered (Brinckmann et al., 2008, Schreijäg et al., 2015). Full understanding of this transition requires a defined transition criterion. The change in fracture mode from ductile to brittle occurs over a temperature range that is closely interconnected with the change in deformation energy. Inside this temperature range, the metal exhibits fracture characteristics from both modes. There will be some ductile fracture near the notch, which changes to cleavage as the crack propagates. This is due to increased hydrostatic stresses as the propagation speed increases (Petch, 1958), implying that fracture will switch from ductile to brittle when the stress ahead of the fracture tip becomes capable of Griffith propagation. Fracture mechanics have gained increased interest due to several incidents where structures fail within the designed region of operation, initiating extensive research on fracture mechanisms, fracture initiation, propagation and arrest, as well as the temperature dependence of these mechanisms. In an attempt to enhance the understanding of the fracture mechanisms small-scale testing has been used to localize testing and to reduce the number of variables tested in each experiment.
In order to consider the presence of cracks in an abandoned gypsum pillar in numerical simulations, a hybrid method FEM/DEM, which allows the transition from continuum to discontinuum, was assumed. By means of a specific numerical code (ELFEN), this approach has been calibrated involving both physical quantities introduced by fracture mechanics and numerical aspects in order to support this hybrid method. Furthermore, the approaches FEM and FEM/DEM have been compared, showing advantages and disadvantages through experimental tests carried out to characterize geomechanical response of the pillar. The interaction domain has been calculated thanks to the implementation of both methods. The meaning of determining this domain is related to the evaluation of failure limit when a coupled system of loads (normal and tangential force and momentum) is acting on pillars. An application to a case study of an abandoned gypsum mine interacting with building in San Lazzaro di Savena (Bologna) is shown.
2 Abandoned Gypsum Mine – specific pillar (P7) chosen
The choice of this pillar is related to its complex and redundant joints’ system, its reduced section and its location. In fact, it was possible to obtain its topography by TLS to furnish a detailed geometrical for the simulation. In the following figure the mine system and the selected pillar is shown.
3 Laboratory tests (UCS, BT, Triax)
A series of laboratory tests permitted to obtain failure parameters to be inserted both in constitutive model for the continuum approach (FEM) than for the crack propagation model for the discontinuum approach (FEM/DEM).
4 A numerical comparison between the two approaches
A coupled system of loads acting on pillar P7 has been simulated. In particular, 3D numerical simulations have been run to calculate an interaction domain represented by normal and tangential force and momentum. Also for the 2D FEM/DEM approach an interaction domain has been built thanks to a simplified solution by choosing two representative sections of pillar P7. In the following the numerical calibration of the continuum and hybrid approach is shown.
This paper briefly reviews the application of various techniques including empirical, analytical and numerical methods in analyzing the surface subsidence associated with sub-level caving. Recent developments in empirical and analytical methods are first introduced. The application of numerical modelling in the characterization of surface subsidence and caving mechanisms is demonstrated using a relatively simple conceptual sub-level cave model and three numerical approaches i) a continuum finite element (FEM) ii) a discontinuum distinct element (DEM) and iii) hybrid finite/discrete element (FDEM) with fracture; the advantages and limitations of each method are discussed. A new deformation monitoring data interpretation technique using the inverse velocity method to predict the time of failure is briefly introduced and its application to sub-level caving is presented using the FDEM numerical method. Recent developments in the field of the discrete fracture network (DFN) and synthetic rock mass, (SRM), approaches and their use in improving our understanding of caving mechanisms are presented. Finally, a new approach to modelling sub-level caving considering intact rock fracture using a DEM UDEC-Trigon method is introduced and preliminary results presented.
Cave mining methods generally include mining operations where after undercutting the orebody caves naturally based on gravity flow; these methods include block caving, panel caving and sub-level caving . Sub-level caving is a cost-effective mining method that enables a high degree of mechanization and automation. Mining using sub-level caving induces a large area of deformation on the hanging wall with a more limited area of damage on the footwall. Various parameters influence the observed hanging wall surface subsidence including depth of active mining, geometry and dip of the orebody, the mechanical properties of the intact rock and the characteristics of pre-existing discontinuities and geological structures. In general, mining using sub-level caving results in two types ground surface deformation zones: (i) the discontinuous zone and (ii) the continuous zone. These two surface deformation zones can be recognized as :
• Discontinuous deformation zone characterized by formation of visible cracks on the ground surface and large horizontal and vertical deformations. Surface disturbance such as tension cracks, topographic steps and chimney caves are normally observed in this zone. The disturbance is more extensive in the hanging wall, although the footwall is also affected by the mining activity.
