Hourly pressure variations were recorded during six years in two synthetic towers, giving a total of 105,000 readings. The readings were converted into a 15-block pressure amplitude spectrum of 12,655 cycles following standard methods employed in the aircraft and automotive industries. The spectrum was applied to center-cracked tension specimens and crack growth rates were measured. The results show that the crack growth rate data can be plotted conservatively against the range of the equivalent stress intensity factor calculated as a root-mean-cube value. Futhermore, it is shown that the 9704 cycles of lowest 1.9 MPa pressure amplitude caused 12 percent of the cumulative fatigue damage, whereas the 13 start-up and shut-down cycles of highest 15.2 MPa amplitude caused 8.2 percent of the damage.

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