Finite Element Analysis and Simulation Study on Micromachining of Hybrid Composite Stacks Using Micro Ultrasonic Machining Process ______

FINITE ELEMENT ANALYSIS AND SIMULATION STUDY ON MICROMACHINING OF HYBRID COMPOSITE STACKS USING MICRO ULTRASONIC MACHINING PROCESS ______

A Thesis

Presented to the

Faculty of

California State University, Fullerton ______

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Mechanical Engineering ______

Panchal, Sagar Rajendrakumar

Thesis Committee Approval:

Sagil James, Department of Mechanical Engineering Nina Robson, Department of Mechanical Engineering Darren Banks, Department of Mechanical Engineering

Spring, 2018

ABSTRACT

Hybrid composites stacks are multi-material laminates which find extensive applications in industries such as aerospace, automobile, and electronics and so on. Most hybrid composites consist of multiple layers of fiber composites and metal sheets stacked together. These composite stacks have excellent physical and mechanical properties including high strength, high hardness, high stiffness, excellent fatigue resistance and low thermal expansion. Micromachining of these materials require particular attention as conventional methods such as micro-drilling is extremely challenging considering the non-homogeneous structure and anisotropic nature of the material layers. Micro

Ultrasonic Machining (μUSM) is a manufacturing process capable of machining such difficult-to-machine materials with ultraprecision. Experimental study showed that

μUSM process could successfully machine hybrid composite stacks at micron scale with a relatively good surface finish. This research uses finite element simulation technique to investigate the material removal during the μUSM process for micromachining hybrid composite stacks. The effects of critical process parameters including the amplitude of vibration, feed rate and tool material on the cavity depth, cutting force and equivalent stress distribution are studied. The outcome of this research can be utilized to further our understanding of performing precision machining of hybrid composite stacks for use in several critical engineering applications.

TABLE OF CONTENTS

ABSTRACT ...... ii

LIST OF TABLES ...... iv

LIST OF FIGURES ...... v

ACKNOWLEDGMENTS ...... vii

Chapter 1. INTRODUCTION ...... 1

2. FINITE ELEMENT ANALYSIS AND SIMULATION ...... 9

Damage Criteria ...... 13 Failure Criteria for CFRP Substrate...... 13 Failure Criteria for Ti Substrate ...... 15

3. RESULTS AND DISCUSSION ...... 16

Variation in Cavity Depth with Time during the μUSM process ...... 17 Variation in Cutting Force with Time during the μUSM process ...... 20 Variation in Equivalent von Mises Stress with Time during the μUSM process . 22 Effect of Vibration Amplitude during the μUSM process ...... 26 Effect of Feed Rate during the μUSM process ...... 30 Effect of Tool Material during the μUSM process ...... 33

4. COMPARISON BETWEEN EXPERIMENTAL AND SIMULATION RESULT...... 36

5. CONCLUSION ...... 38

RESEARCH DISSEMINATION ...... 41

REFERENCES ...... 42

iii

LIST OF TABLES

Table Page

1. Material Properties used in FE Simulation Study of μUSM Process ...... 11

2. Material Properties of CFRP Substrate ...... 11

3. Conditions Used for FE Simulation of µUSM of CFRP/Ti Stacks ...... 12

4. Damage Parameters of CFRP Substrate ...... 14

LIST OF FIGURES

Figure Page

1. Schematic Representation of Micro Ultrasonic Machining Process ...... 6

2. Experimental Setup of Micro Ultrasonic Machining Process ...... 6

3. Microcavities Machined on CFRP/Ti Stacks using µUSM Process a) CFRP Entrance Side and b) Ti Exit Side ...... 7

4. Finite Element Simulation Model of μUSM Process of a) CFRP Substrate and b) Ti Substrate ...... 10

5. Finite Element Simulation Model of μUSM Process showing Damage on a) CFRP Substrate and b) Ti Substrate ...... 17

6. Variation in Cavity Depth during μUSM process on CFRP and Ti Substrates ... 19

7. Cavity Machined (Magnified View) during μUSM Process on a) CFRP Substrate and b) Ti Substrate ...... 19

8. Variation in Cutting Force during μUSM Process on CFRP and Ti Substrates .. 22

9. Variation in Equivalent von Mises Stress during μUSM Process on CFRP and Ti Substrates ...... 23

10. Variation in Normal Stress acting on Ti Substrate during μUSM Process ...... 25

11. Variation in Plastic Strain in Ti Substrate during μUSM Process ...... 25

12. Equivalent von Mises Stress Distribution during μUSM Process on a) CFRP Substrate and b) Ti Substrate ...... 26

13. Effect of Amplitude of Vibration on Cavity Depth during μUSM Process on CFRP and Ti Substrates ...... 28

14. Effect of Amplitude of Vibration on Cutting Force during μUSM Process on CFRP and Ti Substrates ...... 28

15. Effect of Amplitude of Vibration on Equivalent von Mises Stress during μUSM Process on a) CFRP Substrate and b) Ti Substrate ...... 29

16. Effect of Feed Rate on Cavity Depth during μUSM Process on CFRP and Ti Substrates ...... 31

17. Effect of Feed Rate on Cutting Force during μUSM Process on CFRP and Ti Substrates ...... 32

18. Effect of Feed Rate on Equivalent von Mises Stress during μUSM Process on a) CFRP Substrate and b) Ti substrate ...... 33

19. Effect of Tool Material on Cavity Depth during μUSM Process on CFRP and Ti Substrates...... 35

20. Effect of Tool Material on Cutting Force during μUSM Process on CFRP and Ti Substrates...... 35

21. Comparison of MRR during µUSM Process of CFRP/Ti Stack using WC and Cu Tools ...... 37

ACKNOWLEDGMENTS

I would like to thank Dr. Susamma Barua, Interim Dean, and Dr. Sang June Oh,

Interim Associate Dean Engineering and Computer Science, California State University

Fullerton.

I would like to offer my special thanks to Dr. Chean Chin Ngo, Chair, Department of Mechanical Engineering, CSUF for granting me permission and giving an opportunity to pursue my thesis in Advanced Manufacturing.

