【作者】 王俊；

【导师】 杨玉玲；

【作者基本信息】 东北大学 ， 物理学， 2022， 硕士

【摘要】 碳纤维增强复合材料(CFRP)具有密度低、热膨胀系数小、较好的减震性能以及良好的抗疲劳性能等优异性能,广泛用于航空航天的结构部件中。由于CFRP制作成本高,且在受到冲击载荷过程中容易发生脆断和基体分层现象,因此实际应用中CFRP通常与轻质金属材料胶接制备混合结构部件。接头性能是影响结构部件服役安全性和可靠性的关键因素。为了提高胶接接头的强度和牢固性,在胶接之前,必须对CFRP表面树脂层和污染物进行清洗,以获得具有更高润湿性、更高胶接强度的界面。激光清洗不仅可以快速去除CFRP表面树脂及污染物,同时可对CFRP表面进行改性,增加胶粘剂对CFRP的亲和力及胶接强度。然而,目前对CFRP激光清洗机理的研究尚不充分,清洗工艺对接头性能的影响也缺乏系统研究。这些研究对提高航空航天零部件使用寿命具有重要价值。本文首先利用COMSOL Multiphysics模拟软件,采用高斯热源,建立了激光清洗CFRP的仿真模型,研究激光与CFRP相互作用过程中温度场、清洗深度以及热应力场与激光清洗工艺之间的关系,阐明激光清洗机理并为激光清洗工艺提供指导;在模拟计算结果的基础上,开展激光清洗CFRP的实验研究,详细研究激光清洗工艺对CFRP的表面形貌、润湿性以及CFRP与TC4合金胶接接头力学性能的影响机理。主要工作内容及结论如下:建立了激光清洗CFRP的一维热传导方程,并分析了激光清洗CFRP的主要机理,包括激光烧蚀、热应力两种过程。一维热传导方程的初步计算结果表明,激光清洗CFRP时激光功率存在阈值条件。但是由于一维热传导方程过于理想化,为后续的三维模型及实验提供参考。在一维热传导方程基础上,综合考虑热传导、热对流、热辐射等因素,建立了激光清洗CFRP三维仿真模型,研究了激光与CFRP相互作用过程中温度场和热应力场的分布特性以及CFRP表面的激光清洗机理。分析了温度场、热应力场以及清洗深度与激光清洗工艺参数之间的关系。仿真模拟结果表明,在只考虑相变影响的条件下,CFRP表面的激光清洗机理主要表现为激光烧蚀和热应力两种机制同时作用。计算结果表明,当激光功率为74W,扫描速度为1000mm/s时,CFRP内部碳纤维热应力可达到114MPa,该值大于树脂层与基底之间的粘附力,能够彻底清除表面的树脂而不损伤下层的碳纤维,清洗效果最佳。这为优化激光清洗实验工艺提供了重要参考。在此基础上,为激光清洗实验制定了较窄的激光功率窗口(40W-80W),并确定了扫描速度为1000mm/s。在模拟计算结果的基础上,开展了激光清洗实验研究,主要研究了激光功率密度对CFRP表面形貌、成分分布及其对胶接环氧树脂胶的润湿性能。实验结果表明,扫描速度为1000 mm/s时,激光功率为50W时CFRP表面树脂可完全去除且不损伤碳纤维。激光清洗后CFRP表面粗糙度增加,同时,表面自由能随着激光功率密度增大呈现出先减小后增大的变化趋势,CFRP表面粗糙度的变化及表面自由能的提高有效改善了CFRP表面对胶接环氧树脂胶的润湿性能。实验结果表明,除了激光烧蚀和热应力作用外,还可能存在光致分解和光致振动过程,从而使得彻底清洗干净所需的实际激光功率(50W)较模拟计算结果(74W)偏低。最后,研究了不同激光功率密度对CFRP表面硬度、弯曲强度及CFRP与TC4胶接接头力学性能的影响,并对激光清洗前后胶接接头破坏机理进行了分析。研究发现,激光清洗后CFRP表面洛氏硬度提高,随着激光功率密度增大,清洗后的CFRP的弯曲强度呈现先增大后减小的变化特点。激光清洗后的CFRP表面与胶接树脂胶之间的润湿性能显著提高,从而提高了CFRP与TC4接头的胶接强度。胶接接头拉伸实验结果表明,胶接强度随着激光功率密度增大也呈现出先增大后减少的变化趋势。未清洗的CFRP和TC4胶接接头破坏形式表现为界面破坏。而低功率密度(7.0kW/cm2)清洗后的CFRP与TC4的胶接接头则呈现出内聚破坏和纤维撕裂破坏的特征。当激光功率密度大于8.8 kW/cm2时,清洗后的CFRP与TC4胶接接头主要表现出纤维撕裂破坏的特征。更多还原

