Abstract
Fatigue crack propagation is studied in thin wires of about 1 mm in diameter of an ultra high strength steel (σmax>2400MPa). Tests are performed on an electro linear testing machine at a frequency of 20 Hz in ambient air. Crack growth is optically monitored by mean of a long distance microscope with numerical acquisition of crack images. Threshold tests are run in accordance with the load shedding technique with special new requirements to account for the specimen geometry. The influence of load ratio R=0.4 or 0.7 and variable R at constant Kmax is examined. The results support a substantial effect of atmosphere especially on near-threshold propagation and on the threshold of the stress intensity factor range. A substantial contribution of crack closure is put in light. The propagation data are compared to previous data on conventional high strength steels tested in air and in high vacuum with closure correction. Crack path and crack surface morphology microscopic observations allow the identification of the fatigue crack propagation mechanisms.
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