Real-time monitoring of crack growth behaviour during impact and compact tension test with non-destructive testing
The basic-safety-principle concerning components in German nuclear power plants (NPP) requires a high material quality in terms of characteristic toughness values. For this purpose, at present, notched-bar impact tests are carried out for example for the determination of the brittle fracture transition temperature. Compact tension tests are performed to determine mechanical and technological characteristics such as the fracture toughness, crack initiation, the crack growth. Because of the scattering in the data, the quantification of the failure behavior in the transition zone of the toughness is only possible with restrictions. Therefore considerably high safety margins are asked for in the design to avoid failure behavior. So far, the crack growth can only be determined in standardized bending tests and requires partial load relief cycles observing the development of the crack opening displacement by a clip-gauge. The time of the crack initiation can be determined with additional time-consuming microscopic analyses of the crack surface after the bending test. The additional instrumentation of the mentioned tests with NDT should provide further information about the failure process and a more precise and reliable determination of the failure probability. In order to document the principle feasibility, three point bending tests and drop impact tests were carried out using SE(B) samples with several nondestructive sensors applied. These sensors are Electromagnetic Acoustic Transducers (EMATs) to record the ultrasonic time of flight (TOF) of an ultrasonic wave travelling around the crack, and Giant Magnetoresistance (GMR) sensors to measure magnetic flux leakage (MFL) signals emanating from the crack. It was shown that both TOF and the MFL provide information about the crack growth during the bending test. In particular, the TOF was shown to increase with the crack length, thus allowing the determination of the crack length without the partial load relief cycles that were needed before. During the drop impact tests, the high-speed acquisition of MFL signals resolved details of the crack formation process.