Tensile Test Lab Report
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Abstract
This experiment was conducted so as compare the mechanical properties of aluminium and mild steel. The basics on the operation of universal testing machine were also learnt during this experiment. The Universal Testing Machine can be used to determine the tensile strengths of many engineering materials. The design of many engineering structures is based on the tensile properties of the materials used. The stress- strain relationship of various metals can be used to predict the characteristics of materials when subjected to different types of loadings. From this experiment, it can be seen that mild steel have higher tensile and yield strength than aluminium. This explains the wide applications of mild steel in many constructions and other engineering applications that require high strength.
INTRODUCTION
For safe design of structural components in bridges, railway lines, marines ships, aircrafts, pressure vessels etc, the tensile properties of materials used should be analyzed. Hence the tensile strength of the materials should meet the strength requirements of the structural applications. The mechanical properties of the metals determine the kind of engineering application to be used for. Experiments on tensile tests can be used to predict the tensile properties and they are conducted by application of axial or longitudinal forces to a specimen with known dimensions. (Davies, 2004). These forces are applied on the specimen until deformation causes failure. The tensile load and corresponding extensions are then recorded for calculations and determination of stress- strain relationship of the material specimen. The tensile test experiment can be used to determine other mechanical characteristics of the specimen like yield strength, percentage elongation, and ultimate strength among others. The original gauge lengthLo, diameter Do or cross sectional area also used in calculations hence should be recorded. (Micheal F. Asby, 2013)
Aim
To compare and contrast the tensile strengths of mild steel and aluminium specimens
Objectives
To study the deformation and fracture characteristics of mild steel and aluminium when they are subjected to uniaxial loading
To observe the load extension and stress – strain relationships in both aluminium and mild steel
To study the basics of uniaxial tensile testing.
Stress- strain relationship
Tensile loading on material causes the material to undergo deformations. The kind of deformation can either be elastic or plastic deformation. The elastic deformation is characterised by linear relationship between the extension and applied load. Engineering stress σ is given by the ratio of load applied to the original cross sectional area, while engineering strain ε is given by change in length (extension) L over the original length L. (G & Barry, 2012)
Hence;
σ= PAo and (1)
ε= LLo (2)
Where,
σ is engineering stress
P is the applied axial load
Ao is the original cross sectional area
ε is the engineering strain
L is the extension
Lo is the original length
Young's modulus
The engineering stress- strain relationship for elastic deformation is based on Hooke's law. The gradient on this curve gives a modulus of elasticity called The Young's Modulus E.
E=σε , (3)
Where:
E is Youngs modulus
σ is engineering stress and ε is the engineering strain.
In engineering applications of materials/ metals that are subjected to deflections, Young's modulus is of critical importance. (Richard Budynas, 2014)
Figure 1: stress- strain relationship under uniaxial loading. Source (Richard Budynas, 2014)
.
METHODOLOGY
Materials and equipment
Universal testing machine
ruler
Vernier calipers
3 samples of mild steel
3 samples of aluminum
Experimental procedure
By use of Vernier calipers, the thickness and width each samples of aluminium and mild steel were measured. The gage length of each specimen was determined to be 80 mm.
A ruler was used to measure and confirm the gage length of each sample of specimen.
The software for acquiring and recording data was activated and the material corresponding to the specimen was selected in the software.
By zeroing the load cell, the Instron Load Frame could only be set to measure only the tensile load on each specimen inserted.
The jaws were adjusted to fit the size of the specimens. This was followed by attaching the extensometers on the reduced sections of the gage specimen.
To avoid slipping of the specimens, the scroll wheel was used in preloading the machine.
After the specimen was removed, the extensometers were adjusted to zero values and the test commenced to measure strain of the specimen.
The data was recorded by the software on the spreadsheet
By placing each sample in the universal testing machine, the tensile test was conducted and results were recorded in the computer. The data was later retrieved for calculation and plotting of the graphs.
