Tutorial: How to Perform a Complete Failure Analysis on a Broken Engineering Component
A failure analysis investigation is a systematic, evidence-driven process that identifies the physical mechanism and root cause of an engineering component’s premature or unexpected failure. Done correctly, it produces findings defensible in court, actionable by a design team, and reproducible by an independent investigator. This tutorial walks through the complete procedure from component receipt to final report, covering visual examination, fractography, metallographic sectioning, mechanical and chemical testing, fracture mechanics assessment, and root cause determination — following the methodology codified in ASM Handbook Volume 11 (Failure Analysis and Prevention) and augmented with practical field guidance.
- Protect fracture surfaces before anything else — contamination, mechanical damage, and corrosion of the fracture face can destroy irreplaceable fractographic evidence within hours.
- Work from macro to micro: visual examination and macroscopic fractography first, SEM and metallography second. Never section a component before the fracture face is fully documented.
- The fracture origin is the most important location on the entire surface: every observation, section, and test must be referenced back to the origin and its immediate vicinity.
- Fatigue, brittle fracture, ductile overload, corrosion-assisted cracking, and creep each produce distinct and identifiable fractographic signatures at both macroscopic and SEM scales.
- Root cause is rarely a single factor — it is usually an interaction between a material condition, a stress state, and an environmental condition that the design did not anticipate or the manufacturing process did not control.
- The failure analysis report is a legal document in many industries: every claim must be supported by documented, reproducible evidence; no speculation should appear in the Conclusions section.
Step 1: Component Receipt, Documentation, and Evidence Preservation
The single most consequential action in any failure analysis is what happens in the first five minutes after you receive the component. Fracture surfaces are uniquely fragile: corrosion, contamination, mechanical damage from mishandling, and careless attempt to reassemble mating faces can eliminate evidence that cannot be recovered by any subsequent technique, no matter how sophisticated.
- Photograph the component in its as-received condition before any handling — full view, fracture face, and all four orthogonal views. Use a scale bar in every image.
- Do not attempt to fit mating fracture faces together. This destroys asperities and deposits debris that obscures fractographic features.
- Separate mating fracture faces immediately. Store each face in a clean, dry polyethylene bag with silica gel desiccant; insert foam padding between faces if they are in the same bag.
- If the fracture surface is visibly corroding, apply a thin layer of acetone-soluble lacquer or nail varnish to preserve the surface before packaging.
- Record all chain-of-custody information: who received the component, from whom, when, and in what condition.
- Assign a unique investigation number and attach it to the component before any further work begins.
Step 2: Background Information and Service History
No failure analysis should begin examination before gathering the documentary record. The history frequently constrains the field of possible failure modes before a single observation is made: a component that failed after two weeks in service cannot have failed by normal fatigue accumulation over millions of cycles; a component that survived fifteen years before fracture cannot have failed by a manufacturing defect that would have caused immediate failure in proof testing.
The information to collect includes:
- Material specification: Grade, standard, heat number, material test certificate (MTC / EN 10204 document type), original mechanical property results.
- Manufacturing history: Forging/casting process, heat treatment cycle, surface treatment, inspection records, any known non-conformances.
- Design intent: Applied loads (nominal, peak, cyclic range), design stress, factor of safety, operating temperature, pressure, and environment.
- Service history: Hours/cycles in service, known overloads, maintenance events, any previous repairs, and whether similar failures have occurred in the same population of components.
- Failure circumstances: Operating conditions at time of failure, any precursor signs (noise, vibration, leakage), how the failure was discovered, and photographs taken by the operator if available.
Step 3: Visual and Macroscopic Examination
Visual examination under white light, oblique illumination, and low-power stereo microscope (5×–50×) is the foundation of fractography. The objective at this stage is to: identify the fracture origin(s); characterise the crack propagation direction; assess the fracture mode from macroscopic features; and evaluate the overall condition of the component (deformation, wear, corrosion, damage).
