Fatigue Crack Propagation

MSE 527

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Principles of Failure Analysis

General Procedures

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Definition of Failure

When a part or device can no longer perform its intended function, the part has failed.

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Definition of Failure Analysis

A systematic, science-based method employed for investigation of failures occurring during tests or in service.

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Fundamental Sources of Failure

* Deficiencies in design.

* Deficiencies in selection of materials.

* Imperfections in materials.

* Deficiencies in processing.

* Errors in assembly.

* Improper service conditions.

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Impact of Failure Analysis on Society

* Cost of failure.

* Cost of failure analysis.

* Improvement of products.

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14 Stages of Failure Analysis

1. Background data. 8. Metallography

2. Preliminary exam. 9. Failure mode.

3. Nondestructive tests. 10. Chemical analysis.

4. Mechanical tests. 11. Fracture mechanics

5. Sample selection. 12. Simulated tests.

6. Macroscopic exam. 13. Analysis & report.

7. Microscopic exam. 14. Recommendations.

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1. Collection of background data and samples.

* Manufacturing history.

* Service history.

* Photographic records.

* Wreckage analysis.

* Inventory of parts.

* Abnormal conditions.

* Sequence of fractures.

* Sample selection.

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Fracture A preceded fracture B.

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Fracture A preceded fractures B and C.

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2. Preliminary Examinations.

* Most important part of failure analysis.

* Visual inspection of all parts.

* Detailed photography of all parts.

* Study of the fractures.

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3. Nondestructive Inspections.

* Magnetic particle inspection.

* Liquid penetrant inspection.

* Electromagnetic inspection.

* Ultrasonic inspection.

* Radiography.

* Residual stress analysis.

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4. Mechanical Testing.

* Hardness testing.

* Tensile testing.

* Shear testing.

* Impact testing.

* Fatigue testing.

* Fracture mechanics testing.

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5. Selection and Preservation of Fracture Surfaces.

* How? Very Carefully!!!

* Prevent chemical damage to samples.

* Prevent mechanical damage to samples.

* Prevent thermal damage to samples.

* Careful cleaning:

Least destructive technique first.

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6. Macroscopic Examinations

* Use low power stereo-microscopes.

* Determine Origin of failure.

* Determine direction of crack growth:

Chevron patterns, River marks, beach marks etc...

* Determine ductile or brittle fracture.

* Locate other cracks.

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7. Microscopic Examinations

* Light microscopes:

shallow depth of field.

* Transmission Electron Microscopes (TEM):

sample preparation problems.

* Scanning Electron Microscopes ( SEM):

conductivity problems.

coating and replication techniques.

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Dimples typical of a ductile overload fracture by micro-void coalescence mechanism

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Striations typical of fatigue failures

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Cleavage fracture typical of brittle overload fracture

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Rock candy structure typical of intergranular fracture

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8. Metallographic Examination

* Class of Material:

Cast or Wrought

* General Microstructure.

* Crack Path:

Transgranular and/or Intergranular

* Heat Treatment Problems:

Decarburization, Alpha-Case, etc….

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Lap defect in forging

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Interganular crack in copper tube

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Crack branching in martensitic steel

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9. Failure Modes* Ductile:

Plastic Deformation

Equiaxed or Shear Dimples

Dull, Gray and usually Transgranular.

* Brittle:

No Macroscopic Plastic Deformation

Cleavage, Intergranular or Striations

Difficult to diagnose.

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Modes of Fracture

* Monotonic Overload

Brittle

Ductile

* Sub-Critical Crack Growth

Static Loads

Dynamic Loads

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Mechanisms of Fracture

* Overload - Fracture with application of load.

Ductile or Brittle

* Crack Growth - Under Load Over Time.

Fatigue

Stress Corrosion Cracking

Hydrogen Embrittlement

Creep

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Sub-Critical Crack Growth under Dynamic Loads

* Fatigue

* Corrosion Fatigue

* Thermal Fatigue

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Sub-Critical Crack Growth under Static Loads

* Stress Corrosion Cracking

* Hydrogen Embrittlement

* Liquid Metal Embrittlement

* Creep Rupture

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Some Common Fractographic Features

Brittle Overload Cleavage

Ductile Overload Dimples

Stress Corrosion Cracking Intergranular

Hydrogen Embrittlement Intergranular

Creep Rupture Intergranular

Fatigue Striations

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10. Chemical Analysis

* Optical Emission Spectroscopy

* Wet Chemical Analysis

* X-ray, Electron & Neutron Diffraction

* X-ray Fluorescence

* Infrared & Ultraviolet Spectroscopy

* Energy and Wavelength Dispersive X-ray Analysis.

* Surface Analysis Techniques

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11. Fracture Mechanics

* Fracture Toughness Testing.

* Strain Rate Sensitivity.

* Notch Sensitivity.

* Triaxiality

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12. Simulated-Service Testing

* Of Limited Value.

* Simulated Corrosion Tests.

* Deciding between several possible mechanisms.

* Errors by Changing Severity of Conditions.

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13. Conclusions and Report

* Be Clear & Concise.

* Do not Express Opinions Without Facts.

* Consider the Client.

* Site the Sources of External Data.

* Check list is a good idea.

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14. Recommendations

* Should lead to prevention of future failures.

* Should lead to product improvements.

* Do not rush to change material or process specifications without complete analysis of possible interaction with other parts of the system.