We continue our discussion of impact testing by focusing on the test methods and testing machines. As we discussed in part 1, the two most common tests are the Izod test and the Charpy test.
The Izod Test
The Izod impact test was named for its inventor, Edwin Gilbert Izod (1876-1946), and consists of a pendulum with a determined weight at the end of its arm swinging down and striking a specimen while it is held securely in a vertical position. The impact strength is determined by the loss of energy of the pendulum as determined by precisely measuring the loss of height in the pendulum's swing. The specimen, which is usually notched, is gripped at one end only (which is the principal difference between it and the Charpy test).
The principal advantage of this test is that a single specimen can be used multiple times since the ends are broken off one at a time. The principal limitation, and why the test has fallen out of favor for metals testing today, is the length of time required for setup. As such, low-temperature testing is not an option.
The size and shape of the specimen varies according to what materials are being tested. Specimens of metals are usually square, and polymers are usually rectangular being struck perpendicular to the long axis of the rectangle. The specimen-holding fixture is usually part of the machine and cannot be readily cooled (or heated). For this reason, Izod testing in not recommended at anything other than room temperature. The information obtainable from this test includes the impact energy, lateral expansion and fracture appearance.
The Charpy Test
The Charpy test was invented by Georges Augustin Albert Charpy (1865-1945).The Charpy test measures the energy absorbed by a standard notched specimen while breaking under an impact load. The most common method of measuring impact energy in steels today is the Charpy test. The importance of the Charpy impact tests lies in the fact that it can reproduce the ductile-brittle transition transformation in about the same temperature range as it is actually observed in engineering structures.
The Charpy test (Fig. 1) involves striking a suitable test piece with a striker mounted at the end of a pendulum. The test piece is fixed in place at both ends, and the striker impacts the test piece immediately behind a machined notch. The test procedure involves removing the test specimen from its cooling or heating medium (if used) and positioning it on the specimen supports. The pendulum is to be released, as vibration-free as possible, within five seconds after the material is removed from the thermal medium.
Fracturing the specimen removes energy from the hammer, and the height at which the hammer rises after the specimen is broken is measured on the tester. The energy expended is high for a ductile fracture, and the energy will be low for a brittle fracture. The information obtainable from this test includes the impact energy, lateral expansion and fracture appearance.
We'll discuss Charpy testing in more detail next time.
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2. 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
3. Reed-Hill, Robert E., Physical Metallurgy Principles, D. Van Nostrand Company, Inc., 1964
4. Herring, Daniel H., “Understanding Component Failures Parts 1 & 2,” Industrial Heating, July/August 2013
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6. ASTM E23 (Standard Methods for Notched Bar Impact Testing of Metallic Materials)