TESTING OF METALS: Tensile test, Compression Test, Hook’s Law, Strain, Tensile Strain, Compressive Strain, Modulus of Elasticity, Modulus of Rigidity, Bulk Modulus, Linear and Lateral Strain, Poisson’s Ratio, Proof Resilience, Modulus of Resilience

 TESTING OF METALS

Metal testing is accomplished for the purpose of for estimating the behavior of metal under loading (tensile, compressive, shear, tortion and impact, cyclic loading etc.) of metal and for providing necessary data for the product designers, equipment designers, tool and die designers and system designers. The material behavior data under loading is used by designers for design calculations and determining weather a metal can meet the desired functional requirements of the designed product or part. Also, it is very important that the material shall be tested so that their mechanical properties especially their strength can be assessed and compared. Therefore the test procedure for developing standard specification of materials has to be evolved. This necessitates both destructive and non-destructive testing of materials. Destructive tests of metal include various mechanical tests such as tensile, compressive, hardness, impact, fatigue and creep testing. A standard test specimen for tensile test is shown in Fig. 1 Non-destructive testing includes visual examination, radiographic tests, ultrasound test, liquid penetrating test and magnetic particle testing.

Tensile test

A tensile test is carried out on standard tensile test specimen in universal testing machine. Fig. 2 shows schematic set up of universal testing machine reflecting the test specimen griped between two cross heads. Fig. shows the stress strain curve for  ductile material. Fig. shows the properties of a ductile material. Fig. 5 shows the stress strain curves for wrought iron and steels. Fig. 6 shows the stress strain curve for non ferrous material.

Tensile test specimen
Fig 1: Tensile test specimen



Schematic universal testing machine
Fig 2: Schematic universal testing machine



Stress strain curve for ductile material
Fig 3: Stress strain curve for ductile material



Properties of a  ductile  material
Fig 4: Properties of a  ductile  material




Stress strain curves for wrought iron and steel
Fig 5: Stress strain curves for wrought iron and steel



Stress strain curves for non-ferrous material
Fig 6: Stress strain curves for non-ferrous material 

Compression Test

Compression test is reverse of tensile test. This test can also be performed on a universal testing machine. In case of compression test, the specimen is placed bottom cross heads. After that, compressive load is applied on to the test specimen. This test is generally performed for testing brittle material such as cast iron and ceramics etc. Fig. 7 shows the schematic compression test set up on a universal testing machine. The following terms have been deduced using figures pertaining to tensile and compressive tests of standard test specimen.


Schematic compression test set up on a universal testing machine
Fig 7: Schematic compression test set up on a universal testing machine


Hook’s Law

Hook’s law states that when a material is loaded within elastic limit (up to proportional limit), stress is proportional to strain.

Strain

Strain is the ratio of change in dimension to the original dimension.

Tensile Strain

The ratio of increase in length to the original length is known as tensile strain.

Compressive Strain

The ratio of decrease in length to the original length is known as compressive strain.

Modulus of Elasticity

The ratio of tensile stress to tensile strain or compressive stress to compressive strain is called modulus of elasticity. It is denoted by E. It is also called as Young’s modulus of elasticity.
E = Tensile Stress/Tensile Strain

Modulus of Rigidity

The ratio of sheer stress to shear strain is called modulus of rigidity. It is denoted by G.  G = Shear Stress/Shear Strain

Bulk Modulus

The ratio of direct stress to the volumetric strain (ratio of change in volume to the  original volume is known as volumetric strain) is called Bulk modulus (denoted by K).

K = Direct stress/volumetric strain

Linear and Lateral Strain

When a body is subjected to tensile force its length increases and the diameter decreases. So when a test specimen of metal is stressed, one deformation is in the direction of force which is called linear strain and other deformation is perpendicular to the force called lateral strain.

Poisson’s Ratio

The ratio of lateral strain to linear strain in metal is called poisson’s ratio. Its value is constant for a particular material but varies for different materials.

Proof Resilience

The maximum amount of energy which can be stored in an elastic limit is known as proof resilience.

Modulus of Resilience

The proof resilience per unit volume of a material is modulus of resilience or elastic toughness.

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