What is a fracture toughness test?

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 K_Ic, 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:

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, K_Ic, CTOD and/or J.
8. Validation of the results.

Examples of load-displacement traces are shown below.

a) δ_c/J_c/K_Ic behaviour

b) δ_u/J_u behaviour

c) δ_m/J_m behaviour

Trace 'a' is associated with:

• K_Ic, 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.
• δ_c or J_c, 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 J_u 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 δ_m or J_m, 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?)

There are three main standards describring the fracture toughness testing of ductile alloys by generating tearing resistance curves: BS 7448-4 (1997), ISO 12135 (2016) and ASTM E1820 (2018). These standards differ in significant details relating to the generation and analysis of the test data. Find out more about the techniques and methods given in these standards for the determination of tearing resistance curves (R-curves) in single-edge notched bend (SENB) fracture toughness test specimens.

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