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

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70 9.3. Factors influencing the coefficient of friction The coefficient of friction of granular materials of plant origin depends on numerous factors, among which the following are regarded as the most important: moisture content, normal pressure, sliding velocity, surface state and ambient conditions. The influence of these factors on the coefficient of friction will be shown below, taking wheat grain as an example. 9.3.1. Moisture content Already early investigations of grain friction pointed out to moisture content as one of the crucial factors influencing friction. According to Canadian Farm Building Code [29], increase in moisture content of stored grain may result in a sixfold increase in pressure acting on silo wall. Richter [142], based on his investigations on straw, hay and silage, reported an increase in the coefficient of friction with an increase in moisture content. Similar tendency was observed by Brubaker and Pos [24] for friction of wheat against four types of construction materials. These authors suggested that after exceeding 13% of moisture content of grain a particularly fast increase in friction took place. Determination of wheat friction performed by Snyder et al. [153] in a climatic chamber showed that increase in grain moisture content as well as increase of air relative humidity resulted in an increase of coefficient of friction. Hanzelik et al. [61] did not observe any increase in coefficient of friction with an increase in moisture content. Probable reason for this disagreement with other authors was very low normal load applied in their equipment that was an inclined plate. Stewart et al. [157], in their testing on sorgo, confirmed the tendency of faster increase in the coefficient of friction for moisture content above 13%. These authors found also that an increase in moisture content produced a particularly high increase in the coefficient of friction in the case of surfaces of low asperities. Thompson and Ross [161] measured friction of wheat against galvanized steel and found the coefficient of friction versus moisture content characteristic with a maximum for 20% of grain moisture content. These authors suggested that at moisture content from 16% to 20% kernels became soft and deformation around the asperities took place that generated stronger bonds than in the case of hard, dry grains. Further increase in moisture content caused a decrease in the coefficient of friction resulting, according to these authors, from the presence of liquid water in the contact area. Increase of coefficient of friction with an increase in moisture content was confirmed by results of Tsang-Mui-Chung et al. [164], Balassy [11], Scherer and Kutzbach [149]. Lawton [93] confirmed the above stated tendency, however for some materials he found a relationship with minimum coefficient of friction for moisture content of 15%.
71 Molenda et al. [107], for wheat sliding against steel surface, also found the relationship between friction force and moisture content with a minimum at approximately 15%, as shown in figure 9.3. Fig. 9.3. Force of friction versus moisture content for wheat sliding on smooth steel surface. Experimental results and estimated curve [107] 9.3.2. Surface roughness Researchers were usually interested in friction of grain against materials that were commonly used in the construction of agricultural equipment and storage facilities. In earlier investigations wood, steel and rubber were tested, later investigations on galvanized steel, concrete, aluminium, polyethylene or teflon have been conducted. The state of the surface of investigated material was not precisely defined in a majority of published results because of the complexity of a methods of determination of surface properties. Usually the commercial name of material was given, sometimes with a parameter of surface roughness added. As an example, Cyrus [40] reported an increase in the coefficient of friction of wheat kernel against steel with an increase in parameter R a LQ D UDQJH IURP WR P $ VSHFLILF difficulty of investigation of friction is the changing state of surface with prolonging time of measurements. Richter [142] observed that the coefficient of friction decreased with increasing number of performed tests. He suggested polishing sliding surface till the moment of stabilization of frictional force. Thompson and Ross [161] observed considerable variations of coefficient of friction between samples of
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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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16 In turn, the model of Lade, also
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18 same porosity need not have iden
Page 20 and 21: 20 where: A = ∫ E( β) sin βdβ
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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
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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
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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 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 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
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Page 94 and 95: 94 unconfined yield strength to pow
Page 96 and 97: 96 The Chute Index (CI) is recommen
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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 118 and 119: 118 Table 16.9. Mean values (±St.
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120 Table 16.11. 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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