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

Running Head: Title of work Name Name of Instructor Subject Institution Date Laboratory Experiment on stress-strain behaviors of mild steel and high steel bars Abstract The study of strength of materials is an important field in construction industry. This is because it aids in determining the right material to be used. Failure to analyze the characteristic of the material, various challenges may arise such as, collapsing of buildings and structures Hartog (1977, pp113-115). The structures carry loads and they have weight exerted on them. This calls for the knowledge of the characteristic type of material which is used for a particular function. The relationship between the stress- strains has to be established to determine points where a given beam would yield to stress or to ascertain the maximum strength that a given material can with hold. Introduction The knowledge of strength of materials enables the engineers select the proper material to be used for a specific function. Analysis has to done on the behavior of various structural materials. In this experiment, mild steel and high yield steel are used to determine their characteristic behaviors. The mechanical properties such as; the yield stress, young modulus maximum tensile strength, stress at failure and percentage elongation may are determined. Tensile test is performed so as to generate a of stress-strain graph. Graphical analysis assists in the determining of the modulus of elasticity and values of the maximum stresses. Stress of a material is supposed to take values within a given limit of proportionality to avoid the collapse of a given structure. The loading of a given material tends to be proportional to the strain of a given material. This exhibited more so in steel materials. Materials would usually yield to a point normally known as elastic limit, of which the material is able to withstand the load. Further addition of the load causes the material to reach the rupture point, where the material properties are interfered and, the material becomes deformed. Experiment Method Various apparatus were used to perform the experiment. They included: a 500KN Denison testing machine, a Denison extension and extensometer gauge, a grade 250 plain round mild steel bar that has a diameter of 20mm and strength of 250N/mm2. This material follows the specifications of BS-4449. Also another high yielding deformed reinforcing steel bar was also used. It has a grade of 460 and a diameter of 16mm. It also, has a tensile strength of 460N/mm2 and, follows the specification of BS-4449. The bars were placed in the jaws of the machine then; a 50mm extensometer was attached to the load bar in which the reading is adjusted to zero. Different loads were put under the instrument while, the values were recorded till the point of failure is attained. Reading from the extensometer and densitometer were recorded. During the process, the extensometer was removed, while the densitometer continued to perform the experiment. Failures values were noted and, tabulated the values of the load and the extension of each material. The values obtained are plotted to yield a load vs. extension graph and the stress vs. strain graph. Result Table 1 Table of load in (KN) and extension in (mm) of a plain round bar. Load(KN) Extension(mm) Load(KN) Extension(mm) 9.6 0.011 120.2 1.090 20.3 0.043 124.4 1.392 30.4 0.064 128.1 1.750 39.0 0.077 131.7 2.394 51.9 0.099 141.7 4.082 61.0 0.115 149.7 5.973 71.2 0.128 160.1 9.200 82.2 0.144 162.5 10.086 89.6 0.155 164.3 10.742 99.9 0.171 165.7 11.439 109.9 o.187 166.3 11.767 112.2 0.195 167.9 12.751 115.0 0.203 168.9 13.735 116.7 0.211 169.2 14.063 116.9 0.221 169.7 15.047 116.9 0.232 169.9 16.086 117.2 0.243 169.7 18.304 116.7 0.256 169.4 18.656 116.5 0.400 168.9 21.120 116.1 0.505 166.5 23.250 117.9 0.750 162.1 25.064 158.6 26.460 154.3 27.888 143.4 29.812 136.7 31.380 129.0 32.922 Graph 1. Graph of load vs. extension of a plain round bar Table 2 Load vs. extension of a high yield reinforcing bar Load(KN) Extension(mm) Load(KN) Extension(mm) 11.0 0.027 109.0 0.710 20.0 0.069 110.3 2.072 30.0 0.107 112.9 4.158 40.9 0.141 114.5 5.184 50.5 0.168 116.0 5.712 59.6 0.192 117.3 6.125 69.4 0.219 118.6 6.552 80.3 0.243 119.8 7.144 88.9 0.261 120.8 7.410 100.0 0.288 122.6 8.569 102.7 0.296 124.0 9.143 104.4 0.301 126.3 10.783 106.2 0.306 126.5 11.070 107.2 0.312 126.7 11.890 107.8 0.317 126.6 12.177 108.0 0.328 126.3 13.020 108.0 0.336 126.1 13.948 108.1 0.344 125.8 14.212 108.2 0.352 125.3 15.888 107.5 0.357 124.7 16.850 109.1 0.365 123.6 17.200 122.0 17.550 119.9 17.850 117.1 18.200 113.9 18.550 109.9 18.950 103.0 19.250 Graph 2 Graph of load vs. extension of a high yielding reinforcing bar. Graph 3 Round plane stress vs. strain graph Graph 4 Graph of stress vs. strain of a high yield reinforcing bar. Graph 5 Graph of stress vs. strain of a plain round bar using Extensometer values Graph 6 Graph of stress vs. extension of a high yielding bar from the values of an extensometer Calculations 3. Determination of a Yield stress I. Round bar At 0.002 of strain the yield stress of round bar is 342N/mm2 II. High yielding bar At 0.003 strain of high yielding bar is 524N/mm2 4. Elastic modulus Round bar  The elastic modulus of round steel is 193KN/mm2 High yielding bar KN/mm2 Young modulus E from extensometer values KN/mm2 Young modulus of a high yielding bar from the extensometer values KN/mm2 5. Maximum tensile strength Max tensile strength =  Maximum strength for round bar N/mm2 Maximum strength for a high yielding bar N/mm2 6. Stress at failure For round bar 376N/mm2 For high yielding bar 527N/mm2 7. Percentage elongation failure For plain round bar For high yielding bar Discussions As observed from the experiment, the load is proportional to the extension till a certain elastic point reached. At this point, the values represent the maximum loads that can be exerted on the material without fracturing or collapsing. The stress and strain values gotten from the experiment seem to be lower than the standard values for instance; the yield strength of steel which is 212N/mm2 Hartog (1977, pp109-112) because, of the experimental error that may be encountered. The high yielding material seems to be more ductile as compared to the plain round bar. The strain value for round bar at yield point is 0.00208 while at collapse point is 0.16086. On the other hand, the strain value for the high yield bar is 0.03826 at yield point and 0.162575 at collapse point. These values are of importance, because the ductility of a material enables it to withstand some certain load before it ruptures. This may reduce the risks of damaging other materials or causing accidents. In construction industry the knowledge of yield strength is important as it enables the right material to be selected to perform effectively. Proof test of 0.2% is important because some materials such as mild steel usually don’t have yield point Hartog (1977, pp121-123). This test tends to determine the yield point of unequal extension by plotting a straight line. The high yield bar would be suitable for construction as it is able to withstand more loads before it collapses. This is because it has high yield strength of 559.4 N/mm2 whereas the round bar has yield strength of 375N/mm2. Sources of error in the experiment During the experiment the results found were affected by either random or systematic errors. The random errors emanated from the environmental conditions such as, hot weather conditions that altered the setup of the apparatus. Also parallax reading could result making wrong values to be calculated. In addition, the systematic errors that could have resulted included poor calibration of the apparatus. Moreover, the unwanted movements of the apparatus could lead to altered figures. References Hartog, D. J., 1977. Strength of materials: special beam problems. Mc Graw hill, U. S. A Appendices Stress vs. strain values for a round bar Testing machine Read More