Fracture Mechanics & Stress Analysis of Cracks

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Fracture mechanics is a relatively new field of stress analysis which permits a quantitative analysis of cracks and defects. Linear elastic fracture mechanics (LEFM) enabled the safe use of high strength alloys in the Apollo program, jet aircraft and other sophisticated applications. This is because high strength alloys are susceptible to serious strength reduction in the presence of small cracks and flaws. Fracture mechanics provides the engineering basis to quantitatively predict the effects of these cracks.

How much a given crack size will affect strength and how long it will take for a flaw to grow to a size that is dangerous for operation can be answered for many materials. Linear elastic fracture mechanics works best for high strength, brittle alloys, ceramics, and some non-metals (different techniques are used for more ductile materials).

Linear elastic fracture mechanics is used heavily in the analysis and prediction of fatigue crack growth in aircraft and pressure vessels. If an initial flaw or defect size and load spectra are known, fatigue life for a growing crack to a critical size can be estimated. Conversely, actual operating stress ranges can often be estimated by fatigue fracture striation surface measurements.

Linear elastic fracture mechanics analysis of fracture works best for high strength alloys. Tearing instability can be used to evaluate seemingly brittle failure of lower strength, high toughness structures where stored elastic energy becomes important. Low strength, high toughness steel failures can also be evaluated using ideal plastic analysis. These two additional approaches extend the use of fracture mechanics to the more common structural steels of high toughness.

Fracture mechanics is a powerful tool in root cause failure analysis. Notches, flaws and specimen geometry features that concentrate stresses are frequently cited as the sole cause of fatigue failures. Stress levels can be calculated for defects and compared with fatigue crack propagation life estimates. In some cases, measurements of fatigue striation spacing in conjunction with fatigue crack propagation data can yield estimates of operating stress levels. This type of analysis frequently leads to recognition of abnormal overloads, which cause the fatigue. The ability to estimate stresses at failure is also important in root cause failure analysis. This technique has been used often in expert witness cases.

Fracture mechanics can also be used in design. Both crack growth and cumulative damage techniques can be used to predict structural lives and set inspection intervals. Knowledge of the material fracture toughness can be used to estimate critical crack sizes for equipment. Equipment whose critical crack size for failure at operational stresses is below that which can be reliably detected by inspection can be dangerous.


Fracture Mechanics & Stress Analysis Capabilities

Analysis/Capability
Metallurgy
Materials Science
Welding
Corrosion
Fracture Mechanics
Stress Analysis
Strain Gauge Testing
Fatigue Analysis
Full Root Cause Failure Analysis
Product Areas
Trucks
Automobiles
Aircraft
Ships
Pressure Vessels
Machinery
Paper Industry
Chemical Process Industry
Power Generation
Litigation
Experience
Since 1973
Equipment
Large Chamber Scanning Electron Microscope
X ray with SEM
Hydraulic Ram for full Scale Tests
Load Cells
Multi-Channel High Speed Data Acquisition
DC&AC Corrosion Testing Equipment
Optical Microscopes
Specimen Preparation Equipment
Information to Work from
Engineering Drawings & Specifications
Service History
Exemplars or Similar Situations
Service Environment
Failure Determination
Fracture Surface Analysis
Microstructural Analysis
Non-Destructive Evaluations
Stress Analysis
Corrosion Testing
Industry Focus
Litigation
Chemical
Paper
Ship Building
Aircraft
Power Generation
Oil Field
Types of Experience
  • Fatigue Life Prediction
  • Cast Iron Valve Body
    SS Pressure Vessel
    Aircraft Fuselage
  • Fracture Mechanics
  • Trunnion Bolt Failure
    Chain Failure
    Large Steel Casting
  • Tear Instability
  • Continuous Digester
    Boiler System
  • Ideal Plastic Analysis
  • Fan Failure
    Steel Pressure Vessels
    Weld Failures
  • Fatigue Striation Analysis
  • Aircraft Fuselage
    Large Motor Shaft
    SS Exhaust Ducts
  • Fitness For Service
  • Digesters
    SS Pressure Vessel
    Cast Iron Vessels

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