Strona 2_redak - Instytut Agrofizyki im. Bohdana DobrzaÅskiego ...

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80 10.4.1. Friction force mobilization During monotonic uniaxial loading the pressure ratio generally increases to an asymptotic value characteristic for the material. For such a loading it can be assumed that the wall friction is fully mobilized but we can not be sure of the same about the internal friction. Therefore the value of the pressure ratio characteristic for yielding at the wall in the active case (equation 10.4) should be treated as the lower boundary of the possible values of the pressure ratio. Examples of plots of the pressure ratio and the coefficient of friction force mobilization, µ* (µ* ≤ µ), versus normal pressure obtained during loading and unloading loops of rape seeds and wheat grain in the model silo are presented in figure 10.4 [68]. During loading the pressure ratio increased to the asymptotic value typical for the given material while the wall friction coefficient decreased to the asymptotic value of the friction coefficient. On the contrary, during unloading the pressure ratio increased and the friction coefficient decreased. On the contrary, during unloading the pressure ratio increased and the friction coefficient decreased. On the contrary, during unloading the pressure ratio increased and the friction coefficient decreased. Fig. 10.4. Pressure ratio (a) and wall friction coefficient of rape seeds (c) and wheat grain (b, d) during loading-unloading cycles [68]
81 During the first cycle of loading the experimental value of the pressure ratio was close to the predicted value for the active case and yielding at the silo centre or yielding in frictionless wall silo (pointed line, 3 w = 0°). It means that the principal stresses were close to the vertical and horizontal directions. The biggest part of the pressure ratio loop was located in the area of values predicted for the case of yielding at the wall in the active case. During unloading the pressure ratio loop went beyond the value calculated according to Eurocode 1 recommendation (solid line) and arrived into the area of values typical for passive case (above dashed line). The pressure ratio loops resulted from superposition of elastic and plastic interactions between grains. Although the limit states of the model comprise only the plastic interactions, the predicted values of the pressure ratio correspond fairly well to the experimental ones. 10.4.2. Packing structure The pressure ratio is influenced by the procedure of sample preparation [91, 68]. Distributed filling produces higher density as compared to stream filling. This results in a higher angle of internal friction and a lower pressure ratio (fig. 10.5). Filling procedure effects not only the bulk density but also the packing structure of granular material i.e. mutual orientation of particles and distribution of normal direction at contact points. In the case of distributed filling, inter-particle force chains are oriented mainly vertically. Uniaxial compaction strengthens this structure and, as a consequence, the pressure ratio is relatively low. This type of structure results in funnel flow during discharge. On the contrary, concentrated (stream) filling results in looser structure. Particles sliding off along the surface of natural repose cone generate some preferences in particles orientation which additionally influences the stress transition. Finally, the pressure ratio is much higher than in the case of distributed filling and during discharge material tends to mass flow. Fig. 10.5. Effect of the filling procedure on the pressure ratio determined on the model silo
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EC Centre of Excellence AGROPHYSICS
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4 9.3. Factors influencing the coef
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6 BASIC NOTATION c - cohesion [kPa]
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8 Numerous granular materials when
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10 the tensor of plastic strain inc
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12 Fig. 2.1. Yield curves with comp
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14 Stable or unstable behaviour of
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16 In turn, the model of Lade, also
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18 same porosity need not have iden
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20 where: A = ∫ E( β) sin βdβ
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22 (β = 0 o ), positioning the out
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24 grain was poured into the mould
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26 axes of the ellipse characterizi
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28 [176] for the determination of p
Page 30 and 31: 30 A different approach to the desc
Page 32 and 33: 32 where: I - moment of inertia, k
Page 34 and 35: 34 a) b) f s f s m M f n f n Contac
Page 36 and 37: 36 The micropolar model yields resu
Page 38 and 39: 38 Fig. 3.17. Model of shear band f
Page 40 and 41: 40 In the case of agricultural mate
Page 42 and 43: 42 5.2. Density of consolidated mat
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Page 46 and 47: 46 Multiple wetting and drying of g
Page 48 and 49: 48 vertical pressure within the ran
Page 50 and 51: 50 of the curve defined by A, where
Page 52 and 53: 52 6. Vertical consolidation refere
Page 54 and 55: 54 poured into the box without vibr
Page 56 and 57: 56 7.2.2. Bulk density Stress-strai
Page 58 and 59: 58 near the box wall and lifted alo
Page 60 and 61: 60 and the upper boundary was 18.5
Page 62 and 63: 62 A -1 5 -1 0 -5 1 0 5 -5 5 1 0 1
Page 64 and 65: 64 Fig. 8.2. Yield locus and effect
Page 66 and 67: 66 in such a way that friction woul
Page 68 and 69: 68 a) b) µ = tg ϕw ϕ w c) d) e)
Page 70 and 71: 70 9.3. Factors influencing the coe
Page 72 and 73: 72 galvanized steel received from d
Page 74 and 75: 74 9.3.4. Velocity of sliding Becau
Page 76 and 77: 76 1+ sinϕ k = . 1 − sinϕ (10.3
Page 78 and 79: 78 N ϕ VLQϕ 3UHVVXUHUDWLRN
Page 82 and 83: 82 10.4.3. Moisture content With in
Page 84 and 85: 84 Table 10.1. Measured (k s ) and
Page 86 and 87: 86 conventional engineering analyse
Page 88 and 89: 88 a, b - product-dependent coeffic
Page 90 and 91: 90 also be performed as a function
Page 92 and 93: 92 reliable processing and stable v
Page 94 and 95: 94 unconfined yield strength to pow
Page 96 and 97: 96 The Chute Index (CI) is recommen
Page 98 and 99: 98 obtained the maximum deviation o
Page 100 and 101: 100 efficient flow control using op
Page 102 and 103: 102 In the field of experimental in
Page 104 and 105: 104 22. Britton M.G., Moysey E.B.:
Page 106 and 107: 106 69. Horabik J., Rusinek R.: Pre
Page 108 and 109: 108 114. Morland L.W., Sawicki A.,
Page 110 and 111: 110 158. 6WSQLHZVNL$ Influence of m
Page 112 and 113: 112 16. APPENDIX - Physical Propert
Page 114 and 115: 114 16.2. Density and porosity Tabl
Page 116 and 117: 116 Table 16.7. Mean values (±St.
Page 126 and 127: 126 Table 16.16. Cont. 1 2 3 4 Icin
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130 Table 16.20. Mean values (±St.
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136 Table 16.25. Cont. 1 2 3 4 Icin
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140 Table 16.29. Cont. Table sugar
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144 Table 16.35. Mean values (± St
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