Experimental Study of the Process Zone, Nucleation Zone and Plastic Area of Earthquakes by the Shadow Optical Method of Caustics

On a plexiglass sample, a penetrating crack was prefabricated by laser. The crack is inclined towards the major principal stress r1ðry Þ at an angle of about 30 . Using this sample and by means of shadow optical method of caustics and microcrack location, the process zone, nucleation zone and plastic area of earthquakes were studied experimentally, and the strain variation in the shadow area was monitored. From the result, we comprehend the following. When the stress ry was increased to a certain value, shadow areas were formed around the prefabricated crack and at its tips, with that at the tips being larger. These shadow areas become larger with the increase of load and smaller with unloading. In the shadow area the strain was inhomogeneous: it was very large in some places but very small in others. When the shadow area reached a critical state, microcracks appeared at the tips of the prefabricated crack. Microcracks appeared on one side of the prefabricated crack where the strain and the shadow were both smaller. The zone with concentrated microcracks, or the process zone, was always located at the crack tip; this zone together with a half length of the original crack formed the nucleation zone which fell into the shadow area but was smaller than it. The shadow optical area of caustics bears a certain quantitative relation with the plastic area. Therefore, if an appropriate method is available to obtain the shadow optical area of caustics, it would be possible to detect the area with strong differential deformation change, and hence to determine the zone where strong fracture (an earthquake) would take place.