Leeds Beckett UniversityMSc Materials Technology

Metals Lab Report

Table of Contents

Abstract................................................................................................................................ 1

Introduction........................................................................................................................ 1

Methodology....................................................................................................................... 2Apparatus...................................................................................................................................... 2Method............................................................................................................................................ 3

Results................................................................................................................................... 3(1) Graphics about load P (kN), against extension, e (mm), for each of the samples being measured by the Denison extension gauge..........................................3(2) Graphics Stress-Strain converted from the part (1)................................................4(3) Yield stress and 0.2% proof stress.................................................................................6(4) The elastic module using the initial linear-elastic part of the stress-strain graph produced in (2) above and the Young’s modulus using the Extensometer readings over the range 30kN to 83kN................................................................................7(5) The maximum tensile strength.......................................................................................9(6) The stress at failure............................................................................................................ 9(7) The percentage elongation at failure.........................................................................10(8) The true stress................................................................................................................... 10(9) Table of results...................................................................................................................10

Discussion......................................................................................................................... 11

Conclusion......................................................................................................................... 16

Bibliography..................................................................................................................... 17

Abstract

The aims of this Laboratory Report are to determine the characteristics (Yield Stress, Modulus of Elasticity, Maximum Tensile Strength, the Stress at failure, the percentage elongation at failure and the true stress) of two kind of steel according to its carbon percentage. The results showed that how much carbon percentage content more strength and less ductile the steel will get. Some errors may be occurred because the Yield Strength had a considerable increased comparing to normal values from relevant data. All the characteristics were defined, the majority found by graphs.

Introduction

Due its characteristics steel started to be used 4000 years ago on the beginning of the Iron Age. By the 17 th century, iron’s property were well understood but Europe needed a more versatile structural metal, by the 19 th

century people was trying to find a manner to problems evolving iron’s brittleness. In 1856 when Henry Bessemer discovered how to reduce the carbon content in iron using oxygen, the modern steel industry was born. Steels can be divided by its amount of carbon, low-carbon steels contain less than 0.25 wt% carbon, they are applied in a variety of situations such as: structural shapes (I-beam), pipelines, buildings, bridges, and cans. Medium carbon-steels contain carbon concentration between 0.25 and 0.60 wt% and are used to design that need more resistance than Low-carbon steel. This Lab report is about the tensile test for both kind of steel cited before.This study attempts to the steel properties and behavior in function of carbon content to compare to relevant data and using the knowledge got from the lectures presented about steel to explain this behavior.

MethodologyApparatus

1. 500 kN Denison Testing Machine

2. Extensometer and Denison extension gauge (measures cross head movement)

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3. Grade 250 plain round mild steel bar, 20mm diameter.Characteristic strength = 250 N/mm2 Length = 100mmConforms to BS 4449.

4. Grade 460 deformed high yield steel.Reinforced bar, T16, 16mm diameter.Characteristic strength = 460 N/mm2

Length = 80mmConforms to BS 4449.

Figure 1 – The 1500 kN Denison Testing Machine, the extensometer and the specimen.

Method

Each of the bars in turn (first the Grade 250, second the Grade 460) is placed in the jaws of the testing machine.The 50mm extensometer is attached to the bar and zeroed by the Laboratory Technician.Each specimen is deformed up to fracture with a gradually increasing tensile load that is applied uniaxially. According to the increment of the force applied,

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extension readings from the extensometer and the Denison extension gauge are noted.At the yield point, the extensometer is removed to prevent damage to it and readings continue on the Denison extension gauge.The load at failure and the manner of failure are noted.

ResultsAll the graphics bellow were made using the software Excel 2011 by Microsoft Corporation.

(1) Graphics about load P (kN), against extension, e (mm), for each of the samples being measured by the Denison extension gauge.

0 5 10 15 20 25 30 350

20

40

60

80

100

120

140

160

180

Load P (kN) - Extension e (mm)

e (mm)

P (

kN

)

Figure 2 – Graphic Load (P) x extension (e) to low carbon steel specimen.

