The subject of impact testing continues from last month’s discussion by looking at the Charpy “V” notch test in more detail. Let’s learn more.

Charpy Test Specimens

Charpy test specimens normally measure 55 mm x 10 mm x 10 mm and have a notch machined across one of the larger faces. The basic types of notches are the “V” notch, keyhole notch and “U” notch (Fig. 1). The V-shaped notch is 2 mm deep, with 45° included angle and 0.25-mm radius at its root. The U-shaped notch is 5 mm deep with vertical sides and a 1-mm radius U-shaped root. The keyhole notch is also 5 mm deep and contains a 2-mm-diameter hole and vertical sides. The notch shape, depth and tip radius are, therefore, important test parameters.

Since the base of the notch is in a state of triaxial tension (part 1) and the rate of loading is 10 million times faster than a standard tensile test, the notch serves as a stress concentration zone. Some materials are also more sensitive to the presence of a notch than others.

What is impact energy?

Impact energy is a measure of the work done to fracture a test specimen when subjected to a high instantaneous shock load. In a Charpy test, when the striker impacts the specimen, the specimen absorbs the energy of impact and begins to yield, with plastic deformation (and subsequent work hardening) occurring at the notch. When the specimen can absorb no more energy, fracture occurs.

Determination of Charpy Impact Energy

At the point of impact, the striker has a known amount of kinetic energy. The impact energy is calculated based on the height to which the striker would have risen if no test specimen was in place compared to the height to which the striker actually rose. Prior to fracture, tough materials absorb relatively higher amounts of energy than brittle materials, which absorb much lower amounts of energy.

Factors that affect the Charpy impact energy of a specimen include:

  • Yield strength and ductility
  • Stress intensity for the type, size and depth of notch employed
  • Temperature
  • Strain rate
  • Fracture mechanism

Yield Strength and Ductility

For a given material, the impact energy will generally decrease if the yield strength is increased (i.e., if the material undergoes processing of some nature that makes it more brittle and less able to resist plastic deformation). Such processes may include various types of heat treatments, cold working, etc.

Temperature and Strain Rate

Most of the impact energy is absorbed by means of plastic deformation during the yielding of the specimen. Therefore, factors that affect the yield behavior and ductility of the material, such as temperature and strain rate, will influence the impact energy. This type of behavior is more prominent in materials with a body-centered cubic structure (e.g., martensite, ferrite), where lowering the temperature reduces ductility more markedly than for face-centered cubic structures (e.g., austenite).

The notch-bar impact test over a range of temperatures is more meaningful than at a single temperature. Multi-test temperatures at above and below room temperature are employed to determine the ductile-to-brittle transition temperature of a given material (Fig. 2). Steel A shows higher notch toughness at room temperature, but its transition temperature is higher than that of steel B. The material with the lowest transition temperature is generally preferred.[2]

Fracture Mechanism

Metals tend to fail in overload by one of two fracture modes: either ductile rupture by microvoid coalescence or brittle fracture via cleavage. Microvoid coalescence occurs when voids form and strain increases, causing these voids to eventually join together, and failure results. By contrast, cleavage occurs via shear along specific crystal planes. Of the two fracture modes, cleavage involves far less plastic deformation and hence absorbs far less fracture energy.[4,5]

Variables Affecting Transition Temperature

The transition temperature in steel can be affected by a number of factors, one of the most interesting of which is the microstructure. If we compare a quenched-and-tempered microstructure with that of a normalized-and-tempered microstructure, we can see the effect of these different heat treatments on the impact strength and ductile-to-brittle transition temperature (DBTT) characteristics of 4340 medium-carbon alloy steel with high hardenability (Fig. 3). While both heat treatments produced an identical hardness, the microstructure of the quenched-and-tempered material consisted of tempered martensite and fine spheroidized alloy carbides. The normalized structure may consist of some combination of pearlite (in this case), bainite or ferrite, depending on the size and cooling rate of the heat-treated component. As one quickly observes, the transition temperature is approximately 150?C (300?F) lower for the quenched-and-tempered microstructure.

The chemistry of the steel also plays a prominent role. For example, the transition temperature for a 1% manganese, 0.30% silicon mild-carbon steel rises rapidly with increasing carbon content. Thus (all other factors being equal), the higher the carbon content, the more prone a material is to brittle fracture, especially near room temperature and below. Phosphorous has an even stronger effect on raising the DBTT, while increasing manganese contents tends to lower it. Other elements may play important roles, depending on the composition of the steel.

Trends

Impact testing has changed very little over the decades. Until the mid-20th century, there was greater use of Izod testing and keyhole-notch Charpy testing than today. Charpy V-notch is currently the predominant test method, although some U-notch and unnotched specimens are also used in specific applications. The key difference today is the way in which the data that comes from the test equipment is handled.

ASTM E23 (Standard Test Method for Notched Bar Impact Testing of Metallic Materials), perhaps the most widely adopted standard on metals impact testing, establishes guidelines for test specimens (e.g., configuration, orientation, machining), preparation of the apparatus, test-temperature conditions and actual test procedures.

While mechanical pointers are still in use, many equipment manufacturers also include a rotary encoder on the machine axle to collect data. This data can then be used not only to determine the height to which the hammer rose but also to calculate the impact speed of the hammer. The impact values can instantly be displayed on the screen.

One new trend, which is currently being driven by the nuclear industry, is toward a fully instrumented impact tester. In these machines, the striking tup is equipped with strain gauges that are linked to high-speed data-capture equipment. A typical impact test is over in approximately 10 milliseconds or, quite literally, the blink of an eye. With this sophisticated equipment, however, we can capture up to 20,000 data points during this test. This data can be gathered to generate a graph that represents the striking tup position as it travels through the specimen.

Finally, it is interesting that this new technology will not fundamentally change the way in which the test is performed or the equipment required to perform it. Instead, newly developed technology will provide us with more repeatability, greater accuracy and higher resolution, as well as improved speed with respect to interpretation of results.

Next Time: How to compare Charpy and Izod values. Click here for part 3.

 

References

  1. Mr. Craig Darragh, AgFox LLC, technical and editorial contributions, private correspondence
  2. Dieter Jr., George E., Mechanical Metallurgy, McGraw-Hill Book Company, 1961ed-Hill, Robert E., Physical Metallurgy Principles, D. Van Nostrand Company, Inc., 1964
  3. 4. Wilby, A. J. and D. P. Neale, “Defects Introduced into Metals During Fabrication and Service,” Vol. III, Materials Science and Engineering, Encyclopedia of Life Support Systems.
  4. Herring, Daniel H., “Understanding Component Failures Parts 1 & 2,” Industrial Heating, July/August 2013
  5. Herring, Daniel H., Atmosphere Heat Treating, Volume II, BNP Media Group, 2015
  6. Wulpi, Donald J., Understanding How Components Fail, 3rd Edition, ASM International, 2013
  7. ASTM E23 (Standard Methods for Notched Bar Impact Testing of Metallic Materials)
  8. ASTM A370 (Standard Test Methods and Definitions for Mechanical Testing of Steel Products)