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
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 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 it’s brittle and has least toughness which can break easily and cannot withstand the sudden high loads. Specimen with highest absorbed energy means it’s 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.
