Experimental Study On Curing Deformation Mechanism Of Carbon Fiber Reinforced Thermosetting Composites

Composite curing process involves thermal-chemical-mechanical interactions,which usually leads to process-induced deformation after cure.Due to much smaller assembling tolerance of composite structures than the metal counterparts,the curingdeformation significantly increases the difficulties of assembly.The assembly problems have resulted in a remarkable increase in assembling time and cost of large commercialaircraft structures,and consequently limited further large applications of composite materials in aerospace structures.Although many reports conducted on curing deformation analysis and prediction,systematic investigations on the variations of material properties and influential factors were still lacked and thus the accurate prediction of curing deformation became difficult.It is therefore crucial to develop a series of experimental studies for curing deformation based on material properties at various phase stages of the curing process.The systematical investigation on the fundamental mechanisms of the influential factors on curing deformation can provide the essential information for improving the curing deformation predictions.In this thesis,the evolution and interaction relationships among cure degree,phase transformation and mechanical properties of carbon fiber/epoxy prepregs aresystematically studied.The anisotropic thermal expansion of the cooling stage,chemical shrinkage and tool-part interaction of the curing stage,as well as other factors are comprehensively investigated by experimental methods.The FBG sensors were here used to obtain the internal interactions during cure.The specimens with specific shapes were applied,such as the flat plate,the L-shaped corner and the C-shaped sample.The influential mechanisms of these factors on curing deformation areanalyzed combined with the experimental results.The main investigation contents and major achievements can be concluded as follows:The curing kinetic model,phase transition model and modulus prediction model of resin are developed.Then the evolution of the cure degree,phase transfer points and the change in resin modulus during cure,which provide essential information for further investigation,are accuratelypredicted by the established models.The cure degree keeps constant at the end of second isothermal stage.The gelation andvitrification occur during the second isothermal stage.It is identified that anisotropic thermal expansion acts mainly during cooling stage,the chemical shrinkage and tool-part interaction acts mainly during curing stage.Asymmetrical plates and unidirectional L-shaped specimens are used to investigate warpage and spring-inrespectively during cooling stage of the composite curing process with the reheating method and the cure-interrupted method.It is found that the final warpage is mainly determined by the thermal deformation for the asymmetrical laminate;warpage during cure can be predicted by the thermal deformation under the instantaneous glass transition temperature.The thermal spring-in can be predicted by the difference between in-plane and out-of-plane CTE(Coefficient of Thermal Expansion)for the L-shaped specimen.In addition,significant non-thermal spring-in is also observed in the L-shaped specimen.Thus,C-shaped sample modeled on CFRP mould with a constant curvature was then applied to study the contribution of chemical shrinkage.An innovative chipped FBG-based method is developed to determine the effective chemical shrinkage by monitoring gelation and vitrification during cure.The spring-in of the C-shaped specimen can be more accurately predicted using the present method.The cure-interrupted method is used to investigate the contribution of chemical shrinkage to spring-in of C-shaped specimen.It is concluded that the effective chemical shrinkage is transversely isotropic.The spring-in of the C-shaped specimen can be predicted using effective chemical shrinkage and thermal spring-in under instantaneous glass transition temperature.A point pre-cure FBG positioning technique is developed and used to monitor the mold-induced strains of the flat plate and U-shaped specimen throughout the whole curing process using FBGs.The influence of tool-part interaction during curing,in particular of the in-plane interactionand geometrical constraints of the tool,are investigated in combination of the tension-shear test results.The influence of geometrical constraint on curing deformation isassessed for L-,U-,and rectangular shaped specimens.The results revealed that the in-plane tool-part interaction has a cumulative effect along the longitudinal direction and warpage is mainly caused by inter-layer sliding between the prepreg plies,whilst the geometry constraints is independent with the length of the tool-part interface and the warpageisprimarilycaused by stress release and sliding during consolidation.Furthermore,the corner spring-in issignificantly increased due to geometrical constraints.Last but not least,the influence of other factors including thermal lag effect,fiber wrinkage and ply overlapping on curing deformation of large-scaled composite parts are also investigated.The chipped FBG-based method developed in this thesis was here used to capture the non-uniform gelation induced by the thermal-lag effects.It was shown that the strain gradient of the effective chemical shrinkage was the main reason for producing residual stress or warpage.Spring-in angle may be reduced due to fiber wrinkage at the corners.The style of ply overlapping affects the ways of tool-part interaction during heating and then influences the spring-in of composite part after curing.