A metal member with a hole is used for various structures. The metal member having high fatigue strength is needed especially for an aircraft.
JP H07-500,773A, U.S. Pat. No. 6,389,865, U.S. Pat. No. 6,742,376 and U.S. Pat. No. 7,131,310 disclose a method of improving the fatigue life of a metal plate member with a hole. This method will be described with reference to FIGS. 1 to 3.
Referring to FIG. 1, a work piece 290 of a plate shape is provided with a front surface 291 and a back surface 292 on a side opposite to the front surface. In a condition that a metal block 299 is placed against the back surface 292, an indenter set 200 is pressed against the work piece 290 from the side of front surface 291. The indenter set 200 is pushed against a hole formation region of the work piece 290 for the hole to be formed.
Referring to FIG. 2, a dimple 293 is formed in the hole formation region of the work piece 290 by the previous pressing. Then, a through-hole is opened in the hole formation region by using a drill 300.
FIG. 3 shows the work piece 290 before opening the through-hole. A plastic deformation region 294 is formed around the dimple 293. A curve 296 shows a residual stress distribution in the work piece 290 generated after opening the hole. The curve 296 shows the residual stress distribution along a line L200 passing through the center of the dimple 293.
Because a compression residual stress is given around the through-hole by opening the through-hole in the dimple 293 of the hole formation region with a drill 300, the fatigue life of the work piece 290 is improved.