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Agricultural Nets Propr
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POLYMER TESTING Polymer Testing 26 (2007) 970–984 Product Performance Mechanical behaviour and properties of agricultural nets. Part II: Analysis of the performance of the main categories of agricultural nets D. Briassoulis , A. Mistriotis, D. Eleftherakis Department of Agricultural Engineering, Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece Received 12 April 2007; accepted 15 June 2007 Abstract The use of HDPE nets in agriculture has been constantly growing over the last few years. They are used for protecting sensitive crops against hail, insect, birds, wind and excessive solar radiation. In the first part of this work, three new test methods have been introduced, evaluated and proposed specifically for agricultural nets in order to support the quality assessment of the large variety of nets provided by the manufacturers to the farmers. In the present work, 35 commercial agricultural nets belonging to four different main agricultural application categories are investigated by employing the testing methods proposed in part I and their overall mechanical behaviour is analysed systematically. r 2007 Elsevier Ltd. All rights reserved. Keywords: Agricultural nets; Testing methods; Tensile properties; Impact resistance; Mesh breaking; Ageing 1. Introduction Using plastic nets instead of plastic films for protecting crops is an interesting technique which has constantly expanded in many agricultural applications during the last decade as it offers many advantages [1]. A large number of agricultural nets of various fabric types, mesh sizes, yarn types, and colours with a wide range of mechanical and physical properties are currently available in the market [1]. Permeable covers are extensively used in certain types of cultivation such as fruit-tree farming, for bird and insect protection as well as for protection against adverse climatic actions. In particular, nets are used to protect cultivations from damages due to hail, strong winds, rain, insects and birds, and excessive solar radiation. In this respect, agricultural nets support the production of higher quality, lower input products using lower levels of agrochemicals and, in some cases, reducing or eliminating the need for energy consumption. Contrary to the intensive greenhouse production techniques, the use of agricultural nets mostly focuses on the reduction of the use of pesticides or energy consumption, and the production of higher- quality products by mildly moderating the micro- climate under the cover. Thus, nets are used to enhance productivity, quality and homogeneity of plants and fruits by generating moderate green- house and windbreak effects, thus allowing an improved microclimate for the crops. ARTICLE IN PRESS www.elsevier.com/locate/polytest 0142-9418/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2007.06.010 Corresponding author. Tel.: +30 01 5294011; fax: +30 01 5294023. E-mail address: [email protected] (D. Briassoulis).
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Transcript of Agricultural Nets Propr
POLYMERTESTING
Polymer Testing 26 (2007) 970–984
Product Performance
Mechanical behaviour and properties of agricultural nets.Part II: Analysis of the performance of the main categories
of agricultural nets
D. Briassoulis�, A. Mistriotis, D. Eleftherakis
Department of Agricultural Engineering, Agricultural University of Athens, Iera Odos 75, 11855 Athens, Greece
Received 12 April 2007; accepted 15 June 2007
Abstract
The use of HDPE nets in agriculture has been constantly growing over the last few years. They are used for protecting
sensitive crops against hail, insect, birds, wind and excessive solar radiation. In the first part of this work, three new test
methods have been introduced, evaluated and proposed specifically for agricultural nets in order to support the quality
assessment of the large variety of nets provided by the manufacturers to the farmers. In the present work, 35 commercial
agricultural nets belonging to four different main agricultural application categories are investigated by employing the
testing methods proposed in part I and their overall mechanical behaviour is analysed systematically.
r 2007 Elsevier Ltd. All rights reserved.
Keywords: Agricultural nets; Testing methods; Tensile properties; Impact resistance; Mesh breaking; Ageing
1. Introduction
Using plastic nets instead of plastic films forprotecting crops is an interesting technique whichhas constantly expanded in many agriculturalapplications during the last decade as it offers manyadvantages [1]. A large number of agricultural netsof various fabric types, mesh sizes, yarn types, andcolours with a wide range of mechanical andphysical properties are currently available in themarket [1].