I would like to express my very great appreciation to my faculty advisor Dr. Sagil

James for his valuable suggestions, enthusiastic guidance and persistent supervision, which were essential for the completion of this thesis research work. I am obligated to him for his constant inspiration and meticulous efforts in amending errors and suggesting improvements.

I would like to thank Dr. Nina Robson and Dr. Darren Banks for being in the thesis approval committee. I wish to acknowledge the help provided by all members of the Titan Advanced Manufacturing Laboratory who supported me throughout my research work. I would like to thank Administrative support: Crystal Barnett and

Charlotte Sanchez.

Special thanks to my parents and family members for their unconditional love along with their support and encouragement.

vii 1

CHAPTER 1

INTRODUCTION

Fiber reinforced polymers (FRP) are composites of high-strength fibers embedded in a matrix of polymer material (Ku, Wang, Pattarachaiyakoop, & Trada, 2011). The fibers are generally made of materials such as carbon fiber, glass fiber, basalt or aramid which have high strength, high stiffness and low density (Bakis et al., 2002). FRPs have several advantages including low-cost, corrosion resistance, lightweight, inherent durability, high strength, eco-friendly, and biodegradability (Ku et al., 2011). These properties make FRPs ideal choice of material for several applications including aerospace, automobile, construction, medical technologies and so on (Mallick, 2007).

Among the various FRPs, Carbon Fiber Reinforced Polymer (CFRP) has gained particular attention due to its high strength-to-weight ratio, excellent fatigue resistance, high dimensional stability, low thermal expansion, and excellent tensile strength (Meier,

1992). However, CFRP materials have several limitations including low strength in specific directions considering its anisotropic nature, high susceptibility to fracture due to its brittleness and low wear resistance (Campbell, 2010).

To overcome the limitations, FRPs are often stacked with metal alloys to form multi-layers of hybrid composite stacks (Brinksmeier & Janssen, 2002). The addition of thin layers of metal alloys to FRPs enhances their ability to resist high impact loads and improve the elastic modulus without a significant increase in weight. These hybrid 2 composite stacks have been increasingly popular and are used as an attractive alternative for traditional composites and metal alloys (Asundi & Choi, 1997). Studies have reported as much as 35% reduction in mass of the structure by replacing metals and metal alloys with hybrid composite stacks (Sairajan, Aglietti, & Mani, 2016). CFRP composites stacked with thin layers of lightweight metals such as Copper (Cu), Aluminum (Al),

Titanium (Ti) and their alloys have been identified as an innovative material for several critical engineering applications (Cheng, Tsui, & Clyne, 1998; Kuo, Wang, & Liu, 2017;

Schatzel, 2009). The CFRP metal stacks exhibit high load-bearing capability and excellent impact and shock resistance (Garrick, 2007). The fuselage, wings and tail-plane components of modern day aircraft including Airbus A380 or Boeing 787 contain stacks of hybrid composite metal stacks such as CFRP, CFRP/Ti, and CFRP/Al and so on

(Pramanik & Littlefair, 2014). The addition of thin layers of metals or metal alloys on

CFRP enhances the structure’s ability to withstand high mechanical loads, resulting in increased strength-to-weight ratio and thereby reducing the fuel consumption (Shyha et al., 2011). Some of the other critical applications of CFRP metal stacks include modern automobiles where CFRP/Al stack is used for exterior body components, rotor blades of helicopters consisting of CFRP/Al stacks and so on (Möller et al., 2010).

Most of the applications mentioned here require machining or drilling of the hybrid composite metal stacks to the required precision (Brinksmeier & Janssen, 2002;

Hashish, 1991). In the past, there have been several attempts on drilling hybrid stacks such as CFRP/Ti and CFRP/Al through conventional drilling in a single shot operation

(Brinksmeier & Janssen, 2002; Isbilir & Ghassemieh, 2013; K.-H. Park, Beal, Kwon, &

Lantrip, 2014; SenthilKumar, Prabukarthi, & Krishnaraj, 2013; Zitoune, Krishnaraj, &

Collombet, 2010). However, most of these studies have reported difficulties associated with the drilling operation including low tool life, severe damage and delamination to

CFRP layers and clogging of drill flutes (Brinksmeier & Janssen, 2002; Isbilir &

Ghassemieh, 2013; Senthil Kumar et al., 2013). These difficulties can be attributed to the poor machinability of the stacked constituents and the differences in properties across the thickness of the stacks (Brinksmeier & Janssen, 2002). Similarly, studies done on drilling

Fiber Metal Laminates (FML) consisting of stacks of Glass Fiber Reinforced Polymer

(GFRP) and Aluminum sheets have reported drilling-induced damages and delamination

(Giasin & Ayvar-Soberanis, 2017). There have been reports of machining hybrid composite metal stacks using non-traditional machining processes such as Abrasive

Waterjet Machining (AWJM) (Alberdi, Artaza, Suárez, Rivero, & Girot, 2016), Rotary

Ultrasonic Machining (RUM) (Cong, Pei, & Treadwell, 2014), and Electrical Discharge

Machining (EDM) (Ramulu & Spaulding, 2016). Studies on drilling of CFRP/Ti stacks through RUM process suggested longer tool life and better surface quality in RUM compared to conventional drilling (Cong et al., 2014; Cong, Pei, Deines, Liu, &

Treadwell, 2013). Studies on both AWJM process and EDM process reported limitations in machining hybrid composite stacks (Alberdi et al., 2016; Ramulu & Spaulding, 2016).

While AWJM causes delamination of FRP layers, EDM process produces extremely low surface finish along with cracks in the matrix material (Alberdi et al., 2016; Ramulu &

Spaulding, 2016).

Growing demand for micro-sized components with enhanced performance necessitates the need for producing micro-scale features on the composites and the hybrid composite stacks with ultraprecision and high accuracy. Typical examples include micro-

4 holes on Printed Circuit Boards (PCB) for microelectronics and optoelectronics industries

(Rahamathullah & Shunmugam, 2013), micro-perforations on aircraft wings and tail surfaces (Cheng et al., 1998) and micro-perforated composite panel absorbers (S.-H.

