A fracture mechanics interpretation of acoustic emission results

• Published in: Nuclear engineering and design 1989-03, Vol.112, p.139-154
• Main Authors: Neubrech, G.E., Walte, F., Waschkies, E., Klein, M.
• Format: Article
• Language: English
• ISSN: 0029-5493; 1872-759X
• DOI: 10.1016/0029-5493(89)90152-0

The examination of the crack growth processes during thermal shock experiments performed with a corrosive medium on a cladded nozzle of the HDR-RPV was carried out by an acoustic emission and analysis system, that is able to automatically identify acoustic emission events as crack growth or as other processes (such as crack friction). This classification is based on a statistical model that considers different signal parameters (rise time, endurance, energy of the signal) for each acoustic event. More often, additional signal parameters are the position and time of starting. Using an evaluation system it is possible to pick out the events with certain parameters, using many different combinations, and plot and compare them against other results obtained from measurements or calculations. In this way the acoustic emission location method is confirmed by a general comparison with a nondestructive test result. In contrast, the timing of the signal start compared with the cyclic crack loading was unexpected. Most of the acoustic signals did not occur when the thermal load was at a maximum, but during an intervening period during the relief phase which is caused by reheating. In order to find the reasons for this, the signals are separated according to fracture mechanics models. From this it was discovered that the acoustic emission model and the fracture mechanics model are partly in agreement: that is, for the friction aspects. Finally the acoustic emission results suggest the following important and new conclusion regarding the mechanism of crack propagation: in the particular case of the nozzle experiments the crack growth is not primarily controlled by the load parameter delta-K, but occurs only during a short period of adverse temperature. This conclusion should be proved by means of further investigations on the HDR-RPV. The results are conclusive especially when applying the results from samples to components. This paper demonstrates that current ideas on fracture mechanics processes are only possible: • - through the use of acoustic emission testing, • - through a powerful evaluation and presentation system for the measured results which include acoustic emission, • - and not least through the simulation of the real-life conditions found in a nuclear power station (HDR).