PSI - Issue 48

Bernadett Spisák et al. / Procedia Structural Integrity 48 (2023) 326–333 Spisák et al / Structural Integrity Procedia 00 (2023) 000 – 000

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3. The process of mini CT preparation Nuclear power plants have a limited licensed lifetime, therefore surveillance programs were established to monitor the operation of nuclear reactors. The aim of the monitoring programs is to detect changes in mechanical properties, for example strength and toughness, as a result of radiation in age-restricted structural materials. Only a limited amount of valuable representative material is available. In the surveillance programs, as a first step impact tests are carried out with Charpy-V notched (CVN) specimens using a pendulum as impact testing machine. After the test was carried out the half of the CVN specimens can still be used, and in this case can be utilized for miniature CT specimens (Fig 3.).

Fig. 3. Preparation of miniaturized CT specimen

The preparation of miniature samples has certain limitations in terms of machining; practically the only viable way is to use an electric discharge machine (EDM), which provides the necessary precision. 8 mini CT specimens were prepared, 6 of which were planned to be used for fracture mechanics tests and 2 were planned to be used in case of complications, which were not used in the end. The 10x10x4 mm CT specimen was machined from 15H2MFA by cutting and EDM. 4. Validation of GTN parameters with normal and mini CT specimen The GTN parameters obtained with the help of the NT specimens were validated on normal and miniaturized CT specimens. The tests were carried out on an Instron 8850 testing machine according to ASTM E 1820-20. Prior to testing, a crack of a given length was created on the notched specimens by fatigue. The crack propagation was determined by the compliance method both during the pre-tensioning and during the test. The final length of the pre cracks was required to be between 0,45W and 0,55W. The crack propagation of the CT specimen was recorded using an Epsilon CP100856 J-integral extensometer. In addition, the load-crack opening curves were recorded during the tests. The size of the pre-cracks in case of the normal CT were 23.8 mm and 24.2 mm. Taking advantage of the symmetry, it was sufficient to create a quarter model and then place the symmetry conditions on the corresponding nodes. The load was applied to the model using a pin, which was set as a rigid body. Finally, in the crack region an element size of 100 µm was used. The same boundary conditions were applied in case of the normal and the miniaturized CT specimens too. Fig 4. shows the size and the finite element model of the 0.16CT specimen with the used boundary conditions. From Fig 5. (a) it can be seen that the determined GTN parameters are working well in case of the normal CT specimen, however For the mini specimens, it was necessary to use the real geometry dimensions of the pieces and the real pre-crack in order to compare the simulation results with the measurement.