Yannis P. Korkolis

Constitutive Modeling and Rupture Predictions of Al-6061-T6 Tubes Under Biaxial Loading Paths

This brief note reports the results of a set of biaxial experiments on Al-6061-T6 tubes tested to rupture under radial stress paths of combined internal pressure and axial load. The experiments are then simulated with shell-type finite element models, in which several yield functions are calibrated and implemented and their performance evaluated against the experimental results.

Recent developments in hydroforming technology

This article is the review of hydroforming technologies that have been used increasingly in various industries including automotive applications. General concepts and technological developments in tube hydroforming and sheet hydroforming are presented with recent research and development activities. Then, the theoretical background associated with the plasticity and constitutive laws and their implementation to the computational modeling of the hydroforming process are discussed.

Residual Ductility and Microstructural Evolution in Continuous-Bending-under-Tension of AA-6022-T4

A ubiquitous experiment to characterize the formability of sheet metal is the simple tension test. Past research has shown that if the material is repeatedly bent and unbent during this test (i.e., Continuous-Bending-under-Tension, CBT), the percent elongation at failure can significantly increase. In this paper, this phenomenon is evaluated in detail for AA-6022-T4 sheets using a custom-built CBT device. In particular, the residual ductility of specimens that are subjected to CBT processing is investigated. This is achieved by subjecting a specimen to CBT processing and then creating subsize tensile test and microstructural samples from the specimens after varying numbers of CBT cycles. Interestingly, the engineering stress initially increases after CBT processing to a certain number of cycles, but then decreases with less elongation achieved for increasing numbers of CBT cycles. Additionally, a detailed microstructure and texture characterization are performed using standard scanning electron microscopy and electron backscattered diffraction imaging. The results show that the material under CBT preserves high integrity to large plastic strains due to a uniform distribution of damage formation and evolution in the material. The ability to delay ductile fracture during the CBT process to large plastic strains, results in formation of a strong <111> fiber texture throughout the material.

Thermal Effects on the Enhanced Ductility in Non-Monotonic Uniaxial Tension of DP780 Steel Sheet

To understand the material behavior during non-monotonic loading, uniaxial tension tests were conducted in three modes, namely, the monotonic loading, loading with periodic relaxation and periodic loading-unloadingreloading, at different strain rates (0.001/s to 0.01/s). In this study, the temperature gradient developing during each test and its contribution to increasing the apparent ductility of DP780 steel sheets were considered. In order to assess the influence of temperature, isothermal uniaxial tension tests were also performed at three temperatures (298 K, 313 K and 328 K (25 °C, 40 °C and 55 °C)). A digital image correlation system coupled with an infrared thermography was used in the experiments. The results show that the non-monotonic loading modes increased the apparent ductility of the specimens. It was observed that compared with the monotonic loading, the temperature gradient became more uniform when a non-monotonic loading was applied.

Formability Enhancement in Titanium Tube-Flaring by Manipulating the Deformation Path

The tube flaring process has been traditionally used to expand one end of a tube without changing its cross-sectional area. This simple process typically forms the product using a single punch. To delay failure and enhance formability, a two-step flaring process can be used. For example, if a significant elliptical flared shape is attempted in a one-step process, a necking/tearing failure would occur on the major axis of the ellipse. However, if a two-step process, starting with a mildly elliptical punch and followed by the final, sharply elliptical punch is used instead, the desired elliptical shape can be achieved. In this paper, the effects of the punch geometry of the first step on the deformation paths are numerically analyzed. By manipulating the deformation path, failure can be delayed so that higher formability is achieved. The numerical model is validated by comparison with experimental results.

Biaxial unloading and springback behavior of dual-phase DP590 steel using cruciform specimens

The unloading behavior of a dual-phase steel (DP590) from a biaxial state of stress was probed using a newly-designed cruciform specimen. The specimen was designed to develop uniform and relatively large plastic strains (over 15% equivalent logarithmic plastic strain) in the gage section, before failure. Nine radial loading paths in the 1st quadrant of the plane stress space were probed. The experiments involved repeated loading and unloading up to failure. At every unloading, the initial response was found to agree with the linear, orthotropically elastic response of the undeformed material. This first linear response was followed by a second one, at a reduced slope. Beyond that, the recorded response was fully non-linear. The same sequence of events was observed during each reloading. The biaxial non-linear strain recovery components exnl and eynl were measured to be on average approximately 11% of the elastic strains exe and eye, respectively. This ratio was found to increase with plastic deformation. ...

Modeling of hole-expansion of AA6022-T4 aluminum sheets with anisotropic non-quadratic yield functions

In the hole-expansion of anisotropic AA6022-T4 sheets, the strain around the hole is non-uniformly distributed due to the anisotropy of the material. This was examined by performing experiments with a flat-headed punch and using Digital Image Correlation (DIC). In the experiments, failure always initiated at a unique location, oriented at 45° to the Rolling Direction of the sheet. The use of DIC allowed the probing of the full-strain-field in real-time. Subsequently, the experiments were simulated in DYNAFORM using shell elements and the Yld2000-2D anisotropic non-quadratic yield function, properly calibrated for this material. In addition, the hardening curve of the material was inversely identified at large strains from the tail of the tensile test. The strain evolution is compared between the experiments and the model.

Novel Method for Combined Tension and Shear Loading of Thin-Walled Tubes

Two simple experiments for probing the plastic response of thin-walled tubes under pure hoop tension and combined tension and shear are described. They both use ring-like specimens with test-sections machined on them. The specimens are placed over two D-shaped mandrels that are parted in a universal testing machine. The first experiment is the Ring Hoop Tension Test, which can be used to obtain the uniaxial hoop response of the tube. The second is the recently proposed Ring Plane-Strain Tension experiment, which can impose stress-states with an arbitrary combination of tension and shear. It is shown that the results of these experiments can be used to probe the yield locus of the tube material and calibrate suitable anisotropic yield functions. The experiments are proposed as complementary to the established tension-internal pressure loading of thin-walled tubes, but can also be used independently, in the absence of the specialized equipment that the latter experiment requires.

Special Issue: Forming and Joining of Lightweight and Multimaterial Systems

With this special issue of the Journal of Manufacturing Science and Engineering (JMSE) our objective is to collect papers which enhance the understanding of material deformation processes; expand the lightweight material forming and multi-material joining processes and modeling capability; and promote R&D activities on forming and joining new materials and/or new forming and joining technologies at length scales varying from microto macro-scale.

Design of a Continuous-Bending-Under-Tension Machine and Initial Experiments on Al-6022-T4

An experimental technique called Continuous-Bending-Under-Tension (CBT) can produce elongations over two times that of a standard tensile test by preventing the necking instability from occurring. This is achieved by superposing plastic bending on tension along the gage length of the material using three rollers. The specimen is kept under tension as the rollers apply the three point bending and cyclically transverse along the gage length. This localizes the plastic deformation by subjecting the specimen to bending-under-tension. Details on the design of the unique CBT machine and some preliminary results for the CBT research being conducted are presented here. These results include CBT tests where the roller depth was varied to demonstrate the increased elongation compared to the traditional tension test, CBT repeatability, and a modified friction test using the CBT machine.Copyright