Yuxi Jia

Investigation of computer-aided engineering of silicone rubber vulcanizing (I)—vulcanization degree calculation based on temperature field analysis

The equation of vulcanization reaction rate of additive liquid silicone rubber is established by experiment, then the mathematical model of vulcanization ratio is constructed. The new concept of increment of vulcanization ratio is introduced, so the numerical analysis of vulcanization ratio is done. According to statistical theory, the numerical computation expressions of crosslinking structure parameters are derived, while the original molecules are distributed in Flory manner. Handling the reaction heat by the way of internal heat source, the partial differential equation of heat transfer is established in the vulcanization molding area, and necessary approximate treatments are done as well. Adopting weighted margin method, the foundational control equations for calculating the temperature field are derived, and two types of thermal boundary conditions are tackled. On the foundation, the numerical simulation theory of vulcanization molding can be studied, and the computer-aided engineering software of vulcanization molding can be designed.

A multiphasic model for the volume change of polyelectrolyte hydrogels

A multiphasic model for the volume change of polyelectrolyte hydrogels that takes into account conservation of mass and momentum is derived. The gradient of chemical/electrochemical potentials of water and mobile ions is taken as the driving force for the volume change of the polyelectrolyte hydrogel, which is damped by the frictional forces between different phases and balanced by the elastic restoring force of the polymer network. Employing the model constructed here, the free swelling of a spherical polyelectrolyte hydrogel immersed in salt solution is simulated by the finite element method. The simulation shows that the polyelectrolyte hydrogel swells from the surface to the interior when the concentration of the external salt solution decreases. The swelling kinetics for ordinary hydrogels with high frictional coefficient between the polymer network and water is controlled by the collective diffusion of the polymer network, while for fast-response hydrogels it is controlled by the ionic diffusion in the hydrogel.

Investigation of computer-aided engineering of silicone rubber vulcanizing (II)—finite element simulation of unsteady vulcanization field

On the basis of construction of the mathematical model of vulcanization ratio and numerical calculation of crosslinking structure parameters and derivation of the control equation of temperature field, two-dimensional space is divided in the manner of triangular element, then the interpolating function is calculated. Thereafter, the calculation of the element variation and collectivity synthesis is done, and the procedures of the numerical simulation of vulcanization process are described in detail. Finally, the computer-aided engineering software of silicone rubber vulcanizing is designed. The rationality of the simulation theory is verified by hardness test and analytical test.

Effective elastic properties and stress distribution of 2D biaxial nonorthogonally braided composites

Due to the interlacement of tows, the architecture of braided composites is complex, especially for the nonorthogonal braid in which the tow has various cross sections along the towpath. The microscopic mechanical analysis is highly sensitive to the geometric architecture, which causes great difficulties for the research on material behavior of nonorthogonal braid. In this article, a finite element model has been proposed to investigate the effective elastic properties and stress distribution of 2D biaxial nonorthogonally braided composites. This research was conducted for two kinds of 2D biaxial braid, 1 × 1 and 2 × 2, using a parallelogram repeated unit cell that is suitable for the description of stress distribution under different load conditions. The differences between 1 × 1 and 2 × 2 in effective elastic properties and stress distribution were discussed. The effect of braid angle on the mechanical properties was also studied. The result reveals that 2 × 2 has greater in-plane Young’s modulus than 1 × 1 with the same tow, fiber volume ratio, and braid angle, although the situation of the out-plane Young’s modulus is on the contrary.

Lattice Boltzmann study of hydrodynamic effects in lamellar ordering process of two-dimensional quenched block copolymers

By incorporating self-consistent field theory with lattice Boltzmann method, a model for polymer melts is proposed. Compared with models based on Ginzburg-Landau free energy, our model does not employ phenomenological free energies to describe systems and can consider the chain topological details of polymers. We use this model to study the effects of hydrodynamic interactions on the dynamics of microphase separation for block copolymers. In the early stage of phase separation, an exponential growth predicted by Cahn-Hilliard treatment is found. Simulation results also show that the effect of hydrodynamic interactions can be neglected in the early stage. For the late stage of phase separation, it is easy to see the effects of hydrodynamic interactions on the ordering process of lamellae phase. From the analysis of structure factor curves, we find that the growth of domains is faster if hydrodynamic interactions are introduced. Furthermore, the scaling of the pattern dynamics is investigated for the late stage at zero thermal noise. By studying the behavior of scaling exponents of the structure factor and the nematic order-parameter correlation function C(nn), we can see that the effects of hydrodynamic interactions lead to bigger growth exponent for both functions.

Self-assembly of T-shaped rod-coil block copolymer melts.