Permeable covers are extensively used in certaintypes of cultivation such as fruit-tree farming, forbird and insect protection as well as for protection
against adverse climatic actions. In particular, netsare used to protect cultivations from damages dueto hail, strong winds, rain, insects and birds, andexcessive solar radiation. In this respect, agriculturalnets support the production of higher quality, lowerinput products using lower levels of agrochemicalsand, in some cases, reducing or eliminating the needfor energy consumption.
Contrary to the intensive greenhouse productiontechniques, the use of agricultural nets mostlyfocuses on the reduction of the use of pesticides orenergy consumption, and the production of higher-quality products by mildly moderating the micro-climate under the cover. Thus, nets are used toenhance productivity, quality and homogeneity ofplants and fruits by generating moderate green-house and windbreak effects, thus allowing animproved microclimate for the crops.
ARTICLE IN PRESS
www.elsevier.com/locate/polytest
0142-9418/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymertesting.2007.06.010
�Corresponding author. Tel.: +30 01 5294011;
fax: +3001 5294023.
E-mail address: [email protected] (D. Briassoulis).
The design, mechanical and physical properties ofagricultural nets were systematically investigated inthe framework of a research project carried out at aEuropean level1 to support the design of newoptimised net-supporting structural systems, fullyexploiting the advantages offered by these interest-ing novel materials.
Proper evaluation schemes for the most criticalmechanical properties of agricultural nets, directlyrelated to the operational needs of specific agricul-tural applications, have been established in part I ofthis work [2]. Three main test methods weredeveloped or adapted and proposed for characteris-ing agricultural nets with respect to tensile proper-ties, impact resistance and mesh breaking resistance.For the tensile properties, a method based on ISO10319:1993 standard test method for geotextiles,was introduced and evaluated. Impact resistancewas found to be properly assessed using ASTM D1709-01: Standard Test Methods for Impact Re-sistance of Plastic Film by the Free–Falling DartMethod—Method B. Concerning mesh breakingresistance, a method based on the ISO 1806Standard for fishing nets was adapted, evaluatedand proposed.
In this part II paper, the most critical propertiesof a large number of typical agricultural netsbelonging to four different main agricultural appli-cation categories are investigated by employing thetesting methods proposed in part I. The overallmechanical behaviour of these nets is analysedsystematically, offering in this way valuable designcriteria for the selection of agricultural nets.
2. Classification of agricultural nets
A large number of agricultural nets are currentlyavailable and promoted in the market world wide.These nets are usually grouped into a few discretecategories and are used in the correspondingapplications. In the present section, the agriculturalnets under consideration are classified according totheir application in four main categories, namelywindbreak, anti-insect, anti-hail, and shading nets.
The oldest applications of the nets in protectedcultivation were in fruit farming, such as grapes,peaches, apricots, apples and cherries, and forornamentals, such as cut flowers, in order to protectplants and fruits from hail and wind. These nets are
generally manufactured with transparent threadsand appear white with an average net mesh sizeranging approximately from 1.5 to 4mm.
Nets used only for wind protection in the form ofwindbreaks are usually dark colour and the averagenet mesh size ranges from 1 to 3mm.
Black or green shading nets are used over orunder the greenhouse cladding material during thewarmer periods, mainly in Mediterranean countries,in order to reduce the higher air temperatures insidethe greenhouse. Shading nets are also used ascovering material of special structures, the so-calledshade houses, protecting plants from excessive solarradiation and high temperatures. The mesh size ofthese nets is usually in the range of 0.6–4mm withlight transmittance varying from 20% to 70%.
Biological nets are generally used for the protec-tion of plants, mainly in greenhouses: these aredensely meshed anti-virus-vector insect nets calledbio-nets. The net mesh size in this case depends onthe insect species against which the net is designedto protect crops, and is approximately 0.5mm. Thethreads are transparent.
The mechanical properties of nets depend on thecorresponding properties of the fibres and also theweave type. The mechanical properties of the fibresare more or less known for specific materials.
Plastic nets are also characterised by theirstructural characteristics such as the kind anddimensions of fibres, fabrics and meshing. HDPEyarns are of two types: monofilaments, round orflat, and tapes [1].
