Question

Rockwell hardness measures

• A. Indentation depth
• B. Indentation diameter
• C. Indentation area
• D. Impact energy

Hint:

Remember:
- Rockwell \(\rightarrow\) Measures Indentation Depth (fastest, direct reading).
- Brinell \(\rightarrow\) Measures Indentation Diameter (optical microscope required).
- Vickers \(\rightarrow\) Measures Indentation Diagonals to compute projected area.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question asks for the fundamental physical quantity measured by the Rockwell hardness tester to determine the hardness value of a material.
Hardness tests differ not only in their indenters but also in how the resulting indentation is measured (e.g., optically measuring area/diameter or mechanically measuring depth).

Step 2: Key Formula or Approach:
The Rockwell test measures the permanent depth of indentation produced by a force.
The measurement is performed in a two-step loading sequence to eliminate surface preparation errors.
The Rockwell hardness number (HR) is calculated using: \[ \text{HR} = N - \frac{h}{s} \] where:
\( h \) is the permanent increase in depth of indentation under the minor load after removal of the major load (in mm).
\( s \) is the scaling unit factor (usually 0.002 mm for standard Rockwell and 0.001 mm for superficial Rockwell).
\( N \) is a constant dependent on the scale (e.g., 100 for diamond cone scales, 130 for ball scales).

Step 3: Detailed Explanation:

• Sequence of Loading:
- A minor load of 10 kgf (or 3 kgf for superficial) is first applied to establish a zero-reference position and penetrate surface roughness.
- A major load (typically 60, 100, or 150 kgf) is then applied to perform plastic deformation.
- After the major load is released, the minor load is maintained, and the residual depth increase \( h \) is measured.

• Direct Readout: Because the Rockwell system directly measures depth, the machine can automatically convert this into a hardness number on a dial or digital readout.
- This makes it much faster than Brinell (which requires optical measurement of the indentation diameter) or Vickers (which requires optical measurement of the diagonal lengths to calculate the projected area).
- Impact energy is measured by tests like Charpy or Izod, not by hardness tests.


Step 4: Final Answer:
The Rockwell hardness test is defined as a depth-sensing measurement system.
Thus, the correct option is (A).