Determination of the Mechanical Properties of Metallic Materials Using Tensile Testing - Lab Report Example

Determination of the Mechanical Properties of Metallic Materials Using Hounsfield Extensometer Introduction Mechanical properties of materials are determined by a procedure of tensile testing using Hounsfield Extensometer which provides engineers with quantitative data for various works like design in engineering. Samples are prepared according to the relevant British Standard and tested to failure so as to produce stress-strain curve which give information on the mechanical properties of the material being studied (BS EN 10002-1:2001). Callister (2007) argues that a stress-strain curve is produced after calculating the stress (applied force/ cross sectional area of specimen, N/m2) using the original cross sectional area, the strain is calculated by dividing elongation or length increase over the original length of specimen, it is dimensionless . A stress-strain curve is shown in Excel pages in which a straight line portion indicates elastic deformation limit that is reversible when applied load is removed. The elasticity limit is defined by yield stress level (σy) which is determined at the intersection of stress-strain curve and a guide-line that is drawn parallel to the linear portion of the curve as illustrated in fig 1(b) with a 0.002 offset of strain (0.2 percent change in length) (BS EN 10002-1:2001). Some low-carbon steel Stress-strain behavior display a “double yield point” in which the lower yield limit is always considered according to Askeland (1997). Beyond yield point or elastic limit, a plastic and permanent deformation occurs, i.e. not reversible when applied external load is removed. The maximum load to be supported by a sample is called ultimate tensile stress (UTS). Further pulling causes the “necking” of the specimen causing a continual decrease of the cross sectional area lowering force or stress that is required and finally leading to failure or breakdown. Tensile test machine in G118 (Hounsfield extensometer) is controlled to maintain it’s pulling speed regardless of required load to deform the specimen being tested. Askeland (1997) states that, final cross sectional area and specimen length measurements yields information on the level of ductility of material. Ductility is the ability of the material to undergo plastic deformation due to tension. Aims of the practical session 1) To establish the mechanical properties or parameters of various engineering metals by use of a standard tensile test machine in accordance to relevant British Standard 2) To develop the relationship between the material’s behavior and the parameters Objectives 1) To Conduct standard tensile test in accordance to the relevant British Standards on a range of materials like; 0.1 percent carbon steel (normalized), 0.8 percent carbon steel (normalized), 3.5 percent grey cast iron, 70/30 brass (drawn) and 70/30 brass (annealed). 2) To gather the test results, produce stress-strain curves and analyze the mechanical properties 3) To produce a report including application examples of some techniques in industry Laboratory Set-Up & Method 1) The following specimens were provided: 0.1 percent carbon steel (normalized), 0.8 percent carbon steel (normalized), 3.5 percent grey cast iron, 70/30 brass (drawn) and 70/30 brass (annealed). 2) The specimen parallel diameter and length were measured and recorded using a vernier caliper. 3) The specimens were loaded into the holding jaws of the Hounsfield extensometer one at a time. The distance travelled and load was reset to zero each and every time before we started another test. Fig.1 Hounsfield extensometer Fig.2 tested specimens. 4) The specimens were tested to failure by applying the loads or force slowly with a speed of 3mm/s for hard metals and 6 mm/s for soft metals. The Hounsfield extensometer that was used maintained a constant pulling speed in entire test. 5) The extensometer was stopped after every specimen failing. The data gathering software was loaded on the PC which allowed us to save and display results in a graphical format. The test results were then sent to the data acquisition computer. The data acquisition system recording speed was 5 data points per second. 