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    Fatigue-Crack Growth under Variable-Amplitude Loading in ASTM A514-B Steel

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    This paper describes the fatigure-crack-growth behavior of ASTM A514-B steel under variable-amplitude random-sequence stress spectra such as occur in actual bridges. The fatigue-crack growth-rate data were obtained by using wedge-opening-loading specimens tested under variable-amplitude load spectra that are represented by a Rayleigh distribution function. The data show that the average fatigue-crack-growth rates under variable-amplitude random-sequence load fluctuation are approximately equal to the rate of fatigue-crack growth under constant-amplitude cyclic-load fluctuation equal to the root-mean-square value of the variable-amplitude fluctuation. The average fatigue-crack-growth rates, da/dN, under variable-amplitude random-load fluctuation and under constant-amplitude load fluctuation were found to agree closely when da/dN was plotted as a function of the root-mean-square stress-intensity-factor range, ΔKrms. To verify the preceding observation, tests were conducted under the following variable-amplitude load fluctuations represented by the same Rayleigh distribution function: random sequence, ascending sequence, descending sequence, and ascending-descending sequence. All tests were conducted at a constant minimum load equal to 200 lb. The fatigue-crack growth-rate data obtained under these various loading sequences were close and were approximately equal to the average rate of fatigue-crack growth under constant-amplitude load fluctuations having a magnitude equal to the root-mean-square value of the distribution function. Thus, within the limits of the present investigation, the average fatigue-crack-growth rates, da/dN, under variable amplitude (random-sequence or ordered-sequence) load fluctuations and under constant-amplitude load fluctuations can be represented by the equation da/dN=A(ΔKrms)n where ΔKrms is the root-mean-square stress-intensity-factor fluctuation (in ksi n.), and A and n are constants. Further work is needed to establish the applicability of the foregoing observations to various metals and to various continuous distribution functions.


    fatigue properties, mechanical properties, crack propagation, cyclic loads, constant amplitude, variable amplitude, sequencing, fatigue life

    Author Information:

    Barsom, J. M.
    Associate research consultant, U.S. Steel Corp., Monroeville, Pa.

    Committee/Subcommittee: E08.05

    DOI: 10.1520/STP49640S