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By F Shi, W Valery

WMC Fertilizers (WMCF) operates a Beneficiation Plant at Phosphate Hill in Queensland, Australia. The plant consists of two-stages of crushing, followed by a wet scrubbing and desliming circuit. Ore is then fed to a tertiary crusher and a milling/classification circuit, with the final product going to a swing-lift thickener and storage tanks. From the slurry storage tanks, the phosphate rock is fed to a horizontal belt filters which dewater the phosphate rock ahead of a hemi-hydrate phosphoric acid plant. The water soluble recovery of phosphate in the phosphoric acid plant is dependent on the moisture content in the filter cake, as this governs the wash water addition to the hemi-hydrate filters. Since commencement of operation, the moisture content of the filter cake fed to the phosphoric acid plant has been higher than design reducing the phosphoric acid plant recovery and at times reducing the throughput through the phosphoric acid plant. The moisture content in the filter cake can be decreased by reducing the generation of fines in the milling and classification circuit (classified as û38 micron material). This paper identifies the causes of fines generation in the Phosphate Hill beneficiation plant. It discusses the options that were assessed to minimise fines generation, and outlines the results to date of circuit changes undertaken at Phosphate Hill.

Jan 1, 2003

By J. Eloranta

"Previous work indicated that the annual per-capita consumption of raw aggregate material averages about 10 tons; half of which is produced by blasting. Given a population of approximately 320 million in the United States alone, this amounts to 1.6 billion tons of blasted material annually. It was also estimated that up to 20% of the blasted material in aggregate operations is smaller than 6.35 mm (1/4inch) in size. Thus, about 320 million tons (i.e. 1 ton per capita) of byproduct is produced annually and sold at significantly lower prices or potentially discarded in waste dumps."

Jan 1, 2017

By Shibu John

Rising energy demand and steadily decreasing quality of raw materials due to the global resource depletion are great challenges to the steel production today. The FINEX® Process has been developed jointly by POSCO, Korea and PRIMETALS Technologies Austria, to provide the iron making sector with the capability to lower hot metal production costs, environmental pollutions and to increase the flexibility in terms of operation and the choice of raw materials. FINEX® is a new technology combining a gas-based reduction in a series of fluidized bed reactors and a reduction smelting in a melter gasifier. Commenced in April 2007, the first 1.5Mtpa commercial plant has demonstrated the competiveness as an alternative iron making route. Another 2.0 Mtpa, third generation FINEX® (3G) plant was recently installed at POSCO, Pohang Works and has been operating satisfactorily since its blow-in in January 2014. Based on the well-proven plant concept, new process features, the highly competitive production costs and environmental advantages, the FINEX® Process can be considered as a competitive alternative to the conventional blast furnace iron making route.

Aug 17, 2017

By E. Scheepers

Within a phosphorus producing submerged arc furnace it was found that the continuous fluctuation of the furnace between a flowrate-driven state (high throughput) and a thermodynamic-driven state (low throughput) caused both techniques to have similar overall, predictive abilities and resulted in a linear, ARX-type, adaptive prediction model as the model of choice. Secondly, this pre-diction model was developed, tested and then shown to have a reasonable, 8-hour-ahead predictive accuracy (R2, coefficient of determination) of 30% (±6%) on future Pslag values. This inherent relationship exists because, at the precise moment of a Pslag prediction, the furnace already contains the metallurgical memory (input variables to the linear model) needed to ensure that some predictive possibilities will always exists - all as a result of the long residence times in the furnace. Residence time, however, is not a directly adjustable variable but rather a fully dependent variable and a function of an array of interconnected and interactive variables. In fact, this applies to virtually all input variables, reemphasising the importance of innate metallurgical memory. Thirdly, predictive control possibilities were explored by simulating the set-points of two fully independent variables used by operators to control the process: the ratio of fixed carbon-to-P2O5 and the ratio of silica gravel-to-pellets. This linear, predictive control model showed only a slight improvement with an 8-hour-ahead predictive accuracy of 35% (±7%). This highlights how ineffective current adjustments are in optimally steering the process and how difficult even incremental improvements in feed-forward and predictive control can be. Finally, it is shown that fundamental design-, sampling- and process restrictions currently associated with the process will always limit the predictive and especially control accuracy or meaningfulness of any dynamic model. These restrictions include the size of the furnace resulting in long residence times, 8-hours sampling intervals, an extremely complex and interactive process and a 16% spatial analyses variation on the Pslag values ? the very value that the model is to predict. The point is made that, given the current status quo, even the perfect dynamic prediction model can not improve on an 8-hour-ahead prediction of 30% (±6%). This barrier can only be pierced with e.g. tidier and more frequent sampling regimes and other upfront capital investments, a decision that becomes a cost accounting exercise and that can only be taken by the management structure. An investment demanding ever-increasing attention is CFD software and it?s potential to shed more light on the complex interactions within the furnace.

