01 Tension En
-
Upload
kenneth-tan -
Category
Documents
-
view
220 -
download
0
Transcript of 01 Tension En
-
7/28/2019 01 Tension En
1/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 1
T. Udomphol
LLaabboorraattoorryy 11
Tensile Testing
____________________________________
Objectives
Students are required to understand the principle of a uniaxial tensile testing andgain their practices on operating the tensile testing machine.
Students are able to explain load-extension and stress-strain relationships. To evaluate the values of ultimate tensile strength, yield strength, % elongation,
fracture strain and Youngs Modulus of the selected metals when subjected to
uniaxial tensile loading.
Students can explain deformation and fracture characteristics of different materialssuch as aluminium, steels or brass when subjected to uniaxial tensile loading.
-
7/28/2019 01 Tension En
2/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 2
T. Udomphol
1. Literature Review
1.1 Stress and strain relationship
Uniaxial tensile test is known as a basic engineering test to achieve ultimate strength, yield
strength and ductility of interested materials. These important parameters are useful for the selection
of engineering materials for any applications required. A standard specimen is prepared in a round or
a square section along the gauge length as shown in figures 1 a) and b) respectively, depending on the
standard used. Both ends of the specimens should have sufficient length and a surface condition such
that they are firmly gripped during testing. The initial gauge length Lo
is standardized (in several
countries) and varies with the diameter (Do) or the cross-sectional area (A
o) of the specimen as listed
in table 1. This is because if the gauge length is too long, the % elongation might be underestimated
in this case. Any heat treatments should be applied on to the specimen prior to machining to produce
the final specimen readily for testing. This has been done to prevent surface oxide scales that might
act as stress concentration which might subsequently affect the final tensile properties due to
premature failure. There might be some exceptions, for examples, surface hardening or surface
coating on the materials. These processes should be employed after specimen machining in order to
obtain the tensile properties results which include the actual specimen surface conditions.
Figure 1: Standard tensile specimens
Type specimen United State (ASTM) Great Britain Germany
Sheet )/( oo AL 4.5 5.65 11.3
Rod )/( oo DL 4.0 5.0 10.0
Table 1: Dimensional relationships of tensile specimens used in various countries.
-
7/28/2019 01 Tension En
3/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 3
T. Udomphol
When a specimen is subjected to a tensile loading, the metal will undergo elastic and plastic
deformation. Initially, the metal will elastically deform giving a linear relationship of load and
extension. These two parameters are then used for the calculation of the engineering stress and
engineering strain to give a relationship as illustrated in figure 2 using equations 1 and 2 as follow
oA
P= (1)
oo
of
L
L
L
LL =
= (2)
where is the engineering stress
is the engineering strain
P is the external tensile load
Ao
is the original cross-sectional area of the specimen
Lo
is the original length of the specimen
Lf
is the final length of the specimen
During elastic deformation, the engineering stress-strain relationship follows the Hooks Law
and the slope of the curve indicates the Youngs modulus (E)
=E (3)
If the tensile loading continues, yielding occurs at the beginning of plastic deformation. The
yield stress, y, can be obtained by dividing the load at yielding (Py) by the original cross-sectional
area of the specimen (Ao) as shown in equation 4.
o
y
yA
P= (4)
-
7/28/2019 01 Tension En
4/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 4
T. Udomphol
Figure 2: Stress-strain relationship under uniaxial tensile loading
The yield point can be observed directly from the load-extension curve of the BCC metals
such as iron and steel or in polycrystalline titanium and molybdenum, and especially low carbon
steels, see figure 3 a). The yield point elongation phenomenon shows the upper yield point followed
by a sudden reduction in the stress or load till reaching the lower yield point. At the yield point
elongation, the specimen continues to extend without a significant change in the stress level. Load
increment is then followed with increasing strain. This yield point phenomenon is associated with a
small amount of interstitial or substitutional atoms. This is for example in the case of low-carbon
steels, which have small atoms of carbon and nitrogen present as impurities. When the dislocations
are pinned by these solute atoms, the stress is raised in order to overcome the breakaway stress
required for the pulling of dislocation line from the solute atoms. This dislocation pinning is related
to the upper yield point as indicated in figure 3 a). If the dislocation line is free from the solute
atoms, the stress required to move the dislocations then suddenly drops, which is associated with the
lower yield point. Furthermore, it was found that the degree of the yield point effect is affected by the
amounts of the solute atoms and is also influenced by the interaction energy between the solute atoms
and the dislocations.
