STRESS-CORROSION FATIGUE CRACKING

OF COLD EXPANDED COMPONENTS

by

RAMKUMAR KUNNAVAKKAMVINJAMUR, B.E.

A THESIS

IN

MECHANICAL ENGINEERING

Submitted to the Graduate Faculty of Texas Tech University in

Partial Fulfillment of the Requirements for

the Degree of

MASTER OF SCIENCE

IN

MECHANICAL ENGINEERING

Approved

May, 2002

ACKNOWLEDGMENTS

I would like to thank my graduate advisor, Dr. Jahangir Rasty for his support and

guidance throughout my graduate studies and research. I would also like to express my

sincere gratitude to Dr. Atila Ertas and Dr. Stephen Ekwaro-Osire for serving on my

thesis committee.

I would like to thank Dr. Mark Grimson and Dr. Candace Haigler of Electron

Microscopy Laboratory for their assistance while generating the photographs of fracture

surfaces. 1 am also very grateful to Dr. Thomas Burton and Department of Mechanical

Engineering, Texas Tech University for giving me the opportunity to pursue my graduate

studies.

I would also like to acknowledge with thanks helps rendered by Dr. Xiaobin Le,

Mr. Ron Bermett and Mr. Kevin Kerr during testing. I also highly appreciate the help

rendered by Mr. Norman Jackson and machine shop staff during the machining of the

specimens.

Finally, I would like to thank my parents and friends for their support and

encouragement during my academic career.

TABLE OF CONTENTS

ACKNOWLEDGMENTS ii

ABSTRACT vi

LIST OF TABLES viii

LIST OF FIGURES ix

GLOSSARY xi

CHAPTER

I. INTRODUCTION 1

1.1 Background 1

1.1.1 Cold Expansion Technology 1

1.1.2 Sfress Corrosion Fatigue Cracking 2

1.2 Literature Review 4

1.3 Objectives 7

II. EXPERIMENTAL PROCEDURE 8

2.1 Corrosion of Specimens 8

2.1.1 Masking of the Sample 8

2.1.2 Corrosion Cell 9

2.1.3 Corrosion Rate Determination 12

2.1.4 Corrosion of the Samples 12

2.1.5 Cleaning and weighing of Samples 14

2.2 Fatigue Testing of the Specimens 15

111

2.2.1 S ystem Parameters 15

2.2.2 Trial Runs 15

2.2.3 Sample Preparation for Fatigue Testing 17

2.2.4 Testing 21

2,3 Fractographic Analysis 21

2.3.1 Specimen Preparation 21

2.3.2 Analysis 24

III RESULTS AND DISCUSSIONS 28

3.1 Mass Loss Results 28

3.2 Tensile Test Results 32

3.3 Fatigue Test Resuhs 32

3.4 Statistical Analysis 38

3.4.1 ANOVA for 3 - factorial design 41

3.4.2 ANOVA for 3 ' x 2' factorial design 47

3.5 Fractographic Analysis Results 53

3.5.1 (0,0) Specimen 53

3.5.2 (0,10) Specimen 56

3.5.3 (2,10) Specimen 56

3.5.4 (2,20) Specimen 59

3.5.5 (4,20) Specimen 60

rv CONCLUSIONS AND RECOMMENDATIONS 67

4.1 Conclusions 67

4.2 Recommendations ^o

REFERENCES ^^

ABSTRACT

Fasteners like rivets are widely used for assembling parts in the aircraft industry.

The fasteners not only help in easy assembly and dismantling of the parts, but also help in

proper distribution of the load applied on the body. However, in order to fasten two parts

together, a number of holes have to be drilled on both parts and these holes tend to raise

the sfress in the region surrounding them. This sfress concenfration effect of the holes

reduces the fatigue life of the part when cyclic load is applied to the part. To offset the

sfress concenfration effect of the holes, a technique called Split Sleeve Cold Expansion

has been widely used in the aircraft industry. It induces a compressive residual sfress in

the fastener hole and the region aroimd it, which retards the growth of fatigue crack and

thereby improves the fatigue life of the component.

The objective of this thesis work was to analyze the effect of Split Sleeve Cold

Expansion on the fatigue life of AI-7075 T6 specimen subjected to corrosion. In order to

fiilly understand the interaction between cold expansion, corrosion and fatigue life, two

possible cases were considered. In the first case the specimens were cold expanded first

and then corroded to required mass loss level. In the second case the specimen were

corroded prior to cold expansion. The specimens were corroded using a galvanic

corrosion cell. The specimens were then tested for fatigue sfrength.

The results showed that cold expansion improved the fatigue life of xm-corroded

and mildly corroded specimen by a factor of 20. However, fatigue life of cold expanded

specimens dropped exponentially with corrosion. For severely corroded specimens there

vi

was no significant improvement in fatigue life due to cold expansion. The results also

showed that for specimens subjected to the same degrees of cold expansion and

corrosion, the fatigue life of specimens cold expanded after corrosion was lower than the

fatigue life of specimens cold expanded prior to corrosion.

Vll

LIST OF TABLES

2.1 Order of test runs for full factorial 3 ^ x 2 ' experiment 22

2.2 Description of codes used in Table 2.1 23

3.1 Mass Loss (ML), in grams, for various corrosion exposure times (hours) 29

3.2 Tensile Test Data for A1-7075-T6 32

3.3 Fatigue test data for cold expanded specimens subjected to subsequent corrosion 34

3.4 Fatigue test data for corroded specimens subjected to subsequent cold expansion 36

3.5 Fatigue life data for balanced 3 ^ factorial design 42

3.6 ANOVA for balanced 3 ^ factorial design 43

3.7 Fatigue life data for unbalanced 3 ^ factorial design 44

3.8 ANOVA for unbalanced 3^ factorial design (Type I) 45

3.9 ANOVA for unbalanced 3 ^ factorial design (Type III) 46

3.10 Fatigue hfe data for balanced 3 ^ x 2 ' factorial design 48

3.11 ANOVA for balanced 3 ^ x 2 ' factorial design 49

3.12 Fatigue hfe data for unbalanced 3" x 2' factorial design 50

3.13 ANOVA for unbalanced 3 ' x 2' factorial design (Type I) 51

3.14 ANOVA for unbalanced 3 ' x 2' factorial design (Type III) 52

VlU

LIST OF FIGURES

1.1 Split Sleeve Cold Expansion 3

2.1 Masked specimen prior to corrosion 10

2.2 Corrosion cell 11

2.3 Corroded specimen prior to fatigue tests 13

2.4 Insfron testing machine used for fatigue testings 16

2.5 Schematic of fatigue test specimens 18

2.6 Split Sleeve cold expansion tool 20

2.7 Sputter coater 25

2.8 Scarming Elecfron Microscope 27

3.1 Corrosion mass loss versus Time 31

3.2 Fatigue life of cold expanded specimens subjected to subsequent corrosion 35

3.3 Fatigue of corroded specimens subjected to subsequent cold expansion 37

3.4 Comparison of fatigue hfe of specimens cold expanded prior to corrosion and specimens cold expanded after corrosion at CET=2 % 39

3.5 Comparison of fatigue life of specimens cold expanded prior to corrosion

with specimens cold expanded after corrosion at CET = 4 % 40

3.6 Appearance of edge of (0,0) specimen 54

3.7 Appearance of surface along the length of (0,0) specimen 55

3.8 Fatigue striations for (0,0) specimen 57

3.9 Appearance of the edge of (0,10) specimen 5 8

IX

3.10 Corroded edge for (2,10) specimen 61

3.11 Appearance of fracture surface for (2,10) specimen 62

3.12 Appearance of fracture surface for (2,10) specimen on the other side 62

of the hole

3.13 Corroded edge for (2,20) specimen 63

3.14 Corroded edge for (4,20) specimen 64

3.15 Surface on the left side of the hole for (4,20) specimen 65

3.16 Appearance of the surface on the right side of the hole for (4,20) specimen 66

GLOSSARY

(0,0) Unfreated specimens.

(2,0) Un-corroded specimens subjected to 2% cold expansion.

