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Laboratory 6: Impact testing
Mechanical Metallurgy Laboratory 431303 1
T. Udomphol
LLaabboorraattoorryy 66
Impact Testing
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Objectives
• Students are required to study the principle of impact testing using metals which are
susceptible to brittle fracture such as mild steels.
• Types of fracture in metals are investigated using the fracture energy absorption
criterion.
• Students can explain the meaning and use of Ductile-to-Brittle-Transition-
Temperature Curve (DBTT) and explain the relationship between the absorbed
energy of the specimen and its fracture surfaces. Identify the transition temperature
of the tested materials.
• Students are capable of interpreting the obtained experimental data for the selection
of engineering materials.
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1. Literature Review
1.1 Brittle fracture
Fracture in materials was widely investigated especially during the industrial revolution
where extraction processes of iron and steels led to the wide-spread uses of iron and steels for
structural and transportation applications, etc. However, metallurgy of iron and steels was not deeply
understood, which resulted in improper utilization of materials. Moreover, with low engineering
technology, defects were normally observed in jointed metals or assembled parts, which were the
main problems leading to weakening and global failure of engineering structures during services. The
well known case has been the tragic failure of the Liberty ships and T-2 tankers. The Liberty ships
built during the World War II appeared to have cracks along the welds resulting in fracturing into two
halves as they were at the deck prior to services as pictured in figure 1. Brittle fracture has then been
investigated in great details whereas ductile fracture was however studied in a lower extent due to its
less deleterious effects. Since brittle fracture has been one of the most catastrophic types leading to
losses of life and cost, study of brittle fracture especially in steels has therefore been on the main
focus. Investigation into causes and factors affecting fracture behaviour has been of great interest
and solutions to its problems have also been cooperated.
Figure 1: Liberty ship which was broken in two halves along the welds.
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There are three main factors influencing brittle fracture in materials 1) triaxial state of
stresses, 2) low temperature application and 3) high strain rate or rapid rate of loading. Defects such
as cracks and porosity found during casting, rolling or forging produce triaxial state of stresses in the
components. However, for brittle fracture to occur, it is not necessary that all three factors are present
at the same time. It has been observed that the state of stresses and service temperatures are the main
causes of brittle failure whereas strain rate seems to aid the mechanism of brittle fracture to progress
sooner and more severe.
1.2 Charpy impact testing
Charpy impact test is practical for the assessment of brittle fracture of metals and is also used
as an indicator to determine suitable service temperatures. The Charpy test sample has 10x10x55
mm3 dimensions, a 45
o V notch of 2 mm depth and a 0.25 mm root radius will be hit by a pendulum at
the opposite end of the notch as shown in figure 2. To perform the test, the pendulum set at a certain
height is released and impact the specimen at the opposite end of the notch to produce a fractured
sample. The absorbed energy required to produce two fresh fracture surfaces will be recorded in the
unit of Joule. Since this energy depends on the fracture area (excluding the notch area), thus standard
specimens are required for a direct comparison of the absorbed energy.
Figure 2: Charpy impact test, a) test method and b) notch dimensions.
As the pendulum is raised to a specific position, the potential energy (mgh) equal to
approximately 300J is stored. The potential energy is converted into the kinetic energy after releasing
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the pendulum. During specimen impact, some of the kinetic energy is absorbed during specimen
fracture and the rest of the energy is used to swing the pendulum to the other side of the machine as
shown in figure 2 (a). The greater of the high of the pendulum swings to the other side of the
machine, the less energy absorbed during the fracture surface. This means the material fractures in a
brittle manner. On the other hand, if the absorbed energy is high, ductile fracture will result and the
specimen has high toughness.
Generally, fracture behaviour of BCC structured metals such as mild steels varies with
temperature. At low temperature, BBC metals fracture in a brittle mode and becomes more ductile as
the temperature increases. FCC structure metals such as stainless steels, copper and alumminium
however do not show a dramatic change in fracture behaviour with increasing temperature.
Therefore, an investigation of fracture behaviour in BCC structure metals is concerned with the
ductile to brittle transition temperature (DBTT) curve. This curve shows three different regions of
lower shelf, upper shelf and transition region as shown in figure 3. If we first consider fracture
surfaces of samples tested at low temperatures, the brittle fracture surfaces consisting primarily of
cleavage facets and in some cases with small areas of ductile dimple as illustrated in figure 4.
Cleavage fracture requires less energy to produce flat fracture surfaces of the cleavage facets. As the
temperature increases, the area of cleavage facets is reduced as opposed to increasing regions of
ductile dimples or ductile tearing. Within a transition range, the absorbed energy increases rapidly
and the specimen fracture surfaces now show a mixed mode of ductile and brittle features. The
percentage of ductile and brittle features in this region depends on the test temperatures. The higher
the temperature, the more ductile areas will result. In the upper shelf region according to the DBTT
curve, the fracture surfaces become fully ductile (100% fibrous). The fracture surface appears
relatively rough, dull and gray due to microvoid formation and coalescence. This type of fracture
surface provides the highest energy absorption due to extensive plastic deformation.
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Figure 3: Effect of temperature on energy absorption during material fracture.
Figure 4: Fracture surfaces at different temperatures.
