Question

The fracture surface in ductile fracture generally shows

• A. cleavage facets
• B. dimpled microvoids
• C. brittle river patterns
• D. transgranular mirror like regions

Hint:

Microscopic Fractographic Signatures:
- Ductile Fracture \(\rightarrow\) Dimples (microvoid coalescence).
- Brittle Fracture \(\rightarrow\) Flat Cleavage Facets / River Patterns.
- Fatigue Fracture \(\rightarrow\) Beach marks (macroscopic) and Striations (microscopic).

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question asks for the characteristic fractographic feature observed on the fracture surface of a material that has failed via ductile fracture.
Fractography is the study of fracture surfaces to determine the mechanism and cause of failure.

Step 2: Key Formula or Approach:
Ductile fracture is characterized by significant plastic deformation prior to and during crack propagation. The microscopic mechanism of ductile fracture involves a sequence of stages: microvoid initiation, void growth, and void coalescence.

Step 3: Detailed Explanation:

• Mechanism of Void Coalescence:
- Under tensile load, local plastic strains concentrate around second-phase particles, inclusions, or grain boundaries.
- This causes the matrix to detach from these particles, forming microscopic cavities (microvoids).
- As plastic deformation continues, these voids grow and expand.
- Eventually, the thin metal walls separating adjacent voids neck down and shear apart, coalescing the voids into a continuous fracture surface.
- When viewed under a Scanning Electron Microscope (SEM), this fracture surface exhibits a characteristic "dimpled" appearance, where each dimple represents half of a coalesced microvoid.

• Brittle Fracture Features (Incorrect Options):
- Cleavage facets, river patterns, and transgranular mirror-like regions are characteristic of brittle fracture.
- In brittle fracture, cracks propagate rapidly along low-energy crystallographic planes without significant plastic flow.


Step 4: Final Answer:
The microscopic fracture surface in ductile fracture is characterized by dimpled microvoids.
Therefore, option (B) is the correct choice.