-
7/28/2019 01 Tension En
5/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 5
T. Udomphol
Aluminium on the other hand having a FCC crystal structure does not show the definite yield
point in comparison to those of the BCC structure materials, but shows a smooth engineering stress-
strain curve. The yield strength therefore has to be calculated from the load at 0.2% strain divided by
the original cross-sectional area as follows
o
yA
P %2.0%2.0 = ...(5)
Note: the yield strength values can also be obtained at 0.5 and 1.0% strain.
The determination of the yield strength at 0.2% offset or 0.2% strain can be carried out by
drawing a straight line parallel to the slope of the stress-strain curve in the linear section, having an
intersection on the x-axis at a strain equal to 0.002 as illustrated in figure 3 b). An interception
between the 0.2% offset line and the stress-strain curve represents the yield strength at 0.2% offset or
0.2% strain.
Figure 3: a) Comparative stress-strain relationships of low carbon steel and aluminium alloy and b)
the determination of the yield strength at 0.2% offset.
-
7/28/2019 01 Tension En
6/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 6
T. Udomphol
Beyond yielding, continuous loading leads to an increase in the stress required to
permanently deform the specimen as shown in the engineering stress-strain curve. At this stage, the
specimen is strain hardened or work hardened. The degree of strain hardening depends on the nature
of the deformed materials, crystal structure and chemical composition, which affects the dislocation
motion. FCC structure materials having a high number of operating slip systems can easily slip and
create a high density of dislocations. Tangling of these dislocations requires higher stress to
uniformly and plastically deform the specimen, therefore resulting in strain hardening.
If the load is continuously applied, the stress-strain curve will reach the maximum point,
which is the ultimate tensile strength (UTS, TS
). At this point, the specimen can withstand the
highest stress before necking takes place. This can be observed by a local reduction in the cross-
sectional area of the specimen generally observed in the centre of the gauge length as illustrated in
figure 4. After necking, plastic deformation is not uniform and the stress decreases accordingly until
fracture. The fracture strength (fracture
) can be calculated from the load at fracture divided by the
original cross-sectional area,Ao, as expressed in equation 6.
o
fracture
fracture
A
P= (6)
Figure 4: Necking of a tensile specimen occurring prior to fracture
Tensile ductility of the specimen can be represented as % elongation or % reduction in area
as expressed in the equations given below
100%
=
oL
LElongation (7)
-
7/28/2019 01 Tension En
7/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 7
T. Udomphol
100100%0
=
=A
A
A
AARA
o
fo(8)
where Af is the cross-sectional area of specimen at fracture.
The fracture strain of the specimen can be obtained by drawing a straight line starting at the
fracture point of the stress-strain curve parallel to the slope in the linear relation. The interception of
the parallel line at the x axis indicates the fracture strain of the specimen being tested.
1.2 Fracture characteristics of the tested specimens
Metals with good ductility normally exhibit a so-called cup and cone fracture observed on
either halves of a broken specimen as illustrated in figure 5. Necking starts when the stress-strain
curve has passed the maximum point where plastic deformation is no longer uniform. Across the
necking area within the specimen gauge length (normally located in the middle), microvoids are
formed, enlarged and then merged to each other as the load is increased. This creates a crack having a
plane perpendicular to the applied tensile stress. Just before the specimen breaks, the shear plane of
approximately 45o
to the tensile axis is formed along the peripheral of the specimen. This shear plane
then joins with the former crack to generate the cup and cone fracture as demonstrated in figure 5.
The rough or fibrous fracture surfaces appear in grey by naked eyes. Under SEM, copious amounts of
microvoids are observed as depicted in figure 6. This type of fracture surface signifies high energy
absorption during the fracture process due to large amount of plastic deformation taking place, also
indicating good tensile ductility.
