36'1 Fracture toughness

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36.1 Fracture toughness

• There are often cracks or crack-like structures
  in materials.
• Cracks form during processing and service (e.g.
  surface crack by corrosion).
• Under sufficient load, cracks lead to
  catastrophic, fast fracture.

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• Fracture toughness a property to measure the
  tolerance of materials for cracks under load.
• A useful property for brittle materials with
  limited ductility (all ceramics, some metals,
  some polymers).
• Tensile toughness and impact toughness should be
  used for ductile materials (ductile metals and
  polymers).

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A crack in a superalloy
C. Newey and G. Weaver, Material Principles and
Practice.
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A crack in alumina (Al2O3)
B. Lawn, Fracture of Brittle Solids.
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36.2 Intuition and question

• Intuition
• Under a stress, a crack will propagate, leading
  to failure
• The larger the crack, the smaller the stress
  needed to propagate the crack.
• Question
• With a given crack size (c), what is the maximum
  stress (?f) that can be applied without causing
  failure?

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Intuition
?f
2c
?f
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36.3 Answer KIC
?f
Fast fracture occurs when
2c
?f
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?f vs c
KIC (MPa.m1/2)
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36.4 Use of KIC an example
Materials Selection in Mechanical Design. M.F.
Ashby , Butterworth-Heinemann, 1992.
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Selection criterion

• It may leak (ct/2).
• It may distort (? ?y)
• It must not explode

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Performance index
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2
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Materials Selection in Mechanical Design. M.F.
Ashby , Butterworth-Heinemann, 1992.