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

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56 7.2.2. Bulk density Stress-strain behaviour of granular material depends on the bulk density of the sample. Dense samples dilate during shear test while loose samples decrease in volume. In dense samples shear stress attains a peak value, and with continuing shear displacement it drops back to a lower ultimate value and remains at that constant level during further shear. In the loose state most granular materials tend to decrease in volume when subjected to shear under constant normal load. For such samples shear stress gradually increases until it reaches ultimate value. Thereafter, with increasing displacement it remains stable. The density at which material is sheared without change in volume is termed, after Cassagrande [30], the critical density. Fig. 7.5. Relationships of stress versus strain obtained in triaxial compression test for wheat grain of moisture content of 13% and four levels of bulk density Figure 7.5 illustrates relationships between axial total stress and axial strain in triaxial compression of wheat grain of 13% in moisture content at four levels of bulk density [101]. The graphs show that increase in density ρ resulted in a quantitative change in material properties. For denser samples higher were both the maximum value of σ 1 and the maximum value of axial strain ε 1 . For the sample of bulk density of 822kg m -3 the axial stress reached maximum value σ 1max for ε 1 of 0.1. For the sample of the lowest density (of 729 kg m -3 ) the stress σ 1max was attained at ε 1 of 0.22, thus at the strain about four times higher than ε 1 of 0.06 recommended by Eurocode 1.
57 7.2.3. Time of consolidation The time of consolidation is one of the most important factors influencing mechanical properties of powders. For example, a long period of storage of powders in bags or in silo can lead to caking when the material becomes nearly solid, causing serious problems in handling. In the case of food powders moisture is the most severe factor causing caking. In order to determine the effects of time of consolidation, the sample has to be compressed for the prescribed period of time before the shear test is performed. Figure 7.6 shows the results of determination of strength parameters of wheat meal following Eurocode 1 procedure. The sample was sheared immediately after consolidation by twists or remained under reference load for additional two hours. Time consolidation resulted in an increase of the angle of internal friction from 31°±4° to 43°± 2°. After the time consolidation, the sample usually returns to its original state if no chemical or physical changes have taken place. In such a case the material may be used for the measurement of the next point of the σ(τ) characteristic. This may be verified by running normal instantaneous test procedure. If any change in strength properties occurs after the time consolidation, this points out to chemical or other irreversible changes and a new sample should be taken for determination of the next point of the characteristic. 7 Shear stress (kPa) 6 5 4 3 2 1 0h 2h 0 3 4.5 6 Normal stress (kPa) Fig. 7.6. Shear stress versus normal stress relationship from direct shear test for wheat meal without time consolidation and consolidated for two hours 7.2.4. Method of sample deposition. Anisotropy of packing The angle of natural repose Φ and the phenomenon of grains lying nearly along the generatrix of the formed cone was used for producing the preferred grain orientation. Rye grains were poured through a funnel into the shear box inclined to horizontal at an angle β as shown in figure 3.1. The outlet of the funnel was placed
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EC Centre of Excellence AGROPHYSICS
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4 9.3. Factors influencing the coef
Page 6 and 7: 6 BASIC NOTATION c - cohesion [kPa]
Page 8 and 9: 8 Numerous granular materials when
Page 10 and 11: 10 the tensor of plastic strain inc
Page 12 and 13: 12 Fig. 2.1. Yield curves with comp
Page 14 and 15: 14 Stable or unstable behaviour of
Page 16 and 17: 16 In turn, the model of Lade, also
Page 18 and 19: 18 same porosity need not have iden
Page 20 and 21: 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 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 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.:
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106 69. Horabik J., Rusinek R.: Pre
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108 114. Morland L.W., Sawicki A.,
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110 158. 6WSQLHZVNL$ Influence of m
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112 16. APPENDIX - Physical Propert
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114 16.2. Density and porosity Tabl
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116 Table 16.7. 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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