10 Jul 2008
Instrumented Drop Weight Tear Testing
The drop weight tear test (DWTT) has been in use for over 40 years now, as a practical laboratory-scale way of ensuring that steel used in the manufacture of line-pipe is not subject to brittle failure when in service. It is one of a battery of tests that assess the suitability of steel for a particular application, another of which is the Charpy V-notch (CVN) test, from which the upper shelf energy (USE) has commonly been used as a measure of the ductile fracture resistance.
Since the introduction of the DWTT, materials have moved on. In particular, demands for high operating pressures of line-pipes and larger diameters have driven the development of higher strength steels. Forty years ago the work that led to the drop weight tear test was done on X52 steel (360MPa yield strength). Improvements in thermo-mechanical processing has yielded improvements of approximately 10,000psi per decade, to the point where the state of the art is now X100 steels, and the use of X120 steels is being considered.
This development in material technology has placed substantial demands on conventional test techniques and the relevance of some results have been drawn into question.
Since a DWT tester represents a significant investment both in terms of capital cost and operator training, it is important that any equipment being specified now should have the flexibility and the capacity to cover developments in test methodology and the mechanical properties of materials for the expected service life of the apparatus, ten years or more. To this end, this article examines issues concerning the measurement of fracture toughness of steel, and in particular the use of high capacity instrumented DWT testers.
Brittle Fracture
Avoidance of brittle behavior in pipeline steel is of paramount importance to manufacturers. Originally materials were characterized by the so-called Athens test, a full-scale burst test consisting of a test section about 200m in length pressurized with natural gas. The need for a practical, laboratory-scale test was recognized and subsequent work (notably by the Batelle Memorial Institute) resulted in the drop weight tear test, or DWTT, which was adopted by the American Petroleum Institute (API) in 1965 as recommended practice 5L3.
The DWTT involves cutting a full-thickness specimen from the wall of the pipe and putting a notch in it to act as a stress raiser. The test specimen is supported at either end, then hit in the centre, on the edge opposite the notch, by a hammer attached to a falling weight, breaking it into two. The broken surfaces are then inspected, and the percentage of the surface that shows ‘shear’ (or ductile) fracture, as opposed to ‘cleavage’ (or brittle fracture) is assessed. As a quality assurance test, this is usually done at a single specific temperature, and a minimum percentage shear area (commonly 85%) is used as the pass/fail criterion.
The original Batelle work, and investigations done since (at Centro Sviluppo Materiali in Rome amongst other institutions) , have shown good correlation between DWTT results and the results of burst test up to at least X100 grades of steel. Further work on even tougher grades remains to be done.
While being a well-founded, widely used test, there are a number of minor problems with the DWTT. The first is that it is rather labour-intensive, and determination of the percentage shear area is a process that is difficult to automate. Another difficulty that has been observed is that some highly ductile steels show abnormal fracture appearance, which leads to difficulty applying the minimum shear area criterion.
An instrumented DWT tester augments the basic apparatus by measuring the force that the hammer applies to the specimen to break it. From this measure of force (as a function of time), displacement and energy curves can be obtained. Significantly, it is possible to identify the point on the force curve where crack initiation occurs, and from this calculate separate values for initiation energy and propagation energy. Such an apparatus has the potential to circumvent both the problems described above, since it has been shown that a relationship exists between the transition temperature for DWTT crack propagation energy, and the transition temperature for 85% shear area. It will probably be quite some time before these observations feed into international standards, but there is scope for the in-house use of these test methods.
Ductile Fracture
As mentioned in the introduction, Charpy V-notch test USE has been used as a measure of ductile fracture resistance and has provided good service. With the introduction of high strength steels however the applicability of this test has been called into question, and research has shown that Charpy energies above 150J are not representative for ductile fracture resistance. 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 (of course there is the associated benefit of 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.
Use of CTOA
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 modeling 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.
Instrumented DWTT
In summary, the addition of instrumentation to a DWT tester turns an apparatus for conducting a 40-year old quality control test into a research tool that is likely to cater for test requirements well into the future, investigating and characterizing brittle and ductile fracture resistance in line-pipe steel.
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. Furthermore 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.
Nick Osborne, Imatek Ltd
July 2008
Nick Osborne is the Technical Director of Imatek Ltd.
For further information contact Imatek at info@imatek.co.uk