Prany Formal

LAB REPORT 1:

TENSILE TEST OF ALUMINUM 6061-T6511

Date of Lab Experiment: January 19, 2010, January 26, 2010

Date of Submission: February 2, 2010

By

_______________

Prannoy A Agarwala

[email protected]

Submitted to Dr. David Lanning

Department of Aeronautical Engineering

College of Engineering

In Partial Fulfillment

Of the Requirements

Of

ES321

Engineering Materials Science Lab

Spring 2010

Embry Riddle Aeronautical University

Prescott, Arizona

Al 6061 Tensile Test i

Abstract

Aluminum alloys play a critical role in structural components of aircrafts and yachts. A specimen of Aluminum 6061-T6511 has been subjected to a standardized tensile test (ASTM standards). Data such as the Young’s Modulus, Ultimate Tensile strength and a Stress-Strain curve of the tensile test is reported. The published value of the Young’s Modulus for Aluminum 6061 is 10,000ksi. Alloys can exhibit ductile and brittle failure. Ductile failure is accompanied by plastic deformation whereas brittle failure is not. Necking exhibits plastic deformation and hence occurs in materials which are ductile when subjected to stress. The first phase of the experiment involves measuring dimensions, hardness, and roughness of the specimen before and after the tensile test. The second phase of the experiment involves, investigating the fracture surface macroscopically and microscopically with the use of a stereomicroscope and a scanning electron microscope.

The values obtained from the experiment are in accordance with the published values. Phenomena such as necking, strain hardening and a change in the texture of the surface after failure are observed. The specimen exhibits a transitional ductile to brittle fracture.

This paper introduces the theoretical principles behind ductile and brittle fracture and publishes data and results related to the tensile test of the specimen of Aluminum 6061-T6511.

Al 6061 Tensile Test ii

TABLE OF CONTENTS

Abstract ………………………………………….……………………………. … i

List of Tables …..……………………………………………………………….. iii

List of Figures …..………………………………………………………………. iv

List of Graphics ….……………………………………………………………. v

List of Symbols ………………………………………………………………… vi

List of Abbreviations/Acronyms ...…………………………………………… vii

1.0 Introduction ...……………………………………………………………… 1

2.0 Theory ………………………………………………………………………. 2

3.0 Apparatus and Procedures ...……………………………………………….. 9

3.1 Apparatus .…………………………………………………………… 9

3.2 Procedures …………………………………………………………… 13

3.1.1 Phase 1……………………………………………………….. 13

3.1.2 Phase 2 ………………………………………………………. 14

4.0 Results and Discussion ……………………………………………………. 15

5.0 Conclusions and Recommendations ……………………………………….. 22

6.0 References ………………………………………………………………… 23

7.0 Appendix I: Sample Calculations ………………………………………….. 25

8.0 Appendix II: Raw Data …………………………………………………….. 26

Al 6061 Tensile Test iii

LIST OF TABLES

Table 2.1: Aluminum 6061-T651 Data Sheet ………………………………… 4

Table 4.1: Dimensional Data of Al 6061-T6511 Specimen ………………….. 16

Table 4.2: Specimen Hardness Data ………………………………………... 17

Table 4.3: Specimen Roughness Data ………………………………………. 17

Al 6061 Tensile Test iv

LIST OF FIGURES

Figure 2.1: Stress Vs. Strain Curve for a ductile material ….………………………………….. 3

Figure 2.2: Ductile Overload ………………………….……………………………………….. 6

Figure 2.3: Transition from Ductile to Brittle Fracture ….…………………………………….. 7

Figure 2.4: Ductile and Brittle Fracture ………………….…………………………………….. 7

Figure 3.1: Tensile test bar ……………………………….…………………………………….. 9

Figure 3.2: Tensile Testing Machine …………………...……………………………………... 10

Figure 3.3: Surface Roughness Tester ………………...………………………………………. 10

Figure 3.4: Rockwell Hardness Tester ………………...……………………………………… 11

Figure 3.5: Vernier Calliper …………………………...……………………………………… 11

Figure 3.6: Micrometer Screw Gage ………………...………………………………………... 12

Figure 3.7: Ultrasonic Cleaner ……………………...………………………………………… 12

Figure 3.8: Stereomicroscope ……………………...…………………………………………. 12

Figure 3.9: Scanning Electron Microscope …...……………………………………………… 13

Figure 4.1: Stress vs Strain Plot ……………..………………………………………………. 15

Figure 4.2: Plot to find Young’s Modulus ……...……………………………………………. 16

Figure 4.3: Stereomicroscope Image 1, Necking …..………………………………………… 18

Figure 4.4: Fracture surface under stereomicroscope …..……………………………………. 19

Figure 4.5: Fracture surface under Scanning Electron Microscope -Al 6061-T651…...……... 19

