The Behaviour of Metals - Assignment Example

AIM The goal of this report is to explain the behaviour of varies metals under tensile loading and to determine their properties via stress-strain curve. These curves reveal many properties of materials like: elastic modulus, tensile strength yield strength, fracture stress, and elongation percentage. The report, also, aims at giving detailed comparisons between resilience and toughness for each of the test pieces. The report will also include a discussion of varies factors that have the possibility of affecting the experimental measurements. INTRODUCTION Tensile testing, also called tension testing, is a basic material science test which involves subjecting a sample a sample to a controlled tension until failure. The following three properties are directly measured via a tensile test: maximum elongation, ultimate tensile strength, and reduction in area. The following properties can also be determined from these measurements: Youngs modulus, strain-hardening characteristics, and yield strength. Determining such parameters of a material is usually crucial for Engineers in the process of designing or constructing. The 30kN Universal Testing Machine will perform the tensile test on grade 250 mild steel. The specimen will be held and subjected to steadily increasing uniaxial tensile load until failure, i.e. until fracture occurs. The applied load and displacement of specimen are observed through extensometer and recorded. The stress and strain can be calculated by the formulas, and, therefore, plotted against each other as shown below. Engineering Stress: Applied load divided by the original cross-sectional area. Where Engineering Strain – Change in length divided by the original length. Where Formulas used for the results section to calculate Young’s modulus, yield strength, % elongation and Ultimate Tensile Strength (UTS) are shown below as well as a graphical explanation of each calculation. Young’s Modulus: Stiffness of the material when the material is undergoing deformation, i.e. the gradient of the straight line, this is usually measured in GPa. Where Breaking Strength – refers to the amount of stress a material can withstand before breaking. Yield Strength – refers to the Plastic deformation that occurs at a specific amount of stress (0.2% plastic strain). This usually measured in MPa. Tensile Strength (ultimate) – refers to the maximum amount of tension a material can withstand. This usually measured in MPa. % Elongation – This is determined by the change in length divided by the original gauge length. APPRATUS Twin motorises ball screw universal test machine Maker: Instron Model: 3367 Serial number: R4202        Mild Steel (Grade 250)        Aluminium 6061        Copper (C12200)        Brass (C380-58.5%Cu, 39%Zn, 2.5%Pb) PROCEDURE 1. Measure and record the dimensions of the specimen. 2. Calculate and record the cross-sectional area. 3. Place the material in the universal testing machine, with an original length of 25mm. 4. Determine the load applied and displacement values using extensometer. 5. Calculate the stress and strain to plot a graph. 6. Young’s modulus, Ultimate Tensile Strength, yield strength and elongation percentage can be determined from the graph of stress vs strain. RESULTS After undertaking tensile strength tests for each metal, Figures 1 to 4 below were extracted. Figure 1 Stress - strain curve for brass Figure 2 Stress - strain curve for copper Figure 3 Stress - Strain curve for Aluminium Figure 4 Stress - Strain curve for Mild steel Calculation for young’s modulus, yield strength, ultimate tensile strength, percentage elongation and stress at fracture are calculated using the formulae below, respectively for each of the 4 metal samples. E= %EI (Percentage Elongation) = The table below states tensile properties of each metal samples; Test piece and composition Elastic Modulas (GPa) Yield Strength (MPa) UTS (Ultimate Tensile strength, MPa) Fracture Stress (MPa) Elongation (%) Fracture Appearance Mild Steel 146.5 293 370 245 42.16 ductile with some necking Aluminium 55.7 120 133 4 16.32 Ductile with some necking Copper 95 230 244 189 43.52 Brittle Brass 85 180 357 291 56.72 Brittle Table 1 Tensile properties of test specimens Resilience and toughness are some other properties to consider in this test result. Formula for calculating resilience is as follows; While for toughness, the formula used is; Toughness= Material Resilience (MPa) Toughness (MPa) Mild steel 0.29 102.9 Aluminium 0.12 0.64 Copper 0.29 83.16 Brass 0.19 165.87 Table 2 Results of resilience and toughness for four test specimens Numerical calculations of each property covered so far can be done with the reference to the data in appendix section of this report and with the guidance of the graphs of figures 1 to 4. DISCUSSION Tensile properties such as Young’s modulus (E), Yield strength, Percentage elongation, Ultimate tensile strength and fracture strength in each test specimen were