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An automated variable-amplitude fatigue crack growth system has been developed that provides arbitrary waveform generation of either an endpoint or block spectrum using either a triangular or sinusoidal waveform. Up to 90 000 endpoints or 1000 blocks may be supported on a 640 K computer system. A constant peak-to-peak loading rate with upper and lower frequency limits may be specified. The block mode allows testing for a given number of cycles between specified load levels. Ramps, hold-times, and breakpoints may be incorporated in the block mode. The breakpoint feature allows changing to the next block at specified crack lengths. The spectrum may be also scaled as a function of crack length to provide for increasing K, decreasing K, or constant K. Custom hardware and software provide simultaneous command and feedback verification, as well as data acquisition to monitor the crack length. The command-feedback features of this system provide typical load errors of less than 0.2% of range with a guarantee of less than 1%. Either compliance or electric potential drop may be used for nonvisual monitoring of crack growth rates.
fatigue crack growth, variable amplitude, automation, compliance, electric potential drop, FALSTAFF, fatigue testing, fracture testing
Director, Fracture Technology Associates, Pleasant Valley, PA