Abstract
Corrosion nucleated fatigue is a primary failure mode in aerospace structures. Built up aerospace aluminum assemblies are commonly joined by stainless steel fasteners. The complex geometries and high stress levels often lead to coating breakdown at these locations. Such coating failure can give rise to electrolyte ingress, setting up a galvanic couple between aluminum structure and the stainless steel fastener. Several corrosion morphologies in aluminum substructure can form due to the galvanic coupling of aluminum and stainless steel. Corrosion morphologies depend on the alloy microstructure, potential distribution, pH distribution and local chemistry within the fastener-hole geometry. The presence of corrosion on the aluminum substructure affects the structural integrity of the airframe and complicates the prognosis of the remaining life as fatigue cracks can initiate at the corrosion damage during in-flight cyclic loading. Detailed investigation of the fatigue crack formation process in pitted AA 7075-T651 specimens established the primary role of micro-topography along the broad pit surface. However, similar studies in high strength steels that exhibit smoother pit morphology demonstrated a primary role of macro-topography. This discrepancy motivates the current study to investigate the fatigue initiation, propagation and crack growth behavior of AA 7050-T7451 with various corrosion morphologies produced using conditions typical of a galvanic couple. Furthermore, this study seeks to determine the contribution of corrosion geometry, constituent particle distribution, and grain orientation to the local plasticity that dictates fatigue crack formation location.
It is hypothesized that the fatigue crack initiation can be found on the location with most severe corrosion damage shape. In order to test this hypothesis, vastly different corrosion damage morphologies (discrete pits, general corrosion with surface recession, fissures, cigar-shaped pit, and intergranular corrosion) are artificially formed on the surface of the AA 7050-T7451 using electrochemical techniques. The corrosion morphologies are characterized using optical microscopy, white light interferometry, X-ray computed tomography, and scanning electron microcopy. Specimens are cyclically loaded in a longitudinal direction at a relative humidity of > 90% maintained within a sealed acrylic chamber. A programmed fatigue loading sequence with σmax of 200 MPa, R of 0.5 and 20 Hz frequency is used to create marker bands on the fracture surface. The microstructurally small crack growth rates and the fatigue initiation life to create a 10 µm crack are determined using these marker bands. A combination of 2D and 3D images enables the determination of the fatigue initiation sites. These fatigue formation sites are correlated to the macro-scale (> 250 µm) corrosion damage features. The different metrics analyzed for discrete pits are the pit depth, pit density, pit volume, surface area of pit mouth, pit diameter, number of pits per plane, average pit depth per plane, and total pit depth per plane. On the other hand, the macro-scale metrics analyzed for fissure and IGC morphologies are fissure depth, number of fissures per plane, average fissure depth per plane and total fissure depth per plane. Roughness metrics such as average roughness or root mean square (RMS), peak density, maximum valley depth, maximum height, texture aspect ratio, auto correlation length, kurtosis, and arithmetic mean height are analyzed for general corrosion with surface recession. Results show that individual contribution of the macro-scale features does not independently dictate the location of fatigue crack formation location. The fatigue initiation life of specimens with different corrosion morphologies is very low and has near constant values. The crack growth rates of the specimens with different corrosion morphologies converge to similar values. However, the increased initial size for large corrosion damage can lead to mildly lower total life.
The analysis of the contribution of individual macro-scale corrosion metrics is extended to discrete pit specimens tested at low relative humidity (< 5% RH) and high σmax (300 MPa). Results show that despite changes in the fatigue testing environment or the σmax, the individual contribution of the macro-scale corrosion features still does not solely influence the location of the fatigue crack formation. Initiation life of specimens tested at < 5% RH is still very low. The crack growth rate of these specimens is lower, compared to the specimens tested at > 90% RH, leading to higher total fatigue life. The increase of σmax results in a decreased total fatigue life and increased crack growth rates.
This study also looks at the effect of micro-scale (< 250 µm) corrosion damage features. It is hypothesized that fatigue crack initiation can be found on areas with distinct micro-scale corrosion damage feature. The fatigue crack initiation features are classified according to the micro-features such as jut-ins, micro-pits, ligament with high aspect ratio and ligament with low aspect ratio. Most of the fatigue crack initiation occurs at jut-in features along the corroded surface however not all jut-ins initiate fatigue cracks. The underlying alloy microstructure features are also considered; specifically, the constituent particle distribution, grain orientation with respect to the loading direction, grain orientation spread, grain size and misorientation angles. It is also hypothesized that the fatigue initiation site occurs at the area with high density of constituent particles, or at grains with large amount of deformation, or at larger grains, or at grains with high misorientation angles. Results show that individual contribution of the micro-features and underlying microstructure do not independently identify the location of fatigue crack initiation.
Since individually the macro-scale, micro-scale corrosion features and alloy microstructure are shown to not solely prescribe the location of fatigue crack initiation, the combined effects of these parameters are considered via (1) data science approaches, and (2) analytical calculation of microstructure scale driving force. Data science is employed to understand the combined effects of the macro-scale corrosion features. Logistic regression and random forests modeling are used to predict the probability of fatigue crack initiation using the macro-scale metrics as predictor variables. Crystal plasticity modeling is used to analytically evaluate the combined effects of corrosion morphology and alloy microstructure. Crystal plasticity computed local stress/strain state is established for specific corrosion morphology, grain structure, and constituent particle distribution in order to calculate different fatigue indicator parameters (FIP) within the perimeter of the corrosion damage. Results of this study will help inform and validate current micro-mechanical models of initiation and MSC taking into account pertinent FIPs to properly predict crack formation location, fatigue initiation life and total fatigue life of aerospace structures with corrosion damage.
The results and conclusion of this effort will quantitatively characterize the crack formation behavior of a relevant aerospace Al alloy in realistic conditions and leverage this data to further mechanistic understanding of the factors governing the corrosion to fatigue crack transition. This understanding is critical to inform engineering scale prognosis strategies and provide guidance on the critical criteria for designing corrosion mitigation strategies in the context of fatigue damage.
