Question

In a Brazilian test, the rock specimen diameter shall preferably be:

• A. not less than NX core size (54 mm), or at least 10 times the average grain size
• B. not less than NX core size (75 mm), or at least 10 times the average grain size
• C. not less than NX core size (54 mm), or at least 20 times the average grain size
• D. not less than NX core size (75 mm), or at least 20 times the average grain size

Hint:

Standard NX core = 54 mm. Use diameter ≥ 10× grain size to ensure accurate tensile strength measurement.

Answer 1

The Correct Option is A

To determine the appropriate specimen diameter for a Brazilian test, we examine international testing standards and rock mechanics best practices.

1. Understanding the Brazilian Test:
The Brazilian test is an indirect tensile strength test method where a disc-shaped rock specimen is compressed diametrically until failure occurs along its diameter.

2. Standard Requirements:
According to ISRM (International Society for Rock Mechanics) and ASTM standards:

3. Specimen Diameter Criteria:
- NX core size minimum: 54 mm (standard NX core diameter)
- Grain size relationship: At least 10 times the average grain size
- Thickness-to-diameter ratio: Typically 0.5-1.0

4. Why These Dimensions Matter:
- 54 mm diameter ensures sufficient area for valid failure plane development
- 10× grain size requirement minimizes grain-scale heterogeneity effects
- Maintains consistent stress distribution during testing

5. Incorrect Options Analysis:
- 75 mm exceeds standard NX core size (unnecessarily large)
- 20× grain size is excessive for most applications
- Larger sizes may introduce new failure modes

6. Practical Considerations:
- The 54 mm diameter is widely available from standard coring operations
- Meets minimum statistical representative elementary volume requirements
- Allows testing of most common rock types with typical grain sizes

Conclusion:
The rock specimen diameter shall preferably be not less than NX core size (54 mm), or at least 10 times the average grain size.

Final Answer:
The correct option is: not less than NX core size (54 mm), or at least 10 times the average grain size.