Park, 2013). While the micro-perforations on aircraft wings help reduce the airflow turbulence and increase fuel efficiency (Rahamathullah & Shunmugam, 2013), the micro- perforated panels help in acoustic absorption and noise control (Shen & Jiang, 2014). In the majority of these applications, the micro-holes and micro-perforations are drilled on the composite surface and not on hybrid composite stacks. Moreover, the techniques used for drilling these microfeatures on composite materials include mechanical micro-drilling and laser drilling (Matthams & Clyne; Rahamathullah & Shunmugam, 2013). However, these techniques are incapable of accurately machining hybrid composite stacks at the micron scale. Laser drilling of laminar stacks is a considerable challenge considering the extreme differences in material properties across the thickness of the stacks (Hoult,

2014). Additionally, laser machining has limitations in cutting thickness and causes high thermal damage and microstructural changes around the machined region resulting in loss of strength in the materials (Mistry & James, 2017). Mechanical micro drilling processes such as twist drilling and end milling have been traditionally used in PCB industries over the years (Cong et al., 2014). However, these techniques have limitations while drilling difficult-to-machine materials such as titanium (Cong et al., 2014). Limitations of micro- drilling include high cutting forces and cutting temperatures, composite delamination, poor surface finish and significant hole-size variation (Cong et al., 2014).

Problems in micromachining CFRP/Ti stacks can be overcome by using Micro

Ultrasonic Machining (μUSM) process. The μUSM process uses a vibrating micro tool

5 and a slurry consisting of hard abrasive particles to remove material from the substrate surface through repeated impacts (Egashira & Masuzawa, 1999). The schematic of the

μUSM process is shown in Figure 1. The μUSM process is capable of micromachining hard and brittle materials as well as both electrically conductive and non-conductive materials. The μUSM process produces surfaces without any thermal damage, results in crack-free machining and with only minimal residual stresses (Egashira & Masuzawa,

1999). Recently, our research group studied the micromachining of hybrid composite stacks consisting of CFRP/Ti using μUSM process (James & Sonate, 2017). The study used tungsten carbide (WC) and Cu micro tools for micromachining stacks of CFRP/Ti.

Figure 2 shows the experimental setup used for micromachining CFRP/Ti stacks using

μUSM process. The study found that μUSM process is capable of successfully machining

CFRP/Ti stacks at micron scale with a relatively good surface finish and no CFRP delamination. Representative images of micro-holes machined on CFRP/Ti stacks using

μUSM process is shown in Figure 3.

Figure 1. Schematic Representation of Micro Ultrasonic Machining Process (James & Sonate, 2017).

Figure 2. Experimental Setup of Micro Ultrasonic Machining Process (James & Sonate, 2017).

(a) (b)

Figure 3. Microcavities Machined on CFRP/Ti Stacks using µUSM Process a) CFRP Entrance Side And b) Ti Exit Side (James & Sonate, 2017).

Our past experimental studies suggested that simultaneously ensuring both quality and efficiency during the μUSM process is challenging (James & Sonate, 2017). The challenges include proper selection of operating conditions along with appropriate tool and abrasives. Similarly, an experimental study on RUM process of machining CFRP/Ti stacks at macron scale reported that using variable feed rates leads to lower cutting forces, lower tool wear and shorter cycle times compared to fixed feed rates (Cong et al.,

2013). The study recommends using almost ten times higher feed rate while machining

CFRP compared to Ti. These studies suggest that it is critical to use optimized machining conditions while machining hybrid composite stacks during the μUSM process to achieve good surface quality, increased tool life and shorter machining time. Determining optimal machining conditions will require better insight of critical parameters involved in the

μUSM process including cutting force, tool force, and stress distribution and so on.

Considering the complexity and dynamic nature of the process, determining these critical process parameters through experimental studies is extremely inconvenient and time- consuming.

Simulation techniques such as Finite Element Analysis (FEA) are ideal in such cases. However, there have only been very few simulation studies reported on the μUSM

8 process. A simulation study performed by Wang et al. used Smoothed Particle

Hydrodynamics (SPH) mesh-free method to understand the material removal in μUSM process of glass (J. Wang, Shimada, Mizutani, & Kuriyagawa, 2018a). It was found that crack generation influences the material removal process in the workpiece and the wear of abrasive grains. Another study by the same authors investigated the effect of tool materials during the μUSM process and found that WC tool undergoes less wear compared to stainless steel tool (J. Wang, Shimada, Mizutani, & Kuriyagawa, 2018b). A preliminary simulation study on CFRP/Ti machining using μUSM process done by authors found that there are minimal surface residual stresses on the workpiece after machining (Sonate, Vepuri, & James, 2017). To our best knowledge, there has been no study reported on FEA simulation of the μUSM process for micromachining of hybrid composite stacks.

The goal of this research is to use finite element analysis and simulation technique to study the μUSM process for micromachining hybrid composite stacks. In this study,

CFRP/Ti stacks are used as the hybrid composite material. The study investigates the material removal during the μUSM process in terms of critical parameters including cavity depth, cutting force, and equivalent von Mises stress distribution.

CHAPTER 2

FINITE ELEMENT ANALYSIS AND SIMULATION

The μUSM process is simulated using commercial finite element analysis tool of

MSC Marc/Mentat (Santa Ana, CA, USA) (Marc & Volume, 2010). In this simulation study, a two-dimensional (2D) Finite Element (FE) analysis technique is used to investigate the material removal during the μUSM process of CFRP/Ti stack. The FEA model of the μUSM process consists of a vibrating tool, substrate, and abrasive particles.

The FE simulation model used in this study is shown in Figure 4. The tool is considered as a rigid body and is rectangular (size 8 μm x 20 μm) in shape dividing into 1200 elements with a meshing of 20x60. A negative bias factor of 0.5 is provided on the tool to refine the mesh near machining zone. The tool materials used in the study are WC and

Cu. Three spherical particles having a diameter of 2 µm is used as the abrasive medium.

The reason for considering three particles is to understand the neighboring effects of material damage during the machining process. The abrasive particles are made of silicon carbide and are meshed using a quadrilateral mesh (20x20x5). The material properties of the tool and abrasive particles are shown in Table 1.