(4,0) Un-corroded specimens subjected to 4% cold expansion.

(0,10) Non-cold expanded specimens subjected to 10% mass loss due to corrosion.

(0,20) Non-cold expanded specimens subjected to 20% mass loss due to corrosion.

(2,10) Specimens subjected to 2% cold expansion and 10% mass loss due to corrosion.

(2,20) Specimens subjected to 2% cold expansion and 20% mass loss due to corrosion.

(4,10) Specimens subjected to 4% cold expansion and 10% mass loss due to corrosion.

(4,20) Specimens subjected to 4% cold expansion and 20% mass loss due to corrosion.

XI

CHAPTER I

INTRODUCTION

1.1 Background

1.1.1 Cold Expansion Technology

Fasteners like rivets and bolts are widely used for assembling parts in the afrcraft

industry, where a number of parts are riveted together to form aircraft structures. Not

only do these fastened joints enable easy assembly and dismantUng, they are also able to

transfer as well as distribute loads applied onto the structure. However, in order to rivet

the parts to build the structure, a number of holes have to be drilled in the parts. These

holes are a significant source of stress concentration, and in the presence of cyclic tensile

stress, they act as primary sources of fatigue cracks that originate from microscopic

defects in the material. Over a period of time, these cracks begin to grow and result in

fatigue failure of the structure.

Many systems have been conceived to offset this stress concentration effect of

fastener holes. The underlying principle of all these systems is to induce permanent

compressive stress near the hole, which could prevent fatigue cracks that originate from

the edge of the holes. One such system, which has been widely used in the aircraft

industry, is the Split Sleeve Cold Expansion process. This process was conceived by the

Boeing Company and later developed as an integrated system by Fatigue Technology Inc.

(Rufin, 1993 ). This process uses a tapered mandrel in conjunction with a disposable, pre-

lubricated spUt sleeve to compressively pre-sfress a significant zone around the fastener

hole. The compressive stresses are generated by pulUng the tapered mandrel, pre-fitted

with the sleeve, through the hole. The mandrel and the sleeve radially expand the hole

creating an annular zone of compressive stress around the hole. This pre-stressing offsets

the stress concentiration of the hole to produce substantial improvements in the fatigue

performance of the fastened joints. The compressive zone also arrests the growth of pre­

existing cracks in the material. Figure 1.1 shows the process.

1.1.2 Stress Corrosion Fatigue Cracking

Stress corrosion fatigue cracking is essentially fatigue fracture aggravated by the

effects of the envfroimient. American Society for Metals defines it, as "Effect of the

appUcation of repeated or fluctuating stresses in a corrosive envfronment characterized by

shorter life than would be encountered as a result of either the repeated or fluctuating

stresses or corrosive envfronment" (Corrosion, vol 13, ASM Handbook, 1987).

The fatigue hfe of the part is strongly affected by the type of cycUc stressing. The

longer and more frequentiy a fatigue crack is opened, the more will be the effect of

envfronment on shortening the fatigue hfe. In many cases the fatigue crack is initiated

from smaU pits on the corroded surface, which act as stress concentration points. In other

cases the fatigue crack initiates ffrst and is then made to grow more rapidly by moisture

or other corrodents that enter the crack by capillary action.

This type of cracking is most frequentiy encountered in ships, afrcrafts and

structures that are frequently exposed to corrosive envfronments and fluctuating loads

and is a major problem as the fatigue Ufe is reduced cfrasticaUy.

^- Nosrcap Mandfet

/

Split Sleevp

(a)

Ai-«-a ^ r C r f l d K,5>panis,iois

S Z Z S Z ^ ^ ^ ^ ^ & ^

(b)

Figure 1.1: SpUt sleeve cold expansion, (a) Parts of the Split Sleeve Cold Expansion tool, (b) Shows the cold expansion process (Rufin, 1993).

1.2 Literature Review

A large volume of hterature is available on cold expansion due to its widespread

use in the afrcraft industry. The research work dates back to the 1970s when the Split

Sleeve Cold Expansion system was developed by the Manufacturing Research and

Development Organization of Boeing Commercial Airplane Company. PhiUips (1973) of

Boeing Commercial Airplane Company provided an overview of the process and

summarized the results that were obtained from Afr Force Materials Laboratory (AFML)

sponsored tests on aerospace materials. The results showed that compressive residual

stresses induced during the cold expansion process effectively arrests crack growth from

the hole and significantiy improves fatigue Ufe of the specimen.

Petrak and Stewart (1976) did some of the earUest work on cold expansion. They

carried out tests to evaluate the capabiUty of cold expansion and other interference

fastener systems to retard crack growth from fastener holes. They tested Al-7075 T6

alloys with pre-existing cracks in the holes and showed that the cold expansion and

interference fastener systems effectively retarded crack growth.

Later, Chandawanich and Shaipe (1979) studied the effect of cold expansion on

crack initiation and crack growth rate in Al-7075 T6 alloys. They showed that while cold

expansion did not affect the crack initiation, it retarded the crack propagation rate and

thereby increased the fatigue hfe of the specimen. More recentiy, Rachid et al. (2000)

demonstrated that Split Sleeve Cold Expansion affects the crack nucleation stage by

causing changes in the microstructure of the material near the surface. Rufin (1993)

presented a paper describing the effectiveness of SpUt Sleeve Cold Expansion in

improving the fatigue Ufe of an engine. The papers also showcased the abiUty of SpUt

Sleeve Cold Expansion to arrest fatigue cracks in holes with low edge/margin ratio. He

also showed that cold expansion could be used effectively for components working at

high temperatures.

Even though SpUt Sleeve Cold Expansion was initially intended for new

production afrcraft structures, the process has been effectively used for in-service repairs.

Reid (1997) described how cold expansion could be used to enhance the integrity and

durabiUty of repafrs. Gaerke et al. (2000) stucUed the benefits of cold expanding fastener

holes at various stages of fatigue Ufe in AI-2024 T 31 low-load transfer joint. The authors

pre-cycled the specimen to 25%, 50%, 75% baseUne fatigue Ufe of the non-expanded

specimen and then cold expanded the components prior to final cycUng to failure. The

tests showed that part-Ufe cold expansion could provide substantial improvements in

fatigue Ufe.

A number of analytical and finite element methods for predicting the residual

stresses induced by cold expansion have been developed over a period of years due to the

importance of knowing the residual stress field around the hole to predict the crack

growth rate. Ozdemfr and Edwards (1996) developed a method to predict the residual

stress distribution around the hole through the use of Sachs method. They also showed

that the orientation of the spUt on the expansion sleeve affects the distribution of residual

stresses. Dutta (1997) developed a finite element model to identify the process variables

confrolUng the effectiveness of the cold expansion technology. The author along with

Rasty (1999) also developed analytical equations for determination of the elastic-plastic

boundary radius, using a far field strain measurement. The elastic-plastic boundary was

then used for determining the residual stress distribution.

The effect of corrosion on fatigue Ufe of components is widely researched due to

its detifrnental impact on the fatigue Ufe of the component. Sankaran et aL (2001) showed

tiiat pitting corrosion in Al-7075 T6 components can reduce the fatigue Ufe of the

component by 6-8 times. Chen et al. (1996) showed that failure in corroded specimens

occur due to fatigue cracks nucleating from pits formed on the surface due to corrosion.

When the region close to the hole is corroded, then the combined stress concentration

effects of the hole and the rough surface produced by corrosion can severely reduce the

fatigue Ufe of the component. Moesser et aL (1995) studied the effect of corrosion near

die fastener hole on fatigue Ufe of the component.

Due to the serious problems posed by the interaction between corrosion and

fastener hole, the effect of cold expansion in retarcUng stress corrosion fatigue crack

propagation has been researched. Cook et al. (1996) studied the effect of pitting corrosion

on cold expanded holes. They also stucUed the interaction between the tensile residual

stresses induced by cold expansion away from the hole and stress corrosion cracking.

They also demonstrated that when a specimen is cold expanded after corrosion, the

fatigue life of the specimen is lower than when the specimen is cold expanded prior to

corrosion.

fri this thesis work, the effect of cold expansion on stress corrosion fatigue

cracking is studied. UnUke the other works, the extent of corrosion here is quantified by

using mass loss as measuring parameter.

