The effect of corrosion location relative to local stresses on the fatigue life of geometrically-complex, galvanically corroded AA7075-T6

Aluminum components used in aerospace structures are commonly coupled with stainless-steel fasteners. These through-hole geometries on the aluminum substrate cause a concentrated stress field. The high-stresses at the fastener sites can preferentially initiate coating damage allowing for moisture ingress which can lead to the formation of a galvanic couple between the aluminum alloy and the stainless-steel fastener. Corrosion damage is known to favorably initiate fatigue cracks thus severely reducing the total life of the component. This work aims to understand the relative impact and interaction of fastener hole geometry induced stress concentrations and corrosion damage on the fatigue crack initiation behavior. Specifically, by imparting various levels of corrosion severities at different locations within the macro-scale stress field, the relative impact of each on the initiation process can be determined. This work demonstrated a dominant role of the macro-scale stress field on the crack formation location. Specifically, crack formation was found to preferentially occur at high stress regions in lieu of forming at lower stress regions, regardless of corrosion severity. Critically, the findings of this work will inform the means by which coatings are evaluated and will serve as a controlled validation of experiments for fracture mechanics modeling.

Numerical Investigation on the Effects of Circuit Parameters on the Plastic Deformation of Fastener Holes in Thin Aluminum Alloy via Electromagnetic Expansion Process

The size of plastic deformation zone during fastener hole strengthening is a critical indicator of the strengthening effect. In this study, a considerable plastic deformation zone in 1.5 mm aluminum alloy plate with a hole was produced via electromagnetic strengthening. The finite element analysis results showed that the electromagnetic strengthening process could achieve high compressive hoop residual stress around the fastener hole in thin plate without serious axial deformation compared with conventional cold hole expansion process. The simulation results were experimentally validated by the grid method. Furthermore, for the same discharge energy, the size of plastic deformation zone varies with the discharge capacitance, and there was an optimal combination of the discharge capacitance and discharge voltage. What’s more, even the plastic deformation zone was the same at the maxed load, different unloading process during the electromagnetic hole expansion process also had a great influence on the strengthening effect.

The Influence of Laser Shock Peening on Fatigue Properties of AA2024-T3 Alloy with a Fastener Hole

The objective of the present study was to estimate the influence of laser shock peening on the fatigue properties of AA2024-T3 specimens with a fastener hole and to investigate the possibility to heal the initial cracks in such specimens. Fatigue cracks of different lengths were introduced in the specimens with a fastener hole before applying laser shock peening. Deep compressive residual stresses, characterized by the hole drilling method, were generated into the specimens by applying laser shock peening on both sides. Subsequently, the specimens were subjected to fatigue tests. The results show that laser shock peening has a positive effect regarding the fatigue life improvement in the specimens with a fastener hole. In addition, laser shock peening leads to a healing effect on fatigue cracks. The efficiency of this effect depends on the initial crack length. The effect of laser shock peening on the fatigue life periods was determined by using resonant frequency graphs.

Estimating the Throwing Power of SS316 when Coupled with AA7075 Through Finite Element Modeling

Galvanic corrosion is common in applications involving a fastener and panel assembly. Often, the fastener is made from a more noble metal and the panel is made from a less noble metal, selected for their respective mechanical properties. The ability for the more noble material to galvanically couple to the panel’s surface as a function of distance is referenced to as “throwing power,” and was the main subject of this research. In this work, SS316 and AA7075 were investigated as the fastener and panel material, respectively. A Ti-6Al-4V fastener and a sol-gel coated SS316 fastener were also considered to determine the impact of different materials on the galvanically driven throwing power. Along with different fastener materials, different fastener geometries were considered as well. Raised fasteners are generally used in tandem with washers, while countersunk fasteners are not in order to remain flush with the surface. The difference between these two geometries on the throwing power was investigated. It was determined that the SS316 washer was the largest contributor to the galvanic current in the raised fastener assembly, due to its large surface area. At distances of two inches away, the SS316 fastener and washer were able to double the natural corrosion rate of AA7075. A countersunk SS316 fastener, with the same total surface area as that of the raised fastener and washer assembly, was seen to lower the throwing power which forced a large amount of current down the fastener hole. Throughout all of the computational tests, the model relies on the generation of accurate electrochemical kinetics measured in solutions of appropriate composition.

