Q1: Which of the following hardness tests uses the depth of penetration caused by the indentor as the parameter for arriving at the hardness value of a material ?

ANS:D - Rockwell hardness test.

The Rockwell hardness test is a widely used method for determining the hardness of metallic materials. It measures the depth of penetration of an indenter into the material under a specific load, with the hardness value indicated directly on a dial or digital readout. Here's how the Rockwell hardness test works:

1. Preparation: The surface of the material to be tested is typically ground or polished to remove any surface irregularities that could affect the test results.
2. Selection of Scale: There are different scales in the Rockwell hardness test, each designated by a letter. The choice of scale depends on the material being tested and its expected hardness. Common scales include the Rockwell B scale (for softer materials like aluminum and copper alloys) and the Rockwell C scale (for harder materials like steel and cast iron).
3. Application of Preliminary Load: A preliminary load, known as the preload or minor load, is applied to ensure proper seating of the indenter and to minimize surface irregularities. This preload typically ranges from 3 kgf to 10 kgf, depending on the scale being used.
4. Application of Major Load: After the preload is applied, a major load is applied using a calibrated spring or hydraulic system. The major load is determined by the scale being used and the expected hardness of the material. The indenter penetrates into the material, and the depth of penetration is measured.
5. Removal of Major Load: Once the major load has been applied for a specific duration (dwell time), it is removed, and the remaining minor load is maintained. This allows the material to recover slightly from the indentation, which can affect the final hardness reading.
6. Measurement of Hardness: The hardness value is determined by measuring the depth of penetration of the indenter. This measurement is typically taken using a dial or digital readout directly from the testing machine.
7. Reading the Hardness Value: The hardness value is indicated directly on the Rockwell hardness scale, which is calibrated based on the depth of penetration and the hardness scale being used. The higher the hardness number, the harder the material.

The Rockwell hardness test offers several advantages, including speed, ease of use, and the ability to test a wide range of materials and hardness levels. It is commonly used in manufacturing, quality control, and research applications to assess the hardness and mechanical properties of metallic materials.