Fracture mechanics

Fracture mechanics is an analytical tool for quantitatively relating the factors controlling susceptibility to brittle fracture. Fracture mechanics is used only in critical applications justifying in-depth engineering analysis. The principal factors to be considered in fracture mechanics are:

1. Flaw size—The basic theory of fracture mechanics is that brittle fracture initiates at preexisting flaws (cracks). Welded structures are rarely flaw-free. Inspection during fabrication is necessary to limit as-built flaw sizes. Fatigue and stress corrosion cracking can also cause flaws to increase in service, thus periodic inspections are necessary.

2. Stress—Tensile stress is necessary for brittle fracture to occur. Since stress concentrations increase nominal stress to cause high local stresses and greater susceptibility to fracture, they should be minimized. Residual stress from forming and welding can also affect local stress levels. Postweld heat treatment (PWHT) lowers residual stress. Thus, PWHT generally lowers the risk of brittle failure.

3. Fracture toughness—Notch toughness is the ability of a material to deform plastically in the presence of a notch or crack. Thus, tough materials are resistant to brittle fracture. Fracture toughness is a material property just as strength is. Notch toughness is a qualitative term, describing a material’s resistance to fracture. Fracture toughness is quantitative and is measured according to the principles of fracture mechanics.

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