Failure Analaysis

   Failure as a Design      Criterion

   Fracture Mechanics

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Tutorial Questions

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Griffith Equation


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Stress Intensity Factor and Fracture Toughness Testing
- Stresses Close to a Crack Tip
- Fracture of Glass
- High Strength Versus high Toughness
- Quenching and Residual Stress
- Missile Motor Case Fracture
- Fracture Toughness Tests
- Plastic Zone Effect
- Specimen Thickness Effect
- Growth of Semi-Elliptic Flaws
- Leak-Before-Break Concept
- Pressurised Vessels
- Fracture of a Beer Barrel
- Pin-Loaded Lug
- Materials Selection and Temperature
- Chemical Reactor Vessel
- Fracture of Ice

 :: 
Characterising Sub-Critical Growth
 -  Fatigue Life Prediction
 -  Stress Corrosion Cracking

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Theory Resource



Problem 3

This question is designed to illustrate the the paradox between 'classic' mechanical engineering design, using high strength, and fracture mechanics design based on high toughness. It should take about 15 minutes to complete.
A welded structure is to be fabricated from large sheets of 0.45C-Ni-Cr-Mo steel. The detection limit of available NDT techniques limits the critical defect size to sizes > 3 mm, as cracks smaller than this are not detectable. A design stress level of half the tensile strength is proposed.

To save weight in the structure it has been suggested that the steel could be heat treated to a higher tensile strength level. The current grade has a tensile strength of 1520 MPa, and a candidate replacement grade has a 2070 MPa strength level.

Is this change supportable in fracture mechanics terms?

You may assume plane strain conditions in all computations, and the figure below indicates the relationship between fracture toughness and tensile strength for this steel.

Compare the allowable stress levels, and hence weights, in both grades of steel for an allowable initial defect size of approximately 5 mm.


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