Failure Analysis in Metallography

Introduction to Failure Analysis

Failure analysis in metallography involves the systematic investigation of material failures to determine their root causes. This process combines macroscopic examination, metallographic preparation, and microscopic analysis to understand why a component failed and how similar failures can be prevented.

The goal of failure analysis is not just to identify what failed, but to understand why it failed, when it failed, and how to prevent similar failures in the future. This makes it a valuable tool in materials engineering, quality control, and product development.

Types of Material Failures

1. Ductile Failure

Ductile failures occur when materials undergo significant plastic deformation before fracture. Characteristics include:

• Necking and reduction in cross-sectional area
• Cup-and-cone fracture surfaces
• Dimpled fracture appearance at high magnification
• Evidence of plastic deformation in the microstructure

2. Brittle Failure

Brittle failures occur with little or no plastic deformation. Characteristics include:

• Flat, featureless fracture surfaces
• Cleavage facets (in crystalline materials)
• River patterns on fracture surfaces
• Minimal plastic deformation in surrounding material

3. Fatigue Failure

Fatigue failures result from cyclic loading below the material's ultimate strength. Characteristics include:

• Beach marks (macroscopic, optical): Concentric ring patterns visible to the naked eye or under low-power stereomicroscope. Each beach mark records a stretch of cycling under one set of conditions; transitions between beach marks correspond to load or environment changes (start/stop cycles, overload events). Useful for reconstructing the failure history.
• Striations (microscopic, SEM): Fine parallel ridges only resolvable under SEM. Each striation corresponds to one load cycle of crack advance. Used to back-calculate cycle counts and crack growth rates. Beach marks and striations are different scales of feature, not synonyms — a single beach mark may contain thousands of striations.
• Multiple crack initiation sites (typical when stress concentrators or surface defects are present)
• Progressive crack growth patterns radiating from initiation
• Final fast fracture region (often ductile dimples in tough materials, cleavage in brittle ones)

4. Corrosion-Related Failure

Failures caused by environmental degradation. Types include:

• Stress corrosion cracking (SCC)
• Corrosion fatigue
• Intergranular corrosion
• Pitting and crevice corrosion

5. Creep Failure

Failures occurring under sustained loads at elevated temperatures. Characteristics include:

• Grain boundary cavitation
• Elongated grains in the direction of stress
• Intergranular fracture surfaces
• Time-dependent deformation

Failure Analysis Methodology

Step 1: Background Information Collection

Before beginning any analysis, gather comprehensive information:

• Component history and service conditions
• Material specifications and heat treatment
• Manufacturing processes and quality records
• Operating environment and loading conditions
• Failure chronology and witness accounts

Step 2: Macroscopic Examination

Initial visual inspection provides critical information:

• Document fracture surface appearance
• Identify crack initiation sites
• Note deformation patterns
• Record corrosion or environmental damage
• Photograph all relevant features

Step 3: Sample Selection and Preparation

Careful sample selection is important for meaningful analysis:

• Select samples from failure origin and unaffected areas
• Preserve fracture surfaces when possible
• Prepare cross-sections perpendicular to fracture surface
• Follow standard metallographic preparation techniques
• Use appropriate etching to reveal microstructure

Refer to our sectioning, grinding, polishing, and mounting guides for proper sample preparation.

Step 4: Microscopic Examination

Detailed microstructural analysis reveals failure mechanisms:

• Examine microstructure at failure origin
• Compare with unaffected areas
• Identify microstructural anomalies
• Document grain size, phase distribution, and inclusions
• Look for evidence of degradation or damage

Step 5: Fractography

Fracture surface examination (fractography) provides direct evidence of failure mode:

• Use scanning electron microscopy (SEM) for high-resolution imaging
• Identify fracture features (dimples, cleavage, striations)
• Determine crack propagation direction
• Identify secondary cracks and damage
• Correlate with microstructural features

Step 6: Additional Testing

Supplementary tests may be necessary:

• Hardness testing to verify heat treatment
• Chemical analysis to verify composition
• Mechanical property testing on unaffected material
• Corrosion testing if environmental factors are suspected
• Residual stress measurements

Step 7: Root Cause Analysis

Synthesize all information to determine root cause:

• Correlate findings with service conditions
• Identify primary and contributing factors
• Distinguish between design, material, manufacturing, and service issues
• Consider multiple failure mechanisms if applicable

Step 8: Recommendations and Reporting

Document findings and provide actionable recommendations:

