Volume 14
Issue 2
Agricultural Engineering
JOURNAL OF
POLISH
AGRICULTURAL
UNIVERSITIES
Available Online: http://www.ejpau.media.pl/volume14/issue2/art13.html
DETERMINING THE SHEAR STRENGTH AND PICKING FORCE OF ROSE FLOWER
Adel Heidari^{1}, Gholam Reza Chegini^{2}
^{1} Department of Agrtechnology,
College of Aboureihan, University of Tehran, Iran
^{2} Department of Mechanical Engineering of Biosystems, College of Aboureihan, University of Tehran, Tehran, Iran
In this research the effect of bevel angle and shear velocity on shear strength and shearing energy of rose flower (Rosa hybrid L.) stem was studied by using direct shear test. Also, the effects of picking velocity, stretch direction and sample groups (near growth tip, near root internode) on force and energy required to pick up the leaves were studied. All experiments were carried out by Instron Universal Test Machine. The average values of shear strength and energy per unit were estimate 1.63 MPa and 5.16 mJ·mm^{2}, respectively. With increasing the shear velocity, from 10 to 500 mm·min^{1} the average values of shear strength and energy per unit area decreased. Picking force and energy data ranged from 4.5 to 12.2 N and from 8.6 to 16.9 mJ·mm^{2}, respectively. The effects of picking velocity on tensile strength and energy per unit area was significant (P = 1%). The effects of stem position and stretch direction on tensile strength and energy per unit area were significant.
Key words: rose flower, stem, mechanical properties, shear strength, picking force.
INTRODUCTION
Rose flower is one of the most important flowers commercially. Origin of hybrids is Asia Minor. Roses are dicot herbaceous often cultivated as a garden or potted annual flower. The required working hours to produce a rose are 3.770 to 5.240 h·ha^{1}, and cutting production accounts for 11.8% to 19.3% of this time. Therefore, mechanization of cutting production has been strongly desired with rose industries. The acceptability of roses in trading particularly for exporting to European countries depends on adequate post harvest longevity [8]. Cleaning flower stems by removing thorns and foliage without injuring the stem is one of essential post harvest process [16]. In order to design equipments for picking leaf and also other required processing, it is necessary to obtain the physical and mechanical properties of rose flower stem [18].
A review of the literature revealed no information on physical and mechanical properties of rose flower. However research have been conducted to find shear strength of other crops as factors which significantly affect them. Most studies on the mechanical properties of plants have been carried out during their growth using failure criteria (force, stress and energy) [2,12]. Simonton [15] studied physical properties, strength in bending, and strength in compression for Geranium. McRandal and McNulty [11] evaluated the shear strength of grass stems by quasistatic test. Prasad and Gupta [14] determined the shearing force and energy for cutting maize stem. They were reported that the shear strength and shear energy decreased with the shear velocity in the direct shear test. HassanBeygi et al [6] determined the picking force of saffron flower and shear strength of saffron stalk. They were reported that with increasing cutting rate from 20 to 200 mm·min^{1} the average values of shear strength and shear energy per stalk area decreased significantly in the range of 0.179 to 0.158 N·mm^{2} and 0.467 to 0.340 mJ·mm^{2}, respectively. Khazaei et al. [10] studied effect of bevel angle, oblique angle, shear velocity and blade type, on shear strength and shear energy of pyrethrum flower stem. Tensile strength and energy per unit area were evaluated for picking up the flower. İnce et al. [9] determined shearing stress, specific shearing energy, bending stress and modulus of elasticity for sunflower stalk. Tabatabaee Koloor et al. [17] evaluated the effects of moisture levels and the cross sectional area of rice stem as the variety, blade bevel angle, blade type and cutting speed on shearing strength. Hoseinzadeh et al. [7] considered three varieties of wheat and knife bevel angles, four levels of moisture content and three shearing speeds of pendulum on the shearing energy of wheat straw. Esehaghbeygi el al. [5] measured shearing stress of wheat stalk for four moisture content levels, three cutting heights, two types of cutting knives, smooth and serrated edge and three blades oblique angle. Akritidis [1] studied the main mechanical factors influencing the cut of maize stalks by expressed in mathematical equations. Chattopadhyay et al. [3] determined shear properties of sorghum stalks with quasistatic shear test.
The objective of this research was to measure key physical properties of rose flower as follows:
Measuring the required shear strength and energy for cutting rose flower stem.
Measuring the required force and energy for picking of rose leaves.
MATERIAL AND METHODS
The rose flower were harvested from Ashianesabz greenhouse in Tehran, Iran, from healthy mother stock plants by sharp knife at a height of 10 cm above the soil surface in the morning of each testing data. Harvested stems were covered and transported in an insulated container to the Tarbiatmodares University biomechanic research laboratory for testing. Specimens were keeped in the refrigerator at temperature 4°C. Testing was complete as rapidly as possible in order to reduce the effects of drying. An Instron Universal Test Machine with a 500 N load cell was used to measure the shearing, bending and compression force of rose stem test (Fig. 1). Measurement resolution was 0.1 N, moisture contents of all were 76% (w.b.).