1.3 Objectives

The three objectives of this research were: (1) To study the effect of corrosion on

fatigue Ufe of non-treated Al 7075-T6 specimens, (2) to study the effect of cold

expansion on fatigue Ufe of specimens corroded after cold expansion, (3) to study the

effect of cold expansion on fatigue Ufe of specimens corroded prior to cold expansion.

CHAPTER n

EXPERIMENTAL PROCEDURE

2.1 Corrosion of Specimens

2.1.1 Masking of the Sample

This experiment requfred only a smaU region of the specimen around the hole to

be corroded in order to study the interaction between corrosion and cold expansion.

Therefore, it was necessary to protect the remaining regions from corrosion using a

corrosion-inhibiting mask. 3-M AU-Weather Corrosion Protection Tape was used for tiiis

purpose. The tape was appUed on the regions of the sample that were to be protected

from corrosion. Some cUfficulty was encountered in finding a suitable method for

applying the tape.

Initially, the tape was appUed in an overlapping maimer over the sample, leaving

a smaU rectangular region in the middle of the sample exposed. This was found to yield

poor results due to leakage along the overlapping edges. Also, the tape tended to peel off

over prolonged periods of exposure, resulting in corrosion of masked areas of the

specimen.

In order to correct this problem, rectangular strips corresponding to the areas to be

exposed were cut out from the tape before applying it to the specimen. The areas to be

exposed were carefully marked on the front and the backside of the sample. The mask

was then appUed over the sample by carefuUy aUgning the tape and the sample. This

method was found to yield good results for the corrosion rates used in this experiment.

There was no leakage around the mask and the corrosion was restricted to the exposed

region.

However, for very severe corrosion (close to 35 % removal of material by weight)

this method was found to be inadequate. There was leakage around the mask due to

difference in the thickness between the corroded and masked regions. Due to this, some

portions of the masked region were also corroded. But, for the corrosion levels used in

this experiment the masking was found to yield good results. An example of the masked

specimen can be seen in Figure 2.1.

2.1.2 Corrosion CeU

Accelerated corrosion of the samples was carried out using a galvanic corrosion

ceU. The ceU consisted of an 8-gaUon plastic container, electrolyte, anode, catiiode and a

power source. The masked aluminum sample was used as anode and was coimected to

the positive terminal of the constant current power source. Two Al-7075 T6 plates were

used as cathodes to corrode both sides of the specimen. The cathodes were connected to

the negative terminal of the power source. The electrolyte used was a 2% salt solution

prepared by dissolving aquarium salt in distiUed water. The Ph of the electirolyte was

maintained at around 8 and the temperature was maintained at 70 degrees Fahrenheit.

The anode and tiie catiiodes were immersed in the electrolyte and a constant current of 50

mA / in^ was appUed across the electi-odes. Figure 2.2 shows the ceU used for corroding

the specimen.

''//^V';i;^ "p^^'^rf'^f^.

Figure 2.1: Masked specimen prior to corrosion.

10

(a)

(b)

Figure 2.2: Corrosion ceU (a) front view and (b) top view

11

2.1.3 Corrosion Rate Determination

In order to determine the time taken to corrode the sample to requfred mass loss

levels, several trial samples were corroded. The trial samples were 1.6 inches in width

and 5 inches in length. The samples were typicaUy 0.1 inch in thickness. The samples

were masked and corroded for a period of 30 hours and the mass loss was noted at 1-hour

intervals. The recorded data were then used to fit a curve to predict the time requfred to

corrode the sample to requfred mass loss level. This model was tested for accuracy and

found to yield good results.

2.1.4 Corrosion of the Samples

The samples to be tested were corroded one at a time in the corrosion ceU. They

were immersed in the electrolyte and a constant current density of 50 mA / in^ was

maintained across the electrodes. The samples were removed at the end of time requfred

to reach the desfred mass loss level.

As a result of the corrosion, there was a buildup of corrosion products on the

surface of the cathodes. This material was removed periodically using a wfre brush. The

temperature and Ph of the electrolyte were carefuUy monitored and the electrol5l;e was

changed if there were any deviations. The electrolyte was also changed regularly at 10-

hour intervals to remove the corrosion products. Figure 2.3 shows a corroded sample

12

••i-^ ' A ;

•i -*• ' • i - * '" ! '

I * • — • • .

Figure 2.3: Corroded specimen prior to fatigue testing.

13

2.1.5 Cleaning and Weighing of Samples

After removing the samples from the corrosion cell, the samples were unmasked

and cleaned. The corrosion products on the surface of the sample were gentiy removed

using a plastic brush. The samples were further cleaned using 30% nitric acid solution

according to ASTM Gl standard. The samples were then washed in distiUed water and

dried using a hafr dryer.

The samples were then weighed using a Sartorius Analytic scale, model A210P,

with precision of 1/10000 of a gram. The weight of the sample before and after corrosion

was recorded and was used to calculate the percentage mass loss using the formula.

Percentage mass loss = Am /(L * W * T * p) (2.1)

Where Am = weight of material removed by corrosion

L = length of corroded region

W = width of the corroded region

T = thickness of the corroded region

p = density of Al-7075 T6 aUoy.

The mass loss percentage was recorded and tiie samples were marked for identification.

14

2.2 Fatigue Testing of the Specimens

2.2.1 System Parameters

Fatigue testing of the samples was done using a servo hydrauUc testing machine.

The machine consisted of an MTS load frame (MiimeapoUs, MN) with Instron grips and

hydrauUcs (Canton, MA). The testing system was controUed using a desktop PC.

Wavemaker-Runtime (Version 5.1), developed by Inston was used for signal control and

data acquisition. Figure 2.4 shows the system used for fatigue testing.

The fatigue tests were conducted at 10 HZ frequencies with the load varying from

2500 Ibf to 250 Ibf. The maximum and minimum loads to which the samples were to be

subjected were calculated using the modified-Goodman equation.

Wavemaker software was used for generating the program to control the fatigue

test. To get the requfred load cycles, a ramp function of magnitude 1375 Ibf was appUed

followed by a sine function with ampUtude varying from -1-1125 Ibf to -1125 Ibf. This

resulted in the desfred range of 250 Ibf to 2500 Ibf.

2.2.2 Trial Runs

The samples used for trial runs were typicaUy 12 inches in length and 1.6 inches

in width with a thickness of 0.1 inch. A hole of diameter 0.219 inches was driUed in die

samples. Trial runs were performed to determine the area of the specimen to be corroded

so that it fractures across the hole. This was requfred in order to study the effect of cold

expansion.

15

Figure 2.4: Instron testing machine used for fatigue tests

16

Initially, a region of lengtii 2.5 inches and widdi 1.6 inches was corroded to 5%,

10%, 15% and 20% mass loss and tested. The non-cold expanded specimen fractured

across the hole, but when the specimen was cold expanded to 2% or 4% the results were

unpredictable. The specimen fractured away from die hole along the bottom edge of the

corroded region and it was not possible to analyze the effect of cold expansion.

Therefore, the area to be corroded was reduced to a smaU rectangular strip of

length 0.219 inches and width 1.6 inches. This yielded good resuhs for non-cold

expanded samples. When the samples were cold expanded and tested, it was found that

some of the samples fractured due to cracks originating from outer edges.

To ensure that the cracks originate from the hole the edges of the specimen were

masked and the exposed area was reduced to a rectangular strip 0.219 inches in length

and 1 inch in width. This sample fractured consistentiy through the holes from cracks

originating from the holes.

2.2.3 Sample Preparation for Fatigue Testing

The samples to be used for actual testing were cut out from Al-7075 T6 aUoy

sheets into rectangular strips of length 12 inches and width 3 inches along the dfrection of

roUing. These rectangular strips were machined to dog bone shape using HAAS VF-3

Computer Numerical Control MiU (HASS Automation, Inc. L.A., CA). Figure 2.5 shows

the dimension of the specimen. The gage area of the sample was 2.5 inches in length and

1.6 inches in width. A hole of diameter 0.219 inches was driUed in the sample. A total

of 60 samples were machined using the CNC miU.

