Determination of the Tensile Strength of Metals - Lab Report Example

DETERMINATION OF THE TENSILE STRENGTH OF METALS location Determination of the Tensile Strength of Metals Abstract Strength of materials has been of interest in studies done by civil engineers. The study majorly focuses on metals whose various characteristics are determined. The report below is of a lab experiment conducted using three specimens of metals. It mainly focuses the reaction of the three metals when subjected to tensional forces. The metals are first machined carefully to achieve suitable shapes that can fit onto the Instron machine. The aim of this is to ensure accurate data is obtained from the experiment. The tensile tester machine used subjects the specimens upon uniform tensional forces. The metal displacement arising from its extension due to the continuously applied forces at regular intervals is recorded. Each of the three metals have the values of their loads plotted against the displacement. The yielding points and the maximum load carrying capacities of the metals are determined from the graphs. Table of Contents Abstract 2 Table of Contents 2 List Tables 3 Introduction 4 Background Theory 5 Experimental Procedure 9 It is paramount to wear safety glasses before beginning the experiment since broken pieces of metals can cause harm to your eyes. Additionally, gloves help protect the person conducting the experiment from any residue on the machine and samples. 9 At first, the metal specimen were shaped into dog-bone shapes with predefined dimensions according to the Instron (model 5569) machine specifications. The thickness, width and gauge length of the metal samples were measured and the measurements maintained for each of the samples. The impurity defects are noted and taken into consideration. 9 The tensile test machine was first turned on using the switch located to the right side of the machine. The video extensometer was also turned on. The “Bluehill” icon on the desktop was double-clicked before selecting Test on the main page to start a new sample. Naming of the test was done by clicking the Browse selecting the folder to save the test. The button written “Next” was clicked and a preferred method chosen and saved. The General part in the method setup was used for display purposes .The sample parameters and dimensions were specified in the Specimen section. The rectangle is first selected before specifying the width, length and thickness of the sample. The gauge length is the displacement from the clamp. 9 The actual test was described in the Control 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. The end of test criteria was identified at the End of test section. A large load drop was experienced when a sample failure occurs. The machine stopped for the test carried out when the sample load dropped by a certain percentage of the peak load. The method of data acquisition was specified in the Data section, either manually or automatically. The strain tab recognized whether the strain was measured from the extension or the video extensometer. Thereafter the Results and graphs section was where the data shown was selected and how it was displayed. 10 Setting up the Instrument 10 The experiment began by installing the proper load cell, either 2kN or 50kN depending on the sensitivity of the sample and its load range. The load cells were only switched when the machine was off. The handle was used to unscrew and remove the bolts before plugging in the new load cell into the port behind the machine. All the load were removed from the load cells before clicking the Calibrate button on the upper right-hand corner in order to calibrate the load cells. Pins were then used to install the correct types of clamps. Additionally, height brackets were installed when need arose. Finally, the load was zeroed once the clamp was installed. 10 Then the up and down arrows were pressed until the clamps were almost touching. The reset gauge is adjusted to zero the position of the clamps. The up and down arrows were used until the clams were about 100mm apart to form the typical gauge length for the dog bone samples. A sample of metal was placed the tensile test machine grip. Holding sample, the other hand was tightening the machine. 10 At approximately 3 mm away from the gauge-length of the specimen, the specimen was gripped such that the two ends of the specimen were covered by the grip. Slipping course errors, the specimens were held tightly. The specimen was confirmed to be vertically aligned to prevent the occurrence of torsional force instead of axial force. The bottom handle was tightly turned in the “close” direction. The sample was then visually verified whether it was gripped symmetrically at its two ends. Lastly, the zero extension button at the top of the screen was used to zero the extension. 11 In the tensile testing the geometry of the sample was entered. Thereafter it was noted that 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. Note was taken on the failure mode when the specimen fails. 11 The machine had an automatic setting that enables it to stop once the sample was broken. Then the “Return” button on the digital controller was pressed to allow both the upper and lower grips to automatically return 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. The file was then saved once the process was over and the Finish icon clicked. It exported the data into a PDF and individual data files. Thereafter, there was the cleaning up of the broken fragments from the specimen. 11 Results and Analysis 11 Discussion of Results 23 Conclusions and Recommendations 24 Reference 25 Appendix 27 List Tables Fig 1 Various regions and points on the stress-strain curve……………….7 Fig2 Table of results from metal 1 and the chart of load against displacement for the metal 1………………..……………………….16 Fig 3 Table of results from metal 2 and the chart of load against displacement for the metal 2………………………...……………….20 Fig 4 Table of results from metal 3 and the chart of load against displacement for the metal 3……………………..