Guodong Fang

Modeling of one-dimensional thermal response of silica-phenolic composites with volume ablation

The silica-phenolic composites experience volume ablation when exposed to a radiant heat flux. Based on the analysis of mechanisms during volume ablation, a mathematical model was developed to predict the one-dimensional thermal response of the ablative material in this paper. After discretizing the space and time domain, the governing equations were described using the implicit finite differential form. Both the heat-mass transfer process and the moving boundary caused by thermal expansion, as well as the variation of pore pressure due to concentration and flow of the decomposition gases were considered in the formulation of the model. The thermal response of silica-phenolic composites during the volume ablation, including temperature distribution, pore pressure distribution, volume fraction of the phase components and degree of decomposition, were calculated by the proposed model. The time-dependent temperature progressions at different material depths were measured using a solar radiation heating experiment platform. The experimental and calculated temperature profiles are in good agreement.

Modeling of Nonlinear Mechanical Behavior for 3D Needled C/C-SiC Composites Under Tensile Load

This paper established a macroscopic constitutive model to describe the nonlinear stress–strain behavior of 3D needled C/C-SiC composites under tensile load. Extensive on- and off-axis tensile tests were performed to investigate the macroscopic mechanical behavior and damage characteristics of the composites. The nonlinear mechanical behavior of the material was mainly induced by matrix tensile cracking and fiber/matrix debonding. Permanent deformations and secant modulus degradation were observed in cyclic loading-unloading tests. The nonlinear stress–strain relationship of the material could be described macroscopically by plasticity deformation and stiffness degradation. In the proposed model, we employed a plasticity theory with associated plastic flow rule to describe the evolution of plastic strains. A novel damage variable was also introduced to characterize the stiffness degradation of the material. The damage evolution law was derived from the statistical distribution of material strength. Parameters of the proposed model can be determined from off-axis tensile tests. Stress–strain curves predicted by this model showed reasonable agreement with experimental results.

Micro-tomography based Geometry Modeling of Three-Dimensional Braided Composites

A tracking and recognizing algorithm is proposed to automatically generate irregular cross-sections and central path of braid yarn within the 3D braided composites by using sets of high resolution tomography images. Only the initial cross-sections of braid yarns in a tomography image after treatment are required to be calibrated manually as searching cross-section template. The virtual geometry of 3D braided composites including some detailed geometry information, such as the braid yarn squeezing deformation, braid yarn distortion and braid yarn path deviation etc., can be reconstructed. The reconstructed geometry model can reflect the change of braid configurations during solidification process. The geometry configurations and mechanical properties of the braided composites are analyzed by using the reconstructed geometry model.

Ablation mechanism and properties of silica fiber-reinforced composite upon oxyacetylene torch exposure

The mechanism of mass loss and endothermic properties of silica fiber-reinforced phenolic composites during ablation were investigated in this paper. A theoretical prediction model combining the surface ablation theory and heat transfer theory of heat shield was developed to study the surface ablation behavior. In the formulation of the mathematical model, the effect of the moving boundary on the thermal response was considered, which results from the surface recession of the material in the thickness direction during ablation. The surface ablation recession rate and wall temperature of silica fiber-reinforced phenolic composite specimen were measured using an oxyacetylene torch experimental platform. Then, the efficiency of the model was verified by comparing calculation and experimental results. According to the principles of energy conservation on the ablated surface of the material, the proportion formulas of the heat absorption induced by individual endothermic mechanisms and the total heat absorption were derived. Similarly, the proportions of the mass loss caused by individual mass loss mechanisms were also given. Finally, variations of the ablation properties of the silica fiber-reinforced phenolic composites versus thermal exposure time were calculated and analyzed.