17

3,00

Figure 2.5: Schematic of fatigue test specimens. (All dimensions are in inches)

18

The machined samples were divided into two groups. The first set of samples

was prepared by cold expanding the samples to 0%, 2%, 4 % and then corroding

them to 0%, 10% and 20% mass loss. The second set was prepared by corroding the

samples fu"st prior to cold expansion.

The samples were cold expanded using the SpUt Sleeve Cold Expansion (SsCx)

System developed by Fatigue Technology Inc., CA. SpUt sleeves of thickness 0.0080

inches (# 2116-312) were used in combination with mandrel of nose diameter 0.2119

inches (# 2025-174) for 4% cold expansion. For 2% cold expansion, the same sleeve was

used with a mandrel of nose diameter 0.2070 inches (# CBM-6-2-N-1-30-V2). Figure 2.6

shows the system used for cold expansion. The percentage cold expansion was

calculated using the formula.

D +2W-Z), ,^^^ Percentage expansion = — (2.2)

Where D = Nose diameter of the mandrel

D;, = Diameter of the hole

W = Sleeve thickness.

The samples were corroded to three mass loss levels, namely 0%, 10% and 20% mass

loss, using the galvanic ceU described previously. Mass loss levels were determined from

trial runs based on number of cycles requfred to fracture the specimen. The samples were

corroded in the galvanic cell by applying a constant current density of 50 mA/in.sq across

the electrodes.

19

~.''-->^

Figure 2.6: SpUt Sleeve cold expansion tool

20

2.2.4 Testing

The fatigue testing of the samples was carried out using the system described

previously. The order in which the samples were to be tested in the machine was

completely randomized using random number tables. This was done in order to reduce

the effect of extraneous factors that might affect the machine. Table 2.1 shows the order

in which the samples were tested. Table 2.2 shows the coding used in Table 2.1. The

samples were run to a maximum of 2 milUon cycles. The tests were stopped at this point

if the samples did not break due to time constraints.

2.3 Fractographic Analysis

2.3.1 Specimen Preparation

Fractured samples from the fatigue tests were used for conducting fractographic

analysis. Fracture surfaces from the fatigue test samples were machined and mounted on

aluminum pedestals using STR tapes. The specimens were then grounded to the pedestal

using coUoidal silver paste. TypicaUy die specimens used for analysis were 1.6 inches in

width and 0.3 inches in height.

When observed under die microscope, the specimen tends to expand or even melt

due to the heat generated by the electrons bombarding the surface. In order to prevent

over-heating of the surface the specimen has to be sputter coated.

Sputter coating in tiiis case was done using Techics Hununer V Sputter Coater.

This coater uses argon gas as inert gas and plates eitiier gold or gold/paUadium on

21

Table 2.1: Order of test runs for full factorial 3^ x2 ' experiment

Test run

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

Cold expansion

0

1

-1

1

0

0

0

-1

0

0

1

-1

1

0

1

0

0

0

0

1

0

1

1

Corrosion

0

1

1

0

0

1

0

-1

1

1

0

-1

0

-1

1

0

-1

1

0

1

-1

1

-1

Order

1

1

1

1

2

1

1

1

2

1

2

1

1

1

2

2

1

1

2

1

1

2

1

22

Table 2.1.Continued

Test run

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

Cold expansion

1

-1

-1

1

1

0

-1

0

-1

1

1

-1

1

1

-1

0

Corrosion

1

1

-1

0

0

0

• 0

1

1

1

-1

0

0

-1

0

1

Order

1

1

1

2

2

1

1

2

1

2

1

1

1

1

1

2

Table 2.2: Description of Codes used in Table 2.1

Cold expansion (%)

0

2

4

Code

-1

0

1

Corrosion (%)

0

10

20

Code

-1

0

1

Order

Cold expanded first

Corroded first

Code

1

2

23

die specimen. The plating was done in a vacuum chamber, which was maintained at a

vacuum pressure of around 50-70 mTorr. During die process the voltage across the

electrodes in die vacuum chamber was set at 10 Volts and the current was maintained at

10 mA by varying die pressure control dial. The diickness of die plating given by die

sputter coater was controlled by setting die time on die process conti-ol timer. In diis case

die timer was set at 2 minutes to give 200 Angsti-om diick plating on die specimen.

Figure 2.7 shows die sputter coater used for preparing die specimen. Ten specmiens, two

each for (0,0), (0,10), (2,10), (2,20) and (4,20) cases were prepared usmg die sputter

coater.

2.3.2 Analysis

Fractographic analysis was done using a Hitachi S-570 Scanning Electron

Microscope. The specimen to be analyzed was mounted onto a pedestal in the column of

the microscope. This column was typicaUy maintained at a vacuum pressure of 0.0001

Torr in order to aUow passage of electrons without interference.

The Hitachi S-570 uses a tungsten filament, 0.1 nm in diameter, to generate the

electrons. The emission rate from the filament was controUed by varying the filament

current using the filament pot. The filament pot was adjusted to saturate the filament

properly to get a good image. The elecfrons generated by the filament were then

accelerated towards the specimen by applying an accelerating voltage. The accelerating

voltage was typicaUy set at 12 KV.

24

i"

en . » • • ' • ' • • ' ,» » ./ 4*

O:

Figure 2.7: Sputter coater (Techics Hummer v sputter coater)

25

The S-570 uses two electromagnetic lenses to properly focus the electron beam

generated by the filament properly on the specimen's requfred spot. It is important to

aUgn the filament and the lenses to get a good image. This aUgnment was done using

beam aUgnment controls, which centers the filament properly with respect to the

condenser lens. Further, the stigmator associated with the objective lens was also adjusted

using stigmation controls in order to remove astigmatism errors in the electron beam.

The image generated on the screen was then focused using coarse and fine focus

contirols. Further, die working distance, aperture opening, condenser lens current and tilt

angle were also varied in order to get good resolution and depth of focus in the image.

The final image generated on the screen was then captured using a Kodak camera

mounted on the microscope. Figure 2.8 shows a picture of the scanning electron

microscope used.

26

Figure 2.8: Scanning Electron Microscope (Hitachi S-570)

27

CHAPTER ni

RESULTS AND DISCUSSIONS

3.1 Mass Loss Results

The mass loss data for the trial samples were recorded every hour for 30 hours.

Table 3.1 shows the total mass lost by the trail samples and the time taken for it. The data

from this table were used to fit a curve given by the equation 0.00002972 x ̂ -i- 0.00579 x

- 0.0009, which predicts the mass loss as a function of time. A plot of the curve and the

data used to fit the curves is shovra in Figure 3.1. The plot shows that the values

predicted by the fitted curve differ from the actual values ± 10% especiaUy at around 20

hours and above. In order to compensate for this difference between the predicted values

and the actual values the test specimens were weighed three hours before the predicted

time and depending on the mass loss they were further corroded. By this method, it was

possible to corrode the test specimen to within ± 1 % of requfred mass loss.

