Safe Product Design - The Role of the NDE Reliability Analysis

When pushed to the limits of their detection capability, NDE systems do not produce con-sistent hit/miss indications. Their capability of detecting small defects is therefore expressed in terms of POD. An adequate NDE system is required to ensure the structural integrity. In conventional signal response analysis, the POD is expressed as a function of the defect size, and its adequacy for the inspection task is tested against the maximum allowable defect size which will not undermine the structural integrity. Analyses of modern structures show that other parameters, beside the defect size, can both significantly influence the POD and determine the severity of the defect for the structure. Within the multi-parameter reliability analysis, the POD is expressed as a function of those influencing parameters. When determining the adequacy of the NDE system, the capability of detecting a defect has to be expressed and tested against the critical value of exactly that parameter that determines defects severity for the structure. Failing to do so can lead to a rejection of the healthy, or acceptance of the bad part. The principle is demonstrated on the example of the Transmit-Receive Longitudinal (TRL) ultrasonic inspection of the iron cast component for semi-elliptical surface defects

Frequency and Location Dependent Attenuation in Copper Tubes for Radioactive Waste Deposit

The present paper describes the progress in understanding the amplitudes and attenuation changes acting on the inspection, by the Phased Array UT, of copper canisters for the final disposal of spent nuclear fuel. Previous studies showed the existence of a low pass filtering effect on the attenuation coefficient, due to the heterogeneous grain size distribution, along the path of UT waves, thus affecting both the detectability of defects and their POD determination. Consequently, the difference between the first and second back wall echoes were not sufficient to determine the local attenuation (within the inspection range), which affects the signal response obtained by the individual defect. These deviations were taken into account by manual adaption of the modelled data, elaborating an attenuation that is the superposition of a 0 value from back-wall echo and a taking into account the deviations between measured and modelled data values. To confirm the experimental evidence and achieve a deeper insight, experiments were carried out onto test samples cut from a full-size copper canister characterized by different attenuation properties. Data were analysed both in the time and in the spectral domains and a model explaining the observed behaviour was implemented