SEM Fractography
Definition
The use of scanning electron microscopy to image fracture surfaces at magnifications beyond the optical limit, revealing dimple size and distribution in ductile fracture, cleavage facets and river marks in brittle fracture, and striations in fatigue fracture.
- Instrument
- Scanning electron microscope
- Ductile fracture signature
- Dimple size and distribution
- Brittle fracture signature
- Cleavage facets and river marks
- Fatigue signature
- Striations
- Advantage over optical
- Resolves detail beyond the optical microscope's limit
Common questions
How do dimples on a fracture surface indicate ductile failure?+
Dimples form as microvoids nucleate around inclusions or second-phase particles and coalesce under plastic deformation before final separation. Their size and depth can indicate the strain rate and stress state at failure, which helps reconstruct how a component or weapon part broke.
What do river marks reveal about a brittle fracture?+
River marks are ridge patterns on cleavage facets that fan out in the local direction of crack propagation, similar to tributaries joining a river. Tracing them back lets an examiner locate the fracture origin even on a piece with no other identifying features.
Why are fatigue striations forensically useful in failure analysis?+
Each striation typically corresponds to one loading cycle, so counting and measuring striation spacing across a fatigue zone can estimate the number of cycles and time to failure, which matters in cases involving mechanical failure, product liability, or sabotage.
Related terms
- Brittle Fracture
- Fracture with little or no plastic deformation, occurring by rapid cleavage through grains or along grain boundaries. The surface is flat, often...
- Cleavage
- Fracture by separation along specific crystallographic planes, typically {100} planes in body-centred-cubic metals. The fracture surface is flat at the grain scale...
- Cup-and-Cone Fracture
- The classic macroscopic appearance of ductile tensile fracture in round bar: a flat central zone (the cup) formed by void coalescence surrounded...
- Ductile Fracture
- Fracture preceded by extensive plastic deformation, in which microvoids nucleate at inclusions, grow, and coalesce to form the fracture surface. The surface...
- Intergranular Fracture
- Fracture path running along grain boundaries rather than through grains. Indicates boundary weakness from segregation, embrittlement, or environmental attack, and produces a...