Mechanics of Agricultural Materials

488 Pages·1986·11.7 MB·English

by  GYÖRGY SITKEI (Eds.)

Developments in Agricultural Engineering 8
Mechanics
of Agricultural Materials
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by J. Shejbal (Editor)
1980 viii-f 608 pp.
2. Land and Stream Salinity
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4. Grain Handling and Storage
by G. Boumans
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5. Controlled Atmosphere and Fumigation in Grain Storages
by B.E. Ripp et al. (Editors)
1984 xiv-f-798 pp.
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by A. Maton, J. Daelemans and J. Lambrecht
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by E. McKyes
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Developments in Agricultural Engineering 8
Mechanics
of
Agricultural
Materials
GYÖRGY SITKEI
Department of Woodworking Machines, University of Forestry and Wood Science
Sopron, Hungary
ELSEVIER
Amsterdam · Oxford . New York . Tokyo 1986
This is the revised English version of
„A mezögazdasagi anyagok mechanikaja"
published by Akadémiai Kiado, Budapest
English translation by
S. Bars
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Library of Congress Cataloging-in-Publication Data
Sitkei, György.
Mechanics of agricultural materials.
(Developments in agricultural engineering; 8)
Translation of: A mezögazdasagi anyagok mechanikâja.
Bibliography: p.
Includes index.
1. Agricultural engineering. 2. Materials.
3. Agricultural physics. I. Title. II. Series.
S727.S6513 1986 631.3 85-29292
ISBN 0-444-99523-4
ISBN 0-444-99523-4 (Vol. 8)
ISBN 0-444-41940-3 (Series)
© Akadémiai Kiado, Budapest 1986
Joint edition published by Elsevier Science Publishers, Amsterdam, The Netherlands and
Akadémiai Kiado, Budapest, Hungary
Printed in Hungary
CONTENTS
Preface 9
1. The development and importance of agricultural mechanics 11
2. Physical properties of agricultural materials 13
2.1 Shape and size 13
2.2 Surface area 23
2.3 Volume and density 26
2.4 Thermal and hygroscopic expansion 29
3. Mechanical properties 32
4. Thermal properties 34
4.1 Specific heat 34
4.2 Heat-conduction coefficient 35
4.3 Temperature conductivity 40
5. Electrical properties 41
5.1 Dielectric constant and dielectric loss 42
6. Optical properties 48
6.1 Reflectance properties 49
6.2 Transmittance properties 50
7. Water storage in agricultural materials 56
7.1 Physics of water storage 56
7.2 Adsorption and desorption of water 62
7.3 Equilibrium moisture content 64
7.4 Moisture adsorption by hygroscopic materials 67
7.5 Internal moisture movement 69
7.6 Mass-transfer at the surface 70
7.7 Mass-transfer coefficients 72
7.8 Moisture gradients 76
7.9 Contact moisture exchange 78
7.10 The theory of drying 80
7.11 General relationships in the drying process 85
7.12 Heating and cooling of deep piles 97
7.13 Heat production in biological materials during storage 104
7.14 Moisture exchange of fruits and vegetables with the air 109
6 CONTENTS
4 1 1
8. The background of rheology
8.1 Characteristics of biological materials 114
8.2 Ideal materials and their properties 118
8.3 Time-dependent behavior of materials; viscoelasticity 121
22
8.4 Creep I
23
8.5 Recovery I
2
8.6 Relaxation *4
25
8.7 Linearity I
25
8.8 Rheological models I
2
8.9 Rheological equations I ?
8.10 Integral representation of viscoelastic constitutive equations 137
8.11 Behavior of viscoelastic materials under oscillating loads 139
8.12 Nonlinear constitutive equations 143
8.13 Temperature effects 144
8.14 Non-Newtonian fluids; viscosimetry 145
8.14.1 Characteristic flow curves 145
8.14.2 Viscosimetry 149
53
9. Contact stresses I
9.1 Contact stress in elastic bodies 153
9.2 Contact stress in viscoelastic bodies 156
9.3 The theory of the rigid die; the Boussinesq problem 159
10. Impact loading 166
10.1 Impact of elastic bodies 166
10.2 Impact of viscoelastic bodies 169
10.3 Application of cushioning materials 177
11. The finite-element method 186
11.1 Concept of the method 186
11.2 Shapes of elements and the displacement function 187
11.3 Embedding the elements into the continuum 194
11.4 Finite-element formulation 196
11.5 Viscoelastic stress analysis; numerical method 201
11.6 Application of the finite-element method to flow fields 205
12. Application of rheology 208
12.1 Force-deformation relationship 208
12.2 Stress-strain relationships 216
12.2.1 Uniaxial compression 216
12.2.2 Uniaxial tension 218
12.2.3 Shear 220
12.2.4 Bending 222
12.2.5 Elastic-plastic behavior of agricultural materials 227
12.2.6 Hydrostatic compression 228
12.2.7 Viscoelastic properties 230
12.2.8 Dynamic testing 240
13. Mechanical damage 260
13.1 Causes and the forms of appearance of damage 260
CONTENTS 7
13.2 Biological and chemical reactions after damage 262
13.3 Establishing and measuring damage 263
13.4 Effect of moisture changes on damage 265
13.5 Impact damage 268