Depending on the kind of weave, the fabrics aredistinguished into three categories: Flat or Italian,English or Leno, and Raschel [1]. The first two typesare woven, while the third one is knitted. Flat wovenis characterised by a simple orthogonal weavebetween weft and warp. Flat woven nets are lightand stable in their shape, but they are relatively stiffand do not allow large deformations [1]. Englishwoven is a modified flat woven net with a doublefibre in the weft direction, enclosing the warp fibrein between [1]. The resulting net is not highlydeformable and is used when the covering shouldprovide higher stiffness. Raschel looms produce netswith longitudinal ‘‘chains’’ and transversal knittedelements [1]. In Raschel membranes all threads arelinked with each other and they cannot unravel,either under the action of strong wind or hail.
In the following sections, where the mechanicalbehaviour of many characteristic types of agricul-tural nets is analysed systematically, nets are
ARTICLE IN PRESS
1‘Development of protective structures covered with permeable
materials for agricultural use,’ SME-2003-1-507865 (DG 12).
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984 971
classified with respect to their application in the fourmain categories of windbreak, anti-insect, anti-hailand shading nets. Furthermore, the influence of thetype of fibre and fabric on their mechanicalproperties is also investigated.
3. Behaviour of agricultural nets in tension
A large number of agricultural nets, used invarious agricultural applications were tested accord-ing to the revised/adapted procedure of ISO 10319:1993 [2,3] (35 typical agricultural nets were selectedfor testing of their mechanical and physical proper-ties). The tensile tests were carried out by using anINSTRON testing machine Model 4204 at thefacilities of the laboratory of Farm Structures(AUA, Athens) [2].
The tests were conducted in both the weft and thewarp directions. Five samples were tested in eachdirection after pre-conditioning at 23 1C and 50%humidity for 48h. The typical tensile behaviour ofselected characteristic cases of agricultural nets isillustrated below for various categories of application.
3.1. Windbreak nets
The windbreak nets are mostly woven nets(normal or Leno). The behaviour of the windbreaknet SCMD shown in Fig. 1 illustrates the typical
behaviour of the woven windbreak nets in tension(top in warp and bottom in weft direction). Thisbehaviour, also presented in Fig. 2 in terms of theobtained stress–strain curves, suggests a ratherbrittle material behaviour with a significant part ofit being in the linear elastic range in both directions.The net is stronger in the warp direction and hassimilar elongation at break in both directions.
3.2. Anti-insect nets
The anti-insect nets are also mostly woven nets(normal or Leno), with a few exceptions. Thebehaviour of the anti-insect net BIO40 shown inFig. 3 illustrates a typical behaviour of the wovenanti-insect nets in tension (top in warp and bottomin weft direction). The stress–strain curves presentedin Fig. 4 suggest a similar form to the windbreak netcurves, brittle material behaviour with a significantpart of it being in the linear elastic range. The nethas the same strength in both directions but higherelongation at break in the weft direction.
Non-typical tensile behaviour has been foundwith the anti-insect net IMPOL, shown in Fig. 5(top in warp and bottom in the weft direction). Thisnet is not, however, a woven but it is a knitted net,exhibiting a significant lateral contraction due to itsspecial knitting fabrication. The stress–strain curvespresented in Fig. 6 suggest a linear–nonlinear elastic
ARTICLE IN PRESS
Fig. 1. Tensile test of a sample of the windbreak net SCMD (top in warp and bottom in the weft direction).
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984972
ARTICLE IN PRESS
Fig. 3. Tensile test of a sample of the anti-insect net: BIO40 (top in warp and bottom in the weft direction).
0
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Load (
N)
Specimen 1 to 5
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0 20 40 50
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ad
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)
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Extension (mm)
40 50 60 10
Extension (mm)
30
1200
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400
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-200
Specimen #
12345
Specimen #
12345
Fig. 2. Tensile stress–strain curves of the windbreak net SCMD (left in warp and right in the weft direction).
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0 20 30 40 60 70 80
Lo
ad
(N
)
-200
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N)
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10 20 30 40 60 90 100
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Specimen 1 to 5 Specimen 1 to 5
Specimen #12345
Specimen #12345
Fig. 4. Tensile stress–strain curves of the anti-insect net: BIO40 (left in warp and right in the weft direction).
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984 973
behaviour of a brittle material in the warp directionwith higher strength and a clear linear elasticbehaviour in the weft direction.
