Fracture of steel during an earthquake—state-of-the-art in Japan

The history of Japanese research on brittle fracture of building steel is reviewed, and recent experimental and analytical achievements are described. Brittle fracture is triggered by the generation of a ductile crack at a hot spot of strain concentration after it undergoes a noticeable amount of plastic strain. A fracture process composed of ductile crack initiation followed by stable crack growth and finally explosive failure in a brittle mode was observed in the damage of the buildings in Kobe during the 1995 Hyogoken-Nanbu earthquake, as well as in large-scale laboratory tests. Ductile crack initiation in a material is governed by the uniform strain capacity of the material and by stress triaxiality, which is related to the notch sharpness and, to a very small extent, by size. A crack generated upon application of a few or several cycles of plastic strain caused by a severe earthquake is not a fatigue crack, but a ductile crack. The rate of growth of the ductile crack per unit strain is proportional to the depth of the crack. The amount of strain accumulated during the crack growth to final failure is governed by the Charpy impact value, plate thickness and material strength. Governing equations for these three fracture aspects are proposed as empirical formulae.