【Abstract】 Carbon fiber reinforced polymer(CFRP)is widely used in aerospace structural components due to its excellent performances such as low density,low coefficient of thermal expansion.good vibration damping properties and good fatigue resistance.However,CFRP was not used individualy to prepare components.Instead,it is usually bonded with other lightweight-metals such as TC4 due to its high cost,susceptibility to brittle fracture and matrix delamination during impact loading.Hence,joint performance is a key factor affecting the service safety and reliability of the structural components.In order to improve the bonding strength and solidity of the adhesive joint,resin layer and contaminants on the CFRP surface must be cleaned prior to adhesive bonding.Laser cleaning can not only quickly remove resin and contaminants on the surface of CFRP.but also increase the affinity of the adhesive to CFRP and the bonding strength.However,the current researches on the mechanism of laser cleaning CFRP is still insufficient,and the effect of cleaning process on joint performance is also need to be studied systematicly.These studies are of great value for enhancing the service reliability and extending the service life of the aerospace components.In this paper,the simulation model for laser cleaning CFRP was established by using COMSOL Multiphysics simulation software,the relationship between temperature field,cleaning depth,thermal stress field and laser cleaning parameters were investigated.The mechanism of laser cleaning was explained so as to provide reference for the laser cleaning process.Based on the simulation research,the experimental study of laser cleaning CFRP was carried out,the effect of laser cleaning on the surface morphology,wettability and mechanical properties of CFRP was studied in detail.The main work contents and conclusions are as follows:One dimensional heat conduction equation for laser cleaning CFRP was established,on the basis of the simulation results,we can conclude that the mechanisms for laser cleaning can be characterized as laser ablation and thermal stress action.The results also illustrated that threshold power for laser cleaning is needed.However,the results from the one-dimensional heat conduction equation could just be used to provide reference for establishing three dimentional modle and laser cleaning experiments,because it was a simple and ideal model that neglected the influence of phase transformation,heat conduction,heat diffusion.On the basis of the one-dimensional heat conduction eqution,three-dimensional simulation model for laser cleaning was established,invoving the heat conduction,thermal convection,and thermal radiation.The characteristics of temperature field and thermal stress field were analysed to reveal the mechanism of laser cleaning CFRP.The relation between the temeperature,thermal stress,cleaning depth and the laser cleaning parameters was investigated.The simulation results indicated that the mechanisim of laser cleaning CFRP can be characterized as the combined action of laser ablation and thermal stress if only considering the effect of phase transformation but not other factors.Multiple laser power and scanning speed were used to simulate the thermal stress subjected to laser power.The results revealed that a maximum value of thermal stress of 114 MPa would be generated at laser power of 74 W and laser scanning speed of 1000 mm/s.This value is larger than the adhesion between contaminant on the surface of CFRP and the substrate,indicating that the contaminant could be thouroughly removed from the surface to get an excellent cleaning effect.The simulation results provide an important reference for optimizing laser cleaning parameters for the experiments.Meanwhile,a narrower laser power window with range of 40 W to 80 W,and a desired laser scanning speed of 1000mm/s were worked out for the following laser cleaning experiments.Based on the simulation results,laser cleaning experiments were carried out.The effect of laser power on the morphology、chemical components,and the wettability of the cleaned CFRP to the bonding resin were investigated.The results illustrated that a laser power of 50 W is desired at a scanning speed of 1000 mm/s to completely remove the contaminants but without damaging the fibers inside.The surface roughness and surface free energy of the CFRP increased after laser cleaning treatment.Besides the action of laser ablation and thermal stress,there might be photodislocation and photoinduced vibration during laser cleaning process.Which results in the difference between simulation result and experimental result in laser power.Finally,the mechanical properties including hardness,bending strength of the CFRP,and bonding strength of the adhesive joint between CFRP and TC4 alloy were studied.The failure mechanism of the adhesive joint was anylyzed.It was observed that the hardness of the surface of CFRP increased after laser cleaning process.The bending strength of the cleaned CFRP increased first and followed by decreasing with increasing of laser power.The wettability of the CFRP was significantly enhanced subjected to laser cleaning,which in turn improved the bonding strength between CFRP and TC4 alloy.The tensile experiment results showed that the bonding strength presented an increasing followed by a decreasing trend with increasing of laser power during laser cleaning process.Different failure mechanisms were found for different joints.The failure mechanism for the joint between CFRP control and TC4 can be characterized as interfacial failure.The joint between TC4 and CFRP cleaned with a lower laser power density of 7.0 kW/cm2 presented the characteristics of cohesive failure and fiber-tear failure.However,a fiber-tear failure was observed in the joints between TC4 and CFRP cleaned with higher laser power density larger than 8.8 kW/cm2.更多还原