Title:
Compression test.
Objectives:
1. To study the application of the strain gauge to obtain stress-strain curves. 2. To identify and determine the mechanical properties of given metal alloy by carrying out uni-axial tension-compression test.
Introduction:
A compression test is any test in which material experiences the opposing forces inward upon the specimen from opposing side or otherwise compressed, crushed, and flattened. The specimen is generally placed between two plates which are the compression test fixture; the load is distributed among the test sample surfaces and is compressed by applying force using the universal test machine. The test material while then experiences shortening in the applied forces direction and expanding in the perpendicular direction toward the applied force. The goals of compression test in this experiment are to learn the applications of universal test machine and strain gauge to obtain the stress-strain curves and identify the metal alloy. Compression test also allow the determination of the behavior of the metal alloy when it experiences the compressive load by measuring the variables such as stress and strain. In this experiment, uni-axial tension-compression test is used to determine the mechanical properties of the metal alloy.
Figure 1 Compression Test
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Figure 2 Structure of Universal Test Machine
Equipment and Materials:
Universal Testing Machine SHIMADZU 5kN, PC, 2 compression test fixtures (pressure plates), strain gauge, digital vernier caliper, and metal alloy.
Experimental Procedures:
1. A strain gauge was attached carefully on the test specimen. 2. The test specimen was positioned right at the center of the pressure plates. 3. The machine and strain gauge were connected to the PC to provide the data of force and strain with data logger DBU-120A unit. 4. The machine was operated slowly so that the pressure plates touched each other to completely close the gap while observing the indicator where force value showed 0kN. 5. The test specimen was compressed with the force of 2kN by setting the ending load on the data logger. 6. The strain of the test specimen was recorded in table below. 7. Steps 5 to 6 were repeated to obtain different strain by varying the ending load to 4kN, 6kN, 8kN, and 10kN. 8. A stress-strain graph was plotted.
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Results:
Diameter of test specimen, D = 25.35 mm
Force, F (N)
Strain, ε (×10-6 m/m)
0
0
2000
40.0
4000
75.0
6000
105.0
8000
122.0
10000
144.0 Table 1 Experiment Data
Stress, σ (MPa)
Strain, ε (×10-6 m/m)
Yong Modulus, E (GPa)
0
0
0
3.963
40.0
99.1
7.926
75.0
106
11.89
105.0
113.2
15.85
122.0
130.0
19.81
144.0
137.6
Table 2 Stress and Strain
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Graph of Stress Against Strain 25
20
) a P 15 M ( σ , s s 10 e r t S 5
0 0
20
40
60
80
100
120
140
160
Strain, ε (×10-6 m/m)
Graph 1 Stress-strain Curve
Average Young Modulus, E ave = 117 GPa
⸫ The metal alloy is determined to be grey cast iron.
Calculations:
1. Stress, =
=
2. Yong Modulus, =
=
(2 N) (. m)
=
= 3.963 × 10 Pa
(.× ) m m
(4.×− )
3. Average Young Modulus, =
= 99.1 × 10 Pa
(.++.2++.)× 5
= 117 × 10 Pa
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Discussions:
Based on Table 1, due to limitation of the machine and strain gauge, the maximum strain, ε
that obtained is 144.0 μm/m. While in Table 2, the stress, σ can be obtained by using the
relationship of Force, F and cross-sectional area, A of the metal alloy. Young’s modulus can be defined as a measure of the ability of a material to withstand the changes in length when under lengthwise tension or compression. Therefore, Young’s modulus can be determined as the ratio of stress,
σ
and strain, ε. Young’s modulus also referred as the
modulus of elasticity. Average Young’s modulus, E ave is calculated to find out the type of the metal alloy used in this experiment. E ave is found out to be 117 GPa and grey cast iron matched this value. Therefore, the metal alloy is grey cast iron. According to Graph 1, it shows that as the stress applied on the grey cast iron increases, the strain experienced by the grey cast iron also increases. But as the line incline proportionally, it will reach a limit and will increase slowly again and decrease as a curve. Before the line reaches the limit, the area under the line is the modulus of resilience. Modulus of resilience is the maximum energy that can be absorbed by the material before permanent deform. Before the point limit, the grey cast iron is completely elastic. When the line increases proportionally to a point limit, the point is defined as the yield strength of the metal alloy. Yield strength is the maximum stress that can be applied to the grey cast iron before permanent deformation. The line will then incline slowly and reach a maximum point, the maximum point is the ultimate tensile strength. The ultimate tensile strength is the maximum stress that can be absorb by the grey cast iron before structure failure. After that, the line decline slowly and stop at a point, declining line means that the grey cast iron start to break and fracture when it reach a point. But due to lack of data, all the variables can’t be calculated. Grey cast iron is an ideal material for machinery bases and many housing applications due its cheap price. It has an acceptable ductility, tensile strength, yield strength, and impact resistance for most applications. Grey cast iron also has an excellent ability to dampen the vibrations. Moreover, grey cast iron has high thermal conductivity that allows heat to be absorbed easily to its body. Besides that, grey cast iron shows great ability in withstanding the thermal cycling. Thermal cycling is where the material experiences in changing state of warm and cold temperature. This action will create stress
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and premature failure to some types of metal alloy. In the other hands, grey cast iron has low tensile strength and shock resistance compared to others casting irons. Finally, grey cast iron is produced with different composition of materials. The compositions of gray cast iron are iron, carbon, silicon, manganese, sulphur, and phosphorus. Compression test is important in industry. This is because in manufacturing industry, suitable material should be used to build certain objects that meet the mechanical properties of the material. Compression test is useful to determine the compressive strength, yield strength, ultimate strength, elastic limit, and the elastic modulus. With the understanding of these different parameters and values associated with a specific material it may be determined the material used is suited for specific applications or if it will fail under the test. In this experiment, grey cast iron is suitable for the making of engineering components such as Oil Pan, Frames, Hammer, Machine Base, Wheels, Valve, Motor Blocks and many more. Due to the test specimen used is too small, the universal test machine is unable to determine the strain of the test specimen clearly. To solve this problem, strain gauge is used. But a continuous data are unable to be determined, thus causing the graph to be so imperfect. Many parameters fail to be found except the Young’s Modulus. Moreover, more data should be collected to produce more accurate result and graph. This can reduce the error of the experiment. Errors also happen in the strain gauge, when measuring the strain using strain gauge, the value always fluctuate and hard to get an accurate strain values. This causes the error of the experiment to increase as well. It is suggested that more values are collected and calculate the average data to obtain a more accurate data. The precautions in this experiment are ensure that the two pressure plates touch the surface of the test specimen but not over compressed before the experiment start. This will cause the initial value of the data to be incorrect. Besides that, note that the maximum compressive force that the universal compression machine can applied is limited. It is advised that the load should not exceed the limit or else will cause the machine to break down easily.
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Conclusion:
The objectives of this experiment are achieved. The strain gauge is successfully applied and stress-strain curve is achieved. The metal alloy is determined to be grey cast iron. Grey cast iron has good ductility, tensile strength, and yield strength. Grey cast iron also able to dampen the vibrations, has high thermal conductivity, able to withstand the thermal cycling, and has low tensile strength and shock r esistance.
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