The substrate materials of CFRP and Ti are separately analyzed during the simulation. The schematic of the FE simulation model of μUSM Process on CFRP and Ti substrates are shown in Figure 4a and 4b respectively. Both CFRP and Ti substrates are rectangular (size 100 μm x 30 μm) divided into 5000 elements with a meshing of 100x50.

The CFRP substrate is made of carbon fiber/epoxy unidirectional laminates. Each laminate consists of four plies in quasi-isotropic laminate orientations of 0°, 45°, -45°, and 90°. This orientation helps carry the load equally in all directions and is the most used configuration for CFRP laminates (G.-D. Wang & Melly, 2018). The mechanical properties of CFRP composite used for the FE simulation study are shown in Table 2

[38]. The Ti substrate is made of Ti-6Al-4V alloy which is the most commonly used Ti alloy. The material properties of Ti are shown in Table 1. A positive bias factor of 0.5 is provided on both the substrates to refine the mesh near machining zone. 4-node isoparametric, quadrilateral plane strain elements (element type 11) is used with geometric and material non-linearity for the tool, abrasive particles, and Ti substrate. 4- node, plane strain, composite element (element type 151) is used for modeling the CFRP substrate. The automatic global remeshing feature in MSC Marc is used to increase the accuracy of the simulation and reduce computational time.

(a) (b)

Figure 4. Finite Element Simulation Model of μUSM Process of a) CFRP Substrate and b) Ti Substrate.

Table 1. Material Properties used in FE Simulation Study of μUSM Process

Elastic modulus, E Poisson’s Density, ρ Material (GPa) ratio, ν (Kg/m3) Tool – Tungsten Carbide (Li et al., 2015) 600 0.25 15880 Tool – Copper (Ledbetter & Naimon, 1974) 128 0.36 8960 Abrasive – Silicon Carbide (Iyer, 2007) 137 0.37 4840 Substrate – Titanium (Xi, Bermingham, Wang, 113 0.342 4430 & Dargusch, 2013)

Table 2. Material Properties of CFRP Substrate

E11 E22 E33 υ12 υ13 υ23 G12 G23 G13 ρ 127 GPa 9.1 GPa 9.1 GPa 0.31 0.31 0.45 5.6 GPa 4 GPa 5.6 GPa 1600 Kg/m3

The model is analyzed for a total simulation duration of 250 microseconds (µs).

The tool is subject to ultrasonic vibrations according to a sinusoidal function as shown in the equation below for a frequency of 20 KHz. The displacement X of the tool can be expressed as

X =Amplitude × Sin(2 × π × Frequency × Time) (1)

The amplitude values used in this study are 2 μm, 3 μm, and 4 μm. A constant feed rate is given by the movement of the substrate in the positive Y-direction. The feed rate values used in this study are 5 μm/s, 10 μm/s, and 15 μm/s.

The underlying assumptions used in this simulation study are 1) Abrasive particles are assumed to be spherical in shape 2) CFRP substrate is considered as elastic- plastic orthotropic and undergo brittle mode failure 3) Ti substrate is considered as elastic-plastic isotropic and undergo ductile mode failure 4) Three abrasive grains are sufficient to explain the material removal process and 5) Both the tool and abrasive particles do not undergo wear.

The boundary conditions used in the simulation are 1) The tool is considered to be rigid and is constrained to move only in Y-direction 2) The boundary layer of the substrate material is constrained in X-direction and 3) The abrasive grain is considered to be rigid. A gravity force is applied to the abrasive grain with an acceleration of 9.8 m/s2 in the negative Y-axis direction. The conditions used for the FE simulation study of

μUSM Process on CFRP/Ti stacks are shown in Table 3.

Table 3: Conditions Used for FE Simulation of µUSM of CFRP/Ti Stacks

Parameter Unit Value Tool Material WC, Cu Tool Dimensions µm 8x20 Substrate Material CFRP, Ti Substrate Dimensions µm 100x50

Abrasive Material SiC Abrasive Particle Size µm 2 Number of Abrasives 3 Initial Machining Gap µm 3 Frequency KHz 20 Amplitude µm 2, 3, 4 Feed Rate µm/s 5, 10, 15 Duration of Simulation µs 250

Damage Criteria

Damage plays a significant role in the material removal during the μUSM process on CFRP and Ti substrates. In this simulation study, the damage initiation criterion is defined according to the material behavior to determine the condition for the onset of damage on CFRP and Ti substrates. The damage criteria used for CFRP substrate for this

13 study is Hashin Criteria (Hashin, 1980). The damage of Ti substrate in this study is modeled according to the Cockcroft-Latham Criteria (Cockcroft & Latham, 1968).

Failure Criteria for CFRP Substrate

The CFRP composites consist of the carbon fiber reinforcements inside a polymer matrix. Considering the two phases involved, CFRP material could primarily undergo two different failure modes – matrix failure and fiber failure (G.-D. Wang & Melly,

2018). CFRP could also be subject to inter-laminar failure commonly referred to as delamination. Moreover, the damage in CFRP is initiated without any significant plastic deformation, and hence plasticity can be neglected while modeling CFRP (Lapczyk &

Hurtado, 2007). Among the several theories that explain the composite failures, Hashin criterion is the most widely used theory for CFRP failure (G.-D. Wang & Melly, 2018).