Jan 1, 2014

By E. Scheepers

The process under investigation is the large-scale production of phosphorus. Gravity delivers the feed, consisting of pelletised apatite, coke and silica (in the form of gravel), to a submerged arc furnace through ten, evenly distributed feed chutes ensuring constant packed bed volume. Within this packed bed, solid-gas reactions are dominant in generating the gaseous product. These reactions, as well as the packed bed itself, were the main focus in the development of a fully three-dimensional CFD model in which fluid flow and energy distribution are represented as functions of chemical reactions, heating and melting of material and heat losses. The associated complex process interactions necessitated the creation of two process-specific, user-developed models that integrates accurate, thermodynamical data with computational fluid dynamics calculations, as well as reaction kinetics. The boundary conditions are provided by industrial measurements, and the key model parameters of reaction kinetics were obtained by laboratory experiments for different feed mixtures. Thirteen temperature probes installed inside the refractory lining of the furnace especially for this project allowed parameter fitting and validation to be guided within the constraints of real measurements. Simulation results reveal a thin, dome-shaped, reductive zone where conditions are optimum for maximum phosphorus recovery, as well as evidence of a thermal reserve zone similar to an ironmaking blast furnace located above this reduction zone. When compared to actual operating data, the CFD model can successfully predict the specific power consumption, the phosphorus in the slag and gaseous product flowrate under given operating conditions. This CFD model forms part of a larger project that aims to create a Dynamic-CFD hybrid control model.

Jan 1, 2014

By Ron Stewart, Richard Youmaran, Andy Adler

In the current context of heightened concerns with explosives security, there is significant interest in technological controls to improve security. It is important to be able to control what is fired, by whom, where and when. This paper describes research Orica has performed to investigate and test biometric systems to address the question of "by whom". The goal of this research is to incorporate the most suitable biometric system onto the 'blaster' unit of an electronic initiation system. This approach will ensure that only authorized personnel can initiate a blast involving electronic detonators. Requirements analysis: we initially explored many different biometric technologies to evaluate them against the requirements, including security, usability, ruggedness, size, form factor, privacy, and operational temperature range, This analysis identified chip based fingerprint sensors as the best candidate. Development of prototype units: in order to test the identified sensors, we modified standard, commercially-available, electronic blast initiation units ("blaster") to incorporate a fingerprint reader. Testing and evaluation: Biometric We conducted a biometric scenario evaluation in order to determine: 1) security level (measured by false accept rate (FAR)); 2) usability (measured by failure to enroll (FTE) and false reject rates (FRR)), and to 3) discover environment specific issues and challenges (such as temperature, humidity, dirt, or those related to the usage patterns of the user group). Tests were conducted at quarry sites in eastern Ontario, Canada. Results show rates of: FAR= 0%, FTE= 1.67%, FRR= 28.81%. Overall, these results suggest that this fingerprint biometric technology has a good level of usability in this application of electronic blast initiation control.

Jan 1, 2007

By C Bonini, A Markham, P Whittle, B Pearson

"Mine-derived sedimentation in rivers can have significant impacts on the processes and features of the receiving environment and impact on communities reliant on this environment. However, identifying and quantifying the contribution of mine-derived sediment to the receiving environment can be complex. This is particularly the case in remote environments and where little/no baseline assessments have been undertaken. One approach that can be used involves sediment fingerprinting, whereby a unique tracer is identified within the mine-derived sediment to discriminate between mine- and non-mine derived sediment sources downstream. The technique typically involves sampling of sediment at upstream, control and impact sites and sites within the mine deposit. Laboratory analyses are run on all the samples to determine whether the chemical signature (eg presence/concentration of specific metals) differs between each group of samples. In complex environments where the mine-derived signature may be difficult to detect from a basic review of the data, or where sampling constraints may limit the number of samples for analyses, statistical methods can be used to help identify sediment groups. This paper provides a mine case study to show how such an assessment can be undertaken despite the logistical challenges faced while working in remote environments.CITATION:Pearson, B, Markham, A, Bonini, C and Whittle, P, 2013. Fingerprinting and signatures - tracing mine impacts in complex aquatic systems, in Proceedings Water in Mining 2013, pp 287-292 (The Australasian Institute of Mining and Metallurgy: Melbourne)."

