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«by Gaurav S. Goel A dissertation submitted to the faculty of the University of North Carolina at Charlotte in partial fulfillment of the requirements ...»

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A NUMERICAL STUDY OF TILLAGE TOOL WEAR DURING PLOWING

OF SANDY SOIL

by

Gaurav S. Goel

A dissertation submitted to the faculty of the

University of North Carolina at Charlotte in

partial fulfillment of the requirements

for the degree of Doctor of Philosophy in

Mechanical Engineering

Charlotte

2013

Approved by:

Dr. Harish P. Cherukuri

Dr. Ronald E. Smelser

Dr. David C. Weggel Dr. Yogendra P. Kakad ii c 2013 Gaurav S. Goel

ALL RIGHTS RESERVED

iii

ABSTRACT

GAURAV S. GOEL. A numerical study of tillage tool wear during plowing of sandy soil. (Under the direction of DR. HARISH P. CHERUKURI) Wear of tillage tools during tilling operations leads to increase in power require- ments, frequent downtimes for tool replacement, loss of production and an increase in the equipment and operational costs. A large number of variables - such as the moisture content in the soil, soil particle sizes, plowing speed, plowing depth, tool material properties, etc. - influence the tool wear rates. In turn, the tool wear has an effect on the power requirements for the tilling operations since a worn out tool requires larger amounts of power. The purpose of the present work is to (a). develop finite element models for predicting tool wear and power requirements, (b). validate the finite element models with experiments, and (c). study the effect of the above- mentioned variables on tool wear and power requirements. The ultimate objective is to understand the primary mechanisms behind tool wear and devise new designs for tillage tools to mitigate tool wear rates.

The tool wear process is treated as a two-body abrasion where a rigid sand particle scratches against the surface of the tillage tool at a prescribed speed. A finite element model is developed to model the scratching process. The deformations predicted by the finite element models are used in conjunction with the classical ploughing theory and the material removal factor to identify the mechanisms underlying the wear of the tillage tool. The predicted material removal factors are compared with experimental results and a parametric study is carried out to study the effect of various parameters on tool wear.

A two-dimensional finite element model simulating the soil-tool interaction during tilling is also developed. The purpose of this model is to study the feasibility of using the finite element me

–  –  –

My first and foremost affirmation goes to my advisor, Dr. Harish Cherukuri, for his encouragement, advice, and great interest in the pursuit of my research and in the timely suggestions to write my dissertation. I would like to convey my sincere thanks to the Department of Mechanical Engineering and Engineering Science for their support throughout my studies.

Besides my advisor, Dr. Cherukuri, I would like to express my sincere thanks to my dissertation committee members: Dr. Ron Smelser, Dr. Dave Weggel, and Dr.

Yogendra Kakad, on agreeing to serve on my dissertation committee, and encouraging me to work on my research project.

In addition, I would like to acknowledge the Tribology and Surfaces Group, School of Materials Engineering, National University of Colombia, especially Dr. Alejandro Toro, for the support throughout my studies.

My sincere thanks also goes to the faculty in Mechanical Engineering for the teaching assistantships offered from time to time during my course of studies. Additionally, I would like to thank my friends and colleagues for their subsistence support during my stay as a doctoral student.

Also, I would like thank my parents, Prof. H. S. Goel and Prabha Goel, for encouraging me to pursue doctorate studies and supporting me throughout my life.

Last but not the least, I would like to praise my wife Ruchi and kid Shubhi for their patience and encouragement that helped me to complete my research successfully.

vi

TABLE OF CONTENTS

–  –  –

FIGURE 1.4: (a).

The plowing assembly and (b). A 2D schematic of a tillage tool. 4 FIGURE 1.5: A comparison of an unworn tillage tool with a worn tillage tool, (a). 5 top view, and (b). side view.

FIGURE 1.6: Tool material loss is measured as the difference in the weights of a tillage 5 tool before and after the soil tillage.