• Continuous deformation zone characterized by uniform settlement and lowering of the ground surface which can only be detected by periodic monitoring with in general no visible surface cracking.
Recent growth of stage count of hydraulic fracturing has led to the increase in time and cost for a well completion. Simplifying time consuming process is a key for economic success. The use of degradable materials for components of downhole tools could provide several attractive advantages, such as eliminating or simplifying the recovery process of the tools. Polyglycolic acid (PGA), a hydrolyzable polymer, is a material suitable for such components and has already used in some applications including frac balls because of its high mechanical strength and appropriate degradation characteristics. In low-temperature wells close to the glass transition temperature of PGA however, there may be the potential risk, such as tool breakage during installation or stimulation due to the changes in the mechanical properties of PGA at low temperatures.
This paper will focus on the improvement of the impact strength of PGA. Through an extensive study of modifiers, an elastic additive with high compatibility to PGA has been identified. Morphological studies of mixtures showed finely dispersed domains of the additive within a PGA matrix. The impact strength of the mixture was 2 times higher than that of neat PGA. A formulation was found optimizing this impact strength versus other mechanical properties of the mixture, while leaving degradability, processability and machinability basically equivalent with neat PGA. The improvement of the impact strength would broaden the applicability of PGA tooling and help to reduce the cost and time in well completion process at low temperature wells.
Shinohara, Yasuhiro (Steel Research Laboratories, Nippon Steel Corporation) | Nagata, Yukinobu (Steel Research Laboratories, Nippon Steel Corporation) | Tsuru, Eiji (Steel Research Laboratories, Nippon Steel Corporation) | Hara, Takuya (Kimitsu Technological Research Department, Nippon Steel Corporation)
Liang, Z.Z. (School for Civil and Hydraulic Engineering) | Tang, C.A. (School for Civil and Hydraulic Engineering) | Li, L.C. (School for Civil and Hydraulic Engineering) | Zhang , Y.B. (School for Civil and Hydraulic Engineering)
Akselsen, Odd M. (Materials and Chemistry, SINTEF, Trondheim, Norway) | Fostervoll, Hans (Materials and Chemistry, SINTEF, Trondheim, Norway) | Hårsvær, Ansgar (Materials and Chemistry, SINTEF, Trondheim, Norway) | Aune, Ragnhild (Materials and Chemistry, SINTEF, Trondheim, Norway)
In the present investigation, 2 different wires for hyperbaric (underwater) GTA (gas tungsten arc) welding of X70 pipelines have been tested with respect to their weld metal mechanical properties. Welding of full coupons at different pressures (seawater depths of 16, 75 and 200 msw) was done with subsequent weld metal chemical analyses, hardness measurements, tensile testing and Charpy V notch testing as well as microstructure characterization. It is shown that both wires satisfied strength requirements set to X70 grade, representing a weld metal overmatch situation. Both wires gave sufficient impact toughness, but the toughness of the Ni-Mo containing weld was reduced with increasing seawater depth. This observation was strongly linked to the positioning of the Charpy V notch, and crack growth in a brittle, partially transformed region as a consequence of reheating by subsequent stringer beads. The embrittling microstructure consisted of high carbon MA (martensite-austenite constituents islands) decorating prior austenite grain boundaries. This microstructure was less pronounced when welding with the high Ni wire, which may explain why no similar toughness drop was found.
Up to now, subsea pipelines of grades X60 and X65 have Mainly been used in the Norwegian continental shelf installations. These are 10 to 42 in outer diam and their wall thickness ranges from 14 mm to 40 mm. Offshore tie-ins using qualified welding procedures have been made at 40 to 218 msw (meter sea water). X70 has been used only in one case, the Europipe (Aune et al., 2005). Forthcoming installations will include several X70 pipelines. One example is the Langeled pipeline, which will be the longest subsea pipeline in the world and will transport gas from Nyhamna on the West Coast of Norway via Sleipner in the North Sea to Easington in the U.K. Prior to subsea installations of pipelines, a test programme on welding consumables is required.