Study On Mechanisms During Automated Fiber Placement In Situ Consolidation Process Of CF/PPS Thermoplastic Composites

The automated fiber placement(AFP)in situ consolidation process of thermoplastic composites has great potential in improving the efficiency and reducing the cost for composites manufacturing,and has a wide application prospect in the manufacturing of aerospace composites.However,due to the instantaneous high temperature characteristics of the process,the property of the composites is difficult to reach the level of traditional process such as autoclave at a fast placement speed.Therefore,the contradiction between manufacturing efficiency and quality has brought great challenges to the wide application of the process.Based on this,this thesis takes carbon fiber reinforced polyphenylene sulfide thermoplastic composites(CF/PPS)as the research object,aiming at the mechanisms during laser-assisted AFP in-situ consolidation process including temperature field,thermal degradation and crystallization of PPS,and interlaminar bonding were investigated,and the effects of processing parameters on the interlaminar properties of the composites were analyzed.During the process,the placement head moves continuously and the prepreg is layered on the tool/substrate,which is a three-dimensional transient process.A three-dimensional transient temperature field finite element model was established.The temperature history of the prepreg and the influence of tool temperature and placement speed on the peak temperature of the layers was discussed.An on-line temperature field measurement system was built,and the Fiber Bragg Grating sensor with smaller size and higher sampling frequency was used for the temperature field measuring,and then the accuracy of the finite element model was verified.During the process,the increase of prepreg temperature can effectively reduce the viscosity of the matrix,which is conducive to interlaminar bonding.However,while increasing the prepreg temperature,it is necessary to ensure that the thermal degradation of the matrix is within an acceptable range,so that a trade-off can be made between the property and manufacturing efficiency.Based on this,a thermal degradation kinetic model of PPS considering multi-stage reaction under oxidation condition was established combining the material properties and environmental conditions during the process.The accuracy of the kinetic model was verified by the fourth-order Runge-Kutta method.Then the kinetic model was used to predict the thermal degradation of PPS during the instantaneous high temperature of the process.Combined with the surface morphology and element variation of the laminate after consolidation,the thermal degradation mechanism of PPS matrix during the process was revealed.The crystallization temperature window of CF/PPS composites was determined by the crystallization kinetics,and the accuracy of the kinetic calculation results was verified.The effects of laser heating temperature,placement speed and tool temperature on the crystallinity of composites were studied,and the tool temperature was determined to be the most significant influencing factor.Combined with the results of crystallization kinetics,the mechanism of the tool temperature on the crystallinity of the composites was determined: when the tool temperature was within the crystallization temperature window of the composites,the crystallization process of the composites was not only including the non isothermal crystallization during the cooling stage,but also the isothermal crystallization process on the tool.And the isothermal crystallization process can effectively improve the crystallinity of laminates.Based on this,the effects of tool temperature on the interlaminar shear strength(ILSS),flexural strength and Mode I fracture toughness were evaluated.Based on two commonly used prepreg surface models,ideal rectangular surface model and fractal surface model,the effects of processing parameters on the intimate contact between layers were analyzed and compared.The healing process of PPS was investigated by the rheological experiment.The intimate contact process was determined to be the control factor during interlaminar bonding by comparing the time scales of the intimate contact process and the healing process.In the range of the process parameters determined by above processing mechanisms investigation results,the effects of the processing parameters on the ILSS of the laminates were analyzed comprehensively.Taking the ILSS of hot-pressed laminates as the standard value,the experimental value of the degree of interlaminar bonding during the AFP in-situ consolidation process was calculated,and then compared with the theoretical value calculated by two intimate contact models.The applicability of the two models to CF/PPS composites system used in this thesis was determined.