Tensile testing is one of the most important mechanical tests which can be conducted on an
engineering material. To compare and select materials for various applications one must
know the important properties of that material. For example, steel, one of the most important
engineering materials, is manufactured in different grades with different strengths,
toughness, ductility etc. For this reason, it is important to have an effective method of
determining these properties. Tensile testing can accurately measure many of the most
important engineering design properties.
Stress,o (MPa)
Utimate
Strength
Yield Point
Plastic
Deformation
• Young's modulus
• Yield point
• Ultimate tensile strength
• Ductility
• Toughness
• Resilience
Necking
Figure 1: Stress/strain curve showing yield point, ultimate tensile strength, fracture point and deformation regions.
In tension tests, a specimen is subjected to a continually increasing extension while
simultaneous observations are made of the uniaxial load on the specimen. An engineering
stress-strain curve is constructed from the load-elongation measurements. In the first part of
the practical session, you will see how the load versus elongation curve is obtained, and in
the later part of the session, you will use load and elongation results to draw a stress-strain
curve and calculate some of the most important materials properties parameters. This
procedure is expanded on below. Through this process, the following material properties can
be determined:
Strain,c (%)
It is important to note that not all mechanical properties can be determined through tensile
testing. Examples of important properties which must be obtained through other tests
include hardness, fatigue, and impact properties./n3.1. Stress-Strain Conversion
The load-elongation data must first be converted into stress-strain data for the determination
of material properties. Stress (a) can be found using (Eq 1) as the force (F) divided by the
original cross-sectional area (A). The strain (e) can be found using (Eq 2) as the change in
length (AZ) divided by the original gauge length (4). This data can then be used to produce a
stress-strain curve from which various material properties can be found.
0 = 7/10
==
3.2. Young's modulus
Young's modulus is the linear relationship between stress and strain of a material. At low
levels they are proportional to each other through the below (Eq 3) relations. Also known as
Hooke's law.
Eq. 1
a = Ee
Where o is the stress, E is the Young's modulus and is the strain.
Whilst the material exhibits this linear behaviour the materials deformation is reversible. The
Young's modulus is the first linear region of the stress/strain graph. See Figure 1.
3.3. Yield point
Eq. 2
I
The abrupt transition from elastic to plastic deformations is called the yield point. If no yield
point exists, the offset yield strength is calculated for a strain offset of 0.2%. This is where a
line is plotted on top of the stress/strain curve using Hooke's law, and then a parallel line is
D
Eq 3
drawn at 0.2% strain. The corresponding stress is known as the 0.2% offset yield strength. See
Figure 2.
Figure 2: Stress/strain curve showing 0.2% offline./n3.4. Ultimate tensile strength
Ultimate tensile strength (UTS) is the maximum stress the material can handle before failure.
Whilst this point may not represent the fracture point, it is the materials strength limit. See
figure 1.
3.5. Ductility
Ductility is a measure of the degree of plastic deformation at fracture. To calculate the
ductility of a material, the strain at fracture is required. It can be determined by Eq 4.
EL%=100
Eq. 4
Where EL% is the elongation, L, is the length at failure and L is the original length of the
material.
3.6. Toughness
Toughness is the ability of material to absorb energy and plastically deform before fracturing.
It is approximated by the area under the stress strain curve and determined by Eq 5.
24₂ = ¹o de
Where u, is the toughness, &, is the strain at failure and is the stress.
3.7. Resilience
Resilience is the capacity of a material to absorb elastic energy when it is deformed elastically,
and then upon unloading to have this energy recovered. To determine this, the area under
the linear region of curve is needed. This relationship can be simplified as shown in Eq 6.
Eq. 5
ur = fode = a₂²y
Eq. 6
Whereu, is the resilience, &, is the strain at yield, dy is the stress at yield and a is the stress.
Fig: 1
Fig: 2
Fig: 3