Macroscopic Fracture Features and Their Interpretation
| Macroscopic Feature | Appearance | Fracture Mode Indicated | Location Significance |
|---|---|---|---|
| Beach marks (clamshell marks, arrest marks) | Concentric curved bands radiating from origin; smooth between bands | Fatigue crack growth — each band marks a crack arrest event (load change, shutdown) | Converge toward the initiation site; spacing indicates crack growth rate variation |
| Ratchet marks | Ridges or steps on fracture surface perpendicular to crack front | Fatigue with multiple initiation sites joining; severe stress concentration | Present at the origin zone; more ratchet marks = higher stress concentration or overload |
| Chevron / herringbone pattern | V-shaped ridges pointing back toward origin | Rapid brittle fracture (cleavage or quasi-cleavage); the V opens away from origin | Follow the pattern back to apex — that is the origin |
| Shear lip | 45° angled zone of shear fracture at edge(s) of fracture surface | Final ductile fracture overload zone; or fatigue final fracture in ductile material | Present at perimeter of fracture; size indicates relative toughness of material |
| Rubbed / polished zone | Smooth, burnished area on fatigue fracture surface | Repeated face-to-face contact during crack cycling — near-threshold crack growth region | Indicates origin-proximal region; extended rubbing = long crack initiation life |
| Radial lines / fan pattern | Lines radiating outward from a point | Rapid fracture from point origin; crack branched radially outward from initiation | The apex of the fan is the origin |
| Thumbnail / elliptical zone | Semi-elliptical smooth patch at component surface | Fatigue crack growth from a surface initiation site under bending or torsion | Flat edge of the thumbnail lies at the component surface — that is the origin |
Determining Crack Origin from Macroscopic Evidence
The crack origin can almost always be located macroscopically by applying two principles: (1) crack fronts diverge away from the origin, so features that converge (beach marks, chevron Vs) point back to it; and (2) the origin region typically shows the smoothest, most rubbed surface because it experienced the longest crack-face contact time. On complex components with multiple origins, ratchet marks at the boundaries between individual initiation zones mark where separately nucleated crack fronts merged.
Step 4: SEM Fractography — Fracture Mode Identification
Scanning electron microscopy of the fracture surface at 100×–20,000× magnification resolves the microscale features that definitively identify the fracture mechanism. SEM fractography must begin at the macroscopically identified origin and progressively characterise the crack initiation zone, the stable crack propagation zone, and the final fracture zone as three distinct regions with potentially different fractographic signatures.
Fracture Surface Cleaning Protocol
Before SEM examination, the fracture surface may require cleaning to remove loose debris, corrosion products, or biological contamination that obscures the underlying features. The minimum effective cleaning method should always be used:
- Dry nitrogen or compressed air jet: First step for loose debris. Never use compressed air from a shop line (oil contamination).
- Acetone or isopropanol in ultrasonic bath (5–10 min): Removes oil, grease, fingerprint contamination. Safe for most metals.
- Plastic replication then acetone dissolution: For heavily corroded surfaces, apply acetate replication film to extract a mechanical replica of the surface texture while preserving the corrosion chemistry evidence in the parent.
- Cathodic cleaning (electrolytic cleaning in dilute citric acid): For tenacious iron oxide on steel fracture faces, mild cathodic cleaning at 1–3 V can dissolve oxide without attacking the underlying metal. Must be monitored continuously.
Key SEM Fractographic Features
Step 5: Metallographic Examination
Metallography provides the microstructural context for the fractographic observations: it reveals the crack path, confirms the material condition, identifies manufacturing defects, and allows assessment of heat treatment quality and microstructural homogeneity. The fundamental rule is that metallographic sectioning is irreversible and destructive — it must not be carried out until all non-destructive examination (visual, macroscopic, SEM fractography) is complete and documented.
- Section A — Longitudinal through-origin section: Cut parallel to the stress axis and perpendicular to the fracture face, passing through the identified origin. This section reveals the crack path, secondary cracks, initiation feature (inclusion, pit, notch), and surrounding microstructure.
- Section B — Transverse section remote from fracture: Cut well away from the fracture (minimum 3× component diameter) to obtain undamaged reference microstructure representative of the material condition.