28

Table 3.1: Mass loss (ML), in grams, for various corrosion exposure times (hours)

Time (hours)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

M L -Run 1

0.0048

0.010

0.0167

0.023

0.0293

0.0353

0.0422

0.0488

0.0544

0.062

0.0686

0.0753

0.082

0.0888

0.0954

0.1024

0.1094

0.1164

0.1233

M L - Run 2

0.0048

0.0108

0.016

0.0231

0.0293

0.0356

0.0412

0.0475

0.0535

0.0594

0.0653

0.0713

0.0771

0.0831

0.0893

0.0956

0.1019

0.1082

0.1145

M L -Run 3

0.0048

0.0104

0.0164

0.0223

0.0282

0.0341

0.0404

0.0466

0.0529

0.0595

0.0661

0.0727

0.0793

0.0859

0.0929

0.0998

0.1068

0.1138

0.1215

Average M L

0.0048

0.0103

0.0164

0.0228

0.0289

0.0350

0.0412

0.0476

0.0539

0.0603

0.0667

0.0731

0.0794

0.0858

0.0925

0.0992

0.1060

0.1128

0.1197

M L from fitted curve

0.0049

0.0108

0.0167

0.0227

0.0288

0.0349

0.0411

0.0473

0.536

0.0600

0.0664

0.0729

0.0794

0.0860

0.0926

0.0994

0.1061

0.1130

0.1198

29

Table 3.1. Continued

Time (hours)

20

21

22

23

24

25

26

27

28

29

30

ML -Run 1

0.1303

0.1373

0.1443

0.1512

0.1582

0.1652

0.172

0.1791

0.1861

0.1930

0.2

ML - Run 2

0.1208

0.1270

0.1333

0.1396

0.1459

0.1529

0.1598

0.1668

0.1742

0.1815

0.1888

ML -Run 3

0.1291

0.1368

0.1444

0.1521

0.1597

0.1702

0.1785

0.1862

0.1938

0.2018

0.2098

Average ML

0.1267

0.1337

0.1407

0.1476

0.1546

0.1628

0.1701

0.1774

0.1847

0.1922

0.1995

ML from fitted curve

0.1268

0.1338

0.1409

0.1480

0.1552

0.1624

0.1698

0.1771

0.1846

0.1920

0.1996

30

0,25 Actual d i t i points Fitted curve

10 15 20 Corrosion time (hours)

25 30

Figure 3.1: Corrosion mass loss versus Time

31

3.2 Tensile Test Results

The tensile tests were done to estimate the ultimate tensile sttength of Al-7075

T6 aUoy. Table 3.2 shows the results of the tests.

Table 3.2: Tensile test data for A1-7075-T6

Test run

1

2

3

Ultimate tensile sttength (psi)

82,000

81,500

82,670

Average ultimate tensile sttength (psi)

82,056

3.3 Fatigue Test Results

The results of the fatigue tests for the specunens that were cold expanded prior to

corrosion are shown in Table 3.3. Figure 3.2 shows a graphical representation of the data

in Table 3.3.

Figure 3.2 shows that non-cold expanded specimens widi 0% corrosion have an

average fatigue Ufe of approximately 97,000 cycles and as the specmiens are corroded the

fatigue life reduces drasticaUy. At 10% corrosion, die fatigue Ufe of the specmiens is

around 23,000 cycles and at 20% corrosion it is approxunately 14,000 cycles. The data

shows that fatigue Ufe of die specunens decreases exponentially widi increased corrosion.

When the holes in die un-corroded specimens were cold expanded by 2%, die

specimens did not break tiU 2,000,000 cycles at which point die tests were stopped. For

32

calculation purposes, die fatigue Ufe of die specimens was recorded as 2,000,000 cycles.

When die cold expanded specimens were corroded to 10% mass loss, die specimens

fractured at approximately 470,000 cycles. The test data shows diat 2% cold expansion

of die holes improves die fatigue Ufe of die specunens nearly twenty tunes when diey are

corroded to 0% or 10% mass loss. At 20% mass loss level, die effects of 2% cold

expansion on die fatigue life were negUgible and die specimens fractured at around

25,000 cycles.

When the holes in the specimens were subjected to 4% cold expansion, the

specimens did not fracture tiU 2,000,000 cycles for bodi 0% and 10% mass loss levels.

The fatigue test data shows that 4% cold expansion improved die fatigue Ufe of the

specimens approximately four times compared to 2% cold expansion. However, at 20%

mass loss level, the specimens fractured at around 42,000 cycles and the effects of 4 %

cold expansion were minimal. The fatigue ttend Unes for cold expanded specimens show

a drastic reduction in the fatigue life of the specunens with corrosion implying that

corrosion has an exponential effect on the fatigue Ufe of cold expanded specimens. Table

3.4 and Figure 3.3 show the fatigue test data for specimens that were corroded prior to

cold expansion.

Figure 3.4 gives a comparison of the fatigue Ufe for the specimens that were given

2% cold expansion prior to corrosion and the specunens cold expanded after corrosion.

The graph clearly shows that the fatigue life of the specunens that were cold expanded

after corrosion is lower than the fatigue life of specimens that were cold expanded before

corrosion. This effect is particularly visible at 10% corrosion level. The specimens that

33

Table 3.3: Fatigue test data for cold expanded specimens subjected to subsequent corrosion

Cold expansion

Corrosion (% mass loss)

Cycles to Failure Mean cycles to failure Standard deviation

10

20

10

20

10

20

97,457 100,214 88,740

24,389 22,329 23,910 24,109

14,208 16,804 11,300 14,738

> 2,000,000 > 2,000,000 > 2,000,000

430,352

1,300,214 482,502 493,278

29,322 23,501 24,672

> 2,000,000 > 2,000,000 > 2,000,000

1,720,463 > 2,000,000 > 2,000,000

38,701 45,128 43,125 42,108

95,470

23,684

14,217

> 2,000,000

468,710

25,831

> 2,000,000

> 1,906,821

42,265

1,2,3: specimen did not break and the test was stopped at this point * reading was not included while calculating the mean and standard deviation

5989

920

2273

33654

3079

161390

2608

34

2,5

2 4 6 8 10 12 14 16 18 20 Mass loss due to corrosion (%)

Figure 3.2: Fatigue life of cold expanded (CET) specimens subjected to subsequent corrosion.

35

Table 3.4: Fatigue test data for corroded specimens subjected to subsequent cold expansion

Corrosion „ , _ ,, Mean cycles to c^. •, • • • ^ ,„ , , expansion Cycles to Failure ^ ., Standard deviation (% mass loss) ., failure

97,457 0 0 100,214 95,470'* 5989

88,740

> 2,000,000 > 2,000,000 > 2,000,000

> 2,000,000 > 2,000,000 > 2,000,000

20

> 2,000,000 > 2,000,000 ''^

> 2,000,000 > 2,000,000 ''•'

24,389

10 0 23'9?0 ^^'^^^' ^^° 24,109

351,439

10 2 300,209 337,750 25,293 ^" ^ 345,320

354,035

> 2,000,000 > 2,000,000 > 2,000,000

10 4 > 2,000,000 > 2,000,000'°

14,208

14,738

42,814 24,636 29,648 8084

^ 24,232 25,908

28,904

? 6 m " '"^ ''"•

4,5,7,9,11: same values used in Table 3.3 6, 8,10: specimen did not break and the test had to be stopped at this point

36

6 8 10 12 14 Mass loss due to corrosion (%)

Figure 3.3: Fatigue Ufe of corroded specimens subjected to subsequent cold expansion(CET).

37

were cold expanded prior to corrosion have a fatigue Ufe of around 468,710 cycles while

the specimens that were cold expanded after corrosion have a fatigue Ufe of 337,750

cycles. At 0%, 20% corrosion levels the fatigue Ufe of the specimens for both the cases

was almost the same. Figure 3.5 shows the fatigue life graphs at 4 % cold expansion for

the specimens cold expanded prior to corrosion auid specimens corroded before cold

expansion. In this case the fatigue Ufe of the specimens for both the cases was identical as

can be seen from the graphs.

3.4 Statistical Analysis

Statistical analysis was done to study the effect of variables involved, namely cold

expansion, corrosion and the order of cold expansion on the fatigue Ufe of the specimen.

This was done by using Analysis Of Variance (ANOVA) procedure. ANOVA procedure

was performed for balanced and unbalanced 3^ and 3^x2' Factorial designs using

ANOVA and GLM subroutines in SAS. The results of die analysis are discussed here.

38

2.5

CD

o

•i -̂̂ to

(0

o >. o

E 0.5

XV.

XX ^X ^X xX xX x^ X

1 1 —— Fitted curve for specimen cold expanded first ' 1- — - Fitted curve for specimen corroded first ' ; • Actual data point for specimen cold expanded first | ' , Q Actual data point for specimen corroded first

X X . X X.

X X. X >w "X > v

X X. X X.

X X "X X.

X >v X X , X X

X X

N X ^ V

'

^ ^ 1 X nx .

