Tensile Test Report - COMPLETE

TITLE: TENSILE TEST FOR METALS

AIM:

The aim of this experiment is to study the behaviour of engineering materials under tension until collapse / failure / rupture and to determine:

a) Relationship between stress and strain (stress-strain curve) b) Modulus of Elasticity (E)

c) Indicate stress strength in :

i. Yield stress (σY and 0.1% and 0.2% proof stress, σP)

ii. Maximum stress (σMAX)

iii. Rupture or failure stress (σBR = σRUP)

d) Local extension or the strain to a specified length

e) Average of general extension or strain to overall length of specimen (EAV)

f) % reduction in area (∆AO %)

APPARATUS:

a) Testing machine (Universal Testing Machine – 10 ton capacity) b) Extensometer

c) Dividing and marketing machine d) Micrometer

e) Vernier calliper f) Steel ruler

INTRODUCTION:

Mild steel performs some behavior in tension. A convenient way of demonstrating elastic behavior is to plot a graph of the results of a simple tensile test that will be carried out in this experiment on a thin mild steel rod and aluminum rod. The rod may be hung vertically and a series of forces applied at the lower end. Two gauge points are marked on the rod and the distance between them measured after each force increment has been added. The test is continued until the rod breaks.

The stress and the strain values at each incremental stage are calculated and plotted in the graph. The following characteristics will be observed from the graph:

a) Elastic range of stress: the straight-line portion of the graph between the origin and the elastic limit; i.e. the portion over which Hooke’s law is obeyed.

b) Plastic range of stress: the curved portion of the graph between the elastic limit and the failure point; a permanent set (change in length) will be observed if the load is removed in this region.

c) Elastic limit: the stress up to which complete recovery of the strain takes place upon removal of the load.

d) Limit of proportionality: the stress up to which Hooke’s law is obeyed, i.e. the graph is a straight line, usually occurring fractionally below the elastic limit.

e) Upper yield point: the stress at which the first sudden increase in strain occurs. f) Lower yield point: the lowest value of stress during the first sudden yielding; usually

this can only be located by using controlled strain method of testing.

g) Failing stress: the stress at which the rod breaks; allowance has to be made for the reduction in cross-section (known as ‘necking’) which occurs due to plastic yielding immediately prior to failure.

h) Ultimate or maximum stress: the maximum value of stress attained before the final rapid yielding leading to failure.

PROCEDURE:

a) Physical inspection of the given sample is carried out in terms of straightness, uniform diameter and cross-section, cracks and defects.

b) Diameter of the specimen sample is measured in a few cross-section (n). The average diameter (DAV) or original diameter (DO) is determined : (DAV = DO = ∑D i/n)

c) For every 10 mm, the specimen is marked with dividing and marking machine and the overall length, Lt is measured.

d) The sample is placed in the testing machine.

e) Extensometer is fix in the middle of specimen and the readings between the jaws of extensometer, Lg are recorded.

f) The range of load required with basic ultimate tensile strength of given material is selected.

Original cross – sectional area of specimen : AO = π/4 DO2

Estimated Ultimate tensile load = UTS x π/4 DO2 x

1

9.81 x 1000 ton 1 Ton = 1000 Kgf 1N = f x g

g) The specimen is loaded and the behaviour is noted when the load is increased.

h) The extensometer is removed when the strain is not proportional to the stress applied. Loading is continued till failure is reached.

i) The diameter at the neck of the failed sample is measured.

j) Gauge length, Ls = 5.65 √AO is calculated according to B.S. where AO = original area

of the specimen. The value of Ls is placed to the nearest centimetre. The increase in

length of Ls is measured after test.

k) The total length of the specimen after failure is measured.

0 100 200 300 400 500 600 -1 0 1 2 3 4 5 6 T en si le s tr e ss ( M P a ) Tensile strain (%)

Tensile Test - Steel

Specimen Name G5 Steel 0 100 200 300 400 -1 0 1 2 3 4 5 6 T en si le s tr e ss ( M P a ) Tensile strain (%)

Tensile Test - Aluminium

Specimen Name G5 aluminium

For Steel:

Estimated Ultimate tensile load = UTS x π₄DO 2 _x 1

9.81 x 1000 ton 1 Ton = 1000 Kgf 1N = f x g

Estimated Ultimate tensile load of steel = 575 x 71.63 x _{9.81 x 1000}1

= 4.198 ton Area, A0 = π 4 Do 2 ₌ π 4x 9.55 2 = 71.63 mm2 LS original= 5.85

√

= 4.48% , Where  Lt = Lt final – Lt initial

c) % increase in local elongation as per B.S. = Ls_{Ls x 100%}

= 57.00−49.51_49.51 x 100% = _{49.51 x}7.49 100%

d) % decrease in cross-sectional area = A_{original− Aneck} A_original x 100% = A_o−A_f Ao = Do 2 −Df 2 D_o2 = 9.55 2 −5.852 9.552 x 100% = 62.48% For Aluminum:

