TPT November 2008

T echnology U pdate

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. These variables include crack-tip stress or strain, crack-tip opening displacement or crack-tip opening angle (CTOA), and crack- tip force or energy release rate. 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 steel 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.

 DWTT results

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 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. This article was supplied by Nick Osborne, technical director of Imatek Ltd Imatek Ltd – UK Fax : +44 1438 829054

Email : info@imatek.co.uk Website : www.imatek.co.uk

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