Hashin criterion takes into account four distinct failure modes for the CFRP composites which are fiber tension, fiber compression, matrix tension, and matrix compression as expressed by (Hashin, 1980)

퐹푖푏푒푟 푡푒푛푠푖표푛 휎 ≥0

휎 휎 + 휎 ≥ 1 푓푎푖푙푢푟푒 + = (2) 푋 푆 < 1 푛표 푓푎푖푙푢푟푒

퐹푖푏푒푟 푐표푚푝푟푒푠푠푖표푛 휎 <0

휎 ≥ 1 푓푎푖푙푢푟푒 = 푋 < 1 푛표 푓푎푖푙푢푟푒 (3)

푀푎푡푟푖푥 푡푒푛푠푖표푛 휎 + 휎 >0

휎 + 휎 휎 + 휎 휎 휎 + 휎 + + 푌 푆 푆 (4) ≥ 1 푓푎푖푙푢푟푒 = < 1 푛표 푓푎푖푙푢푟푒

푀푎푡푟푖푥 퐶표푚푝푟푒푠푠푖표푛 휎 + 휎 <0

( ) (5) − 1 + + + =

≥ 1 푓푎푖푙푢푟푒 < 1 푛표 푓푎푖푙푢푟푒

where σij is the stress components, subscripts T and C are the tensile and compressive strengths of the laminate, XT and YT are the allowable tensile strengths, XC and YC are the allowable compressive strengths, and S12, S13, and S23 are the allowable shear strengths.

The input values for Hashin criterion damage parameters used in this study are listed in

Table 4 (Phadnis, Makhdum, Roy, & Silberschmidt, 2013).

Table 4. Damage Parameters of CFRP Substrate

XT YT XC YC S12 S23 2720 MPa 111 MPa 1690 MPa 214 MPa 115 MPa 40 MPa

Failure Criteria for Ti Substrate

In this study, the failure of Ti substrate during the μUSM process is modeled according to Cockcroft-Latham criterion. This criterion is generally applicable for ductile fracture and considers the fact that failure occurs when the plastic work per unit volume exceeds a critical value (Kuhn, 2013). While this theory is primarily applied to tensile tests, it has also been extended for applications involving compression, shearing and fatigue-based failures (Kuhn, 2013). Cockcroft-Latham damage indicator is a post-

15 processing value used in MSC Marc to indicate a possible damage area (Marc & Volume,

2010). The normalized Cockcroft-Latham failure criterion can be expressed as

휎 휀̇푑푡̅ ≥ 퐶 (6) 휎 where 휎 is the maximum principal stress, 휎 is the effective von Mises stress, 휀̇ ̅ is the effective plastic strain rate and C is the material constant threshold for damage. The value of material constant threshold C, used in this study for Ti failure is 0.61 (Mirahmadi,

Hamedi, & Ajami, 2014; Mirahmadi, Hamedi, & Parsa, 2014).

CHAPTER 3

RESULTS AND DISCUSSION

The results of the finite element simulation of μUSM process on CFRP and Ti substrates using WC tool for a vibration frequency of 20 KHz, the amplitude of 2 μm and feed rate of 5 μm/s are shown in Figures 5a and 5b respectively. The color bar used in these figures represent the damage value ranging from 0 to 1. From the figures, it is observed that the CFRP substrate undergo more damage than Ti substrate during the machining process. CFRP substrate undergoes damage in brittle mode through microchipping and microcracking. The material removal in Ti substrate happens in ductile mode through plastic deformation. It is also observed that the cavity produced on

Ti is uneven compared to that of CFRP. The following sections describe the effects of critical process parameters including vibration amplitude, feed rate, and tool material on the cavity depth, cutting force, and equivalent von Mises stress distribution during the

μUSM process.

(a) (b) Figure 5. Finite Element Simulation Model of μUSM Process showing Damage on a) CFRP Substrate and b) Ti Substrate.

Variation in Cavity Depth with Time during the μUSM process

During the μUSM process, the material removal happens due to the impact of the abrasive grain on the substrate surface. The cavity depth is measured to analyze the extent of material removal from the substrates during the machining process. In this study, the cavity depth is defined as the vertical distance from the entrance of the cavity

(the surface which has not been machined) to the bottom of the cavity. The variation in cavity depth with respect to time during the μUSM process on CFRP and Ti substrates is shown in Figure 6. This study uses a WC tool with a vibration amplitude of 2 μm, the feed rate of 5 μm/s and simulation during of 250 μs. From the figure, it can be seen that the initial impact happens after a duration of approximately 40 μs. It is due to the initial machining gap provided at the start of the simulation. The figure suggests that during the

μUSM process, the cavity depth on CFRP substrate is larger than that on the Ti substrate.

Figures 7a and 7b show the magnified view of the cavity depth obtained during the

μUSM process on CFRP and Ti substrates respectively. The larger cavity depth on CFRP

18 can be explained by the fact that it undergoes brittle mode failure giving rise to microchipping and microcracking resulting in increased material removal.

The relatively lower cavity depth on Ti substrate is attributed to the material removal mechanism involved in the process. The Ti substrate undergoes plastic deformation in ductile mode. The plastic deformation phase during the ductile mode machining is characterized by microplowing, microcutting and occasionally microcracking mechanisms. As the abrasive particles interact with the Ti surface with energy exceeding the yield point, the substrate material starts to undergo plastic deformation through microplowing. The microplowing mechanism causes the material to pile-up causing an uneven surface. The subsequent hammerings of the tool on the abrasive particles cause deeper penetration and microcutting of the Ti material. As the microcutting progresses over the time the materials are removed in the form of microchips from the substrate. It should be noted that not all impacts of tool results in material removal from the Ti substrate as the energy of these impact could only cause elastic deformations. The elastic deformations are characterized by either no wear, adhesion of abrasive particles on the surface as well as rebounding of the deformed surface as the tool moves away from the substrate surface. Also, during both microplowing and microcutting regimes, the plastically deforming Ti material offers significant resistance to the penetrating abrasive particles and the tool. The combination of these effects causes lesser material removal from the substrate and results in shallow cavity formation. If the abrasives are penetrated further into the bulk of the substrate, the microcutting mechanism changes to the microcracking mechanism. However, the

19 duration of the present simulation study is limited to the microplowing and microcutting regimes.

Figure 6. Variation in Cavity Depth during μUSM Process on CFRP and Ti Substrates.

(a) (b)

Figure 7. Cavity Machined (Magnified View) during μUSM Process on a) CFRP Substrate and b) Ti Substrate.

Variation in Cutting Force with Time during the μUSM process

During the μUSM process, the abrasive particles could have two different forms of interactions with the substrate – a) non-contact mode and contact mode. During the non- contact mode, the abrasive particles are in the suspended state at the time of tool strike.

