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Fine Coal Evaluation by Gravity Methods

Sep 13, 2010

For some time fine coal (-0.5 mm) flot ation has been considered as a bulk flotation problem. Combinations of hydrocarbon oils and frothers have been used as surface active agents in coal flotation. The selectivity of the flotation process has always been poor in such cases. With the increase in fine coal production due to mining methods and coal handling systems, optimisation of fine coal treatment.has become important. Petrographic and mineralogical composit- ions of coal particles have been reported to influence their flotability. The problem becomes more complex due to surface oxidation of coal and the presence of slime forming clay minerals in the run of mine coal. In this paper, the flotation problem is examined on the basis of various constituents of coal and their associated gangue minerals. The selection of reagents has been suggested on the basis of their surface chemical properties. Factors such as surface modific- ation of coal and gangue materials, flotation kinetics and hydrocarbon oil - coal inter- action have been critically evaluated to improve fine coal flotation recovery.

Jan 1, 1978

By Graham Davey

High-speed stirred milling is a relatively new method of grinding in the metalliferous process industries. However, it has been widely used in the industrial mineral field for more than 30 years. Many existing metallurgical operations are looking to maximize economic return from existing deposits this requires new thinking and technologies. The operating experience of two copper operations is discussed here to show the effect of the introduction of the Metso Stirred Media Detritor (SMD), a new type of high speed stirred mill. Both operations have derived significant metallurgical benefits from this new technology. The benefits and process difficulties are explained. The initial drive in high speed stirred milling technology was toward ultra fine grinding where the mill product has an 80 percent passing size of 15 µm (937mesh) or less. More recently, a number of installations have occurred in a more conventional regrind setting. Few conventional copper concentrators require such fine grinding to attain satisfactory copper grades and recoveries. So other advantages must be considered here rather than just the ability to grind fine with good power efficiency.

Jan 1, 2004

By E. W. Davis

I . OPERATING TESTS 1. CHARACTER OF ORE UNDER INVESTIGATION On the eastern end of the Mesabi Range, in Northern Minnesota, is a large formation of siliceous rock which contains bands and fine grains of magnetite . The magnetite comprises about 35 per cent. of the rock. Fig.. 1 show's graphically that the tonnage of minus 200-mesh material produced varies directly with the tonnage fed to the mill. There is undoubtedly some limit to this relation, but there seems to be no indication of it at 18 T. per hr. Some of the conclusions drawn from this test are that: (a) The ball-mill is naturally a machine of very large capacity; (b) if it is not possible to deliver a large tonnage of original feed to the mill, a closed circuit should be provided so that the mill may crush its own oversize.

Jan 2, 1919

By Franz Simon

Conventional fine gold electrolytes for barrel applications working electrolytically actually permit significant precious metal savings by corresponding optimizations but the most gold is still deposited on areas of the components where it is not absolutely essential for the desired bonding and soldering applications, i.e. in high current density areas. Therefore an electrolyte working electrolessly has been developed which inevitably leads to a much more uniform thickness distribution since in an electroless process there are no areas favoured by metal deposition and current distribution. The new electrolyte will be presented and its performance will be compared with conventional electrolytes as regards thickness distribution. The potential gold savings will be discussed.

Oct 1, 2003

Fine Gravity Separation in the Reflux Classifier, Exploiting a High Shear Rate, Laminar Flow Mechanism The Reflux Classifier is used in the gravity separation of particles over a broad range of particle sizes. The device consists of a fluidised bed zone, with a system of parallel inclined channels above. The inclined channels provide a significant hydraulic advantage over conventional fluidised beds via the so-called Boycott Effect (Boycott, 1920). This paper describes a new advance in fine gravity separation based on the high shear rate and laminar flow profile that arises when closely spaced inclined channels are deployed. As reported recently (Galvin et al, 2009a), a powerful density based separation mechanism develops from this arrangement, suppressing the effects of particle size. In this paper we focus on two areas, firstly the application of the separator in the laboratory for batch fractionation of particles of varying size and density to determine the grade recovery curve and hence the liberation, and secondly the step change in separation performance achieved on a continuous steady state basis when closely spaced inclined channels are used. The variation in the separation density with particle size decreases significantly, and the Ecarte Probable Error (Ep) remains at low levels over a broad size range.

Sep 13, 2010

By A. D. Taylor

In recent years there has been an increasing effort to attract students into engineering. Obviously, the effort is necessary because young people find the image of the engineer unattractive. Some of the things they find unattractive are real, some are not, and part of the problem is too little knowledge of the field. First, I would like to comment on some of the ideas which appear to be justified. Engineering is a difficult course of study. True, and nothing can or should be done to change this. The education is continually being improved, but it is unlikely to be made easier.

