In Rockwell hardness testing method, the hardness of a material is measured by-
Depth of penetration of indenter
Understanding Rockwell Hardness Testing
Rockwell hardness testing is a widely used method for determining the hardness of materials. It is a quick and direct reading hardness test. Unlike some other hardness tests that measure the size of the indentation, the Rockwell method measures hardness based on the depth of penetration of an indenter into the material under specific loads.
How Rockwell Hardness is Measured
The Rockwell hardness test involves applying two loads:
- A minor load is applied first to seat the indenter and remove the effect of surface finish.
- A major load is then applied for a specific dwell time.
- The major load is removed, leaving the minor load still applied.
The Rockwell hardness number is calculated from the permanent increase in the depth of penetration resulting from the application and removal of the major load. A smaller increase in depth indicates a harder material, while a larger increase indicates a softer material.
The formula for the Rockwell hardness number (HR) is generally given as:
\( \text{HR} = N - \frac{h}{S} \)
Where:
- \(N\) is a constant (depends on the scale used)
- \(h\) is the permanent increase in depth of penetration caused by the major load (in mm or inches)
- \(S\) is a constant (depends on the scale used)
This formula clearly shows that the hardness number is directly related to the depth of penetration \(h\).
Analyzing the Options
Let's look at the given options in the context of Rockwell hardness testing:
- Depth of penetration of indenter: As explained above, the Rockwell test directly measures the permanent increase in the depth of the indentation to determine hardness. This aligns with the principle of the test.
- Surface area of indention: Measuring the surface area of the indentation is the principle behind methods like the Brinell hardness test. This is not how Rockwell hardness is determined.
- Projected area of indention: Measuring the projected area (area projected onto a plane perpendicular to the load) is the basis for tests like the Vickers hardness test. This is different from the Rockwell method.
- Height of rebound: Measuring the height of rebound after a standard mass is dropped onto the material surface is the principle behind dynamic hardness tests like the Leeb rebound hardness test (often associated with Shore scleroscope, though Shore can also be indentation based). This is not the method used in Rockwell testing.
Based on the working principle of the Rockwell hardness test, the hardness of a material is measured by the depth of penetration of the indenter.
| Hardness Test Method | Measurement Principle | Basis for Hardness Number |
|---|---|---|
| Rockwell | Indentation under load | Depth of penetration (specifically, increase in depth from minor to major load) |
| Brinell | Indentation under load | Surface area of indentation |
| Vickers | Indentation under load | Projected area of indentation |
| Leeb/Shore (Rebound) | Impact/Rebound | Height of rebound |
Conclusion on Rockwell Hardness Measurement
In summary, the defining characteristic of the Rockwell hardness test is its reliance on the depth of penetration of the indenter under different loads to calculate the hardness value. This makes it a distinct method compared to tests that measure the size or area of the indentation.
Revision Table: Key Aspects of Hardness Testing
| Term | Definition/Relevance to Rockwell Test |
|---|---|
| Rockwell Hardness | A hardness scale based on the depth of residual indentation. |
| Indenter | The tip (usually cone or ball shaped) that penetrates the material. |
| Minor Load | Small initial load applied to seat the indenter. |
| Major Load | Larger load applied after the minor load; causes primary indentation. |
| Depth of Penetration | The key measurement in Rockwell testing, specifically the increase in depth due to the major load. |
| Hardness Number | The calculated value representing the material's resistance to indentation. |
Additional Information on Hardness Testing Methods
Hardness testing is a crucial part of material characterization. Different methods are suitable for different materials and applications.
- Brinell Hardness: Uses a large ball indenter and high loads. Suitable for softer to medium hardness materials. The hardness number is calculated from the load divided by the surface area of the spherical indentation.
- Vickers Hardness: Uses a square-based diamond pyramid indenter. Versatile for a wide range of materials and hardnesses. The hardness number is calculated from the load divided by the projected area of the indentation. Often used for microhardness testing.
- Knoop Hardness: Uses a rhombic-based diamond pyramid indenter that produces an elongated indentation. Primarily used for microhardness testing on brittle materials or thin layers where shallow indentations are needed. Hardness is based on the projected area.
- Shore Scleroscope: A dynamic test where a diamond-tipped hammer drops onto the surface. Hardness is related to the height the hammer rebounds.
- Leeb Hardness: Another dynamic test where an impact body with a carbide ball strikes the surface. Hardness is calculated from the ratio of rebound velocity to impact velocity.
Each test has its advantages and limitations, making the choice of method dependent on the material properties, sample size, and required precision.
Important Questions from Testing of Materials
The radius of curvature at the root of the V-notch in an Izod impact test specimen is
The ability of a material to be drawn into a thin wire without breaking is termed as:
___________ is the collective term for the physical manifestations of the defects like cracks, spalling, pop out, staining and corrosion.
The impact test is done to test
The property of the material or a structure indicating the extent to which it can deform beyond the limit of yield deformation before failure or fracture is termed as:
