RD&D-Programme 2004 - SKB

Within eddy current testing, the project at Uppsala University has been concluded with good
results /6-7/. The results show that the testing results are not dependent on the material’s
microstructure, i.e. welds and parent metal give equivalent results. The distance from the surface
to the discontinuity can be determined by means of the signals’ phase angle, regardless of the
shape of the discontinuities. At the Canister Laboratory, equipment for eddy current testing
(ET instrument with probe and data collection system) has been acquired, and evaluation of
full-scale welds is currently under way.
Programme
Development of methodology and equipment for nondestructive testing (NDT) of FSW is
under way at the Canister Laboratory and is expected to be completed during 2004.
As a part of the programme for determining the reliability of the NDT methods (NDT
Reliability), we will study radiographic inspection of welds. The basis for this is that SKB
has not conducted any studies on radiographic testing, something which SKI has emphasized
is important. A 35 degree incident angle to the weld is used in radiographic testing of EBW
welds, which gives good results. One question is whether we should use other and multiple
beam directions in order, for example, to be able to determine defect position. These questions
are of interest for FSW welds in particular. Practical radiographic tests are time-consuming and
expensive and not possible in a reasonable time perspective, since possible beam directions are
limited by the X-ray chamber.
This part of the project includes the following:
• Measurement of the spectrum of the radiation source and the detector at the Canister
Laboratory. This survey is focused on the linear accelerator’s characteristics, the detector’s
sensitivity and spatial resolution, and a comparison of system performance with standards for
traditional film-based radiography.
• Adaptation of BAM’s simulation model with the aid of the above-measured parameters.
• Simulation tests to study the detection possibilities at different configurations.
Reconstructions of actual defects imaged by means of high-resolution microfocus computer
tomography and placed in a CAD model of the weld are used as input data in the simulation.
The simulations are validated by comparing simulated images with real images obtained in
SKB’s system.
• Evaluation of alternative radiographic methods and detectors.
A second part of the project concerns determination of the reliability of the NDT methods. In
an initial stage, test objects are being examined according to established testing procedures at
the Canister Laboratory and the results are being carefully documented. The test objects consist
of lid welds with a large number of discontinuities created by varying important parameters
beyond the process window for the particular welding method. The purpose of this approach
is to ensure that different types of possible discontinuities are present in the test objects. In the
next stage the incidence of defects is investigated in detail by computer tomography complemented
by high-resolution microfocus computer tomography. In addition, investigations are also
conducted by means of other NDT methods and destructive tests on metallographic sections and
tensile specimens. These data are then processed statistically. In order to describe the reliability
of the NDT methods, POD (probability of detection) curves and ROC (risk of false calls) curves
are used in accordance with relevant recommendations in the standard MIL-HDBK-1823 /6-8/.
The statistical processing of the data will shed light on the reliability of the NDT methods both
individually and in combination. Simulation tests will also be performed in the project in order
to determine POD in the use of radiographic technique. Initially, the information that exists on
the real defects with respect to such important parameters as orientation, size, shape factors, gap
width etc will be processed. The next step is to statistically determine within what limits these
parameters vary and randomly generate defects within this parameter window. The advantage
is that a virtually unlimited number of relevant defects can be generated and imaged in the
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