Self-assembled behavior of T-shaped rod-coil block copolymer melts is studied by applying self-consistent-field lattice techniques in three-dimensional space. Compared with rod-coil diblock copolymers with the anchor point positioned at one end, the copolymers with the anchor point at the middle of the rod exhibit significantly different phase behaviors. When the rod volume fraction is low, the steric hindrance of the lateral coils prevents the rods stacking into strip or micelle as that in rod-coil diblock copolymers. The competition between interfacial energy and entropy results in the formation of lamellar structures and the increasing thickness of the lamellar layer with increasing rod volume fraction. When the rod volume fraction is high, the graft density of the planar interface is decreased, which results in space-filling requirements and stretching penalty, thus leading to the stability of nonlamellar structures with curing interface. Furthermore, our results also suggest that the effect of the chain architecture on the self-assembled behavior is remarkable when the rod volume fraction is low, whereas the effect is weak when the rod volume fraction is high.

Comparison of the multiphasic model and the transport model for the swelling and deformation of polyelectrolyte hydrogels

Polyelectrolyte hydrogel is a ternary mixture of water, polymer network and mobile ions. The present paper examined two popular models describing the swelling and deformation behaviors of polyelectrolyte hydrogels, i.e. the multiphasic model and the transport model. The water flow, the network deformation and the ionic diffusion are coupled in the multiphasic model, and the gradient of the fluid pressure is taken as the driving force for the network deformation. However, the water flow is neglected in the transport model with the ionic osmotic pressure taking the role of fluid pressure. Two simplified experiments, i.e. the free swelling of a hydrogel sphere in response to the concentration change of the external salt solution and the bending deformation of a hydrogel strip under an external electric field, are simulated by the two models. Simulation shows that the two models lead to the same predictions for the swelling equilibrium of the hydrogel sphere but different predictions for the swelling kinetics of the hydrogel sphere and the deformation of the hydrogel strip under the external electric field. These are due to the fact that the two models are equivalent in thermodynamic equilibrium situations, but in thermodynamic non-equilibrium situations, the transport model is no longer applicable as it neglects the water flow in the hydrogel and takes the ionic osmotic pressure as a mechanical parameter to play the role of swelling pressure. The present work will be helpful for understanding the hydrodynamics of polyelectrolyte hydrogels and the application of the two models.

Numerical Simulation of Moldable Silicone Rubber Vulcanization Process Based on Thermal Coupling Analysis

Abstract The kinetic model of liquid silicone rubber hydrosilation is determined with measuring apparatus for vulcanization extent. The definition of increment of vulcanization ratio is introduced, so the numerical computation expression of the full dose of vulcanization ratio is obtained. The foundational control equation of the vulcanization temperature field accompanied by reaction heat is derived. Then two types of thermal boundary conditions are analyzed. The implementation procedures of the finite element simulation are described in detail. Finally, the finite element simulation software of hot vulcanization process of moldable silicone rubber is designed, and the rationality of the simulation theory and algorithm is verified by one typical engineering example and its hardness test as well as its engineering application.

A new structure-related model to predict the permeability of non-crimp fabric preform

The permeability of the fabric preform is a critical input parameter for analyzing the liquid composite molding impregnation process. However, the permeability prediction is challenging due to its complex dependence on the fabric structure. In this paper, a novel analytical model is developed to predict the permeability of non-crimp fabric preform based on the relation between the pressure drop and geometric parameters of the microchannel cross section. The model takes into account four structural parameters including the width, the height, the semi-major axis length of the ellipse section of fiber bundle and the distance between fiber bundles. The permeability of the unit cell is calculated by the presented analytical model and the finite element simulation, respectively. The results show that the channels between fiber bundles play an important role in determining the fabric permeability. The structural parameters of the unit cell have important effects on the permeability. The new structure-related permeability model can accurately predict the permeability of the non-crimp fabric preform in a certain range.

Lightning Damage of Carbon Fiber/Epoxy Laminates with Interlayers Modified by Nickel-Coated Multi-Walled Carbon Nanotubes

The numerical model of carbon fiber reinforced polymer (CFRP) laminates with electrically modified interlayers subjected to lightning strike is constructed through finite element simulation, in which both intra-laminar and inter-laminar lightning damages are considered by means of coupled electrical-thermal-pyrolytic analysis method. Then the lightning damage extents including the damage volume and maximum damage depth are investigated. The results reveal that the simulated lightning damages could be qualitatively compared to the experimental counterparts of CFRP laminates with interlayers modified by nickel-coated multi-walled carbon nanotubes (Ni-MWCNTs). With higher electrical conductivity of modified interlayer and more amount of modified interlayers, both damage volume and maximum damage depth are reduced. This work provides an effective guidance to the anti-lightning optimization of CFRP laminates.