Figure 4.6: Fracture surface under SEM (2500x) ………………………………..………….. 20

Figure 4.7: Backscatter image of fracture surface using SEM ………………..……………... 21

Al 6061 Tensile Test v

LIST OF GRAPHICS

Figure 2.1: Stress Vs. Strain Curve for a ductile material ………………..…………………… 3

Table 2.1: Aluminum 6061-T651 Data Sheet ……………………………..………………….. 4

Figure 2.2: Ductile Overload ……………………………………………….…………………. 6

Figure 2.3: Transition from Ductile to Brittle Fracture …………………….…………………. 7

Figure 2.4: Ductile and Brittle Fracture …………………………………….…………………. 7

Figure 3.1: Tensile test bar ………………………………………………….…………………. 9

Figure 3.2: Tensile Testing Machine ………………………………………...………………. 10

Figure 3.3: Surface Roughness Tester ………………………………………...…………….. 10

Figure 3.4: Rockwell Hardness Tester ……………………………...……………………….. 11

Figure 3.5: Vernier Calliper ………………………………………...……………………….. 11

Figure 3.6: Micrometer Screw Gage ………………………………...………………………. 12

Figure 3.7: Ultrasonic Cleaner ……………………………………...……………………….. 12

Figure 3.8: Stereomicroscope ………………………………………..……………………… 12

Figure 3.9: Scanning Electron Microscope ………………………….……………………… 13

Figure 4.1: Stress vs Strain Plot ……………………………………………………………. 15

Figure 4.2: Plot to find Young’s Modulus …………………………………………………. 16

Table 4.1: Dimensional Data of Al 6061-T6511 Specimen ………….………………….. … 16

Table 4.2: Specimen Hardness Data ………………………………….….……………......... 17

Table 4.3: Specimen Roughness Data ………………………………….…………………… 17

Figure 4.3: Stereomicroscope Image 1, Necking ………………………..………………….. 18

Figure 4.4: Fracture surface under stereomicroscope …………………….………………… 19

Figure 4.5: Fracture surface under Scanning Electron Microscope -Al 6061-T651………….. 19

Figure 4.6: Fracture surface under SEM (2500x) ……………………………….. ………... 20

Figure 4.7: Backscatter image of fracture surface using SEM …………………… ………. 21

Al 6061 Tensile Test vi

LIST OF SYMBOLS

σ……………………………...Stress...............................................Lb/in2

ε………………………………Strain………………………………..—

ν………………………Poisson’s Ratio……………………………..—

Al 6061 Tensile Test vii

LIST OF ABBREVIATIONS/ACRONYMS

ASTM………American Society for Testing and Materials……………………--

SEM……………….Scanning Electron Microscope…………………………...--

Al 6061 Tensile Test 1

1.0 Introduction

Ever since its discovery, Aluminum has been critical to human beings. Scientists and engineers have spent decades on studying and understanding the behavior and chemistry of metals under different conditions. With time, human beings have also learnt how to create alloys, which are matrices of different metals and elements.

Alloys such as bronze and brass have been known to man since prehistoric times. Alloys have an advantage over simple elements because their composition can be adjusted based on how they are being used. Qualities such as better resistance to corrosion, improved strength, better ductility and other benefits allow alloys to be used in the production and manufacture in a number of applications today.

Alloys of aluminum are extensively used in the production of yachts, aircrafts and also for welding. Many structural components in aircraft such as the fuselage and wings use these aluminum alloys. Hence, it is critical for us to understand how these alloys behave under extreme loads and conditions.

Aluminum alloy 6061 is one such alloy that has been employed over the years for the aforementioned applications.

The purpose of this lab, then, is to subject a test specimen of Aluminum 6061-T6511 to a tensile test and record data related to the test. After fracture, the fracture surface and the region around the fracture surface is viewed under and electron microscope and a light microscope.

The results of the tensile test of Aluminum 6061-T6511 are summarized. Details such as the Modulus of Elasticity, Proportional Limit and the Ultimate Tensile Strength are reported. Also, a discussion of the fracture of the specimen is carried out.


2.0 Theory

Stress(σ ¿: Stress is defined as the force experienced by an element over the cross sectional area subjected to that force. Its units are expressed as force/area, for example – pounds per square inch(psi) or Newtons per square metre(N/m2).

Strain(ε): Strain represents the elongation experienced by a material due to application of a stress. It is dimensionless or unit less.

Young’s Modulus(E): Also known as the modulus of elasticity, is defined as the ratio of the axial stress over the axial strain. The three are related by the Hooke’s Law.

Shear Modulus(G): Also known as the modulus of rigidity, is defined as the ratio of the shear stress over the shear strain.

Poisson’s ratio(ν): Defined as the ratio of the transverse strain to the axial strain experienced by a material.

Stress-Strain curve: The stress strain curve is a graphical plot, representing the stress experienced by a material on the y-axis, and the strain experienced by the material due to the stress on the x-axis.

Elastic Deformation: When the stress applied to a material is removed, the material(solid) regains its original shape.

Plastic Deformation: Is the deformation experienced by a material when it no longer behaves as an elastic material. In other words, once the stress is removed from the material, it does not regain its original shape.

Fracture: Is the point at which a material breaks into two or more pieces due to an application of stress.

Ultimate Tensile Strength: Is the maximum stress a material can experience before fracture or failure. It is the maximum stress on the stress-strain curve.

Offset Yield Strength: Is found using the stress-strain curve, and is the stress required to produce a 0.2% plastic strain.

Proportional Limit: Defined as the maximum stress at which stress is directly proportional to the strain.

Hardness: Is the property of a material to resist indentation by a hard object.

Roughness: It can be defined as the measure of the texture of a surface.

Ductility: Is the ability of a material to deform before failure.


Theoretical principles:Stress exists in two forms – normal or axial stress, represented by sigma (σ), and shear stress, represented by tau (τ). Shear stresses are caused due to forces acting in the plane of the cross section while normal stresses are caused due to forces acting perpendicular to the plane of the cross section.

The Stress-Strain curve illustrated in figure 2.1 is a means of expressing stress versus strain for materials. The stress vs strain curve is distinct for each kind of material and thus, is of significant importance as it is used to express the different material properties of materials.

Figure 2.1: Stress Vs. Strain Curve for a ductile material. (Source: Interactive Nano-Visualization in Science and Engineering Education,

2010)

The Young’s Modulus is usually found by measuring the slope of the linear portion of the stress-strain curve(i.e. the elastic region). In Table 2.1 below, we see that the typical value for the Young’s Modulus for Al 6061-T651 is around 10,000ksi. The Young’s Modulus is related to stress and strain by the Hooke’s Law.


