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What is a fracture toughness test?

TWI Frequently asked questions

by Isabel Hadley

The intention of a fracture toughness test is to measure the resistance of a material to the presence of a flaw in terms of the load required to cause brittle or ductile crack extension (or to reach a maximum load condition) in astandard specimen containing a fatigue precrack. The result is expressed in terms of toughness parameters such as KIc, critical J or critical CTOD.

The measurement of fracture toughness is covered by several standards, such as BS7448 and ASTM E1820 (see FAQ: Are there any differences between fracture toughnesstests carried out to BS 7448 andthose using ASTM E1820?

A fracture toughness test typically consists of the following steps:

  1. Machining of a standard test specimen (typically a single edge-notched bend or compact tension specimen), which is notched in the area of interest.
  2. Growth of a fatigue precrack by application of cyclic loading, usually at room temperature.
  3. Attachment of displacement measuring gauges across the crack mouth
  4. Maintenance of a stable specimen test temperature, typically the minimum service temperature of the component of interest
  5. Application of a monotonically increasing load, whilst monitoring both load and crack mouth opening.
  6. Breaking open of the specimen to allow detailed measurement of the crack front (occasionally, this happens during the test itself).
  7. Calculation of the relevant toughness parameters, KIc, CTOD and/or J.
  8. Validation of the results.


Examples of load-displacement traces are shown below.

faqih010f1a.gif
a) δ c/J c/K Ic behaviour

faqih010f1b.gif
b) δ u/J u behaviour

faqih010f1c.gif
c) δ m/J m behaviour

Trace 'a' is associated with:

  • KIc, the plane strain fracture toughness, a measure of the resistance of a material to crack extension when the crack tip stress state is predominantly plane strain.
  • δc or Jc, the critical values of CTOD and J associated with brittle extension of a crack, under conditions where minimal ductile crack extension occurs (<Δa<0.2mm)

Trace 'b' would tend to generate δ u or Ju results, these being the critical values of CTOD and J associated with brittle extension of a crack under conditions where ductile crack extension (Δa ≥0.2mm) precedes brittle extension. Traces like this are characteristic of ferritic steels in the transition region.

Trace 'c' shows behaviour associated with δm or Jm, the values of CTOD and J at the first attainment of a maximumload plateau. In such cases, propagation of the crack is by ductile tearing and the toughness of the material can be re-analysed if required using more advanced techniques (see FAQ: What is a tearingresistance curve?)