………………….22 Table 1: Recorded results Introduction The tensile testing experiment involves the carefully designed experiments that are majorly conducted in the laboratories in order to help determine the mechanical properties of materials. The design of the experiments are in such a manner that they replicate the service conditions closely. Some load is applied on materials in different modes. These include shear, tensile and compressive. The load application process involves have many factors taken into considerations in the real life scenario. The mechanical design process consider these properties very important for material selection. Background Theory The tensile testing experiment has been done severally in the laboratories for many years now. Let’s consider the main machine to be used in the experiment, the tensile testing machine. It involves specimens with standard dimensions and shapes being subjected to axial loads. A specimen is shaped like a dog bone for it to fit well into the machine. During a typical tensile experiment, the specimen is gripped at its two ends and pulled apart to elongate at determined rate to its breakpoint. In the process, any material with high ductility may cause difficulties in reaching its break point. The experiment uses the tensile tester shown below. It is manufactured by Instron (model 5569). Its feature of variable pulling rate and a higher maximum load (50kN) makes it the frequently used machine for the experiment. The Instron (model 5569) is as shown below. Mechanical testing of different metal types requires the change of experimental setup according to the ASTM standards. During the tensile test, a plot of stress (σ) versus strain (ε) is conducted for analytical purposes. The instrument’s software is used to automatically generate the plot. Stress is measured in N/m2 or Pa, in the metric system, whereby 1 N/m2=1Pa. The mathematical equivalent of the expression is ………………………………………….(i) Where -stress (N/m) F- force (N/) A- Area () While strain is given by, ……………………………………………(ii) L- final displacement (m) and - initial resting point (m). Strain has no units it’s a ratio of length and length A stress-train curve shown in figure 2 below would easily explain what would be expected during the experiment. Fig 1. Various regions on the stress-strain curve (Davis, 2004). The curve obtained from the actual experiment may not be the exact copy of the one shown above. It will only closely resemble it for metallic elements. The “Engineering Stress-strain” curve shown in figure 2 above indicates how there is a dramatically reduction in the cross-sectional area of a material. The reduction happens once the material reaches its ultimate stress strength of the stress-strain curve in a process is known as necking. A constant cross-sectional area is assumed throughout the experiment during the plotting of the stress-strain curve. The “true” curve of stress-strain could still be constructed 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. ……………………………………………………………………………....(iii) ……………………………………………………………………………..….(iv) Whereby L0 is the initial length of the specimen, L being the instantaneous length while σ refers to the instantaneous stress. As shown in figure 2 above, a stress-strain curve can be divided into four regions including the elastic, yielding, strain hardening and necking regions. The area under the graph is the amounts of energy. Hence, the total area under the curve (up to the fracture point) is termed to be the modulus of toughness. The modulus of toughness is the amount of energy needed to break the sample. The energy can be compared to the impact energy of the sample determined from the impact tests. The linear region of the curve has an area known as the modulus of resilience. The modulus of resilience is the minimum energy needed to deform the sample. The linear part of the curve is the elastic region. Normally, Plastic region is the region where a material behaves elastically. When the material is in the elastic region, a release of the force returns the material to its original shape. Thus, the curve slope can be determined using equation 5, and it forms the elastic modulus, E. Elastic modulus is an intrinsic property of a material expressed in Pascals (Pa). ………………………………………………………………………(5) Experimental Procedure It is paramount to wear safety glasses before beginning the experiment since broken pieces of metals can cause harm to your eyes. Additionally, gloves help protect the person conducting the experiment from any residue on the machine and samples. At first, the metal specimen were shaped into dog-bone shapes with predefined dimensions according to the Instron (model 5569) machine specifications. The thickness, width and gauge length of the metal samples were measured and the measurements maintained for each of the samples. The impurity defects are noted and taken into consideration. The tensile test machine was first turned on using the switch located to the right side of the machine. The video extensometer was also turned on. The “Bluehill” icon on the desktop was double-clicked before selecting Test on the main page to start a new sample. Naming of the test was done by clicking the Browse selecting the folder to save the test. The button written “Next” was clicked and a preferred method chosen and saved. The General part in the method setup was used for display purposes .The sample parameters and dimensions were specified in the Specimen section. The rectangle is first selected before specifying the width, length and thickness of the sample. The gauge length is the displacement from the clamp. The actual test was described in the Control 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. The end of test criteria was identified at the End of test section. A large load drop was experienced when a sample failure occurs. The machine stopped for the test carried out when the sample load dropped by a certain percentage of the peak load. The method of data acquisition was specified in the Data section, either manually