^ ̂ ̂ __ — , - « 2 4 6 8 10 12 14 16 18 20

Mass loss due to corrosion (grams)

Figure 3.4: Comparison of fatigue life of specimens cold expanded prior to corrosion and specimens cold expanded after corrosion at cold expansion (CET) = 2%

39

2,5

g Fitted curve for specimen cold expanded first Fitted curve for specimen corroded first Actual data points for specimen corroded first actual data points for specimen cold expanded first

6 8 10 12 14 Mass loss due to corrosion (grams)

20

Figure 3.5: Comparison of fatigue Ufe of specunens cold expanded prior to corrosion and specunens cold expanded after corrosion at cold expansion (CET) = 4 %. Curves are almost similar.

40

3.4.1 ANOVA for 3^ factorial design

Fatigue test data from Table 3.3 were used for analyzing die effect of cold

expansion and corrosion on die fatigue Ufe of die specimen. Table 3.5 shows die data

used for analysis for balanced3^ factorial design. Table 3.6 shows die results of die

ANOVA analysis.

The results show that cold expansion and corrosion have a significant effect on

the fatigue Ufe of the specimen, and there is a significant interaction between cold

expansion and corrosion. This can be inferred from the high F-values and low P-values

for cold expansion and corrosion. The results also show that corrosion is a sUghdy more

dominant factor compared to cold expansion.

Table 3.7 shows the data used for the unbalanced 3^ factorial design. The results

of this analysis shown in Table 3.8 and Table 3.9 further confirm the results discussed

above.

41

Table 3.5: Fatigue Ufe data for balanced 3^ factorial design

Cold expansion (%)

0

2

4

Corrosion (%) 0

97,457 100,214 88,740

2,000,000 2,000,000 2,000,000

2,000,000 2,000,000 2,000,000

10

24,389 22,329 23,910

430,352 482,502 493,278

1,720,463 2,000,000 2,000,000

20

14,028 16,804 11,300

29,322 23,501 24,672

38,701 45,128 43,125

42

Table 3.6: ANOVA for balanced 3^ factorial design

Source of variation

Model

Cold expansion

Corrosion

Cold expansion

* Corrosion

Error

Corrected total

Degree of freedom

8

2

2

4

18

26

Sum of squares

2.095 e 13

7.629 e 12

8.276 e 12

5.050 e 12

5.282 e l l

2.148 e 13

Mean square

2.619 e 12

3.814 e 12

4.138 e 12

1.263 e 12

2.934 e 10

F-value

89.26

129.98

141.02

43.02

-

-

P-value

< .0001

< .0001

< .0001

< .0001

-

43

Table 3.7: Fatigue life data for unbalanced 3^ factorial design

Cold expansion (%)

0

2

4

Corrosion (%) 0

97,457 100,214 88,740

2,000,000 2,000,000 2,000,000

2,000,000 2,000,000 2,000,000

10

24,389 22,329 23,910 24109

430,352 1,300,214 482,502 493,278

1,720,463 2,000,000 2,000,000

20

14,028 16,804 11,300 14,738

29,322 23,501 24,672

38,701 45,128 43,125 42,108

44

Table 3.8: ANOVA for unbalanced 3^ factorial design (Type I)

Source of variation

Model

Cold expansion

Corrosion

Cold expansion

*

Corrosion

Error

Corrected total

Degree of freedom

8

2

2

4

22

30

Type I sum of squares

2.2401 e 13

7.491 e 12

8.911 e 12

5.997 e 12

5.7304 e l l

2.297 e 13

Mean square

2.8001 e 12

3.746 e 12

4.456 e 12

1.499 e 12

2.6047 e 10

-

F-value

107.50

143.82

171.07

57.56

-

P-value

< .0001

< .0001

< .0001

< .0001

-

-

45

Table 3.9: ANOVA for unbalanced 3^ factorial design (Type III)

Source of variation

Model

Cold expansion

Corrosion

Cold expansion

* Corrosion

Error

Corrected total

Degree of freedom

8

2

2

4

22

30

Type III sum of squares

2.2401 e 13

8.738 e 12

9.166 e 12

5.997 e 12

5.7304 e l l

2.297 e 13

Mean square

2.8001 e 12

4.368 e 12

4.583 e 12

1.499 e 12

2.6047 e 10

F-value

107.50

167.73

175.95

57.56

P-value

< .0001

< .0001

< .0001

< .0001

-

-

46

3.4.2 ANOVA for 3 ' x 2' Factorial designs

Fatigue test data from Table 3.3 and Table 3.4 were used for analyzing die effects

of cold expansion, corrosion and the order of cold expansion on the fatigue life of die

specimen. Table 3.10 shows die data used for 3^x2' factorial analysis and Table 3.12

shows the results of the ANOVA analysis.

The results show that corrosion and cold expansion have a significant effect on

the fatigue life of die specimen. This can be inferred from the high F-values. Corrosion

has a F- value of 556 while cold expansion has a F-value of 516. The F-values also show

that corrosion is a more dominant factor when compared to cold expansion. The order of

cold expansion has only a sUght effect on the fatigue life of the specimen compared to

corrosion and cold expansion. This can be inferred from the low F-value of 1.17 and high

P-value of 0.2857. The results also show that there is a significant amount of interaction

between corrosion and cold expansion. The low F-values show that there is not much

interaction between cold expansion and the order of cold expansion and between

corrosion and the order of cold expansion.

Table 3.12 shows die data for unbalanced 3^ x2^ factorial design, and Table 3.13

and Table 3.14 show the results of the ANOVA analysis. The results confirm die

observation made above.

47

Table 3.10: Fatigue Ufe data for balanced 3^ x 2' factorial design

Cold expansion

(%)

0

2

4

Order of treatment A

Corrosion (%) 0

97,457 100,214 88,740

2,000,000 2,000,000 2,000,000

2,000,000 2,000,000 2,000,000

10

24,389 22,329 23,910

430,352 482,502 493,278

1,720,463 2,000,000 2,000,000

20

14,028 16,804 11,300

29,322 23,501 24,672

38,701 45,128 43,125

B Corrosion (%)

0

97,457 100,214 88,740

2,000,000 2,000,000 2,000,000

2,000,000 2,000,000 2,000,000

10

24,389 22,329 23,910

351,439 345,320 354,035

2,000,000 2,000,000 2,000,000

20

14,028 16,804 11,300

25,636 24,232 25,906

28,908 27,310 26,001

48

Table 3.11: ANOVA for balanced 3^x2' factorial design

Source of variation

Model

Cold expansion

Order

Corrosion

Cold expansion * Order

Cold expansion * Corrosion

Order* Corrosion

Cold expansion * Order * Corrosion

Error

Corrected total

Degree of freedom

17

2

1

2

2

4

2

4

36

53

Sum of squares

4.315 e 13

1.520 e 13

1.729 e 10

1.639 e 13

6.589 e 10

1.131 e 13

3.149 e 10

1.452 e l l

5.301 e l l

4.3683 e 13

Mean square

2.538 e 12

7.600 e 12

1.729 e 10

8.193 e 12

3.295 e 10

2.827 e 12

1.574 e 10

3.630 e 10

1.472 e 10

F-value

172.39

516.10

1.17

556.43

2.24

191.96

1.07

2.47

P-value

< .0001

< .0001

0.2857

< .0001

0.1214

< .0001

0.3539

0.0624

-

49

Table 3.12: Fatigue Ufe data for unbalanced 3^ x2 ' factorial design

Cold expansion

(%)

0

2

4

Order of treatment A

Corrosion (%) 0

97,457 100,214 88,740

2,000,000 2,000,000 2,000,000

2,000,000 2,000,000 2,000,000

10

24,389 22,329 23,910 24,109

430,352 1,300,2214

482,502 493,278

1,720,463 2,000,000 2,000,000

20

14,028 16,804 11,300 14,738

29,322 23,501 24,672

38,701 45,128 43,125 42,108

B Corrosion (%)

