Deformation behaviour simulation of an apple under drop case by ...

Journal of Food Engineering 104 (2011) 293–298

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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng

Deformation behaviour simulation of an apple under drop case by finite element method H. Kursat Celik a, Allan E.W. Rennie b, Ibrahim Akinci a,⇑ a b

Department of Agricultural Machinery, Faculty of Agriculture, Akdeniz University, 07070 Antalya, Turkey Lancaster Product Development Unit, Department of Engineering, Lancaster University, LA1 4YR, Lancaster, United Kingdom

a r t i c l e

i n f o

Article history: Received 29 January 2010 Received in revised form 22 October 2010 Accepted 21 December 2010 Available online 28 December 2010 Keywords: Drop test Finite element method Organic materials Golden Delicious apple

a b s t r a c t This study focuses on the deformation behaviour of organic materials under a drop case. A Golden Delicious apple was chosen as the sample organic material. A three-dimensional (3D) scanner, high speed camera and finite element method (FEM) simulations were utilised to investigate drop case deformation events of the organic material. In total, 22 steps after impact were set up in the simulation by comparing high speed camera screen steps. Maximum equivalent stress of 0.416 MPa and maximum contact force (resultant normal force from rigid plane at impact) of 250.980 N were obtained from simulation results. Visual investigations and simulation results relating to deformations observed under the drop case, agreed well. This study contributes to further our understanding of fruit and vegetable damage/deformation by using computer aided engineering applications. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The design and development of agricultural/food machinery systems are related with properties of agricultural biological or/ and organic material directly or indirectly. Hence, it is very important to conduct research about the properties of these materials, such as fruit and vegetables. These properties are also a sign of the quality of the agricultural products. Much research has signified that the quality of the fruit or vegetables could be determined by their internal and external characteristics. These characteristics can be presented as size, shape, smell, appearance, product presentation and texture (Wang and Sheng, 2005; Abbott and Lu, 1996; Chen and De Baerdemaeker, 1993a; Petrell et al., 1980; Abbott et al., 1968). Most especially, the issue of mechanical damage has a huge significance on the quality of these agricultural products. Mechanical damage mostly occurs during harvesting, handling, transportation and storage. In addition to mechanical effects, agricultural materials may be exposed to various thermal, electrical, optical, and acoustical effects during such processes (Sitkei, 1986). Thus, the damage inflicted can contribute to decreasing quality of the product through biological degradation, such as rotting. Therefore, prediction of the level of damage, stress distribution and deformation of the organic material under the external forces has become a very important issue. ⇑ Corresponding author. Tel.: +90 242 310 2464; fax: +90 242 227 4564. E-mail address: [email protected] (I. Akinci). 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.12.020

In fact, it is very difficult to measure internal stresses, which are caused by mechanical effects, due to the biological cell structure of fruit and vegetables. Numerical methods can be utilised as an alternative solution for prediction of the stresses. Analytical methods for stress investigations are available today only for a few simple cases, and so their applications are limited. Available technology, the proliferation of computers and software allow engineers to solve complicated problems using computer aided design (CAD) technology and numerical methods in a virtual way without committing to physical manufacture and testing (Topakci et al., 2010). The practical cases occurring in agricultural engineering mostly represent complex problems, which may be solved only by numerical methods (Sitkei, 1986). One of these approaches is to estimate the stress field of organic materials using finite element method (FEM) (Cardenas and Stroshine, 1991). FEM is a numerical procedure that can be used to obtain solutions for complicated or large scale engineering problems involving stress analysis, heat transfer, electromagnetism, and fluid flow. The method has been improved since the 1950s and during the 1960s, investigators began to apply FEM to other disciplines of engineering (Moaveni, 1991). Many studies, which are conducted using FEM, can be found in the scientific literature, related to the mechanical damage and physical property investigations of organic materials (Kabas et al., 2008; Chen and De Baerdemaeker, 1993a,b; Lu and Abbott, 1997; Verstreken and De Baerdemaeker, 1994; Chen et al., 1996; Langenakens et al., 1997; Celik et al., 2008). These studies show that FEM can be a suitable method for calculating some of the

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properties and to determine deformation behaviour of the organic materials. Additionally, most of the research on fruit impact damage has been performed on apples, particularly over the past 45 years (Zeebroeck et al., 2007). However, the usage of the numerical methods/ simulations on the impact damage on apples is quite limited alongside the other methods. The aim of this study is to understand the deformation behaviour of a Golden Delicious apple under drop case scenario utilising FEM simulations and visual investigations. The apple was dropped from a height of 0.5 m in an assumed rigid plane under the earth’s gravitational effect. The moment of impact and the apple’s deformation behaviour was recorded by using a high speed motion camera. In the study, a 3D solid model of the apple was created by using a 3D scanner and 3D parametric solid modelling software. A commercial FEM code was utilised for drop test simulations. The original prints of the simulation and visual investigations were presented by data supporting the comparisons. 2. Materials and methods 2.1. Material

Fig. 1. Force–deformation curve for Golden Delicious apples.

apple. The FEM code calculates impact and gravity loads automatically; no other loads or restraints are used. The FEM code then calculates the velocity (V) at impact from: V = (2 g h)1/2, where, g is gravity [m s2] and h is drop height [m]. The FEM code solves a dynamic problem as a function of time. To simulate an object dropping and hitting a hard surface, the FEM code has to solve the following general equation iteratively, because as the object falls and hits the floor, the forces and stiffness keep changing (SolidWorks Product Document, 2009).

½Mfag þ ½Cfvg þ ½Kfxg ¼ fFg A Golden Delicious apple was used as the sample organic material in this study to investigate deformation in the drop case scenario. This apple for the experiment was specifically chosen for having the potential of reflecting the deformation behaviour on its clean and light-coloured skin which is suitable for use with a black and white screen high speed motion camera. One apple was randomly picked from the shelves of a supermarket which was previously kept in a cold storage unit. The mechanical properties of the apple were taken from a previous study to use in the FEM simulation. Some of the mechanical properties and a chart, which details relations between force and deformation, are shown in Table 1 and Fig. 1, respectively (Celik et al., 2008). 2.2. Three-dimensional solid modelling of the apple Agricultural biological and organic materials have complicated and complex freeform surfaces. Hence, creating the 3D solid model of them is a difficult task to undertake, necessitating the need to reflect the apple surface using the 3D scanning technique. As such, a NextEngine Model 2020i Desktop 3D Scanner was used as the means for generating a surface model of the apple. Then the surface model was processed into a 3D solid model using Solidworks 3D parametric solid modelling software. After measurements, the mass of the solid model weighed 152.759 g. The 3D solid modelling process and the dimensions of the apple are presented in Fig. 2. 2.3. Drop case scenario of the apple Drop test studies evaluate the effect of the impact of a part or an assembly with a rigid or flexible planar surface. Dropping an object on the floor is a typical application. In this study, Solidworks Simulation FEM code was utilised for the drop case simulation of the

Table 1 Some mechanical properties of a Golden Delicious apple. Parameter

Magnitude

Tolerance

Force [N] Strain [%] Elastic modulus [N mm2] Poisson rate [] Density [g cm3]