or automatically. The strain tab recognized whether the strain was measured from the extension or the video extensometer. Thereafter the Results and graphs section was where the data shown was selected and how it was displayed. Setting up the Instrument The experiment began by installing the proper load cell, either 2kN or 50kN depending on the sensitivity of the sample and its load range. The load cells were only switched when the machine was off. The handle was used to unscrew and remove the bolts before plugging in the new load cell into the port behind the machine. All the load were removed from the load cells before clicking the Calibrate button on the upper right-hand corner in order to calibrate the load cells. Pins were then used to install the correct types of clamps. Additionally, height brackets were installed when need arose. Finally, the load was zeroed once the clamp was installed. Then the up and down arrows were pressed until the clamps were almost touching. The reset gauge is adjusted to zero the position of the clamps. The up and down arrows were used until the clams were about 100mm apart to form the typical gauge length for the dog bone samples. A sample of metal was placed the tensile test machine grip. Holding sample, the other hand was tightening the machine. At approximately 3 mm away from the gauge-length of the specimen, the specimen was gripped such that the two ends of the specimen were covered by the grip. Slipping course errors, the specimens were held tightly. The specimen was confirmed to be vertically aligned to prevent the occurrence of torsional force instead of axial force. The bottom handle was tightly turned in the “close” direction. The sample was then visually verified whether it was gripped symmetrically at its two ends. Lastly, the zero extension button at the top of the screen was used to zero the extension. In the tensile testing the geometry of the sample was entered. Thereafter it was noted that 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. Note was taken on the failure mode when the specimen fails. The machine had an automatic setting that enables it to stop once the sample was broken. Then the “Return” button on the digital controller was pressed to allow both the upper and lower grips to automatically return 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. The file was then saved once the process was over and the Finish icon clicked. It exported the data into a PDF and individual data files. Thereafter, there was the cleaning up of the broken fragments from the specimen. Results and Analysis The results obtained from the displacement produced from the applied load were tabulated as shown below. Fig. 3 Table of results from metal 2 and the chart of load against displacement for the metal 2 Fig. 5 Table of results from metal 3 and the chart of load against displacement for the metal 3 Recorded results The load at which metal 1 yields 6.0kN The maximum load for metal 1 8.3kN The load at which metal 2 yields 1.1kN The maximum load for metal 2 1.5kN The load at which metal 3 yields 10.5kN The maximum load for metal 3 19.0kN Discussion of Results The results clearly show that metal 2 that yields at an approximate load of 1.1kN experiences yielding before the other two metals. Metal 1is the next in the list with a yielding point at an approximate load of 6.0kN hence the strongest. Lastly, metal 1 tops the list with a yielding point at 10.5kN of load. Similarly, the maximum load carrying capacities of the metals follow the same trend. Metal 3 has the highest load carrying capacity with a maximum load capacity of 19.0kN. Metal 1 follows with a maximum load of 8.3kN. The metal at the bottom of the list is metal 2 which has a maximum load support of 1.5kN. Additional sources compared to the features of the three metals indicate that metals 1, 2 and 3 could be Al, Cu and steel respectively. The steels tensile abilities (truant) is very high compared to the other metals while copper, being malleable, has a lower maximum load carrying capacity. Al forms the intermediate metal in the list. Conclusions and Recommendations Different metals have their tensile strengths varying according to the amount of load they can effectively support per unit area. Metals with high tensile strengths have various uses in the field of structural engineering. Whenever a metal is subjected to a much higher external load than its maximum load carrying capacity, it experiences failure. Upon failure, the metal develops cracks and even breaks. The yielding point and maximum load that any metal can support are taken into consideration when selecting the suitable metal to use in designing structures. Engineers take into consideration the tensile strengths of metals in order to develop strong and durable structures. Reference Brandon, D., Chaim, R. and Rosen, A. (n.d.). Strength of metals and alloys. London: Freund. Davis, J. (2004). Tensile testing. Materials Park, Ohio: ASM International. Gensamer, M. (1941). Strength of metals under combined stresses. [Cleveland]. Gifkins, R. (1983). Strength of metals and alloys (ICSMA 6). Oxford [Oxfordshire]: Pergamon Press. Grover, H. (1954). Fatigue of metals and structures. Washington: U.S. Govt. Print. Off. Han, P. (1992). Tensile testing. Materials Park, Ohio: ASM International. Ivanova, V. and Gordienko, L. (1968). New ways of increasing the strength of metals. London: Iron & Steel Institute. Murakami, Y. (2002). Metal fatigue. Oxford: Elsevier. Tensile testing. (2003). Metal Powder Report, 58(12), p.6. Truckenmiller, W. (1949). Tensile testing experiments. Ann Arbor, Mich.: Cushing Malloy. Appendix Double click the table to view all the data and calculations in excel spread sheets In setting up the experiment: It is paramount to wear safety glasses before beginning the experiment since broken pieces of metals can cause harm to your eyes. Additionally, gloves help protect the person conducting the experiment from any residue on the machine and samples. A photograph of a tensile machine Instron 5569 Read More