Yuanli Bai

Controlled fragmentation of multimaterial fibres and films via polymer cold-drawing

Polymer cold-drawing is a process in which tensile stress reduces the diameter of a drawn fibre (or thickness of a drawn film) and orients the polymeric chains. Cold-drawing has long been used in industrial applications, including the production of flexible fibres with high tensile strength such as polyester and nylon. However, cold-drawing of a composite structure has been less studied. Here we show that in a multimaterial fibre composed of a brittle core embedded in a ductile polymer cladding, cold-drawing results in a surprising phenomenon: controllable and sequential fragmentation of the core to produce uniformly sized rods along metres of fibre, rather than the expected random or chaotic fragmentation. These embedded structures arise from mechanical–geometric instabilities associated with ‘neck’ propagation. Embedded, structured multimaterial threads with complex transverse geometry are thus fragmented into a periodic train of rods held stationary in the polymer cladding. These rods can then be easily extracted via selective dissolution of the cladding, or can self-heal by thermal restoration to re-form the brittle thread. Our method is also applicable to composites with flat rather than cylindrical geometries, in which case cold-drawing leads to the break-up of an embedded or coated brittle film into narrow parallel strips that are aligned normally to the drawing axis. A range of materials was explored to establish the universality of this effect, including silicon, germanium, gold, glasses, silk, polystyrene, biodegradable polymers and ice. We observe, and verify through nonlinear finite-element simulations, a linear relationship between the smallest transverse scale and the longitudinal break-up period. These results may lead to the development of dynamical and thermoreversible camouflaging via a nanoscale Venetian-blind effect, and the fabrication of large-area structured surfaces that facilitate high-sensitivity bio-detection.

Experimental study on the mechanical properties of AZ31B-H24 magnesium alloy sheets under various loading conditions

In order to fully characterize the plasticity and fracture of magnesium AZ31B-H24 sheets, a set of mechanical experiments (105 in total) were performed under different loading conditions, including monotonic uniaxial tension, notch tension, in-plane uniaxial compression, wide compression (or called biaxial compression), plane strain compression, through-thickness compression, in-plane shear, punch test, and uniaxial compression–tension reverse loading. Both the plastic strain histories and stress responses were obtained under the above loading conditions, which give a comprehensive picture of mechanical behaviors of this material. An orthotropic yield criterion involving two linear anisotropic transformation tensors, CPB06ex2, in conjunction with its associated flow rule, and a modified semi-analytical Sachs isotropic hardening model was fully calibrated to describe both the anisotropy in plastic flow and tension–compression asymmetry in stress–strain behaviors. An all-strain based modified-Mohr–Coulomb fracture model, transformed from a stress triaxiality based model, was applied to describe the calibrated fracture locus. Applying a linear transformation to the plastic strain tensor, a non-conjugated anisotropic equivalent strain was proposed to characterize anisotropic fracture behaviors. Good correlations were achieved between experimental results and model predictions in terms of material yield strengths, strain hardening curves, plastic flow directions and ductile fracture strains.

Fracture of 1045 steel under complex loading history

In the paper, a 3D fracture locus of 1045 steel under proportional loading conditions was calibrated using round, plane strain, and tubular specimens. The fracture locus includes both the stress triaxiality and Lode angle dependence. As an extension to the conventional linear damage evolution assumption, a new form of ductile fracture model considering the loading history effect was proposed, in which two weighting functions were introduced to the damage indicator calculation. One considers the non‐linear damage evolution under proportional loading, the other accounts for the effect of change in loading directions. A round of comprehensive fracture tests on 1045 steel was conducted to validate the proposed fracture model. These tests include monotonic loading tests for 3D fracture locus calibration and other tests with complex loading histories, for example two‐stage‐tension test, compression‐tension test and torsion‐tension test.