The particles gain kinetic energy and impact the substrate surface with high velocity resulting in material removal. The contact mode considers the abrasive particles are resting on the substrate surface while the vibrating tool intermittently hammers them. The hammering action of the abrasive particles results in the penetration into the bulk of the substrate. The present simulation study considers the abrasive particles in suspended state initially. However, as the simulation begins the particles settles on the substrate surface due to the force of gravity. It means that the abrasive-substrate interaction in this study would be primarily in contact mode where the tool repeatedly hammers the particles. The intermittent hammering of the tool on the abrasive particles resting on the substrate surface results in the generation of a positive pulsating cutting force in each cycle of ultrasonic vibration.

Cutting force is a hugely significant factor in any machining process which affects the machining performance, quality of machining and tool wear. The problem of tool wear is further escalated at micron scale since even small wear can considerably affect the machining accuracy. It is understood that the cutting force during the μUSM process is much lower than that during conventional micromachining processes such as micro- drilling. It is because during the μUSM process the tool does not directly contact the substrate. Nevertheless, cutting force affects the machining quality and tool life during the

μUSM process. In this study, the cutting force is defined as the maximum value of the

21 magnitude of the normal force acting on the substrate during the μUSM process. The variation in cutting force with respect to time during the μUSM process on CFRP and Ti substrates is shown in Figure 8. The study uses WC tool with an amplitude of 2 μm, the feed rate of 5 μm/s and simulation during of 250 μs. From the figure, it can be seen that the cutting forces follow a pulsating pattern during the machining process. The cutting force acting on CFRP substrate is lower than that on Ti substrate. This can be explained as follows. The material removal of CFRP happens in brittle mode due to microchipping followed by microcrack formation. The material is chipped off the surface as the cutting force exceeds the critical limit based on the Hashin failure criterion. The chipping of the brittle material followed by microcrack growth gradually decreases the resistance of the substrate against the tool. This in turn reduces the cutting force during the μUSM process on CFRP substrate. Moreover, the cutting forces tends to be minimal (almost zero) on

CFRP when the tool is away from the abrasive particle. This is due to the periodic removal of CFRP material due to microchipping.

During the μUSM process on Ti substrate, the material removal happens in ductile mode due to plastic deformation. As the tool strikes intermittently, the abrasive particles penetrate into the bulk of the titanium substrate causing microplowing and microcutting. However, due to the ductile behavior of Ti along with its superior fracture toughness, the abrasive grain is deflected off the surface resulting in material displacement with minimal removal. Further, the plasticity of the Ti material resists the growth of microcracks. Consequently, a higher cutting force is generated as the substrate material opposes the tool motion. The higher cutting forces result in the reduced surface finish during the μUSM process on Ti. It is also noted that the cutting forces on

22 successive peaks increases during machining. It suggests that the material could be subject to work hardening which could favor microchip formation. Additionally, the Ti material shows a positive cutting force even when the tool is away from the abrasive. It can be attributed to the fact that the Ti material continues to interact with the abrasive particles while undergoing plastic deformation and material flow.

Figure 8. Variation in Cutting Force during μUSM Process on CFRP and Ti Substrates.

Variation in Equivalent von Mises Stress with Time during the μUSM process

The μUSM process is capable of producing surfaces with minimal residual stresses. However, stress formation plays a crucial role in removal material during the

μUSM process. The material removal during the μUSM process happens due to the intermittent hammering action of abrasive particles on the substrate surface. As the abrasive particle penetrates into the bulk of the substrate, stresses are generated at the abrasive-substrate interface. Tiny highly stressed regions are formed on the substrate

23 surface leading to micro-crack formation. Figure 9 shows the variation in the magnitude of equivalent von Mises stress during the μUSM process on CFRP and Ti substrates. The study considers the maximum value of the equivalent von Mises stresses acting on the substrate. For this study, the amplitude and feed rate values used are 2 μm and 5 μm/s respectively. The tool used is made of WC material, and the simulation duration is 250

μs. The figures suggest the formation of intermittent peaks of von Mises stresses during the machining process. It is seen that the peak values of stresses acting on Ti substrate

(~700 MPa) are considerably larger than that on CFRP substrate (~400 MPa).

Figure 9. Variation in Equivalent von Mises Stress during μUSM Process on CFRP and Ti Substrates.

The higher values of peak von Mises stress on Ti can be attributed to its ductile behavior and plastic deformation during the machining process. On the other hand, the

CFRP material undergoes brittle mode failure due to microchipping which results in lesser cutting forces and lowers the von Mises stresses. Additionally, the peak values of

24 the von Mises stress gradually decreases during successive impacts on CFRP substrate. It can be attributed to the contribution of prior impacts towards damaging the material.

However, a similar trend is not observed in the case of Ti substrate as the material is subject to plastic deformation and work hardening resulting in an almost similar peak stress values during successive impacts. Figure 10 shows the variation in normal stress acting on the Ti substrate during the machining process. It can be clearly seen that the compressive normal stress exceeds the yield strength of the material (~970 MPa) suggesting yielding and plastic deformation. The variation in plastic strain in the Ti substrate during the μUSM process is shown in Figure 11. The figure suggests that the substrate material is subject to constant plastic flow during the machining process.

Figure 12 shows the distribution of von Mises stresses on CFRP and Ti substrates during the μUSM process at various instances of simulation. The figure suggests the locations of microcrack initiation on the CFRP substrates. It can be seen that the CFRP substrates are subject to localized peak stresses with median microcracks originating from the point of contact with the abrasives. The Ti substrate shows plastic deformation due to high compressive stresses. It is seen that the Ti substrate undergo both elastic and plastic deformations during the process with the abrasives showing a greater tendency to stick to the substrate surface. Moreover, the stresses are distributed in the lateral direction on the Ti substrate over a larger area. It provides additional explanation for larger values of cavity depth observed on CFRP compared to Ti during the μUSM process.

Figure 10. Variation in Normal Stress acting on Ti Substrate during μUSM Process.