- Sectioning method: Use a precision abrasive cut-off saw with continuous coolant. Avoid angle grinders, band saws, or any method that generates significant frictional heat — these can alter the microstructure within 0.5–2 mm of the cut surface.
- Mounting: Hot-mount in Bakelite or cold-mount in epoxy (for heat-sensitive materials or thin sections). Ensure the face of interest is perpendicular to the mount axis.
- Grinding and polishing sequence: 120 → 240 → 400 → 600 → 1200 grit SiC paper, each stage perpendicular to the previous; 6 μm diamond paste; 1 μm diamond paste; optional 0.05 μm colloidal silica (OPS) for deformation-free final polish.
- Etching: 2–3% Nital (HNO₃ in ethanol) for carbon and alloy steels; Keller’s reagent for aluminium alloys; Kroll’s reagent for titanium; 10% oxalic acid electrolytic for sensitised austenitic stainless steel. Etch time depends on alloy and microstructural feature of interest.
What to Observe in the Metallographic Section
| Observation | What to Record | What It Reveals |
|---|---|---|
| Crack path morphology | Transgranular, intergranular, or mixed; branching; crack tip blunting or sharp | Mechanism: cleavage (TG), SCC/HE/temper embrittlement (IG), fatigue (TG with blunted tip), overload (TG, blunted) |
| Microstructure at origin | Phase identification, grain size, inclusions, defects, HAZ features | Confirms initiating feature (inclusion, pore, decarburised layer, untempered martensite, sensitised grain boundary) |
| Grain size (ASTM E112) | ASTM grain size number or mean intercept length | Compliance with specification; abnormal grain growth indicates overheating; very fine grains indicate cold work or normalising |
| Hardness traverse | HV profile from surface to core, across HAZ, through case depth | Confirms heat treatment; detects decarburisation (low surface hardness), grinding burn (tempered zone, then re-hardened zone), case depth for carburised/nitrided components |
| Inclusion content | ASTM E45 or ISO 4967 inclusion rating; type, size, distribution, orientation | Material quality; large MnS stringers in fatigue-loaded forgings; Al₂O∓ clusters in clean steels |
| Secondary cracks | Number, orientation, relationship to microstructure | Multiple secondary cracks parallel to main crack → SCC; branching → SCC; perpendicular secondary cracks → fatigue crack branching |
| Weld microstructure (if applicable) | HAZ width, CGHAZ grain size, columnar grain orientation, fusion boundary condition | Weld procedure compliance; HAZ hardness >350 HV in susceptible steel → hydrogen cracking risk; lack of fusion → fatigue initiation site |
Step 6: Mechanical Testing
Mechanical testing verifies whether the failed material met its specified properties at the time of failure. The most important test in most investigations is hardness, because it is non-destructive (relative to the total material available), spatially resolved, and correlates with yield strength, tensile strength, and fracture toughness through well-established empirical relationships.
Hardness Testing Strategy
A systematic hardness survey should always be conducted: surface-to-core traverse on the longitudinal section (detects decarburisation, case depth, or through-hardening verification); across the fracture region versus remote base metal (detects local softening or hardening); and a high-resolution microhardness map around the origin if a specific feature (inclusion, HAZ zone, nitrided layer) is suspected. All hardness values should be compared against the material specification and the applicable design code (e.g., NACE MR0175 limits 250 HV or 22 HRC maximum for sour service components).
When sufficient material is available, tensile testing (ASTM E8 or ISO 6892-1) confirms yield strength and ultimate tensile strength, and the fracture appearance (cup-cone with fibrous fracture — ductile; flat, granular fracture — brittle) provides independent corroboration of the failure mode. Charpy impact testing (ASTM E23 or ISO 148-1) at the service temperature, or a temperature series to establish the ductile-to-brittle transition temperature, is essential for any brittle fracture failure where operating temperature is close to or below the design DBTT.
