Study On Soft-mold Aided Co-curing Process For Advanced Composite Grid-stiffened Structure

Advanced composite grid-stiffened structure (AGS) is a new type of lattice structures which combines the characteristics of material technique and structure design. Due to the advantages such as light weight, high strength, corrosion resistance, high damage tolerance and environmental robust, the structure has been widely used in aviation, aerospace, shipping etc. and became a kind of competing engineering structure, However, its application would be discouraged because of the extremely complicated coupling behavior between curing, temperature and pressure field etc. during the manufacturing process, and due to the complex fabrication process, low quality product, difficult process design and etc. factors. Therefore, the research of technical mechanics during the manufacturing process for AGS has been carried out in the thesis. Firstly, based on the review of existing manufacturing process, an improved technical scheme called soft-mold assisted co-curing process was proposed, which employs an integrated silicon mold as the mandrel. Then, the multi-fields coupling behavior was studied by experiments and numerical simulations, respectively to examine the feasibility of the provided scheme. Lastly, by the integrated software package AGSCURE developed by author, a numerical simulation of a typical AGS component in the scheme was worked out, and some suggestions for the co-curing process execution were proposed. The research contents in the thesis are a part of the project of National Key Science Foundation in China "The research of multifunction super lightweight structural design theory and optimization for the near space aerocraft"(90816025) and the National Natural Science Foundation in China "The study of soft-mold aided co-curing process and parameters optimization for the advanced composite grid-stiffened structures"(10702012).The main research contents and achievements can be summarized as follows:1. Experimental study on soft-mold aided co-curing process and establish the on-line monitoring scheme with embedded fiber Brag grating sensors.(1) The experimental study of AGS specimens manufactured by the soft-mold aided co-curing process indicates that (a) reducing the requirements of molds assembly and position by employing a silicon rubber mandrel,(b) simplified the de-molding operation,(c) enhancing the compaction of fibers bed and quality of the interface between skin and grids. Hence, it demonstrates the feasibility of the provided scheme. (2) An on-line monitoring scheme employing embedded improved fiber Brag grating sensors was proposed for acquiring the variations of temperature and strain in AGS during the manufacturing process, and the acquired data shows the feasibility and effectiveness of the soft-mold aided co-curing process.2. Numerical simulate the multi-fields coupling behavior of soft-mold aided co-curing process of AGS.(1) In the study of advanced composite manufacturing process, most of the analysis models just focus on advanced composite structure parts and ignore the effect of molds and aided materials on the curing behavior of advanced composite. However, the simplified model is inappropriate for the thermal conduction research of soft-mold aided co-curing process for AGS. Therefore, the author suggested that the issue would be solved by defining a contact constraint between the molds and AGS. It was proved that the model would counterpoise the requirements between analysis precision and simulation efficiency by the simulation of the soft-mold aided co-curing process for an AGS component.(2) The variation of resin thermo-mechanical properties, mismatch of thermo-mechanical properties between reinforced fiber and resin matrix and different of stiffness between sub-structures etc. are the sources of residual stresses in an advanced composite structure. In this thesis, an equivalent elastic method combining the laxation effect of resin and mesomechanics model of composite for depicting the anisotropic viscoelastic properties of advanced composite materials was proposed and applied in the analysis for improving the accuracy and effectiveness of the residual stresses prediction. Employing the model, the feature of residual stresses development in AGS during the soft-mold aided co-curing process was investigated by a series of numerical simulations.(3) A precise technologic gap is the guarantee for applying a uniform and synchronous pressure on AGS during the soft-mold aided co-curing process. Therefore, an improved means for design the silicon rubber mandrel was proposed which combines the elastic contact theory and finite element method, the effect of technologic gap on the distribution of contact pressure provided by the swelling soft-mold was investigated by a series of numerical simulations, and the advantages of it was shown by contrasting to the traditional thermal elastic means.3. Develop an integrated software package AGSCURE for simulating the scheme of soft-mold aided co-curing process, and discuss the effect of technical parameters on the product quality of AGS. (1) Based on the results of experiments and numerical simulations, an integrated software package AGSCURE for simulating the soft-mold aided co-curing process was developed. The software package would be useful for the process design for AGS.(2) Based on a parametric study employing the AGSCURE, the discipline of geometric size of AGS on the curing and resin flow behavior was investigated. For example, the distribution of temperature is mainly effected by the width of ribs while the variation of resin viscosity is mainly effected by the height of ribs, and the autoclave pressure can’t drive the resin flow and fiber bed compaction in an AGS effectively.The scheme, analysis models, method and conclusions of soft-mold aided co-curing process proposed in the thesis would be benefit for engineers in design a low-cost and integral manufacturing process of AGS.