RESULTS AND ANALYSIS
Figure 2 table of dimensional results
MILD STEEL ALUMINIUM
Load at Break (Standard)
3,357.43
N
-801.0313
N
Extension at Break (Standard)
26.83716
mm
6.76516
mm
Data point at Break (Standard)
3222
813
Tensile strain (Extension) at Break (Standard)
0.26837
mm/mm
0.06765
mm/mm
Tensile extension at Break (Standard)
26.83716
mm
6.76517
mm
Tensile stress at Break (Standard)
335.743
MPa
-80.10313
MPa
Figure 3: results of mild steel and aluminium samples
mild steel sample
aluminium sample
Time
Extension
Load
stress
strain
Extension
Load
stress
strain
(s)
(mm)
(N)
(MPa)
(mm/mm)
(mm)
(N)
(MPa)
(mm/mm)
0
0
0.90
0.05
0
0
0.611
0.024
0
10
0.83
4694.34
238.89
0.010
0.832
2687.750
106.634
0.010
20
1.67
4831.41
245.87
0.021
1.665
2884.170
114.427
0.021
30
2.50
4781.08
243.30
0.031
2.498
2981.600
118.292
0.031
40
3.33
4918.83
250.31
0.042
3.332
3048.760
120.957
0.042
50
4.17
4926.58
250.71
0.052
4.165
3071.700
121.867
0.052
60
5.00
5257.07
267.53
0.062
4.998
3112.230
123.475
0.062
70
5.83
5437.01
276.68
0.073
5.832
2877.540
114.164
0.073
80
6.66
5575.88
283.75
0.083
6.665
-645.521
-25.610
0.083
81
6.75
5584.21
284.18
0.084
6.748
-780.168
-30.952
0.084
81.1
6.76
5584.04
284.17
0.084
6.757
-791.985
-31.421
0.084
81.2
6.77
5591.60
284.55
0.085
6.765
-801.031
-31.780
0.085
81.3
6.77
5587.98
284.37
0.085
6.772
-809.438
-32.114
0.085
100
8.33
5775.18
293.89
0.104
110
9.16
5847.52
297.57
0.115
120
10.00
5911.04
300.81
0.125
130
10.83
5965.41
303.57
0.135
140
11.67
6010.53
305.87
0.146
150
12.50
6042.57
307.50
0.156
160
13.33
6072.26
309.01
0.167
170
14.16
6092.93
310.06
0.177
180
15.00
6113.24
311.10
0.187
190
15.83
6129.65
311.93
0.198
200
16.67
6140.36
312.48
0.208
210
17.50
6146.37
312.78
0.219
220
18.33
6148.14
312.87
0.229
230
19.16
6149.17
312.93
0.240
240
20.00
6147.15
312.82
0.250
250
20.83
6142.22
312.57
0.260
260
21.66
6130.59
311.98
0.271
270
22.50
6120.44
311.46
0.281
280
23.33
6099.74
310.41
0.292
290
24.16
6050.83
307.92
0.302
300
25.00
5940.21
302.29
0.312
310
25.83
5675.33
288.81
0.323
320
26.67
4725.52
240.48
0.333
322.2
26.84
358.03
18.22
0.336
322.2
26.85
79.03
4.02
0.336
322.2
26.85
-7.95
-0.40
0.336
Figure 4: graph of stress v strain for mild steel
Figure 5: graph of stress v strain for aluminium sample
Figure 6: graph of stress versus strain for both aluminium and mild steel.
DISCUSSION
The data obtained from the universal testing machine shows the difference in rates of extensions in mild steel aluminium samples. From data on cross- sectional area, length, extension and axial loads, the strains and stress for both sample specimens were calculated. When subjected to same amount of load, there was relatively high extension in aluminium than in mild steel. This can be attributed to the difference in micro- crystalline structures of the two sample materials. Mild steel reached yield point at stress of 240 MPa while aluminium reached yield strength at 105 MPa. Hence it can be seen that mild steel has high tensile strength compared to aluminium. When the gradients of both mild steel and aluminium were calculated, mild steel had a higher gradient than aluminium. The gradients of stress- strain curves give the Young's Modulus, which affect the deflection of material under different loads. Further loading of both specimens beyond the yield point gave a stack difference; mild steel reached fracture point at approximately 335 MPa while aluminium reached fracture at – 80 MPa. Mild steel has Body Centered Cubic (BCC) structure while aluminium has Centered (FCC) structure. Changes in length indicate the ductility of the material when loaded. There were large amounts of necking observed in mild steel than there was in aluminium. Precipitation hardening done to aluminium and its alloys hinders the elongation of the specimen.