3.3. Anti-hail nets
The anti-hail nets are mostly woven nets (normalor Leno). The behaviour observed with the anti-hailnet FRU44 in tension, shown in Fig. 7 (top in warpand bottom in the weft direction), is characteristicof a typical woven anti-hail net. The stress–strain
curves presented in Fig. 8 suggest a clear linearelastic behaviour of a brittle type in the warpdirection and linear–nonlinear elastic in the weftdirection with higher strength.
3.4. Shading nets
The shading nets are usually knitted nets(Raschel). The behaviour observed with the knittedshading net OMBR70, in tension is shown in Fig. 9(top in warp and bottom in the weft direction).
ARTICLE IN PRESS
Fig. 5. Tensile test of a sample of the anti-insect net IMPOL (top in warp and bottom in the weft direction).
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0 20 50 60
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ad
(N
)
0
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0 30 50
Lo
ad
(N
)
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-10010
Extension (mm)
30 40
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100
10
Extension (mm)
20 40 60 70
Specimen 1 to 5 Specimen 1 to 5
Specimen #12345
Specimen #12345
Fig. 6. Tensile stress–strain curves of the anti-insect net IMPOL (left in warp and right in the weft direction).
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984974
This is a typical behaviour for shading nets, relatedto the knitted fabrication type, as most of these netsexhibit significant lateral contraction either in bothdirections or mainly in the weft direction. Thestress–strain curves presented in Fig. 10 give adifferent picture as compared to that of the anti-hailnets, with a clear linear elastic behaviour of a brittlenature in the weft direction and linear–nonlinearelastic in the warp direction. The net is stronger inthe warp direction.
3.5. Overall behaviour of agricultural nets in tensile
testing
The overall mechanical behaviour of 35 differenttypes of agricultural nets belonging to four differentapplication categories, investigated in the labora-tory in tension, suggests that:
Shading and anti-hail nets appear to be ratherflexible with the modulus of elasticity varying inthe range of 2–30N/mm (with one anti-hail
ARTICLE IN PRESS
Fig. 7. Tensile test of a sample of the anti-hail net FRU44, (top in warp and bottom in the weft direction).
0
0 10
Lo
ad
(N
)
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0 30 40
Lo
ad
(N
)
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Extension (mm)
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Extension (mm)
20
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-100
Specimen 1 to 5 Specimen 1 to 5
Specimen #
12345
Specimen #
12345
Fig. 8. Tensile stress–strain curves of the anti-hail net FRU44, (left in warp and right in the weft direction).
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984 975
net exception in the weft direction of 41N/mm)(Figs. 11 and 12). Windbreak and anti-insect nets onthe other hand, exhibit a relatively stiffer behaviourwith higher modulus of elasticity values, rangingfrom 47 to 120N/mm (with three exceptions). Inmost cases, a drastic differentiation is observed inthe elastic modulus measured in the warp and theweft directions, within the broader characterisationof the net as flexible or stiff.
In terms of fabrication, it appears that the vastmajority of the woven nets exhibit higher moduli ofelasticity than the knitted nets (Figs. 13 and 14).
Knitted nets (e.g., shading nets and some cases ofknitted nets of other categories like a couple of lowmodulus anti-insect nets IMPOL and REMA) maybe considered to be more flexible in general (with afew exceptions; all woven anti-hail nets exhibitmodulus of elasticity values closer to those of theknitted nets).
In terms of tensile strength in the warp direction,windbreak nets are the strongest (10–16N/mm) whilethe anti-hail nets are the weakest (3.5–7.0N/mm,Fig. 15). The shading and the anti-insect nets exhibita mixed picture as these categories include both weak
ARTICLE IN PRESS
Fig. 9. Tensile test of a sample of the shading net OMBR70, (top in warp and bottom in the weft direction).
0
0 10
Lo
ad
(N
)
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0 20 40 50 60
Lo
ad
(N
)
Specimen 1 to 5 Specimen 1 to 6
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Extension (mm)
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30 70 8010
Specimen #
12345
Specimen #
12456
Fig. 10. Tensile stress–strain curves of the shading net OMBR70, (left in warp and right in the weft direction).
