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What Is Low-Temperature Notch Toughness and
How Is It Determined?
A ductile metal as judged by ordinary tensile or bend
tests may fail in the "brittle fracture" mode with little or
no plastic deformation when subjected to critical testing
or service conditions. The critical conditions depend on
"fracture toughness" of the metal. The brittle fracture is
considered more dangerous than the ductile fractures
exhibited in ordinary tensile or bend tests, because high-
velocity failures take place in steel structures. The
following three factors markedly influence the behavior
of a metal in terms of brittle fracture:
"Fracture toughness" is a generic term for measures of
resistance to extension of a crack. The common methods
of measuring the fracture toughness of welded joints are
the Charpy V-notch impact test, the Crack Tip Opening
Displacement (CTOD) test. Drop Weight test, and
Compact Tension test. Of these testing methods, the
Charpy V-notch impact test is most commonly used to
determine fracture toughness (also known as "low-
temperature notch toughness") of welds.
The Charpy V-notch impact test uses the standard
specimen shown in Fig. 1. When the thickness of the
weld is not large enough for removing this size of
specimen, a thinner specimen (sub-size specimen) may
be used. The specimen is positioned with the ends of the
notched surface straddling two supports and is struck
opposite the notch by a wedge-shaped hammer at the
end of a pendulum. The energy absorbed in breaking the
specimen is calculated from the mass and length of the
pendulum, the initial height of the hammer and the
height of the hammer after the fracture. For carbon and
low alloy metals that exhibit a change in failure mode
(from shear fracture to brittle fracture) with decreasing
temperature, the test is performed at several
temperatures.
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Fig. 1 - A Standard Charpy V-notch impact test specimen (Above) and a typical impact test machine (Left)
The most commonly reported result is the absorbed energy, but the percent shear fracture and the lateral expansion may also be reported. Typical transition temperature curves of carbon or low alloy weld metal are shown in Fig. 2.
Fig. 2 - Typical transition temperature curves of a carbon or low alloy weld metal (S: shear, B: brittle) |