University of JohannesburgScience of Materials 3A

Charpy-Impact TestPractical Report

K.Y. Toni 201102831GROUP F10-03-2013

Abstract

This report seeks to show and explain the results of an experiment that was conducted. In the experiment two different types of metal’s specimens were tested at different temperatures. The aim was to determine their brittle to ductile transition temperatures. The two types of materials being tested were Aluminium and Mild Steel. The test used to conduct the experiment was the Charpy Impact test according to ASTM 23. The Charpy test calculates a materials ability to absorb energy. This ability is linked to ductility. The specimens as expected proved to be more ductile at high temperatures and less ductile at low temperatures.

Table of ContentsAbstract.................................................................................................................................................. i

Table of Figures..................................................................................................................................... iii

List of Tables......................................................................................................................................... iv

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

1.1 Aim..............................................................................................................................................1

1.2. Historical Background.................................................................................................................1

2.0 Literature Review.......................................................................................................................2

3.0 Experimental Set-up..................................................................................................................5

3.1 Apparatus....................................................................................................................................5

3.2 Method........................................................................................................................................5

4.0 Discussion and Interpretation of Results...................................................................................8

4.1 Results.........................................................................................................................................8

4.1.1 Steel (EN 3)...........................................................................................................................8

4.1.2 Aluminium............................................................................................................................8

4.2 Interpretation of Results..............................................................................................................9

4.2.1 Mild Steel..............................................................................................................................9

4.2.2 Aluminium..........................................................................................................................10

4.2.3 Aluminium vs. Mild Steel....................................................................................................10

5.0 Conclusion...............................................................................................................................12

References...........................................................................................................................................13

Table of Figures Figure 1 Brittle-Ductile Transition Behaivour...............................................2Figure 2 U and V notched Specimen Dimensions........................................3Figure 3 Types of Fractures..........................................................................4Figure 4 Charpy Impact Tester.....................................................................5Figure 5 Experimental Set Up Apparatus.....................................................6Figure 6 Notch Position and Dimensions......................................................6Figure 7 Initial and impact position.............................................................7Figure 8 Mild Steel Brittle-Ductile Transition................................................8Figure 9 Aluminium Brittle-Ductile Transition..............................................9Figure 10 Mild Steel Specimens.................................................................10Figure 11 Aluminium Specimens...............................................................11

List of Tables

Table 1 Notch Dimensions...........................................................................3Table 2 EN 3 Impact Test Results.................................................................8Table 3 Aluminium Test Results...................................................................8

1.0 Introduction

1.1 Aim

The aim of the test is determine or measure toughness of materials, the impact loading and multi axial stress states through energy differences. The objective is to indicate or show how the material or specimen being tested will respond to a suddenly applied shock or stress. All this is achieved by measuring the energy absorbed in breaking the piece. This is a destructive mechanical test that uses a pendulum hammer [1].

1.2. Historical Background

The development of material testing was driven by the rapid expansion of the railway network between 1830 and 1900. This was due to a number of failures that occurred in the rails and axles of the rail cars. All the failures were sudden they occurred unexpectedly. All this led to the development of the different tests and impact testing was one of these tests [2].

The first scientist to take note that as the strain rate increased, so did the temperature at which brittle fracture occurred, was Consideré in 1904. In 1905 based on the ideas of S.B. Brussell, a French Scientist Georges Augustine Albert Charpy professor in General Chemistry at Ecole Polytechnic in Paris developed his impact test. The purpose of the Charpy test was to evaluate the behaviour of materials under dynamic conditions. By 1933 the American Society for Testing and Materials (ASTM) had developed a standard called E23 for the Charpy test [2].

During World War II the Americans manufactured 3000 Liberty Ships, of which 1200 of the ships failed in some way or another. Some of the ships broke into two and some failures occurred whilst the ships were docked. There were three things in common in all the failures: they were sudden, brittle and occurred at stresses well below the yield stress of the material. It was then discovered that the Charpy Impact test could actually help detect such failures. This is when the full benefit of the test was realised[2].

