Since NDT methods are non-destructive, they are predestined for in-situ investigations since the process or reaction taking place can be monitored without affecting the sample.
In this context the process is observed under real conditions, such as high temperature, in a reaction cell, in a controlled gas atmosphere, under the effect of mechanical loads, etc. Typical in-situ investigations (shown schematically in Fig. 7) include the thermo-mechanical behavior of structural materials, phase transitions, physical reactions (e.g. melting, sintering, diffusion,...), chemical reactions, etc.
XCT is one of the most powerful methods for in-situ investigations of a material since it gives the complete 3D information without, in most of the cases, affecting the studied system. Laboratory XCT systems with resolutions close to 1 µm can be used for many in-situ investigations, but due to the typical measurement time of 30-200 min to acquire a complete tomogram, usually only interrupted in-situ investigations are possible. S-XCT reveals substantial advantages when fast processes on a timescale below 10 min or processes at high temperatures have to be investigated. Therefore, continuous in-situ XCT is usually performed at synchrotron beamlines.
Concerning in-situ XCT at different loading levels the major experimental challenge is the combination of a loading test machine and an NDT-device (e.g. [ , ]). No ‘‘standardised’’ test set-ups are established yet. Therefore, experimental feasibility and reliable data generation is still an experimental challenge. In-situ techniques are often combined with theoretical calculations and simulations, which can lead to considerable advances in the understanding of many material properties and processes. Laser ultrasonics as a non-contact NDT method is perfectly suited for in-situ measurements. The acquisition time (see Fig. 7) is determined by the repetition rate of the pulse laser used to generate the ultrasound pulses, which lies in the range between 10 and 1000 Hz. In-situ measurements have been performed on solidification of Al or on determining phase transitions in heated steel sheets.