4D microstructural and electrochemical characterization of dissimilar metal corrosion in naval structural Joints

Dissimilar metal corrosion in aircraft and naval structures has proven to be a persistent challenge. Decades of research in the area have shown that such complex contact surfaces are subject to a combination of corrosive environments and mechanical loads. Hence, this multi-faceted problem must be understood from electrochemical, microstructural and mechanical standpoints to comprehensively understand corrosion damage in these systems. Please click Additional Files below to see the full abstract

Tensile and fracture behavior of silica fibers from the Venus flower basket (Euplectella aspergillum)

The extraordinary mechanical properties shown by many biological materials stem from their unique hierarchical structures. The spicules of the deep-sea glass sponge, Euplectella aspergillum, show layered architecture and are known to improve toughness in various loading conditions. In the present work, we conducted a systematic study on the tensile and fracture behavior of E. aspergillum\u27s spicules. Tensile tests were performed on three different gage lengths of spicules that compose anchoring structures of E. aspergillum. The cross-sectional area of each spicule was accurately measured using an x-ray microscope. The effect of gage length and varying cross-sectional area on the tensile behavior of spicules is discussed in detail. The interplay between two failure initiation sites, namely, weakest-link flaws and minimum cross-sectional area locations was observed. Weibull statistics were used to quantify the variability in the strengths of spicules as a function of their gage length. The Weibull modulus was observed to decrease with the increase in gage length. The possibility of a bimodal flaw population for a higher gage length of spicules due to the interplay between failure initiation sites is also discussed. The fractography study was utilized to understand the failure and toughening mechanisms of spicules in tensile loading. The fracture toughness of the central core was quantified from fracture surfaces using a linear elastic fracture mechanics model. The present study shows that the failure of spicules initiates in the outer layers and proceeds progressively to the center. The layers surrounding the central core of the spicule resist the crack propagation and increase its toughness

Hot Rolling of ZK60 Magnesium Alloy with Isotropic Tensile Properties from Tubing Made by Shear Assisted Processing and Extrusion (ShAPE)

In the present work, we utilized Shear Assisted Processing and Extrusion (ShAPE), a solid-phase processing technique, to extrude hollow tubes of ZK60 Mg alloy. Hot rolling was performed on these as-extruded tubes (after slitting them longitudinally) to thickness reductions of 37%, 68%, and 93% to investigate their viability as rolling feedstock material. EBSD analysis showed the formation of twinned grains in the ShAPE processed material and a gradual re-orientation of the basal texture parallel to the extrusion direction with each rolling step. Moreover, an equiaxed grain size of 5.15 ± 3.39 μm was obtained in the ShAPE extruded material, and the microstructure was retained even after 93% rolling reduction. The rolled sheets also showed excellent tensile strengths and no mechanical anisotropy, a critical characteristic for formability. The unique microstructures developed and their excellent mechanical properties, combined with the ease of scalability of the process, make ShAPE a promising alternative to existing methods for producing rolling feedstock material

Bond coat assisted enhancement in microstructural, mechanical and corrosion behavior of AZ91 magnesium alloy cold spray coated with aluminum alloys

Cold spray coating technique was used to deposit coatings of AA6061 aluminum alloy on AZ91 magnesium alloy substrates. A bond coat of commercially pure aluminum was used to facilitate deposition of the AA6061 powders. Two cold spray coating iterations were conducted to optimize the coating’s microstructural features such as porosity, surface roughness, and interface microstructure. Mechanical properties such as hardness, wear resistance, and adhesion behavior were characterized and used as metrics to compare the coatings with the substrate. The corrosion behavior of the coated and uncoated samples was characterized using multiscale and multimodal corrosion measurements comprising macroscopic potentiodynamic polarization and scanning electrochemical cell microscopy. The coated substrates exhibited a significant improvement in mechanical and corrosion performance as compared to the uncoated substrate

Bioinspired honeycomb core design: An experimental study of the role of corner radius, coping and interface

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. The honeybee’s comb has inspired the design of engineering honeycomb core that primarily abstract the hexagonal cell shape and exploit its mass minimizing properties to construct lightweight panels. This work explored three additional design features that are part of natural honeybee comb but have not been as well studied as design features of interest in honeycomb design: the radius at the corner of each cell, the coping at the top of the cell walls, and the interface between cell arrays. These features were first characterized in natural honeycomb using optical and X-ray techniques and then incorporated into honeycomb core design and fabricated using an additive manufacturing process. The honeycomb cores were then tested in out-of-plane compression and bending, and since all three design features added mass to the overall structure, all metrics of interest were examined per unit mass to assess performance gains despite these additions. The study concluded that the presence of an interface increases specific flexural modulus in bending, with no significant benefit in out-of-plane compression; coping radius positively impacts specific flexural strength, however, the corner radius has no significant effect in bending and actually is slightly detrimental for out-of-plane compression testing