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The thermal protection system (TPS) for the reusable reentry spacecraft is designed to protect the structure from extremely high temperatures for many times launch and reentry. To meet the requirements, the TPS requires excellent thermal insulation performance, mechanical properties and strong ability to recover deformation with a minimum lifetime of dozens of missions. In this paper, we fabricated carbon-bonded carbon fiber composites (CBCFs) with a 3D network structure, and the deformation mechanisms under cyclic compression were systematically analyzed. This work demonstrated the excellent resilience behavior of CBCFs with cyclic loading. Quantitative numerical modeling and experimental tests revealed that the CBCFs could completely recover the deformation with strain up to 40% and maintained structural integrity with low energy dissipation (

Carbon-bonded carbon fiber composites (CBCFs) were widely used as thermal insulation materials due to their light weight and ultra-low thermal conductivity. The CBCFs with density of about 0.256 g/cm3 were tested in compression with the modulus and strength evaluated. The in-plane and out-of-plane tests revealed obvious anisotropic behavior of the material, which could be attributed to the fibers distribution. The unloading-reloading tests showed more evidence for the difference of the mechanical behaviors between in-plane and out-of-plane. In addition, we presented a finite element model to predict the mechanical properties of the CBCFs, and the deformation mechanisms as well. The numerical results showed the compressive modulus and strength increased with density following exponential functions. Moreover, the effects of fiber length and the fiber orientation on the mechanical properties were also discussed numerically. The results of this paper are helpful for the design and optimization of these materials for potential applications.

Molecular dynamics (MD) simulations based on a united atom (UA) approach are performed to analyze the mechanical behaviors of polyethylene (PE) under high speed shock compression. With shock loads in a wide range from 0.3 km/s to 2 km/s, the behaviors of shock wave propagations in the polymer are presented. It is found that shock front thickness decreases as the shock load increases and even approaches to zero when the shock load reaches 2 km/s. Hugoniot curves in u s -u p and P-u p are presented, which agree qualitatively with the experimental results. In addition, the molecular morphological evolution is analyzed by the statistical method, which indicates that the major molecular deformation mechanism is bending instead of torsion. More evidences show that how internal mechanism associated with distributions of bond lengths, bond angles, dihedral angles and normalized mean-square radii of gyration.

Is the interfacial strength of graphene/polymer composite on the normal direction higher or lower than that of the polymer matrix? This issue has attracted fewer attention than the interfacial shear properties, but plays a key role in understanding the complex interfacial properties of the graphene/polymer composite as well as the CNT/polymer composite. This work studies the mechanical properties of the graphene/polymer interface on the normal direction by using molecular dynamic simulations. A representative structure of graphene and polymer matrix was constructed to assess the force-separation response of the interface between the graphenes and the polymer matrix including polyethylene, polyurethane and polystyrene. Simulations of this composite systems under uniaxial tension were conducted with pristine or modified graphenes at different stretching velocities. The comparisons of obtained results show that the ratio of the interfacial strength vs the polymer strength plays a key role in determining whether the damage takes place in the polymer matrix or at the interface, hence determining whether the interface is strong or weak. Thus a criterion is proposed to determine the failure behavior of the interface on the normal direction. Furthermore, a theoretical model is established to calculate the interfacial strength in place of the MD results to extend the application of the criterion.

The nanohybrid shish-kebab (NHSK) structure, in which randomly dispersed and wavy carbon nanotubes (CNTs) serve as shish and crystal lamellas periodically decorated onto the CNTs serve as kebabs, has attracted extensive attentions due to its more excellent enhancement in comparison with pristine CNT for polymer matrix composites. In this paper, a new method, based on a modified random sequential adsorption (RSA) algorithm, is proposed to construct the 3D finite element model of the NHSK reinforced composites. The Young’s modulus calculated by this model agrees with the experimental result. Moreover, we present a comparison between the pristine CNT reinforced composite and the NHSK reinforced composite in terms of elastic–plastic behavior. Different load transfer mechanisms are analyzed for the two kinds of composites. Furthermore, we explore the effects of the crystal kebab’s material property, diameter and the periodicity of the kebabs on the elastic–plastic properties of the composite. The results show that the effective mechanical properties of the composites have a large improvement with the existence of crystal kebabs in comparison to the pristine CNT reinforced composite, which indicates that the NHSK structure can improve the load transfer efficiency between the CNT and the matrix and act as another enhanced phase in the composite. The proposed model can be used for the virtual design and optimization of CNT/semicrystalline polymer composites since it has proven capable of assessing the effects of different material and geometrical parameters on the elastic–plastic properties of the composites.

A periodic molecular dynamics (MD) model is proposed to investigate the mechanical properties of the interface between a functionalized single-walled carbon nanotube (SWNT) and matrix, with “end effect” eliminated. The load transfer mechanism in the layer of interphase surrounding SWNT is investigated as the SWNT is sliding in the polymer. It is indicated that the deformation of the interphase is mainly driven by the functional groups, which determines the interfacial properties that is found to be sensitive to the sliding velocity. The effective interfacial shear strengths in different sliding stages and variable sliding velocities are evaluated, and an empirical model is proposed to describe the effect of the sliding velocity and the amount of functionalization on the interfacial strength. Based on the calculation of shear properties of polyethylene (PE) bulk and an equivalent shear load transfer model, we did an original investigation of the interfacial properties determined by the amount of functionalization of SWNT and the sliding velocity. In addition, a criterion is proposed to characterize the strength of the interface. The results show that the assumption of perfect bonding is reasonable for the modeling of these functionalized SWNT/PE composites at a higher scale.

