Objectives

The objectives of this experiment are:

1. To develop an understanding of fracture toughness.

2. To investigate the notched bar impact work and strength of various engineering

materials.

3. To investigate the influence of the notch shape on the notched bar impact work.

4. To test ability of different types of spec impacts using two material for impacts tests;

mild steel and carbon steel.

Theory

1. General

Toughness is the capacity of a material to absorb energy and deform plastically before

fracturing. Since the amount of plastic deformation that occurs before fracture is a

measure of the ductility of the material and because the stress needed to cause fracture is

a measure of its strength, it follows that toughness is associated with both the ductility

and strength of the material.

Unlike other testing applications, impact test involves the sudden and dynamic

application of the load. For this purpose, in general, a pendulum is made to swing from a

fixed height and strike the standard impact specimen. We are used the most common

method for the measurement of impact strength, that is Charpy tests.

In Charpy test, the specimen is fixed in horizontal position as shown in Figure 1. The

pendulum strikes the impact specimen on the unnotched face. The Charpy impact

specimen is square in cross-section with V-notch or U-notch.

Because the Charpy impact specimen does not have to be clamped in position, it is much

easier to test specimens at temperature other than room temperature using this method.

The Charpy impact test can be used to assess the relative toughness of different materials.

It is used as a tool for materials selection in design. It may also be used for quality

control, to ensure that the material being produced reaches a minimum specified

toughness level.

Figure 1: Configuration of Charpy Test

2. Principles of Measurement

In an impact test a specially prepared notched specimen is fractured by a single blow

from a pendulum striker and energy required being a measure of resistance to impact.

The impact test involves a pendulum (Figure 2) swinging down from a specified height

h0 to hit the specimen and fracture it. The height h to which the pendulum rises after

striking and breaking the specimen is a measurement of the energy used in the breaking.

Figure 2: Schematic of Impact Test

If no energy were used, the pendulum would swing to the same height h0 it started from, i.e.

the potential energy mgh0 at the top of the pendulum swing before and after the collision

would be the same.

The greater the energy used in the breaking, the greater the loss of energy and so the lower

the height to which the pendulum rises. If the pendulum swings up to a height h after

breaking the specimen, then the energy used to break it is

E = mgh - mgh [Nm or J]

This energy value called impact toughness or impact value.

Specimens and Equipments

1. Pendulum impact tester G.U.N.T. WP400

2. Vernier calliper.

3. Impact specimens: mild steel (V- and U-notch), carbon steel (V- and U-notch)

Procedures

1. The thickness of the specimen and the dimensions of the unnotched length were

measured (Figure 3).

2. The pendulum was raised to the left until it indicates the maximum energy range on

the upper indicator unit.

3. The specimen was placed horizontally across supports with the notch away from the

pendulum (Figure 4).

4. The pendulum was released.

5. The indicated value from the indicator unit was recorded.

6. The brake was applied until the pendulum has returned to its stable hanging vertical

position.

7. The specimen was removed from the testing area and observes the failure surface.

Figure 3: Standard impact test Figure 4: Specimen placement

specimen with V-notch for impact test

RESULT

Specimen Datum 1

(Nm)

Datum 2

(Nm)

Datum 3

(Nm)

Average

datum

(Nm)

Datum

specimen

(Nm)

Impact

(Nm)

Mild steel

U- notch 5 4.5 4.5 4.7 22.9 18.5

Mild steel

V-notch 4.5 4.5 4.5 4.5 >25 >20.5

Carbon

steel

u- notch

4.5 4.5 4.5 4.5 23 18.5

Carbon

steel

v-notch

4.5 4.5 4.5 4.5 22.2 17.7

Dimension

Specimen

Un notched

length

(l) (mm)

Thickness

(h) (mm)

Un notched cross

section area (so) (mm)

Notch cross section area

(mm)

Mild steel

u notch 10.00 5.02 4.86 5.14

Mild steel

v notch 10.00 5.00 1.98 8.02

Carbon steel

u notch 10.00 5.00 4.84 5.16

Carbon steel

v notch 10.00 5.02 1.78 8.22

Calculation

Indicator value = impact test with specimen

Impact = Datum specimen average Datum

Impact = indicator value average data (Nm)

Discussion

1. By comparing the materials used and the shape of the notch, please select the best

specimen candidate that will theoretically give the highest impact energy. Describe the

hypothesis.

From the results of the experiment, we can see differently how the specimens fracture.

By comparing all the specimens used, we can conclude that the best specimen that gives

highest impact energy is mild steel (v-notch). We can also conclude that mild steel is

tougher than carbon steel and the v-notch provides better performance against impact.

The more energy absorbed by the specimen, the more toughness the materials that will be.

2. Compare the actual result from the experiment with your hypothesis. Discuss.

By the hypothesis, that is the more energy absorbed by the specimen, the tougher the

materials will be is compared to the actual experiment results and it showed that the

hypothesis is similar to the result obtained. It is stated that mild steel is tougher than

carbon steel and the result from the experiment also shows the same. This is because mild

steel is more ductile than carbon steel and carbon steel is a more brittle specimen than

mild steel.

3. What is the information that can be obtained by analyzing the fracture surface of the

specimen? Discuss on the fracture surface obtained in this experiment and the

correlation it have with the impact energy data.

Fracture surface information:-

Highly ductile fracture: - specimen neck down to a point.

Moderator ductile fracture: rough plastic deformation.

Brittle fracture without any plastic deformation.

By observing the surface of specimens, we see that the carbon specimen broke completely

into 2 parts and the broken surfaces looks smoother than the mild steel. This happened due

to the ductileness of carbon is lower than mild steel.

A higher toughness material will absorb more energy upon impact and will therefore

result in a low height to which the pendulum arm will swing to following impact. The more

energy absorbed by the specimen, the more toughness the material will be. Specimen with lowest

absorbed energy means its brittle and has least toughness which can break easily and

cannot withstand the sudden high loads. Specimen with Highest absorbed energy means its

ductile and has highest toughness which can withstand the sudden high loads.

4. Assume that the impact energy obtained from the experiment have possible error.

Discuss the factors that can affect the experiment result from:

a. Specimen

More energy needed in the ductile fracture because it is a tough material.

b. Equipment

Brittle fracture happens quickly without warning while the ductile fracture

took a longer time before the whole process to happen.

5. Give two examples of applications for Charpy impact test in the industry?

Charpy test is widely used during construction of pressure vessels and also in

construction of bridge which is very important to determine which material is the most

suitable against air factor and load on it.

Conclusion

Impact is a high force or shock applied over a short time period. Such a force or

acceleration can sometimes have a greater effect than a lower force applied over a

proportionally longer time period. At normal speeds, during a perfectly inelastic collision, an

object struck by a projectile will deform, and this deformation will absorb most, or even all,

of the force of the collision. Viewed from the conservation of energy perspective, the kinetic

energy of the projectile is changed into heat and sound energy, as a result of the deformations

and vibrations induced in the struck object. However, these deformations and vibrations

cannot occur instantaneously. A high velocity collision (an impact) does not provide

sufficient time for these deformations and vibrations to occur. Thus, the struck material

behaves as if it were more brittle than it is, and the majority of the applied force goes into

fracturing the material. From the Charpy impact test that we have done, carbon steel

undergoes brittle fracture while the mild steel undergoes ductile fracture. More energy is

absorbed by mild steel shows that it is more suitable to be use in the structural construction

that expose to high load for example vehicles body such as car body or bus body.

Reference http://www.twi.co.uk/content/jk71.html

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