Fig. 1. Instron Universal Test Machine (IUTM) with a 500 N load cell and Shear testing device 
Shear strength of stem. A commercial single, sickle knife section and a counter shear were used as the cutting tool. The knife section was attached onto the moveable crosshead of a Universal Testing Machine through a load cell 500 N capacity. An apparatus was constructed to hold the counter shear and stalk specimen (Fig. 2). The specimen was positioned on the specimen support so that it was cut at the location of approximately 20 mm from the base of the stalk sample to approximate the typical cutting height used in field harvesting operations.
Fig. 2. Shear testing device 
The knife was fabricated out of high carbon steel and had a 40 degree bevel angle and edge radius of 0.05 mm. Cutting was achieved by lowering the crosshead of the Universal Testing Machine at a constant cutting velocity. The knife section passed through the specimen along the ledger plate of the stationary counter shear. This simulated a single edge cutting process of a reciprocating knife cutter bar. Twenty replications were used for each treatment and for all tests, a constant clearance distance of 0.2 mm was chosen. The force displacement curve was recorded during the cutting process by a computer data acquisition system at 100 Hz (Fig. 3). The forcedeformation curves were used to evaluate shear force and energy. Shear force was given by the maximum recorded force and shear energy was calculated by measuring the surface area under the forcedeformation curve. The indices which determine the shearing behavior of the plant material are maximum shear strength and specific cutting energy. The maximum shear strength is expressed by [12,13]:
(1) 
where:
τ – maximum shear strength (MPa),
F – maximum shear force (N),
A – crosssectional area of stalk at the shear plane (mm^{2}).
Fig. 3. Typical force – deformation curve for shearing of the rose stem 
The specific cutting energy is found by determining the area under the shear force – displacement curve. In this case, it was found by [12,13]:
(2) 
E – energy per unit area (mJ·mm^{2}),
X – travel distance (mm),
n – number of elements under force – deformation curve.
f – read force in the center of pseudoelastic portion of the force – displacement curve (N).
For all tests, twenty replications (for different rose stems) were used for each treatment to satisfy the objective; the effects of oblique angle at 20, 30 and 50 degree levels, and shear velocity at 10, 100 and 500 mm·min^{1} levels for different diameter were evaluated.
Picking force of rose leaves. A device similar to the one shown in figure 4b held the leaf and connected it to the Instron Test Machine cross head also, the end of stem was clamped by the lower Instron jaw (Fig. 4a).
Fig. 4. a – Instron test machine and tools used to picking leaf (1 – device, 2 – leaf, 3 – rose flower stem, 4 – Instron base plate); b – device to held and connects the leaf to Instron test machine 
In this study, the effects of picking velocity at 10, 100, and 500 mm·min^{1}, tensile direction, up and down direction and two sample groups, near growth tip and near root internodes were studied on maximum picking force and energy. Each test was replicated 15 times (for different diameter stems). After each test, a picture was taken from petiole section area and at the end, pictures were evaluated in MATLAB program and were determined their section area. The tensile strength was computed on the basis of the maximum picking force on crosssectional area of stem at the point of picking the leaf.
RESULTS AND DISCUSSION
Shear strength. The results of statistical parameters of mean, min, max and STD (Standard deviation) for shear strength and energy per unit area have been presented in Table 1. The mean values of shear strength and energy per unit area were obtained 1.63 MPa and 5.16 mJ·mm^{2}.
Table 1. Summary of results of shear strength and energy per unit area for cutting of rose stem 
Maximum 
Minimum 
Mean 
STD^{x} 

Shear strength (MPa) 
3.57 
0.85 
1.63 
0.375 
Energy per unit area (mJ·mm^{2}) 
9.01 
3.02 
5.16 
1.05 
x – standard deviation 
Table 2 shows the results of the variance analysis (ANOVA) of the effect velocity and oblique angle on shear strength and energy per unit area. Shear velocity had a significant effect on both shear strength and energy per unit area. As shown in (Fig. 5), the average values of shear strength decrease with increasing shear velocity. Also, results of Duncan's multiple range test show with increasing shear velocity from 10 to 500 mm·min^{1} the shear strength decreases from 2.75 to 1.2 MPa. The difference between mean values of shear strength at 100 and 500 mm·min^{1} levels was not significant at 5% significance level. The decrease in shear strength with increase in shear velocity may be due to the viscoelastic behavior of plant material.
