Seminar by Yannis P. Korkolis
Ductility enhancement of AA6022-T4 during continuous-bending-under-tension (CBT)
Yannis P. Korkolis
Department of Mechanical Engineering
University of New Hampshire
Deformation during the tensile test is limited by the occurrence of structural instability in the form of localization of deformation and necking. However, the material at the vicinity of the neck is severely more deformed that the rest of the specimen. Therefore, if a way to stabilize the deformation and avert necking is found, the entire specimen can uniformly deform to the limits of material ductility. This can then be used to identify the hardening response of a material past the limit of uniform elongation, as well as lead to innovative manufacturing processes for hard-to-form materials. Such a stabilized tensile test is the Continuous-Bending-Under-Tension (CBT) test, in which 3 rollers traverse a strip loaded in tension, cyclically bending and unbending it. CBT experiments were performed on 1-mm thick AA6022-T4 sheets using a custom CBT testing machine. The axial force recorded during the experiments was oftentimes below the yield force in uniaxial tension (depending on the process parameters), but the overall elongation in CBT was multiple times that recorded in uniaxial tension. The strain paths were found to be uniaxial, while the texture in the CBT specimen was measured to be similar to that found in the neck of a tensile specimen. The experiments were simulated using finite elements and a combined isotropic-kinematic hardening model of the Chaboche family, which was calibrated from cyclic tension-compression tests performed on the same alloy. The simulations reveal the strain development during CBT testing and can be used to understand the mechanism of the ductility enhancement observed.
In the talk, I will also discuss my vision for future research directions.
Yannis Korkolis graduated from the National Technical University of Athens (NTUA) with a 5-year diploma in Mechanical Engineering. Following two years in the industry and the military, he returned to NTUA to obtain a Master’s degree in Solid Mechanics. Subsequently, he pursued a PhD in Engineering Mechanics at the University of Texas at Austin, working on the formability and hydroforming of anisotropic aluminum tubes. He graduated in Aug. 2009 and since then he has been an Assistant (2009-2015) and Associate (2015-present) Professor of Mechanical Engineering at the University of New Hampshire. Since 2015 he is also the Graduate Coordinator of the Mechanical Engineering Department at UNH. He teaches courses in Solid Mechanics, in Manufacturing and in Design. His research interfaces constitutive modeling, formability and ductile fracture with materials forming and additive manufacturing processes including pulsed tube hydroforming, microforming and continuous-bending-under-tension. His particular approach involves multiaxial experiments, oftentimes using unique, custom-built equipment, combined with the use of recent advanced material and numerical models.