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984976
ARTICLE IN PRESS
10.000
20.000
30.000
40.000
50.000
60.000
70.000
OMBR50
OMBR70
OMBR90
HEX
LIBC30
LIBC50
LIBC60
LIBC70
AGR30
AGR50
AGREWSHNB
SHNT
FRU26
FRU44IR
ID
ANTH
GRKR
BIO40
BIO50
IMPOL
INSK
INSM
INST
REMA
SC50
SCMDW
INTBR50
SC75
Ela
sti
c M
od
uls
- w
arp
(N
/mm
)
shading
80.000
0.000
windbreakanti-insectanti-hail
Agricultural nets
Fig. 11. Modulus of elasticity of agricultural nets in the warp direction.
20.000
40.000
60.000
80.000
100.000
120.000
140.000
OMBR50
OMBR70
OMBR90
HEX
LIBC30
LIBC50
LIBC60
LIBC70
AGR30
AGR50
AGREWSHNB
SHNT
FRU26
FRU44IR
ID
ANTH
GRKR
BIO40
BIO50
IMPOL
INSK
INSM
INST
REMASC50
SCMDW
INTBR50
SC75
Ela
sti
c M
od
uls
- w
eft
(N
/mm
)
0.000
Agricultural nets
shading anti-hail anti-insect windbreak
Fig. 12. Modulus of elasticity of agricultural nets in the weft direction.
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984 977
ARTICLE IN PRESS
10.000
20.000
30.000
40.000
50.000
60.000
70.000
80.000
FRU26
FRU44IR
ID
GRKR
BIO40
BIO50
INSK
INSM
INST
SC50
SCMD
BR50SC75
LIBC30
LIBC50
LIBC60
LIBC70
OMBR50
OMBR70
OMBR90
HEX
AGR30
AGR50
AGREWSHNB
SHNT
ANTH
IMPOL
REMA
WIN
T
Ela
sti
c M
od
uls
- w
arp
(N
/mm
)
0.000
Agricultural nets
wovenknitted
Fig. 13. Modulus of elasticity of agricultural nets in the warp direction based on the type of fabrication.
0.000
20.000
40.000
60.000
80.000
100.000
120.000
140.000
FRU26
FRU44IR
ID
GRKR
BIO40
BIO50
INSK
INSM
INST
SC50
SCMD
BR50SC75
LIBC30
LIBC50
LIBC60
LIBC70
OMBR50
OMBR70
OMBR90
HEX
AGR30
AGR50
AGREWSHNB
SHNT
ANTH
IMPOL
REMAW
INT
Ela
sti
c M
od
uls
- w
eft
(N
/mm
)
knitted
Agricultural nets
Woven
Fig. 14. Modulus of elasticity of agricultural nets in the weft direction based on the type of fabrication.
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984978
nets (3.5–5.8N/mm) as well very strong nets(8.0–26.0N/mm). In the weft direction, the tensilestrength of the nets appears to be analogous to theone observed in the warp direction except for six netsfrom the shading nets category that exhibit weakbehaviour in the weft direction (2.5–5.5N/mm),while they are rather strong in the warp direction(8.0–12.0N/mm) (Fig. 16).
In terms of fabrication, most woven nets exhibithigher strength than the corresponding knitted nets.This differentiation is more pronounced in the weftdirection (Figs. 17 and 18). There are exceptionsthough with some woven nets, mainly the anti-hailnets, appearing to be as weak as the average knittednets in the weft direction or as weak as some of theweakest knitted nets in the warp direction. Likewise,some of the strongest knitted shading nets (e.g.,LIBC70) appear to be as strong as the middlestrength woven nets.
The mechanical properties of the agricultural netsmeasured in the laboratory may also be used todefine the orthotropic properties of the equivalentorthotropic membranes simulating the mechanicalbehaviour of the nets in numerical finite elementmodelling, according to the analysis of [2]. Thedefinition of the orthotropic properties of the nets
requires, apart from the determination of themoduli of elasticity (Figs. 11 and 12), also thedetermination of the Poisson’s ratio n.