obtained as an average depending on the date generated using Microsoft-Excel. In contrast to the average range of values for four test specimens and standard data, the variance in two sets of data might be because of number of tensile test trials, as a good estimate of data is obtained from averaging multiple experimental trials. Equipment used in this experiment might have been calibrated inaccurately. For example, Extensometer might be initially set to zero with and uncertainty of 0.1%; hence, readings for extension not accurate as compared to the standard tests taken more precisely. Because of these uncertainties, values for strain could be slightly different from the standard test results for all four test specimens. Another reason for the results to not be justifiable might be when resilience of brass and copper was calculated. As a result of their yield strength point being difficult to note, an offset of 0.02 % parallel to the linear slope of stress against strain was also doubtable when it was used to calculate resilience. From the findings obtained in the results section of this report, it can clearly be seen that the graphs for copper and brass exhibit a property of brittle material. This is because of Young’s modulus for both copper and brass, that is 95 GPa and 85 GPa respectively are relatively low compared to Mild steel, 146.5 GPa and aluminium, 55.7 GPa. The result of low Young’s modulus describes stiffness of a material, Low Young’s modulus value shows a stiffer material and vice versa for less stiffer material. Stiffer materials are subjected under high stresses with, very definite, small change in length. Aluminium has lower Young’s modulus, but its stress at fracture is relatively low compared to copper, brass and mild steel, and for that reason shows a ductile characteristic rather than brittle. Ductile material undergoes significant plastic deformation, and these can be demonstrated with the aid of the figures 1-4 in the results of this report. Mild steel has a higher yield point and undergoes significant plastic deformation; such a material is useful in drawing into sheets of metals or wires because of it ductile characteristic. Resilience of a material refers to the measure of energy absorbed by a material when it is elastically deformed and recovers the energy upon unloading. Graphically, Resilience is area under the linear graph of stress against strain. Linear part of the graph describes Young’s modulus of elasticity. Resilience differs from metal to metal, for instance resilience for mild steel and copper are same, 0.29 MPa. Despite copper and mild steel recorded to have same resilience, their Young’s modulus and yield strength are different. Resilience is entirely dependent on yield strength and young’s modulus. Aluminium and brass have resilience that is noticeably different; such a difference could be different yield points/ yield strength, a material can start yielding, and that is plastic deforming at a small or large change in length with respect to its corresponding stress value. In a manufacturing industry, resilience property of a material is a good measure of material considering the amount of energy it can absorb while loading until the yield point. An example of such a material is Aluminium, which is used as a source of raw material to manufacture springs or other objects relative to elastic potential energy. Materials with high elastic potential energy can be useful in absorbing significant energy and conserve it when unloaded. Toughness refers to the measure a materials ability to absorb energy up to a point of fracture. It is the entire region under a stress-strain curve. Toughness should not be confused with ductility, a tough material has a combination of high strength and high ductility. In table 2 of this report’s findings, toughness for each material is recorded with Brass having the highest value to Aluminium with the lowest. CONCLUSION It can be concluded, from the findings in this report, that, upon numerous tries on testing tensile properties for each specimen, very accurate stress against strain graph can be plotted, which can be the source of all other characteristic calculation of a material example; Toughness, Resilience, Yield strength, fracture stress, ultimate tensile strength and young’s modulus of elasticity. With the test undertaken using a uniaxial tensile load, results of test specimens obtained in this report might be different when same tests are undertaken using a Bi-axial tensile load, although each graph for tensile stress against strain for four materials are similar in terms of the shape of the graph and its characteristics. Percentage elongation for each material can be considered as a brief guide to the extent a material elongates until point of fracture. Table 1 in results section of this report summarises values of percentage elongation for each test specimen; these values show how ductile a material is or how brittle a material is. Read More