Izod impact tests Metals

The Izod impact test is based on an old, established test originally designed for metals in which a notched-bar specimen is tested in cantilever fashion with an excess energy pendulum machine. Izod (ASTM D256), Charpy (ASTM D6110 and Research Report D20-1034, to become a new standard), and the tensile-impact (ASTM D1822) tests can be performed with pendulum machines. 

This action eliminates the need for a costly mechanical roughening process that most other materials require. The depositing of a metal surface on plastic parts can increase environmental resistance of the part, also its mechanical properties and appearance. As an example a plated ABS part (total thickness of plate 0.015 in.) exhibited a 16% increase in tensile strength, a 100% increase in tensile modulus, a 200% increase in flexural modulus, a 30% increase in Izod impact strength, and a 12% increase in deflection temperature. Tests on outdoor aged samples showed complete retention of physical properties after six months. 

The most common impact tests for plastics and metals are Izod and Charpy tests (see Figs. 3-77 and 3-78). A heavy hammer at the end of a pendulum arm swings down and strikes a cantilevered specimen (in the Izod test) or a beam specimen supported on both ends (in the Charpy test). The specimens are unnotched unless the data reports that they are to be notched for information on specimens and test procedures see ASTM D 256. 

As for metals, the presence of a sharp notch can change the fracture of a polymer from ductile to brittle. For this reason, a standard impact test for a polymer is the Charpy or Izod test, where a notched bar of polymer is struck by a pendulum and the energy dissipated in fracture is calculated. 

Impact behavior is one of the most widely specified mechanical properties of the polymeric materials. However, it is also one of the least understood properties. Predicting the impact resistance of plastics still remains one of the most troublesome areas of product design. One of the problems with some earlier Izod and Charpy impact tests was that the tests were adopted by the plastic industry from metallurgists. The principles of impact mechanisms as applied to metals do not seem to work satisfactorily with plastics because of the plastics complex structure. 

The test method may specify either a notched sample or an unnotched sample. If a notched specimen is specified, the dimensions of the notch are also given. Figure 15.31 is an example of these dimensions for a metal sample. In the Charpy test the notched sample is struck from behind the notch. In the Izod test the notch is facing the hammer. The notch, called a stress raiser, concentrates the stresses applied by the impact load. It makes the material brittle and increases the elastic limit in the notch area. Without the notch, many materials will bend without fracture, and the capacity to absorb energy will not be accurately determined. 

Both Izod and Charpy tests are in regular use for impact measurements on plastics. One disadvantage of the Izod test is that clamping forces introduce unknown stresses around the fracture zone, so that the test is less suitable than the Charpy test for research purposes. The Charpy test has been widely adopted for fracture mechanics measurements on both metals and polymers. 

The quality known as toughness describes the way a material reacts under sudden impacts. It is defined as the work required to deform one cubic inch of metal until it fractures. Toughness is measured by the Charpy test or the Izod test. 

All the available data on the impact properties of the rare earth metals are from Love (1959, 1960). The metals were prepared by calcium reduction of the anhydrous fluorides. Izod specimens having a standard V-notch were tested with a 100 inch-pound capacity impact machine that had a striking velocity of 11.3 feet per second. The specimens were all tested in the as-cast condition. Data from Love (1959) were from subsize-width specimens and the impact energy, taken in inch-pounds, was corrected from the subsize notch cross-section to standard notch cross-section and quoted in foot-pounds. 

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