PROBLEM TO BE SOLVED: To accurately measure the displacement of a ring-shaped stress load member by an optical system by forming a light permeable hole to a part of the upper holding member supporting the ring-shaped stress load member. ;SOLUTION: A test piece 1 to be used is constituted of a discshaped flat plate and a testing jig is constituted of an upper holding member 3, a ring- shaped stress load member 2, the test piece 1, a ring-shaped support member 7 and a lower holding member 8. In this set state, permeated lights 13, 14 piercing the jig are formed between the members 2, 3 as well as between the members 7, 8. Optical sensors 15, 16 are arranged on the side opposite to the light incident side of light permeation holes 13, 14 and the light permeation holes 13, 14 are irradiated with laser beam and the positions of the lights passing through the light permeation holes 13, 14 are detected by the sensors 15, 16 and the distance A between the detection positions on the side of the members 3, 8 is calculated. When stress is applied to the test piece 1 and the test piece is bent downwardly, the position of the light passing through the light permeation hole 13 is shifted downwardly and the distance A becomes short by the sensors 15, 16 and the change quantity of the distance A is detected as the bending quantity of the test piece 1.;COPYRIGHT: (C)1997,JPO
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