Nov 26, 2013

THE refining period of an open-hearth heat blends imperceptibly into the finishing period, during which final adjustments are made in slag composition, in bath action and temperature, and in composition of the steel, in order to prepare it for casting into ingots. Operations must be governed by the consideration that alterations in the character of the heat during the finishing period must be held to a minimum. In a broad sense, the finishing and deoxidation period of a heat may be said to be- gin when the action of the last ore addition has ceased, and to end when the teeming of the ingots is completed. INGOT STRUCTLIRES A survey of the relative advantages and disadvantages of different types of ingot structure is required because deoxidation treatments during the finishing period control the amount of gas evolved during solidification and consequently determine the type of ingot structure. Four classes of ingots are recognized, de- pending upon the degree of deoxidation to which the steel has been subjected. These types are killed, semikilled, capped, and rimmed; typical examples of their structures are represented by ingots 1, 2, 5, and 7, respectively, of Fig 9-1. The actual ingots from which these cross sections were sketched were all poured to the same height (indicated by the dotted lines) in the same type and size of bottle-top molds. The eight drawings of the figure represent selected types of ingot structure in a continuous series from "dead killed" to "wild," resulting from variations in the total quantity of gas evolved, from none at all to copious amounts, during solidification of the steel in the mold. Killed and semikilled steels are adjacent at the "quiescent" end of the ingot structure series where only small amounts of gas are evolved. The term "quiescent" describes the action of the molten metal during and subsequent to the teeming of killed and semikilled steels. Capped and rimmed steels are adjacent at the

Jan 1, 1964

By G. K. Burgress, H. S. Rawdon, R. W. Waltenberg, J. J. Crowe

The main objects of this investigation were to determine, from neasurements taken at representative rail mills…

Jan 1, 1915

Discussion Of the paper Of GEORGE K. BURGESS, J. J. CROWE, H. S. RAWDON, and R. G. WALTENBERG, presented at the Pittsburgh meeting, October, 1914, and printed in Bulletin NO. 93, September, 1914, pp. 2433 to 2437. ALBERT SAUVEUR, Cambridge, Mass.-The Bureau of Standards has once more rendered a public service. Manufacturers and consumers of steel rails alike should welcome the important results now laid before them by Dr. Burgess and his co-workers. Personally I welcome these results all the more that in the past, as some of the members may remember, I have never failed, when occasion arose, to call the attention of rail manufacturers to the importance of controlling the finishing temperature of their rails. The pioneer metallographic work conducted at the South Works of the Illinois Steel Co. in the early '90s clearly brought out the close relations existing between the manufacture, properties, and finishing temperature of steel rails and I think I may say without fear of being contradicted that the results of this early investigation had considerable influence in drawing the attention of manufacturers to the importance of finishing temperature and of heat treatment in general. The results now before us should be, it seems to me, of great value to rail manufacturers in their efforts to produce a better and, above all, a safer rail. It seems to be demonstrated in this paper that there is no difficulty in installing in rail mills pyrometric outfits, making it possible to record, without interfering in the mill operations, the temperature of the rails as they leave the finishing rolls, or of the ingots as they are withdrawn from the soaking pits, or at any intermediate point in the process of manufacture. The inference necessarily follows that manufacturers should see to it that such pyrometric installations are made, in order that they may actually know, instead of guessing at, the temperature of their rails;, and, for obvious reasons, rail consumers should insist upon such installations. In the rolling of heavy sections especially should care be taken to lower their finishing temperature, seeing that the present tendency is to finish them at higher temperature, whereas, owing to the slower cooling of their central portions, which makes for coarseness of grain, they should, if anything, be finished at a lower temperature than lighter rails.

