Interpretation of crack propagation events on the basis of acoustic emission signals and other non-destructive test results concerning cracks in a RPV nozzle under cyclic thermal shock loading

Abstract

Cyclic thermal shock tests have been performed on a RPV nozzle (HDR) with cracks in order to apply and to assess technics available for the description of real cracks (non-destructive testing, fracture mechanics calculation). In this paper the application of dye penetrant testing, potential drop testing and acoustic emission measurements on the nozzle is considered. Some experimental results are described in detail; assessments have been possible on the basis of destructive testing (fractography). The determination of crack parameters for correlating these NDT results with fracture mechanics data are discussed such as crack depth, a, cracklength, crack area and crack growth. Crack growth is considered over a period of cycles, a( N), per cycle, d a/d N, and during a cycle, d a/d t. A selection of numerical NDT findings is presented and compared with the results of fracture mechanics calculations. The investigation leads to the following conclusions: 1. 1. The crack dimensions found by NDT are not quite conservative (fractography). 2. 2. As the performed fracture mechanics calculations overestimated the crack growth advanced NDT data like d a/d N indicate starting points for improvements of the calculations (influence of mean stress). 3. 3. Although acoustic emission measurements give no quantitative fracture mechanics parameters qualitative correlations yield decisive understanding of dominating crack growth mechanisms (corrosion). 4. 4. Reversely different comparisons between acoustic emission and fracture mechanics results seem to confirm the classification of acoustic emission signals (crack surface friction, crack growth).