Pei Yongmao

Finite Element Modeling of 3D Orthogonal Woven C/C Composite Based on Micro-Computed Tomography Experiment

AbsractTwo-dimensional images of C/C 3D orthogonal woven composite were captured by X-ray micro-computed tomography (μCT). The μCT data reveal comprehensive meso-geometrical information about the carbon fiber tows, carbon matrix, and void defects etc. The fibers tows are characterized consisting of the cancroids of a tow, the area and aspect ratio of its cross-section. A statistical analysis of the volume fraction and positioning of the void defects in the 3D orthogonal woven architecture is performed based on 2-D micro tomography images. The tabulated statistics are sufficient to generate the virtual specimen, which shares the same statistical characteristics of the C/C composite and the void defects are included. Three-point bending experiment and simulation are carried out and the results show that the finite element model including the void defects gives more accurate results. The finite element model will give some highlights to the numerical simulation approach of the C/C textile composite under thermal, mechanical and oxygen coupled service environment. And the numerical techniques for modelling such kind materials with woven architecture and void defects are recommended.

Effect of Residual Stress on the Magnetoelectric Properties of Terfenol-D/PZT/Terfenol-D Laminates

The effect of residual stress on the magnetoelectric properties of terfenol-D/PZT/terfenol-D laminates is studied. The sandwich structure composites with two longitudinally magnetized terfenol-D plates and one transversely polarized Pb(Zr0.52Ti0.48)O3 plate are manufactured under different uniform and constant magnetic fields. The magnetic plates will deform before adhesion. Therefore, residual stress is induced by the mismatched strain between laminates when the magnetic field disappears. The experimental results show that magnetoelectric coefficient is improved about 130% for sandwich structure composites with residual stress. It can be explained that the proposed method can improve the interface mechanical coupling effect. Therefore, the magnetic energy can be transferred effectively to electric energy through the mechanical deformation. At the same time the strain derivative de/dH is enhanced by residual stress. Thus, the electric polarization response is increased under a same disturbed magnetic field.

An Improved Method of Designing Isotropic Multilayered Spherical Cloak for Electromagnetic Invisibility

The classic anisotropic spherical cloak can be mimicked by many alternating thin layers of isotropic metamaterials [Qiu et al. Phys. Rev. E 79 (2009) 047602]. We propose an improved method of designing permittivity and permeability in each isotropic layer, which eliminates the jumping of the refractive index at the interface. Multilayered spherical cloaks designed by the present method perform much better than those by Qiu et al., especially for forward scattering. It is found that the ratio of layer thickness to the operating wavelength plays an important role in achieving invisibility. The presented cloak should be discretized to at least 40 layers to meet the thickness threshold corresponding to 10% scattering.

Numerical Analysis of Thermodynamic Behaviour of Through-Thickness Stitched Sandwich Laminate

Effects of stitching angle on mechanical properties, thermal protection capability and induced thermal stress of stitched sandwich laminate (SSL) are numerically analyzed by ABAQUS codes. Interest centers on the potential for microcracking in the vicinity of the through-thickness stitches and the skins/foam interfaces. Two numerical models, in-depth heat transfer and thermoelastic deformation, are coupled to yield the transient response of the SSL. Six different stitching angles are considered and the simulation results showed that: the heat conductivity ability of the SSL is improved as the stitching angle increasing, which alters the mechanical behaviour and the thermal stress state of the SSL.

Development of designing lightweight composites and structures for tailorable thermal expansion

Thermal expansion coefficient is an important parameter of thermal physical properties. Materials with zero thermal expansion (ZTE) are urgently needed in engineering applications, particularly in aerospace, precision instruments, and civil engineering, where the control of thermally induced expansion and stress over a wide range of temperatures is of significance. On the other hand, thermal expansion compensation requires materials to possess negative thermal expansion (NTE) and even a desirable positive thermal expansion (PTE). For example, thermal expansion modulation can be achieved via composites with an appropriate ratio of NTE inclusions and PTE substrates. In general, engineering applications have a serious need for materials and structures with tailorable CTEs. Hence, development of material with tailorable thermal expansion is of important scientific significance and engineering applications. At present, the available range of CTE in engineering materials is quite narrow. To obtain wide range of CTE, most of current researches are mainly concentrated on bulk materials and composites. Specifically, few bulk materials with low and negative CTEs have been reported. FeNi-based Invar alloysexhibit low CTEs because of the magneto-volume effect which, however, only exists under the Curie temperature (about 100°C). A family of ceramics with isotropic NTE and metal oxides with anisotropic NTEs, has been widely studied. Unfortunately, the inherent brittleness and low fracture toughness of the NTE ceramics restrict their usage in load-bearing applications. Fiber-reinforced composites are capable of providing tailorable CTEs because some fibers, such as carbon and graphite fibers, possess negative axial CTEs. However, largely different CTEs between the fiber and the matrix always result in matrix cracking under large temperature variation. Besides, through topology optimization of the three-phase composites, ZTE and the maximum uniaxial/biaxial stiffness characteristics can be obtained. However, the optimized geometries are still too complex for practical manufacture and engineering applications. It is important to integrate tailorable CTE with lightweight and robust mechanical properties for the materials. Lightweight composites attract intensive studies by virtue of their low density, excellent mechanical properties as well as tailorable thermal expansion coefficient. This paper firstly summarizes the significant engineering applications of thermal expansion controlling as well as the bulk materials with low and negative thermal expansion. Moreover, research achievements of the laminate, particle reinforced and topologically optimal composites with tailorable thermal expansion are detailly introduced. Finally, we present the latest research progress including geometry design methods, mechanisms, fabrication process and experimental characterization of tailorable thermal expansion for lightweight lattice composites. The design of lattice materials which are assembled through dual-material members, and honeycomb-like hybrid lattices which are composed of inner triangle and outside honeycomb initiates the macro scale lightweight lattice composites for tailorable CTE. However, the lattice composites with bending-dominated members display less stiffness compared with stretch-dominated lattices. Moreover, the complicated geometrical connections of the members lead to manufacture complexity. Triangle lattice composite exhibits tailorable CTE and are used as the basic unit to construct other lattice configurations such as the near zero thermal expansion lattice structures. Thus, numerous studies find the planar lattices and structures design based on this unit. With the development of advanced manufacturing technology, the study of the lightweight lattice composites will be promoted more deeply.