The changes encountered in cross sectional area cannot be influenced by engineering stress- strain relationships; the changes can only be possible for true stress- strain curves. Normally, true strains are of higher values than those of engineering strains. This can be explained by the fact that true strains take place in transverse directions of the gage length. High values of stress and strains in mild steel are attributed to strain hardening. Strain hardening or work hardening in mild steel occurs at higher values of stress than aluminium. In the graph, it can be seen that for engineering stress- strain curves, the curves drop downwards after necking has occurred. However, this phenomenon cannot be seen in normal true stress- strain curves, the curves would reach the highest region of fracture.
Engineering stress and strains were calculated after the extensometers on the Instron machine measured the strain that was applied on each sample specimen. The data on strain was obtained on the cross head after necking had occurred. The engineering stress was then calculated by dividing the applied load by the original cross- sectional area. For engineering strains, the changes in length (extensions) were divided by the original length. In calculations of true stress, the load applied could be divided by the instantaneous area. True strain is calculated by dividing the change in length by the instantaneous final length.
CONCLUSION
Many engineering applications that require high tensile strength normally use mild steel. This is because of the crystalline structure of mild steel that allows it to withstand high axial loads before fracture can occur. Aluminium however has found many uses in designs that require low density materials like in aerodynamics and some motor vehicles. Aluminium experiences high ductility rates compared to mild steel and have therefore low level values of Young's Modulus, a factor that determines deflections in structural components. This experiment therefore gives close relationship of tensile strength to the theoretical data.
REFERENCES
Davies, J. (2004). Tensile Testing (2nd Edition ed.). ASM International.
G, J., & Barry. (2012). Mechanics of Materials (8th Edition ed.). CL Engineering.
Marc, K. K. (2008). Mechanical Behavior of Materials (2nd ed.). Cambrige University Press.
Micheal F. Asby, K. J. (2013). Materials and Design (3rd Edition ed.). Butterworth.
Richard Budynas, K. D. (2014). Mc-Graw Hill Series in Mechanical Engineering (10th Edition ed.). Mc-Graw Hill Series.
Richard, A. (2002). Advanced Mechanics of Materials. (R. J. Schmidt, Ed.) Wiley.
APPENDIX
Terminologies
Engineering strain – it s calculated by dividing the change in length (extension) by original length.
Engineering stress – it is obtained by dividing the applied axial load by the original cross sectional area. Engineering stress-strain curve – is a graph showing the relationship between engineering stress and engineering strains.
Hooke's law -this law explain the linear relationship observed in the elastic regions of a stress strain curves. The gradient along this curves give the Young's modulus.
Modulus of elasticity – also called the Young's modulus, is the ratio of stress to strain and can be calculated on the stress- strain curves by determining the gradients of the curves.
Necking – this refers to the gradual reduction of the cross sectional area along the gage length and starts at the tensile point. It results in formation of cups and cones and is experienced in ductile materials.
Plastic deformation – this phenomenon occurs when the material is loaded beyond the yield point then offloaded.
% Reduction in area – can be determined by dividing the change in cross sectional area over the original area multiplied by 100% when a tensile test is performed on the specimen.
Tensile strength - refers to the maximum stress that a material can withstand during the tensile tests.
Tensile test - refers to the methods of determining the mechanical properties of material when subjected to uniaxial load. The results can be used to determine the Young's modulus, tensile strength, ductility, toughness and ultimate tensile strength of the materials.
True strain – refers to the ratio of extension to the final instantaneous length of the material
True stress – is the ratio of the applied load over the instantaneous cross- sectional area.
Yield strength – this refers to the amount of stress required to initiate plastic deformation.
Ultimate tensile strength
As shown in figure 2 above of the engineering stress- strain relationship, when loading is continued past the yielding point, a permanent deformation of the material is realized. At this point, the material is said to be strain or work hardened and this phenomena is dependent upon the micro- crystalline structure and chemical composition of the material. It is at this point that the material can withstand the highest possible stress and is characterised by reduction of cross sectional area at the center of the specimen- a process known as necking. (Marc, 2008)
Figure 6: stress- strain relationship for mild steel and aluminium. Source (Auther & Richard, 2002)