Jan 1, 1970

By F. F. Aplan

Pollution control will be of utmost importance in the 1970's. In this area, the mineral process engineer has a special competence to understand and remedy many pollution situations. There is an old adage that goes, "Build a better mouse- trap and the world will beat a path to your door." Would that it were so, but the reality of the situation is such that, just as a supplier must sell his machine, a professional man must sell his competence. It is a sad but true fact that mineral processing engineers have not done a very good job in selling their extensive competence in the control of a broad range of pollution problems. Recently one of our students visited a paper pulp mill where he was shown a foam fractionator used to remove paper pulp fibers from plant discharge waters. "Foam fractionator hell, that's a flotation cell!'' was his comment. Ah so, it would appear that large numbers of technical people consider themselves experts on "foam fractionation" Or is it that we have been very remiss in demonstrating the usefulness of our equipment and processes and our professional competence to solve, these pollution problems?

Jan 1, 1970

By Graham Davey

"Low speed stirred milling has been employed for many years in the metalliferous and industrial minerals industries. High speed stirred milling is a relatively new method of grinding in the metalliferous process industries. However it has been it has been in wide spread use in the industrial mineral field for over 30 years for various minerals. Many existing metallurgical operations are looking to maximize economic return from existing deposits, requiring new thinking and technologies. By combining low speed and high-speed milling fine and ultrafine grinding efficiency and operating costs can be improved.The correct selection of different fine grinding technologies is covered, when each mill is applicable and when a combination of the VERTIMILL® Grinding Mill and SMD provides the best economic solution.INTRODUCTIONUltra-fine grinding of minerals has been in wide spread use in the industrial mineral field for over 30 years. Within the last 10 years the metalliferous industry has shown interest in the ability to produce ultrafine particles, as chemical, physio-chemical and physical separation processes have advanced to be able to treat ultrafine metalliferous particles. Also as the primary mineral deposits, around the world are depleted, new more difficult deposits must be mined. Existing operations are also looking to maximize economic return from existing deposits, requiring new thinking and technologies.The traditional approach for gold containing refractory material has been to liberate the locked gold by chemically destroying the pyrite through oxidation. Roasting, pressure oxidation, and bacterial oxidation all employ various degrees of temperature, pressure and catalysis to react the pyrite with oxygen to produce iron oxide and sulfur by-products. However all of these methods are economically impacted due to specific operational added requirements and emissions controls. Pressure and bacterial oxidation have very high capital and operating costs."

Jan 1, 2006

By The Staff

THE object of the work was to increase the capacity of the plant and, if possible, to reduce costs of the actual unit grinding while doing so. The accompanying assays of an infra-sizer analysis of the cyanide tail (Table l) illustrate the problem in hand. The cyanide tail is a combination of two products, gangue and FeS2. The assay alters by virtue of a change in gold value due to grinding and the relative quantity of FeS2 in the sizes, which itself varies with the grind. The disposal tail is the residue after the cyanide tail is scavenged by flotation, most of the pyrite being removed for roasting. It presents a somewhat different problem, since grinding gangue is all gain, but -10 micron FeS2 does not float nearly as well as + 10 product. This results in compromise in the final size to be aimed at. Extra grinding has hardly any appreciable effect on the gold extraction from the FeS2. (See Milling Investigations Into the Ore as Occurring at the Lake Shore Mine; Trans. C.l.M.M., Vol. XXXIX, 1936).

