Abstract
The material studied was aircraft-quality, silicon-modified AISI 4340 (300-M) steel tempered to two tensile strength levels of 2000 and 1200 MPa. Above 10/sup -5/ mm/cycle, growth rates are similar in air and hydrogen at 50 Hz. At 5 Hz, the presence of hydrogen gas results in faster growth rates. Below 10/sup -6/ mm/cycle, the presence of hydrogen gas results in slower near-threshold growth rates, with threshold /DELTA/K/sub o/ values 13% lower in air. The decelerating influence of gaseous hydrogen is observed for both tempers at 50 Hz, at high and low load ratios, and is associated with no apparent change in fracture morphology. 19 refs.
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