–  –  –

FIGURE 2.5: Two different views of a worn tillage tool surface from field tests.

28 FIGURE 2.6: Particle size distribution of the soils taken for the experimental study 29 on tool wear.

FIGURE 2.7: A profilometric graph of a tool surface showing an average of 5-6 30 scratches per centimeter.

FIGURE 2.8: Nominal stress vs nominal strain curves for the tillage tool material 32 heat-treated at three different temperatures.

FIGURE 2.9: True stress vs true strain curves for the tillage tool material heat-treated 32 at three different temperatures.

FIGURE 4.1: A 2D schematic that shows the front and side views of a sand particle 42 scratching the tool surface with one of the large sand particle sizes (0.





5 mm).

–  –  –

FIGURE 4.5: Biased meshing of a tool block with C3D8R linear brick elements and 46 sand particle meshing scratch test model.

FIGURE 4.6: Frictionless contact is established between the sand particle and the 47 tool block surface.

Sand particle size is 0.5 mm.

FIGURE 4.7: Frictionless contact is established between the sand particle and the 48 tool block surface.

Sand particle size is 1.1 mm.

FIGURE 4.8: A sand particle size of 0.

5 mm that penetrates the tool block up to 48

0.05 mm, (a) at the beginning of the indentation (b) and at the end of indentation.

FIGURE 4.9: A sand particle size of 1.

1 mm that penetrates the tool block up to 0.11 49 mm (a) at the beginning of the indentation (b) at the end of indentation.

FIGURE 4.10: A sand particle size of 0.

5 mm that scratches the surface of tool block 49 up to 1.25 mm (a) at the start of scratching (b) at the end of scratching.

FIGURE 4.11: A sand particle size of 1.

1 mm that scratches the tool block up to 1.25 50 mm (a) at the start of scratching (b) at the end of scratching.

FIGURE 4.12: The contact between the sand particle and the tool block is removed 50 as the particle moves away from the block surface.

Sand particle size of 0.5 mm.

FIGURE 4.13: The contact between the sand particle and the tool block is removed 51 as the particle moves away from the block surface.

Sand particle size of 1.1 mm.

FIGURE 4.14: A magnified view of the deformed tool surface that shows groove for- 51 mation and material pile-up on the groove edges with a sand particle size of

0.5 mm.

FIGURE 4.15: Figure shows that the permanent deformation is observed on the tool 52 block with a sand particle size of 1.

1 mm.

FIGURE 4.16: A front view of the block section that shows permanent deformation 52 (a) with a large sand particle size of 0.

5 mm (b) with a very large sand particle size of 1.1 mm.

FIGURE 4.17: The amount of tool material loss with single sand particle sizes ranging 55 from 0.

5 mm to 2.0 mm with an approximate penetration depth (i.e., 10% of the particle size).

–  –  –

FIGURE 5.4: In the initial time step, contact is created between the rigid tool and 67 the soil block.

FIGURE 5.5: Boundary conditions used in the plane-strain tilling analyses.

68

–  –  –

FIGURE 5.7: An enlarged view of a section of the soil block subjected to gravitational 69 loading prior to tilling.

FIGURE 5.8: Soil deformation when the tool has traveled approximately 10 cm along 70 the length of the soil block.

–  –  –

FIGURE 5.11: A comparison of the power consumed for different tractor speeds.

72 FIGURE 5.12: A comparison of power needs predicted numerically and experimen- 73 tally, showing the effects of soil moisture content.

–  –  –

Abrasive wear is one of the primary mechanisms by which tillage tools experience material loss during soil tillage. It has been observed that the tool material loss can be as much as 10 gm per kilometer of tilling. The worn tool (Figure 1.1) leads to poorly tilled soil and consequently needs to be replaced. The replacement can be as frequent as two to three days. Such frequent requirements coupled with the fact that the replacement tools may not be readily available leads to frequent downtimes and consequently significant increases in operational costs. The costs can be further higher if the wear is severe enough that the entire plowing assembly needs to be replaced. Tool wear is also associated with the power required to till the soil. As the tool wears, more energy is required to achieve the same tillage quality. This has the effect of increasing the overall cost of tillage operation.