Physical Properties Metric English Comments

Density 2.7 g/cc 0.0975 lb/in³

AA; Typical

Mechanical Properties

Hardness, Brinell 95 95 AA; Typical; 500 g load; 10 mm ball

Hardness, Knoop 120 120 Converted from Brinell Hardness Value

Hardness, Rockwell A 40 40 Converted from Brinell Hardness Value

Hardness, Rockwell B 60 60 Converted from Brinell Hardness Value

Hardness, Vickers 107 107 Converted from Brinell Hardness Value

Ultimate Tensile Strength

310 MPa 45000 psi AA; Typical

Tensile Yield Strength 276 MPa 40000 psi AA; Typical

Elongation at Break 12 % 12 % AA; Typical; 1/16 in. (1.6 mm) Thickness

Elongation at Break 17 % 17 % AA; Typical; 1/2 in. (12.7 mm) Diameter

Modulus of Elasticity 68.9 GPa 10000 ksi AA; Typical; Average of tension and compression. Compression modulus

is about 2% greater than tensile modulus.

Notched Tensile Strength

324 MPa 47000 psi 2.5 cm width x 0.16 cm thick side-

http://asm.matweb.com/search/GetUnits.asp?convertfrom=109&value=324






http://asm.matweb.com/search/GetUnits.asp?convertfrom=43&value=2.7


notched specimen, Kt = 17.

Ultimate Bearing Strength

607 MPa 88000 psi Edge distance/pin diameter = 2.0

Bearing Yield Strength 386 MPa 56000 psi Edge distance/pin diameter = 2.0

Poisson's Ratio 0.33 0.33 Estimated from trends in similar Al alloys.

Fatigue Strength 96.5 MPa 14000 psi AA; 500,000,000 cycles completely reversed stress; RR Moore

machine/specimen

Fracture Toughness 29 MPa- m½

26.4 ksi-in½

KIC; TL orientation.

Machinability 50 % 50 % 0-100 Scale of Aluminum Alloys

Shear Modulus 26 GPa 3770 ksi Estimated from similar Al alloys.

Shear Strength 207 MPa 30000 psi AA; Typical

Table 2.1: Aluminum 6061-T651 Data Sheet. (Source: Aerospace Specification Metals, Inc, 2010)

Figure 2.1 illustrates the elastic region of a material. This region is the region where elastic deformation takes place. As defined earlier, it is the region where the stress and strain are directly proportional to each other. Also, on removal of the load the material regains its original shape. The elastic limit or the proportionality limit is the point where the linear relationship between stress and strain ends. It is not well defined for every material and hence is not published on data sheets, compared to say the ultimate tensile strength. The reason for elastic deformation can be attributed to chemical bonding. As a tensile load is applied to a material, the bonds stretch and when the load is removed the bonds regain their equilibrium lengths. This is true for elastic deformation only (Fischer 2009).

Plastic deformation occurs when the metal or material no longer behaves as an elastic material. Qualities such as ductility and malleability are due to the plastic deformation of metals. All materials behave very differently in the plastic region. A phenomenon known










as strain hardening is experienced by metals when a tensile stress is applied. This strain hardening actually strengthens metals. This phenomenon happens because as the crystals within the metal matrix are dislocated, a resistance to further dislocations is developed resulting in the observed strengthening of the metal. Another phenomenon observed under the application of tensile stress, is necking. Necking is the reduction of cross sectional area, where a large amount of strain is localized. It occurs around the region where fracture would eventually take place. In Engineering Materials, there are two kinds of fracture which take place, ductile and brittle. The main difference between the two is the amount of plastic deformation that can be withstood. Ductile materials can withstand plastic deformation whereas brittle materials fracture rather rapidly after the elastic region. In ductile materials, crack propagation is slow and the amount of plastic deformation is large. Only when a larger stress is applied will the crack propagate. In brittle materials, crack propagation is rapid and is not accompanied by a large amount of plastic deformation. The cracks continue to grow until fracture.

Figure 2.2: Ductile Overload. (Source: Argo-Tech Materials Laboratory, 2010)

Figure 2.2 is a scanning electron microscope image captured by ATC Labs based out of Cleveland, Ohio of a ductile material after fracture. At the microscopic level, ductile surfaces appear to have a large number of voids and dimples in a very irregular manner. The presence of ductile dimples indicates a ductile overload as seen in Fig 2.2. Also, as the smaller microvoids grow, they start linking up to form voids leading to cracks which lead to fracture. In comparison, brittle fracture seems to occur due to granular fracture. There are two kinds of granular fracture, transgranular or intergranular fracture. The former is when the crack travels through the grain of the material while the latter is when the crack travels through the grain boundaries. Fig 2.3 shows a ductile to brittle fracture


transition. On the right hand side of the image we notice ridges, which is nothing but granular fracture. The transition occurs due to a change in density or temperature.

Figure 2.3: Transition from Ductile to Brittle Fracture. (Source: The Hendrix Group, 2010)

When observed at a macroscopic level, ductile fracture surfaces are rough and a noticeable shear tip is seen. Usually, ductile failure occurs due to shear stress resulting in a large necking region. As seen in Fig 2.4, the fracture on the left is ductile fracture, the shear tip can be seen, as well as the rough and irregular surface. Brittle fracture on the other hand seems to be a cleaner kind of fracture where the amount of necking occurring is smaller. This is due to the fact that a large amount of plastic deformation does not occur. The surface of fracture is also smoother.

Figure 2.4: Ductile and Brittle Fracture. (Source: Charles Sturt University, 2010)


The tensile test is used to determine the properties of a material. The specimen used in the test is prepared in accordance with the American Society for Testing and Materials (ASTM) standards.

The fracture surfaces are usually observed under stereomicroscopes at the macroscopic level and scanning electron microscopes at the microscopic level. The magnification produced by stereomicroscopes is in the range of 10-2000x and visible light is used to produce the image. Scanning electron microscopes (SEM) have the ability to magnify up to 300,000x. They use high-speed electron to produce an image. The lens of an SEM is electromagnetic, while that of a stereomicroscope is glass.

