Introduction: Tensile test
Tensile testing is one of the most fundamental experiments that must be held to learn the behaviour of each material and its properties to design safe s tructures. Additionally, during the learning, new composites materials could be designed to fit the appropriate field of engineering. The main objective of that laboratory session was to make the tensile strength experiment on three common materials, namely cold drawn carbon steel 0.4%, aluminium and aluminium and free free machining brass and brass and ident ify ify its mechanical properties. Such values as ultimate strength (∂ u), maximum load (Pmax), modulus of elasticity (E) and yield strength were found for each material from obtained stressstrain curves. Actually, the main reason for conducting that tensile e xperiment is to learn more about the relationship of normal stress and normal strain for each material individually. After obtaining all values required for the analysis, 3 materials were compared. All specimen were tested once for each material in the Tinius Olsen H25KS load frame machine. All three testing materials were cylindrical with the same gauge length of approximately 27mm The initial diameter (4.8mm) of the specimen was measured by caliper. The cross section was circular with enlarged end grips to ensure that the highest stresses to occur within the gauge length region to avoid rupture in or near the grips. The specimen were then put into the machine mountings to fi x the piece of material. After the setting of jogging speed (in our case 50mm/min), the machine started to apply normal tension, which resulted in elongation of the material and eventually led t o fracture. Thus, the forces over elongation plots were printed out in all three cases considered.
Tinius Olsen Model H25K-S UTM (2013)
Results
Stress-Strain Diagram for Cold drawn plain carbon steel 0.4% 1,338
1,138
) a P M ( σ
s s e r t S
938
738
538
338
138 00,000
00,000
00,000
00,000
00,000
00,000
Strain ε (mm/mm)
Figure 1 Stress-Strain Diagram for cold drawn plain steel 0,4%
Stress-Strain Diagram for Free machining brass 775 675 575 ) a P 475 M ( σ
s s 375 e r t S
275 175 075 00,000
00,000
00,000
Strain ε (mm/mm)
Figure 2 Stress-Strain Diagram for free machining brass
00,000
Stress-Strain Diagram for Aluminium 420 370 320 ) a 270 P M ( σ
220
s s e r t 170 S
120 070 020 00,000
00,000
00,000
00,000
00,000
00,000
00,000
00,000
00,000
Strain ε (mm/mm)
Figure 3 Stress-Strain Diagram for Aluminium Table 1 Comparison between measured and true values of ultimate strength for different materials
Material Cold drawn plain carbon steel 0.4% Aluminum Free machining brass
Ultimate strength σu measured (MPa)
Ultimate strength σu tabulated (MPa)
Deviation (%)
1098,33
600 (Callister, 2011)
45,37
354,01
300 (Callister, 2011)
15,26
600,98
469 (E-Z lok, 2013)
21,96
Table 2 Comparison between measured and true values of yield strength for different materials
Material Cold drawn plain carbon steel 0.4% Aluminum Free machining brass
Yield strength σy measured (MPa)
Yield strength σy tabulated (MPa)
Deviation (%)
958
515 (Callister, 2011)
46,24
475
310 (Callister, 2011)
34,74
320
241 (E-Z lok, 2013)
24,69
The Young’s modulus was measured by calculating the slope of the linear region of stress-strain
diagram. For that purpose several points on this line was taken to calculate the slope. The following equation was used:
(1)
Table 3 Calculated Young’s modulus on the linear region of Stress -Strain diagram for different materials
Steel Young's modulus (GPa)
Brass Young's modulus (GPa)
Aluminium Young's modulus (GPa)
18,651
18,651
9,325
26,111
20,516
16,786
22,381
18,651
14,921
26,111
18,651
16,786
22,381
Average value (GPa)
13,056
22,381
19,117
11,191
22,381
Average value (GPa)
Average value (GPa)
13,677
22,914
Table 4 Comparison between measured and true values of Young’s modulus for different materials
Cold drawn plain carbon steel 0.4%
Free machining brass
Aluminium
Average value (GPa)
Average value (GPa)
Average value (GPa)
22,914 Tabulated value (GPa)
19,117 Tabulated value (GPa)
13,677 Tabulated value (GPa)
207 (Callister, 2011)
97 (Callister, 2011)
69 (Callister, 2011)
Deviation (%)
Deviation (%)
Deviation (%)
803,38
407,40
404.50
From table 4 it can be seen that the difference between true and measured values is immense. It is clearly indicates on some kind of faulty of apparatus which was used during the experiment.