In terms of fabrication, it is interesting to notethat the Poisson’s ratio n ¼ 0 is applicable to allwoven nets (normal or Leno), which represent themajority of the windbreak, anti-hail and anti-insectnets. The only knitted windbreak net (WINT) alsohas n ¼ 0.0, and so all windbreak nets have a zeroPoisson’s ratio, while one knitted anti-hail net(ANTH) has a Poisson’s ratio n ¼ 0.0 (it shouldbe noted though that this is a tape type of net). Onthe other hand, the majority of the shading netsexhibit high Poisson’s ratios. In particular, allshading nets are knitted (Raschel) nets withPoisson’s ratio n ¼ 0.5 (a few of them) or withanisotropic Poisson’s ratio with lower value in thewarp direction (the majority of them). There is onlyone exception with the shading net (HEX) withPosison’s ratio n ¼ 0.0.
3.6. Ageing effects
The durability of the nets, evaluated in terms oftheir ageing behaviour, may affect the long-termcapacity of the covering material to carry efficiently
ARTICLE IN PRESS
15.000
20.000
25.000
30.000
OMBR50
OMBR70
OMBR90
HEX
LIBC30
LIBC50
LIBC60
LIBC70
AGR30
AGR50
AGREWSHNB
SHNT
FRU26
FRU44IR
ID
ANTH
GRKR
BIO40
BIO50
IMPOL
INSK
INSM
INST
REMA
SC50
SCMDW
INT
BR50SC75
Str
en
gth
- w
arp
(N
/mm
)
0.000
5.000
10.000
Agricultural nets
shading anti-hail anti-insect windbreak
Fig. 15. Tensile strength of agricultural nets in the warp direction.
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984 979
ARTICLE IN PRESS
0.000
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15.000
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FRU26
FRU44IR
ID
GRKR
BIO40
BIO50
INSK
INSM
INST
SC50
SCMDBR50
SC75
LIBC30
LIBC50
LIBC60
LIBC70
OMBR50
OMBR70
OMBR90
HEX
AGR30
AGR50
AGREWSHNB
SHNT
ANTH
IMPOL
REMAW
INT
Agricultural nets
Str
en
gth
- w
arp
(N
/mm
)
woven knitted
Fig. 17. Tensile strength of agricultural nets in the warp direction based on the type of fabrication.
15.000
20.000
25.000
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OMBR50
OMBR70
OMBR90
HEX
LIBC30
LIBC50
LIBC60
LIBC70
AGR30
AGR50
AGREWSHNB
SHNT
FRU26
FRU44IR
ID
ANTH
GRKR
BIO40
BIO50
IMPOL
INSK
INSM
INST
REMA
SC50
SCMDW
INT
BR50SC75
Str
en
gth
- w
eft
(N
/mm
)
anti-insect
0.000
5.000
10.000
Agricultural nets
shading anti-hail windbreak
Fig. 16. Tensile strength of agricultural nets in the weft direction.
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984980
the loads and transfer them to the supportingstructure. This is of major importance with respectto the design and function of the net-structuralsupporting systems [4]. A characteristic example ofthe effect of weathering on the tensile strengthand elongation at break of a typical agriculturalnet (coded as K) exposed for a period of 10 yearsunder north European conditions (The Nether-lands) is shown in Fig. 19. It is apparent that themost critical property affected by ageing in thiscase is the tensile strength, which drops down toone-third of its initial value after 10 years ofexposure. It should be expected that under southEuropean climatic conditions with much higherUV radiation the weathering effects would bemore dramatic. As a result, it is considered veryimportant in the design process [4] to estimate/predict the expected duration of life of thesematerials to avoid premature failure of the agricul-tural production.
4. Impact resistance of agricultural nets
As was discussed in [2], impact resistance ofagricultural nets can be tested by ASTM D 1709-01:
Standard Test Methods for Impact Resistance ofPlastic Film by the Free-Falling Dart Method—Method B [5]. The impact resistance behaviour of acomplete set of 35 different commercial nets wasobtained using this method. These tests wereevaluated and verified in terms of consistency and
ARTICLE IN PRESS
0
2
4
6
8
10
12
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0 2 4 6 8 10 12
Years of exposure
Ten
sile s
tren
gth
(M
Pa)
0
5
10
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20
25
30
35
Elo
ng
ati
on
at
bre
ak (
%)
Strength
Elongation at break (%)
Fig. 19. Effect of ageing on the mechanical properties of a typical
agricultural net under conditions of The Netherlands.
