출처: 로베르토 레온, 버지니아 공대, 블랙스버그, 버지니아 토목 및 환경 공학부
구조물에서 발생할 수 있는 더 교활한 유형의 고장 중 하나는 부서지기 쉬운 골절이며, 이는 주로 품질이 좋지 않거나 재료 선택이 좋지 않아 발생하기 때문입니다. 부서지기 쉬운 골절은 갑자기 많은 물질 비탄력성없이 발생하는 경향이 있습니다. 예를 들어 뼈 골절을 생각해 보십시오. 이러한 오류는 3차원 적재 조건, 로컬 스트레인 농도가 높고 설계자가 논리적이고 직접적인 힘 경로가 제공되지 않은 경우 재료가 전단 응력을 개발할 수 있는 능력이 거의 없는 상황에서 발생합니다. 이러한 유형의 고장의 예는 다층 강철 구조물에서 1994 년 노스 리지 지진의 여파로 관찰되었다. 이 건물에서는 연성 동작을 표시하지 않고 여러 키 용접이 골절됩니다. 용접은 냉각으로 인한 3차원 응력뿐만 아니라 재료 와 형상 모두에서 로컬 불연속성을 도입하는 경향이 있기 때문에 골절은 연결 근처 또는 기본 재료 조각 사이의 인터페이스에서 발생하는 경향이 있습니다.
매우 낮은 작동 온도(즉, 알래스카 파이프라인)를 볼 수 있는 구조물에 대한 재료를 지정할 때(즉, 알래스카 파이프라인) 많은 하중 사이클(고속도로의 교량) 또는 용접이 광범위하게 사용되는 경우 재료의 견고성 또는 골절에 대한 저항성을 특징으로 하는 간단한 테스트를 수행해야 합니다. 토목 공학 분야에서 테스트는 이 실험실에 설명된 Charpy V-notch 테스트입니다. Charpy V-notch 테스트는 충격 부하를 받을 때 에너지를 흡수하는 재료의 능력을 매우 단순하게 측정하기 위한 것입니다.
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!