Expected Results:

The values published for Aluminum 6061-T651 in Table 2.1, give an idea on what value the Modulus of Elasticity, Ultimate Tensile Strength, the Proportional Limit and the yield at 0.2% strain should be for our specimen. Also, as Aluminum is a ductile metal, its alloy should exhibit ductile fracture. However, there is a possibility for the alloy to exhibit a ductile to brittle transition fracture as shown in Fig 2.3. This is because of the presence of elements such as Silicon, and Nickel in the alloy.

Equations used:

Stress is measured using the equation:

σ=ForceArea Equation 2.1

Strain is measured using the equation:

ε = change∈lengthoriginal length Equation 2.2

Young’s Modulus of Elasticity is calculated using the equation:

σ = Eε Equation 2.3

% Elongation of gage section:

% elongation= Final length−Original lengthOriginal length

× 100 % Equation

2.4

% Reduction in Area:


%reduction=Original area−Final areaOriginal area

× 100 % Equation

2.5

Justification:

Aluminum alloys are of great value, due to their relatively low weight, high yield strength and their ability to withstand a relatively large amount of plastic deformation. This makes them ideal for use in structural components in the aircraft and yachts. Aluminum alloy T6061 is widely used in the fuselage and wings of aircraft.

3.0 Apparatus and Procedures:

3.1 Apparatus:

The following equipment was used during the lab: One (1) Aluminum 6061-T6511 tensile test bar, One (1) Tensile testing machine, One (1) Extensometer, One (1) sheet of 600 grit sandpaper, One (1) TR200 Surface Roughness Tester, One (1) Rockwell Hardness Tester, One (1) Vernier Calliper, One (1) Micrometer Screw Gauge, One (1) Band saw, One (1) Ultrasonic cleaner, One (1) Stereomicroscope, One (1) Scanning Electron Microscope.

The Aluminum 6061-T6511 tensile test bar is prepared according to American Society for Materials and Testing (ASTM) standards. Figure 3.1 illustrates the bar.


Figure 3.1: Tensile test bar.

The tensile testing machine used is illustrated in Figure 3.2. The machine measures force in pounds. The extensometer measures extension in inches.

Figure 3.2: Tensile Testing Machine. (Source: Direct Industry, Instron, 2010)

Gage sectionGrip section

Specimen length

Gage section width

length


The surface roughness tester used is illustrated in Figure 3.3. It reads measurements in micrometers.

Figure 3.3: Surface Roughness Tester. (Source: M & I Instruments, Inc, 2010)

The hardness tester used is illustrated in Figure 3.4. It gives readings in the Hardness Rockwell B scale.

Figure 3.4: Rockwell Hardness Tester. (Source: FAR Asia Co., Ltd, 2003)

The Vernier Calliper used is illustrated in Figure 3.5. The readings are taken in inches.


Figure 3.5: Vernier Calliper. (Source: Painter Guitars, 2010)

The screw gage used is illustrated in Figure 3.6 on the next page. It gives readings to a thousandth of an inch.

Figure 3.6: Micrometer Screw Gage. (Source: eHow, 2010)The ultrasonic cleaner used is illustrated in Figure 3.7.


Figure 3.7: Ultrasonic Cleaner. (Source: MIT, 2010)The stereomicroscope used is illustrated in Figure 3.8. The images are captured on computer.

Figure 3.8: Stereomicroscope. (Source: Nikonians, 2010)The scanning electron microscope used is illustrated in Figure 3.9. The images are captured on computer. The units is expressed in micrometers.


Figure 3.9: Scanning Electron Microscope. (Source: Carleton College, 2010)

3.2 Procedures3.2.1 Phase One

The following procedures were used for Phase One of the experiment that was conducted on January 19, 2010:1.0 The surface of the tensile test bar was smoothened with sandpaper. All

large scratches were removed.2.0 The dimensions of the tensile test bar was measured and recorded

using a set of vernier callipers and a micrometer screw gage. The gage section width, thickness, length and the specimen’s length were noted.

3.0 Three roughness measurements were made across the gage section using the roughness tester. The direction in which the measurement was made was perpendicular to the loading direction. The measurements were recorded.

4.0 Three Rockwell Hardness tests were made. The load used was set to a 100 kilograms. The readings were recorded.

5.0 Two pencil marks were made on the gage section approximately 2.5 inches apart. The actual distance between the marks was then recorded.

6.0 The bar was then tensile tested, by placing the grip sections in the appropriate slots and attaching an extensometer on the gage section to measure the elongation.


7.0 The data was recorded computationally. The excel spreadsheet with the tensile test data was then obtained.

8.0 Repeat step 2. Dimensions after failure were recorded.9.0 Repeat step 3. Roughness tests around the region of failure were

undertaken and the readings were recorded.10.0 Repeat step 4. The hardness tests are carried out near the region of

failure to check for the phenomenon of strain hardening.3.2.2 Phase Two

The following procedures were used for Phase Two of the experiment which was conducted on January 26, 2010:1.0 The tensile test bar is cut using a band saw. The bar is cut at

approximately 1.5 inches from the failure surface.2.0 The 1.5 inch specimen was then washed in an ultrasonic cleaner for

about 5 minutes.3.0 The failure surface was then viewed under a stereomicroscope to

observe the failure surface macroscopically to look for details such as dimples, and necking. Digital photographs were taken.

4.0 The failure surface was then viewed under a scanning electron microscope. The objective was to observe whether the specimen underwent a ductile or brittle fracture. Digital photographs of the specimen under the SEM were taken.

4.0 Results and Discussion:


0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 0.022

-10000

0

10000

20000

30000

40000

50000

60000

Stress vs Strain

Yield @ 0.2% offset

Ultimate Tensile Strength

Stre

ss

σ(ps

i)

Strain (ε)

Proportional Limit

Figure 4.1: Stress vs Strain Plot.

