Fig. 1 Continuous images of crack propagation in an Al-Cu alloy. Initiation of a crack from a dull crack tip (a), and the sharp tip of the propagating crack (b) are clearly visible.
@X-ray CT is a very promising tool for achieving 4D observation. However, it has taken about 1 to 10 ks for taking one image, and it has been necessary to suspend loading and constant temperature control (by fixing displacement and rapidly cooling to the room temperature, respectively) and stop the progress of the phenomenon, in order to make 4D observations(Review paper 1
). Continuous and uninterrupted imaging at high time resolution would be very significant. Figures 1 and 2 are examples of continuous observation of crack propagation behaviors at a high time resolution of 22.5 s/scan, which was achieved at ESRF through white X-ray irradiation and the use of a high-speed camera that could take 60 frames/s (Paper 1
). This was the first observation of the sharpening of a crack tip (a characteristic for crack propagation) inside materials.
Fig. 2 Analysis of crack propagation in an Al-Cu alloy. In the left graph, the opening profile of the crack was converted into dimensionless values. As the load increased, the profile approached the shape of a propagating crack. The right figure is a log-log plot of Â and r, which were converted into dimensionless values. It also shows the transition of the singular field from that of a static to that of a propagating crack.
@The analysis in Fig. 2 (E: Youngfs modulus, Â: crack tip opening displacement, Ð0
: flow stress, and r: distance from the crack tip) showed a slow transition from the HRR singular field of a static crack to the RDS singular field of a propagating crack. Under high loads, the singular fields changed into RDS at the tip of the crack, and into HRR in front of the tip.
Fig. 3 Time resolutions of present and future X-ray CTs, and various dynamic phenomena in each field
@In order to actualize true 4D observation, much faster imaging than that described above is needed, such as an ultrahigh speed of one 3D image per second (Fig. 3). For this, it is indispensable to create a scintillator (X-ray ¨ conversion element of visible light) of a completely new order . We are thinking of achieving this goal by developing a nano-level microstructured scintillator jointly with researchers in the fields of electrochemistry and microprocessing.