Impact Test

Impact test measure the strength of a material under dynamic loading, where most of the structural components are subjected to dynamic loading. So tensile strength along will not be of sufficient use as a seeing parameter.

● In impact test the material is subjected to sudden load, a hammer is made to swing from a fixed height and strike the standard impact specimen. Impact strength of a material is defined as the capability of the material to absorb energy without failure under impact loading.
Impact testing machine
Impact testing machine

● Depending on the nature of the standard impact specimen, there are two most common method for the measurement of impact strength. They are Izod and Charpy impact tests.

Izod Impact Test :

The Izod specimen may have either square or round cross section. These are usually V-shape notch. The depth of notch is 2mm and included at an angle of 45°. In this test a hammer strike the specimen which is fixed in vertical position. The notch faces the hammer.

Impact test specimen
(a) single notch square Izod
(b) single notch round Izod
(c) beam V- notch
(d) Charpy U- notch

Charpy Impact Test :

The Charpy impact specimen are usually square in cross section and V-shape or key hole shaped notch. The specimen is fixed in horizontal position. The hammer strike the impact specimen on the unnotched face.

Impact Strength :

Impact strength of a material is governed by many factor such as temperature, heat treatment, chemical composition and grain size. Temperature has marked influence on impact strength. As temperature comes down impact strength also decrease. There is sudden drop in impact strength when material is cooled below a particular temperature.

The temperature at which ductile materials changes  to brittle is known as transition temperature. The temperature at which fracture is 50% brittle is called fracture appearance transition temperature (FATT). Carbon raises transition temperature significantly in steels. The deleterious effect of carbon is counteracted by manganese.

● The manganese to carbon ratio should be at least 3 for satisfactory impact strength. Other elements which raises the transition temperature are phosphorous, silicon and molybdenum. Heat treatment also affects impact strength. A tempered martensitic structure provides the best combination of tensile and impact strength.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.

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