0

97,457 100,214 88,740

2,000,000 2,000,000 2,000,000

2,000,000 2,000,000 2,000,000

10

24,389 22,329 23,910 24,109

351,439 300,209 345,320 354,035

2,000,000 2,000,000 2,000,000

20

14,028 16,804 11,300 14,738

42,814 25,636 24,232-25,906

28,908 27,310 26,001

50

Table 3.13: ANOVA for unbalanced 3^ x2 ' factorial design (Type I)

Source of variation

Model

Cold expansion

Order

Corrosion

Cold expansion * Order

Cold expansion * Corrosion

Order * Corrosion

Cold expansion * Order * Corrosion

Error

Corrected total

Degree of freedom

17

2

1

2

2

4

2

4

44

61

Type I sum of squares

4.602 e 13

1.561 e 13

6.495 e 9

1.725 e 13

1.278 e l l

1.287 e 13

2.852 e 10

1.254 e l l

5.752 e l l

4.6599 e 13

Mean square

2.707 e 12

7.806 e 12

6.495 e 9

8.625 e 12

6.380 e 10

3.218 e 12

1.426 e 10

3.134 e 10

1.307 e 10

F-value

207.06

597.03

0.50

659.69

4.88

246.16

1.09

2.40

-

P-value

< .0001

< .0001

0.4846

< .0001

0.0122

< .0001

0.3449

0.0645

51

Table 3.14: ANOVA for unbalanced 3 ' x2 ' factorial design (Type III)

Source of variation

Model

Cold expansion

Order

Corrosion

Cold expansion * Order

Cold expansion * Corrosion

Order * Corrosion

Cold expansion * Order * Corrosion

Error

Corrected total

Degree of freedom

17

2

1

2

2

4

2

4

44

61

Type III sum of squares

4.602 e 13

1.715 e 13

1.236 e 10

1.814e 13

5.394 e 10

1.288 e 13

2.232 e 10

1.254 e l l

5.752 e l l

4.6599 e 13

Mean square

2.707 e 12

8.574 e 12

1.236 e 10

9.071 e 12

2.697 e 10

3.219 e 12

1.116elO

3.134 e 10

1.307 e 10

F-value

207.06

655.80

0.95

693.77

2.06

246.22

0.85

2.40

-

-

P-value

< .0001

< .0001

0.3362

< .0001

0.1392

< .0001

0.4328

0.0645

52

3.5. Fractographic Analysis Results

Fractographic analysis was done for (0,0), (0,10), (2,10), (2,20), (4,20) specmiens

from fatigue test. The remaining specimens namely, (2,0), (4,0), (4,10) were not

examined as they did not fracture. Results of the analysis are discussed below.

3.5.1. (0.0) specimen

Figure 3.6 shows the appearance of the edges of an un-corroded, non-cold

expanded specimen. Figure 3.6a shows the comer and 3.6b shows the edge of the hole.

In the figure the edges of the specimen are clearly defined and do not show any damage

other than that due to fracture

Figure 3.7 shows the variation in the pattern seen on the fracture surface along the

length of the specimen. Near the hole, the fracture surface has a bright and shiny

appearance and shows cleavage fracture type features. This can be seen in Figure 3.7a.

As the crack propagates, the surface shows features that are characteristics of ductile

fracture. Figure 3.7b shows a surface similar to the previous one except for the presence

of some voids, which are characteristic of ductile fracture. Further along the length, the

fracture becomes quasi-cleavage type, showing features associated with both cleavage

and ductile fractures. This can be seen in Figure 3.7c, in which the surface shows a

number of dimples along with some cleavage fracture features. Further along the length,

the fracture becomes completely ductile as seen in Figure 3.7d. The same pattern is

repeated on the other end of the hole.

53

.'.»&

, ^ C -'•̂

. • : 1 ^ - : « ; \ ^ * •/•' ^ i _ . . / '

000101 12KV X£50""i£0Ufti

(a)

J00r07 12KV X780

(b)

880132 leKV Xe.80K 15,0Uin

(c)

Figure 3.6: Appearance of the edge of a (0,0) specimen, (a) Comer of specimen near the hole, (b) Edge of the specimen near the hole, (c) Side edge of the specimen at a distance 0.1 inch from the hole. The surface shows no damage other than due to fracture.

54

•̂WJ

008103 Ic'KV >i700 43i.

(a) (b)

(c) (d)

Figure 3.7: Appearance of the surface along the length of the (0,0) specimen, (a) Cleavage fracture type surface near the edge, (b) Cleavage Fracture type surface at a distance of 0.1 inch from the hole. In addition some voids can be seen on the surface, (c) Fracture surface at a distance of 0.3 inches from hole showing quasi-cleavage fracture pattern, (d) Fracture surface at a distance of 0.5 inches from hole showing ductile fracture pattem.

55

Figure 3.8 shows the striations formed due to fatigue testing on the surface of the

specimen. Near the hole no striations were observed as shown in Figure 3.8a. As die

crack propagates the striations become increasingly visible and larger as can be seen in

Figures 3.8b, c, d and e. In Figure 3.8b taken at distance of 0.05 inches from die hole die

striations could be seen only at a magnification of around 5000, while at a distance of 0.3

inches from the hole the striations were visible at a magnification of 2000. Near the

ductile region no striations were observed as seen in Figure 3.8f

3.5.2. (0.10) specimen

Figure 3.9 shows the corroded surface of the specimen near the hole. Figure 3.9a

shows the edge of the hole. The figure shows ridges along the edge, which represent the

source of the crack. The edges here show some damage due to corrosion as can be seen in

Figures 3.9a and b. The fracture and striation pattems seen here were similar to the

patterns observed in the (0,0) specimen.

3.5.3 (2.10) specimen

This specimen showed the same features as seen in the other two specimens.

Figure 3.10 shows the corroded edge near to the hole. Figure 3.10a shows the edge close

to the hole. The figure shows a number of ridges on die edge, which represent the source

of the crack.

56

(a) (b)

(c) (d)

Figure 3.8: Fatigue striations for (0,0) specimen, (a) No striations are seen near the hole, (b) Striations at a distance of 0.05 inches from the hole, (c) Striations at a distance of 0.1 inch from the hole, (d) Striations at a distance of 0.2 inches from the hole. Fatigue striations become increasingly larger as the crack propagates.

57

668134 1£KV X S . 8 8 K " i s l S u m

(e) (f)

Figure 3.8 (continued): (e) Striations at a distance 0.3 inches from die hole, (f) No striations could be seen in ductile fracture region

^^-f*J>.V*^ •t! -

800138 12KV x e e i e ' .50ISII1

(a) (b)

Figure 3.9: Appearance of edges of (0,10) specimen, (a) Edge of the hole, (b) Comer of the hole. They show extensive damage due to corrosion compared to Figure 3.6.

58

On closer observation of the corroded surface, a number of secondary cracks

could be seen near die hole. This is shown in Figure 3.10b. Figure 3.10c shows a higher

magnification image of die corroded surface. The figure shows sharp cracks formed on

the surface due to corrosion, similar to intergranular fracture.

Figure 3.11 shows the fracture pattem formed on the surface due to the

appUcation of cycUc load. The pattem seen here was sunilar to one seen in (0,0)

specimen. Figure 3.11a shows the cleavage fracture region, while Figure 3.11b shows the

ductile fracture. The specimen showed a gradual transition from cleavage to ductile

fracture on one side of the hole. The other side had only ductile fracture pattem. Figure

3.12 shows the fatigue pattem formed on the surface due to cycUc loading on the other

side of the hole. Fatigue striations seen here were similar to one observed in (0,0)

specimen.

3.5.4 (2.201 specimen

Figure 3.13 shows the corroded surface of the specimen. The surface of the

specimen was more severely damaged when compared to a 10% corrosion specimen.

Figure 3.13a shows the edge of die hole. Figures 3.13a, b and c show widespread damage

to the specunen along die sides. Figures 3.13b and c show die damage on die waUs of die

hole due to corrosion and fatigue cracking. The fracture pattem formed on die surface of

the specunen is sunilar to one formed on die (2,10) specunen. The surface shows a

gradual transition from cleavage to ductile fracture on one side of die hole, while on die

other side it undergoes only ductile fracture as in (2,10) specunen.

