|Alternative Title||Failure behaviors and mechanisms of CFRP Laminates subjected to combined thermal and mechanical loadings|
|Thesis Advisor||黄晨光 ; 宋宏伟|
|Place of Conferral||北京|
|Keyword||Cfrp层合板 热-力性能 激光烧蚀 失效行为 多尺度模型|
Due to superior characteristics such as light weight, high specific strength, high specific stiffness and excellent designability, carbon fibers reinforced plastic (CFRP) is being increasingly used in industrial sectors such as aeronautic and aerospace engineering. CFRP composites are usually or accidentally subjected to the combination of complicated or extreme thermal and mechanical loadings. In these cases, it is important to systematically investigate the high temperature thermal-mechanical behavior and ablation mechanism of CFRP composites. When a polymer-matrix CFRP composite is exposed to high temperature environment, complex chemical processes and physical processes may take place. The chemical processes involve viscous softening, fusing, pyrolysis and vaporization of the matrix, the generation, development and oxidation of pyrolytic carbon residue, oxidation and sublimation of carbon fibers. The physical processes involve thermal expansion and contraction, thermally-induced strains, fibers-matrix interfacial debonding, matrix cracking and delamination damage. As a result, the thermo-mechanical properties of CFRP are irreversibly affected by these physical and chemical processes, which significantly reduce the load-bearing capacity of CFRP composites and its structures. Moreover, the thermophysical and mechanical properties of CFRP are not only dominated by the temperature, but also affected by the heating history. The above complicated processes or factors make theoretical modeling of failure behavior and ablation mechanism of CFRP composite under complex thermal and mechanical loadings a very challenging work.
In this paper, the thermo-mechanical properties and laser ablation behaviors of CFRP laminates under complex environments are investigated. The main research includes the following aspects:
1. Experiments on the failure behavior of CFRP laminates under thermal/ mechanical loadings in conventional heating environment are performed. The experiments include tensile and compressive tests at room temperature, tensile and compressive tests at elevated temperatures, and thermal buckling tests. Then, the failure modes and failure strength of CFRP laminates subjected to combined thermal and mechanical loadings are analyzed. In these tests, the effects of different thermo-mechanical loadings, temperatures and heating rates on the failure behavior of CFRP laminates are discussed. Experimental results show that irreversible heat damages of epoxy resin matrix and carbon fibers are obtained under high temperature, and the failure behavior can be attributed to the pyrolysis, oxidization, intralamina cracks and interlamina delaminating. Meanwhile, the residual strength of CFRP laminates after laser irradiation with different laser power is tested, and test results are compared with those obtained from uniform heating conditions. Obvious difference in both the temperature and heating rate are found in these two experimental conditions, and the difference between two heating rates is over two orders of magnitude. The local high temperature caused by the laser irradiation results in the sublimation of carbon fibers.
2. A multi-level analysis model, which is capable of describing mesoscopic and macroscopic behaviors of CFRP laminates under thermo-mechanical loadings, is established. First, using the mesoscopic analysis model and thermal gravimetric analysis (TGA) results of carbon fibers and epoxy matrix, the kinetic pyrolysis parameters and the relationship of different components with temperature and heating rates are obtained. Based on the multi-level analysis model, the macroscopic thermophysical and mechanical property of single CFRP lamina can be readily achieved. Then, the aforementioned multi-level model is combined with the classic laminates theory to study the thermo-mechanical failure behavior and failure threshold of CFRP laminates under high temperatures. Compared with the experimental results and previous analysis model, the newly developed model is able to accurately describe the failure behavior of CFRP laminates under thermal and mechanical loadings with various heating rates.
3. Systematic experiments are carried out to obtain the ablation behavior of CFRP laminates irradiated by continuous laser power at complex environments, such as static nitrogen environment, static air environment, open airflow environment as well as the supersonic wind tunnel environment. The effects of laser power on the ablation of CFRP laminates are discussed, and the three-dimensional ablation morphologies, cross-sectional ablation lines, temperature histories, ablation ratio and ablation rate are obtained. Experimental results show that the surface tangential airflow causes denudation of materials, which further accelerates the ablation of the materials and changes ablation morphologies. Compared with the experimental results in supersonic wind tunnel environment, much smaller ablation rate and ablation affected area are observed in the open airflow environment. As a result, the commonly adopted experimental measure of open airflow is not suitable for the simulation the ablation behavior of CFRP laminates in a real supersonic airflow environment.
4. Analytical model considering multiple mechanisms is established to simulate the ablation behavior of CFRP laminates irradiated by high power laser, and the effects of different environments and laser parameters are investigated. First, using the thermochemical analysis results under different environments, the ablation processes of CFRP composites are decoupled and the reaction mechanisms are quantified. Then, taking advantage the thermochemical ablation model and multi-level analysis model, the ablation behavior of CFRP laminates is simulated. Death-and-birth elements are adopted in the numerical model to accommodate the regression of ablation boundary and heat conduction. To update the thermophysical properties, ablation reaction, and corresponding reaction heat of each element, the form element method is also used. Finally, based on the finite element software ANSYS and APDL programming platform, the ablations processes of CFRP laminates are simulated. Compared with experimental results, the numerical model is capable of describing and predicting the ablation behavior and ablation threshold of CFRP laminates irradiated by high power laser.
|赵伟娜. 复杂热-力载荷下CFRP层合板的失效行为及破坏机理研究[D]. 北京. 中国科学院大学,2018.|
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