Abstract
The fatigue-crack propagation behavior of heat-treated 4340 steel has been studied as a function of tempering temperature from 400 to 800 F and at +80 and -50 F test temperatures. The Griffith-Irwin fracture-mechanics analysis of the data was used for through-thickness cracks in center-notched sheet specimens. Special emphasis was placed on the phenomenon of tempered martensite embrittlement, which occurs in the 500 to 700 F range of tempering temperatures, in order to see whether it can be detected by fatigue testing. The fatigue-crack-growth test results could be represented by an empirical equation of the form d2a/dN = C(ΔK)m. The exponent m was found to vary with tempering temperature and test temperature. The fatigue-crack-growth rate at -50 F was higher than at room temperature. No evidence of tempered martensite embrittlement was observed, since the crack-growth rate was actually a minimum in the embrittlement range, the crack-growth rates decreasing as the tempering temperature increases to 600 F, and then increasing again with higher tempering temperatures. The effect of ethyl alcohol on fatigue-crack growth and the relationship between KIc and the fracture-mode transition are discussed.
Published Version
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