Objective:

1. To develop an understanding of fracture toughness.

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

3. To test ability of different types of spec impacts using two materials for impact test; mild

steel and carbon steel.

Theory:

Impact test is a test used in studying the toughness of material. The definition of

toughness is defined as the capacity of material to absorb energy and deform plastically

before fracturing. Toughness is associated with both ductility and strength of materials.

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 isa measure of

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

the material. 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. There are two types of method to test impact test which is Izod

test and Charpy test. These two methods are different in placing the specimens. In Izod

test, the specimen is placed in vertical position and the notch area is facing the pendulum.

Meanwhile in Charpy test, the specimen is placed horizontally with unnotched area facing

the pendulum. Moreover, Izod impact specimen only has a V-notch specimen while

Charpy impact specimen has both U-notch and V-notch specimen. Charpy test result can

indicate how brittle the materials are. The most common method for the measurement of

impact strength that is Charpy tests is used in this experiment.

Specimen and Equipments:

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

2. Vernier caliper

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

Procedure:

1) The dimensions of the unnotched length and the thickness of the specimen are measured.

2) The pendulum is raised to the left until it indicates the maximum energy range on the upper

indicator unit.

3) The specimen is placed horizontally across supports with the notch away from the pendulum

4) Pendulum is released.

5) The indicated value from the indicator unit is recorded.

6) The brake is applied until the pendulum has returned to its stable hanging vertical position.

7) The specimen is removed from the testing area and failure surface is observed.

Results:

All the measurements of specimens are recorded in Table 1.

I. Thickness, h [mm]

II. Unnotched length, l [mm]

Specimen Dimension R1(mm) R2(mm) R3(mm) Average(mm)

Mild steel

U-notch

h 5.00 5.02 5.00 5.01

l 5.68 5.66 5.66 5.67

Mild steel

V-notch

h 5.02 5.00 5.00 5.01

l 8.02 8.00 8.00 8.01

Carbon steel

U-notch

h 5.02 5.00 5.00 5.01

l 5.90 5.88 5.90 5.89

Carbon steel

V-notch

h 5.00 5.00 5.00 5.00

l 7.46 7.50 7.48 7.48

Table 1: Measurement of Mild Steel and Carbon Steel

Specimen

Without specimen Datum with

specimen

(Nm)

Impact value

(Nm) Datum 1

(Nm)

Datum 2

(Nm)

Datum 3

(Nm)

Average

(Nm)

Mild steel

U-notch 4.90 5.00 5.00 4.97 25.40 20.43

Mild steel

V-notch 5.00 5.00 5.10 5.03 25.50 20.47

Carbon steel

U-notch 5.00 5.10 5.00 5.03 7.90 2.87

Carbon steel

V-notch 5.00 5.00 5.10 5.03 5.70 0.67

Table 2: Data of impact energy for Mild Steel and Carbon Steel

Discussion:

By comparing all the specimens used, it can be conclude that the best specimen that

gives highest impact energy is mild steel (v-notch). Mild steel is tougher than carbon steel as it

has lower carbon composition in it and the v-notch provides better performance against impact

as it gives bigger unnotched cross-section area compared to u-notch mild steel bar. Toughness

is a property, which is capacity of a material to resist fracture, when subjected to impact.

Tougher materials such as mild steel need higher energy or impact to break or fracture. So, this

means that it can absorb more energy applied on it. So, hypothesis that can be made from this

experiment is the more energy absorbed by the specimen, the more toughness the materials

will be. The actual results obtained from the experiment 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 as mild steel absorb more energy compared to carbon

steel. This is because mild steel is more ductile than carbon steel and carbon steel is a more

brittle as the composition of carbon is higher in carbon steel.

In this experiment, carbon steel specimens broke completely into two parts with

smoother broken surface while mild steel only undergo plastic deformation and did not

fractured. This is because mild steel is more ductile than carbon steel. Besides that, u-notch

specimen of carbon steel gives smoother surface of fracture compared to the v-notch specimen

while u-notch specimen of mild steel bend more than the v-notch specimen. So, it can be said

that u-notch specimens cannot absorb high energy produce by the pendulum during the impact

as u-notch specimen has smaller unnotched cross-section area. Smaller unnotched cross-

section area can only absorb small amount of energy from the impact. The correlation that could

be made is a higher toughness material will absorb more energy upon impact and will therefore

result in a high height to which the pendulum arm will swing to following impact and gives high

impact value reading. 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 upon collision.

Factors that can affect the impact energy value obtained could come from two aspects in

this experiment which are error because of the specimens condition and also the equipment

used. The specimen used in this experiment rust a little so it may affect the plastic deformation

or fracture condition result. The pendulum impact tester used in the experiment is old and does

not in it most effective condition where it cannot give precise reading. This condition is proven

right based on Table 2 where the average datum readings give some value of impact. If the

pendulum impact tester is in a good condition, the reading should be zero as there is no

collisions occur.

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

constructions 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 collision, an object struck by a

projectile will deform, and this deformation will absorb most, or even all, of the force of the

collision. However, these deformations cannot occur instantaneously. A high velocity collision

(an impact) does not provide sufficient time for these deformations 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 but because of low

energy supply from the pendulum, the mild steel just undergoes plastic deformation. 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 and high impact collision such as vehicles body.