High-temperature constitutive model for three-dimensional needled C/C-SiC composite under tensile loading

Tensile experiments of three-dimensional needled C/C-SiC composite from room temperature to 1800℃ were performed to investigate tensile behavior. The damage characteristics and macroscopic mechanical behavior of the composite are relevant to the testing temperature and off-axis angles of the tensile loading. The tensile strength increased while the modulus decreased with the increase of temperature. A high-temperature nonlinear constitutive model was established to analyze the nonlinear stress–strain relationship of the composite. Plastic strain accumulation and stiffness degeneration were described by the plasticity and damage theories. The effect of temperature on the tensile behavior of the composite was particularly considered in this model by introducing a thermal damage variable. The proposed constitutive model can predict the stress–strain behavior of the material subjected to different off-axis tensile load, and at different temperatures. Fairly good agreement was achieved between the predicted and experimental results.

Compressive and flexural behavior of carbon fiber-reinforced PPS composites at elevated temperature

ABSTRACT The effect of temperature on the mechanical behavior of carbon fiber reinforced polyphenylenesulfide (PPS) composites was investigated by compressive and flexural tests from ambient temperature up to 150°C. The failure morphologies of the C/PPS composites were analyzed to identify the variation of failure modes. Related results showed that the mechanical behavior of C/PPS composites decreased severely with the increase of temperature due to the softening of matrix. The PPS resin film tensile test was carried out and the PPS matrix behavior was recognized as the main factor to dominate the mechanical behavior of composites under compressive/flexural loading at elevated temperatures. It can be found that there was an approximate linear relationship between the compression properties of C/PPS composites and the PPS matrix. The dependence of failure modes of composites on temperatures was closely related to the mechanical behavior of PPS matrix.

Mechanism-based strength criterion for 3D needled C/C–SiC composites under in-plane biaxial compression

ABSTRACT Biaxial compressive experiments for 3D needled C/C–SiC composites parallel to the nonwoven cloth were conducted. The failure mechanisms and mechanical properties of the composites were greatly related to the biaxial compressive stress confinement ratio, R. It was found that out-of-plane shear failure controlled the failure of the composites. The failure shear plane was aligned with one of the major loading axes for R (0:1 or 1:3), while the failure shear plane occurred along both loading axes for R (1:2 or 1:1). Compared with uniaxial strength, the biaxial compressive strength increased obviously, which also significantly depended on the value of R. Based on the failure modes, a modified twin-shear strength criterion was established to predict the failure surface of 3D needled C/C–SiC composites under biaxial compressive loading.

Graded negative Poisson’s ratio honeycomb structure design and application:

A sub-domain size optimization method is conducted to design degraded negative Poisson’s ratio honeycomb structures, which could fully utilize the material behavior, such as minimum weight, maximum stiffness and maximum strength etc. The flexural stiffness of negative Poisson’s ratio honeycomb structure is greatly associated with the geometry size, which are obtained by geometry size parameter analysis from the representative volume cell of negative Poisson’s ratio honeycomb sandwich structure. A three-point bending specimen is designed by sub-domain size optimization method, which is verified by bending experiments. The optimal three-point bending specimen with graded negative Poisson’s ratio honeycomb core have the largest bearing capacities and relatively lower weight. The variable thickness wing structure with graded negative Poisson’s ratio honeycomb core is also designed by using the sub-domain size optimization method.

Mechanical analysis of three-dimensional braided composites by using realistic voxel-based model with local mesh refinement:

The elastic and failure analysis of three-dimensional braided composites is conducted by using realistic geometry model with local mesh refinement. The realistic geometry model is reconstructed by using micro computed tomography images. The voxel meshes are utilized to overcome the difficulties of mesh discretization for realistic geometry model of three-dimensional braided composites with complex meso-geometrical configurations. In order to improve the computational efficiency, the local voxel meshes at the braid yarn boundaries are refined to capture the detailed geometries of braid yarn and reduce the number of mesh. The stress averaging technique is applied to alleviate the local artificial stress spurious introduced from voxel meshes at braid yarn boundaries. Three kinds of computation models are used to predict the tensile properties of the braided composites, which are also compared with experimental results. The effects of braid yarn twist angle and mesh sizes on the predicted tensile behavior of the braided composites are studied further. A systematic way is provided to analyze mechanical properties of three-dimensional braided composites by using realistic voxel-based model.