The range of coals from brown coal to anthracite represents a group of intermediate products from the continuous chain of structural and chemical transformations of polymeric carbonaceous substance under the regional metamorphism [11. Since coals consist of amorphous and crystalline phases distributed in specific ways, from the viewpoint of the theory of composite media their rheological behavior can be represented by structural models of the Burgers type.This model was obtained in experimental work on models of creep in coal [2,3]. The treatment in [4] of a composite material capable of recovery coincides with the analytical equations for a Burgers body given in [5]. In these papers the total stress on the model is represented as the sum of two additive components.O=Ot+O tt. (I)The total deformation of the model is correspondingly divided into three components according to their "memory" for external actions-the elastic component e3, the elastic aftereffect ez, and the residual component e 1 (see Fig. 1). The e quation of state of a Burgers body with linear parameters, in the linear and symbolic representation, is Mechanical Rheological Transformed analogy equation equation + + Spring E2 a = E~ea ~ -= E., % § _t. Spring Et oS=EIe2 =' : E1% d r + + Shock absorber Vh dt : P ~h 81 Shock absorber ~lt dt vh + + (2) We use the Carson transformation [5]: g(a) .... Pi e-'pr g(r)dT" 0The total deformation of the model in the image plane iswhence, for the elastic component, we get ea (p) --where k = Ez/~ z, l = E1/rh, m = F~/rll. -4-p (p+ l) ~(v) p2 @p(k.+. 1 + m) + /el ,Institute of Mining, Siberian Branch, Academy of Sciences of the USSR, Novosibirsk.
A simple example for understanding symmetry in fracture mechanics for rocks is hydrostatic compression. It is well known that in this case fracture occurs with the maximum pressures for given conditions. A stress state with unequal components generates shear strains, the rock appears to be less stable, and the level of failure pressure is reduced. Fracture resistance of rock masses thus depends on the symmetry of loads applied to them.
FACTORThe method of studying creep under the effect of a bending load [1][2][3][4] is simple and reliable owing to fulfilment of the St. Venant principle. It gives all the necessary information for studying uniaxial creep: stress, modulus of elasticity, and change in deformation in time, described by the equatione(t)=e(0) 1 + 1--a where s (0 ) is the deformation at the moment of stress, a and 6 are parameters of the creep nucleus, and t is the time in seconds.The value of s(0 ), determined from the equationdepends on the loading parameters, because Eben d=K 4 B Ha ,~ y 'where L, B, and H are the length, breadth, and height of the beam being bent, Ap is the increment of the transverse load, Ay is the increment of sag, and K is a constant --I + 2.95 (H2/L 2) -0.02 H/L.Erzhanov [1] used the method of repeated loading so as to exclude the effect of irreversible deformations on gbend. It should be emphasized that although reference [1] mentions the scale factor, it was not studied in detail.The aim of this report is to determine the optimal specimen size at which repeated loading by bending, giving minimum elastic deformation, does not distort the modulus of elasticity. In this case, study of creep does not require special determination of the modulus of elasticity, and the test results become more representative.We performed tests on specimens of Tastagol' iron ores with 59.2% and 34.2% Fe, and coals and sandstones of the Prokop'ev-Kiselev coalfield in the Kuzbass.Anisotropy of the properties was allowed for by specific orientation of the schistosity during bending of the specimens relative to the effective stresses. The beam span L was kept constant for each series of experiments, while the square cross section varied; this enabled us to take account of the scale factor's effect.The duration of the nominally instantaneous load was stable; thus, with the same stress level, we obtained different loading rates for different cross sections of the specimens.
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Research on the mechanical properties of rocks will be effective if the model of the medium on which the method of investigation is based is both simple and general. As the basic model we have adopted the L model of a plastic body due to Revuzhenko and Shemyakin, which satisfactorily reflects the properties of both broken and solid rock.The shape and size of the fracture products in this model are not random, but are associated with the state of stress of the rock for particular boundary conditions and correspond to a network of slip lines, for which the parameters are constant for a given rock.They are inherited in the stage of elastoplastic behavior from the elastic stage and in the stage of fracture from the elastoplastic stage.The structural blocks of the rock permit them to move and rotate relative to one another in elastic behavior within a particular block.In the transllmiting state the resistance and deformability of the rock depend on the cohestion and friction at their boundaries.The seismic method of estimating the mechanical properties of rocks is permissible provided that we can diagnose the transition of the rock in a field of rising stress from the elastic to the elastoplastic and then to the postlimltlng state.Our experiments, aimed at developing methods for diagnosing the state of the elements of the rock, revealed that the seismic method is indeed applicable.The experiments were performed on samples of Norilsk ores, diabase, and concrete, loaded uniaxially, with measurement of the deformations and elastic vibrations arising in the sample. Figure 1 shows an oscillogram of the elastic vibrations.The amplitude and shape of the vibrations vary over wide limits.The number of periods was counted by an F-5041 frequency meter and chronometer in the sign-changing cycle counting mode.
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