Fracture Mechanics Assessment
When a fracture origin contains a defect of measurable size, or when the fracture mechanics regime of the failure must be confirmed, the following relationships are applied:
Fracture Mechanics Relationships:
Stress Intensity Factor (Mode I, tension):
K_I = Y × σ × √(π × a)
where:
K_I = stress intensity factor (MPa√m)
Y = geometry correction factor (dimensionless; ~1.0–1.2 for surface crack)
σ = applied stress (MPa)
a = crack half-length or depth (m)
Fracture occurs when K_I ≥ K_Ic (plane strain fracture toughness)
Critical crack size at failure:
a_c = (1/π) × (K_Ic / (Y × σ))²
Fatigue crack growth rate (Paris Law):
da/dN = C × (ΔK)^m
where:
da/dN = crack growth rate per cycle (m/cycle)
ΔK = stress intensity range = Y × Δσ × √(πa) (MPa√m)
C, m = material constants (e.g., for ferritic steel in air: C ≈ 6×10⁻¹², m ≈ 3.0)
Integrated from initial crack size a_i to critical size a_c:
N_f = ∫[a_i to a_c] da / [C × (ΔK(a))^m]
Fatigue cycles estimated from striation spacing (SEM):
da/dN ≈ striation spacing (nm to µm, measured at magnification)
N_propagation ≈ (a_c − a_i) / (mean striation spacing)Back-calculating the initial crack size from the measured final fracture dimensions and the known applied stress (from service records or stress analysis) is a powerful verification step: if the calculated initial crack size corresponds to a known defect type (e.g., 0.5 mm surface pit, consistent with the observed corrosion pit at the origin), this provides strong corroboration of the root cause narrative.
Step 7: Chemical Analysis
Chemical analysis serves two distinct purposes in failure analysis: confirming that the material was the correct specified grade, and identifying any environmental species on the fracture surface or corrosion deposit that were involved in the failure mechanism.
Bulk composition (OES): Optical emission spectrometry on a freshly ground surface provides full elemental analysis in under two minutes. Compare each element against the specification range. A steel certified as Grade 4140 but found to contain 0.05% Ti and 0.12% Nb is not 4140 — it is a different grade entirely, with different hardenability and potentially different susceptibility to temper embrittlement or hydrogen-assisted cracking.
Fracture surface chemistry (SEM/EDS): Energy-dispersive X-ray analysis during SEM examination identifies deposits on the fracture face. Chlorine peaks in a stainless steel fracture surface confirm a chloride-induced SCC mechanism. Sulphur deposits in a carbon steel fracture surface from a gas well indicate H₂S service. Copper deposits in a steel fracture from an electrical system indicate liquid metal embrittlement by molten copper. These elemental identifications are often the clinching evidence that converts a plausible hypothesis into a definitive root cause finding.
Step 8: Stress Analysis and Fracture Mechanics Assessment
The investigator must confirm that the failure is consistent with the applied loads — that the fracture mechanics calculation supports the observed fracture. If the fracture surface area is known and the material’s fracture toughness KIc is established from specification or testing, the applied stress at fracture can be back-calculated. If this back-calculated stress is consistent with the documented service load, the fracture is explained. If it requires a stress far exceeding the nominal design load, an unrecognised stress concentration, residual stress, or dynamic overload must be invoked. If it requires a stress far below the nominal load, material below-specification toughness, an environmental reduction in effective KIc, or an incorrect geometry factor assumption must be investigated.
Step 9: Evidence Synthesis and Root Cause Determination
Root cause determination is the analytical core of the investigation. It requires integrating all observations — fractographic, microstructural, mechanical, chemical, and stress-analytical — into a single, internally consistent failure narrative that explains every piece of evidence and is not contradicted by any piece of evidence.
The root cause in engineering failures is almost always multi-factorial. The standard framework identifies:
- Primary cause: The physical mechanism responsible for final fracture (e.g., fatigue crack growth to critical size).
- Initiating cause: The feature or condition that started the crack (e.g., a 0.8 mm stress-corrosion pit at a weld toe).
- Contributing factors: Conditions that accelerated the initiation or propagation (e.g., residual tensile stress from inadequate PWHT; chloride concentration from inadequate drainage of the joint).
