Group 1 Impact Test Lab Report

ABSTRACT Charpy Impact testing is used in studying material toughness and most importantly it indicates a material ability to absorb energy and plastically deform without fracturing. In this study, three stainless steel specimens at different temperature -78¡C, 80¡C and 7¡C respectively were utilized. They were one at a time properly placed within a Charpy testing machine where a swinging pendulum (hammer) was used to impart the load on the specimens. After the impact, the hammer angle swing was recorded as an indication of energy absorbed by the material, the difference in the kinetic energy of the pendulum at the impact, and the potential energy of the pendulum at its maximum angle achieved after impact. In addition, the samples fracture sites were photographed and compared for further material behaviors analysis. Subsequently, it has been observed that the higher the temperature, the more the fracture of the metal exhibited ductile behavior. Moreover, further results revealed the coldest steel specimen of -78¡C has a maximum angle after impact of 131.5¡C and 6.9 J of absorbed energy, whereas the 7¡C sample has C has a maximum angle after impact of 123.5¡C and 24.1 J of absorbed energy. Lastly, the steel sample of 80¡C has a maximum angle after impact of 65¡C and 175.8 J of absorbed Energy. It is then evident to say that the energy absorbed by the material specimens increases as the temperature of the specimen increases; ductile materials under certain conditions can display brittleness. i TABLE OF CONTENTS ABSTRACT .................................................................................................................................... i INTRODUCTION .......................................................................................................................... 1 METHODOLOGY ......................................................................................................................... 3 RESULTS ....................................................................................................................................... 5 DISCUSSION ................................................................................................................................. 7 CONCLUSION ............................................................................................................................. 10 REFERENCES ............................................................................................................................. 11 APPENDIX A – WORK STRUCTURE ...................................................................................... 12 INTRODUCTION In Metallurgy, toughness indicated a material ability to absorb energy and plastically deform without fracturing, it can also be defined as the aptness to absorb strain energy which represent the amount of energy per unit volume before rupturing. Therefore, toughness requires a great balance of strength and ductility. To help determine service life of a part, it is crucial to acquire the proper knowledge of its material toughness. The ability of the material to absorb energy when tested at low strain rate to failure in a static tensile test is not quantitatively comparable to the ability of the material to absorb energy at high strain rate in an impact testing. A qualitative comparison can be made by comparing the area under the stress-strain curves with the energy absorb by impact testing. Impact testing are used in studying the toughness of material, the most common method is the notched-bar impact test for which two types of specimens prevail, Charpy and Izod. It is a standardized high strain-rate test which determines the energy absorbed by a material during fracture. In this study Three steel specimens are subjecting to an impact load; they will fail if the load exceeds the breaking strength of the material. By using a swinging pendulum to impart the load, the energy required to fracture the specimens can be calculated by observing the height the pendulum swings after fracture, as shown in Figure 1. The absorbed energy of the specimens is calculated using Equation 1 below. ‫ܧ‬ൌ ‫ܴ݃ܯ‬ ሺ ߙ െ  ߚሻ Where: • ‫ = ܧ‬Absorbed energy • ‫ = ܯ‬Mass of the pendulum • ݃ = Acceleration of gravity • ܴ = Radius of pendulum • ߙ = Angle of pendulum before release • ߚ = Maximum angle of pendulum after release 1 Equation 1 [1] Throughout this experiment the impact test analysis of the steel specimens has been properly conducted. It has been observed that ductile materials under certain conditions display brittleness. In addition, the relationship between energy absorption capability and temperature has been made brought to the light. Charpy impact test indicates the energy absorbed by the material specimens increases as the temperature of the specimen increases. 2 METHODOLOGY To start the experiment, we obtained a sample of stainless steel under room temperature which came in a cuboid shape. The dimensions of the specimen – the cross-section area, length, notch width, and notch angel – were then obtained for later calculations. To set up the experiment, we used the Charpy testing machine. Figure 1. Charpy Testing Machine The hammer (pendulum) from the machine was pulled up high and anchored. We then obtained a stainless-steel specimen that has been cooled down to -78¡C and place it under the Charpy testing machine. Once taking the sample out to room temperature, the experiment must be conducted under five seconds to reduce the uncertainty in temperature change. The notch on the specimen should be facing away from impact side. The hammer was then released and hit the 3 sample. After the impact, the hammer will still swing up to a certain angle. This angle was observed and recorded from the machine. Figure 2. The Plate that Shows the Angel of the Hammer after the Impact The experiment was repeated with other sample with different temperatures of 80¡C and 7¡C. The fracture sites of the sample were also photographed in order to compare the differences in material behaviors. 