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CONDITIONS OF FAILURE IN FATIGUE CRACKED 4340 STEEL.
NAVAL RESEARCH LAB WASHINGTON D C
For a fatigue-cracked tension specimen, the conditions of plastic failure, i.e., the stress conditions under which the section under the notch becomes plastic, have been developed by Hill. The analysis assumes the material to be ideally plastic, but, nonetheless, should be adequately approximate for heat-treated 4340 steel. Tension tests have been completed, and the results have been used to predict the notch strength for 4340 steel heat-treated to strength levels characteristic for this steel. The calculated notch strengths have been compared with the notch strengths which were earlier measured and reported. For limited conditions the calculated and measured values are found to be in adequate agreement. Differences in the calculated and measured values in the plastic failure interval can be attributed to the strain-hardening to which the 4340 steel is subject. The conventional tensile properties of 4340 steel at the several strength levels examined agree with the data in the literature except that the ductility is reduced. The true-stress natural-strain data indicate a transition in the strain-hardening exponent n from high to low values as the tensile strength increases from the minimum to the maximum values measured.
AT49 5 2110