Estimated Ultimate tensile load = UTS x π₄ DO 2 _x 1

9.81 x 1000 ton 1 Ton = 1000 Kgf 1N = f x g

Estimated Ultimate tensile load of aluminum = 310 x 74.36 x _{9.81 x 1000}1

= 2.350 ton Area, A0 = π 4 DAV 2 ₌ π 4 x 9.73 2 = 74.36 mm2 LS original = 8.37

√

= 300−290₂₉₀ x 100%

= 3.45% , Where  Lt = Lt final – Lt initial

c) % increase in local elongation as per B.S. = Ls_{Ls x 100%}

= 75.00−72.18_72.18 x 100% = _72.182.82 x 100%

= 3.91%, Where Ls = LS final - LS original

d) % decrease in cross-sectional area =

A_{original− Aneck} A_original x 100% = A_o−A_f Ao = Do2−Df2 D_o2 = 9.73 2 −8.372 9.732 x 100% = 26.00 %

RESULTS :

Diameter of the sample:

Material Diameter Di (where i = n) Average Diameter

DO =DAV (mm)

1 (mm) 2 (mm) 3 (mm)

Steel 9.54 9.58 9.52 9.55

Aluminum 9.73 9.72 9.73 9.73

Length of the sample : INITIAL

Material Length, l (mm) Local Length (mm)

Steel 290 50

Aluminium 290 70

Weight of the sample:

Material Weight, w (g)

Steel 168.02

Aluminium 60.51

Diameter of the sample:

Material Diameter Di (where i = n) Average Diameter

DO =DAV (mm)

1 (mm) 2 (mm) 3 (mm)

Steel 5.70 5.93 5.91 5.85

Aluminum 8.35 8.40 8.37 8.37

Length of the sample :

Material Length, l (mm) Local length (mm)

Steel 303 57

Aluminium 300 75

Weight of the sample:

Material Weight, w (g)

Steel 168.02

Aluminium 60.51

Material Ultimate tensile strength UTS

(N/mm2₎

Estimated Ultimate Tensile load ( ton )

Modulus of Elasticity, E (kN/mm2₎ Steel 575 4.198 300 Aluminium 310 2.350 100 FINAL

DISCUSSION:

1. From the results obtained in the experiment, it is shown that the modulus of elasticity of steel, which is 300 kN/mm2 _{is higher than that of aluminium, which is 100 kN/mm}2_.

For the percentage of elongation, the percentage of elongation done by steel is 4.48% while aluminum has a lower value, which is 3.45%.

2. An extensometer is used to measure the changes in the length of a specimen. It is useful for stress-strain measurements. There are many types of extensometer but they are mainly split into two categories which are Contact and non-contact. Contact extensometer is used for applications where high precision strain measurement is required. For certain special cases, non-contact extensometers are used when it is impractical to use a feeler arm or contact extensometer. Contact extensometers can be further categorized into two types as shown in below.

a) These are clipped onto a specimen (hence the name "clip-on" extensometer) before carrying out a tensile test. These devices are used for applications where extremely high precision is required over a relatively small extension (a few mm). They have the disadvantage that they can damage the specimen at the contact point due to the

clamping forces required and the weight of the device itself which can influence certain test specimens.

b) For automated testing clip-on devices have been largely replaced by digital "feeler arm" extensometers. These can be applied to the specimen automatically by a motorised system and produce much more repeatable results than the traditional clip-on devices. They are counter balanced and so have negligible effect on the specimen. Better linearity, reduced signal noise and synchronisation with the corresponding force data are big advantages due to the lack of analogue to digital converters and associated filters which add time lags and smooth the raw data. In addition these devices can remain on the specimen until failure and measure very high extensions (up to 1000 mm) without losing any accuracy.

3. There are tests normally carried out to determine the suitability of a metal for engineering uses such as construction and fabrication, there are compression test, hardness test, fracture test and other tests.

4. When carrying out the tests, errors cannot be avoided as there may be imperfections in the specimen itself. The defection in the specimen may cause the results obtained to be inaccurate as its behavior cannot be determined precisely. Besides, technical problem such as failure to calibrate the machine may influence the results too. Hence, in order to minimise such errors, several precautions are needed to be taken into consideration.

a) When the tensile test is running, do not get too close to the machine in order to avoid injury from a sudden small explosion when the specimen fractures. b) The diameter and length of the specimen should be measured at least three

times to get the average reading so that a more accurate value can be obtained. c) Parallax errors should be avoided when taking measurements with the line of

sight always perpendicular to the scale of the measuring instruments.

CONCLUSION :

From the experiment, we can conclude that aluminium possess a higher ductility compared to steel due to a higher elongation and narrower necking. Meanwhile, the toughness of the steel is higher compared to aluminium due to the higher Modulus of Elasticity, E. Ductility and toughness of metal samples can be determined by using tensile test.