42.120 5.243 1.512 0.384 0.796

±1.647 ±0.500 ±0.065 ±0.007 ±0.031

ð1Þ

where M = mass matrix, C = damping matrix, K = stiffness matrix, F = external force vector, a = acceleration vector, v = velocity vector, x = displacement vector. In the simulation, the body moves in the direction of gravity as a rigid body until it hits the rigid plane (no rotations are considered until the initial impact occurs). Loss of energy in a drop test normally occurs due to damping, friction, or plastic deformation. However, the FEM code does not support damping in a drop test analysis. In this study, we do not define friction and we use a linear isotropic material. Therefore, the impact causes no energy loss and the model continues to bounce off the impact plane for an indefinite period of time. The drop test requires a dynamic analysis solver and the software solves this analysis using an explicit method of direct-time integration. This is a computationally intensive, but numerically stable technique for solving problems in a dynamic analysis (CosmosWorks Product Document, 2008). The aim of the study was to examine the deformation behaviour of the organic material in the drop case. To achieve this aim, both visual examinations and FEM simulations were used. During the transfer(s) from orchard to retailer, the potential drop heights of between 0.05 m and 1.2 m have previously been researched in the scientific literature related with drop tests of apples (Stropek and Golacki, 2007; Moaveni, 1991; Menesatti and Paglia, 2001; Holmes et al., 1998). Potential dynamic apple loading situations and associated drop heights are given in Table 2 (Lewis et al., 2007). Hence, a suitable assumption was created by considering potential drop heights from the scientific literature for the drop height in this finite element analysis (FEA). The drop case scenario assumed that the apple was dropped from 0.5 m high onto a rigid planar surface under the earth’s gravitational effect. This drop case was recorded by using a high speed motion camera: Kodak motion corder analyser SR-series, which is designed to be a valuable addition to the engineer’s problem solving kit (Kodak product user manual, 1998). In the study, dropping moments were recorded by using 3000 fps resolutions with 128 120 frame size and a black and white screen. In fact, it is not possible to simulate all real life responses of the materials in the deformation simulations. Due to the limitations and unpredictable material and dynamic conditions, some assumptions have to be made to gain approximate solutions. As a result of these similar limitations, the apple was assumed to be an entire solid together with skin and an isotropic material model


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Fig. 2. 3D Solid modeling of the apple.

Table 2 Potential dynamic apple loading situations and associated drop heights. Point in journey

Process stage

Potential drop height (m)

Orchard Packing house

Picking bucket Bulk bin Repack Sorting (Conveyors, etc.) Putting on display

0.6 0.6–1 0.05–0.15 0.05–0.15 0.05–0.3

Distributor Retailer

was assumed in the FEM simulation. It was quite difficult to control the impact position. Hence, the apple was allowed to free fall and the simulation was set up according to the resulting impact position of the apple. Steps of 22 were set after the first impact moment in the simulation for the drop case. Mesh construction

was created in the SolidWorks Simulation FEM code and second order tetrahedral solid element type was used for the mesh construction. Total elements of 37203 and total nodes of 43828 were obtained for mesh construction of the solid model. Drop case scenario and mesh construction of the apple are presented in Fig. 3.

3. Results and discussion After completing the FEM pre-processor operations, solving operation was generated. The deformation behaviour, equivalent stress distributions and contact forces (resultant normal forces from rigid plane at impact) of the apple were obtained for each step. According to the FEM simulation results, the maximum equivalent (Von Mises) stress of 0.416 MPa and maximum contact

Fig. 3. Drop case and mesh construction of the apple.

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Table 3 Stresses and contact forces. Step Number

Max. global stress (Von Mises) [MPa]

Contact force [N]

Step number

Max. global stress (Von Mises) [MPa]

Contact force [N]

Step-01 Step-02 Step-03 Step-04 Step-05 Step-06 Step-07 Step-08 Step-09 Step-10 Step-11

0.234 0.305 0.341 0.367 0.403 0.416 0.413 0.385 0.353 0.300 0.219

35.264 83.533 136.200 186.780 228.390 250.980 246.070 213.050 157.970 94.121 40.247

Step-12 Step-13 Step-14 Step-15 Step-16 Step-17 Step-18 Step-19 Step-20 Step-21 Step-22

