Fonte: Roberto Leon, Departamento de Engenharia Civil e Ambiental, Virginia Tech, Blacksburg, VA
Um dos tipos mais insidiosos de falhas que podem ocorrer nas estruturas são as fraturas frágeis, que são principalmente devido a materiais de má qualidade ou má seleção de materiais. Fraturas frágeis tendem a ocorrer de repente e sem muita inelasticidade material; pensar em uma fratura óssea, por exemplo. Essas falhas ocorrem frequentemente em situações em que há pouca capacidade para o material desenvolver tensões de tesoura devido às condições de carregamento tridimensional, onde as concentrações de tensão locais são altas, e onde um caminho de força lógica e direta não foi fornecido pelo designer. Exemplos desse tipo de falha foram observados após o terremoto de Northridge em 1994 em estruturas de aço de vários andares. Nestes edifícios, várias das soldas de chave fraturadas sem apresentar qualquer comportamento dúctil. As fraturas tendem a ocorrer perto de conexões, ou em interfaces entre peças de materiais básicos, pois a soldagem tende a introduzir descontinuidades locais em ambos, materiais e geometria, bem como tensões tridimensionais devido ao resfriamento.
Ao especificar materiais para uma estrutura que verá temperaturas de operação muito baixas (ou seja, o gasoduto do Alasca) muitos ciclos de carregamento (uma ponte em uma rodovia interestadual), ou onde a soldagem é usada extensivamente, é necessário ter um teste simples que caracterize a robustez do material, ou resistência à fratura. No campo da engenharia civil esse teste é o teste de entalhe em V de Charpy, que é descrito neste laboratório. O teste de entalhe Em V charpy destina-se a fornecer uma medida muito simplista da capacidade do material de absorver energia quando submetido a uma carga de impacto.
After repeating the experiment for may specimens and temperature values, you can plot the temperature dependence of the energy absorbed and clearly see the existence of an upper and lower shelf (or flat horizontal portions). These shelves indicate that there are clear minima and maxima that can be achieved for a given material and processing. The main interest is in carefully quantifying the transition temperatures to minimize the risk that these fall within the operating temperatures of the structure being designed. Similar materials undergoing different heat and mechanical treatments will show somewhat similar upper and lower shelves, but also a distinct shift in the transition temperature. Moving the transition zone to the left will tend to lower the fracture risk for a structure; however, that entails significant additional costs in terms of processing.
It also should be noted that the Charpy test is useful for characterizing brittle materials, which will show very little ductility. In practice, Charpy tests are used for all types of materials, including very ductile metals. This use is fundamentally incorrect because the deformation processes driving a brittle failure are different from those in a ductile failure. It has not been possible to derive a simple test that can be used in a production setting, like the Charpy one, for semi-ductile or ductile materials. Thus, it is likely that the Charpy tests will remain popular in the near future.
Impact testing, in the form of Charpy and Izod tests, is commonly used to measure the resistance of metallic materials to brittle fracture. The Charpy test uses a small beam specimen with a notch. The beam is loaded by a large hammer attached to a frictionless pendulum. The combination of the strain rate from this loading sequence and the presence of the V-notch that creates a local large stress concentration result in fast crack propagation and splitting of the specimen.
The test determines the energy absorbed by the material during fracturing by comparing the potential energy at the beginning and ending of the test as measured from the position of the impact hammer. The magnitude of the energy absorbed is dependent on the volume of the material in the small beam specimen, so the results are valid only in a comparative sense.
Fracture mechanics is a very important field of studies in all materials, as it reminds us that all materials contain flaws that the shape and size of the flaw are important, and that one needs to address in design the issue of stress concentrations.
One demonstration of the importance of temperature dependence was in World War II when some Liberty ships and T-2 tankers literally split in half while still in port. For the Liberty ships, this failure had to do with stress concentrations that were induced during welding, as well as embrittlement of the steel hull due to welding operations and accompanied by cold sea temperatures.
The Charpy V-notch test is part of many ASTM standards, and as such, is present in many products that we use everyday. A particularly important application is in bridge design where most steels are specified to pass a low temperature and a high temperature Charpy limit (i.e., 20 ft-lbs at -40°F and 40 ft-lbs at 80°F).
Fracture energy is a very important material property. If one tests a flawless glass plate with surface energy γs= 17×10-5 in-lb/in2 and E=10×106 psi, the theoretical fracture strength would be about 465,000psi, given Griffith's equation (σf = (2Eγs/πa)0.5). If one introduces a flaw, even with a magnitude as small as 0.01in, into the glass plate, the fracture strength is reduced by three orders of magnitude to only 465psi, which is much more like what we see in real life.
Other temperature dependent applications for which a Charpy v-notch test would be important include testing equipment for space travel, where the temperature varies overa great range, as well as for sledding equipment in Antarctica and other polar regions, where temperatures dip well below zero.
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.
Thanks for watching!
