Military aircrafts operate frequently in a highly corrosive environment. Corrosion in aluminum aircraft structures with holes and adhesive-bonded lap joints promotes multi-site cracking, which can lead to failure of major aircraft components. To expedite and facilitate the corrosion repair process, an emerging solid-state process, additive friction stir deposition (i.e. AFSD) is applied for the hole restoration followed by the fatigue performance evaluation. Because of the friction stir induced material flow of the deposited and substrate materials, the resulting microstructure and its associated properties are position-dependent. The weak metallurgical bond at the bottom of the repaired hole coupon can promote a crack initiation under cyclic loading and the total life consists of crack initiation, short crack growth, and long crack propagation. This paper describes the use of a multiphysics modeling approach to characterize the process-driven properties evolution and to evaluate the effects of a kissing bond on the total life of hole restoration coupons.
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The behavior of short fatigue cracks under high temperature conditions is of significant importance for lifetime predictions and defect assessment of components in aircraft and gas turbine applications. The cyclic R(esistance) curve provides a possibility to describe phenomenologically the crack growth behavior below the long crack threshold. Within this paper experimentally determined cyclic R-curves of the nickel based alloy IN718 for varying load ratios (Rσ = -1, 0 and 0.5) at 650 °C are compared to each other. Furthermore, an approach supporting a mechanism-based understanding of the cyclic R-curves, taking plasticity induced crack closure (PICC) into account, is developed. Finite element analysis is used to estimate the amount of PICC in the short crack regime. For a systematic investigation of the impact of different ductility properties on short crack behavior, results from an additively and two conventionally (cast and wrought) manufactured material variants of IN718 are part of the investigations.
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This work presents a flexible experimental setup to study dynamic fragmentation of additively-manufactured metallic materials using two different configurations: (i) rapid axial penetration of thin-walled tubes, and (ii) rapid radial expansion of rings. In the first approach, the experiment consists of a light-gas gun that fires a conical nosed cylindrical projectile that impacts axially on a thin-walled cylindrical tube fabricated by 3D printing. The diameter of the cylindrical part of the projectile is approximately twice greater than the inner diameter of the cylindrical target, which is expanded as the projectile moves forward, eventually breaking into fragments. In the second approach, using a similar technique, a ring is inserted over a high-ductility tube, which expands after penetration by the conical projectile, pushing the metallic ring radially outwards, ultimately breaking into multiple fragments. The experiments have been performed for impact velocities ranging from 180 m/s to 390 m/s. A salient feature of this work is that we have characterized by X-ray tomography the porous microstructure of selected specimens before and after testing. Moreover, two high-speed cameras have been used to film the experiments and thus to obtain time-resolved information on the mechanics of formation and propagation of fractures.
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In this study, a plasticity-induced crack closure model, FASTRAN, was used to predict the fatigue life of Inconel 718, 17-4 precipitation hardening (PH) stainless steel (SS), and Ti-6Al-4V alloys fabricated via additive manufacturing (AM) systems. Results indicated that in the presence of large defects (e.g., lack-of-fusion defects), the total fatigue life of AM specimens is dominated by crack growth. Results indicated that variations in the fatigue lives of specimens in machined and as-build surface conditions can be predicted based on the characteristics of AM process-induced defects and surface profile. Effect of build orientation on fatigue life was also captured based on the size of defects projected on a plane perpendicular to the loading direction. In addition, maximum valley depth of the surface profile can be used as an appropriate parameter for the fatigue-life prediction of AM specimens in their as‐built surface condition.
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Non-combustible Mg alloy components fabricated by laser powder bed fusion in as-built conditions have an average ultimate tensile strength (UTS) of 320 MPa, a significantly larger value than its casting counterparts, which present an average UTS of 200 MPa. In addition, it was determined that stable crack extension always starts at the outer surface due to the coarsened microstructure regions present in the area. Therefore, this paper will use fracture mechanics to predict the UTS value by determining the size of the coarsened microstructure region and considering it as a surface crack with the √area parameter. Then, by using a fixed fracture toughness value, the UTS will be predicted. Furthermore, a processing parameter known as contour, which is used for remelting the outer surface of the specimen, can also smoothen the microstructure and potentially increase the UTS value. Results showed that the √area of the surface crack responsible for fracture was 730 μm for the no-contour specimen and 630 μm for a contour specimen. Subsequently, using Murakami’s theory, the predicted UTS is 320 MPa and 345 MPa respectively. Finally, tensile testing was performed to confirm the prediction, showing similar results with an average deviation of 2.9%.
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