Figure 4.1 is the plot for the Stress vs Strain for the tensile test of Aluminum 6061-T6511. From the plot we see that there is an elastic region, a plastic region and finally fracture. The plastic region shows that the alloy does show ductility as discussed in section 2.0 of this report. Comparing the values from Figure 4.1 to the published values in Table 2.1, we observe that the Ultimate Tensile Strength calculated experimentally is close to the published value. In fact, the published value for Al 6061-T651 is 45000psi, and that obtained from the experiment is approximately 49000psi, an error of about 8.89%. The Tensile Yield Strength published in Table 2.1 is 40000psi, the offset yield strength from the experiment was about 46000psi (from Figure 4.1). Also using a straight edge, and solving for the slope of the linear portion of the curve. We find the Young’s Modulus equal to about 10000ksi, which is in agreement with published values. We notice that the strain experienced at rupture was around the 0.02, very close to published values.

Overall, the stress strain plot obtained experimentally is agreeable to published stress strain plots of Aluminum 6061.


-0.0005 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.0040

5000

10000

15000

20000

25000

30000

35000

40000

f(x) = 10350661.5459156 x + 637.71933228307Stress vs Strain

Stre

ss σ

(psi)

Strain (ε)

Figure 4.2: Plot to find Young’s Modulus.

Figure 4.2 plots the linear portion of the stress vs strain curve for the tensile test of Aluminum 6061-T6511. The equation y = 10,350,661.55x + 637.72 is the equation for the trend line of the Elastic Region of the stress strain curve. It follows that the Young’s Modulus is approximately 10,350ksi. Again, this value is in accordance with published data as shown in Table 2.1, where the Young’s Modulus is gives as 10,000ksi.

Dimensional DataDimension Type Before Failure After FailureGage Section Width 0.4963 in 0.48 inGage Section Length 2.045 in 2.155 inGage Section Thickness 0.1283 in 0.1197 inSpecimen Length 8.22 in 8.468 in

Table 4.1: Dimensional Data of Al 6061-T6511 Specimen

Table 4.1, gives the record of the measurements made of the specimen, before and after the tensile test. As expected from theory, we see that the gage section width decreased after the test; the gage section length increased after the test; the gage section thickness


decreased after test and the overall specimen length increased after the test. This dimensional data shows that necking did occur in the specimen before fracture. Hence, the theoretical principles hold true for the experiment.

This dimensional data allows us to calculate the % elongation of the gage section and the %reduction in Area of the gage section. We use equation 2.4 to calculate %elongation of the gage section and find it equal to 5.38%. The data published in Table 2.1, is the percent elongation at break for a 1/16in specimen and is equal to 12%. Our specimen, is about 1/10th of an inch, and hence has a lower percent elongation at failure. We can also calculate the % reduction in area of the gage section using data from table 4.1 and making use of equation 2.5. The % reduction in area of the cross section is equal to 9.89%. This shows significant necking in the region around fracture. Again, theoretical principles is backed by experimental data.

Hardness DataSerial No. Before Failure After Failure

1 59.2 HRB 65 HRB2 61.1 HRB 65.5 HRB3 60.6 HRB 64.7 HRB

Table 4.2: Specimen Hardness Data

Table 4.2 gives the record of the hardness data calculated before and after the test. From published values of the specimen hardness in table 2.1, where hardness in the Rockwell B Scale was measured to be 60HRB, we see that our specimen for the test has similar hardness, around the 60HRB mark. After the tensile test, we see that there is significant hardening of about 4HRB around the fracture region. This, is in agreement with theory where we had discussed about the phenomenon of strain hardening. Strain hardening occurs in our test specimen.

Roughness DataSerial No. Before Failure After Failure

1 0.309 μm 1.146 μm2 0.316 μm 1.510 μm3 0.281 μm 1.509 μm

Table 4.3: Specimen Roughness Data

Table 4.3 shows the roughness data accumulated before and after failure. We notice that the surface of the specimen around the fracture region got significantly rougher after fracture. This can be explained due to dislocation of the metal matrices due to the applied


stress. This dislocation of the structure caused irregularities in the material causing the surface to become significantly rougher.

Figure 4.3: Stereomicroscope Image 1, Necking.

Figure 4.3, captures the fracture surface perpendicular to the loading direction. The image is taken by a stereomicroscope. Firstly, we observe the cross sectional area of the specimen decrease near the fracture region. This decrease in area clearly indicates necking in the specimen. Secondly, we notice a shear tip on the edge. This shear tip indicates that the fracture could be ductile. However, as we observe the shear tip is not very distinct, suggesting a fracture which could be a ductile to brittle transition.

Figure 4.4, is another image of the fracture surface, in higher magnification. We notice development of ridges right next to the fracture surface, suggesting that the fracture was pretty violent. There seems to be a shear tip on the edge of the fracture surface. However, the rest of the fracture seems is not as rough as a pure ductile fracture. There seems to be an element of brittle fracture. There is also a presence of dimples on and around the fracture surface. Macroscopically the ridges can indicate a presence of granular fracture. To confirm this fracture we would have to take a look at the surface at a microscopic level using a scanning electron microscope. This is illustrated in Figure 4.5.


Figure 4.4: Fracture surface under stereomicroscope.

Figure 4.5: Fracture surface under Scanning Electron Microscope -Al 6061-T651.


As mentioned on the previous page, we now take a look at the fracture surface microscopically in Figure 4.5 under an SEM. The image is magnified by a 1000 times and the charge of electrons used is 13kV. At first look at the image, we notice a few distinct features, such as the presence of a ridge which resemble a rift valley at the center of the image, we also notice an irregular distribution of voids and dimples. The surface seems to be very irregular in terms of whether it is purely ductile fracture or brittle fracture. The surface seems to exhibit features of both kinds of fractures. As discussed in section 2.0, and illustrated in figures 2.2 and 2.3, the fracture seems to be a transitional ductile to brittle fracture. The size of the central rift valley is approximately 25 micrometers, using the scale of 10 micrometers as a reference. Also the voids seem to be more profound around the central rift valley. This could be due to the presence of different types of atoms. Let us take a closer look at the rift valley under a higher magnification in the next image.

