Hardness represents the resistance of a material to indentation, penetration and scratching. In hardness testing, a loaded ball or diamond is pressed against the surface of a material which causes the plastic deformation of the same. This deformation is measured by one of the following methods:
(i) Brinell Hardness test-
In this method, a steel hardened ball is pressed into the surface of the material under a specified load. The load is held in position for a fixed period and then released. This leaves a permanent impression in the surface of the material. Then either measure the diameter or the depth of the impression.
The Brinell specimen Hardness Number (BHN) is defined as the ratio of the applied load to the spherical area of the impression.
Where, P is in Newton.
Conversion tables are also available to determine the hardness number.
(ii) Vicker Pyramid Diamond Method
This method is also similar to the Brinell method except that the indenter is a 136° pyramid diamond on a square base. As hardness of diamond is excessively high. It can be used for the whole range of materials.
The Vicker Pyramid Number (VPN) is defined as the ratio of applied load to the impressed area. The area is calculated by measuring the length of the diagonal of the square impression on the surface of the material.
(iii) Rockwell Hardness Method –
The scale ranges between 0‐100. It uses either a diamond 120° cone indenter or ball indenter made of hardened steel.
Depending on the combination of indenter and load there are several Rockwell hardness scales. Three most commonly used Rockwell hardness scales are given in table.
The applied load depends on the hardness of material. As a thumb rule the load used for measuring the hardness of steel = 30D2 kg; where D is the diameter of the ball. If D = 10mm the load to be used = 3000kg.
Static tests are useful only when the loads are static in nature. These tests do not indicate the resistance of a material against shock or impact loads to which usually the automobile parts are subjected to. In such cases, an impact test has to be undertaken. An impact test indicates the toughness of a material which is defined as the energy absorbed by the specimen without fracture.
The following are the main types of impact tests undertaken:
(i) Izod Impact Test
Figure shows an Izod impact testing machine. It consists of an anvil in which a notched specimen can be fixed. The specimen is taken of some standard dimensions. While fixing, care is to be taken to have the notch on the side of the falling hammer and the level with the level of top face of the hammer.
(ii) Charpy Impact Test
This test is similar to the Izod impact test except that instead of fixing the notched specimen in the anvil, it is supported at each end as a beam as shown in Figure. The hammer strikes at notch in the centre. Impact tests are important as they can reveal the temper brittleness in heat treated materials such as nickel chrome steels.
Testing of materials:
Testing of materials is very important part of the from the point of view of design and manufacturing. It provides the information of material properties, help in ensuring the quality, help in preventing the failures and also helps to make choices among different available materials.
There are mainly two types of testing performed on materials named Mechanical testing and Non-destructive testing. Here, only mechanical testing will be discussed.
Mechanical testing is a destructive type of testing that utilizes static or dynamic forces to reveal the properties of the material. Mechanical testing includes the different types of testing such as Tensile test, hardness testing, impact test, fatigue test, creep test, bend test, etc.
1. Tensile testing:
This test is performed on the universal testing machines (UTM). In this test, the specimen is subjected to uniaxial tensile force in a controlled way until its failure. This test helps us in accessing the following properties ductility, yield strength, tensile strength, Young's modulus (E), and Poisson's ratio (μ).
Select the standard specimen and grip it in the crossheads with proper adjustment. While setting up the job, use the adjusting knob to make zero at lower points to zero to remove the dead weight of the lower table. Now, lock the job, fix the extensometer between the gauge length ( to find the extension), and apply the
Ductility: It relates the elongation during the tensile test of the material and it is defined as the percentage elongation.
Tensile strength: It is defined as the maximum load per unit cross-section area which the material can bear before breaking. It is given as:
Yield Strength: It is the strength of the material above which permanent deformation takes place in the material under stress.
Young's Modulus (E): It is also known as the modulus of elasticity (E) of the material and it represents the stiffness of the material. It is the measure of the regain of shape and size of the material on the removal of the load.
A compression test is also carried on the universal testing machine (UTM). Here, the load applied is compressive in nature and specimen is loaded till it fails.
The compression test is generally carried out for the Brittle materials.
2. Hardness Test
Hardness is the surface property of the material which shows the resistance of a material against indentation, penetration and scratching.
Hardness test are as follows:
1. Brinell Hardness test: In Brinell hardness test a steel or tungsten carbide ball is used to make a impression in the material under a specified load.
2.Vickers Hardness test: It uses the pyramid indenter of square shape and length of the diagonals of the indentation is measured to calculate the hardness number. It is suitable for very hard and tough materials.
Vickers hardness number is given by:
Where F is the applied force (in kg) and D is the average diameter of the diagonals measured.
3. Rockwell Hardness test: Rockwell test sues the diamond cone-shaped or spherical ball type of indenter for the indentation purpose. There are many scales in the Rockwell testing but C scale is the most commonly used scale and hardness on it is denoted as HRC.
3. Toughness or Impact test
Toughness tests are carried at high strain rates and the energy absorbed by the materials in breaking the specimen is considered the toughness of the material. There are two types of impact tests named Izod and Charpy tests.
- For Izod impact testing, the specimen is kept vertically as a cantilever beam. The specimen is kept in such a way that the notch side faces the striking hammer.
The Charpy test shows whether a metal is either brittle or ductile and it is used for predicting ductile to brittle transition.
In the Charpy test, the specimen is placed horizontally and fixed at both ends i.e. it is a simply supported beam. Striking hammer strikes from the opposite side of the notch.
4. Flexural or Bending Test
The bend test is a qualitative test that can be used to access the ductility and soundness of a workpiece. Generally, it is used for the butt-welded joints to control their quality.
In bend tests, the Rectangular specimen is supported at both ends, and then the load is applied vertically at one or two points. The fracture stress in bending is called as the modulus of rupture, flexural strength.
5. Shear Test:
The shear test is used to determine the shear strength of the material which is the maximum shear stress that the material can bear before the appearance of any failure. It plays a key role in the design of fasteners such as bolts and screws.
6. Fatigue Test:
Fatigue is the permanent failure of the material due to fluctuating stresses and failure takes place below the yield point of the metal. The number of cycles at which failure occurs is measured and these can vary from a couple of hundreds to millions of cycles.
The failure of the specimen under rotating loading is termed fatigue failure. Rotating loading results in completely reversed stresses.
The results of the fatigue test are plotted as an S–N curve which is the graphical representation of stress amplitude and the number of stress cycles (N) before the fatigue failure on a log-log graph paper.
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