- Root cause (organisational): The underlying process, design, or management failure that allowed the initiating cause and contributing factors to exist (e.g., absence of a post-weld inspection procedure for the joint geometry; procurement specification that did not require impact testing at service temperature).
Step 10: The Failure Analysis Report
The failure analysis report is the final deliverable and in many industries constitutes a legal document. It must be written so that another qualified investigator could independently repeat the examination and reach the same conclusions from the documented evidence. Every claim in the Conclusions section must be traceable to documented evidence in the body of the report.
Report Structure
- Executive Summary (1 page maximum): Failure mode, primary root cause, and the single most important corrective action. Written last but placed first. Must be comprehensible to a non-specialist reader.
- Background and Service History: Component identity (drawing number, material specification, heat number), manufacturing history, service history, failure timeline, and circumstances of failure discovery.
- Visual and Macroscopic Examination: All photographs with annotations; fracture surface description; origin location with reference to component geometry; deformation pattern; corrosion condition.
- Metallographic Examination: Section locations (referenced to photographs); microstructure description; grain size; phase identification; defect observations; hardness traverse (table and graph); etching reagent used.
- Chemical Analysis: Compositional results versus specification (table); EDS results from fracture surface deposits (spectra and elemental maps); phase identification by XRD if applicable.
- Supplementary Testing: Tensile and impact results versus specification; fracture toughness data; SEM fractographic analysis with annotated micrographs; fracture mechanics calculations.
- Discussion: Integration of all evidence; failure mechanism explanation; reconciliation with service history; explicit assessment of alternative hypotheses and why they are excluded.
- Conclusions: Numbered, declarative statements. Primary cause first, then initiating cause, then contributing factors. No new information; only what the evidence supports.
- Recommendations: Specific, actionable, and categorised by urgency (immediate safety actions; short-term engineering changes; long-term process/specification improvements). Include recommended inspection of similar components in service.
- Appendices: Full photographic record; all raw data (hardness tables, compositional print-outs, EDS spectra); chain-of-custody record; credentials of investigators.
Common Failure Modes: Quick Reference
| Failure Mode | Macroscopic Features | SEM Features | Typical Root Causes | Key Tests |
|---|---|---|---|---|
| Fatigue | Beach marks, ratchet marks, smooth origin zone, shear lip at final fracture | Parallel striations ≅ μm spacing; secondary cracks; dimples in final zone | Stress concentration, surface damage, corrosion-fatigue, overload, design error | SEM fractography, hardness at origin, FEA stress analysis, fatigue crack growth rate calculation |
| Brittle cleavage | Chevron / herringbone pattern, bright crystalline, minimal deformation, fast fracture | Cleavage facets, river marks, tongues, no dimples | Low temperature, high strain rate, hydrogen embrittlement, below-spec toughness, high residual stress | Charpy impact (DBTT), fracture toughness KIc, hardness, composition check |
| Ductile overload | Cup-cone (round section), slant fracture (plate), gross deformation, fibrous grey surface | Equiaxed MVC dimples, inclusions at dimple bases | Overload, incorrect material (low strength), severe stress concentration, temperature excursion | Tensile test, hardness, composition OES, inclusion rating |
| Stress-corrosion cracking (SCC) | Branching cracks, corrosion deposits, minimal deformation, often at weld or bent zone | Intergranular facets (most steels/brass), transgranular in some stainless; corrosion deposits by EDS | Susceptible material + tensile stress + corrosive environment (Cl⁻ for SS; H₂S for high-strength steel; NH₃ for brass) | SEM/EDS deposits, pH / Cl⁻ of environment, slow strain rate (SSR) test, composition |
| Hydrogen embrittlement (HE) | Minimal deformation, often delayed fracture, intergranular dominant | Intergranular + quasi-cleavage; no EDS deposits; white band in steel (hydrogen white-attack) | Acid pickling, electroplating, cathodic overprotection, high-pressure H₂, hydrogen-containing welding | Slow strain rate test, hydrogen permeation, hardness (>350 HV susceptible), microstructure |
| Creep / stress-rupture | Wedge cracking at grain boundaries, intergranular cavitation, macroscopic deformation, surface oxidation | Grain boundary voids and wedge cracks; may show dimples at final rupture | Service temperature above design limit, stress above creep design allowable, material degradation (graphitisation, sigma phase) | Microstructure (void density), hardness (creep softening), remnant life Larson-Miller, replica metallography |
Related Technical Content
Frequently Asked Questions
What is the first thing to do when you receive a failed component?
