Testing of Hardness and Impact Strength Study notes for Mechanical Engineering

By BYJU'S Exam Prep

Updated on: September 25th, 2023

In materials engineering and metallurgy, hardness and impact strength are critical mechanical properties that determine the resistance of a material to deformation and its ability to withstand sudden loading or impact. These properties play a vital role in selecting appropriate materials for various industrial applications, ensuring structural integrity, and optimizing performance. This study material delves into the principles, testing methods, and significance of hardness testing, including Brinell, Rockwell, and Vickers hardness tests, as well as impact strength testing, which involves Charpy and Izod impact tests. Aspiring engineers and materials enthusiasts will gain a comprehensive understanding of these essential mechanical tests and their importance in material evaluation and quality control.

In the realm of material science and mechanical engineering, understanding the hardness and impact strength of materials is paramount. Hardness testing helps gauge a material’s resistance to indentation and wear, while impact strength testing measures its ability to absorb energy under sudden loading conditions. Engineers rely on these tests to select appropriate materials for specific applications, ensuring they can withstand the operational demands and environmental stresses they may encounter. Whether you’re preparing for examinations or simply seeking to enhance your knowledge of material properties and testing, this study material on hardness and impact strength testing will prove invaluable in comprehending the nuances of material behaviour and mechanical performance.

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Table of content

Hardness Testing

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 plastic deformation of the same. This deformation is measured by one of the following methods:

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 on 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.

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 the hardness of a diamond is excessively high. It can be used for a 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.

Rockwell Hardness Method

The scale ranges between 0‐100. It uses either a diamond 120° cone indenter or a ball indenter made of hardened steel.

Depending on the combination of indenter and load there are several Rockwell hardness scales. The three most commonly used Rockwell hardness scales are given in the table.

The applied load depends on the hardness of the 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.

Impact Testing

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:

Izod Impact Test

The 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 the top face of the hammer.

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 a notch in the centre. Impact tests are important as they can reveal the temper brittleness in heat-treated materials such as nickel chrome steels.

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