Jan 1, 1940

By Augustine O. Orumwense, Micheline Boisclair

"The use of stirred mill to complement tumbling mills in grinding fine grained minerals is increasingly becoming an attractive processing option. Parameters such as agitation speed, grinding media type and size, feed pulp density, and grinding time were studied in order to elucidate their impact on stirred milling. Test results showed that the parameters have a profound effect on stirred mill efficiency and operating cost. A comparative test with a tumbling ball mill was also made and the specific energies of both mills in fine grinding were analyzed. A difference of over 40% in energy savings particularly when the product size is smaller than 20 µm was obtained with the stirred mill.INTRODUCTIONIn industrial processes where very fine and ultrafine particles are required such as in chemical, ink, food, paint and lacquer, pulp and paper, plastics, pharmaceutical, metallurgical and ceramic industries, the use of stirred mills for grinding is well established. Mineral processing plants are gradually shifting focus to the use of stirred mills for fine grinding and /or regrinding purposes as the target liberation size gets very small. A number of plants processing finely/sparsely disseminated ores are known to have adopted stirred milling technology for their fine grinding needs (Enderle et al., 1997; Harbort et al., 1999; Gao et al., 2000; Davey, 2003 & 2006; Pease et al., 2006).There are different kinds/designs of stirred mills available on the market but, all can be divided into two sub groups namely fluidized and non-fluidized types. The non-fluidized stirred mills consist of Tower mill, Vertimill, Sala agitated mill (SAM), and ANI-Metprotech (SVM). These mills are characterized by their low-speed and the use of fairly coarse grinding media size, 3 – 20 mm. Hence, they are mostly suitable for relatively coarse grinding application. The fluidized type includes the Isamill, Netzsch mill and Stirred mill detritor (SMD). These mills have a characteristic high agitator speed, use fine grinding media size of 0.5 – 5 mm, and are employed in very fine grinding operations. The superior fine grinding capability of stirred mills is attributable to two main process parameters, which are the use of fine grinding media and a high mill speed."

Jan 1, 2008

By G. Papachristodoulou, O. Trass

"Performance data for both wet and dry grinding of materials have been obtained in this novel, planetary ring-roller mill over wide ranges or operating conditions. The Szego mill consists of a stationary cylindrical shell inside which grooved, radially mobile rollers rotate, driven by a central shaft. The mill has several internal design variables such as number, diameter and length of rollers as well as size, shape and pitch oi helical grooves. It is compact, has a high specific capacity, modest power consumption, and can grind many materials down to ultrafine powders. Illustrative data on product fineness and mill power consumption are given along with results from studies on grinding kinetics.INTRODUCTIONThe extensive use and importance of grinding as a unit operation, coupled with the rather low energy efficiency of conventional equipment provide a challenging incentive to investigate novel methods of comminution. Yet, most current development work is taking place in mill performance modelling and optimization, or with various detailed improvements rather than in the field of new equipment development.This paper is a report on the development of a ring-roller mill invented by the late L.L. Szego which embodies several novel concepts. The mill has a broad range of applications, may be operated wet or dry, and can grind materials down to ultrafine sizes with modest energy consumption. It has been developed by General Comminution Inc. in close cooperation with the University of Toronto."

Jan 1, 1986

By A. G. Doll

"Many power-based grinding models exist, and most operators are familiar with Fred Bond's ""third theory"". Bond's model is most commonly used to describe primary and secondary grinding to product sizes above, for example, 100 µm. Operators sometimes use Bond's equation to describe grinding in situations where it is not appropriate, such as fine grinding below 50 µm. Using an alternative model would be a better choice in this situation.Bond's equation is one in a large family of models. Related equations better suited to fine grinding include the “signature plot”, Von Rittinger's model and Charles' equation. These models have a similar form to Bond's and can be fit to industrial regrind mills and laboratory tests using the simple regression tools in the charts of computer spreadsheets. Operators will find that fine grinding calculations are both easier and more accurate when using the alternative equations to fit their regrind milling surveys or when performing laboratory regrind mill scale-up for plant design.None of these models are new: Charles' equation was published in 1957 and Von Rittinger's model was proposed in 1867. A quick refresh how to apply these equations can turn these oddities from the undergraduate curriculum into useful tools for plant optimization. They can be fit to any type of mill including stirred and tumbling ball mills.IntroductionBuilding a case for fine grind modelling requires reviewing some theory before building a framework for doing fine grinding calculations. The actual calculation procedure recommended is easy and can be applied by plant metallurgists. Don’t be frightened by the theory; the end result is reasonably simple.The observation that mineral breakage is related to the absorption of energy is usually attributed to Von Rittinger (1867). Lynch and Rowland (2005) describe that the ability to empirical test and calibrate energy models of breakage didn’t exist until the widespread adoption of electric motors in the mining industry during the 1930’s and 1949’s. A series of technical papers published by the Allis Chalmers company during the 1940’s demonstrate that they were attempting to do such a calibration using Von Rittinger’s model which is usually stated as: “the energy consumed in the size reduction is proportional to the area of new surface produced.”"

Jan 1, 2017

By R. Buisman, K. Reyneke, M. Fullam, J. V. Brown

Most mineral processes currently in use, such as flotation, spirals, gravity concentrators, etc., are only effective over a small range of sizes, hence the upstream need for technology to accurately classify fines by size. The Pansep’s design is revolutionary. Slurry is fed onto connected mesh sections that move without bending or vibration, in a circular motion. Both sides of the screen mesh are utilized, so Panseps are self-cleaning and has half the footprint of other mesh-type screens. Developed in South Africa in 1997, the Pansep has proved itself very effective as a wet fines screen to effectively classify fine mineral fractions from 50 to 2,000 micron (325 to 10 mesh).

