The Tensile Test
For better consideration of properties of things or materials and their behaviour under pressure, tensile testing forms the basis of the knowledge. Two materials were selected for testing in the lab that is aluminium and Mild steel (with 0.1, 0.4 and 1% of carbon). These were tested thrice for the sake of precision in the readings with the most commonly used apparatus. The properties to be measured included cross-sectional area, Tensile strength, Yield Strength, the Percentage reduction in area and Percentage elongation at break. These calculations helped in comparing the materials selected for the experiment and describing their properties regarding malleability and brittleness. The tensile testing of these materials indicated that mild steel was the strongest material among the three with maximum ultimate tensile strength (451.77MPa). The area reduction was largest in mild steel (62%). Aluminium had a Maximum yield strength of 273.5MPa. Hence concluded that Aluminium and mild steel were revealed to be ductile.
Introduction and theory
Tensile testing is among the best necessary tests in the field of engineering. It delivers valued facts related to the material under discussion and its concomitant properties. Engineering structures are designed and analyzed based on these properties. Also, fresh materials are developed for the required usage. In tensile testing, a material is exposed to controlled tension while waiting for failure. The outcomes of the test are then utilized to pick the material for use and foretell the behaviour of the material under certain circumstances. Properties to be analyzed by tensile testing include elongation, area reduction and tensile strength. Among the two types of tensile testing, Uniaxial is used to measure mechanical characteristics while Biaxial testing is used for composite materials and textiles. The elements that are tested include metals, paper, adhesives, fabrics, etc.(Ritchie et al., 1973).
The purpose of tensile testing is to check the strength of the material and its susceptibility to tolerate pressure. It could be based on elongation value. It can be used to compare different materials.
Choice of material is the most important step in the process of designing. The right choice of material is the most basic task for a specific design. The tensile test is a customary process in engineering to describe the elasticity variables of elements.
The Selection of the material is the main task of the designing process. Engineering has to decide which material that they are going to use in the designing process. The tensile test helps to decide which material should the engineer use to design a particular product. It helps to characterize the plastic and elastic variables that relate to the mechanical behaviour of the stuff.
Tensile testing was piloted by Load Frame equipment and analyzed by data acquirement software. Three, unlike materials, were used comprising Aluminium, Mild Steel with different percentages of carbon. The gage section was reduced as the samples were rod-shaped. This is to ensure that greater stresses occur not near the grips but within the gage. The samples were prepared according to the dimensions of the test, according to the ASTM standards.
The universal testing machine is the commonly used machine for tensile testing. The machine is adjusted with crossheads for length and tension of test specimen. Some machines are hydraulic, and some are electromagnetic. The machine is analyzed by four parameters: Force capacity, Speed, Accuracy, and precision. The most important factor for the functioning of the machine is alignment(Tetelman et al., 1968).
Material selection for the sample is one of the most important tasks of the overall designing process. Engineers need to decide that which material is appropriate in the making of a particular product. The Tensile test helps engineer for the selection of a particular material. The test helps to characterize elastic and plastic variables that are linked to the mechanical behaviours of the material.
The procedure involves putting the test sample into the testing instrument and then slowly puts pressure on it until it breaks or fractures. In this process, the change of length is measured against the applied force. The data is analyzed, and the gauge area is measured by force applied. The measurements of the change of length are used to compute the value of ε (engineering strain).
Whereas ΔL represents gauge length’s change, and L is the length after applying the force and L0 is the initial gauge length. The calculations of the force help to measure the value of σ (engineering stress).
The above graph shows the tensile stress of steel with different composition of carbon. It shows the expansion of the material when the load is applied to it. The graph shows that the expansion is less at the start but increases significantly when applied load is more significant then 10000 Newton.
Where F represents the tensile-force, and A represents the total area of the material. The machines do these measurements as the force upsurges so that the data-points could be plotted in Young’s modulus (stress-strain curve).
The comparison was made of the three materials that were tested for tensile strength, i.e., Aluminium, Mild steel with different compositions of carbon. The two materials showed the relation between them in a very precise way.