• Prepare comprehensive written report
• Include high-quality micrographs and photographs
• Provide specific recommendations for prevention
• Suggest material or design improvements
• Document lessons learned

Common Failure Analysis Techniques

Fracture Surface Analysis

The fracture surface contains the most direct evidence of failure mode. Key features to identify:

• Crack initiation site: Usually at stress concentrators, defects, or surface damage
• Propagation zone: Shows the mechanism of crack growth
• Final fracture zone: Indicates the final failure mechanism
• Secondary features: Corrosion products, debris, or environmental deposits

Microstructural Analysis

Microstructural examination reveals material condition and degradation:

• Grain size and distribution
• Phase composition and morphology
• Inclusion content and distribution
• Heat treatment condition
• Evidence of deformation or damage
• Corrosion or environmental attack

Cross-Sectional Analysis

Examining cross-sections through the failure provides three-dimensional context:

• Crack path through microstructure
• Relationship to grain boundaries or phases
• Depth of environmental attack
• Deformation patterns
• Microstructural gradients

Failure Analysis Case Studies

Case Study 1: Fatigue Failure of a Shaft

A rotating shaft (typically 4140 or 4340 carbon/low-alloy steel in the Q&T condition, or fully hardened tool steel) failed after extended service. Analysis revealed:

• Multiple fatigue crack initiation sites at keyway corners
• Beach marks (macroscopic) at low magnification, with striations resolvable under SEM showing per-cycle crack advance
• Final ductile overload region with dimpled fracture morphology
• Root cause: Stress concentration at sharp keyway corners combined with cyclic loading
• Solution: Redesign with radiused keyway corners and shot peening

Case Study 2: Brittle Fracture of a Weld

A welded structure failed catastrophically. Investigation showed:

• Cleavage fracture initiating from weld defects
• Coarse grain structure in heat-affected zone
• Low toughness at service temperature
• Root cause: Inadequate post-weld heat treatment and low-temperature service
• Solution: Proper PWHT and material selection for low-temperature applications

For the prep workflow specific to weld cross-sections — fusion zone, HAZ, and base metal each requiring its own etch — see the welding analysis guide. Weld defect investigation per ASTM E340 uses macroetching to expose fusion-zone penetration, HAZ extent, and weld-line discontinuities at the macroscopic scale before microstructural work.

Case Study 3: Stress Corrosion Cracking

Stainless steel component failed in service. Analysis identified:

• Intergranular crack propagation
• Corrosion products in crack
• Sensitized microstructure (grain boundary carbides)
• Root cause: Sensitization from welding and exposure to corrosive environment
• Solution: Use low-carbon or stabilized grades (304L, 321, 347) and proper welding procedures

Diagnostic etch that catches this pre-failure: 10% oxalic acid electrolytic at 6 V for 90 s — the canonical sensitization detection per ASTM A262 Practice A. The etch reveals the "ditched" structure characteristic of continuous chromium-carbide precipitation along grain boundaries in sensitized 304/316. Any stainless component that has been welded and is destined for hot-water, chloride, or general corrosive service should pass A262 Practice A before commissioning. See the stainless steel preparation guide for the full electrolytic-etch workflow.

Cross-references for the other case studies: Case Study 1 (fatigue shaft) prep workflow lives in the carbon and low-alloy steel guide for through-hardened 4140/4340-class shafts, and the tool steel guide if the shaft is fully hardened. Case Study 2 (welded structure brittle fracture) prep workflow lives in the welding analysis guide.

Common Prep Artifacts in FA — Don't Mistake These for Defects

The most expensive failure-analysis mistakes are not technical fractography errors — they are prep artifacts misreported as real component defects. A pull-out crater misidentified as gas porosity will reject a perfectly good casting; a smeared surface misread as "no microstructure" will hide a real sensitization or decarburization problem. Each of the five common FA prep artifacts has a clean diagnostic question and a known fix. The longer-form troubleshooters below cover each in detail:

• Edge rounding — the coating disappears as the field of view approaches the mount; thickness measurements drift between operators. Almost always a mount-material problem (phenolic instead of glass-filled epoxy), not a polishing problem.
• Mirror finish that won't etch (smearing) — mechanical polishing has homogenized the surface so chemical etchants find no boundaries to attack. Common on Cu, Al, Mg, and pure Ni. Fix: chemo-mechanical final polish with H₂O₂ in colloidal silica.
• Comet tails behind hard particles — unidirectional scratches trailing carbides, slag inclusions, MMC reinforcement, or cast-iron graphite. Fix: lower polishing force, harder pad, 90° rotation between intervals.
• Embedded SiC dark specks on soft metals — random dots scattered across a polished Al, Mg, Pb, or Sn sample. Liberated SiC grit pressed into the soft matrix during grinding. Fix: switch to alumina (ALO) papers instead of SiC for soft non-ferrous.
• Pull-out vs. real porosity — the diagnostic that determines whether you reject a casting or reprep your sample. Smooth, rounded pit walls = real porosity. Irregular, fresh-fracture walls = pull-out artifact. Examine unetched first.