For brittle metals or metals that failed at relatively low temperatures, the fracture surfaces
usually appear bright and consist of flat areas of brittle facets when examined under SEM as
illustrated in figure 7. In some cases, clusters of these brittle facets are visible when the grain size of
the metal is sufficiently large. The energy absorption is quite small in this case which indicates
relatively low tensile ductility due to limited amount of plastic deformation.
-
7/28/2019 01 Tension En
8/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 8
T. Udomphol
Figure 5: Cup and cone fracture [3]
Figure 6: Ductile fracture surface (Ductile metals) Figure 7: Brittle fracture surface (Brittle metals)
-
7/28/2019 01 Tension En
9/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 9
T. Udomphol
2. Materials and equipment
2.1 Tensile specimens
2.2 Micrometer or vernia calipers
2.3 Universal testing machine
3. Experimental procedure
3.1 The specimens provided are made of aluminium, steel and brass. Measure and record
specimen dimensions (diameter and gauge length) in a table provided for the calculation of
the engineering stress and engineering strain. Marking the location of the gauge length along
the parallel length of each specimen for subsequent observation of necking and strain
measurement.
3.2 Fit the specimen on to the universal Testing Machine (UTM) and carry on testing. Record
load and extension for the construction of stress-strain curve of each tested specimen.
3.3 Calculate Youngs modulus, yield strength, ultimate tensile strength, fracture strain and %
elongation of each specimen and record on the provided table.
3.4 Analyze the fracture surfaces of broken specimens and sketch and describe the results
3.5 Discuss the experimental results and give conclusions.
-
7/28/2019 01 Tension En
10/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 10
T. Udomphol
4. Results
Details Aluminium Steel Brass
Diameter (mm)
Width (mm)
Thickness (mm)
Cross-sectional area (mm2)
Gauge length (mm)
Youngs modulus (GPa)
Load at yield point (N)
Yield strength (MPa)
Maximum load (N)
Ultimate tensile strength (MPa)
% Elongation
Fracture strain
Work hardening exponent (n)
Fracture mode
Fracture surfaces
(Sketch)
Table 2: Experimental data for tensile testing.
-
7/28/2019 01 Tension En
11/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 11
T. Udomphol
Engineering stress-strain curve of aluminium
Describe the engineering stress-strain curve
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
-
7/28/2019 01 Tension En
12/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 12
T. Udomphol
Engineering stress-strain curve of steel
Describe the engineering stress-strain curve
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
-
7/28/2019 01 Tension En
13/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 13
T. Udomphol
Engineering stress-strain curve of brass
Describe the engineering stress-strain curve
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
-
7/28/2019 01 Tension En
14/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 14
T. Udomphol
5. Discussion
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
-
7/28/2019 01 Tension En
15/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 15
T. Udomphol
6. Conclusions
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
-
7/28/2019 01 Tension En
16/17
Laboratory 1: Tensile testing
Mechanical metallurgy laboratory 431303 16
T. Udomphol
7. Questions
7.1 What is work hardening exponent (n)? How is this value related to the ability of metal to be
mechanically formed?
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
7.2 If the tensile specimen is not cylindrical rod shaped but a flat rectangular plate, how do you
expect necking to occur in this type of specimen?
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
____________________________________________________________________________________________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
-
7/28/2019 01 Tension En
17/17
Laboratory 1: Tensile testing
M h i l t ll l b t 431303 17
7.3 Both yield strength and ultimate tensile strength exhibit the ability of a material to withstand
a certain level of load. Which parameter do you prefer to use as a design parameter for a
proper selection of materials for structural applications? Explain
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
8. References
8.1 Hashemi, S. Foundations of materials science and engineering, 2006, 4th
edition, McGraw-
Hill, ISBN 007-125690-3.
8.2 Dieter, G.E.,Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-
100406-8.
8.3 W.D. Callister, Fundamental of materials science and engineering/an interactive e. text,
2001, John Willey & Sons, Inc., New York, ISBN 0-471-39551-x.