Jan 4, 1915

By Joseph A. Sopko

The first documented procedures for designing frozen earth structures are over 50 years old, implemented long before the introduction of the Finite Element Method (FEM). Since that time, the development of three-dimensional models has made the traditional and often over-conservative methods relatively obsolete. This paper discusses a new approach that is largely dependent on the analysis of internal stresses with the frozen structure and comparison to frozen soil laboratory tests. A specific design process is discussed related to initial field investigation, laboratory testing, structural and thermal analysis, and performance monitoring. The design results are compared to case histories in the field that verify the procedure. BACKGROUND One of the first documented methods to designing frozen earth retaining structures was by Sangar (1968). Methodical refinements were published by (Harris 1995) and published by the International Symposium on Ground Freezing in 2002 (Andersland et al. 1991). The structural design or thickness of the frozen mass was based on the unconfined compressive strength of the frozen soils. Two simple equations for determining the thickness of a frozen shaft are shown in Fig. 1 and the first two equations below.

Jan 1, 2019

By K. F. Unrug

In the paper a numerical modeling method is used for evaluation of a roof support system, which is commonly used in the US coal industry. The effectiveness of mechanical roof bolting with or without, header boards and steel straps are compared. This analysis takes into account the response of a shaly roof to changing environmental conditions in mines related to the seasonal changes in temperature and humidity of the intake air. Discussed are principles of roof support in coal mines and based on them a critical evaluation of header boards interaction with bolts and roof itself is carried out. Header boards create a soft inclusion within a much more stiffer system comprising bolt-rock. As an alternative to header boards, steel straps are analysed and corresponding results are presented. It is demonstrated that application of steel strap is beneficial from the stand point of stress distribution and maintaining tension on roof bolts. The laboratory investigations and modeling work previously undertaken by the authors, clearly indicate the detrimental effect of header boards on effectiveness of roof bolting. There are significant practical implications of the research considering that header boards are commonly used in mines. This practice has been probably originated from the wooden support used by mining in "pre-bolting era?, and it has stayed with coal mining based on tradition more than principles of engineering design. It is demonstrated that header board$ are detrimental and actually contribute to development of roof problems.

Jan 1, 1989

By W. Jomaa, V. Songmene, O. Mechri, Laval Université, P. Bocher, J. Levesque

"A 2D finite element modelling (FEM), based on pure Lagrangian approach, is developed for studying the serrated chip formation during high speed machining (HSM) of 7075-T651 alloy. The solution was achieved using a commercial software package, ABAQUS/Explicit v6.13. The Johnson-Cooke (JC) constitutive equation is used to describe the work material behaviour. Moreover, the JC damage equation is implemented into the FEM to take into account for the shear localization during the serrated chip formation. Modified Coulomb friction model has been used to characterize the sliding/sticking effects on the tool-chip interface with the friction coefficient and shear flow stress determined by force calibration and machining data, respectively. The proposed FEM is validated using experimental data obtained in HSM of 7075-T651. The FEM was able to predict accurately serrated chip morphology. The intensity of chip segmentation was found to increase with cutting speed.INTRODUCTIONHigh speed machining (HSM) is one of the most used processes for manufacturing aircraft parts with aluminum alloys. For aluminum alloys, the use of HSM increases metal removal rate and reduces the formation of built up edges and burrs. However, it was found that serrated (also named segmented or sawtooth chip) and discontinuous chips can be produced during HSM of some high strength aluminum alloys, such as 7075-T6 alloy (Rao & Shin, 2001). Many parameters were found to affect the segmented/discontinuous chip formation during the machining of aluminum alloys.There are two cases in which the serrated chip can forms: the first is when machining hardened materials (Morehead, Huang, & Luo, 2007), and the second, when machining ductile materials with a cutting speed higher than a critical value (Barry & Byrne, 2002). Different theories and models were developed in order to explain the formation mechanism of such kind of chips. According to Astakhov (2006), the segmented chip formation is attributed to variations in stress and plastic deformation states, and resulting temperatures on the cutting zone. Liyao et al. (2013) stated that the segmented chip formation was mainly attributable to adiabatic shear instability/catastrophic strain localization. Others (Vyas & Shaw, 1999) suggest that the fracture/crack propagation mechanism is at the root of chip generation."

Jan 1, 2016

By B. MohammadSadeghi, S. Bakhshivash, M. Haghshenas, F. Rahimi

"Constrained groove pressing (CGP), as a severe plastic deformation (SPD) technique, is one of the applicable methods for sheet metal forming capable of producing ultra-fine grained materials (UFGMs). In CGP, processing and die design parameters can significantly affect the deformation response of sheet metal specimen. Having said this, in the present research the effects of die design variables and processing parameters (i.e. shear angle and friction) on the deformation behaviour of an Al-Mg-Si Alloy are studied thoroughly. Finite element analysis (FEA), was used to evaluate the effectiveness of mentioned parameters on the deformation homogeneity, strain distribution and pressing load variations. Also, mechanical properties of the deformed sample in term of micro-hardness was analyzed. It is observed that with increasing the shear angle, effective stress, strain and pressing load are enhanced. Moreover, by increasing the coefficient of friction, pressing load enhances. The color coded micro-hardness measurement showed a good agreement with the strain distribution obtained from FEA (higher hardness for the zones associated with higher shear plastic strain values).INTRODUCTIONWork hardening results in enhanced mechanical properties upon cycles of deformation (Hosford & Caddell, 2007). However, the formability of the processed products are affected by work hardening reducing the possibility of processing in subsequent deformation cycles. Therefore, many industrial processes cannot be performed on sheet metals to produce products with desired mechanical and physical properties (Valiev, Islamgaliev, & Alexandrov, 2000). Severe plastic deformation (SPD) is an effective grain refining technique which can impose large plastic strains through cold working on a metallic specimen (Bisadi, Rastgou, Payganeh, Zeynali, & Ghahremani, 2010). SPD techniques are mainly categorized into three types; sheet, bulk and tube forming. Among SPD sheet forming methods, Constrained Groove Pressing (CGP) (Shin, Park, Kim, & Park, 2002), Accumulative Roll Bonding (ARB) (Saito, Tsuji, Utsunomiya, Sakai, & Hong, 1998) and Repetitive Corrugation and Straightening (RCS) (Zhu, Lowe, Jiang, & Huang, 2001) possess potential applications in manufacturing industries."

Jan 1, 2016

By D. Kruse, M. Sanaei

"3D finite element models of the Segmentally Lined Shaft (SLS) structure are simulated as means of numerical analyses of the system’s structural behavior. Top-down constructed rings, comprised of precast concrete segments, create a large diameter, relatively shallow underground space that can be utilized for various applications. To further investigate the effects of soil conditions and different loadings on the shaft structure, the FE model was run under various boundary conditions. Simulations include a) effects of various soil layers on the SLS, including a circumferential boundary of modified ground, b) effects of various soil layers on the SLS without any ground modification, c) Seismic analysis of the SLS with ground modification and d) Seismic analysis of the SLS without any ground modification. Comparing the analyses outcomes emphasizes the significance of the inclusion of the ground modification boundary in the structural performance of the SLS. INTRODUCTION TO SLS Underground structures are classically built rectangular in shape, which normally requires a two-step construction process beginning with installation of temporary shoring to support earth loads during the excavation, followed by the construction of the permanent structure. Attractive and practical alternatives are circular shafts referred to as segmentally lined shafts (SLS). The structure consists of a stacked series of rings formed from an assemblage of precast concrete segments. Top-down constructed systems of large diameter, relatively shallow SLS structures provide both temporary earth support during excavation and permanent structural support for many civil and building applications. The circular shafts sustain the lateral earth pressure mainly through hoop-stress mechanism, resulting in improved stability and structural efficiency. Cylindrical shell structures with large diameter to wall thickness ratios are prone to buckling under external pressure. In typical designs of small diameter shafts and tunnel linings with small diameter to wall thickness ratios, nonlinear buckling analysis (i.e., nonlinear geometric analysis) is usually not considered, since the buckling capacities for those structures are much higher than typical pressures encountered at their design depths. With an economically viable wall thickness, buckling becomes a critical issue, which should be addressed, for circular shafts with very large diameters. In the design and construction of the SLS, the stability of the structure can be improved by modifying the soil surrounding the circumference of the shaft to a limited thickness and to about 10 ft below the finished shaft structure. This thick wall of soil mixed with cement creates a composite integrated structure while serving additional purposes such as ground control and ground water seepage cutoff."

