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##### STP536

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

**Abstract**

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.

**Keywords:**

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