Practical Investigation: Tensile Testing and Material Properties

This report details a practical experiment on tensile testing, conducted to determine the mechanical properties of various metals including stainless steel, galvanized steel, and aluminum. The experiment involved measuring the dimensions of the specimens, applying tensile force until failure, and recording the corresponding elongation. The results, presented in tables, include stress, strain, Young's modulus, elastic limit, yield stress, and ultimate tensile strength for each material. Observations regarding the fracture behavior and difficulties encountered during the experiment are discussed. The report also includes a discussion on the Young's modulus, the effect of heat treatment on material properties, and the phenomenon of strain hardening, along with applications of the tested materials.

Introduction
Tensile testing is a basic material science and engineering test in which a sample piece is
placed in a tension until it fails. This testing is used to measure the different mechanical
properties of material.
Aim
This report aims to discuss about the various materials under tension and focused on finding
out the mechanical properties of metals.
Material Required
1. Bench top tensile tester
2. Vernier Calliper
3. 3 different pieces
 Stainless steel
 Aluminium and
 Galvanized steel
Procedure
 First, measure all the dimensions of all the specimens given
 Measure and mark the gauge length of all three specimens.
 Clamp properly testing piece on a bench top tensile tester
 Note all the data from the experiment.
Observation and difficulties
During the experiment the gauge length of metal pieces were marked by marker. It is
observed that stainless steel and the galvanized steel were broken from mid span whereas
aluminium was broken at the end. The stainless steel was elongated and needed more force as
the dimension was changed more compared to those of other two test pieces. There were
some difficulties in measuring after the fracture of the test pieces so measurements are
approximate.
Results and Discussion

Table: 1 Stainless Steel
Force(kN) ∆L Strain(mm/mm) Stress(MPa)
0.3 0.1 0.00095 7.2
0.1 0.5 0.0047 24
2.5 2 0.076 60
3.89 8 0.114 93.5
4.25 12 0.133 102
4.36 14 0.171 104
4.54 18 0.190 109
4.61 20 0.214 100
4.62 22.5 0.216 111
4.57 22.7 0.223 109
4.3 23.5 0.224 103
4.03 26.6 0.224 88
3.70 23.06 0.224 76
0 23.06(fail) 0.224 76
Dimension required for stainless steel
Total length = 105mm
Elongation = 126.3-105 = 21.3 mm
Thickness = 1.04 mm
Breadth = 40mm
Elastic Limit = 93.5 MPa, Yield Stress = 102 MPa, Ultimate tensile Stress = 111 MPa
Young’s Modulus = Stress
Strain (Upto elastic limit) = 93.5
0.114 = 820 MPa.
Table 2: Galvanized Steel

Force(kN) ∆L Strain(mm/mm) Stress(MPa)
0.15 0.6 0.0057 3.8
0.46 1.0 0.0095 25.5
0.8 1.2 0.0114 20.4
1.15 1.4 0.0133 29.3
1.4 1.5 0.0142 35.71
1.57 1.7 0.016 40
1.65 4.5 0.0428 42
1.7 5.70 0.0542 43.3
1.74 9.3 0.0885 44.3
1.72 10.1 0.0961 43.8
1.54 11.5 0.1095 39.2
1.2 11.8 0.1123 30.6
0 11.8(fail) 0.112 0
Dimension
Thickness = 0.98mm
Width = 40 mm
Total length L = 105 mm
Elongation = 115-105 = 2.62mm
Elastic Limit = 25 MPa, Yield Stress = 25.5 MPa, Ultimate strength = 44.3 MPa
Young’s Modulus = Stress
Strain (Upto elastic limit) = 25.5
0.0095 = 2.684 GPa.

Table: Aluminium
Force(kN) ∆L(mm) Strain(mm/mm) Stress(MPa)
0.5 0.6 0.0057 12.2
0.8 0.8 0.00761 19.6
0.95 1.2 0.0114 23.8
0.97 1.5 0.0142 23.7
0.98 2.4 0.0228 24
0.97 2.3 0.0219 23.7
0.93 2.6 0.0247 22.7
0.87 2.7 0.0257 21.3
0.72 2.8 0.0266 17.6
0.36 2.9 0.0276 8.8
150 2.9 0.0276 3.67
0 3(fail) 0
Dimension
Total length L = 105
Elongation = 107.6-105=2.62
Thickness = 1.02mm
Width = 40mm
Elastic Limit = 19.6 MPa, Yield Stress = 23.8 MPa, Ultimate strength = 24 MPa
Young’s Modulus = Stress
Strain (Upto elastic limit) = 19.6
0.00761 = 2.575 GPa.

Conclusion
From the tests conducted it can be inferred that:
 Stainless steel is the strongest among the three materials. Stainless steel is steel which
also has chromium in it. Chromium reduce the rusting of iron. Chromium also forms
Chromium carbide precipitates which goes into the grain boundaries and
strengthens it and makes it resistance to deflections. Chromium carbide is also a
refractory which means it can retain its strength at high temperatures
 Galvanized steel is basically steel coated with a layer of zinc. It is corrosion resistant
even in water, but not so great in salt water. Since this is just a coated steel, stainless
steel which is microscopically different due to addition of chromium is better in terms
of yield strength
 Aluminium is more malleable then steel but less strong. But Aluminium has better
strength to weight ratio, which means if a piece of aluminium and a piece of steel has
same strength, Aluminium will weigh less than steel.
Improvements:
Materials can be improved when heat treated. Quenching is done to introduce martensitic
structure with refined grains. More refined grains means more grain boundaries to stop
the dislocations once they start to propagate. Heat treatment is also done to residual stress
in the material.
Answer to the questions
1. The Young’s Modulus is the relationship between stress and the strain which
measures the stiffness of the solid material. It is different from one material to
another because of the bonding energies of the material and their atomic structure
of the material. Because of this reason stainless steel has high young modulus than
that of galvanized steel and aluminium. This shows greater the bonding strength
the higher the stiffness. The above three metals has metallic bonding but the
spacing and tightness of the atoms determines their young’s modulus .Stainless
steel is heavier than the aluminium as the steel contains dense and tightly packed
atoms. Due to this reason stainless steel and galvanized steel have high load
withstanding capacity compared to that of aluminium.
2. If the yield strength of a material is increased three times by heat treatment,
Young modulus of material does not change because it is the only the ratio of
stress over strain. Also, the crystalline structure of the atom and interatomic force
does not increase.

3. If the material is loaded above yield strength but below the ultimate tensile
strength and unloaded depending upon the load it will cause plastic deformation
and the material will not return to its original dimensions. Again when the
material will be loaded it will yield at a higher value than previous yield point.
This is called strain hardening.
(Jones, 2012) explained that when the material is subjected to σ2 (higher stress than
yield) and unloaded it will return to ε3 strain. And again when loaded it will start
from there and will reach σ4 value which will be its new yield point.
This happened because when plastic deformation occurs it causes dislocations.
These dislocations forms barrier to any further movement and requires greater
force to overcome. This phenomenon is called Strain Hardening.
4. Applications of
a. Stainless Steel – Utensils, Building reinforcements, Medical apparatus etc.
b. Galvanized Steel – Pipes, body of vehicles, computer parts, ornaments etc.
c. Aluminium – Solar panels, refrigerators, heat exchangers, sheet metal
products, constructions etc.
Reference
Jones, D. R. (2012). Engineering Materials 2: An Introduction to Microstructures and Processing.
Butterworth-Heinemann.