0.000
5.000
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35.000
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FRU26
FRU44IR
ID
GRKR
BIO40
BIO50
INSK
INSM
INST
SC50
SCMDBR50
SC75
LIBC30
LIBC50
LIBC60
LIBC70
OMBR50
OMBR70
OMBR90
HEX
AGR30
AGR50
AGREWSHNB
SHNT
ANTH
IMPOL
REMAW
INT
Agricultural nets
Str
en
gth
- w
eft
(N
/mm
)
knittedwoven
Fig. 18. Tensile strength of agricultural nets in the weft direction based on the type of fabrication.
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984 981
repeatability. The impact resistance of all tested netsis presented in Table 1.
These results confirm that anti-hail nets (all, butone, are woven nets) behave well, with their impactresistance varying within a narrow range (76–113 J),and satisfy the needs for which they are designed(i.e., they resist equivalent hailstones with diametersin the range of 55–60mm). Using other types of netsis shown to also cover the anti-hail needs to an evengreater degree than the corresponding anti-hail nets(most windbreak, anti-insect and several shadingnets were found to resist equivalent hailstones witha diameter X75mm). In fact, only a couple ofknitted shading nets (AGR50, AGREW) and one
knitted anti-insect net (REMA) appear to be weakerthan the anti-hail nets with respect to impactresistance (equivalent hailstones with a diameter of47–48mm). Apart from the fact that the twoweakest nets in terms of impact resistance are bothknitted nets, no systematic correlation of hailresistance with the type of fabrication (woven orknitted) has been observed.
5. Mesh breaking load of netting
In part I of this work [2] a testing method forestimating the mesh breaking resistance of nets wasproposed based on the ISO 1806 Standard for fishing
ARTICLE IN PRESS
Table 1
Impact resistance of agricultural nets
Mass of dart (g) Equivalent hailstone
diameter (mm)
Equivalent hailstone
energy (J)
Comments
Shading nets
OMBR50 444 52 39
OMBR70 X1568 X75 X166 The samples do not break
OMBR90 X1568 X75 X166 The samples do not break
HEX 1324 71 136
LIBC30 717 60 68
LIBC50 X1568 X75 X166 The samples do not break
LIBC60 X1568 X75 X166 The samples do not break
LIBC70 X1568 X75 X166 The samples do not break
AGR30 418 51 37
AGR50 510 48 29 Method A
AGREW 452 47 25 Method A
SHNB 852 63 82
SHNT ?
Anti-hail nets
FRU26 584 56 54
FRU44 826 62 80
IRID 800 62 77
ANTH 634 58 59
GRKR 700 59 66
Anti-insect nets
BIO40 X1568 X75 X166 Cracks along the circular grip
BIO50 X1568 X75 X166 Cracks along the circular grip
IMPOL 593 57 54
INSK X1568 X75 X166 Cracks along the circular grip
INSM X1568 X75 X166 Cracks along the circular grip
INST X1568 X75 X166 The samples do not break
REMA 356 49 30
Windbreak nets
SC50 X1568 X75 X166 Cracks along the circular grip
SC75 X1568 X75 X166 Cracks along the circular grip
SCMD X1568 X75 X166 Cracks along the circular grip
WINT X1568 X75 X166 Cracks along the circular grip
BR 50 X1568 X75 X166 Cracks along the circular grip
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984982
nets [6]. The use of free (non-constrained) specimenswas selected as a more suitable test (as comparedto testing constrained specimens) for simulatingfailures during installation or deployment/removalof agricultural nets. The measured maximum load tofail the specimen was used as a measure of the mesh-breaking strength. The results obtained, shown inTable 2, suggest that mesh breaking resistancedepends on the mesh direction and the net type.
6. Conclusions
In the present work, the new testing methodsproposed in part I [2] have been applied to a broadvariety of agricultural nets. The important mechan-ical properties of a wide range of representativecommercially available agricultural plastic nets weredetermined by employing the proposed testingmethods. A general correlation between the typeof fabrication, the specific application and themechanical behaviour of the agricultural nets hasbeen established.