Figure 11. Variation in Plastic Strain in Ti Substrate during μUSM Process.

t=0 μs t=45 μs t=95 μs t=145 μs t=195 μs (a)

t=0 μs t=45 μs t=100 μs t=165 μs t=195 μs (b)

Figure 12. Equivalent von Mises Stress Distribution during μUSM Process on a) CFRP Substrate and b) Ti Substrate.

Effect of Vibration Amplitude during μUSM process

During the µUSM process, the tool attached to the sonotrode vibrating at ultrasonic frequency transmits ultrasonic energy to the abrasive particles. This energy is further transferred to the substrate and is used for material removal. The ultrasonic energy is directly proportional to the amplitude of vibration. Thus amplitude is a critical factor affecting the material removal process during the µUSM process. For studying the effect of amplitude on machining force, the µUSM process is simulated for various amplitudes while maintaining a feed rate of 5 µm/s and a duration of 250 µs. The effect of amplitude on the machining process is studied by analyzing the cavity depth, cutting force and the equivalent von Mises stress distribution.

Figure 13 shows the effect of amplitude on cavity depth during the µUSM process of CFRP and Ti substrates. It is observed that with the increase in amplitude, the cavity depth increases. It can be explained by the fact that as the amplitude increases, the corresponding ultrasonic energy transmitted to the substrate increases resulting in more

27 material removal. It is seen that the cavity depth in CFRP is more than that in Ti for the same amplitude. It is noted that the CFRP material shows a significant increase (average

65%) in cavity depth for increasing amplitudes. However, a similar trend is not observed in Ti material in which there is only minimal increase (average 45%) in cavity depths.

The significant increase in cavity depths in CFRP with increasing amplitudes suggests that it is easier to machine brittle materials at higher amplitudes using µUSM process. For ductile materials, the increased ultrasonic energy corresponding to higher amplitudes is mostly wasted in plastic deformation and ductile flow phenomena. It results in only minimal increase in cavity depth for Ti substrate.

Figure 14 shows the variation in cutting force with respect to varying amplitude.

From the figure, it is seen that the cutting force increases with increase in the amplitude.

It is because higher amplitudes transfer more ultrasonic energy to the abrasive particles, which results in higher cutting force during the µUSM process. It is also seen from the figure that the relative increase in cutting force is higher in the case of Ti compared to that of CFRP. It suggests that machining at higher amplitudes is not recommended for Ti materials as the cutting force increases significantly resulting in higher tool wear and shortened tool life. The variation in equivalent von Mises stress with an increase in the amplitude of vibration is shown in Figures 15a and 15b. Similar to the trend observed in the cutting force study, the increases in stress values with respect to increasing amplitudes is more pronounced in the case of Ti substrate compared to CFRP substrate. It suggests that the Ti substrate is subject to higher compressive stresses and subsequently more plastic deformation and work hardening at high amplitudes during the µUSM process.

Figure 13. Effect of Amplitude of Vibration on Cavity Depth during μUSM Process on CFRP and Ti Substrates.

Figure 14. Effect of Amplitude of Vibration on Cutting Force during μUSM Process on CFRP and Ti Substrates.

(a)

(b)

Figure 15. Effect of Amplitude of Vibration on Equivalent von Mises Stress during μUSM Process on a) CFRP Substrate and b) Ti Substrate.

Effect of Feed Rate during the μUSM process

Feed rate is a significant parameter influencing the machining performance during the µUSM process. In this study, the effect of feed rate on cavity depth, cutting force and

30 equivalent von Mises stress is analyzed by maintaining a constant amplitude of 2 µm and a simulation duration of 250 µs. During each simulation, a constant feed rate is provided to the substrate in the positive Y-axis direction. The feed rate values used for this study are 5 µm/s, 10 µm/s, and 15 µm/s. Figure 16 shows the variation in cavity depth with respect to different feed rates used during the µUSM process. It is seen that the cavity depths increase with an increase in feed rate. It is because as the feed rate increases, the velocity between the tool and substrate increases. It results in higher cutting force during the machining process resulting in increased material removal.

Figure 17 shows the variation in cutting forces during the µUSM process of CFRP and Ti substrates for different feed rates. It is seen that the relative increase in cutting force for increasing feed rate is significantly higher (30% or more) in Ti substrate compared with CFRP substrate. Moreover, higher cutting forces result in poor surface finish and reduced tool life. This suggests that machining of Ti substrate should be performed at a lower feed rate. However, machining of CFRP at higher feed rate does not increase the cutting forces significantly (less than 10%). This would mean that CFRP substrate can be machined efficiently at higher values of feed rate. The variation in equivalent von Mises stress with time for different feed rates during the µUSM process of

CFRP and Ti substrates are shown in Figures 18a and 18b. The figures suggest that the feed rate has a positive influence on the equivalent stress formation on the substrates. The stress on CFRP substrate is less influenced by increasing the feed rate. Similar to the trend observed for the cutting force study, a more pronounced influence in von Mises stresses are observed on Ti substrate with increase in feed rate values.

Figure 16. Effect of Feed Rate on Cavity Depth during μUSM Process on CFRP and Ti Substrates.

Figure 17. Effect of Feed Rate on Cutting Force during μUSM Process on CFRP and Ti Substrates.

(a)

(b)

Figure 18. Effect of Feed Rate on Equivalent von Mises Stress during μUSM Process on a) CFRP Substrate and b) Ti Substrate.

Effect of Tool Material during the μUSM process

Tool material plays a significant role in the μUSM process affecting the machining quality and performance. Material properties influencing tool selection during

μUSM process include elastic strength, fatigue strength, wear resistance, hardness and toughness. There are several tool materials traditionally used in the µUSM process including metals/metal alloys such as steel, brass, Cu, Ti and carbides/ceramics including

WC, alumina and so on. This study uses Cu and WC as the tool materials during the

µUSM process. Figure 19 shows the variation in cavity depth during the µUSM process using Cu and WC tools. From the figure, it is seen that the cavity depth is more while using WC as tool compared to Cu. It can be attributed to the higher hardness of WC tool compared to Cu tool. The tool with higher hardness is capable of penetrating the abrasive particle more into the bulk of the substrate. It would lead to more substantial material removal and hence increased cavity depth. It is also seen that the cavity depth is more for

CFRP compared to Ti in both the cases. It can be explained by the fact that while CFRP undergoes brittle mode machining, the abrasive penetration into Ti material becomes difficult as the plastic deformation hinders the tool motion.