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As the ductile and brittle behaviours of BCC structure metals vary with temperatures, it is
important to identify the operating temperature that can avoid brittle failure. If the operating
temperatures are too low (lower shelf), the potentially dangerous cases of brittle fractures will take
place. Moreover, dissimilar metals possessing different microstructures provide different levels of
energy to be absorbed during fracture. Therefore, the use of structural materials especially BCC
metals should concern about their service temperatures such that brittle fracture can be avoided.
Temperatures at which material fracture appear fully ductile are considered to be safe whereas the
application in the transition temperature involves certain degrees of risk.
1.3 Criteria for the determination of transition temperature
As mentioned previously, the absorbed energy of BCC metals changes drastically within the
transition region, we therefore have to identify a transition temperature, which can be used to
determine the suitable service temperature of particular materials in order to avoid metal failure in a
catastrophic manner. There are several criteria for the identification of the transition temperature.
(See figure 6).
• T1 Transition temperature is the temperature at which the test sample absorbs the most
fracture energy and possesses 100% fibrous fracture surfaces. This means brittle fracture is
neglected in this case and is considered to be the safest among other criteria. The T1
transition temperature is also called the fracture transition plastic or FTP.
• T2 Transition temperature is the temperature at which the percentage of cleavage and ductile
fractures are equal. This transition is also called fracture appearance transition temperature or
FATT because the fracture surface area is used as an indicator to determine the transition
temperature.
• T3 Transition temperature is the temperature correlating to an average absorbed energy value
of upper and lower shelf energy absorption. At or above this temperature, there is a
correlation that less than 70% of the brittle cleavage fracture that indicates a high probability
at which failure will not occur if the stress does not exceed about one-half of the yield stress.
• T4 Transition temperature is the temperature at which the absorbed energy (C
v) equals 20J.
This criterion was introduced to determine toughness value of steels used during the World
War II. It is based on the idea that brittle fracture will not occur if the sample has the
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absorbed energy above 20J. However this criterion might show no significant meanings for
other materials.
• T5 Transition temperature is the temperature at which there is none of the ductile dimples
appearing on the fracture surfaces. This temperature is also called nil ductility temperature or
NDT since there is no plastic deformation during fracture.
Figure 6: Different criteria used to determine the transition temperature.
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2. Materials and equipment
2.1 Standard Charpy impact specimens
2.2 Micrometer or vernia caliper
2.3 Impact testing machine
2.4 Water
2.5 Ethanol
2.6 Liquid nitrogen
2.7 Cryogenic equipment
2.8 Hot plate
2.9 Beaker
2.10 Thong
2.11 Low temperature gloves
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3. Experimental procedure
3.1 Exmaine standard Charpy impact specimens of 10x10x55 mm3 dimensions with a notch of
45o angle and 2 mm depth located in the middle as shown in figure 2.
3.2 A pair of specimens will be tested at individual temperatures using the mediums as listed in
table 1.
Temperature -196oC -78
oC 0
oC 25
oC 100
oC
Medium Liquid
nitrogen
Liquid
nitrogen +
ethanol
Ice Room
temperature
Boiling water
Table 1 Temperature and mediums used for Charpy impact testing.
3.3 Room temperature test is first carried out by placing the Charpy impact specimen on the anvil
and positioning it in the middle location using a positioning pin where the opposite site of the
notch is destined for the pendulum impact (see figure 2)
3.4 Raise the pendulum to a height corresponding to the maximum stored energy of 300J.
Release the pendulum to allow specimen impact. Safely stop the movement of the pendulum
after swinging back from the opposite side of the machine.
3.5 When the pendulum is still, safely retrieve the broken specimen without damaging fracture
surfaces. Record the absorbed energy in table 2. Repeat the test at the same test condition
using another specimen to average out the obtained values.
3.6 Charpy impact testing at temperatures other than room temperature is carried out following
2.3-2.5. Prior to specimen impact, specimen is submerged in the medium for at least 5
minutes to ensure uniform temperature across the specimens. Specimen impact must be
within 5 seconds after removing from the medium. Record the absorbed energy in table 2.
Repeat the test at the same test condition using another specimen to average out the obtained
values.
3.7 Plot the ductile to brittle transition temperature curves of mild steel and stainless steel.
Analyze and discuss the experimental results. Give conclusions.
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4. Results
Mild steel Stainless steel Temperature
(oC)
Specimen 1 Specimen 2 Average Specimen 1 Specimen 2 Average
-196
-78
0
25
100
Table 2: Absorbed energy of each specimen tested at different temperatures.
Figure 7: Ductile to brittle transition temperature curves.
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Figure 8: Fracture surfaces of Charpy specimens tested at different temperatures.
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5. Discussion
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6. Conclusions
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7. Questions
7.1 Why do mild steel and stainless steel behave differently according to this experiment?
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7.2 What temperatures do you think mild steel and stainless steel will be safely used? Explain.
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7.3 Do you think aluminium will experience ductile to brittle transition? Why?
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8. References
8.1 Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-
100406-8.
8.2 Hashemi, S. Foundations of materials science and engineering, 2006, 4th edition, McGraw-
Hill, ISBN 007-125690-3.
8.3 Noble, B., Tensile and impact properties of metals and polymers, TQ education and training
led product division, 1996.