A routine is compiled to describe the spatial distributions of interpenetrating phases by solving the phase field equation. And a 3D random finite element (FE) model based on the phase field method is presented which can characterize the realistic microstructure of the interpenetrating phase composite (IPC) for the first time. Compared with the previous models, the present model is easier to generate at a low cost. The statistical criterion has been employed to determine the critical size of the 3D random FE model. The sample capacity has also been discussed. In addition, the cross-cubic model and the tetrakaidecahedron model are also conducted by finite element method (FEM) for comparison. Furthermore, the Young’s modulus and elastoplastic properties of IPC are predicted by FE models and theoretical methods, and the results of the 3D random FE model accord well with experimental data.

A new multi-scale model is proposed to investigate the relationship between the mechanical properties and microstructure of fiber-reinforced silica aerogel composites. The multi-scale model consists of the aerogel model in nanometers and the composite model in micrometers. The aerogel model is generated to represent the cluster structure of silica aerogels based on a modified diffusion-limited cluster aggregation (DLCA) algorithm, in which the size-dependent interactions between the primary particles are obtained from theoretical derivations. A continuum damage constitutive model is established to represent the behavior of the silica aerogel matrix in the composite by implementing the aerogel model with the discrete element method (DEM). After that, a modified embedded element technique (EET) is proposed to generate the finite element (FE) model of the silica aerogel composite with curve fibers. This multi-scale model is used to investigate the tensile behavior of fiber-reinforced silica aerogel composites, while a comparison with experimental results is presented. The numerical results show that the primary particle size has a significant effect on the Young׳s modulus and the tensile strength of the composites. Moreover, our present model is employed to explore the relationship between the mechanical properties of the composite and the fiber characteristics, including the fiber volume fraction, length and curvature. The predictions indicate that the characteristics of the reinforcing fibers are significant for the mechanical properties of silica aerogel composites. The present multi-scale model can be extended to study the mechanical properties of other aerogel composites.

A geometric generation algorithm is proposed to produce the 3D finite element model to analyze the polymeric composite with randomly dispersed and wavy CNTs, typical in CNT/polymer composite. Considering the model randomness, the statistical criterion has been employed to determine the sample capacity, and then the critical size of FE models has been determined based on the degree of departure from isotropy and the statistical inhomogeneity. Moreover, a comparison is conducted among five types of FE models when CNT distribution is completely random; the differences are not evident among their predictions. However, when predicting the micro-field distribution inside materials, the beam models present a huge difference to the solid models. Furthermore, we explore the effect of the CNT length, orientation and waviness on the elastic–plastic properties of composite by using the FE model describing CNTs as effective continuum solid fibers, and then compare the FE predictions with experimental data.

A new automatic searching & coupling (ASC) technique is proposed to generate the 3D representative volume element (RVE) to analyze the random fiber composite (RaFC) with a wide range of fiber aspect ratios (FAR), particularly the RaFC with a high FAR. Compared with the conventional model, the present model is easier to generate and more time-saving as it eliminates the drawback of free meshing. In addition, the ASC technique can remove the additional stiffness introduced by the embedded element technique (EET), and hence can improve precision and convergence. Moreover, our technique facilitates the direct application of the 3D periodic boundary conditions (PBC) to the RVE. Using the finite element (FE) method, the elastic properties of RVE are calculated, and the critical RVE size is determined. Furthermore, the 3D elastic properties of RaFC are predicted, which accord well with experimental data. Besides, the RVE is employed to explore the influences of FAR, fiber volume fraction (FVF) and the fiber incline angle on the elastic properties of RaFC, and a comparison with theoretical results is also presented.

A new two-level model is proposed to investigate the relationship between the mechanical properties and microstructure of silica aerogels. This two-level model consists of the particle-particle interaction model and the cluster structure model. The particle-particle interaction model is proposed to describe interactions between primary particles, in which the polymerization reaction between primary particles is considered. The cluster structure model represents the geometrical structure of silica aerogels, and it is established using a modified diffusion-limited colloid aggregation (DLCA) algorithm. This two-level model is used to investigate the tensile behavior of silica aerogels based on the discrete element method (DEM). The numerical results show that the primary particle size has significant effects on the elastic modulus and tensile strength of silica aerogels. Moreover, the power-law relationships between tensile properties and aerogel density are dependent on the variation of the primary particle radius with density. The present results can explain the difference among different experimental exponents to a certain extent. In comparison with experimental data within a wide density range, this two-level model provides good quantitative estimations of the elastic modulus and tensile strength of silica aerogels after the size effects of the primary particle are considered. This paper provides a fundamental understanding of the relationship between the mechanical properties and microstructure of silica aerogels. The two-level model can be extended to study the mechanical properties of other aerogels and aerogel composites.