2.0 Literature Review

Impact testing involves the sudden and dynamic application of the load. The impact Charpy impact test measures the resistance of a material to rapid suddenly applied loads [3]. Toughness is the materials capacity to resist fracture when subjected to impact and the impact tester works by measuring the energy absorbed by a specimen of a material, this energy tells us how tough a material is. It is important to note that if a specimen absorbs low energy it is brittle and therefore not tough but if it absorbs high energy it is ductile and therefore tough.

As this test is used to determine the materials resistance to shock, the resistance is expected to be low at low temperatures and higher at higher temperatures. When the Charpy test is conducted on the same type of

material but at different temperatures one is interested at in finding out the materials transition temperature. This is the temperature were the material shifts in a sudden manner from ductile to brittle as shown in Figure 1 below

Figure 1 Brittle-Ductile Transition Behaivour

There are two different types of the Charpy impact test. First type uses a V-notch specimen, where the specimen is notched with a 2mm deep 45˚ angle with the notches bottom radius at 0.25mm as shown in Figure 6. The second type is uses a U-notch specimen, where the specimen is notched as shown in Figure 2 and Table 1 below.

Figure 2 U and V notched Specimen Dimensions

A highly ductile specimen facture is expected to neck down to a point as shown in Figure 3(a) below. A brittle fracture is expected to break without any plastic deformation as shown in Figure 3(c) below. A moderate ductile fracture is expected to produce a rough plastic deformation as shown in Figure 3(b) below.

Ductile materials are materials that have the property of Ductility. Ductility is a measure of a materials ability to be stretched or drawn [4]. In relation to toughness, tough materials are materials that can withstand an impact without breaking and ductile materials are materials that will bend or distort without breaking when impacted since toughness is the energy required to fracture a given volume of material [4]. Brittleness on the other hand is the tendency of a material to fracture or fail upon the

Table 1 Notch Dimensions

Figure 3 Types of Fractures

application of a relatively small amount of force, impact, or shock which is the opposite of toughness [1].

3.0 Experimental Set-up

3.1 Apparatus

Charpy-impact tester, shown below in Figure 4 Glass beaker Ellen keys Hot water Ethanol Liquid nitrogen

Tongs 5×Aluminium specimens 5×Mild steel specimens Thermocouple

3.2 Method

i. The aluminium and mild steel specimens were cooled to temperatures of -50℃ by being placed inside a glass beaker as shown in Figure 5 below with ethanol and having liquid nitrogen poured inside the beaker.

ii. After the temperature of the specimens was tested with the thermocouple shown in Figure 5 and found to be ± -50℃ the specimen would be immediately moved by a tong to the impact tester where it would be tested.

iii. The machine was then set up to be ready for the test by zeroing its scale and moving the pendulum hummer to its starting position.

iv. Each specimen was placed on the testing machine’s anvil and counter bearings using tongs due to the low temperatures. It was then centred, with the notch facing away from the hammer as shown in Figure 6 below.

v. The pendulum was then released and it hit the specimen on the un-notched side through the counter bearings with one blow as shown in Figure 7 below.

Figure 5 Experimental Set Up Apparatus

Figure 6 Notch Position and Dimensions

vi. The energy value displayed on the scale was recorded.vii.

The pendulum was then set up to its initial position again for the next test.viii. This procedure was repeated for all the specimens at their different

temperatures (-50℃ to 50℃).ix. To raise the temperature of the other specimens to the higher

temperatures boiling hot water was used or poured to the ethanol, liquid nitrogen mixture.

x. Before each test enough time was allowed for the whole specimen to be at the desired temperature.

xi. The fractured surface would then be examined after each test.xii. All in all 10 tests were conducted, 5 for the aluminium specimens

and 5 for the mild steel specimens.

