The Effect of High Altitude Environments on the Damage Structure Evolution during Fatigue Fracture of Legacy Aerospace Aluminum Alloy 7075-T651

Many aircraft structural components are frequently made from 7xxx series aluminum alloys (Al-Zn-Mg-Cu), which can suffer from fatigue. It has been reported that 80% of flight times were spent at a “cruising altitude” (above 30,000 ft), where 17% of significant load peaks occurred and 42% of such significant events occur at “high altitude” (above 10,000 ft). Orders of magnitude reduction in the fatigue crack growth rates (da/dN) occur during fatigue testing in high altitude environments (e.g. low temperature and low partial pressure of water vapor (PH2O)) pertinent to high altitude flight. Critically, despite a constant exposure parameter (PH2O/frequency), differences in da/dN are observed between tests at 23°C and low temperature environments (below -30°C). Several hypotheses have been proposed to explain the apparent temperature dependence: surface reaction rates between the water vapor and aluminum, hydrogen diffusion, hydrogen-dislocation interaction, and dislocation or damage structure evolution. Each of these hypotheses will be examined in turn. It is also necessary to evaluate the efficacy of current testing protocols on rigorously capturing environmental crack growth data, including the existence of the threshold transition regime (TTR).

To address these knowledge gaps the following approaches were employed. An evaluation of alternate testing protocols in an attempt to avoid the complicating TTR behavior. Use of various testing protocols to develop a more complete mechanistic understanding of the temperature dependent fatigue behavior of AA7075-T651 despite a constant exposure parameter (PH2O/f). The development of a systematic and multi-scale set of characterization techniques (EBSD, HR-EBSD, FIB, TEM, and PED) to evaluate the local crack wake damage structure to gain insights into the high altitude environmental cracking processes.

The results indicate low temperatures found in high altitude environments have a strong influence on the fatigue behavior. Not only does temperature influence the driving force threshold for the transition from a slip band cracking (SBC) mechanism to a flat transgranular cracking mechanism, but also lessens the impact water has on the fatigue behavior, insinuating low water vapor levels may not need to be taken into consideration for aerospace applications in high altitude environments. Furthermore, the multi-length scale characterization protocol revealed the majority of the fatigue damage structure was localized to the first ≈ 500 nm of depth from the crack wake surface, which the development of said structure has a similar appearance to Beilby layers.

The definition of the layer lessened with a decrease in driving force and temperature. The presence of hydrogen appeared to cause an elimination of high dislocation density and precipitates within the layer, but again lessened with smaller driving forces. Additionally, the presence of hydrogen precipitated the first ≈ 100 nm of depth to form a recrystallization layer, appearing at both high and low driving forces, but was unable to form at low temperatures. The TTR event observed with fatigue cracking of AA7075-T651 in low water environments was determined to be an intrinsic response which cannot be eliminated by a reduction of sample thickness or a change in the loading protocol.