TPI April 2009

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.

DWTT results ▲ ▲

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),

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Tube Products International April 2009

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