Table 5 Maximum load applied on the specimen for different materials
Material
Maximum load (N)
Cold drawn plain carbon steel 0.4%
19875
Aluminum
6406
Free machining brass
10875
Table 6 Initial diameter, diameter after rupture and percent reduction area for different materials
Material Cold drawn plain carbon steel 0.4% Aluminum Free machining brass
Initial diameter (mm)
Final diameter (mm)
%RA
4,8
4,1
27,04
4,8
3,5
46,83
4,8
4,0
30,56
Results analysis and discussion 1. According to the results obtained, from the stress strain plots, it can be seen that all three materials yielded and the results showed quite significant deviation from the theoretical results. The most resistant material to tensile deformation was plain carbon steel with calculated yield strength of 958MPa (theoretical 515MPa). Aluminum yield strength was measured to be 475Mpa (theoretical 310MPa) and the calculated yield strength of brass was 320MPa (theoretical 241MPa). Yield strength is basically the value of stress after which the plastic deformation take place. During yielding, materials undergo plastic deformation with a slight increase in load. Yield strength is a very significant value that must always be taken into consideration for engineering construction design. When designing a construction, the materials that are used must withstand the forces applied to avoid plastic deformation. In other words, the appropriate material with the high yield strength must be used or the cross sectional area could be enlarged so that the forces applied must produce normal stresses that are below the yield strength. 2. Comparison of all specimen for the ultimate strength showed that the cold drawn carbon steel is the material that can sustain the largest stress before failure with the measured value of approximately 1098Mpa, compared to theoretical data of 600MPa (46% difference). Brass and aluminum tensile strengths were calculated to be 600 MPa and 354 MPa respectively. Tensile strength is the maximum stress that is applied before the material ruptures or fails. Due to the mechanical properties the significance of the tensile strength is widely found in the design of brittle materials and similar compositions. Since rupture in brittle materials occur in the highest stress value, tensile strength is very significant data that must be obtained in order to avoid failure of brittle materials. 3. The analysis of elastic modulae provided the most deflected results. Fo r instance the most resistant material was found to be carbon steel again with the Young’s modulus of about 23GPa. The free machining brass could withstand about 19 GPa of the stress before plastic deformation, whereas aluminium’s measured value was in the region of 13.6GPa.
Young's modulus is an individual property of a material. It is actually the ability to resist the deformation due to the force applied. Materials with higher elastic modulus are more resistant to the deformation (metals, heavy alloys), and hence, it would take more stress to reach for the material to reach the same strain with higher Young's modulus compared to the one with lower. Engineers are more likely to use
materials which would withstand large stresses within elastic region and thus materials with high elastic modulus is more preferable in structural design. 4. Aluminum was claimed to be the most ductile material since it could undergo larger deformations before rupture occurred with the area reduction of approximately 47%. Ductile materials are mostly preferred in tension engineering design due to its mechanical properties. Firstly, ductility means the efficient way to absorb energy and consequently, this results in much more larger deformation if overloaded, before fracture (Hibbeler, 2011). Regarding the brittle materials, they can hardly reach the yielding region, which means that they cannot withstand overloading since it will lead directly to the rupture without deformation, when tension is considered. However, when compression is analyzed, brittle materials have an advantage in a face of ductile materials as they are more resistant to axial compression deformations rather than ductile ones. Thus, in structural design, in compression regions brittle materials are in favor. 5. Figure 4 shows the fracture profile of plain carbon steel. It can be seen that this profile is almost straight line. This is because steel failed due to the normal stresses, which act perpendicular to the surface of the specimen i.e. perpendicular to line of fracture. The necking indicates that material is ductile.
Figure 4 Fracture profile of plain carbon steel
Figure 5 represents rupture profile of the brass. In this case profile is not straight line anymore (in comparison with steel). This is due to the fact that shear and normal stress contribute equally in breaking of brass. In some regions brass failed due to normal stresses, in another due to the o
shear stresses, which are maximum at 45 degree angle. It is shown on figure 6.
Figure 5 Fracture profile of fr ee machining brass
Figure 6 Fracture surface of brass
The fracture profile of the Aluminium is shown on the figure 7. According to this figure rupture occurred at 45o degree angle. Thus Aluminium failed due to the shear stresses. The material necks significantly compared to the plain carbon steel and brass. In these metals necking is not so pronounce.
Figure 7 Fracture profile of Aluminium
Conclusion During this laboratory session, three diff erent materials including carbon steel, brass and aluminum were tested by the tensile strength experiment. The behavior of both brittle and ductile materials under tensile forces was observed. According to the graphs and other obtained data during the experiment three materials have yielded. Carbon steel was found to be the most resistant material against tensile stress. Whereas, the aluminum was determined as the most ductile material, being able to withstand more deformations in comparison with the other two materials. Furthermore, the fracture profiles of each material have been demonstrated via images. These parameters play very important role in choosing the material for engineering purposes. It is important to mention that the safety issues are also very significant and because of that engineers are testing and analyzing the behavior of a variety of materials by such experiments as tensile stress experiment.
Reference list th
Callister, W. and Rethwisch, D. (2011). Materials Science and Engineering, 8 ed. Wiley, Hoboken th
Hibeller, R. (2011). Mechanics of Materials, 8 edition, Prentice Hall, Boston E-Z Lok. (2013). Free-Cutting Brass, UNS C36000. Retrieved 16 October 2013, from http://www.ezlok.com/TechnicalInfo/MPBrass.html Tinius Olsen. (2013). Benchtop Testers Model H25K-S UTM. Retrieved 13 October 2013, from http://www.tiniusolsen.com/products/bench-machines/bench-h25k-s.html