It may be concluded that: The windbreak, anti-insect and anti-hail nets are mostly woven nets(normal or Leno), with a few exceptions. Theshading nets are knitted nets (Raschel).
In terms of fabrication, it may be assumed thatthe Poisson’s ratio n ¼ 0 is applicable to the wovennets (normal or Leno), that represent the majorityof the windbreak, anti-hail and anti-insect nets (i.e.there is no lateral contraction associated with thetensioning of these nets), with a few exceptions. Ahigh value of Poisson’s ratio (a value of n ¼ 0.40;reflecting the fact that tensioning is associated withsignificant lateral contraction) may be assumed inthe cases of the knitted nets (mainly shading nets) ingeneral.
The vast majority of the woven nets exhibithigher moduli of elasticity than the knitted nets,which may be considered to be more flexible ingeneral (with a few exceptions).
Most woven nets exhibit also higher strength thanthe corresponding knitted nets. This differentiationis more pronounced in the weft direction. There are,however, several exceptions.
In terms of impact resistance, no systematiccorrelation of hail resistance with the type offabrication (woven or knitted) has been observed.The three weakest nets are, however, all knittednets.
The reported values of the mechanical propertiesof the tested nets describe the overall mechanicalperformance and the range of variability of the keymechanical properties of existing agricultural netswith respect to the corresponding applications andtype of fabrication. The test procedures described inthis work (Parts I and II) constitute a usefulguidance for the proper testing and reporting ofthe mechanical properties of agricultural nets. Thevalues obtained may be used as reference or typicalvalues by the relevant industry. Furthermore, theanalysed overall mechanical behaviour of these netsoffers valuable design criteria for the appropriateselection of agricultural nets.
Acknowledgements
This work has been carried out in the frameworkof the European Research Project: ‘Developmentof protective structures covered with permeablematerials for agricultural use,’ SME-2003-1-507865(DG 12), funded by the EU. Special acknowledge-ments are due to our partners Arrigoni (Italy) andHowitec (The Netherlands), for providing thetesting materials as well as their useful suggestionsand comments. Useful comments were also receivedby Instron (UK) and a number of testing laboratoriesin Athens, Greece (O.P.E., EUCAT, ELKEDE,KDEP). Also, thanks are due to Mr L.Xatzis andMs M.Psyhogiou (AUA) for their contribution tothe laboratory measurements.
References
[1] S. Castellano, G. Scarascia Mugnozza, G. Russo,
D. Briassoulis, A. Mistriotis, S. Hemming, D. Waaijenberg,
Plastic nets in agriculture: a general review of typologies and
applications, Biosystems Engineering, in print.
[2] D. Briassoulis, A. Mistriotis, D. Eleftherakis, Mechanical
behaviour and properties of agricultural nets—Part I: Testing
ARTICLE IN PRESS
Table 2
Mesh-breaking resistance of agricultural nets
Warp maximum
load (N)
Weft maximum
load (N)
Shading nets
OMBR 50 37 51
HEX 102
AGR30 55 47
AGR50 49 46
AGREW 39 66
SHNT 45 40
Anti-hail nets
Typical? anti-hail net 54 48
D. Briassoulis et al. / Polymer Testing 26 (2007) 970–984 983
methods for agricultural nets, Polymer Testing 26 (2007)
822–832.
[3] International Organization for Standardization, ISO 10319-
1993: Geotextiles—Wide Width Tensile Test, International
Organization for Standardization, 1993.
[4] D. Briassoulis, A. Mistriotis, Design methodology for protecting
agricultural structures covered with nets, under preparation.
[5] ASTM D 1709-01: Standard Test Methods for Impact
Resistance of Plastic Film by the Free-Falling Dart Method,
American Society for Testing and Materials, 2001.
[6] International Organization for Standardization, ISO 1806,
2002: Fishing Nets—Determination of mesh breaking load of
netting, International Organization for Standardization,
2002.
ARTICLE IN PRESSD. Briassoulis et al. / Polymer Testing 26 (2007) 970–984984
