Patent ID: 8354156

Claim:
A prepreg base material comprising: reinforcing fibers arranged substantially in one direction and a matrix resin between the reinforcing fibers, wherein the prepreg base material has substantially throughout its entire surface incisions, each incision extending in a direction substantially crossing the reinforcing fibers, wherein substantially all of the reinforcing fibers are divided by the incisions, a length L of each of reinforcing fiber segments formed by adjacent incisions is 10 to 100 mm, a thickness H of the prepreg base material is 30 to 150 μm, a fiber volume content Vf of the reinforcing fibers is 45 to 65%, an edge of one of the incisions and edges of the adjacent incisions overlap each other when the one incision and the adjacent incisions project in a direction of the fibers so that the one incision allows the reinforcing fibers between the adjacent incisions to cut into segments with a length shorter than the length L of the fiber segments cut by the adjacent incisions, wherein each of said incisions is in the form of a slit segment having a selected length, and when said slit segment is projected in the arranging direction of said reinforcing fibers, a projected length Ws of said slit segment in a perpendicular direction to the arranging direction of said reinforcing fibers is 1 to 10 mm, and wherein said slit segments are provided from top and bottom faces of said prepreg base material in a thickness direction thereof without penetrating said prepreg base material; a cut depth Hs of each of said slit segments is 0.4 to 0.6 times the thickness H of said prepreg base material; an interval La between any slit segment A in said top face and another slit segment B in said top face adjacent to said slit segment A in an orientation direction of said reinforcing fibers is 10 to 100 mm; the geometrical center of a slit segment C provided in said bottom face is located at a position where its distance measured from said slit segment A toward said slit segment B in the orientation direction of said reinforcing fibers is 0.4 to 0.6 times La; a part of said reinforcing fibers existing between said slit segment A and said slit segment B in said top face is cut by both said slit segment A in said top face and said slit segment C in said bottom face or both said slit segment B in said top face and said slit segment C in said bottom face; and geometrical shapes of slit segments in said top face and/or geometrical shapes of slit segments in said bottom face are the same as each other.