Failure Analysis, Forensic Engineering and Fitness-for-Service
Failure Analysis, Forensic Engineering and Fitness-for-Service tests structured investigation: preserve evidence, establish the mechanism, find the root cause, then decide whether remaining plant can keep running and under what inspection regime.
The method
Secure and record the scene before anything moves; note service history and ask what changed. Examine the fracture surface at low magnification for origin and crack-growth direction (chevrons point back to the origin), then use SEM. Take metallographic sections away from the fracture, and chemical and mechanical samples from the same heat. Classify the mechanism: ductile rupture (dimples), cleavage (rivers and facets), fatigue (beach marks from the load spectrum, striations from individual cycles, ratchet marks from multiple origins), intergranular failure (hydrogen, temper embrittlement, creep cavitation), stress corrosion cracking (branched cracks from a surface), corrosion fatigue, and wear. Map cause to design, material, fabrication or service, and only then propose corrective action.
Fitness-for-service
BS 7910 and API 579-1/ASME FFS-1 provide assessment levels: Level 1 screening, Level 2 with material-specific toughness, Level 3 with detailed stress analysis, resolved on a failure assessment diagram using Kr (toughness ratio) and Lr (load ratio). Critical flaw size is compared with what NDT can reliably detect, and remaining life comes from crack-growth integration. Inspection intervals are commonly set at about half the calculated remaining life.
Worked example
An edge crack grows from 2 mm to 10 mm under stress range 100 MPa, Y = 1.12, C = 2x10^-12 and m = 3 in MPa root-m and m/cycle units. The stress intensity range is 8.9 MPa root-m at 2 mm (growth 1.4x10^-9 m/cycle) and 19.9 at 10 mm (growth 1.6x10^-8 m/cycle). Integrating gives about 1.6x10^6 cycles; at 2000 cycles per day that is roughly two years, so inspection is scheduled at about one year.
Traps
Diagnosing from a single photograph; acid-cleaning or wire-brushing a fracture surface; confusing beach marks with striations; assuming a single origin; using tensile data in a fatigue assessment; breaking chain of custody; writing an expert report that advocates for the client rather than assisting the court.