Jan 1, 2001

By R. Owen

The vision-based approach to process control involving fine particles in harsh on-line environments is discussed in this paper. The sensor equipment at the process location, consisting of a light source and a CCD camera provides RS-170 B&W video output. The video can be seen on a process monitor in the control room and is sent to a computer based Vision Processor. The Vision Processor contains a video digitizer and Canty developed software that extracts process information. The iron mine application controls thickener agent feed rate and thus settling pond overflow clarity. The sensor is mounted in the overflow flume of the outdoor settling pond and is used year around in the Minnesota weather. Alternate control system needs a low flow water sample which is subject to freezing in winter. The computer based vision processor provides 4-20 mA analog outputs for real time control as well as file archive for a history record.

Jan 1, 2001

By M. Leroux, J. A. Finch

"Sizing below 38 µm (400 mesh) is an increasing requirement in mill testing laboratories. The Cyclosizer is gaining wide acceptance in this role. A recent alternative is to use electro-formed sieves (micro-sieves), which are available with openings down to 5 µm. The advantage of micro-sieving is a cut size which is independent of mineral density, easier retention of the fi nest fraction and reduced purchase cost. A simple method of microsieving using an ultrasonic bath, which performs readily to 10 µm, is described.A systematic study of Cyclosizing and micro-sieving is reported for three samples; silica, copper oxide and a lead concentrate. Micro-sieving gave comparable results. Comparison with a standard sizing method, the Sedigraph 5000D, revealed misplaced material in each fraction by both methods. The range for micro-sieving was from 20% to 60% depending on the sieve and material; for the Cyclosizer it was a consistent 40-50% for all cones and all materials. The Cyclosizer can be sensitive to feed mass and particle size for dense materials. It is recommended that for minerals of specific gravity greater than 5, no more than 35 g of -38 µm be used.The disadvantage of micro-sieving is that it requires continuous operator attention and sieving time depends on sample mass. To fractionate 35 g-as used on the Cyclosizer and considered at reasonable mass for mill testing purposes-through 25-, 15- and 10-µm sieves requires about 80 minutes.IntroductionParticle size has a pronounced influence on mineral separation processes. Consequently, the measurement of particle size plays an important part in mill testing. By far the most common method is by screening. This, however, is usually limited to 38 µm (400 mesh) or occasionally 25 µm (500 mesh). Increasingly, mineral products are becoming finer than this limit and consequently finer sizing methods are required."

Jan 1, 1982

"Sedimented hydrothermal precipitates of silica, now fine-grained quartz, are described from a Pb-Zn-bearing vein system. Their discrimination from the wall-accreted fine-grained quartz that is common in the shallow parts of epithermal systems is discussed. Local fine-grained (<0.3 mm) quartz accumulations occur in a suite of veins with otherwise normal drusiform and crustified infills. This sediment may fill the vein above chokes in steep segments (axial plugs), or form continuous layers within the footwall material where the vein is inclined at low to moderate angles. Pads of fine-grained sediment are deposited on the upwards-facing surfaces of wall-rock clasts and of drusiform crystals growing from the vein walls. The asymmetry of these deposits differentiates them from the more common symmetrical, crustified walllining deposits of fine-grained quartz and opaline silica, and requires a gravity-controlled, sedimentary depositional mechanism.Thin section examination shows that much of the fine-grained quartz is present as elongate prisms. The prism habit was present soon after deposition, but it is not possible to identify unequivocally the original sedimented grains. Sedimentation of vein- and wall-rock-derived detritus is also recognized. Deposition of the fine-grained sediment accompanies normal in situ drusiform growth of quartz and other vein minerals. The mammilated and spherulitic textures associated with accretion of amorphous quartz are entirely absent from the Tyndrum veins with sedimented materials. The literature on modern hydrothermal waters and physical chemistry of silica precipitation shows that precipitates of silica suspended in hydrothermal fluids are amorphous and derived from colloidal silica. It is argued that much of the suspended silica was present as quartz at the time of deposition or was converted from amorphous silica almost immediately after deposition.Hydrothermal sediments and their distribution potentially provide information on flow directions and velocity in veins. Plugs of sediment obstruct vertical flow and lead to local flows with large horizontal components. It is shown that sedimentation features occur in other deposits and can involve other minerals. If the features seen here can be scaled up to the size of major veins, they may result in up and downdip variations in mineral assemblages and abundances, which may affect grade."

Jan 1, 2007

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 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