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62 Tube Products International April 2009
The trouble with using the Charpy test for high strength
specimens is that the crack initiation energy is very high
compared to the total test energy: indeed, sometimes
it is greater than the available impact energy, and the
specimen simply bends instead of cracking.
To address this problem, researchers have turned to
looking at ways of extracting energy measurements
from the DWT test, since this uses more representative
sample sizes. An associated benefit is being able to use
a single test to determine two material properties.
Pendulum DWT testers provide a simple way of
measuring the total energy absorbed by a specimen,
and are successful up to a point, but when used with
very high strength steels suffer from the same failing
as the Charpy test: with a single measurement it is
impossible to separate the plastic deformation, crack
initiation and crack propagation contributions to this
value.
Instrumented DTW testers readily provide this type of
data, and crack propagation energy can be directly
derived from test results. As an aside, work done by
Pohang University in South Korea has demonstrated
that while Charpy USE has a very weak correlation
with DWTT propagation energy, it has a very strong
correlation with DWTT initiation energy, supporting
the hypothesis that for high strength steels almost
all the energy in a Charpy test goes into initiating the
crack.
The breakdown in the usefulness of Charpy USE as a
predictor of fracture toughness has led investigators,
since the 1980s, to look towards more theoretical
approaches based on fracture mechanics variables
such as crack-tip stress or strain, crack-tip opening
displacement or crack-tip opening angle (CTOA),
crack-tip force or energy release rate, to name just a
few. Important work at the Centro Sviluppo Materiali
in Rome, amongst other institutions, has concluded
that the most appropriate variable for modelling stable
crack growth is the CTOA at a specified distance from
a crack tip, or CTOAsc.
There are a number of ways of measuring CTOA, one of
which is direct measurement using a high-speed video
camera.
A well known indirect method is the two-specimen
CTOA test or TSCT. This uses absorbed energy values
for multiple DWTT-like specimens with different notch
depths to derive the CTOA value.
Work at Pohang University in South Korea and others has
shown a strong correlation between CTOA and DWTT
propagation energy (specifically, a linear relationship
between the propagation energy and sin(2 CTOAsc)).
Although more work needs to be done to validate this
relationship for a range of materials and specimens, this
work suggests that it is possible to make a measurement
of CTOA, an important material parameter, using a
single specimen in an instrumented DWT tester.
Of course, instrumenting a DWT tester is not a trivial
matter: the forces that are generated when impacting
high strength steels samples with thicknesses up
to 50mm can exceed 1 MegaNewton: not only do
these forces have to be measured accurately at high
bandwidth, but the compliance of the apparatus
needs to be low enough to make these measurements
meaningful.
The drop weight has to be precisely guided to ensure
that the hammer is kept perpendicular to the plane of
the impact.
Considering that on the higher capacity machines the
total impact energy is 100,000J or more, and over its
lifetime the apparatus must endure tens of thousands
of such impacts, the design represents a challenging
combination of heavy engineering and precision.
Imatek Ltd – UK
Fax: +44 1438 829054
Email:
Website:
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