Kaijuan Chen

A STUDY OF TWO ALTERNATE TANGENT MODULUS FORMULATIONS AND ATTENDANT IMPLICIT ALGORITHMS FOR CREEP AS WELL AS HIGH-STRAIN-RATE PLASTICITY*

Abstract Two alternate tangent modulus formulations based on the use of material characteristics expressed as υp = g( ϵ υp , σ ) and σ = h ( ϵ υp υp ) , respectively, are prese for the analysis of material response under conditions such as high temperature creep and high strainrate dynamic plasticity. In each formulation, implicit algorithms of generalized midpoint radial mapping are presented to compute stress histories at a material point. Several examples to illustrate the stability, accuracy, and convergence of the presented computational methodologies are included. In each instance, the two alternate tangent modulus approaches lead to results of comparable accuracy and compare excellently with each other as well as other available independent solutions.

Viscoelastic–Viscoplastic Cyclic Deformation of Polycarbonate Polymer: Experiment and Constitutive Model

A series of uniaxial tests (including multilevel loading–unloading recovery, creep-recovery, and cyclic tension–compression/tension ones) were performed to investigate the monotonic and cyclic viscoelastic–viscoplastic deformations of polycarbonate (PC) polymer at room temperature. The results show that the PC exhibits strong nonlinearity and rate-dependence, and obvious ratchetting occurs during the stress-controlled cyclic tension–compression/tension tests with nonzero mean stress, which comes from both the viscoelasticity and viscoplasticity of the PC. Based on the experimental observation, a nonlinear viscoelastic–viscoplastic cyclic constitutive model is then constructed. The viscoelastic part of the proposed model is constructed by extending the Schaperys nonlinear viscoelastic model, and the viscoplastic one is established by adopting the Ohno–Abdel-Karims nonlinear kinematic hardening rule to describe the accumulation of irrecoverable viscoplastic strain produced during cyclic loading. Furthermore, the dependence of elastic compliance of the PC on the accumulated viscoplastic strain is considered. Finally, the capability of the proposed model is verified by comparing the predicted results with the corresponding experimental ones of the PC. It is shown that the proposed model provides reasonable predictions to the various deformation characteristics of the PC presented in the multilevel loading–unloading recovery, creep-recovery, and cyclic tension–compression/tension tests.

3D Printing of Highly Stretchable, Shape-Memory, and Self-Healing Elastomer toward Novel 4D Printing

The three-dimensional (3D) printing of flexible and stretchable materials with smart functions such as shape memory (SM) and self-healing (SH) is highly desirable for the development of future 4D printing technology for myriad applications, such as soft actuators, deployable smart medical devices, and flexible electronics. Here, we report a novel ink that can be used for the 3D printing of highly stretchable, SM, and SH elastomer via UV-light-assisted direct-ink-write printing. An ink containing urethane diacrylate and a linear semicrystalline polymer is developed for the 3D printing of a semi-interpenetrating polymer network elastomer that can be stretched by up to 600%. The 3D-printed complex structures show interesting functional properties, such as high strain SM and SM -assisted SH capability. We demonstrate that such a 3D-printed SM elastomer has the potential application for biomedical devices, such as vascular repair devices. This research paves a new way for the further development of novel 4D printing, soft robotics, and biomedical devices.

Uniaxial cyclic deformation and internal heat production of ultra-high molecular weight polyethylene

The cyclic deformation and corresponding internal heat production of ultra-high molecular weight polyethylene (UHMWPE) polymer were investigated under the uniaxial strain-controlled and stress-controlled cyclic loading conditions. It is seen that the UHMWPE behaves basically a cyclic stabilizing feature since the responding stress amplitude does not remarkably change during the cyclic loading, except for that at high strain rate (where an obvious cyclic softening is caused partially by the thermal softening); an apparent mean stress relaxation occurs in the asymmetrical strain-controlled cyclic tests, and the degree of mean stress relaxation increases with the increasing mean strain; an obvious ratchetting takes place in the asymmetrical stress-controlled cyclic tests, and the ratchetting strain depends greatly upon the applied mean stress and stress amplitude, as well as the prescribed stress rate. Moreover, it is found that the temperature on the surface of specimen increases apparently in the uniaxial strain-controlled cyclic tests and the temperature variation becomes more remarkable when the prescribed strain rate is higher. However, the temperature variation is not so apparent in the uniaxial stress-controlled cyclic tests due to much smaller responding strain amplitude.

High-Speed 3D Printing of High-Performance Thermosetting Polymers via Two-Stage Curing

Design and direct fabrication of high-performance thermosets and composites via 3D printing are highly desirable in engineering applications. Most 3D printed thermosetting polymers to date suffer from poor mechanical properties and low printing speed. Here, a novel ink for high-speed 3D printing of high-performance epoxy thermosets via a two-stage curing approach is presented. The ink containing photocurable resin and thermally curable epoxy resin is used for the digital light processing (DLP) 3D printing. After printing, the part is thermally cured at elevated temperature to yield an interpenetrating polymer network epoxy composite, whose mechanical properties are comparable to engineering epoxy. The printing speed is accelerated by the continuous liquid interface production assisted DLP 3D printing method, achieving a printing speed as high as 216 mm h-1 . It is also demonstrated that 3D printing structural electronics can be achieved by combining the 3D printed epoxy composites with infilled silver ink in the hollow channels. The new 3D printing method via two-stage curing combines the attributes of outstanding printing speed, high resolution, low volume shrinkage, and excellent mechanical properties, and provides a new avenue to fabricate 3D thermosetting composites with excellent mechanical properties and high efficiency toward high-performance and functional applications.