Doruk BİLEKE lab report 202 PDF

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DORUK BİLEKE 53584 MECH 202-EXPERIMENT 3-4 CHARPY IMPACT TEST AND SEM IMAGE

Charpy impact test is a standard test to determine how much energy absorbed by the material while breaking it under an impact load which is invented by Albert Charpy in 1900. An arm with a hammer at the end of it working as a pendulum with known length and weight is set to a specified height where it has the maximum potential energy. In our experiment that maximum energy is 15 J. When the arm released, it rotates about a fixed point and it impacts the specimen to break it. After the impact, the arm continues its motion with the remainder of kinetic energy until it fully turns to potential which is the result of the test. There is a pointer which indicates the potential energy at that height and by subtracting the initial and the final energies, the impact energy absorbed by the specimen is found. It is usually used in quality control process in industries because of its fast and economical application for instance in the construction of pressure vessels and bridges to determine how storms will affect the materials used. Also, it was used in World War 2 to understand the reason behind the failure of ships. As a disadvantage, it is used more as a comparative test rather than a definitive test.

Figure 1

Charpy impact test is applied most commonly on metals, but also used on ceramics, polymers and composites. Mostly used to investigate how brittle materials behaves under dynamic loadings. Main information that can be obtained by this test is whether the material is ductile or brittle. We can determine that with looking to the impact energy where ductile materials absorb much more energy than brittle ones. Due to that difference of absorbing energy, if the pendulum climbs to higher height that means the material is ductile and vice versa. As well as measuring the toughness of the material the yield strength can be estimated since there is a connection between them which can’t be expressed by a definite formula. Also, strain rate can be analyzed by the numerical result of the test. After the test, by observing the fracture surfaces we can determine the material’s ductility too. Rough surface tells that material is ductile while smooth surface indicates that is brittle. An example can be seen below as the Figure 2a shows a ductile material’s fracture surface and Figure 2b shows brittle material’s.

Figure 2a

Figure 2b

Standard specimen of the test is a bar with dimensions 55x10x10 mm with a notch machines at the midpoint of the opposite face of the impacted one. The reason having that notch is simply to guide the specimen while breaking to have fracture surface on that plane. These notches are differing as V-notch and U-notch or keyhole notch which are 2mm deep, with 45° angle, 0.25mm radius at the base and 5mm deep, 1mm radius at the base respectively.

Figure 3

There are several factors affecting the test which can be sorted as specimen’s geometry, notch’s shape and depth, temperature and finally fracture mechanism. Firstly, the size of the specimen can affect the impact energy directly in such a way that the change in the ratio of its dimensions to cause the specimen to fracture easier or harder. Then, the notch’s depth and tip radius is another important factor since it creates a stress concentration and regarding to the material the results can vary. So, while preparing the specimen, the values must be the same since small differences would have impacts on the result of the test. Therefore, the standardization of the specimen provides a proper comparison between tested materials. Other factors that increase stress concentration are cracks and voids in microstructure. As the strain increases, voids come together and can cause a sudden fracture before it is fully plastically deformed. Finally, the most important factor that affects the toughness of the material is temperature. When the test is applied to identical specimens in different temperatures, the impact energies decreasing as well as the temperature which induces ductile to brittle transition. This is an essential information before determining the working temperature margin of the material without a fracture. That effect of the temperature can cause a material to fail under the same load since it becomes more brittle while it is working properly at higher temperature. Some materials such as carbon steel undergo that ductile to brittle transition with decreasing temperature. The impact energy of low-strength metals is usually high while highstrength materials is low and their fracture manner wouldn’t change with temperature, because of that Charpy impact test is mostly applied on materials with low-strength such as body-centered cubic (BCC) transition metals. Thus, we can come up with materials with FCC structure, high and low strength materials are insensitive to temperature while materials with BCC structure are which causes to ductile to brittle transition. After the Charpy impact test we took the specimen to observe its fracture surface under Scanning Electron Microscope (SEM). First I want to start with a brief information about SEM. Before it starts, the sample with conductive carbon tape attached on it placed into the microscope. Then SEM sends focused electron beam with high energy onto the samples surface which enables us to see the microstructure with a very high magnification as zooming in 30000 times when it is compared with optical microscope. That electron beam sent from SEM generate variety of signals due to the electronsample interaction and we can use these signals to determine texture, elemental composition, crystalline structure and orientation of materials of the sample. The electron beam is made by voltage difference created within the microscope and accelerated towards the sample surface by electron gun.

Figure 4

The electrons are focused to a specified point by using a positively charged limiting filter which is called condenser ring. The working principle of the condenser ring is with the positive charge, it can push the electrons to a certain point without scattering. After the beam is sent to the sample’s surface, three reactions occur there and electrons reflected with different angles to different places. SEM collect the data from these reflections and created an image for the samples surface. As the result of the first reaction which is the immediate turning back of electrons after they crashed with the electrons rotating around the atoms of the sample which are collected by backscatter electron detector. It is placed near to the electron gun since reflected electrons comes back with linear motion. Second reaction occur as electrons in the beam have an energy transfer with electrons of the atoms of the sample. The electrons sent by the microscope join to the orbits and another electron which is initially at the orbit reflects. These electrons are collected by secondary electron detector. Final reaction which’s data collected by X-Ray detector is sent electrons stuck between the atoms so they couldn’t reflect however, they spread their energy. Data are collected from different depth where secondary electrons came from the top of the surface while backscatter electrons came right under them and X-Ray detector collects from the deepest distance. As I said we can come up with texture and elemental analysis with these data. Secondary electrons are the main factor to determine the fracture surface of the sample while backscatter electrons help us to determine elemental composition. We can understand each elements intensity with the different values of energy and velocity after they backscattered since all element have different amount of electron around them and the values depend on these amounts. According to those data we can have a map-like image of placing of each element throughout the samples surface. When we have done all these steps we come up with an image of out tested material. The image shows us a very typical brittle fracture surface which frequently have smooth and shiny surfaces. If the material is ductile we must have lots of dimples and small black holes everywhere on the surface instead of having those smooth, shiny sections. As we can see there are dimples in the significant location which are reasonable since the material is an alloy and it will sure have a trace of ductility. We couldn’t expect 100 % brittle material since it is not single crystal. When I compare Figure 5a and Figure 5b which are the images of ductile and brittle fracture surfaces under SEM, I can easily say that our sample in Figure 5c is certainly brittle.

Figure 5a

Figure 5b

Figure 6c

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