PURPOSE:To obtain an even test piece, by measuring the length of a precracking by a bending displacement of a fracture toughness test piece to apply a specified repeated load until the length reaches a specified value. CONSTITUTION:A fatigue tester 11 is driven to form a fatigue precracking at a chip 9a of a fracture toughness test piece 9. In other words, a vibration exciter 13 is vibrated to provide a repeated load on the top of the test piece 9 with a working element 13a. A deflection detector 14 measures a displacement of the working element 13a at the vibration exciter 13. A control circuit detects the length of a cracking generated in the test piece 9 based on the deflection amplitude of the test piece 9 determined from the results of detection from the detector 14. In this manner, a specified precracking is formed in the test piece 9 and an output is sent out from the control circuit. Then, the subsequent fracture toughness test piece 9 is placed on a support base 10 to form a precracking again.
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