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Steel Designers' Manual - 6th Edition (2003)
Materials testing for fracture properties 259
Fig. 7.7 Typical fracture toughness test specimens. (a) Three-point bend beam; (b) compact
tension specimen
tension specimen (see Fig. 7.7) is cyclically loaded within prescribed limits until a
sharp fatigue crack is formed. The specimen is then subjected to a displacementcontrolled
monotonic loading until either brittle fracture occurs or a prescribed
maximum force is reached. The applied force is plotted against displacement and,
provided specific validity criteria are met, a plane strain K Ic may be found by analysis
of the data. When the validity criteria are not met, the data may be re-analysed
to evaluate a critical CTOD or critical J for that material. The determination of
critical CTOD requires the relationship between applied load and the opening of
the mouth of the crack, measured using a clip gauge. Critical J calculations need the
load against load line displacement response, so the detailed arrangements of the
two types of test are slightly different.
The specimen size requirements for a valid K Ic are that:
Ê K
thickness, width and ligament ≥ 2.5Á
Ë s
The size requirements for the J value to be valid are that:
Ê
thickness, width and ligament ≥ 25Á Ë
J
s
Ic
y
Ic
y
ˆ
˜
¯
(7.6)
(7.7)
This implies that significantly smaller specimens are required for critical J value tests
than for KIc tests.
ˆ
˜
¯
2