Airbag Mapped Mesh Auto-Flattening Method

Current software cannot easily model an airbag to be flattened without wrinkles. This paper improves the modeling efficiency using the initial metric method to design a mapped mesh auto-flattening algorithm. The element geometric transformation matrix was obtained using the theory of computer graphics. The algorithm proved to be practical for modeling a passenger-side airbag model. The efficiency and precision of modeling airbags are greatly improved by this method.

Linkage between Ductile Fracture and Extremely Low Cycle Fatigue of Inconel 718 Under Multiaxial Loading Conditions

Ductile fracture and extremely low cycle fatigue (ELCF) [1] are two common failure modes in aircraft engines and turbomachinery designs [2]; however, the linkage between these two failure modes under multi-axial loading conditions has never been systematically studied. Inconel 718 (IN718) is one type of high temperature alloys widely used in turbomachines. Specially designed specimens and tests were used to achieve desired multi-axial loading conditions. Two groups of tests were conducted: (a) round bar specimens with different notches; (b) plane strain specimens. Similar types of tests were conducted for IN718 under both types of failure modes (ductile fracture and ELCF). It is found that the ductile fracture of IN718 under multi-axial loading conditions is strongly dependent on stress triaxiality, but weakly dependent on the Lode angle parameter [3]. A 3D fracture locus was calibrated using modified MohrCoulomb (MMC) criterion proposed by Bai and Wierzbicki [4]. It is found that the same phenomenon of stress state dependency exists in the ELCF, which need to be addressed. The mechanism linkage between these two failure modes was explored.

Design of a 3kW 150k RPM super high-speed permanent magnet synchronous motor

This paper discusses the design procedure of a super high speed permanent magnet synchronous machine. A 3kW radial PMSM designed for operation up to 150,000 r/min is proposed in the paper. Thanks to the super high speed, the designed motor can achieve high energy density at 2.5 kW/kg(housing weight included) and efficiency is over 97%. The super high speed brings challenges both on electrical design and mechanical design. Proper design of the stator lamination, including bore diameter, outer diameter, slot, yoke thickness and airgap is explained. At high frequency, proximity effect and skin effect becomes significant, the effects have been analyzed and multi-strand Litz wires are chosen for this design. Double-layer distributed winding is used instead of concentric winding to reduce the harmonic EMF. The rotor structure is designed to endure the mechanical stress at high RPM. The design is optimized in ANSYS Maxwell and simulation results verified the performance of the motor. A controller for the high speed drive has been developed for the motor.

Tissue Performance Characteristics: Closing the Fidelity Gap in Medical Simulations

Medical simulations often lack the fidelity necessary to train higher-level procedures such as surgery. To work around these gaps, organizations with a training mission like the Department of Defense (DoD) often resort to biologic tissues from cadavers and animals.

Hybridized fabrication of robust low-loss multimaterial chalcogenide fiber for infrared applications

Double-crucible cane fabrication of highly purified chalcogenide-glass was combined with multimaterial thermal fiber drawing to produce robust low-loss 0.2 NA chalcogenide fibers monolithically provided with a polymer jacket and featuring losses <;1 dB/m across the infrared.

Analytical Solution on the Failure of Strips Under Bending and Tension

The analysis of stretch-bending of a strip is important in understanding failure initiation in the sheet metal forming process. Most of the current work in this area has been purely experimental or numerical simulation. In the paper, a procedure for constructing an analytical solution for failure of strips under bending and tension was developed. One important aspect of the derivation was the consideration of the logarithmic strain instead of engineer strain, as was done in the existing solutions found in the literatures. The other important aspect is the assumption of tension/bending damage coupling. The input data of the analytical solution are the geometry of the tool (die radius), sheet thickness, parameters of the plasticity hardening rule, magnitudes of fracture strains under uni-axial tension and plane strain, and the tension/bending damage coupling coefficient. The present closed-form solution was validated by test results of the present authors as well as data from the open literature. A good correlation is obtained between the measured and predicted radius to thickness ratio (R / t) to fracture under a given pre-tension. The results of the paper will be useful in the design of stamping and deep drawing operation.