Effects of frequency and amplitude variations of superimposed minor loading sequences on the corrosion fatigue behavior of aircraft structural aluminum alloys

Economic considerations have forced users of commercial jet transports to retain the services of aircraft that have surpassed the designed lifetime of structural components of the aircraft. With the increased service time that these aircraft are experiencing, many of the aircraft are becoming increasingly susceptible to corrosive attack. Additionally, the increased public awareness of commercial aircraft incidents has mandated that the behavior of so called "aging" aircraft structures be better understood.;Fatigue crack growth tests were conducted using four aluminum alloys that are commonly used in the structural components of commercial jet transports. Two of these materials represented materials that were commonly used in the construction of the what are now termed "aging" aircraft. The other two materials are typically used in the new commercial transports. These materials were subjected to trapezoidal loading cycles with superimposed minor loading cycles. The amplitude and frequency of the minor loading cycles were varied in order to determine the effects of such loading variations on the corrosion fatigue behavior of the alloys.;It was determined that the minor cycles did not influence the crack growth rate of the material below some specific minor cycle amplitude. Indications were seen that the amplitude at which the minor cycles become active may be influenced by the presence of the corrosive environment. The effect of the active minor cycles on the fatigue crack growth process could be seen in the crack growth rate curves and also in the fractographic analysis of the specimen surfaces.;The combined influence of the frequency and environment on the fatigue behavior of the alloys was noted. Once the minor cycles had become active, a lower minor cycle frequency led to a larger impact of the corrosive environment.;It is believed that the identification of the activity or inactivity of minor cycle loading will aid in the development of spectrum loading determined from actual components. The actual spectrums may be simplified based on the fact that specific minor cycles are known to be inactive, due to insufficient amplitude to effect the fatigue crack growth process.