When the FA conclusion turns on the presence or absence of a defect, run through the five questions above before signing the report.

Best Practices in Failure Analysis

Preservation of Evidence

• Protect fracture surfaces from damage and contamination
• Document everything before destructive testing
• Maintain chain of custody for legal cases
• Store samples properly to prevent further degradation

Systematic Approach

• Follow a structured methodology
• Document all observations and measurements
• Use standardized terminology and classifications
• Maintain objectivity and avoid premature conclusions

Quality Documentation

• High-quality photographs at all stages
• Clear, labeled micrographs with scale bars
• Detailed written descriptions
• Comprehensive reports with conclusions and recommendations

Interdisciplinary Collaboration

• Work with design engineers, materials scientists, and manufacturing experts
• Consider all aspects: design, materials, manufacturing, and service
• Use expertise from multiple fields
• Ensure recommendations are practical and implementable

Tools and Equipment

Effective failure analysis requires appropriate tools:

• Macroscopic examination: Stereo microscopes, digital cameras, measuring tools
• Metallographic preparation: Sectioning, mounting, grinding, and polishing equipment
• Optical microscopy: Light microscopes with various illumination modes
• Electron microscopy: SEM for high-resolution fractography
• Hardness testing: Various scales for material characterization
• Chemical analysis: EDS, XRF, or wet chemistry methods

See our equipment overview guide for more information on metallographic equipment.

Standards and References

Standards directly relevant to failure analysis practice:

General preparation and terminology

• ASTM E3 — Standard Practice for Preparation of Metallographic Specimens
• ASTM E407 — Standard Practice for Microetching Metals and Alloys (the canonical etchant reference; assigns numeric IDs to standard etchants like Nital, Glyceregia, Keller's, Kroll's, Marble's)
• ASTM E883 — Standard Guide for Reflected-Light Photomicrography
• ASTM E1823 — Standard Terminology Relating to Fatigue and Fracture Testing

Defect-specific diagnostic standards

• ASTM A262 — Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels (Practice A: oxalic acid electrolytic etch — the canonical sensitization diagnostic)
• ASTM A923 — Detecting Detrimental Intermetallic Phase in Duplex Austenitic/Ferritic Stainless Steels
• ASTM A763 — Detecting Susceptibility to Intergranular Attack in Ferritic Stainless Steels
• ASTM A247 — Evaluating the Microstructure of Graphite in Iron Castings (nodularity rating; required to be performed on as-polished, unetched specimens)
• ASTM E45 — Determining the Inclusion Content of Steel (worst-field method; A-D classification charts)
• ASTM E340 — Macroetching Metals and Alloys (weld penetration, HAZ extent, segregation, flow lines — relevant to weld and forging FA)
• ASTM E1077 — Estimating the Depth of Decarburization of Steel Specimens
• ASTM E1351 — Production and Evaluation of Field Metallographic Replicas (in-service non-destructive examination)

Coatings and surface treatments

• ASTM E1920 — Standard Guide for Metallographic Preparation of Thermal Sprayed Coatings

Reference handbooks

• ASM Handbook Volume 11 — Failure Analysis and Prevention
• ASM Handbook Volume 12 — Fractography
• ASM Handbook Volume 9 — Metallography and Microstructures (recipe reference for etchants and prep ladders)

Refer to our ASTM standards reference for the full list with descriptions.

Conclusion

Failure analysis is a critical discipline that combines metallographic techniques with engineering analysis to understand material failures. By systematically investigating failures, engineers can identify root causes, prevent future occurrences, and improve material and component design.

Success in failure analysis requires:

• Thorough understanding of metallographic techniques
• Knowledge of material behavior and failure mechanisms
• Systematic, methodical approach
• Attention to detail and preservation of evidence
• Clear communication of findings and recommendations

For more information on metallographic techniques, see our guides on sectioning, grinding, polishing, and microstructural analysis.