Tensile and Roughness/Hardness Tests

Tensile and Roughness/Hardness Tests 1
TENSILE AND ROUGHNESS/HARDNESS TESTS
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Tensile and Roughness/Hardness Tests 2
Tensile and Roughness/Hardness Tests
AIM
The aim of the first experiment was to perform tensile tests on steel reinforcing bar
samples and draw load vs. extension graphs so as to compare and cross-examine the physical and
mechanical properties of the steel reinforcing bars. The purpose of the second experiment was to
explore the toughness of carbon steel at low and normal temperatures and the hardness of steel
when exposed to different heat treatments.
INTRODUCTION
Materials behave differently when subjected to tensile loads. Typically, metals extend
when tensile loads are applied on them. The metals continue elongating until they break or
fracture. Tensile test helps in understanding the behaviour of metals under tensile loads
(Michigan Technological University, (n.d.)). This is essential is determining the ability of the
metal to support design loads. Tensile test also provides information about the modulus of
elasticity, yield strength, strain and ultimate tensile strength of the metal (Instron, (n.d.)).
The results of Charpy impact test are affected by factors associated with metal ductility
and toughness. In this test, a typical 10x10mm bar of steel is subjected to sudden impact until it
breaks then the energy absorbed by the bar is measured. The specimen tested is a simply
supported beam. The energy absorbed by the specimen usually depends on temperature. When
temperature is low, the fracture of the carbon steel is brittle hence it absorbs little energy. This
leaves it with a shiny crystalline surface. The impact strength of carbon steel decreases
significantly at low temperature. As temperature increases to high temperature, the fracture
becomes ductile thus ripping the carbon steel sample into spikes making it to absorb significant

Tensile and Roughness/Hardness Tests 3
amount of energy. The factors that affect the hardness and toughness of carbon steel include:
carbon content, temperature and heat treatment.
LITERATURE REVIEW
Tensile test (also referred to as tension test) is a very common test used to investigate the
mechanical and physical properties of steel and other metals. The test is simple (straightforward)
and quite inexpensive (Test Resources, (n.d.)). The test helps in understanding how the metal
reacts and behaves when a tensile load is applied on it. The greater the tension the more the
elongation of the metal. Data obtained from tensile test is used to draw a stress-strain curve. This
curve shows how the metal reacted to the forces that were applied on it.
The ability of materials to resistant loads depends on their properties. There are materials
that are more resistant to static loads but less resistant to sudden loads (impact) and vice versa.
Impact resistance refers to the resistance of a material to sudden load or shock. Charpy impact
test is commonly used for illustrating the effect of temperature on the fracture toughness of steel
and also to establish the resistance of steel specimen towards an impact or a sudden load. The
test determines the amount of energy that a material absorbs when subjected to shock. The test is
very simple and that is one of the reasons why it is commonly used. In Charpy impact test, a
small horizontal beam with a notch is subjected to a sudden load by a large hammer that is fixed
to a pendulum. This loading sequence provides a strain rate that is combined with the notch on
the specimen where a large stress concentration is created enables propagation of fast crack and
fracturing of the specimen.
When a steel specimen is subjected to an impact and it experiences a very little plastic
deformation or forms a small fracture, it is said to be brittle but when it experiences extensive

Tensile and Roughness/Hardness Tests 4
plastic deformation or forms a bigger fracture, it is said to be ductile. The surface of a brittle
fracture appears to be clear and shiny whereas the surface of a ductile surface appears to be
fibrous and greyish (Mishra, (n.d.)).
APPARATUS
The apparatus used for tensile tests are Denison Universal Testing Machine, weighing
balance and micrometer. The apparatus used for toughness and hardness test are electric furnace
at 800 °C, chiller fluid, beaker containing cold water to be used for quenching, Charpy impact
apparatus, heat-resistant mat, Vickers hardness apparatus, microscope, eye protection, tongs,
gloves, one steel spanner and four 0.4% carbon steel impact samples (10x10x55mm with
0.25mm tip radius and 2mm deep notch).
PROCEDURE
Section A: Tensile test
Plain bar:
The diameter of the plain bar was measured using the micrometer. This was done by
taking two diameter measurements at different points along the bar’s length. The two measured
diameters of the bar were used to calculate the average cross sectional area of the bar. The
overall length and weight of the bar were also measured and recorded. The average cross
sectional area of the bar was also calculated using density for steel of 7850 kg/m3 and the value
compared with the one that had been determined before.
Ribbed bar: