The MISO technique, invented by the authors to observe dynamically the behaviors of solidification front and solidification cracking during welding, has been expanded to measure the critical strain required for initiation and propagation of solidification crack. For this purpose, dotty marks in high speed cine photos resulting from ruggedness on macrostructure on weld metal surface have been utilized as the re-ference points of gauge length to measure the dynamical change of the strain. The optimum gauge length has been shown to be 1 to 2 mm on the basis of feasibility, although the critical strain has been increased with shortening of gauge length. This technique has been applied to a tensile hot cracking test under the condition of high strain rate for carbon steels and stainless steels, and the critical strain measured has been shown to be more precise and reasonable than that measured by the Varestraint cracking test.