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0 5 10 15 20 250

20

40

60

80

100

120

140

Load P (kN) - Extension e (mm)

e (mm)

P (

kN

)

Figure 3 – Graphic Load (P) x extension (e) to medium carbon steel specimen.

(2) Graphics Stress-Strain converted from the part (1).

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.350

100

200

300

400

500

600

Stress - Strain

ε

(N

/mm

²)

σ

Figure 4 – Graphic Stress x Strain to low carbon steel specimen.

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0.000 0.050 0.100 0.150 0.200 0.2500

100

200

300

400

500

600

700

Stress - Strain

ε

(N

/mm

²)

σ

Figure 5 – Graphic Stress x Strain to medium carbon steel specimen.

-0.05 0.15 0.350

100

200

300

400

500

600

700

Stress-Strain

0.4wt% carbon0.1wt% carbon

Strain

Stre

ss (

N/m

m2

)

Figure 6 – Graphic Stress x Strain to compare both specimens.

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(3) Yield stress and 0.2% proof stress.

0.000 0.001 0.002 0.003 0.004 0.0050

50

100

150

200

250

300

350

400

Stress - Strain

ε

(N

/mm

²)

σ

Figure 7 – Graphic Stress x Strain to low carbon steel specimen and straight line parallel to the initial part of the stress-strain curve, from the value of 0.2% strain.

σy = 370.78N/mm2

Considering the moment when the extensometer was removed that is the Yield point, the Young’s modulus is:σy = 370.78 N/mm2

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0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.0080

50

100

150

200

250

300

350

400

450

500

550

600

Stress - Strain

ε

(N

/mm

²)

σ

Figure 8 – Graphic Stress x Strain to medium carbon steel specimen and straight line parallel to the initial part of the stress-strain curve, from the value of 0.2% strain.

σy = 549.40 N/mm2

Considering the moment when the extensometer was removed that is the Yield point, the Young’s modulus is:σy = 545.78 N/mm2

(4) The elastic module using the initial linear-elastic part of the stress-strain graph produced in (2) above and the Young’s modulus using the Extensometer readings over the range 30kN to 83kN.For the low carbon steel we have:

E=σ2−σ 1ε2−ε1

E=0.3179504−00.001710−0

=185.9kN /mm ²

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According to the extensometer over the range 30kN to 83kN:

0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.0040

50

100

150

200

250

300

350

400

Stress - Strain (extensometer readings)

ε

(N

/mm

²)

σ

Figure 9 – Graphic Stress x Strain to low carbon steel specimen according to extensometer readings.

E=σ2−σ 1ε2−ε1

E= 285.17−64.610.001197−0.000169

=214.55 kN /mm ²

For the medium carbon steel we have:

E=σ2−σ 1ε2−ε1

E=0.5040323−00.0036−0

=140.01 kN /mm ²

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According to the extensometer over the range 30kN to 83kN:

0 0.001 0.002 0.003 0.004 0.005 0.0060

100

200

300

400

500

600

Stress - Strain (extensometer readings)

ε

(N

/mm

²)

σ

Figure 10 – Graphic Stress x Strain to medium carbon steel specimen according to extensometer readings.

E=σ2−σ 1ε2−ε1

E= 0.448085−0.1512100.00197125−0.00060375

=217.09 kN /mm ²

(5) The maximum tensile strength.

For the low carbon steel:Maximum tensile strength is 540.7384N/mm2 when the load is 169.9kN and the deflection is 16.086mm.

For the medium carbon steel:Maximum tensile strength is 638.6089N/mm2 when the load is 126.7kN and the deflection is 11.89mm.

(6) The stress at failure.

For the low carbon steel:The stress at failure is equal to 410.57N/mm2

For the medium carbon steel:The stress at failure is equal to 519.15N/mm2

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(7) The percentage elongation at failure.