Figure 7 Initial and impact position

4.0 Discussion and Interpretation of Results

4.1 Results

4.1.1 Steel (EN 3)

Table 2 EN 3 Impact Test Results

Temperature(℃)

-51.5 -23.9 -1.9 27.8 52.1

Energy (kgf.m)

1 1 1.2 2.6 15

4.1.2 Aluminium

Table 3 Aluminium Test Results

Temperature(℃)

-48.9 -24.4 -0.5 27.4 49.5

Energy(kgf.m)

3 2.8 2.8 2.8 3

-60 -40 -20 0 20 40 600

2

4

6

8

10

12

14

16

1 1 1.2

2.6

15

Mild Steel Brittle to Ductile Transition

Temperature( )℃

Char

py E

nerg

y (k

gf.m

)

Figure 8 Mild Steel Brittle-Ductile Transition

4.2 Interpretation of Results

4.2.1 Mild Steel

Looking at the energy results in Table 2 above, the transition temperature in the mild steel can be seen. At low temperatures mild steel absorbed very little impact energy which meant it was brittle. With the five tests that were conducted the change in the amount of energy absorbed was not significant until between 27.8℃ and 52.1℃. This behaviour of mild steel was somehow puzzling at first between -51.5℃ and -1.9℃ due to the fact that there was no significant change in energy absorption as shown in Error: Reference source not found below. This means at low temperatures steel has more or less the same toughness.

The results in Table 2 also show that if we were to continue increasing the temperature the mild steel would have become more ductile, one can easily prove this through extrapolation. When looking at Figure 8 the brittle to ductile transition is visible which means one of our expectations was met.

-60 -40 -20 0 20 40 602.65

2.7

2.75

2.8

2.85

2.9

2.95

3

3.05

3

2.8 2.8 2.8

3

Aluminium Brittle-Ductile Transition

Temperature( )℃

Char

y En

ergy

(kgf

/m)

Figure 9 Aluminium Brittle-Ductile Transition

Figure 10 Mild Steel Specimens

4.2.2 Aluminium

The aluminium specimen shown in FIGURE below showed almost no brittle

to ductile transition during its five tests as shown in Table 3 above. In fact there was very little change in its ability to absorb impact energy throughout the tests and this behaviour was not expected. All materials are expected to behave differently under different temperatures. This lack of change shows that aluminium’s toughness remains the same regardless of the temperature.

Due to lack of time and experimental constraints it was impossible test the aluminium at temperatures way higher than 50℃ and lower than -50℃, but from the results in Table 3 one can see that extrapolation would yield the same results. According to the results in Table 3 aluminium will fail at the same impact load regardless of the temperature. When looking at Figure 9 our expectation was not met as the experiment failed to yield results that can help determine aluminium’s brittle to ductile transition temperature.

4.2.3 Aluminium vs. Mild Steel

Mild steel proved to be tougher than aluminium at high temperatures but more brittle at lower temperatures. This leads to a recommendation that one should use mild steel at high temperatures over aluminium if that

person is looking for a tougher material. At temperatures below 0℃ this is the other way, a person should use aluminium over mild steel if toughness is of the main design factors as from the experiment conducted it was proven that aluminium is tougher than steel at such temperatures.

Figure 11 Aluminium Specimens

5.0 Conclusion

The aim of the experiment was met and the results were satisfactory even though they were partly in line with what was expected. All materials are expected to show a significant change in ductility at different temperatures but during this experiment aluminium proved this theory incorrect. The aluminium absorbed the same amount of energy throughout the experiment even though it was tested at different temperatures. With mild steel the experiment went as expected as there was a change in the materials ability to absorb energy. Mild steel proved to be more ductile at high temperatures and thus absorbed more energy whilst it was brittle at low temperature and thus absorbed less energy. The other finding was that mild steels ability to absorb energy only changes when it is above 0℃. This means it remains brittle at temperatures below 0℃ and thus absorbs the same amount of energy.

References

[1] BUSINESSDICTIONARY.COM, “BUSINESSDICTIONARY.COM,” [Online]. Available: http://www.businessdictionaty.com/definition/Charpy-impact-test.html..

[2] L. Tóth, H. Rossmanith, D. François and A. Pineau, “Historical Background and the Development of the Charpy Test,” Elsevier Science Ltd.: 3 20., Oxford, 2002.‐

[3] [Online]. Available: http://www.youtube.com/watch?vtpGhqQvftAo.

[4] K. G. Budinski and M. K. Budinski, Engineering Materials: Proprties and Selection, New Jersey: Pearson, 2010, p. 64.

[5] T. F. Kilduff and J. A. Jacobs, in Engineering Materials Technology, New York, Pearson Prentice Hall, 2005.

[6] [Online]. Available: http://www.wmtr.com/content/charpy.html.