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Fatigue crack propagation studies were conducted on specimens of four high-strength marine alloys to determine the sensitivity of fatigue-crack growth rates to seawater and electrochemical potential. The materials studied included HY-130 steel, 17-4 PH steel in several combinations of melt processing and temper, Ti-6A1-2Cb-1Ta-0.8Mo, and 5456-H116 aluminum. Fatigue testing was conducted at low cyclic frequency, and the fatigue data are presented in terms of fatigue-crack growth rate (da/dN) versus crack-tip stress-intensity factor range (ΔK). Specimens were exposed to fresh flowing natural seawater under freely corroding and potentiostat-controlled electrochemical conditions while undergoing corrosion-fatigue testing. The results of this investigation reveal significantly distinct differences among the four alloys under the conditions of corrosion fatigue. Both seawater and potential acted to accelerate crack growth rates in the ferrous alloys, which proved to be much more sensitive to seawater and negative potential than the nonferrous alloys studied. The titanium alloy exhibited no measurable sensitivity to either seawater or negative potential. The aluminum alloy exhibited only moderate sensitivity to seawater and beneficial effects from both positive and negative potentials. These exploratory studies indicate that high-strength marine alloys exhibit widely differing responses to corrosion-fatigue crack growth and that high-strength steels currently aimed for marine service are among the alloys most deleteriously affected.
corrosion fatigue, crack propagation, high-strength alloys, fracture mechanics, steels, titanium alloys, aluminum alloys, seawater, electrochemistry
Head, Fatigue Criteria Section, Engineering Materials Division, Naval Research Laboratory, Washington>, D.C.
Research chemist, Naval Research Laboratory, Marine Corrosion Research Laboratory, Key West, Fla
Research chemist, Pullman Kellogg, Research and Development Center, Houston, Tex