Experiment No. 1
THE IMPACT TEST
EVALUATION OF TOUGHNESS OF MATERIALS
The suitability of any material for structural applications may be determined by
mechanical tests. Such testing includes a determination of the ability of a material to
withstand various stress systems which 'may be tensile, compressive, shear or cyclic in
nature. The behavior of any material is also influenced by the presence of defects, the
intrinsic microstructural features and the rate of application of stress, which may vary
from exceedingly slow rates of deformation (creep) to impact loading.'.
Time is an important factor in determining the amount of plastic deformation
(strain) that a material exhibits under stress. An increase in the rate of force application
in materials which are strain rate sensitive will cause an increase in the yield stress, the
ultimate tensile stress and a reduction in the elongation to fracture (Figure 1).
The magnitude of the response of a material to time dependent strain is markedly
affected by the temperature at which the test is conducted and by the internal structure of
the material. The latter would include such variables as dislocation arrangement, size and
distribution of the strengthening precipitates, and the grain structure of the material.
The Charpy impact test is the simplest of all testing procedures used to deter-mine
the toughness of a material. itprovides several useful properties of the material such as
toughness, ductility and fracture characteristics. In practice stress systems are complex
and hence, caution must be exercised in applying/using the information obtained frim the
2. Purpose of the Experiment
The primary purpose of the experiment is to demonstrate how an impact test is
performed and toughness of a material evaluated, and the fracture characteristics detailed.
Carbon steel and wrought precipitation hardened aluminum alloy are the two candidate
materials which will be tested and the toughness properties of these materials evaluated.
3. Background of the Experiment
Although most material properties change gradually as the
temperature is reduced, some materials have been found to exhib it dramatic property
changes over a small temperature range. A ductile-to-brittle transition is one such
property change of engineering importance. The characteristics of this transition are
shown in Figure 2.
At the higher temperatures the specimen deforms plastically prior to catastrophic
failure. At the lower temperatures, the specimen exhibits little or no plastic deformation
and fails predominantly by brittle ineer-crystalline clevage. In the transition zone
between these two extremes, the material fails by a mixture of ductile and brittle fracture.
A transition temperature, Tc, sometimes taken as the temperature in the. middle of the
transition zone characterizes this behavior of a material for a given testing condition. To
protect a component from brittle failure it is necessary that the transition temperature
of the material be below the lowest expected service temperature impact testing of
notched specimens using standard Charpy tests is normally used for determining the
ductile-to-brittle transition temperature. Since this represents a high strain rate and since
the transition temperature is raised by increasing the strain rate, a maximum Tc is
obtained by this testing procedure. Using this characteristic of the material in design
criteria helps to ensure that the structure will not undergo brittle failure.
The principal drawback of the Charpy tests is that the results from specimens with
standard notches are not reliable indicators of the behavior of real structures. It is
therefore difficult to- apply the results in the design of structures; though long
experiencxe with some steels has led to reasonable'. correlations with in-service
Low temperature brittleness is observed primarily in BCC and HCP metals, and
many polymeric materials. Most ceramics are normally brittle at ordinary temperatures.
The mechanical properties of FCC metals are little affected by temperature changes, as
shown in Figure 3. The ductile-to-brittle transition is also sensitive to the composition
and internal structure of the material. Nickel is often added to steels to lower the
transition temperature. coarse grained materials are also more prone to brittle failure.
The ductile-to-brittle transition approach has been responsible for many -drastic
service failures in BCC metals. During World War II, many welded Liberty ships broke
in half due to the combination of low temperature and the ductile-to-brittle behavior. A
contributory cause was the sharpy notched hatch cover design. The presence of a notch
makes the situation worse by raising the transition temperature. The transition
temperature is very dependent on the sharpness of the notch, and the sharper the notch the
higher the transition temperature.
4. Experimental Procedures
The Charpy impact test machine is a simple user-friendly instrument. A schematic
of the test machine is shown in Figure 4. The output of the test is ENERGY ABSORBED
from which the data analysis will be made. Basically, the test procedure is a very simple
one. The test specimen (Figure 5) is secured into a test fixture in the impact ma6hine-
Next, the specimen is impacted by the movable hammer or pendulum. The energy
absorbed or required. in fracturing the specimen is recorded from the pointer which
moves over a graduated scale.
A step by step summary of the test procedure is as follows:
Step 1. Set up the Charpy V-notch Impact test machine
Step 2. Insert a carbon steel sample in the test fixture
Step 3. Release the pendulum and record the energy absorbed,
Step 4. Repeat the test for the non-ferrous material aluminum
Step 5. Repeat the test f or '.the carbon steel and aluminum
specimens at the temperature of dry ice.
Step 6.Record the temperature of dry ice and the energy
Step 7. Repeat the test for the steel and aluminum specimens at the
temperature of liquid nitrogen.
Step 8. Record the temperature of liquid nitrogen and theenergy
Step 9. Repeat the impact test on the steel and aluminum
specimens at an elevated temperature.
Step 10. Record the elevated temperature and the energy absorbed
Step 11. Perform atleast two tests at each temperature.
5. Data Analysis
At this point you have in your possession data pertaining to test temperature or
temperature of the test specimen anmd corresponding energy absorbed.
1. Plot energy absorbed versus temperature. Draw a smooth curve
through the points.
2. Plot energy-absorbed versus temperature for the aluminum
specimen. Draw a smooth curve through the points.
3. Determine the transition temperature, Tc for the aluminum
4. Determine the transition temperature, TC" for the steel specimens
5. Describe the change in fracture appearance with temperature of the steel specimens
6. Describe the change in fracture appearance with temperature of the aluminum
7. Describe the difference in response to temperature of the carbon steel and
A technical report of the following format ois required
(a) ABSTRACT: This should be succint summary to explain what was done, why it
was done and what results were achieved.
(b) INTRODUCTION: This section should include the objective and explain the
theory and background associated with the
experiment. It should not be a repeat of what is already given in the handout.
(c) MATERIALS AND EXPERIMENTAL PROCEDURES: This section should
detail a description of the materials use d and what exactly was done.
(d) RESULTS: This section documents the experinmental findings in 'a concise and
logical sequence. Do not comment 'on the results.
(e) DISCUSSION OF RESULTS: This section is an interpretation of your
experimental results. Rationalize/explain any peculiarities in your data.
(f) CONCLUDING REMARKS: Concise statements in a logical sequence describing
fundamental conclusions from this experiment
(g) REFERENCES: If you referrred to any reference in the mai; body of the report,
this should be listed here. ASME f ormatting is required.
(h) APPENDICES: This section of the report must include all the tables and graphs.
(I) GRAPHS: in the report should be full page. Use only regular/standard graph
paper. Smooth curve should-b@rawn through the data points. You will/could
have scatter in data, but you certainly should not draw your graph Point-topoint.
Axes and units should be clearly labelled and marked.