Figure 4.6: Fracture surface under SEM (2500x)

In figure 4.6, we take a look at the rift valley which we spotted in figure 4.6 under higher magnification (2500x). The so called valley indicates brittle fracture in that particular region. The valley is formed due to the propagation of voids along the grain boundaries or along the grains of the material. As discussed in section 2.0, this could be intergranular or transgranular fracture. Also the presence of distinct voids around the valley, indicates the presence of ductile fracture. This transition from ductile to brittle fracture and vice versa can be attributed to the presence of different atoms in the alloy, as Al 6061 has the


presence of elements like Nickel and Silicon. To confirm this theory we use the same image in Figure 4.6 and increase the charge of the electron to 20 kV, using the back scatter of electrons to produce images of particles.

Figure 4.7: Backscatter image of fracture surface using SEM.

Figure 4.7 illustrates the image captured using the backscatter of electrons to produce images of the particles present in the specimen. We also notice the presence of another rift valley indicating granular failure. The dark patches indicate the presence of dimples, which are seen in ductile fracture. As mentioned before, there is a presence of a large number of particles of different sizes and densities, leading to the transitional ductile to brittle fracture.

Overall, the achieved results were in accordance with the published values and theoretical principles. Although, the ultimate tensile strength was higher than that published. This could be attributed due to the preparation of the alloy. The slightest change in composition of the elements in the alloy can lead to significantly different results.


5.0 Conclusions and Recommendations:

This lab was performed to tensile test a specimen of Aluminum 6061-T6511. The experiment was performed over two phases. The first phase involved taking dimensional, hardness and roughness data of the test specimen, and then subjecting it to a tensile test. After the tensile test, the dimensions, hardness and roughness were measured. The second phase of the lab was to inspect the fracture surface under a stereomicroscope and a scanning electron microscope to discuss the kind of fracture that took place.

The results obtained were in agreement with the published values, such as the Young’s Modulus was found to be equal to 10,350ksi, the Ultimate Tensile Strength was found to be equal to 49000psi, the Yield at 0.2% offset was found to be equal to 46000psi, and the material was found to exhibit a ductile to brittle transitional fracture. The most significant result of the tensile test was that Aluminum 6061-T6511 exhibited both ductile and brittle properties in fracture allowing it to undergo plastic deformation even though there is a presence of brittle failure.

One of the most significant limitations encountered in the lab was the pressure of time on individual groups when it came to taking images of the fracture surface under the stereo and scanning electron microscopes. This could have been overcome if groups more time was available to perform the experiment.

The presence of scratches on the surface of the specimen could have also led to a discrepancy between published and experimental values. This limitation can be overcome by ensuring the removal of scratches by the use of sandpaper.


6.0 REFERENCES

Aerospace Specification Metals, Inc (2010). ASM Material Data Sheet. Retrieved January 28, 2010, from http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061T6

Argo-Tech Materials Laboratory. (2003). SEM Imaging. Retrieved January 28, 2010, from http://www.atclabs.com/Photos.htm

Boyer, H.E., Gall, T.L. (1985). Metals Handbook. American Society for Metals, Materials Park, OH.

Carleton College. (2010). Scanning Electron Microscopy(SEM). Retrieved January 30, 2010, from http://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html

Charles Sturt University. (n.d.). Retrieved January 28, 2010, from http://www.hsc.csu.edu.au/engineering_studies/lifting/3210/index.html

Direct Industry. (n.d.). Tensile/Compression testing machines – INSTRON. Retrieved January 29, 2010, from http://www.directindustry.com/prod/instron/tensile-compression-testing-machine-18463-41713.html

Far Asia Co., Ltd (China). (2003). Rockwell Hardness Tester. Retrieved January 29, 2010, from http://www.farasia.com.cn/products/prod0071.htm

Fischer, Traugott (2009). Materials Science for Engineering Students. Burlington, MA: Elsevier Inc.

Hendrix Group, Inc. (2004). The Hendrix Group – Ductile/Brittle Fracture. Retrieved January 29, 2010, from http://www.hghouston.com/x/50.html

Interactive Nano-Visualization in Science and Engineering Education. (n.d.). Retrieved January 27, 2010, from http://invsee.asu.edu/srinivas/stress-strain/ssgraph.jpg.

Holt, J.M. (1996). Structural Alloys Handbook. CINDAS/Purdue University, West Lafayette, IN.

Massachusetts Institute of Technology. (2010). CPRL. Retrieved January 29,2010, from http://web.mit.edu/cprl/www/other.shtml

M & I Instruments Inc. (2008). Surface Roughness Tester TR200. Retrieved January 29, 2010, from http://www.microinstruments.ca/Products/SurfaceRoughnessTesters/SurfaceRoughnessTestersTR200.htm


(n.d.). Tools. Retrieved January 29, 2010, from http://www.painterguitars.com/Dave/tools.htm


7.0 Appendix I: Sample Calculations

Dimensional DataDimension Type Before Failure After Failure

Gage Section Width 0.4963 in 0.48 inGage Section Length 2.045 in 2.155 inGage Section Thickness 0.1283 in 0.1197 inSpecimen Length 8.22 in 8.468 in

Cross sectional area used for computing stress = gage section width X gage section Thickness= (0.1283in)(0.4963in) = 0.0637in 2

Final Area = (0.48in)(0.1197in) = 0.0575in 2

% reduction in area = (0.0637 – 0.0575)/(0.0637) X 100 % = 9.89%

%elongation of gage section = (2.155 – 2.045)/(2.045) X 100 % = 5.38%


8.0 Appendix II: Raw Data

TESTNUM

POINTNUM TIME POSIT FORCE EXT

Stress σ(psi) Strain(ε)

525 1 7.3930.0420

521.424

9

-0.00054

8 336.3406-5.4829E-

06-5.49387E-

06

525 212.57

70.0459

527.426

750.00132

5430.56125

71.32501E-

051.32766E-

05

525 314.15

70.0512

2533.342

040.00223

9523.42293

62.23882E-

05 2.2433E-05

525 417.49

7 0.062340.512

520.00300

6635.98938

23.00638E-

053.01239E-

05

525 518.73

70.0663

2547.264

610.00370

1741.98760

33.70085E-

053.70825E-

05

525 619.80

10.0700

2554.233

110.00427

7851.38317

94.27656E-

054.28511E-

05

525 721.19

70.0746

562.615

50.00515

4982.97490

45.15379E-

05 5.1641E-05

525 822.25

90.0780

2569.670

560.00603

1 1093.72946.03102E-

056.04308E-

05

525 922.79

80.0798

7575.758

980.00661

61189.3089

56.61589E-

056.62912E-

05

525 1023.13

80.0811

2582.280

220.00771

2 1291.68327.71247E-

057.72789E-

05

525 1123.67

70.0829

7589.277

570.00860

81401.5316

38.60795E-

058.62517E-

05

525 1224.37

80.0851

2599.117

130.00986

91555.9989

79.86896E-

05 9.8887E-05

525 1324.91

70.0869

75107.25

43 0.01113 1683.7403 0.00011130.0001115

23

525 1425.25

7 0.0882113.38

60.01182

51779.9993

10.0001182

450.0001184

82

525 1525.61

80.0894

5120.18

130.01286

61886.6768

10.0001286

630.0001289

2

525 1626.13

80.0909

75129.37

160.01402

72030.9520

10.0001402

680.0001405

48

525 1726.68

10.0928

25140.17

790.01553

52200.5942

90.0001553

450.0001556

56

525 1827.03

70.0940

75146.29

510.01664

92296.6266

90.0001664

940.0001668

27

525 1927.37

7 0.0953154.12

930.01742

62419.6115

70.0001742

610.0001746

09

525 2027.91

70.0968

25163.13

20.01870

52560.9422

10.0001870

540.0001874

28

525 2128.25

90.0980

75170.11

490.01958

32670.5640

40.0001958

270.0001962

18

525 2228.43

70.0986

75176.43

420.02070

72769.7672

50.0002070

660.0002074

81

525 2328.79

80.0999

25183.79

220.02192

22885.2779

60.0002192

20.0002196

58

525 2428.97

80.1005

25189.86

620.02274

42980.6307

80.0002274

440.0002278

99525 25 29.31 0.1017 198.58 0.02381 3117.4316 0.0002381 0.0002386


7 75 04 4 9 35 12

525 2629.67

7 0.1027204.61

11 0.024813212.1049

30.0002480

960.0002485

92

525 2729.85

7 0.1033210.52

64 0.025653304.9662

80.0002565

030.0002570

16

525 2830.19

70.1045

5220.19

280.02711

23456.7155

30.0002711

240.0002716

66

525 2930.55

90.1057

75229.78

710.02860

23607.3324

80.0002860

180.0002865

9

525 3030.91

7 0.107238.38

580.02981

73742.3210

20.0002981

720.0002987

69

525 3131.07

70.1076

25245.23

890.03106

93849.9043

50.0003106

910.0003113

12

525 32 31.440.1085

5252.40

940.03205

63962.4706

10.0003205

60.0003212

01

525 3331.79

80.1097

75263.02

80.03362

84129.1682

10.0003362

770.0003369

5

525 3432.13

80.1110

25273.48

790.03521

84293.3743

40.0003521

770.0003528

81

525 3532.49

80.1122

5284.09

210.03671

6 4459.84510.0003671

640.0003678

98

525 3632.85

70.1134

75294.49

440.03827

94623.1452

80.0003827

890.0003835

55

525 3733.19

7 0.1144303.59

81 0.039544766.0609

7 0.00039540.0003961

91

525 3833.55

90.1156

25 315.140.04151

44947.2534

60.0004151

380.0004159

68

525 3933.91

70.1168

75327.04

280.04329

65134.1091

90.0004329

570.0004338

23

525 4034.25

9 0.1181338.51

260.04498

65314.1691

20.0004498

620.0004507

62

525 4134.61

70.1193

25350.92

020.04697

85508.9512

80.0004697

830.0004707

22

525 4234.97

80.1205

5362.85

170.04875

15696.2592

60.0004875

110.0004884

86

525 4335.31

70.1214

75371.24

850.04995

7 5828.0770.0004995

730.0005005

72

525 4435.49

7 0.1221380.75

620.05134

6 5977.33430.0005134

630.0005144

9

525 4535.85

70.1233

5392.87

530.05327

46167.5867

30.0005327

430.0005338

09

525 4636.19

70.1245

75406.01

87 0.055126373.9195

70.0005512

020.0005523

05

525 4736.55

7 0.1258418.44

070.05711

26568.9283

30.0005711

230.0005722

65

525 4836.91

70.1267

25426.82

310.05829

16700.5189

80.0005829

110.0005840

77

525 4937.07

70.1273

5436.76

360.05979

96856.5711

20.0005979

890.0005991

85

525 5037.43

70.1285

75449.32

990.06178

27053.8445

10.0006178

180.0006190

53

525 5137.79

90.1298

25463.07

930.06375

67269.6900

20.0006375

560.0006388

31

525 52 37.960.1304

25471.62

03 0.064877403.7723

80.0006487

040.0006500

02


525 5338.31

90.1316

5486.45

170.06712

87636.6047

30.0006712

750.0006726

18

525 5438.67

70.1328

75501.45

620.06952

27872.1544

40.0006952

170.0006966

07

525 5539.01

70.1338

25509.33

360.07073

77995.8181

80.0007073

70.0007087

85

525 5639.19

70.1344

25519.80

790.07228

18160.2499

60.0007228

140.0007242

59

525 5739.55

