Frequently Asked Questions
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 a standard 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 BS 7448 and ASTM E1820 (see FAQ: Are there any differences between fracture toughness tests carried out to BS 7448 and those using ASTM E1820?
A fracture toughness test typically consists of the following steps:
- Machining of a standard test specimen (typically a single edge-notched bend or compact tension specimen), which is notched in the area of interest.
- Growth of a fatigue precrack by application of cyclic loading, usually at room temperature.
- Attachment of displacement measuring gauges across the crack mouth
- Maintenance of a stable specimen test temperature, typically the minimum service temperature of the component of interest
- Application of a monotonically increasing load, whilst monitoring both load and crack mouth opening.
- Breaking open of the specimen to allow detailed measurement of the crack front (occasionally, this happens during the test itself).
- Calculation of the relevant toughness parameters, KIc, CTOD and/or J.
- Validation of the results.
Examples of load-displacement traces are shown below.
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 δu 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 maximum load 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 tearing resistance curve?)