The Experiment on the Determination of Tensile Strength of Metals - Lab Report Example

Determination of the Tensile Strength of Metals location Determination of the Tensile Strength of Metals Abstract The experiment on the determination of tensile strength of metals discussed below involves the study of the behaviour of three different metals. The metals are suitably shaped and machined to have uniform dimensions. It helps to minimize the experimental errors that might arise in case non-uniform metal samples were to be used. The experiment solely uses tensile tester that subjects the metal samples upon uniform tensional forces. The forces are applied continuously at regular intervals. As a result, the displacement of the metal arising from its extension is recorded. The values obtained are plotted on different graphs for each metal. The graphs are of the applied load against the displacement. Finally, the yielding points of the metals are determined from the charts together with the maximum load carrying capacity of the metals. Contents Abstract 2 Contents 3 List of Figures/Tables 4 Introduction 5 Background Theory 6 Experimental Procedure 9 Specimen Preparation 9 Bluehill Software Setup 9 The method setup included 9 Instrument Setup 10 Tensile Test 11 End of Test 11 Results and Analysis 12 Discussion of Results 22 Conclusions and Recommendations 23 List of Figures/Tables Figure 1 A photograph of a tensile machine Instron 5569 Figure 2 Various regions and points on the stress-strain curve Figure 3 Table of results from metal 1 and the chart of load against displacement for the metal 1 Figure 4 Table of results from metal 2 and the chart of load against displacement for the metal 2 Figure 5 Table of results from metal 3 and the chart of load against displacement for the metal 3 Introduction In determining the mechanical properties of materials, carefully designed experiments are conducted in the laboratories. The experiments are so designed that they replicate the service conditions in a very close range. Load application on materials involves many factors considered in the real life. The modes, in which loads are applied, include some typical examples like tensile, shear and compressive. These properties are of much importance in material selection for mechanical design. There are other factors like time and temperature that complicate the design process. Background Theory As from the topic of this laboratory experiment, the report is confined to the tensile properties of metals. Figure 1 below shows a tensile testing machine with features similar to the one used to conduct this experiment. A specimen of standard dimensions and shape is subjected to an axial load. A doge-bone shaped specimen is gripped at its two ends and pulled apart during a typical tensile experiment to elongate at a determined rate to its breakpoint. In the process, a highly ductile material may make it difficult to reach its break point. The tensile tester used here for the experiment is manufactured by Instron (model 5569) with a variable pulling rate and a maximum load of 50kN. Different types of mechanical testing can be accommodated by the machine only by changing the setup of the experiment according to the ASTM standards. A plot of stress (σ) versus strain (ε) is conducted during the tensile test experiment for analytical purposes. The plot can automatically be generated by the software provided by the instrument manufacturer. From the metric system, stress is measured in N/m2 or Pa, whereby 1 N/m2=1Pa. The value of stress is calculated by the division of the force (F) applied to the machine particularly in the axial direction normally by its cross-sectional area (A). The area needs to be measured prior to running the experiment.Thus, the mathematical equivalent of the expression is (Equation 1) Strain has no units since it involves the division of lengths which have the same units. Thus, (Equation 2) A stress-train curve would typically look like the one shown in figure 2 below. The curve shown above is just a typical example of stress-strain curve which may not be the exact curve obtained from an actual experiment. The curve is typical for metallic elements. Figure 2 above shows an “Engineering Stress-strain” curve whereby the cross-sectional area of a material reduces dramatically once the material reaches its ultimate stress strength of the stress-strain curve. The process is known as necking. A plot of stress-strain curve from the computer assumes a constant cross-sectional area throughout the experiment, even during necking. The result of the process is a downward sloping curve. By installing a “gauge” that measures the change in cross-sectional area of the specimen throughout the experiment, the “true” stress-strain curve can be constructed. The “true” stress-strain curve is possible to construct it theoretically even without the measurement of the cross-sectional area of the specimen during the experiment of the tensile testing. It is achieved by assuming a constant volume of the material throughout the experiment. From the concept, both the “true” strain (εT) and “true” stress (σT) can be calculated by the use of equation 3 and 4 respectively. (Equation 3) (Equation 4) Whereby L0 refers to the initial length of the specimen, L being the instantaneous length and σ refers to the instantaneous stress. From Figure 2 above, it is visible how a stress-strain curve can be divided into four regions. These include elastic, yielding, strain hardening and necking. The area formed under the curve represents the energy totals necesary to accomplish the different phases. The area that is under the curve particulerly (up to the fracture point) is termed to be the modulus of toughness (The amount of energy needed to break the sample). It could be compared to the impact energy of the sample used determined from the results of the impact tests. Area that lies under the region that is liner onthe curve forms the modulus of resilience. (This is the minimum Quantity of energy necesary to deform the sample). Elastic region forms the linear part of the curve of Figure 2. Any point past this area forms the plastic region (The region where a material behaves elastically). Upon the release of the force when the material is in its elastic region, the material returns back to normal original shape. Thus, the curve slope can be determined using equation 5, and it forms the elastic modulus, E. It is an intrinsic property of a material expressed in Pascals (Pa). (Equation 5) Experimental Procedure Important! Wearing of safety glasses is compulsory before starting the experiment. Broken metal pieces can cause harm to your eyes. Gloves should also be worn to protect against any residue on the machine and samples. Specimen Preparation The metal specimen were shaped into dog-bone