59

3.5.5 (4.20) specimen

Figure 3.14 shows die corroded surface of die specimen close to the edge. Unlike

odier specimens, large cracks Uke die one shown in Figure 3.14b were observed in (4,20)

specimen. These cracks were found to originate near the hole and ran through the enter

length of the specimen. Figures 3.14c also shows a number of secondary cracks formed

near the edges, which were corroded.

Also, die (4,20) specimen showed brittle fracture region away from the edge of

the hole. Figures 3.15a and b show surfaces having cleavage fracture pattems in the midst

of ductile fracture regions (Figure 3.15c and d). On the other side there was a gradual

transition from cleavage to ductile fracture as seen in Figure 3.16 .

60

•m^.'s^^t..'-^^.

^ \

066103 12KV X1.00K 30uir.

(a) (b)

000104 12KV X2.50K 12.0uffi

(C)

Figure 3.10: Corroded edge for (2,10) specimen, (a) Edge of the hole, (b) Edge at a higher magnification, (c) lUgher magnification image of the crack Edge and surface close to the hole show number of cracks, which were not seen in (0,0) specimen.

61

000111 12KV X680 50UB 088131 12KV xsee

(a) (b)

Figure 3.11: Appearance of the fracture surface for (2,10) specimen, (a) Cleavage type fracture pattem near the hole, (b) Ductile fracture type pattem at a distance of 0.4 inches from the hole. There is a gradual transition from cleavage type fracture to ductile fracture.

860128 12KV X500"

(a) (b)

Figure 3.12: Appearance of die fracture surface for (2,10) specunen on die odier side of the hole, (a) Ductile fracture type pattem near die hole, (b) Ductile fracture type pattem at a distance of 0.4 inches from die hole. Dunples get elongated as die crack propagates.

62

(a) (b)

080105 12KV X1.50K 29.Sum

(c)

Figure 3.13: Corroded edge for (2,20) specimen, (a) Edge of the hole, (b) Higher magnification image of the edge of the hole, (c) Higher magnification image of the surface of the hole. Side waUs of the hole show extensive cracking.

63

(a) (b)

000121 12KV X500 60um

(C)

Figure 3.14: Corroded edge for (4,20) specimen, (a) Near the hole, (b) Crack on the surface, (c) Surface near the comer of the hole shows number of smaUer cracks.

64

e s e i i s 12KV K50e"

(a) (b)

(c) (d)

Figure 3.15: Surface on the left side of the hole for (4,20) specimen, (a) shows a flat region similar to cleavage fracture region, (b) Higher magnification image of the same, (c) Shows ductile fracture region around the cleavage fracture region, (d) Higher magnification image of the same. Cleavage fracture region was found away from the hole and was surrounded by ductile fracture region.

65

000143 I2KV X500 ekun,

(a) (b)

006147 12KV X580

(C) (d)

Figure 3.19: Appearance of the surface on die right hand side of die hole for (4,20) specimen, (a) Cleavage fracture pattem near the edge, (b) Cleavage fracture pattem at a distance of 0.05 inches from the hole, (c) Quasi-cleavage fracture pattern at a distance of 0.15 inches from the hole, (d) Ductile fracture pattem at a distance of 0.4 inches from the hole. The surface shows a gradual ttansition from cleavage fracture to ductile fracture.

66

CHAPTER IV

CONCLUSIONS AND RECOMMENDATIONS

4.1 Conclusions

The following conclusions were drawn from the study.

1. Cold expansion of specimens proved to be an effective means of enhancing the

fatigue life of the specimens. The benchmark specimens that were not subjected to cold

expansion, or corrosion, showed an average fatigue life of 95,500 cycles. 2% cold

expansion of specimens resulted in a 20-fold improvement in the fatigue life of

benchmark specimens. These specimens had an average fatigue life of over 2,000,000

cycles. Higher cold expansion of 4% also showed improvements in fatigue life of

specimens beyond 2,000,000 cycles.

2, Corrosion as measured by percentage mass loss of the specimens, resulted in a

reduction in the fatigue life of the specimens. In benchmark specimens, 10% corrosion

resulted in the reduction of fatigue life to 23,700 cycles from 95,500 cycles. The effect of

corrosion was also apparent in cold expanded specimens. The fatigue life of 2% cold

expanded specimens dropped from over 2,000,000 cycles to 468,700 cycles after a mass

loss of 10% due to corrosion. However, the fatigue life of 4% cold expansion specimens

oidy dropped from over 2,000,000 cycles to 1,900,000 cycles after a 10% mass loss due

to corrosion. This data clearly shows that higher cold expansion provides more resistance

against fatigue failure, even when the material is subjected to a subsequent corrosion. The

data also shows that 10% corrosion causes the fatigue life of non-cold expanded

67

specimens to be reduced by a factor of 4. The same observation can be made regarding

2% cold expansion specimens where 10% corrosion results in a 4-fold reduction in the

specimens fatigue life.

3. Two independent mechanisms contribute to the observed reduction in the

fatigue Ufe of the corroded specimens. Ffrst is the obvious reduction in the materials

cross sectional area which gives rise to higher sttess ampUtudes and hence lowered

fatigue life. The second mechanism has to do widi stress corrosion cracking (SCC), a

phenomenon where die effect of corrosion is accentuated by the presence of a stress field.

It is not clear at this point how much of the above reduction is due to the decrease in

materials cross sectional area, and how much of the reduction is due to degradation of the

material properties due to corrosion. Further studies are requfred in this area to

understand it clearly.

4. The effect of higher mass loss due to corrosion, as expected, was further

reduction in the fatigue life of the specimens. For benchmark specimens, the fatigue life

was reduced from 95,500 cycles to 14,200 cycles for 20% corroded specimens as

compared to 23,700 cycles for 10% corroded specunens. At higher corrosion rates die

effect of cold expansion on the fatigue life of die specimens was also negUgible. 2% cold

expansion specimens subjected to 20% mass loss fractured at around 26,000 cycles while

4% cold expansion specmiens subjected to 20% mass loss fracttued at around 42,000

cycles. Again, it is not clear at tins point how much of die above reduction in fatigue life

is due to die decrease in materials cross sectional area, and how much of die reduction is

due to degradation of die material properties due to corrosion itself.

68

5. The data from Table 3.4 also indicates that cold expansion can dramatically

increase the fatigue Ufe of specimens, which have been afready corroded. A 2% cold

expansion of 10% corroded specimen, increased its fatigue life from 23,700 cycles to

337,800 cycles. Cold expansion of the same specimen by 4% resulted in a fatigue life

improvement beyond 2,000,000 cycles. One observation here is that in general the fatigue

life of a specimen was higher if the cold expansion was done before the corrosion than if

the cold expansion was done after corrosion. This data supports the theory that the

material properties in the corroded zone undergo a degradation, which when foUowed by

cold expansion, will produce an adverse effect on the abiUty of the material to resist crack

propagation.

4.2 Recommendations

Corrosion of specimens in this study was conducted to simulate envfronmental

effects on a material operating in a corrosive envfronment. However in die real world,

corrosion occurs simultaneously with die cycUc loading on die material. Therefore in an

ideal case, corrosion of the material should occur as die material is cycUcaUy loaded.

ReaUzing the difficulties associated widi conducting such an experiment, an alternative

approach for futtu-e sttidies might be to intermpt die corrosion cycle periodically,

foUowed by cycUcaUy loading die specimen to 10% of its expected Ufe, foUowed by

anodier increment of corrosion, foUowed by anodier increment of cycUc loading, and so

on until the specimen fractures.

69

Also, just as it has been shown that cold expansion can be beneficial during

various stages of a components fatigue Ufe (Gaerke et al., 2000), it would be interesting

to study the effect of cold expansion at different intervals during the corrosion cycle. For

example, if a specimen is to be corroded by 10% and then cold expanded by 2%, it would

be interesting to stop the corrosion at 2%, 4%, 6% and 8% (in four different specimens),

conduct a 2% cold expansion, and continue the corrosion process to a total of 10%. Such

a study will show the effect of cold expansion timing on the corrosion evolution.

70

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73

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