For the low carbon steel:The percentage elongation at failure is:

elongation%=32.922100

∗100=32.92%

For the medium carbon steel:The percentage elongation at failure is:

elongation%=19.2580

∗100=24.06%

(8) The true stress.

For the low carbon steel:The true stress is given bellow:The initial area is 314.2mm2 for the specimen with 20mm diameter; this initial area was reduced to 132.73mm2 because its diameter reduced to 13mm during the test.The load at failure is equal to 129kN; to find the true stress is necessary to divide the load at failure by the final area.

σ T=129000132.73

=971.90 N /mm ²

For the medium carbon steel:The true stress is given bellow:The initial area is 198.4mm2 for the specimen with 16mm diameter; this initial area was reduced to 132.73mm2 because its diameter reduced to 13mm during the test.The load at failure is equal to 103kN; to find the true stress is necessary to divide the load at failure by the final area.

σ T=103000132.73

=776.01N /mm ²

(9) Table of results.

Steel σy (N/mm2)Extensometer

σy (N/mm2)0.2% proof

E (graphic) (kN/mm2)

E (extensometer) (kN/mm2)

0.1wt% carbon 370.78 370.78 185.90 214.55

0.4wt% carbon 545.36 549.40 140.01 217.09

Table 1 – Table of results 1: Yield stress, E (graph) and E (extensometer reading).

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Steel

Maximum Tensile Strength (N/mm2)

Elongation %Stress at failure

(N/mm2)

True stress (kN/mm2)

0.1wt% carbon 540.74 32.92 410.57 971.900.4wt% carbon 638.61 24.06 519.15 776.01Table 2 – Table of results 2: Maximum tensile strength, elongation, stress at failure and true stress.

DiscussionInitially the definitions of: stress, strain and Young’s Modulus. Stress is the force acting per unit area, Strain is defined as the change in dimension per unit length, and the Young’s Modulus or Modulus of Elasticity is the slope of a tensile stress-strain curve in the linear part (Askeland, Fulay and Wright, 2012).The Modulus of Elasticity is an important characteristic because it expresses how easy a material can be deformed elastically (Ahmed, 2014b).The specimens tested are known as Ferrous Metals because its composition (90% is iron and steel), specifically a plain carbon steel because it contents iron and up to 1% carbon (Ahmed, 2014).Callister states that low carbon steels normally have Yield Strength of 275 MPa, Tensile Strengths between 415 and 550 MPa, and Ductility of 25%EL (Callister, 2007).Using 0.4wt% carbon steel with AISI Number 1040 as example. The properties for Tensile Strength, Yield Strength, and Ductility are given bellow (Callister, 2007).Tensile Strength (MPa): 605-780Yeld Strength (MPa): 430-585Ductility (EL%): 33-19The Young’s modulus or Modulus of Elasticity for steel is 207 GPa or 207 kN/mm2 (Callister, 2007). The figure bellow shows the properties according to the lecture 3, the numbers are similar to the numbers above.

Figure 11 – Properties of metals (Ahmed, 2014a).

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First, comparing the properties showed above for low carbon steels with the 0.1wt% specimen tested. The Yield Strength showed at Table 1 of 370.78 MPa is almost 35% higher than the value of 275 MPa, the maximum tensile strength (540.74 MPa) is inside the limit between 415 and 550 MPa and the ductility for the specimen is 32.92% whilst the normal ductility according to Callister is 25%.The results for the first specimen (0.1% carbon content) showed that the Yield Stress is higher than normal and the ductility is also elevated, the possibility to admit more carbon content on this specimen is ignored because its ductility is higher than normal.Analyzing the second specimen tested with the expected values, according to figure 10, the ductility and the Yield Strength are elevated which represents the same situation as first specimen.The Modulus of Elasticity for both specimens showed almost the same value according to extensometer reading as expected whereas using Gauge readings the results were not reliable. The carbon content has no effect on the stiffness of the steel as shown the figure bellow.

