Toughness of a material can be measured using the Charpy V-notch test, a simple test that characterizes the material's robustness or resistance to fracture.
Brittle failures are one of the most insidious structural failures, coming with no warning. To avoid this, applications involving very low operating temperatures, repeated cycles of loading, or extensive welding must make us of tough materials. Tough materials are much less likely to fail in a brittle manner.
Toughness can be measured using the Charpy V-notch test. Testing involves hitting a notched specimen with a swinging hammer of known weight, calculating the energy absorbed by the specimen during impact, and observing the fracture surface.
This video will illustrate how to perform the Charpy V-notch test and analyze the results.
A tough material is one that is both strong and ductile. It can absorb more energy than materials that are less tough before failing. Along with the chemical composition of a material, changes in material processing and the loading situation can cause changes in the toughness of a material.
The Charpy V-notch test is used to predict whether a material will behave in a brittle or ductile manner in service. Each test specimen has standardized dimensions with a V-notch designed to significantly increase the localized stress. During testing, the specimen is supported in the test machine with the notch facing away from the direction of loading. A hammer of a known weight and height is swung, striking the specimen. The notched side of the specimen experiences tension. This results in a crack propagating through the thickness of the specimen to failure.
The potential energy of the hammer becomes kinetic energy as it swings toward the specimen. As the hammer hits the specimen, a small amount of energy is absorbed. Change in potential energy can be calculated knowing the height of the hammer before and after striking the specimen. The energy lost by the hammer is equal to the energy absorbed by the specimen. Energy absorbed during failure indicates the toughness of the material. This is related to the area under the stress-strain curve, with the toughest materials able to absorb both high stress and high strain.
Charpy V-notch impact test values are accurate for specific testing conditions but can also be used to predict the relative behavior of materials.
In the next section, we will measure the toughness of two different kinds of steel at both high and low temperatures using the Charpy V-notch impact test.
Caution: this experiment involves heavy moving parts and extreme temperatures. Follow all safety guidelines and procedures during testing. Before the day of testing, have specimens of the desired materials machined to the standard dimensions for Charpy testing.
For this demonstration, we will test two different types of steel, ASTM A36 and C1018. To prepare the specimens, use the cold box to cool one specimen of each metal to minus 40 degrees Celsius. Use a hot plate to heat another specimen of each metal to 200 degrees Celsius. Keep a third set of specimens at room temperature.
Now, prepare the testing machine. First, check that the path of the hammer is clear of any obstructions, and then lift the hammer until it latches. Secure the lock to prevent an accidental release of the hammer. Confirm that the area is clear, then remove the lock and press on the lever to release the pendulum. The hammer should swing down freely with very little friction, so that negligible energy is lost as indicated on the dial. Use the break to stop the pendulum so that you can resecure the hammer, and then use tongs to center a specimen on the anvil with the notch facing away from the impact side.
When the specimen is ready, set the dial on the machine to 300 foot pounds. Once again confirm that the area is clear, and then release the pendulum. The hammer will impact the specimen, and as it swings up on the opposite side, move the dial to indicate the amount of the energy that the specimen absorbed. Record the value from the gauge, and then use the machine break to stop the hammer from swinging. Engaging the break will invalidate the gauge reading, so do not take the reading after the break has been applied.
Once the pendulum has stopped, retrieve the specimen and determine the percent of area of the fractured face that has fibrous texture. Repeat the test procedure for the remaining samples. When you have finished the final test, leave the hammer in the down position.
Now, take a look at the results.
Compare representative samples of a face centered cubic material from each of the temperature groups. These samples show little variation across the range of temperatures tested.
Now, compare samples of a body centered cubic material from each of the temperature groups. Samples that were tested at elevated temperature show more ductility and plastic deformation, whereas samples from the low temperature group display signs of brittle fracture.
The transition to brittle failure can be seen by plotting the absorbed energy as a function of sample temperature for many tests. For body centered cubic materials, there is a clear upper plateau in absorbed energy at elevated temperatures, a low plateau at reduced temperatures, and a transition region in between. Face centered cubic materials do not display the same transition at reduced temperatures.
Now that you appreciate the Charpy V-notch impact test for its use in predicting the toughness of materials in service, let's take a look at how it is applied to assure sound structures every day.
Extreme temperature environments, like space exploration, where the temperature varies over a great range, as well as dog sledding, where temperatures dip well below zero, require tough materials.
A particularly important application is in bridge design, where steels are required to meet ASTM standards, which include both low and high temperature Charpy limits.
You've just watched JoVE's introduction to the Charpy impact test. You should now understand how to perform the Charpy impact test on materials at a variety of temperatures, and how these results relate to the material toughness.
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