Before touching the fracture surface, photograph the as-received condition under multiple lighting angles with a scale bar in every image. Record all identifying information: component designation, material specification, service history, and operating conditions at time of failure. Protect fracture surfaces from contamination and mechanical damage immediately — place mating faces in separate clean polyethylene bags with silica gel desiccant. Never attempt to fit mating fracture faces together; this destroys fractographic evidence. Any cleaning must wait until all as-received evidence is photographed and documented. Assign a unique investigation number and document chain of custody before any further action.
How do you distinguish fatigue from brittle fracture macroscopically?
Fatigue fractures show beach marks (concentric curved bands radiating from the origin), a rubbed/polished zone in the crack growth region, and a distinct final fracture zone. Ratchet marks at the origin indicate multiple initiation sites. Brittle fracture shows no beach marks; instead the surface is covered by a chevron or herringbone pattern pointing back to the origin (cleavage), or by granular intergranular facets visible to the naked eye. Brittle fracture typically shows no macroscopic deformation. The shear lip — a 45° angled zone at the component edge — is present in fatigue (at the final fracture zone) and in ductile overload, but absent in fully brittle fracture. The relative size of the stable crack growth zone versus the final fracture zone also indicates the applied stress relative to the material’s fracture toughness at the time of failure.
What SEM features identify ductile, brittle, and fatigue fracture modes?
Ductile overload fracture shows microvoid coalescence (MVC) dimples — equiaxed cup-shaped cavities with inclusions or particles at their bases. Brittle cleavage fracture shows flat crystallographic facets with river marks (convergent step features) and tongues; in BCC metals, cleavage occurs on {100} planes. Intergranular brittle fracture (SCC, HE, temper embrittlement) shows the three-dimensional polyhedral grain boundary faces without dimples. Fatigue fracture shows characteristic parallel striations — each striation corresponding to one load cycle — oriented perpendicular to the local crack growth direction, often with secondary cracks parallel to the striations. Striation spacing can be measured to estimate crack growth rate da/dN and compared to Paris law predictions for the material.
How do you prepare a metallographic section without destroying fracture evidence?
Always section well away from the fracture face — never cut through the origin region before fractography is complete and documented. The correct sequence: photograph and document the fracture fully first; mark the section line at minimum 5–10 mm from the fracture face; use a slow-speed abrasive cut-off saw with coolant to minimise thermal damage; protect the fracture face with lacquer before sectioning if the cut must be close; then take a longitudinal section through the origin after the fracture face is fully archived. Mount in Bakelite or epoxy, grind through 120/240/400/600/1200 grit SiC, polish to 1 μm diamond, then etch with Nital 2–3% for steels. The longitudinal section through the origin is the most informative section for revealing the crack path, secondary cracks, and initiating microstructural feature.
What hardness measurements should be taken during failure analysis?
Perform Vickers hardness (HV10 or HV30) traverses across: the fracture origin region and adjacent base metal (detects softening, overheating, decarburisation); the HAZ and base metal in welded components (identifies hard zones >350 HV at risk of hydrogen cracking, or soft over-tempered zones); surface versus core (confirms case depth in surface-hardened components, detects decarburised layers); and any discoloured or heat-tinted regions (identifies overheating or fire damage). Compare all measured values against the material specification and applicable code limits (e.g., NACE MR0175 maximum 250 HV for sour service). Use microhardness (HV0.1–HV0.5) to map individual phases and thin surface layers such as nitrided cases or decarburised zones.
What chemical analysis methods are used and what does each confirm?
