Question

Pre-cracked specimens are used in the Instrumented Charpy test to measure its

• A. Toughness
• B. Dynamic fracture toughness
• C. Transition temperature
• D. Compressive strength

Hint:

Remember: Notch \(\rightarrow\) Impact Energy (Qualitative toughness). Pre-crack \(\rightarrow\) Fracture Toughness (Quantitative material property). Instrumentation provides the load-deflection data needed for the calculation.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The standard Charpy test measures impact energy absorption of a notched specimen, which is a qualitative measure of toughness. To obtain a quantitative fracture mechanics property, the test must be modified to meet the stringent requirements of fracture toughness testing.

Step 2: Detailed Explanation:

• Fracture Toughness (K\(_{Ic}\), J\(_{Ic}\)): This is a material property that quantifies a material's resistance to the propagation of a pre-existing crack. A key requirement for a valid fracture toughness test is the presence of a sharp, natural crack, as machined notches are not sharp enough. This is achieved by creating a fatigue

pre-crack at the root of the machined notch.

• Instrumented Charpy Test: This test adds a load cell to the Charpy pendulum's striker. This allows for the recording of a dynamic load-displacement curve during the impact event.

• Pre-cracked Instrumented Charpy Test: By combining a pre-cracked specimen with an instrumented striker, the test can provide the data needed to calculate the

dynamic fracture toughness (denoted as K\(_{Id}\) or J\(_{Id}\)). This is a true material property that can be used in engineering design and failure analysis, unlike the simple impact energy value from a standard Charpy test.

• Other Options: itemize

• (A) "Toughness" is a general term; dynamic fracture toughness is a specific, quantitative measure.

• (C) Transition temperature is typically determined using standard (not pre-cracked) Charpy specimens by plotting impact energy vs. temperature.

• (D) Compressive strength is measured in a compression test, not an impact test.
itemize

Step 3: Final Answer:
The use of a pre-cracked specimen in an Instrumented Charpy test allows for the measurement of the material's dynamic fracture toughness.