Figure 12 – Graphs showing the effects of increasing carbon content (Ahmed, 2014a).

The figure above also demonstrates the effects according to the ductility and the tensile strength, looking at figure 5 this effect showed above occurred in

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the same way on test. The tensile strength was higher for the 0.4% carbon content specimen whilst its ductility was decreased comparing to 0.1% carbon content specimen.The fracture behavior in metals occurs according the movement of atoms, once the yield point is achieved the plastic deformation starts by dislocation slip. There is a movement order of difficult to follow starting from the easiest difficult until the more difficult to move, it explains the behavior of the stress-strain curve in the plastic part, it is a rising curve initially but decreasing gradually until the fracture point. A characteristic of fracture is the appearance of the “neck” due the deformations in the site of final fracture, the neck appears due the coalesce of cracks. This ductile characteristic is important because this behavior can gives warning that the material is going to failure, otherwise brittle materials failure suddenly and cause accidents fast (Sturges, n.d.a).The pictures bellow show the neck during the test.

Figure 13 – Neck during test.

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Figure 14 – 0.1% carbon content specimen after failure.

The figure bellow show the 0.4% carbon content specimen after failure. The ductile is considerate a moderately ductile fracture due the reduction in area, the first specimen was 20mm diameter before failure and 13mm diameter after failure, in other words, the area reduced 58%. For the second specimen was 16mm diameter before failure and 13mm diameter after failure, the area has reduced 34%. This shows the ductility in function of area, the results showed the ductility in function of elongation. The fracture has cup and cone appearance due the shear stress with an angle of 45 degrees between the atoms in the end of fracture.

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Figure 15 – 0.4% carbon content specimen after failure.

The figure above showed as expected less deformation, it is more difficult to see the neck. It is due the higher percentage of carbon relation the other specimen.The two kind of bars tested are normally used for the reinforcement of concrete specially to increase the tensile resistance of concrete and the ductility. The bars are chose according to its Yield Strength to attend the project, the Yield Strength is important because from this point the material start to get plastic deformation, the 0.2% proof is used to determine the Yield Strength. When it comes to steel in concrete is important to mention the importance of ductility because concrete is a brittle material and brittle fracture are general catastrophic.It is important to cite the importance of the structure of iron’s atoms, the transformation to increase steels resistance is possible due the characteristics of the arrangement of atoms and its behavior when temperature changes, changing the phase and returning to make possible the structure changes from ferrite to austenite. As austenite will hold up to 1.7% carbon and our steel is 0.8% carbon, to make it possible a lamellar structure called pearlite is necessary to occur, heat energy can make the carbon excess diffuse through pearlite regions until the carbon amount required (Sturges, n.d.b).Some problems may be happened during of before the test because the Yield Strength for both specimens showed different results from expected. Should be analyzed the machine and specimens, the surface of the specimens or its composition can be altered during the process of fabrication.It is important to state the reliability of the extensometer comparing to the Dennison Extension Gauge that gave different results to Young’s Modulus than expected for both specimens.

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Conclusion

The objectives of laboratory were achieved; all properties to be studied were analyzed and compared with other data. The results showed that the unique problem occurred with the Yield Strength. Moreover, more experiment with each kind of steel should be made to analyze the problem and find a solution to explain the reasons for the Yield Strength to be higher than normal.

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Bibliography

[1] Ahmed, A. (2014a). Lecture 3 - Ferrous & Non-Ferrous Metals.

[2] Ahmed, A. (2014b). First lecture.

[3] Askeland, D., Fulay, P. and Wright, W. (2012). The Science and

Engineering of Materials. 6th ed. Stamford: Cengage Learning.

[4] Callister, W. (2007). Materials science and engineering. New York: John

Wiley & Sons.

[5] Sturges, J. (n.d.a). Dislocations and Strengthening of Materials.

[6] Sturges, J. (n.d.b). Microstructure and Phase Transformations in Metal & Alloys.

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