Strain Behavior of Mild Steel and High Yield Steel Bars - Lab Report Example

Lab Report Table of Contents Page 0 …………………………………………………………..3 2.0 Introduction………………………………………………………...3 3.0 Apparatus…………………………………………………………..4 4.0 Procedure…………………………………………………………...4 4.1 Experimental Set up…………………………………………...4 5.0 Results……………………………………………………………....5 6.0 Analysis……………………………………………………………..7 6.1 Elastic Modulus for Extensometer……………………………...11 7.0 Discussion………………………………………………………….13 8.0 Conclusion…………………………………………………………13 References……………………………………………………………..14 Appendices…………………………………………………………….14 Lab Report: Stress-Strain Behavior of Mild Steel and High Yield Steel bars 1.0 Abstract The purpose of this experiment is to investigate the mechanical behavior of structural steel under different loading weights. The specimens are then tested under the Denison Testing Machine, Extensometer and Denison extension gauge. The major observations made with the experiment from the physical properties are that; the characteristic tensile strength of the two sample bars. The bars exhibited necking down and ultimately fracture. This is similar feature observed in standard mild steel bars which are tested under increasing loading in order to study their flexural behavior. The samples under this study exhibited a relatively large deflection; high ductility and deformation prior to collapse. This report therefore presents a description of the properties of the plain round bar and the reinforcing bar. The calculations from these observations are highlighted. 2.0 Introduction A full understanding and behavior of the construction materials is an important aspect in engineering design. The physical properties of structural materials are expected to meet the demand of the fundamental assumptions defining structural rules that guide designs. Mild steels are employed in a wide array of engineering applications. Therefore, the study of mechanical characteristics at various strain rates is important. This is in order to improve the safety of equipment against impacts and crashing. The characteristics of steel are mainly determined by its chemical composition, manufacturing method and heat treatment. But, there are some physical properties that also influence the behavior of steel; such as Young’s Modulus of elasticity, ultimate strength and yield strength and percentage elongation. Knowledge of the behavior of structural steel under load is crucial to ensure that structural collapse does not occur and that serviceability requirements are put in place. To this end, the following mechanical properties of a material are required. The yield stress The elastic modulus The maximum tensile strength The stress at failure The % elongation at failure 3.0 Apparatus 1. 500kN Denison testing machine 2. Extensometer and Denison extension gauge 3. Grade 250 plain round mild steel bar, 20mm diameter Characteristic strength=250 N/mm2 Conforms to BS 4449 4. Grade 460 deformed high yield steel. Reinforcing bar T16, 16mm diameter. Characteristic strength = 460 N/mm2 Conforms to BS 4449 4.0 Procedure Each of the bars in turn is placed in the jaws of the testing machine The 50mm extensometer is attached to the bar and set to zero. The load is then applied and recorded in constant increments up to failure. For every load applied, the extension readings from the extensometer and the Denison extension gauge are noted. At the yield point, the extensometer is removed to avoid damage on it while readings continue on the Denison extension gauge. The load at failure point and the manner of failure are observed. 4.1 The Experimental Set Up 5.0 Results For 20mm Plain Round Bar Table 1: Results Load Dennison Extensometer (kN) Extension Extension   Gauge (mm)   (mm)   9.6 0.011 0.0021 20.3 0.043 0.0169 30.4 0.064 0.031 39 0.077 0.0431 51.9 0.099 0.0608 61 0.115 0.0743 71.2 0.128 0.0899 82.2 0.144 0.1077 89.6 0.155 0.1197 99.9 0.171 0.1444 109.3 0.187 0.1729 112.2 0.195 0.1815 115 0.203 0.1889 116.7 0.211 0.1923 116.9 0.221 0.1921 116.9 0.232 0.1934 117.2 0.243 0.194 116.7 0.256 0.2111 116.5 0.4 0.3555 116.1 0.505   117.9 0.721 120.2 1.09 124.4 1.392 128.1 1.75 131.7 2.394 141.7 4.082 149.7 5.973 160.1 9.2 162.5 10.086 164.3 10.742 165.7 11.439 166.3 11.767 167.4 12.423 167.9 12.751 168.9 13.735 169.2 14.063 169.7 15.047 169.9 16.086 169.7 18.304 169.4 18.656 168.9 21.12 166.5 23.25 162.1 25.064 158.6 26.46 154.3 27.888 143.4 29.812 136.7 31.38 129 32.922 For 16mm Reinforcing Bar Table2:Results Load Dennison Extensometer (kN) Extension Extension   Gauge (mm)   (mm)   11 0.027 0.0089 20 0.069 0.0285 30 0.107 0.0483 40.9 0.141 0.0714 50.5 0.168 0.0897 59.6 0.192 0.1045 69.4 0.219 0.1235 80.3 0.243 0.1425 88.9 0.261 0.1577 100 0.288 0.1815 102.7 0.296 0.2244 104.4 0.301 0.2516 106.2 0.306 0.2675 107.2 0.312 0.2823 107.8 0.317 0.2892 108 0.328 0.2953 108 0.336 