Elsevier

Powder Technology

Volume 380, March 2021, Pages 638-648
Powder Technology

Measured damage resistance of corn and wheat kernels to compression, friction, and repeated impacts

https://doi.org/10.1016/j.powtec.2020.11.012Get rights and content

Highlights

  • Damage probability models were developed for predicting grain kernel damage.

  • Corn and wheat kernels were susceptible to impact and compression loading.

  • Corn and wheat kernels had high resistance to wear damage.

Abstract

The damage resistance of grain kernels to external loading is useful for optimizing agricultural equipment design and adjusting operational settings. This study aims to quantify and compare the damage resistance of corn and wheat kernels under different types of loading and fit the data with statistical models. The damage resistance of corn and wheat kernels to compression, friction, and repeated impacts was measured using a universal testing machine, pin-on-disk tribometer, and Wisconsin breakage tester/rotary blade impactor, respectively. The average fracture force and fracture energy under compression (± one standard deviation) were, respectively, 309.4±107.1 N and 48.7±27.1 mJ for corn, and 83.4±23.8 N and 28.7±13.5 mJ for wheat. The wear damage was insignificant for corn-acrylic, corn-steel, and wheat-acrylic wear tests. For the wheat-steel wear test, the average work done by the friction force to cause pericarp damage was 3.85±1.50 J. A lognormal distribution was used to fit the compression test data, and a three-parameter Weibull distribution was used to fit the repeated impact test data. The statistical models fit the damage probability based on the loading force or input energy with R-squared values over 0.98. It was found that corn and wheat kernels were susceptible to compression and impact loading, while both had high resistance to wear damage.

Introduction

Grain kernels are subject to a combination of compression, impact, and friction loads during agricultural production [1]. The level of grain kernel damage is greatly affected by various physical and mechanical properties of the grain such as moisture content, shape, size, and damage resistance [2]. Damage resistance is defined as the ability of grain to withstand applied loads without plastic deformation or failure. The damage resistance of different grain types under different types of loading has been investigated by many researchers as it is useful for optimizing equipment design and adjusting operational settings [[3], [4], [5]].

Damage caused by a compression load is common in harvesting and handling. One typical example is grain kernels jamming between machine components. This damage is undesired as it reduces grain quality with the generation of cracked and broken kernels. However, for some operations, such as dehulling [6], milling [7], and oil extraction [8], the process equipment is specifically designed to damage the structure of kernels using compression. Measurements of compression resistance can be used to guide the design of such equipment to improve product quality and energy efficiency. The compression loading behavior of different types of grain, including corn [9,10], wheat [11,12], soybeans [13,14], rice [15], and barley [16], has been widely studied by researchers. The universal testing machine is one of the most commonly used test devices to measure the compression resistance of grain kernels. During the test, an individual kernel is compressed between two parallel plates and the force applied to the kernel and the kernel deformation are recorded. The compression resistance is quantified either as the peak force or the energy absorbed by the kernel when the damage occurs [3]. It has been found that the compression resistance is affected by the type and variety of the grain [10,12], the test settings (e.g., loading orientation [17] and loading rate [18]), and the grain's physical properties (e.g., moisture content [19] and composition [20]).

Impact loading is another major cause of grain kernel damage. This type of damage can be significant when kernels are impacted by machine components moving at a high speed, such as threshing by a combine harvester [21] or conveying by a bucket elevator [22]. Kernels are also subject to damage when they are accelerated to a high speed and impact on a hard surface, such as conveying by a pneumatic conveyor [23] or free-falling from a high elevation [24]. Thus, the cylinder speed of combine harvesters, the speed of conveyors, and the falling height of grain streams should be limited based on the grain's impact damage resistance to avoid excess damage. Knowledge of impact resistance can also be used in the grain cleaning process. It has been reported that insect-infested wheat kernels have lower impact resistance than intact kernels, which is the fundamental principle behind the operation of the entoleter [25]. Various devices have been developed for impact damage tests including the centrifugal impactor [26], the rotary hammer impactor [27], the drop bar impactor [28], and the pneumatic projector [29]. The level of impact damage is correlated with the impact speed and/or impact energy. The major factors affecting the level of impact damage are moisture content [30], impact speed [31], impact orientation [32], impact surface properties [24], and impact angle [33].

Little research has been conducted to investigate the damage to grain caused by frictional forces, and wear damage is not specifically mentioned in the grain inspection handbook published by the United States Department of Agriculture [34]. This lack of attention may be because wear damage is not as significant as compression and impact damage during agricultural production. However, data on wear resistance could be useful for optimizing the dehulling process of grains with husks (e.g., wheat and rice) [35], the wheat scouring process [36], and the rice polishing process [37,38]. One parameter used to quantify wear resistance is the abrasive hardness index (AHI), which is defined as the time required to abrade 1% of the kernel (by mass) as fines [39]. A kernel with a larger AHI value is tougher and less likely to be damaged by frictional forces. The AHI can be measured with the tangential abrasive dehulling device (TADD) developed by Oomah et al. [40], for example, in which several grams of grain are placed in bottomless cups and slide against a rotating disk. The degree of wear is determined by the mass loss of the grain sample. It is worth noting that the AHI is a measure of the wear resistance of the grain in bulk. A careful review of the literature did not find a test that quantifies a single kernel wear damage. The conventional method to determine wear of inorganic materials during sliding is the pin-on-disk test [41]. For this test, the materials of interest are manufactured as a pin with a rounded tip and a flat circular disk. The pin slides against the rotating disk positioned perpendicular to the pin. The amount of wear is quantified by the change in sample weight or dimension, which can be correlated with the amount of work done by the frictional force.

In previous studies, usually only one type of loading was investigated [1,3,4,6,10]. The objective of the current study is to measure the damage resistance of corn and wheat kernels to compression, friction, and repeated impacts and fit the data with statistical expressions. In addition, comparisons of the damage resistance are made between different types of loading.

Section snippets

Test material

The corn and wheat samples used in the current study were acquired from a local farm (Covington, IN). The moisture contents of the samples were measured using the whole kernel oven method described in ASABE Standard S352.2 [42]. The moisture content of the corn was 14.7% wet basis and the moisture content of the wheat was 13.9% wet basis. For the damage resistance tests, only whole kernels, i.e., fully intact kernels with no external cracks or kernel pieces chipped away, were used. Broken

Compression testing

The fracture force and fracture energy for 100 corn kernels and 100 wheat kernels were measured. and reported in Table 2. The higher average fracture force and energy of the corn indicate that the corn has a higher resistance to compression loading than the wheat. These measurements are consistent with previous studies [10,11,20,56], but there are some differences. These differences are due to differences in sample size, grain variety, and moisture content. In addition, some former studies

Conclusions

The fracture force and fracture energy due to compression for corn and wheat were found to be fit well with a lognormal distribution. A three-parameter Weibull distribution, which was motivated by the impact damage model of Vogel and Peukert, provided an excellent fit to the single and repeated impact test data of corn and wheat. The difference between the damage fraction curves acquired from the Wisconsin breakage tester and the rotary blade impactor indicates that impact location has a

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by CNH Industrial. The authors would like to thank Dr. Farshid Sadeghi for allowing the use of the tribometer and his graduate students Arman Ahmadi and Akshat Sharma for the device training. The authors would like to thank Dr. Mark Casada for allowing the use of the Wisconsin breakage tester. The authors would like to thank Dr. Richard Stroshine for his comments on an early version of the manuscript. The authors would also like to thank Dr. Eric Veikle for

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