Jan 1, 2016

By Y. L. Su, R. Stefanko, Y. J. Wang

This chapter describes a new approach to the solution of stress distribution around underground openings utilizing the finite element method and employing stochastically simulated rock properties. It is well-known from extensive laboratory tests that the properties of rocks, e.g., Young's modulus and Poisson's ratio, are not single-valued but form some distribution. If the scatter of test data is attributed to the variability of the rock itself rather than due to an inadequacy in test procedure, then stress solutions have to consider these variations. Utilizing this basic philosophy, the present study was initiated. A long circular opening subjected to plane strain in a hydrostatic stress field was chosen as a specific example to illustrate this approach. The technique adopted in solving stress distributions was based on the finite element method. This method is essentially a generalization of standard structural analysis procedures in which the continuum is idealized as an assemblage of "elements," interconnected at a finite number of nodal points. It is capable therefore of handling different material properties for individual elements in the model, if desired. In this study, each individual element was assumed to be isotropic, homogeneous, and elastic. However, values for Young's modulus and Poisson's ratio were randomly varied from element to element according to an assumed frequency distribution.

Jan 1, 1970

By W. G. Pariseau

Recent advances in rock-mass mechanics and computational technology now allow for the stability analysis of excavations in "wet" mines where mining-induced changes in rock mass deformation and fluid flow are coupled and time dependent. In addition to water pressure and flow in jointed, possibly porous, rock masses, all of the usual dry-mine parameters, including the premining stress state and rock-mass properties, are taken into account. Finite-element analyses of shafts and tunnels, as circular, elliptical, rectangular and arched-rectangular hole shapes excavated in an initially stressed and pressurized rock mass, indicate that the usual guidelines for optimum shape and orientation of single openings apply. Here, optimum means maximizing the minimum wall safety factor or, equivalently, minimizing the peak stress concentration. The results also show that failure originates in elements at the wall and the minimum safety factor at the wall in the wet case is always less than the dry case, regardless of drainage time allowed.

Jan 1, 1997

By W. G. Pariseau

Recent advances in rock mass mechanics and computational technology now allow for stability analysis of excavations in "wet" mines where mining-induced changes in rock mass deformation and fluid flow are coupled and time-dependent. All the usual dry mine parameters including the premining stress state and rock mass properties are taken into account, in addition to water pressure and flow in jointed, possibly porous, rock masses. Finite element analyses of shafts and tunnels as circular, elliptical, rectangular and arched-rectangular hole shapes excavated in an initially stressed and pressurized rock mass indicate that the usual guidelines for optimum shape and orientation of single openings apply. Here, optimum means maximizing the minimum wall safety factor, or equivalently, minimizing the peak stress concentration. The results also show that failure originates in elements at the wall and that the minimum safety factor at the wall in the wet case is always less than the dry case regardless of drainage time allowed.