4 RESULTS At the conclusion of the Charpy impact testing of the stainless-steel specimens at three different temperatures, the amount of energy absorbed by each specimen was calculated. Calculating the potential energy of the impact tester pendulum before being released, and after being released, and then taking the difference in potential energies as the energy transmitted and absorbed by the specimen. This calculation is simplified as shown in Equation 2. ‫ܴ݃ܯ‬ ‫ ܧ‬ൌ ሺ ߙ െ  ߚሻ Equation 2 [1] Where: • ‫ = ܧ‬Absorbed energy • ‫ = ܯ‬Mass of the pendulum • ݃ = Acceleration of gravity • ܴ = Radius of pendulum • ߙ = Angle of pendulum before release • ߚ = Maximum angle of pendulum after release The results of the Charpy impact test indicate the energy absorbed by the material specimens increases as the temperature of the specimen increases, as indicated numerically below in Table 1, and graphically in Figure 3. Table 1. Results Constants g M α R Temperature 2 9.81 m/s 17.35 kg 135 degrees 0.9144 m Maximum Angle After Impact Absorbed Energy 80 °C 7 °C -78 °C β β β 5 65 degrees 123.5 degrees 131.5 degrees 175.8 J 24.1 J 6.9 J Figure 3. Results Graph Having only three data points to examine, a positive identification of the ductile to brittle transitional range is unlikely, however according to the results, this range appears to be between 20¡C and 80¡C. With regard to the plane strain fracture toughness, a material textbook states that: “Attempts have been made to correlate plane strain fracture toughnesses and Charpy V-Notch energies, with only limited success. Plane strain fracture toughness tests are not as simple to perform as impact tests; furthermore, equipment and specimens are more expensive.” [1] Nevertheless, using the yield strength of stainless steel and the crank length as shown in Equation 3 an approximation of the plane strain fracture toughness was attempted and calculated ‫ܽܲܯ‬ to be ͳͺǤ͹ͷ ξ݉. ‫ܽܲܯ‬ ‫ܭ‬ூ௖ ൌ ͳǤͳ ‫ͳʹ כ‬ͷ ‫ܭ‬ூ௖ ൌ ܻߪξߨܽ  ‫ܽܲܯ‬ ‫ כ‬ξߨ ‫Ͳ כ‬ǤͲͲʹ݉ ൌ ͳͺǤ͹ͷ ξ݉ 6 Equation 3 [2] DISCUSSION As expected, the samples exhibited similar behaviors with the theory in which materials becomes more ductile when the temperature is increased. The ductile-brittle transition happens mainly because of the restriction in motion of molecules. When materials are cooled down to certain temperature, depending on the material, “molecules cannot easily slip or slide on a microscopic scale” [2]. When they are not able to move easily, the bonds between them are more prone to break since it does not have the flexibility or ductility to fluctuate back and forth. The -78¡C sample showed a rather sharp and clean fracture. This was the sign the material having brittle behavior. It can be seen in Figure 4 that the sample did not deform much before fracture. Since it became more brittle and molecules were restricted to move, only a small amount of energy was required break the bonds between atoms; hence the low absorbed energy. Figure 4. Side View of Sample of -78¡C 7 The 7¡C sample also showed a rather sharp cut which can be viewed below in Figure 5. It was difficult to determine the difference in behavior when comparing this and the previous sample. However, with close inspection, it can be seen from the sideview at the fracture point of the 7¡C sample stretched a bit more than the colder companion. Figure 5. Sample of 7¡C The highest temperature sample of 80¡C exhibited the clearest sight of material being ductile. It can be easily seen from Figure 6 that the sample had gone through tremendous strain before fracture. This is because the effects the high temperature has on the sample. When the sample was subjected to hot temperature, the molecules had more room to move back and forth. They did not have as much restrictions as the cold sample. Due to this fact, the sample took more energy to break the bonds before fracture; hence the high absorbed energy. 8 Figure 6. Sample of 80¡C Exhibiting Ductile Behavior 9 CONCLUSION During the course of this experiment, the energy able to be absorbed by each specimen before fracturing was determined by the Charpy impact test. The greater the absorbed energy, the higher ductility the specimen was capable of before fracture. From the experiment, the stainlesssteel test samples exhibited brittle behavior at very low temperatures and became more ductile at elevated temperature. The controlled variable that was changed between tests was the temperature at which the specimens were tested at. As temperatures rose, the absorbed energy increased, indicating a clear effect of temperature on the energy absorption capability. The specimen that was submerged in dry ice, measuring at temperature –78¡C, absorbed the least amount of energy before ultimately breaking. Therefore, an inverse relationship can be seen between the temperature and energy required to fracture the material. The temperature on the environment also impacts the mobility of the defects within the material. Reducing the temperature causes the defects in the substance to not move as much as compared to materials at elevated temperatures. Another way to determine whether the substance was brittle/ductile is to physically examine the fracture region of the specimen after the Charpy test. It can be concluded that higher the temperature, that the fracture showed more ductile behavior, indicating that the defected molecules had more space to move back and forth in elevated temperature. As the data shows, the test specimens clearly underwent a ductile to brittle transition as their temperature rose. The transitional period is dependent on the temperature of the material. 10