6) The broken specimens from the jaws were removed and the final diameter of specimen at the neck region was measured and recorded. Test Results & Discussion The gathered data of every specimen was used to draw a graph of force applied against the measured elongation of the specimen (mm). The readings were converted into engineering stress values (σ in N/m2 or Pa) and engineering strain (ε). The engineering stress was calculated by dividing applied force over the original cross sectional area. The engineering strain was gotten by dividing elongation or extension over the original length. The engineering σ-ε curves were plotted in graphical format in Excel and corresponding yield and ultimate tensile stresses for each material were identified. The graphs are in Excel pages with their corresponding yield and ultimate tensile stresses. Stresses Ultimate Tensile Stress(MN/m2) Yield Stress(MN/m2) Worked Brass (60-40) 1. 566 442 2. 547 453 3. 359 446 Annealed Brass (60-40) 1. 484 368 2. 494 391 3. 482 373 3.5%CarbonSteel 1. 266 266 0.8%CarbonSteel 1. 1021 936 2. 1030 953 3. 1056 952 0.1%CarbonSteel 1. 520 373 2. 515 375 3. 513 377 The ductility of the material was calculated for each material using the elongation difference. The formula used was: . The change in cross sectional area was also used to calculate ductility: . The ductility of the materials is; the results were tabulated as shown in table below. Specimen Original Diameter (mm) Final Diameter (mm) Original Area Final area Ductility (%) Worked Brass (60-40) 5 3.28 19.64 8.45 57 Annealed Brass (60-40) 5 2.9 19.64 6.60 66.4 3.5%CarbonSteel 5 - 19.64 - 0.8%CarbonSteel 5 4.11 19.64 13.27 32.4 0.1%CarbonSteel 5 3.15 19.64 7.79 60.3 Discussion The ductility of the metal lowers with increase of the carbon content. This shows that when carbon content is increased in steel metals, the ability of the metal to undergo the plastic deformation reduces greatly. This affects properties like brittleness, when carbon content is high, brittleness also increases. 0.1 percent steel has the highest ductility among the tested specimens; this is due to low carbon content in it. The carbon content in metals also affects the tensile strength of the steel. Ultimate tensile strength increases with increase with increase in carbon content. 0.8 percent carbon has highest ultimate strength. Generally, as carbon content increases, metals becomes harder and stronger but difficult to weld and also less ductile. Yield strength of steel increases with increase in carbon content, this is because carbon atoms do fit into the sites of interstitial crystal lattice of body centered arrangement of atoms of iron. The interstitial carbon to a great extent does reduce dislocation mobility which subsequently has hardening effect on iron. For the dislocations to move, high level of stress has to be applied for dislocations to move because interstitial atoms of carbon cause iron BCC lattice cells distort. Engineering stress (MN/m2)vs Engineering Strain(m) suffered by the 3.5 carbon steel Discussion 3.5 carbon steel has increased the yield stress. Yield strength of steel increases with increase in carbon content, the carbon atoms do stay into the sites of interstitial crystal lattice arrangement of body centered atoms of iron. The cold working on brass increases its hardness. Hardness of brass is proportional to yield stress. Cold worked brass have high ultimate tensile and yield strength as compared to Annealed brass. A cold worked brass has increased the yield stress. Annealing increases the ductility of the brass. This is compared to 66.4 percent of annealed brass to 57 percent of cold worked. The annealed metals have the microstructures altered hence affecting the structure and characteristics of brass. Conclusion There is high ductility in steel with high carbon content than in brass. Annealed brass also has high ductility compared to cold worked brass. This shows that heating of brass makes them effective towards drawing and forming than cold worked ones. Tensile testing is mainly carried out to produce data to be used for purposes of design or procedure of material joining. With tensile testing data, structure limiting values that can make withstand failure are known. The test is important so as to control the safety of the buildings and other structures build using these steels. Reference Askeland, D 1996, The Science & Engineering of Materials. Chapman & Hall, Michigan. Callister, W 2006, Materials Science & Engineering, John Wiley & Sons, Michigan Higgins, R 2006 Materials for Engineers & Technicians, Elsevier, England. The British Standard Institution 2001, Tensile testing of metallic materials, The British Standard Institution, London. Appendices Fig 1 (a) – Adapted from Fig 6.11, Callister, 2007, page 145 on Typical engineering stress-strain curve where M and F denote the ultimate tensile and fracture stress respectively. Fig 1 (b) Adapted from Fig 6.15, Callister, 2007, page 149, on the Determination of the Young’s modulus on the stress-strain curve. Read More