Jan 1, 1995

By W. G. Pariseau

Conventional barrier pillars in underground coal mines are intended to protect mains and sub-main entries for relatively long periods of time. Inter-panel barrier pillars that may be used in especially deep coal mines, say, below 800 m (2,400 ft), are intended to protect longwall panel faces and adjacent panel entries for the life of nearby panels. Barrier pillar failures have serious consequences, especially when failure is sudden and catastrophic. Design of barrier pillars is thus of considerable importance to safety and efficiency of operations. Locally, failure occurs at the limit to elasticity when stress tends to exceed strength. Strength of rock is stress dependent, so an analysis of stress is central to rational design of barrier pillars. There are several approaches to stress analysis for coal mine pillar design that range from the simplified but popular tributary area method including variations that use empirical definitions of the tributary area (volume) to more sophisticated numerical methods such as the boundary element method and the finite element method. Despite ease of use, empirical methods are limited and always suspect when applied to conditions beyond their scope of definition, while boundary element methods do not generally include pillars in the solution domain. The finite element method is not so restricted and thus is a natural choice for conventional and inter-panel barrier pillar safety analyses. Caving of roof strata certainly affects the distribution of stress about longwall faces and nearby barrier pillars. Conventional wisdom that considers roof strata to respond as cantilever beams indicates the longer the span the greater are loads on longwall faces and face supports. Failure of roof beams, caving, shortens spans and relieves face and support loads. The same view is applicable to barrier pillars in that long roof spans outside of barrier pillars leads to higher pillar loads than short spans. Hence, caving should be beneficial to barrier pillar safety. However, the benefit may be small depending on the extent of caving and only in the form of a slight reduction in overburden weight transmitted to the barrier pillar, especially in case of inter-panel barrier pillars. FINITE ELEMENT MODEL Finite element models for stress analysis in rock mechanics are formulated in consideration of physical laws, kinematics, and material laws. These models meet the essential requirements for equilibrium of stress, satisfy strain displacements relations, and in a general sense, relate stress to strain through material laws, often referred to as constitutive relations. The last may be complex and replete with nonlinearities. A de facto engineering material law is the elastic model in the form of Hooke?s law for linear, homogeneous materials that may be anisotropic. A limit to elasticity is imposed by strength in the form of a failure criterion or yield function that accommodates triaxial stress states. Examples used in rock mechanics are the well-known Mohr-Coulomb (MC)criterion, the popular Hoek-Brown (HB) criterion, and the Drucker-Prager (DP) criterion. The first is linear in stress, while the second is nonlinear. Both do not include any effect of the intermediate principal stress. The last is also linear in stress but includes the intermediate principal stress. All show an increase in the major principal stress at failure with confining pressure. In the UT2/3 finite element programs, the failure criterion Y is a nonlinear, anisotropic form and is also the plastic potential [1]: [YJI=+ 11NYJI=+- (1) ] where N is an exponent, usually 2, and J2 and I1 are nonlinear anisotropic forms of the second invariant of deviatoric stress and the first invariant of principal stress. The form (1) reduces to DP when N=1and isotropy prevails. When Y = 1, the limit to elasticity is reached and plastic deformation is possible. The material constants that enter (1) are all computed from unconfined compressive, tensile, and, in case of anisotropy, shear strengths. These latter strengths may be estimated from conventional laboratory test data. Of course, joints in field-scale rock masses induce different values. How different is often a matter of conjecture. For comparative studies, laboratory rock properties seem adequate. Inelastic, that is, plastic behavior occurs with strain beyond the elastic limit. Strain hardening, ideal plasticity, and strain softening are possibilities. Ideally brittle behavior with complete loss of strength is an extreme form of strain softening [2]. Caving may also be considered as strain softening with a less drastic reduction of material properties. As intact roof strata fail, fall, and form a rubble pile below, a new material is formed with greatly reduced elastic moduli and strengths, say, 1/100th of the original intact moduli and strengths. From another viewpoint, caving is simply a natural cut and fill process. Input for stress analysis by the finite element method includes: 1. Preexcavation stress state 2. Geology (stratigraphic column) 3. Structure (major faults, joint sets) 4. Elastic moduli 5. Strengths, and specific gravities of each rock type 6. Excavation sequence Output from an analysis includes distributions of: 1. Stress 2. Strain 3. Displacement 4. Local factor of safety (element fs) The last is defined as the ratio of strength to stress and in view of the failure criterion (1), is the ratio [fsJstrengthJstress=] where the numerator is the maximum possible value and the denominator is the actual value of the square root of the second invariant of deviatoric stress. This invariant is a measure of shear stress intensity but also reduces to uniaxial tensile and compressive strengths as the case may be. An fs greater than 1 indicates an elastic state of element stress; a value of 1 indicates the limit to

Jan 1, 2009

By K. C. Viveros, R. R. Ambriz, C. García, F. De Villèle, A. Talha, D. Jaramillo

Cold hole expansion on base metal, weld metal and heat affected zone of 6061-T6 aluminum alloy welds was simulated by means of finite element method. Three dimensional compact test models with a previous fatigue crack length with a relation of ??= a/W = 0.3, were used to analyze the hole expansion process. Considering the local mechanical properties for each zone of the welded joint, the optimal expansion degree (1.64 to 2.46 %) was determined. The residual stress field, through thickness, after the expansion process in all welding zones, were correlated with the crack opening displacement obtained by finite element analysis. It was observed, that compressive residual stress fields for a post-expansion, in different crack lengths, provided a crack closure mechanism for base metal, weld metal and heat affected zone. Contrary, the crack remained open when residual stresses are positive. The results are given as tangential stresses versus distance from the edge of the cold expansion and crack opening displacement as a function of applied load.

Jan 1, 2014