shapes with predefined dimensions according to the Instron (model 5569) machine specifications. The width, gauge length and thickness of the metal samples were measured in mm with each sample having approximately same dimensions for each sample. Sample like impurities defects was noted. Bluehill Software Setup The tensile test machine was turned on. The location of the switch is to the right side of the machine. Also we made sure we had turned on the video extensometer. The “Bluehill” icon on the desktop was double-clicked before selecting Test on the main page to start a new sample. The Browse button was clicked before naming the test in order to select the folder to save the test. Then I clicked next. After choosing the method preferable to use, the new method was saved. Any changes were saved immediately. The method setup included • General: It was used for display purposes. • Specimen: It specifies the sample parameters and dimensions. Tensile testing required a dog-bone sample. The thickness, width, and gauge length of the sample were specified after selecting the rectangle. The gauge length forms the distance between the clamps before starting the test. • Control: the actual test was described here before selecting the displacement mode and specifying the extension rate. Most tests used 5mm/min or 50min/mm depending on the rate of the test needed, either fast or slow. • End of test: It helped in identifying the criteria for the end of the test. A large load drop was experienced when a sample failure occurs. For the test carried out, the machine stopped when the sample load dropped by a certain percentage of the peak load. • Data: It helped in specifying whether the data was acquired manually or automatically. The strain tab recognized whether the strain was measured from the extension or the video extensometer. • Results and graphs: it was used to select the data shown and how it was displayed. Instrument Setup We made sure that the proper load cell was installed, either 2kN or 50kN depending on the sensitivity and the load range of the sample. Switching of load cells was done only when the machine was off. The bolts were unscrewed and removed using the handle. Then the new load cell was plugged into the port behind the machine. The load cell was calibrated by clicking on the button on the upper right-hand corner. All loads are first removed from the load cell before clicking the Calibrate icon. The correct types of clamps were installed using the pins. Height brackets were also installed when needed. Then the load was zeroed once the clamp were installed. The up and down arrows were pressed until the clamps were just touching. The reset gauge was pressed to zero the position of the clamps. The up and down arrows were used until the clams were about 100mm apart. This forms the typical gauge length for the dog bone samples. A metal sample was placed between the grips of both the tensile test machine. While holding the sample vertically with on hand, the other hand was turnning the handle in a closing direction of top grip tightly as possible. Then the specimen was gripped such that the two ends of the specimen were covered tightly by the grip, by approximation of 3 mm away from its gauge-length. It was important to grip the specimens tightly to prevent any possible slipping which would otherwise result in experimental errors. We made sure that the specimen was vertically aligned; otherwise a torsional force would result instead of an axial force. The bottom handle was turned in the “close” direction as tight as possible. The sample was verified visually if it was gripped symmetrically at its two ends. Then the extension was zeroed by pushing the zero extension button. The load was also zeroed. It was important to wait a few seconds for the computer to settle at zero. Tensile Test The geometry of the sample was first entered. After clicking on the start button, both the upper and the bottom grips started moving in the opposite directions according to the specific pulling rate. The experiment was observed at a distance of about 2m away that is considered safe at an angle. Notes was taken on the failure mode when the specimen fails. A graph plot of that is tensile stress (Mpa) Vs the tensile strain in mm/mm was generated in real time duration during the experiment. End of Test The machine stopped automatically once the sample was broken. Then the “Return” button on the digital controller was pressed. The upper and the lower grips were automatically returned to their original positions. Then the two handles were turned in the open directions to remove the sample. The previous step was repeated for any additional tests. When finished, the file was saved, and the Finish icon clicked. It exported the data into a PDF and individual data files. The broken fragments from the specimen were cleaned up. Then the machine was then turned off, and the program exited. Results and Analysis The tables below show the results obtained from the variations of displacement achieved by load applied at specific time intervals. The load at which metal 1 yields = 10.5kN The maximum load for metal 1 = 19.0kN The load at which metal 2 yields = 1.2kN The maximum load for metal 2 = 1.5kN The load at which metal 3 yields = 6.0kN The maximum load for metal 3 = 8.3kN Discussion of Results The results indicate metal 2 yields at approximate load of 1.2 kN. It is thus faster than the other metals while being followed by metal 3 that yields at approximately 6.0 kN. Metal 1 has the highest yielding point of about 10.5 kN. It can also be noted that the maximum load that the metals can carry follow the same trend as their yielding points. Metal 1 tops the amount of maximum load it can support at about 19.0 kN, followed by metal 3 at 8.3 kN. Metal 2 is at the bottom of the group with a maximum load capacity of 1.5 kN. From the research on appropriate literature, the metallic materials 1, 2 and 3 may be from steel, copper and aluminum respectively. Usually the steel is the metal with highest tensile strength; thus it has a very high maximum load to carry. Copper on the other hand is malleable, making it reach it maximum load carrying capacity faster than both the steel and aluminum. Conclusions and Recommendations The tensile strength of metals varies according to the amount of load they are supposed to carry out. Failure usually occurs where a metal (material) is subjected to an external load much higher than its maximum carrying capacity. Through the experiment, it is easier to determine the best metal to use in carrying external loads in structures being constructed by the civil engineers as in the process they tend to avoid failure by all means. Bibliography Hwang, J. (2013). Characterization of minerals, metals, and materials 2013. Hoboken, N.J.: Wiley. Read More