Specimen and automated test method development for investigation of fatigue crack initiation

An economical and effective test specimen has been developed for use in fatigue crack initiation studies. A dynamic materials-testing system was similarly developed for this purpose, consisting of a servohydraulic dynamic testing unit, high-resolution photomacrographic equipment, and computer control systems. The specimen and system permit fully-automated fatigue crack initiation testing, and crack lengths of approximately 0.001 inches have been detected with the peak load amplitude applied to the specimen. This paper presents the development of the specimen, the design constraints imposed by the system and the required test results, and the prototype test program and subsequent specimen modifications. Final specimen dimensions, the correlation between notch root stress and applied load, and recommendations for specimen use are also provided.