13.6 Effects of various parameters on sensitivity to damage 276
14. Aerodynamic and hydrodynamic properties and phenomena 284
14.1 Aerodynamic drag coefficient 284
14.2 Terminal velocity 285
14.3 Aerodynamic resistance of granular bulk materials 288
14.4 Separation of foreign materials 298
14.5 Pneumatic conveying of agricultural materials 302
14.5.1 State diagram for pneumatic transport 303
14.5.2 Characteristic numbers and similarity laws 305
14.5.3 Distributions of air velocity and material in transport tubes 306
14.5.4 Pressure loss in transport tubes 309
14.5.5 Limiting velocity for pneumatic transport 313
14.5.6 Unsteady states of motion 313
14.5.7 Pneumatic transport of forage materials 317
14.6 Fluid-bed conveying 319
14.7 Conveying by throwing 322
14.8 Pneumatic conveying of non-Newtonian materials 326
14.9 Flow in perforated ducts 328
14.10 Ventilation of bales and stacks 333
14.11 Non-Newtonian flow in tubes 337
14.12 Air resistance of fruit-tree crowns 345
14.13 Hydrodynamic properties 347
15. Friction problems 352
15.1 General laws of friction 352
15.2 Friction coefficients of agricultural products 354
15.3 Rolling of agricultural products 361
15.4 Angle of internal friction and angle of natural repose 362
15.5 State diagram for granular bulk materials 367
15.6 Stress state of granular bulk materials 369
15.7 Pressure distribution in bins 372
15.7.1 Lateral pressure coefficient 372
15.7.2 Calculation for flat bins 377
15.7.3 Calculation for high bins 378
15.8 Flow of granular materials from an orifice 383
15.9 Flow of granular materials in chutes 388
15.10 Further friction problems 392
16. Wafering and pressing of agricultural materials 403
16.1 General relationships for pressing processes 403
16.2 Energy requirements of pressing 412
16.3 Rebound of material after pressing 417
16.4 Pressure distribution in the space before a compressing piston 420
16.5 Pressure conditions in pelleting machines 427
8 CONTENTS
16.6 Effects of various parameters on the pelleting process 432
16.7 Mechanical dewatering of agricultural materials 434
17. Cutting of agricultural materials 439
17.1 Cutting methods 439
17.2 Deformations caused by cutting 442
17.3 Energy requirements of cutting 445
17.4 Free cutting 450
17.5 Energy requirements of forage harvesters 455
18. Grinding (comminution) of agricultural materials 458
18.1 Mechanism of comminution 459
18.2 General relationships for hammer mills 461
18.3 Size distribution of comminuted products 464
18.4 Energy requirements of hammer mills 468
18.5 Closed-circuit grinding 471
18.6 Grinding of forage materials 472
References 475
Subject index 485
PREFACE
The importance of economical production of agricultural materials, especially
crops and animal producst serving as base materiasl for foodstuffs, and of their
technological processing (mechanical operations, storage, handling, etc.) is ever-
increasing. During technological processes agriculturla materiasl may be exposed
to various mechanica,l therma,l electrical, optical and acoustical (e.g. ultrasonic)
effects. To ensure optimal design of such processes, the interactions between
biological materiasl and the physical effects, acting on them, as well as the gen-
eral laws governing the same, must be known.
The behavior of most agriculturla materiasl deviates essentially from that of
the generally known elastic materials. The flow properties of cereals, of granular
materiasl and of those materiasl which fall into the category of non-Newtonian
liquids, also deviate essentially from the corresponding ideal liquid behavior.
The work of agricultural engineers has been aggravated by these circumstance,s
and this is the main reason why agriculturla engineering has relied for a long
time on empirical data.
The mechanics of agricultural materials, as a scientific discipline, is still being
developed at present, and in many cases has no exact methods as yet. However,
the methods developed so far can already be utilized successfully for designing
and optimizing machines and technological processes.
The present work is the first attempt to summarize the calculation methods
developed in the main fields of agricultural mechanics, and to indicate the material
laws involved, on the basis of a unified approach, with all relevant physicomechan-
ical properties taken into account.
The author expresses sincere gratitude to his co-workers at the Agricultural
College of Kφnnend, who have contributed through important results of their
own research work to the general development of the subject. Special thanks
are due to Professor J. Janik, Director, Dr. I. Bajsz, Professor of Agricultural
Mechanics, and Dr. J. Fehιr, Senior Assistant Lecturer.
I express my thanks also to Professor J. Galambos, who assisted by giving
valuable counsel during review of the manuscrip,t and concerning the final con-
struction of the book.
G. Sitkei