Various Types of 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 specimen
Fig. 1 Tensile test specimen

Tensile test

A tensile test is carried out on standard tensile test specimen in universal testing machine. Fig. 2 shows a schematic set up of universal testing machine reflecting the test specimen griped between two cross heads. Fig. 3 shows the stress strain curve for ductile material. Fig. 4 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.

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 crossheads. 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.

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.

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

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.

Testing of Hardness

It is a very important property of the metals and has a wide variety of meanings. It embraces many different properties such as resistance to wear, scratching, deformation and machinability etc. It also means the ability of a metal to cut another metal. The hardness of a metal may be determined by the following tests.

(a) Brinell hardness test
(b) Rockwell hardness test
(c) Vickers hardness (also called Diamond Pyramid) test
(d) Shore scleroscope

Fig. 8 below shows Rockwell hardness testing machine.

Rockwell hardness testing machine
Fig. 8 Rockwell hardness testing machine

Testing of Impact Strength

Schematic impact testing machine setup
Fig. 9 Schematic impact testing machine setup

When metal is subjected to suddenly applied load or stress, it may fail. In order to assess the capacity of metal 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. Izod test and the Charpy test are commonly performed for determining impact strength of materials. These methods employ same machine and yield a quantitative value of the energy required to fracture a special V notch shape metal. The most common kinds of impact test use notched specimens loaded as beams. V notch is generally used and it is get machined to standard specifications with a special milling cutter on milling machine in machine shop. The beams may be simply loaded (Charpy test) or loaded as cantilevers (Izod test). The function of the V notch in metal is to ensure that the specimen will break as a result of the 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 therefore important to observe 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. Fig. 9 shows the impact testing set up arrangement for charpy test. 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 in this case is a function of the height of fall and the weight of the pendulum used in the test setup. 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 standard metal specimen. Advance testing setups of carrying out such experiments are generally equipped with scales and pendulumactuated pointers, which provide direct readings of energy absorption.

Testing of Fatigue

Material subjected to static and cyclic loading, yield strength is the main criterion for product design. However for dynamic loading conditions, the fatigue strength or endurance limit of a material is used in main criteria used for designing of parts subjected to repeated alternating stresses over an extended period of time. Fig 10 shows a fatigue test set up determining the fatigue strength of material. The fatigue test determines the stresses which a sample of material of standard dimensions can safely endure for a given number of cycles. It is performed on a test specimen of standard metal having a round cross-section, loaded at two points as a rotating simple beam, and supported at its ends. The upper surface of such a standard test specimen is always in compression and the lower surface is always in tension. The maximum stress in metal always occurs at the surface, halfway along the length of the standard test specimen, where the cross section is minimum. For every full rotation of the specimen, a point in the surface originally at the top centre goes alternately from a maximum in compression to a maximum in tension and then back to the same maximum in compression. Standard test specimens are tested to failure using different loads, and the number of cycles before failure is noted for each load. The results of such tests are recorded on graphs of applied stress against the logarithm of the number of cycles to failure. The curve is known as S-N curve.

Schematic fatigue test setup
Fig. 10 Schematic fatigue test setup

Testing of Creep

Metal part when is subjected to a high constant stress at high temperature for a longer period of time, it will undergo a slow and permanent deformation (in form of a crack which may further propagate further towards creep failure) called creep. Creep is time dependent phenomena of metal failure at high constant stress and at high temperature such subjecting of at steam turbine blade. A schematic creep testing setup is shown in Fig. 11. Test is carried out up to the failure of the test specimen. A creep curve for high temperature and long time creep is shown in Fig. 12. The curve shows different portions of the primary secondary and tertiary creep which ends at fracture in metals.

Schematic creep testing setup
Fig. 11 Schematic creep testing setup
Creep curve for a high temperature and long time creep test
Fig. 12 Creep curve for a high temperature and long time creep test
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