Fig. 5. Effect of shear velocity on shear strength of rose stem 
Table 2. Result of analysis of effect of velocity oblique angle on shear strength and energy per unit area of rose stem 
Source of variation 
DF^{e} 
Shear strength (MPa) 
Energy per unit area (mJ·mm^{2}) 
Treatment 
9 
3.157^{a} 
9.572^{a} 
Velocity (V) 
2 
2.31^{a} 
3.232^{b} 
Oblique angle (O) 
2 
10.18^{a} 
34.398^{a} 
V×O 
4 
0.066^{ns} 
0.328^{ns} 
Error 
81 
0.215 
0.931 
a – significant at 1%, b – significant at 5%, e – degree of freedom, ns – not significant 
The dependency of shear strength and energy per unit area on shearing velocity and oblique angle was approximated to [13]:
σ_{s} = 4.99 – 249.78·V_{s}_{ }+ 22429.18·V_{s}^{2} – 6.88·O_{a}+ 3.315·O_{a}^{2}, R^{2} = 0.987 (3)
E_{s} = 6.182 – 347.33·V_{s} + 31575.85·V_{s}^{2} – 6.99·O_{a} +7.82·O_{a}^{2}, R^{2} = 0.982 (4)
where:
σ_{s} – shear strength (MPa)
E_{s} – shearing energy per unit area (mJ·mm^{2})
V_{s} – shear velocity (mm·min^{1})
O_{a} – oblique angle (rad).
Fig. 6. Effect of shear velocity on energy per unit area of rose stem 
Picking force of leaf. The average value for stalk diameter was 7.167 mm for near root internode and 5.427 for near growth tip. Variance analysis showed significant differences in both stem diameter and petiole section area related to each group. Mean values of petiole section area were 0.0462 cm^{2} and 0.091 cm^{2} for near growth tip and near root internode, respectively. Figure 7 shows that with increasing of stalk diameter, rose section areas petiole increased.
Fig. 7. Effect of stalk diameter on petiole section area of rose stem 
Figure 8 shows the effect of stem diameter on picking force, energy and energy per unit area of rose flower. Picking force and energy increased with increasing the stem diameter, but the value of shear energy per unit area decreased. Based on variance analysis (Table 3), picking force and energy were affected by picking velocity significantly (P = 1%).
Fig. 8. Effect of stem diameter on picking force, absorbed energy and energy per unit area 
Duncan's multiple range test variance analyses results show with increasing the picking velocity from 10 to 500 mm·min^{1}, the mean values of picking force and energy increase from 5.975 to 9.998 N and from 9.451 to 14.449 mJ, respectively. There was no significant difference between mean values of picking force and energy at 10 and 100 mm·min^{1} levels. El Hag et al. [4] found that the effects of loading rate on tensile strength of cotton stem were influenced further by density of stem. For high density specimens 0.400 g·cm^{3}, the tensile strength increased directly with the rate of loading, but for lower density specimens 0.3759 g·cm^{3} the tensile strength decreased with increase in loading rate from 7.6 to 25.4 mm·min^{1} and then increased with further increase of loading rate.
Table 3. Result of analysis of variance (ANOVA) of effect of velocity, Location, on shear strength and energy per unit area of rose stem 
Source of variation 
DF 
Picking force (N) 
Picking energy (mJ) 
Treatment 
11 
190.78^{a} 
128.41^{a} 
Location (L) 
1 
7.244^{ns} 
73.59^{a} 
Tensile direction (D) 
1 
1813.43^{a} 
998.24^{a} 
Velocity(V) 
2 
85.46^{a} 
129.79^{a} 
L × D 
2 
1.917^{ns} 
23.89^{ns} 
L × V 
2 
4.05^{ns} 
19.25^{ns} 
D × V 
2 
45.41^{a} 
2.45^{ns} 
L × D × V 
2 
3.09^{ns} 
6.88^{ns} 
Error 
48 
321.68 
8.79 
a – significant at 1%, b – significant at 5%, ns – not significant 
Result of the variance analysis for location of samples showed there is not significant effect on picking force, but for energy was significant at 1% level. Mean values for picking force and energy varied from 7.335 to 8.061 N and 10.592 to 12.807 mJ, for top and bottom location, respectively. Picking direction had significant difference at 1% levels on force and energy values. Results of Duncan multiple range tests shows that mean values for up and down direction were 13.21 N and 2.22 N, respectively.
CONCLUSION
The average values of shear strength and energy per unit area were decreased due to the shear velocity increasing. So the shear strength and energy per unit area of 1.63 MPa and 5.16 mJ·mm^{2} were recommended for picking the rose flower.
With increasing the shear velocity, from 10 to 500 mm·min^{1} the average values of shear strength and energy per unit area decreased.
The shear strength decrease with increasing shear velocity which may be due to the viscoelastic behavior of plant material.
With increasing stem diameter, picking force and energy increased, but the value of shear energy per unit area decreased.
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Accepted for print: 4.05.2011
Adel Heidari
Department of Agrtechnology,
College of Aboureihan, University of Tehran, Iran
Pakdasht, 3391653775, Iran
email: heidariadel@ymail.com
Gholam Reza Chegini
Department of Mechanical Engineering of Biosystems, College of Aboureihan, University of Tehran, Tehran, Iran
Telephone: 098 21 360 406 14
Cell phone: 0989126356329
email: chegini@ut.ac.ir
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