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Research Papers

A Computational Model for Surface Welding in Covalent Adaptable Networks Using Finite-Element Analysis

[+] Author and Article Information
Kai Yu

The George W. Woodruff School
of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332

Qian Shi, Tiejun Wang

State Key Laboratory for Strength and
Vibration of Mechanical Structures,
School of Aerospace Science,
Xian Jiaotong University,
Xian 710049, China

Martin L. Dunn

SUTD Digital Manufacturing
and Design (DManD) Centre,
Singapore University of Technology
and Design,
Singapore 138682, Singapore

H. Jerry Qi

The George W. Woodruff School
of Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332
e-mail: qih@me.gatech.edu

1Corresponding author.

Contributed by the Applied Mechanics Division of ASME for publication in the JOURNAL OF APPLIED MECHANICS. Manuscript received March 31, 2016; final manuscript received May 17, 2016; published online June 22, 2016. Editor: Yonggang Huang.

J. Appl. Mech 83(9), 091002 (Jun 22, 2016) (11 pages) Paper No: JAM-16-1163; doi: 10.1115/1.4033682 History: Received March 31, 2016; Revised May 17, 2016

Covalent adaptable network (CAN) polymers can rearrange their macromolecular network by bond exchange reactions (BERs), where an active unit attaches to and then replaces a unit in an existing bond and forms a new bond. When such macromolecular events occur on the interface, they can contribute to surface welding, self-healing, and recycling of thermosetting polymers. In this paper, we study the interfacial welding and failure of CANs involving both interfacial normal and shear stresses. To do this, we incorporate our recently developed multiscale model for surface welding of CANs with a cohesive zone modeling approach in finite-element method (FEM) simulation. The developed FEM paradigm involves a multiscale model predicting the interfacial chain density and fracture energy, which are transferred to a cohesive zone model to establish the surface traction-separation law. The simulations show good agreement with experimental results on the modulus and strength of welded samples. They also provide understanding of the interactions between surface welding and material malleability in determining the final mechanical properties of polymer structures. The developed FEM model can be applied to study other complex welding problems, such as polymer reprocessing with nonregular particle size and shape.

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Figures

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Fig. 1

A schematic of the developed multiple length scale constitutive model for the surface welding effect of CANs. (a) ABER occurs on a macromolecular chain, (b) this event can occur within a network and at the interface where macromolecular chains cross the interface, (c) the welding occurs when the two surfaces are in contact, and (d) two samples are welded together[4].

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Fig. 2

Mechanical properties of welded samples. (a) Experimental procedure to weld two pieces of epoxy sample. (b) Typical stress–strain curves of the thermosetting samples after being welded for different time periods. Inset view shows the appearance of the welded sample during experiment. The welding temperature is 180 °C, and the applied pressure is 40 kPa.

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Fig. 3

(a) Experimental procedure to welded three epoxy cylinders and (b) typical force–displacement curves as a function of welding time. The welding temperature is 180 °C.

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Fig. 4

Schematic view of the linear traction separation law

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Fig. 5

Schematic view of the DCZM

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Fig. 6

FEM simulations on the surface welding of flat interface. (a) Comparison of experimental and simulated stress–strain curves as a function of welding time. (b) The crack propagation in both simulation (top figure) and experiment (bottom figure). States ① and ② correspond to the data point ① and ② in (a). (c) Interfacial normal stress and shear stress distribution at different states of crack propagation. The black line indicates the state right before the break of the first contact pair. A positive normal stress indicates stretching. (d) Summary of the strength of a welded sample as a function of welding time and temperature. For (a)–(c), the welding temperature is 180 °C, and the welding pressure is 40 kPa. For (d), the welding temperature is 180 °C or 160 °C, and the welding pressure is 40 kPa.

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Fig. 7

Analysis of the mechanical properties of welded polymer structure. (a) Snap shots during the simulation and experiment. (b) Interfacial normal and shear stress distribution before and after unloading step. The welding time is 60 min. (c) Interfacial normal and shear stress distribution after unloading as a function of welding time. (d) Summary of maximum stretching force and shape fixity as a function of welding time in both experiments and simulation.

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Fig. 8

(a) FEM simulations of the welding enabled polymer reprocessing. The welding pressure is 90 kPa, and the welding time is 60 min. (b) The porosity and Mises stress distribution in the unloaded structure with different welding time and pressure applied.

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Fig. 9

Normalized stretch ability of reprocessed sample in both experiment and simulation

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Fig. 10

(a) Normalized stretch ability as a function of reprocessing pressure and time. (b) Porosity of reprocessed sample and the maximum interfacial residual stress as a function of reprocessing pressure and time. The maximum residual stress is located on the end points of interface.

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Fig. 11

The simulated stress–strain curves of welded CANs with different sizes of imperfection

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