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

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72 galvanized steel received from different sources. These authors reported that coefficient of friction and standard deviation decreased in subsequent measurements. Snyder et al. [153], on the contrary, observed a decrease in the coefficient of friction with prolonged frictional contact. Washing the frictional surfaces with carbon tetrachloride reduced the coefficient to its original value. The substance found after evaporation of the solvent was determined to be cutin, a wax-like substance found on the surface of grain kernels. The effect of cutin on the friction coefficient was more noticeable after the metal surface was allowed to age for one or more days. Molenda et al. [107] determined the friction coefficient of pairs of wheat grains on steel plates of different roughness. Results of these tests are presented in figure 9.4. In the case of smooth surface, a minimum of of 0.06 was found at moisture content of 15.2%. For the plate of surface roughness R t RI P WKH YDOXHVRI FRHIILFLHQW RI IULFWLRQ consistently increased with increasing moisture content. For the remaining plates the coefficient of friction increased with increasing surface roughness, but no clear tendency with an increase in moisture content was observed. The apparent inconsistency of observed frictional behaviour is probably a result of varied degree of contribution of basic phenomena in the resultant coefficient of friction. In the case of smooth surface the effect of adhesion prevailed. With increasing surface roughness the first deformation of surface asperities became relevant, while cutting of grain surface took place by asperities of plates of the highest levels of surface roughness. µ P µ P µ P µ P µ P 0RLVWXUHFRQWHQW Fig. 9.4. Coefficient of friction versus grain moisture content for wheat grains sliding on steel plate of five levels of surface roughness [107]
73 9.3.3. Normal pressure Normal pressure was shown to be another important factor influencing friction. Thompson and Ross [161] applied normal pressure in a range from 7 to 172 kPa and observed a decrease in the coefficient of friction of wheat against steel with an increase in pressure. The authors suggest that with an increase in normal pressure contact stress increased less than proportionally. Hertz’s theory [104] postulated that with an increase in normal force N normal contact stress increases as N 1/3 . This way, according to Thompson and Ross [161], frictional forces in the contact area that are proportional to normal loads are weaker than proportional to normal stress. In such a way Hertz’s theory would explain the decrease in the coefficient of friction with an increase in normal pressure. A similar tendency was reported by Fiala [52] for different agricultural materials, Lobotka [97] in the case of corn, and Zhang et al. [174] for wheat. Moore et al. [113] stated an opposite tendency, one of increase in the coefficient of friction with an increase in normal wall load in a silo made of corrugated steel. Typical results of friction of wheat against smooth and corrugated galvanized steel are shown in figure 9.5 after Molenda et al [111]. The tests were performed using the tilting table method and soft red winter wheat of 11.5% moisture content. Fig. 9.5. Coefficient of friction of wheat on smooth and corrugated galvanized steel determined using tilting table method. Mean values and 95% confidence intervals for five levels of normal pressure [111]
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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
Page 22 and 23: 22 (β = 0 o ), positioning the out
Page 24 and 25: 24 grain was poured into the mould
Page 26 and 27: 26 axes of the ellipse characterizi
Page 28 and 29: 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
Page 44 and 45: 44 (cereal grain, sand, soil), the
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 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 80 and 81: 80 10.4.1. Friction force mobilizat
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.
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122 Table 16.13. Mean values (±St.
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126 Table 16.16. Cont. 1 2 3 4 Icin
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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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