Study On Scale Effects And Slenderness Ratio Effects Of Rock Mass Characteristic

Scale effects is an universal phenomenon in material science field whichis caused by heterogeneousness of material (This is mainly caused by crackfor rock mass). Theoretical analysis on scale effects is carried through firstlyfrom different angle in this paper. For example, mechanism on scale effects isstudied with defect distribution rule, heterogeneous model and localweakening model from statistical angle. The deformation process of rock massis analyzed with dissipation theory. At the same time definition of strength isadvanced.On the basis of above-mentioned theory, Lime rock specimens indifferent scale series from Hebei Jingxing is tested under uniaxial compression.Besides, through numerical simulation and test of coal specimens from Datongand Luan scale effects rule is found out. And that, physical and mechanicalcharacteristics of specimens are studied under uniaxial compression. Theresults are as follows:Testing on lime rock specimens from Hebei Jingxing:1,Geometrical comparability series(1) Stress-strain curve on large scale specimens presents an obviousreturn circle; (2) Crack propagation: It doesn't develop any more at the case of uniformcycling load after crack happens. Specimens also can't failure on thiscondition. If increasing load further, crack continues to expand. This showsstress concentration is more serious in specimens and stress gradient is moresteep.(3) Large and small specimens both produce crack paralleling to loadingdirection under uniaxial compression. Local point strain is a stochasticvariable and its change is complex with increase of observed stress. Theaverage of axial local point strain is linear with observed stress. The slope ofeach cycle is approximately parallel. The average of transverse local pointstrain increases flexuously with loading or unloading. Differently, the slope ofincrease in loading process is larger than that in unloading process. Theobserved strain is 10~2 times as local point strain at the same observed stress.This indicates that macroscopical symmetrical rock is quasi-brittle materialwith microcosmic heterogeneousness and its macroscopical mechanicalbehavior is the results of reciprocity and magnification between microcosmiccharacteristic, not simple sum.(4) The degree of irregularity of strain increases with the number of cycle.But degree of irregularity of transverse strain fluctuates larger at the samecycle. Moreover degree of irregularity of transverse strain is larger than that ofaxial strain. This shows that deformation and failure of rock is process ofevolvement of small crack. Small crack evolves from even and out-of-orderstatus to heterogeneous and ordinal status. (5) Ultrasonic speed decreases with the number of cycle. The decreasecourse is such: Firstly, ultrasonic speed fluctuates in a steady scope in somecycles; And then ultrasonic speed quickly decreases to a certain more smallvalue; Afterwards in succedent cycles, ultrasonic speed fluctuates about thevalue. That is to say, ultrasonic speed fluctuates and decreases at intervals ofcycles. In deformation process of rock, quantitative change and qualitativechange of its small crack evolvement carry through crosswise.(6) Strength and modulus of rock decrease with exponential function asscale of specimens increases. Strength is positive correlative to modulus. It ispresumed that the increase of modulus is caused by that of strength. Strengthis related to original crack fractal dimension and ultrasonic speed and itincreases as ultrasonic speed increases.2,Different length but with the same section series(1) Strength almost doesn't vary in 1-3 slenderness ratio. The dispersionsof strength is the smallest at 3 slenderness ratio. Strength decreases withincrease of slenderness ratio. The dispersions of strength and local weakeningdegree is larger, stability is worse, and brittleness is larger When slendernessratio is larger than 3.(2) Modulus, deformation modulus and Poisson's ratio increase withincrease of slenderness ratio. And utmost axial strain decreases with increaseof slenderness ratio. Moreover, the dispersions of mechanical parametersincreases with increase of slenderness ratio.(3) It varies with the difference of rock category that the critical slenderness ratio at which strength decreases obviously.Numerical simulation: studying uniaxial compression characteristics ondifferent schemes specimens (with different crack) of two kinds of scale serieswhich heterogeneousness is 100 and 2.(1) Scale, homogeneousness, direction of crack, length of a single crackand density of crack have influence on strength of specimens. The order ofinfluence degree from bigness to smallness is homogeneousness, density ofcrack, direction of crack, length of a single crack and scale. It shows that scaleeffects happening in actual engineering is in fact effects of structure of rockmass.(2) Fracture area is more obvious with increase of scale of specimens inthe same homogeneousness. And the fracture area of specimens is moreconcentrated. Failure shape of specimens is more close to brittle failure withincrease of scale of specimens. In the same way failure area is not single andis more complex with decrease of scale of specimens. And failure shape ismore close to quasi-brittle failure. It varies with homogeneousness that thescale of specimens at which failure shape changes.(3) The failure shape of homogeneous specimens with crack is controlledby crack. But that of heterogeneous specimens with crack is controlled bycrack and heterogeneousness.(4) Acoustic emission doesn't appear until homogeneous specimens fail.Acoustic emission energy increases with increase of scale of specimens. Butfor heterogeneous specimens, acoustic emission appears stochatically and evenly at the beginning of loading and keeps in the entire loading process.Acoustic emission energy releases post at the time of failure for bothhomogeneous and heterogeneous specimens.(5) Stress is linear with strain before failure and stress falls sharply afterfailure for homogeneous column or square specimens. Stress is non-linearwith strain before failure and the decrease degree of stress weakens withincrease of scale of specimens.The total conclusion from above-mentioned study: scale effects is in factthe effects of structure. Scale effects expression and choosing rule of itsparameters are produced. The expression has specific physics concept and thechoosing of parameters is simple. And it is reliable validated by engineeringexamples.