Influence Of Multi-scale Structure Of Fiber Reinforced Resin Matrix Composites On Mechanical Properties

Fiber reinforced resin matrix composites have varieties of applications in aerospace, automobile, architecture and sports equipment industries because of their great performances, such as high specific strength and modulus, easy to design, good fatigue resistance and good corrosion resistance. Especially in aeronautical structures, composites have become one of the most important materials. Fiber composite material has an internal structure of multi-scaled and high heterogeneity of component materials, this results in that the mechanical properties of the fiber-reinforced composite material have a high degree of correlation with its structure. Aimed at the large aircraft development projects established by country mid-long-term scientific and technological development plan, research on the correlation between structure and property of fiber composite, to achieve high performance, low-defect, low-cost objectives, is of great practical significance.Textile structural composites and laminated structural composites have been widely applied in the aerospace industry engineering structural components. Due to the textile structure of reinforcements in textile structural composites, the material internal stress field is highly non-homogenized and severe stress concentration exists in some regions. The position of the stress concentration is highly relevant to the type of textile structure and external load. The laminated structure composite materials can provide enough carrying capacity in the plane direction, but are highly sensitive to the out-plane load due to the weaker performance of the interface region between layers. Delamination is likely caused under the impact load seriously degrade the mechanical properties of laminated structural composites in the successor service process, forming a potential safety hazard. One of the effective ways to solve the above problems is to carry out the fiber reinforced resin matrix composite structure performance numerical analysis and study the correlation among the mechanical behavior, damage mechanism and the internal structure of the composite. This work is helpful for optimizing the structure design and can provide the theoretical basis and technical support for composite products which meet the performance requirements and size requirements, and predicting and avoiding potential safety hazards. Despite the large number of studies which have been carried out at home and abroad, compared with the complex physical processes, numerical analysis is still relatively simplified and lacks a comparative study between the different material structures. Against these deficiencies, combined with the subject background and project sources, this paper focused on textile structural composites and laminated composite. A numerical analysis method was used to study the effect of material organization structure on the material microscopic stress distribution as well as macro-mechanical response. At the same time, in the different structure levels, the correlation of structure and mechanical properties for fiber composite was further studied systematically. The main work and conclusions of this article are as follows:Firstly, for the smallest structural scale in the multi-scale structure composite, fiber scale, the numerical analysis of the mechanical properties of unidirectional composites was carried out, which is the basis of the subsequent analysis on the bigger structural scale. A hexagonal array of fiber was adopted, which is closer to the transversely isotropic. A periodical representative unit cell model was presented with the periodic boundary conditions imposed on it, which ensures not only displacement continuity but also stress continuity in the corresponding boundary of unit cell. The enhanced phase, matrix phase and interface phase in composite were modeled through material constitutive of transversely isotropic(or isotropic), isotropic and cohesive zone model respectively. The microscopic stress distribution in composite under different loads was simulated through finite element model. The equivalent elastic modulus of unidirectional composites was analyzed through homogenization method of composite micromechanics. Based on the analysis of microscopic stress field, the failure strength of the unidirectional composite was predicted by the progressive damage method. The effect of the interface strength on the overall failure of unidirectional composite was discussed. It is shown that the property of fiber dominates the failure strength of the material under the longitudinal load, and the influence of interface strength can be ignored. Under transverse load, matrix damage and interface failure interactively appear, and interfacial properties have an important impact on the whole failure of the material, especially in the transverse shear loads. The improvement of the interfacial bond strength can significantly improve the performance of the material to resist shear failure.Secondly, on the basis of analysis of unidirectional composites, for greater structural scale-fiber bundles scale, the correlation analysis on structure-properties of textile composites was carried out. This paper selected typical two-axis braided composites,1×1braid and2×2braid as the research object. Compared with the orthogonally braided composites, non-orthogonally braid composites have an irregular geometry configuration of fiber bundle, and the internal stress distribution analysis is more difficult. For non-orthogonally braided composites:the traditional rectangular unit cell was abandoned; a periodical parallelogram cell, more suitable for the analysis of the stress field, was adopted; and then the periodic boundary conditions were imposed. Taking into account the change in cross-sectional of the actual non-orthogonally braided fiber bundles, the finite element model, closer to the real configuration, was constructed. On this basis, the effect of textile structure on the elastic properties was discussed, as well as the stress distribution in the materials under different loads. Furthermore, statistical analysis of the stress concentration in the materials was carried out. It is found that the microscopic mechanical analysis is highly sensitive to the geometric architecture; the effect of braid angle on the mechanical properties is significant. The differences between1×1and2×2in effective elastic properties and stress distribution are caused by the structural differences:2×2has greater in-plane Young’s modulus than1×1, the situation of the out-plane Young’s modulus is on the contrary.Thirdly, on the basis of conclusion of unidirectional fiber composites mechanics analysis, in the single ply scale, the mechanical properties of laminated composites were carried out. Starting from the indentation damage mechanics, the damage criterion and material performance degradation, corresponding to different damage in the composites under indentation load, were proposed. And a numerical model of laminated composites subjected to quasi-static indentation was established on the basis of progressive damage mechanics. The evolution of various damages within laminated structure under the quasi-static indentation was analyzed, as well as the effect of laminated structure on damage. It is found that the matrix damage and delamination propagate slowly before the first reduction of load, and then propagate quickly. Furthermore, under the condition of same laminates thickness, the effect of single ply thickness on the indentation damage was discussed. It is shown that the increase of single ply thickness can enlarge the matrix damage and the delamination degree, whereas it can restrain the development of fiber fracture.Finally, on the basis of quasi-indention static analysis, the dynamic mechanical response of laminated composite under the low-speed impact was analyzed. The intra-ply damage including matrix crack and fiber fracture was represented by the continuum damage mechanics (CDM) which takes into account the physical progressive failure behavior in the ply, using the damage variable to describe the intra-ply damage state. The delamination at the interface between two plies was characterized by the cohesive zone model (CZM) which takes into account the normal crack and the tangent slip, using a specific correlation between traction and separation displacement to describe the initiation and development of delamination. A numerical method for the evaluation of composite laminates damage under the low-velocity impact was proposed by CDM and CZM. The correlation between the mesoscale structure and the macroscopic response under impact was constructed effectively by the finite element analysis. The effect of the interlaminar toughness on the impact damage was investigated, which is as yet seldom discussed in detail. The results reveal that as the fracture toughness enhances, the delamination area and the dissipated energy caused by delamination decrease. The contribution of different types of delamination is evaluated and the tangential slip is the dominant delamination during the impact process. Furthermore, the effect of ply orientation on the impact resistance was discussed. It is inferred that more orientation of fiber can make the impact resistance larger.