The cross-sectional area was measured before and after applying force, and it was observed to be almost same for Aluminium and steel with different percentage of carbon.
The highest tensile strength was found to be of Steel with difference percentage of carbon with a mean value of 176.16MPa with the ultimate tensile strength of value 451.77 MPa.
Aluminium showed more yield strength than Mild steel with different percentage of carbon, with an average of 273.5 MPa.
Above table shows the maximum amount of load that aluminium can hold and the expansion of aluminium when maximum load is applied.
Tensile Stress = 7519 / 5.06 = 1485.9
Reduction in the area:
When pressure force is applied to the materials, the highest reduction in area was found to be 62% in mild steel with different percentage of carbon.
Tensile Stress of Steel with 0.1% carbon = 7950 / 4.90 = 1622
Tensile Stress of Steel with 0.4% carbon = 13317 / 5.01 = 2658
Tensile Stress of Steel with 1% carbon = 18331 / 5.02 = 3651
Elongation at break:
The force was applied to check the elasticity; the minimum was found in aluminium with a value of 17%.
The Carbon made fibres, and its composite materials have the more tensile strength and more modulus of elasticity as compare to other materials. All of them breaks up in a brittle manner, and the curve remains linear until the material breaks or fractures without any bending of the curve at the high-loads. Therefore there will be no everlasting modifications in the original shape in this test, and hence there will be no ductility.
Figure 1(Jen et al., 2011)
The Young’s Modulus (Stress-strain graph shown below) shows that the aluminium material has more plastic deformation as compared to that of steel with different percentage of carbon materials and the higher percentage elongation reflects this result. After having fractures on its surface, the surface of aluminium turns out to be very rough and irregular. The two halves of the fractured material have shown the cup and the cone shape which are inclined at the angle of 45 degrees on the surface of the fractured material surface. In the uniaxial tensile test, the orientation has represented that the angle of the principle shared the stress and the surface of the material obeys this principle. Crystalline boundaries slip into each other before the fracture due to the shear stress(Verbridge et al., 2006).
The Characteristics of the ductile materials, a common property of the aluminium, the aluminium material have more toughness and hardness as compare to steel with different percentage of carbon material which is represented by the larger area in the stress-strain graph shown above. Although it is smaller than that of copper material, the plastic area of the steel sample is large enough that it is deliberated as to have some ductile properties. The fractured surface has a cup and cone geometry at much lesser context as compared to that of the aluminium material. The steel material has a more necking region as that of the aluminium material. It results in the reduction of the overall area of the fracture as shown in the below figure. Mild Steel material shows a rapid transition whereas aluminium shows a gradual transition. Necking is also a property of a ductile material.
Figure 5(Tepfers, 1979)
The Experimental value of Modulus of Elasticity obtained is nearly one order of magnitude lesser as that of the values in the engineering material. This is around 176Gpa for mild steel, 105 GPA for the aluminium material. The Yield Strength for mild steel, are aluminium 31.9 Mpa and 273 Mpa respectively. The calculation of the Modulus of the material by using uniaxial tensile stress experiment is not considered as accurate, and its value is calculated by the natural frequency of the material with the help of oscillation test. The main reason for this is because
- By noticing the small movements and motions of the material is inexact due to the inaccuracy of the measuring tools
- The features, for example, creep could take part in the strain.
- When applying large forces the tools could star flexing, and the movement of the tools and instruments is being read inaccurately as a movement of the material/sample.
The above graph shows the tensile stress of aluminium. It shows the expansion of the material when the load is applied to it. The graph shows that the expansion is less at the start but increases significantly when applied load is greater than 7000 Newton.
The final readings of the tensile stress taken are very near to that of the theoretical values. The difference of the reading of the theoretical and practical values for the Young Modulus has suggested a very little confidence in the results. In conclusion, copper could be considered as more Ductile material as that of steel with more toughness and steel is deliberated to have a more tensile stress and higher yield with the equal modulus.
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