Optical emission spectrometry (OES) on a freshly ground surface confirms bulk chemical composition versus the specified grade — detecting wrong material, ladle analysis deviations, or specification non-compliance. Energy-dispersive X-ray spectroscopy (EDS) in the SEM identifies elemental composition of fracture surface deposits, corrosion products, inclusions, and second-phase particles at the micron scale — essential for identifying stress-corrosion agents (chlorine for SCC in SS, sulphur for H₂S service), confirming inclusion types (MnS, Al₂O₃, TiN), or identifying contaminants. X-ray diffraction (XRD) identifies crystalline phases in corrosion products and scale. Wet chemical analysis or ion chromatography quantifies solution chemistry (chloride, sulphate, H₂S) in corrosion-related failures. Each method provides a different layer of chemical evidence that together builds the environmental and material history.
What are the most common root causes of fatigue failures?
The most common root causes are: (1) Stress concentrations — sharp notches, machining marks, weld toe geometry, or sudden section changes that locally amplify stress above the fatigue limit; (2) Surface damage — corrosion pits, fretting, hydrogen-assisted surface cracking, or loss of beneficial compressive residual stress; (3) Occasional overloads — a single load cycle exceeding the fatigue crack initiation threshold can start a crack that grows over subsequent normal service cycles; (4) Material defects — large MnS stringers in longitudinally loaded forgings, surface laps, seams, or decarburisation; (5) Design errors — underestimated fatigue stress concentration factor Kt, or failure to account for combined bending and torsion loading; (6) Corrosion fatigue — aqueous environments eliminate the endurance limit of steels entirely, dramatically accelerating both initiation and propagation in humid or submerged service.
What is the structure of a formal failure analysis report?
A formal failure analysis report contains: (1) Executive Summary — one page, failure mode, root cause, and primary recommendation; (2) Background and service history; (3) Visual and macroscopic examination with annotated photographs; (4) Metallographic examination — microstructure, grain size, hardness traverse with data tables; (5) Chemical analysis — composition versus specification, EDS results; (6) Supplementary testing — tensile, impact, fracture mechanics calculations; (7) Discussion — integration of all evidence, failure mechanism, explicit assessment of alternative hypotheses; (8) Conclusions — numbered declarative statements of primary cause and contributing factors, no new information; (9) Recommendations — specific and actionable, categorised by urgency; (10) Appendices — full photographic record, raw data, chain-of-custody. The report must be technically defensible and reproducible by another qualified investigator from the documented evidence alone.
How do you distinguish stress-corrosion cracking (SCC) from hydrogen embrittlement (HE)?
Both can produce intergranular fracture in high-strength steels, but key distinguishing features are: SCC shows branching secondary cracks along the crack path; corrosion products (oxides, chlorides, sulphides confirmed by EDS) on fracture surfaces; and failure requires a specific susceptible material-environment combination (austenitic SS in chlorides, high-strength steel in H₂S per NACE MR0175, brass in ammonia). Hydrogen embrittlement produces intergranular fracture with minimal corrosion product on the fracture face; may show quasi-cleavage regions; is associated with electroplating, acid pickling, cathodic protection overprotection, or high-pressure hydrogen. HE often shows delayed fracture behaviour (failure occurring hours after loading). Confirmatory tests: slow-strain-rate (SSR) testing in the suspect environment for SCC; hydrogen permeation testing and stepped-load testing for HE susceptibility. Hardness above 350 HV strongly indicates HE susceptibility in high-strength carbon and alloy steels.
What is fracture mechanics life assessment and when is it used in failure analysis?
Fracture mechanics life assessment determines whether a crack of known size would propagate to failure under the documented service loads, or back-calculates the initial crack size and growth history from the observed fracture dimensions. The stress intensity factor K₁ = Yσ√(πa) governs fracture when it reaches K₁c (fracture toughness). For fatigue, the Paris law da/dN = C(ΔK)m is integrated from initial to critical crack size to predict propagation life or infer cycle count from striation spacing. This is used in: aerospace investigations (where every failure must be reconciled with recorded load spectra); pressure vessel assessments (fitness-for-service per BS 7910 or API 579); cases where the operator disputes the service history; and any investigation where quantitative confirmation of the root cause scenario is required for legal proceedings or regulatory reporting.
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