Figure 1.1: A worn-out tillage tool that needs replacement, Santa F´ de Antioquia, e Colombia.

2 Sand and gravel are the primary constituents of a typical soil that cause wear in tillage tools. During the soil-tool interaction, the tool wear rates and power consumption are also influenced by a number of other operating parameters associated with the tilling operation. The moisture content of the soil has the effect of reducing the shear strength of the soil. The plowing depth, the plowing/tillage speed and the tool properties are also possible factors influencing tool wear. Consequently, it is important to develop a detailed model of soil-tool interaction to understand the effect that these parameters have on tool wear and on the power requirements for tilling.

1.1 Materials and Methods

The tillage process examined here is carried out with a single tillage tool (also termed a chisel) attached to a plowing assembly. The complete assembly plows the soil at a constant depth of 300 mm from the surface of the soil. This process involves a tractor carrying three individual plowing assemblies that travel at an average linear speed of 3-5 km/hr (Figure 1.2).

Figure 1.2: An arrangement of three plowing assemblies attached to a tractor used for soil tillage.

Soil tillage is a complex tribological phenomenon that involves friction and wear with negligible lubrication. The plowing assembly carrying a tillage tool is moved 3 horizontally at a constant (tillage) speed with a prescribed depth (also called the plowing depth). Figure 1.3 shows this. The tool movement causes shear failure in the soil. This results in the soil breaking into smaller aggregates along the tool surfaces.

The broken soil particles slide along the tool surfaces. The various soil particles induce tool surface deformation to differing degrees of intensity with some causing large enough stresses and deformation to cause material separation at the tool surface and thus resulting in tool wear. Plowing operations and the quality are influenced by a number of factors [44]. These variables can be intrinsic or external to the system, and they affect the rate of tool wear during tilling operations [40, 59, 70].

Figure 1.3: Plowing operation in a silt loam soil, Santa F´ de Antioquia, Colombia.

e In this work, observations are included from field tests conducted by the Tribology and Surfaces Group, School of Materials Engineering, National University of Colombia-Medellin. These tests were carried out to investigate the effect of various parameters, such as plowing depth, soil resistance to penetration, soil moisture content and tillage speed on the tool life. Similar studies from both experimental and analytical points of view have been made previously [5,27,29,44,54]. The observations from the field tests carried out in Colombia are outlined in the following sections.

4 1.1.1 Tillage Tool Material Figure 1.4 shows a schematic of a tillage tool attached to a plowing assembly.

The tillage tool is made from heat-treated alloy steel. The purpose of heat-treating the tool material is to improve its hardness and to lower tool wear. The tillage tools used in the field tests conducted in Colombia were made of heat-treated DIN 30MnB5 alloy steel.

–  –  –

Figure 1.4: (a).

The plowing assembly and (b). A 2D schematic of a tillage tool.

The tests were conducted with a tillage tool whose dimensions are 6 cm in width and 12 cm in length. An unworn tool and a worn tool are shown in Figure 1.5. The worn tool is shown along with the unworn tool to highlight the severity of wear that is experienced in tilling operations. Clearly, the worn tool, if not replaced, compromises the tillage quality. The tool wear is measured by the amount of lost material. This amount is estimated by measuring the difference in the weight of a tool before and after the tillage process (Figure 1.6) [61].

1.1.2 Soil Material

–  –  –

Figure 1.5: A comparison of an unworn tillage tool with a worn tillage tool, (a).

top view, and (b). side view.

Figure 1.6: Tool material loss is measured as the difference in the weights of a tillage tool before and after the soil tillage.

–  –  –

Figure 1.7: A view of a field on completion of a tillage process in Colombia.



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