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1. Quantification of the local mechanical behavior in dissimilar metal welds using digital image correlation instrumented cross-weld tensile testing

   https://doi.org/10.1007/s40194-024-01738-0  

   Siefert, W.; Buehner, M.; Alexandrov, B. T. (March 2024, Welding in the World)

   The local yielding behavior in base metal, heat-affected zone, fusion boundary region, and weld metal of low-alloy steel/Alloy 625 filler metal welds was quantified using digital image correlation instrumented cross-weld tensile test. The tested welds exhibited undermatching, matching, or overmatching weld metal yield strength with significant gradients in the local yielding behavior. An undermatching weld yielded at 69 MPa below the base metal yield stress, accumulating to 0.72% total strain. The base metal in an overmatching weld had 110 MPa lower yield strength than the weld metal. The strong strain hardening response in the Alloy 625 weld metal, within the uniform elongation range, and its constraining effect on the fusion boundary region and heat affected zone, led to extensive strain accumulation, necking, and final failure in the base metal of all tested welds. The yielding behavior of the tested welds was compared to stress-based criteria, utilizing minimum specified and as-delivered yield and ultimate tensile strength, and to strain-based criteria. The capability of digital image correlation instrumented cross-weld tensile testing to quantify local yielding and strain accumulation demonstrates potential application in proving conformity to stress-based and strain-based design criteria of dissimilar and matching filler metal welds. 

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2. Anomalous mechanical behavior of nanocrystalline binary alloys under extreme conditions

   https://doi.org/10.1038/s41467-018-05027-5  

   Turnage, S. A.; Rajagopalan, M.; Darling, K. A.; Garg, P.; Kale, C.; Bazehhour, B. G.; Adlakha, I.; Hornbuckle, B. C.; Williams, C. L.; Peralta, P.; et al (July 2018, Nature Communications)

   Abstract Fundamentally, material flow stress increases exponentially at deformation rates exceeding, typically, ~10³ s⁻¹, resulting in brittle failure. The origin of such behavior derives from the dislocation motion causing non-Arrhenius deformation at higher strain rates due to drag forces from phonon interactions. Here, we discover that this assumption is prevented from manifesting when microstructural length is stabilized at an extremely fine size (nanoscale regime). This divergent strain-rate-insensitive behavior is attributed to a unique microstructure that alters the average dislocation velocity, and distance traveled, preventing/delaying dislocation interaction with phonons until higher strain rates than observed in known systems; thus enabling constant flow-stress response even at extreme conditions. Previously, these extreme loading conditions were unattainable in nanocrystalline materials due to thermal and mechanical instability of their microstructures; thus, these anomalies have never been observed in any other material. Finally, the unique stability leads to high-temperature strength maintained up to 80% of the melting point (~1356 K). 

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3. Using tensile and compressive stress superposition during incremental forming to manipulate martensitic transformation in SS304

   https://doi.org/10.1088/1757-899X/1307/1/012006  

   Mamros, E M; Maaβ, F; Tekkaya, A E; Kinsey, B L; Ha, J (May 2024, IOP Conference Series: Materials Science and Engineering)

   Abstract Single point incremental forming (SPIF) is a flexible manufacturing process that has applications in industries ranging from biomedical to automotive. In addition to rapid prototyping, which requires easy adaptations in geometry or material for design changes, control of the final part properties is desired. One strategy that can be implemented is stress superposition, which is the application of additional stresses during an existing manufacturing process. Tensile and compressive stresses applied during SPIF showed significant effects on the resulting microstructure in stainless steel 304 truncated square pyramids. Specifically, the amount of martensitic transformation was increased through stress superposed incremental forming. Finite element analyses with advanced material modeling supported that the stress triaxiality had a larger effect than the Lode angle parameter on the phase transformation that occurred during deformation. By controlling the amount of tensile and compressive stresses superposed during incremental forming, the microstructure of the final component can be manipulated based on the intended application and desired final part properties. 

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4. Plastic Flow of AA6013-T6 at Elevated Temperatures and Subsequent Reaging to Regain Full Strength

   https://doi.org/https://doi.org/10.1007/978-3-030-36408-3_56  

   Katherine E. Rader, Jon T. (March 2020, Minerals, Metals and Materials Series: Light Metals 2020)

   Combining a retrogression heat treatment with simulta- neous warm forming can increase the formability of peak-aged, high-strength aluminum alloys while allowing peak-aged strength to be recovered through a single reaging heat treatment after forming. This process is termed retrogression-forming-and-reaging (RFRA). This study investigates the applicability of RFRA to AA6013- T6 sheet material. Elevated-temperature tensile tests were performed at temperatures from 230 to 250 °C and strain rates from 3.2
   10 −3 to 10 −1 s −1 . Tensile tests were followed by reaging with a simulated paint-bake heat treatment. Flow stress at a true strain of 0.10 ranges from 230 MPa (250 °C and 3.2
   10 −3 s −1 ) to 290 MPa (230 °C and 10 −1 s −1 ), significantly lower than the room-temperature yield strength of 360 MPa in the T6 condition. The average elongation to rupture and reduc- tion in area from elevated-temperature tests are 22% and 56%, respectively, which are similar to the room- temperature values for the T4 condition. Elevated- temperature testing reduced material hardness compared to the original T6 condition. Subsequent reaging with a simulated paint-bake raised hardness to 96% of the T6 condition in un-deformed material, but slightly decreased the hardness of the deformed material. Recommendations for implementing RFRA of AA6013-T6 are presented. 

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5. Mechanical properties of polymeric microfiltration membranes

   https://doi.org/10.1016/j.memsci.2019.117351  

   Elele, E.; Shen, Y.; Tang, J.; Lei, Q.; Khusid, B.; Tkacik, G.; Carbrello, C. (August 2019, Journal of membrane science)

   The influence of the pore topology and polymer properties on mechanical characteristics of asymmetric polyethersulfone (PES) and symmetric polyvinylidene fluoride (PVDF) microfiltration membranes was investigated by conducting elongation, creep, stress relaxation, small-amplitude oscillatory and bubble point pressure tests. The main aspects of the membrane stress-strain curves were found to be similar despite significant differences in the pore topology and polymer properties. While the Kelvin-Voigt model for solid polymers described the membrane viscoelastic response below the transition to ductile yielding, the stress-strain curves of membranes and solid polymers above the yield point appeared to be drastically different. All tested membranes demonstrated weak strain hardening, low sensitivity to strain rate, significant elastic recovery, stress relaxation and reduction of the bubble point pressure with accumulation of plastic deformation. Therefore, tensile stresses exerted on a membrane under assembling and process conditions should be smaller than the yield stress to assure that they will not impair filter performance. The novelty of our approach is the use of models for perforated plates to evaluate membrane mechanical properties as ductile yielding for both proceeds via localized plastic deformation around pores. Presented results provide a reliable framework for development of membranes with properties tailored to applications. 

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