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Impact Testing On Metals

 Metals sometimes fail when subjected to suddenly applied load or stress and in order to assess their capacity to stand sudden impacts , the impact test is employed. The impact test measures the energy necessary to fracture a standard notched bar by an impulse load and as such is an indication of the notch toughness of  the material under shock loading.

Two major tests for determining impact toughness are Izod test and the Charpy test. Both of these methods use the same type of machine and both yield a quantitative value of the energy required to fracture.




The two most common kinds of impact test use notched specimens loaded as beams. The beams may be simply loaded ( Charpy test ) or loaded as cantilevers (Izod test). The notch is usually a V-notch cut to specifications with a special milling cutter. The function of the notch is to ensure that the specimen will break as a result of  Impact load to which it is subjected. Without the notch, many alloys would simply bend without breaking, and it would therefore be impossible to determine their ability to absorb energy. It is important to note that the blow in Charpy test is delivered at a point directly behind the notch and in the Izod test the blow is struck on the same side of the notch towards the end of the cantilever. 

The specimen is held in a rigid vice or support and is struck a blow by a traveling pendulum that fractures or severely deforms the notched specimen. The energy input is a function of the height of the fall and the weight of the pendulum. The energy remaining after fracture is determined from the height of rise of the pendulum due to inertia and its weight. The difference between the energy input and the energy remaining represents the energy absorbed by the specimen. Modern machines are equipped with scales and pendulum - actuated pointers, which yield direct readings of energy absorption.  

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