0.2968 108.1 0.344 0.3143 108.2 0.352 0.4144 107.5 0.357   109.1 0.365 109 0.71 110.3 2.072 112.9 4.158 114.5 5.184 116 5.712 117.3 6.125 118.6 6.552 119.8 7.144 120.8 7.41 122.6 8.569 124 9.143 126.3 10.783 126.5 11.07 126.7 11.89 126.6 12.177 126.3 13.02 126.1 13.948 125.8 14.212 125.3 15.888 124.7 16.85 123.6 17.2 122 17.55 119.9 17.85 117.1 18.2 113.9 18.55 109.9 18.95 103 19.25 6.0 Analysis Figure 2 Figure 3 Figure 4 Figure 5 The yield stress Plain Round Bar (20mm) 348.089172 N/mm² Reinforcing Bar (16mm) 497.611465 N/mm² 0.2% Proof stress Plain Round Bar (20mm) 357.324 N/mm² Reinforcing Bar (16mm) 511.046 N/mm² Elastic Modulus Plain round Bar E= (318.15-226.75)/ (10-7) = 30.47 Reinforcing Bar E = (399.58 – 296.58) / (8-6) = 51.5 6.1 Elastic Modulus for Extensometer Figure 6 Figure 7 Reinforcing Bar E= (442.376 – 345.342)/ (9-7) = 48.517 Plain Round Bar E = (348.089 – 261.78)/ (11 - 8) = 28.7 Maximum Tensile Strength Reinforcing Bar = 497.611 N/mm² Plain Round Bar = 348.089 N/mm² Stress at Failure Reinforcing Bar = 512.539 N/mm² Plain Round Bar = 410.828 N/mm² Percentage Elongation at Failure Reinforcing Bar (127 – 80) / 80 * 100 = 58.75% Plain Round Bar (145 – 100) / 100 *100 = 45% 7.0 Discussion In view of the stress-strain curves resulting out of this experiment. The samples behave as if they have a definite spring constant according to Hooke’s Law. The stress-strain curve is linear in the elastic region. In the stress-strain curve so long as loading of the mild steel is done within the elastic region; the strains are totally recoverable and the sample will return to its original measurements as the load is relaxed to zero. Immediately the load value exceeds the yield point, gross plastic deformation, which is permanent, occurs to the specimen as load is gradually returned to zero afterwards. The stress-strain curves drawn from the tests in this experiment; point out to the fact that the low-carbon steel has a definite yield point. Also, determination of the offset yield strength was also done through drawing a parallel line to the elastic portion of the curve, starting from the 0.2% strain level. The comparison between the two yield points from direct observation and the yield offset method revealed similar results. Furthermore, the ultimate tensile strength values of the two tested samples show a 30% difference between these values while the percentage elongation has a 23% difference. From direct observation the modulus of elasticity for each experimental value differs from the expected by at least a third of the value. Consequently, the range for modulus of elasticity is very large. Comparing the reinforcing bar and the plain round bar, it can be noted that the reinforcing bar has a higher modulus of elasticity. The reinforcing bar values is higher than the plain round bar by at least 40%. 8.0 Conclusion The objective of the experiment was achieved. The modulus of elasticity, obtained from the slope of the linear region of the stress-strain graph, is a property that can be used to characterize the two sample materials. Two samples of mild steel bars metals, underwent the stress-strain tensile test performed using the Dennison Extension Gauge apparatus. From the data collected; a stress vs. strain graph was produced for each sample. With prior knowledge, the modulus of elasticity is the slope of the linear region of a stress strain graph. Through Excel, each modulus of elasticity was found by applying a linear curve fit to the selected data set for each sample. In a broad sense, the tensile strength and modulus of elasticity of reinforcing bar is higher than that of plain round bars. However, the modulus of elasticity for each sample was more than half of the expected values provided in the instructions manual. Possible errors that contributed to the large deviation in the experimental and expected modulus of elasticity values include the age of the samples while the tensile force was being applied, measurement uncertainties within the Dennison Extension gauge and sample slipping. Reference PUNMIA, B., JAIN, A. K., & JAIN, A. K. (2004). Basic civil engineering: for B.E. / B.Tech first year courses of various universities including M.D.U. and K.U., Haryana. New Delhi, Laxmi Publications. Askeland, D.R., (2007) The Science and Engineering of Materials, Fourth Ed. Appendix1: Experimental Set up Appendix 2: Results for 20mm Plain Round Bar Appendix 3: Results for 16mm Reinforcing Bar Appendix 4: Load Vs Extension Data Curve (For 20mm Plain Round Bar) Appendix 5: Load Vs Extension Data Curve (For 16mm Reinforcing Bar) Appendix 6: Stress against Strain (16mm Reinforcing Bar) Appendix 7: Stress Against Strain (20mm plain round Bar) Appendix 8: Stress against Strain (16mm Reinforcing Bar) for Extensometer Appendix 9: Stress Against Strain (20mm plain round Bar) for Extensometer Read More