70.1356

75534.27

86 0.074528387.4195

50.0007452

010.0007466

92

525 5839.91

7 0.1369547.26

330.07660

48591.2606

90.0007660

360.0007675

68

525 5940.07

80.1375

25555.81

880.07789

28725.5706

10.0007789

20.0007804

78

525 6040.43

70.1387

5572.59

780.08042

38988.9770

70.0008042

330.0008058

41

525 6140.80

20.1396

75581.18

220.08189

49123.7392

50.0008189

450.0008205

83

525 6240.95

9 0.1403592.29

130.08343

9 9298.13690.0008343

880.0008360

57

525 6341.31

70.1415

25605.39

140.08547

79503.7899

30.0008547

660.0008564

75

525 6441.49

70.1421

5613.36

980.08669

29629.0394

40.0008669

190.0008686

53

525 6541.67

70.1427

5620.95

860.08778

89748.1720

10.0008778

850.0008796

41

525 6641.85

70.1433

75629.29

760.08918

79879.0833

20.0008918

660.0008936

5

525 6742.01

80.1439

75637.70

880.09033

810011.127

20.0009033

80.0009051

87

525 6842.19

7 0.1446645.68

720.09175

410136.376

70.0009175

430.0009193

79

525 6942.55

70.1455

25658.41

210.09344

510336.139

90.0009344

490.0009363

18

525 7042.73

70.1461

5666.67

910.09481

610465.919

60.0009481

560.0009500

52

525 7142.89

70.1467

75674.44

1 0.0959310587.770

50.0009593

040.0009612

23

525 7243.07

70.1473

75683.03

970.09722

810722.758

80.0009722

80.0009742

24

525 7343.25

8 0.148691.75

38 0.09848 10859.5580.0009847

990.0009867

69

525 7443.43

7 0.1486699.87

650.09974

110987.071

70.0009974

10.0009994

04

525 7543.64

70.1492

25708.57

620.10115

711123.644

80.0010115

730.0010135

96

525 7643.77

70.1498

5716.98

740.10259

211255.688

60.0010259

20.0010279

72

525 7743.96

20.1504

75725.16

770.10355

111384.108

70.0010355

150.0010375

86

525 7844.13

80.1510

75733.50

670.10501

411515.019

10.0010501

350.0010522

36

525 7944.47

8 0.1517741.29

750.10611

911637.324

10.0010611

930.0010633

15525 80 44.65 0.1526 752.89 0.10790 11819.421 0.0010790 0.0010811


7 25 72 1 6 11 69

525 8144.83

80.1532

25760.86

1 0.1090811944.443

1 0.00109080.0010929

81

525 8245.01

80.1538

5769.22

890.11046

912075.807

60.0011046

890.0011068

98

525 8345.19

70.1544

75777.10

63 0.1116212199.471

30.0011162

030.0011184

35

525 8445.37

7 0.1551784.86

820.11284

512321.322

20.0011284

480.0011307

05

525 8545.53

9 0.1557792.68

790.11406

912444.079

50.0011406

930.0011429

74

525 8645.71

70.1563

25799.80

050.11517

512555.738

40.0011517

50.0011540

53

525 8745.89

70.1569

25807.86

550.11633

512682.346

60.0011633

550.0011656

82

525 8846.25

70.1578

5818.39

75 0.1178812847.683

80.0011787

980.0011811

56

525 8946.43

70.1584

75825.66

880.11903

1 12961.8340.0011903

120.0011926

92

525 9046.59

70.1590

75833.11

330.12034

713078.702

40.0012034

710.0012058

78

525 9146.77

7 0.1597840.32

70.12137

1 13191.9470.0012137

050.0012161

33

525 92 46.960.1603

25847.80

040.12271

413309.268

70.0012271

380.0012295

92

525 9347.13

80.1609

25855.31

710.12369

213427.269

70.0012369

160.0012393

89

525 9447.31

90.1615

5862.37

210.12469

713538.023

10.0012469

670.0012494

61

525 9547.49

70.1621

75869.93

20.12588

513656.703

40.0012588

470.0012613

64

525 9647.65

7 0.1628876.81

380.12701

813764.737

50.0012701

780.0012727

18

525 9748.01

70.1637

25888.37

020.12876

313946.156

60.0012876

310.0012902

07

525 9848.19

70.1643

25895.75

7 0.1297514062.118

7 0.00129750.0013000

95

525 9948.37

70.1649

5903.57

660.13089

2 14184.8760.0013089

230.0013115

41

525 10048.53

70.1655

5910.93

450.13206

214300.384

80.0013206

190.0013232

61

525 10148.71

70.1661

5918.36

460.13330

514417.027

10.0013330

470.0013357

13

525 10248.89

90.1667

75926.25

640.13450

214540.916

90.0013450

180.0013477

08

525 103 49.08 0.1674934.06

160.13577

214663.447

20.0013577

20.0013604

35

525 10449.25

7 0.168942.08

320.13684

114789.375

10.0013684

110.0013711

48

525 10549.43

70.1686

25949.94

610.13795

614912.811

70.0013795

590.0013823

18

525 10649.61

70.1692

25958.31

40.13926

315044.175

30.0013926

270.0013954

12

525 10749.95

70.1701

25969.40

850.14084

415218.343

90.0014084

350.0014112

52


Hardness Data

Serial No. Before FailureAfter Failure

1 59.2 HRB 65 HRB2 61.1 HRB 65.5 HRB3 60.6 HRB 64.7 HRB

Roughness Data

Serial No. Before FailureAfter Failure

1 0.309 μm 1.146 μm2 0.316 μm 1.510 μm3 0.281 μm 1.509 μm

Note : the raw data shown above is one part of the entire data collected in the lab. More raw data is contained in the CD attached with this lab report.

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