Papers by Author: Jia Yu

Abstract: It is widely acknowledged that decisions made in the early design stages have a greater influence on the final product than those made in the later stages. In the early stages of a conventional design process, composite structures are designed without sufficient consideration being given to limitations of composite manufacturing process. Quite often some of composite designs cannot be produced with special performance requirement or cannot be produced at a reasonable cost. To resolve this drawback and achieve the manufacturable designs for composite product, a knowledge-based decision-making tool was introduced and applied to support the composite design optimization. Firstly, the current development of DFM techniques in composite domain, as well as the main problems in performing DFM for composites is analyzed. Secondly, some of key issues in the composite manufacturing process that should be considered during the composite design process are presented. And then, a knowledge-based DFM system for composite manufacturing process is proposed, focusing on the system decision task, the system framework and the flowchart. Finally, the system is illustrated with a case in autoclave process.

Abstract: Microarc oxidation (MAO) is a relatively convenient and effective technique to deposit ceramic coatings on the surfaces of Al, Ti, Mg and their alloys. This technique can introduce various desired elements into titania-based coatings and produce various functional coatings with a porous structure. Microarc oxidized (MAO) TiO2-based coatings on titanium alloy were formed in electrolytes containing aluminate and ZrO2 particles. The phase composition of the samples was analyzed by glance-angle X-ray diffraction and the surface morphologies of the samples were observed by a scanning electron microscopy (SEM). In addition, the element concentrations on the surfaces of the samples were measured by an energy dispersive X-ray spectrometer. The experiment results indicated that: MAO coatings, which are porous structures and exhibit good interfacial bonding to the substrate, may possess specific surface structures such as crystal phase, non-equilibrium solid and complex mixed-compounds since complex plasma physical and chemical reactions.

Abstract: Ti3SiC2 is one of the nano-layered ternary ceramics Mn+1AXn, where M is a transition metal, A is an A-group (mostly IIIA or IVA) element, and X is C or N. With the filler of Ag-Cu-Ti, the brazing of Ti3SiC2 has been conducted at 800°C–950°C for 5-10min under 3.5×103Pa in a vacuum. The phase composition and microstructure of the joints were investigated by XRD, SEM AND EPMA. The diffusion of Cu element in fillers through the reaction zone toward Ti3SiC2 is the main controlling step in the bonding process. Joint strengths were evaluated by three point bending test. The maximum flexural strength of joints reaches is 306±11MPa, which is lower than the bending strength of Ti3SiC2, obtained under the same condition.

Abstract: Unidirectional carbon fibre reinforced resin matrix composite (CFRP) with different fibre volume fraction are stretched dynamically under static load in SEM, initiation and propagation mechanism of crack is in-situ observed, and tensile fracture of specimens is also observed. The results show that: Microscopic cracks are mainly originated from fracture of fibre, numerous fibre cracks transfixion each other in form of matrix or interface cracking, and cause failure of CFRP. Microscopic crack propagation path is related to the thickness of matrix layer between fibres. Propagation of single fibre crack at interface accord with description of microscopic crack deflection criterion for fibre reinforced composite, but the crack deflection criterion cannot descript microscopic crack propagation mechanism of unidirectional CFRP effectively, because distribution discreteness of fibre and its strength are not considered.

Abstract: In this paper, the finite element software——ABAQUS Computing Platform is used to build two-dimension unit cell of the composite one-way slab model. Compute it by two ways: 1)supposing that the fiber and matrix are elasticity, but they are different for material parameters, and this assumption is generally used. It can be computed directly in the ABAQUS software; 2)supposing that the fiber is elasticity, but the matrix is the viscoelasticity. For the matrix, its viscoelastic character can be simulated by the solid constitutive model with three parameters. The maximum stress (MS) judgment criterion is used to determine whether the matrix is destroyed. If the matrix is destroyed, the stiffness of the whole matrix would be degraded by means of the degradation factor, which determined by the Camanho parameter degradation pattern. Compare the stress and strain curves got through the numerical simulation and through the experiments, and find that the second situation is similar to the experiment’s result.

Abstract: The preparation technologies of thermal barrier coating by the Electron Beam Physical Vapor Deposition (EBPVD) technique were briefly introduced in this paper. And design principal of thermal barrier coating is discussed, at the same time the selection of raw material was also taken into account. On the basis of several assumed perfect conditions, a reasonable Finite Element Analysis (FEA) physical model was built up in order to exactly describe the whole deposition process. Taking the advantage of the large-scale commercial software of FEA, the distribution of residual stress and the possible displacement tendency were obtained. The analysis results show that: with the increasing substrate preheating temperature, the inter-laminar shear stress increases but the axial residual stress decreases. And the probability of cracking after de-bonding tends to enhance as the thickness of deposition coating is increased. Also it is verified that the FE model has produced little numerical error.