0.122 0.031 0.032 0.028 0.024 0.039 0.044 0.027 0.027 0.026 0.018

5.887 0 0 0 0 0 0 0 0 0 0

the point when the maximum stress is reached. Contact force is zero at step 13 as there is no contact with the rigid surface at this step. However, after step 13 there are instances of stress observable, ranging from the value 0.018–0.044 MPa even when there is no contact force existing at all. A comparison for approximate damage can be conducted using the FEA results and the chart which is given in Fig. 1. According to the force–deformation curve, the maximum force was 42.120 N. This value is lower than the simulation result that was 250.980 N. Hence, it can be said that the fruit damage for this drop case is in existence. It should be highlighted that damage shape and magnitude of the organic material can be changeable under different impact conditions and orientations. In the physical test section of this study, it is important to consider the maximum deformation case in dropping an apple. The drop angle which is used in the simulation was chosen to simulate one of the worst drop cases (i.e. the smallest contact area between the apple and the drop substrate). To investigate different orientated drop cases, the simulation parameters can be redefined by researchers and results can be interpreted visually if desired. Besides, as the previous studies highlighted, it should not be ignored that the simulation results and bruise susceptibility change not only depend on drop angle, but also depend on humidity, harvest time, apple variety and some mechanical properties such as elastic modulus and Poisson ratio (Garcia et al., 1995; Pang et al., 1994; Ragni and Berardinelli, 2001).

4. Conclusions

Fig. 4. Stress magnitudes of the apple according to drop steps.

force of 250.980 N occurred in step 6. The other step’s stress values, contact force values and their charts are given in Table 3 and Fig. 4, respectively. The apple’s drop case was compared with simulation result plots, where each simulation step was compared with images of the high speed camera’s steps. As a result of the comparison, it can be agreed that the simulation and the camera’s steps work in unison with each other. In addition to this, the simulation’s results allow us to investigate the section stress distributions. Comparison of the camera images and simulation prints are given in Fig. 5. As can be seen in Fig. 5, the first stress values occur at step 1, as soon as the apple has impacted the rigid surface. The contact force reaches its maximum value at step 6 and then the apple starts to bounce as a reaction to the rigid surface. Meanwhile, step 6 is

Visual investigation of physical events is a very important issue in all disciplines of engineering, especially to understand and examine material behaviour, which under the loading case, can be very difficult for the complex structures associated with organic materials. Generally, visual investigations do not allow for examinations of what happens inside of the material under the loading case. However, these simulations can be very useful in that process. Most especially during long transportation times of these organic materials, some mechanical damage may occur due to vehicle vibrations, collisions between the materials and drop cases. Therefore, for instance, the design of specific packaging systems in accordance with predicted damage scenarios for different organic material may be a viable option. The other important issue is impact damage cases between organic materials during harvesting and post-harvesting processes. It is also difficult to predict how the deformations occur. However, computer aided engineering (CAE) applications can help in these issues. Hence, in this study, it is focused on how the CAE applications help to simulate deformation behaviour of the organic material during harvesting, handling, transportation, and storage. According to this study, some concluding points can be presented as follows: The sample apple was modelled as solid 3D using reverse engineering tools, so that all surface details of the apple could be obtained for the solid model as accurate using a 3D desktop scanner; The drop case scenario assumed that the apple was dropping from 0.5 m high onto a planar rigid surface under the earth’s gravitational effect and this scenario was recorded with 3000 fps and 128 120 frame size by using a high speed camera. This experiment in this study presented that use of a high speed camera record can be very useful to understand and investigate deformation behaviour of these kind of organic materials under drop cases;


297

Fig. 5. Drop simulation and high speed camera comparison.

The FEM simulation was set up according to the drop case scenario with 22 steps after impact moment and it was solved. Maximum equivalent stress of 0.416 MPa and maximum contact force of 250.980 N were obtained for the step 6 in the FEM simulation, and each step was compared with step images of the camera; It is agreed in the comparison of the simulation and experimental study that simulation plots work in unison; Although there are some limitations, it can be agreed that CAE applications are very useful in predicting deformation of round fruits such as apples.

Acknowledgements This study was part-supported financially by the Scientific Research Fund of Akdeniz University. Additionally, the authors wish to acknowledge the UK EPSRC-supported Engineering Instrument Loan Pool at Rutherford Appleton Laboratories for the loan of the high speed camera equipment utilised in this research.

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