Galvanic Corrosion Between Coated Al Alloy Plate and Stainless Steel Fasteners, Part 1: FEM Model Development and Validation

Aerospace structures often involve dissimilar materials to optimize structural performance and cost. These materials can then lead to the formation of galvanic couples when moisture is present. Specifically, noble metal fasteners (such as SS316) are often used in aluminum alloy load-bearing structures, which can lead to accelerated, localized corrosion attack of the aluminum alloy due to the cathodic current supplied by the SS316 fastener. This localized attack is difficult to predict, and tests are often expensive, so modeling of these galvanic couples could be of great utility. The work reported here focuses on the galvanic coupling between fasteners installed in a panel test assembly, and the resultant corrosion damage down the fastener holes. This arrangement is a common assembly geometry in aerospace applications. A specific sol-gel coating was applied to the fasteners, to determine its effectiveness on mitigating galvanic corrosion; bare fasteners were also tested, to investigate a worst-case scenario. Geometric constraints in the model were made to match those of an experimental test panel, which was exposed to ASTM B117 salt fog for 504 h. The electrochemical boundary conditions were generated in solutions appropriate to the material and environment to which it would be exposed. Anodic charge passed during exposure was calculated from image analyses of the corrosion damage in the experimental test, and the results were compared with the model. The Laplacian-based model provides a very good first approximation for predicting the damage within the fastener hole. Validation was provided by both experimental results generated in this study as well as comparison to results in the literature that used similar, but not identical, conditions.

Understanding the influence of laminate stacking sequence on strain/stress concentrations in thin laminates at repair holes with large scarf angles

Scarf repair is widely used in the restoration of structural performance of damaged aircraft secondary structures. Such repairs result in reduced thickness sections which are significantly larger than those associated with typical fastener holes. Significant literature exists on the distribution of strain/stress concentration in fastener hole geometries, both straight sided and countersunk, but is lacking for the geometries associated with shallow scarf angles and thin laminates. Hence, herein three-dimensional finite element models are developed to understand the influence of stacking sequence and scarf angle on strain/stress concentrations. The results demonstrate and quantify for the first time that strain concentrations are not only dependant on the laminate membrane stiffness but also on laminate bending stiffness, due to the anisotropy created as a result of scarfing angle, hole geometry and laminate thickness. Scarfing is demonstrated, for typical repair geometry associated with foreign object damage (hole diameter 20 mm, scarf angles 3° to 7°), to elevate strains by up to 2.5 times when compared to equivalent diameter straight-sided holes in laminates of thickness ≈1 mm.

Net section fracture assessment of steel bolted joints with shear lag effect

Bolted connections are willingly used in steel structures because of their easiness of fabrication and assembly, but often they are the weakest component in the construction. In case of tensile lap connections, fracture of net cross section usually determines a joint capacity. Additionally, possible eccentricities can affect the distribution of stresses in the cross section and hence its load capacity. Analysis of fracture is a completely different issue compared to well-known and established problems of stability or plastic resistance. Paper relates to steel angle tension members connected by one bolt. It starts from the description of experimental investigations which results were used for hierarchical validation of computational models. Choice between two types of material models (elastic-plastic and Gurson–Tvergaard–Needleman) and building of FE models, representing different degrees of complexity, were described. Paper ends with parametric study taking into account influence of the edge distance from the centre of a fastener hole to the adjacent edge of angle. The paper’s aim is to verify and present the methodology for fracture prediction in steel angle tension members, which can be next extended for bolted joints with larger number of bolts and different geometrical configurations.

Strength of composite plate with parallel offset misalignment double-bolted joint under bending moment

The connection concept of placing only two bolts in offset misalignment against the bending load along the wing span was used for an aerobatic airplane designed in Thailand as a KIT plane to minimize the impact of drilling numerous holes. This concept can deviate the force direction on the holes. The two suitable drilling positions should have the lowest resultant force and highest strength of fiber reinforcement structure. To investigate the strength of the fastener hole when the force deviated from the original orientation, specimens, made from twill weave carbon fiber-epoxy and laid up at ±45-degree orientation, were tested under bearing load according to ASTM D5961 standard. The experimental results revealed that the bearing strength of CFRP material decreases when the force deviation angle increases, so zero-angle deviation of the resultant force on the drilling hole is the most suitable orientation to absorb bolt bearing load. The most suitable pattern of two offset misalignment holes is the greatest horizontal distance at zero vertical distance when it was considered only the effect of the bearing strength and the deviation angle. Moreover, the failure pattern begins to deviate along the fiber orientation when the inclination angle increased.