The variation in cutting force during the μUSM process using the WC and the Cu tools are shown in Figure 20. The figure suggests that the cutting force is lesser while using the WC tool compared to the Cu tool. It is because WC tool is hard and causes more penetration into the bulk of the substrate resulting in reduced reaction forces. On the other hand, Cu tool results in lesser penetration into the substrate. The resistance of the substrate on the Cu tool results in increased cutting force during the machining process. Additionally, it is also observed that the abrasive particles have a higher tendency of sticking to the Cu tool. The higher cutting forces suggest that the Cu tool would undergo significantly higher wear during the μUSM process. However, the aspect

34 of tool wear is beyond the scope of the current study as the simulation assumes the tool to be rigid.

Figure 19. Effect of Tool Material on Cavity Depth during μUSM Process on CFRP and Ti Substrates.

Figure 20. Effect of Tool Material on Cutting Force during μUSM Process on CFRP and Ti Substrates.

CHAPTER 4

COMPARISON BETWEEN EXPERIMENTAL AND SIMULATION RESULTS

From the simulation results, it is seen that the tool material has a significant influence on the material removal process during the µUSM process of CFRP/Ti stacks. According to the simulation study, the cutting force on both CFRP and Ti during the µUSM process using the WC tool is lower than that while using the Cu tool. It suggests that the corresponding cavity depth and the material removal are higher for WC tool compared to the Cu tool. The experimental studies on the µUSM process of CFRP/Ti stacks showed that the Material Removal Rate (MRR) is higher in the case of WC tool compared to the

Cu tool as shown in Figure 21. The experimental observations are thus consistent with the present simulation results. It should be noted that the experimentation involves multiple impacts of several irregularly shaped particles in comparison to the simulation study which only has multiple impacts of three spherical abrasive particles. Both the experimentation and simulation results suggest that the material removal is faster using

WC tool compared to the Cu tool.

Figure 21. Comparison of MRR during µUSM Process of CFRP/Ti Stack using WC and Cu Tools (James & Sonate, 2017).

CHAPTER 5

CONCLUSION

This study reports the Finite Element (FE) Analysis on micromachining of hybrid composite stacks using Micro Ultrasonic Machining (µUSM) process. The materials used for the hybrid composite stack in this study are Carbon Fiber Reinforced Polymer

(CFRP) and Titanium (Ti). The effects of critical process parameters including the amplitude of vibration, feed force and tool material on the cavity depth, cutting forces and equivalent von Mises stresses are studied. The major conclusion that can be drawn from this study are as follows:

a) During the µUSM process, the CFRP substrate undergoes more machining than Ti substrate. The CFRP substrate undergoes damage in brittle mode through microchipping and microcracking. The Ti substrate undergoes plastic deformation in ductile mode through microplowing and microcutting mechanisms. Moreover, the cavities produced on Ti is uneven compared that of CFRP.

b) Study on cavity depth suggested that the cavity depth on CFRP substrate is larger than that on the Ti substrate. It is inferred that the plastic deformation of the Ti material offers more resistance to the tool motion, preventing abrasive penetration into the bulk of the substrate.

c) Study on cutting forces showed that the generation of positive pulsating cutting forces in each cycle of ultrasonic vibration. The cutting force acting on CFRP substrate is lower than that on Ti substrate. The higher cutting forces on Ti is attributed to its ductile behavior and superior fracture toughness, causing increased resistance to the tool motion. The cutting force on CFRP gradually decreases due to microchipping and microcracking of the material.

38 d) Study on equivalent von Mises stresses revealed the formation of intermittent peaks of von Mises stresses during the machining process. The peak values of stresses acting on Ti substrate is observed as considerably larger than that on CFRP substrate. The normal stress analysis suggested that the compressive stresses acting on Ti are exceeding the yield point of the material. e) The study on the effects of the amplitude of vibration showed that the cavity depth increases with the increase in amplitude. CFRP material shows a significant increase in cavity depth for increasing amplitudes compared to Ti. It is inferred that machining of brittle materials is easier at higher amplitudes using µUSM process. Higher amplitude machining is not recommended for ductile materials such as Ti as the cutting force increases significantly resulting in higher tool wear and shortened tool life. f) Study on effects of feed rate suggested that the relative increase in cutting force for increasing feed rate is significantly higher in Ti substrate compared with CFRP substrate. It suggests that machining of Ti substrate should be performed at a lower feed rate. However, machining of CFRP at higher feed rate does not increase the cutting forces significantly, and thus CFRP substrate can be machined efficiently at higher feed rates. g) The study on the effects of tool material during the μUSM process suggested that the cutting force is lesser while using the WC tool compared to the Cu tool. The higher cutting forces mean that the Cu tool would undergo significantly higher wear during the μUSM process. h) Both the experimentation and simulation results suggest that the material removal is more using WC tool compared to the Cu tool.

RESEARCH DISSEMINATION

1. Panchal, Sagar and James, S., “Finite Element Analysis and Simulation Study on Micromachining of Hybrid Composite Stacks using Micro Ultrasonic Machining Process”, Journal of Ultrasonics (Submitted)

2. Panchal, Sagar and James, S., “Finite Element Analysis and Simulation of Micro Ultrasonic Machining of CFRP/Ti Stacks”, Society for the Advancement of Material and Process Engineering (SAMPE) International Conference, May 21- 24, 2018, Long Beach, CA

3. Panchal, Sagar and James, S., “Finite Element Analysis and Simulation of Micro Ultrasonic Machining of CFRP/Ti Stacks”, Poster, AeroDef Aerospace Defense Manufacturing Conference, March 27-28, 2018, Long Beach, CA

4. Panchal, Sagar and James, S., “Finite Element Analysis and Simulation of Micro Ultrasonic Machining of CFRP/Ti Stacks”, Presentation, Student Research Competition (SRC) at California State University Fullerton, March 1, 2018, Fullerton, CA

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