| Carbon fiber-reinforced composites are widely used in aircraft manufacturing due to their lightweight,high strength,corrosion resistance,and other characteristics.With the advancement of technology,the use of composite materials in aircraft is increasing and the demand for damage repair of composite structures is increasing.Focusing on the curing process of the AS4/3501-6 laminates,the effects of thermal process parameters such as heating rate,convective heat transfer coefficient,and configuration process parameters such as scarf angle and adhesive layer thickness on the thermal curing process and the residual strength of the repaired laminate are studied by means of experiments and numerical simulation,so as to provide experimental and numerical bases for optimizing repair parameters and improving the quality of scarf repair.In the experimental study,a circular platform scarf repair and a tapered scarf repair were designed for the two damaged laminates including impact damage,and the scarf repair experiments with different repair methods were carried out.Then,aiming at the repaired laminates with two different repair methods,the ASTM D7317 experimental standard was used to conduct compression experiments on two repaired laminates,and the experimental data were compared to obtain the residual strength of different scarf repair methods.And the circular platform scarf repair method was determined as the main research object of this thesis.In the numerical simulation analysis,a three-dimensional finite element model of coupled thermal-chemical-force multi-physical fields in the thermal curing process of the composite scarf repair was established based on the time-varying characteristics of the thermal and mechanical parameters of the material,in which the composite curing shrinkage and thermal expansion and contraction effect were taken into consideration.And effects of curing parameters on the temperature field,curing degree field,residual stress field during the curing process as well as on the residual strength of the repaired laminate were studied.The main research contents are as follows:(1)To obtain a heat transfer-curing model,the time-varying thermal parameters such as thermal conductivity and specific heat of the material were incorporated into the thermochemical model and the influence of thermal process parameters on the temperature field and curing degree field during the curing process were numerically analyzed.The study results show that: different thermal environments have little effect on the peak temperature of the patch,and the autoclave heating process effectively improves the temperature distribution within the patch;for the heat-blanket curing method,increasing the heating rate has little effect on the temperature,but can shorten the curing time of the patch;for the autoclave curing method,the change in convective heat transfer coefficient has little effect on the time required to complete curing,but can effectively reduce the temperature gradient within the patch.Therefore,the temperature distribution in the plate can be improved by using the autoclave curing method or by increasing the convective heat transfer coefficient under the autoclave curing method,and the curing cycle can be shortened by increasing the heating rate under the heat-blanket curing method.(2)Based on the control equation with time-varying properties of elastic modulus during the resin curing process,the mechanical parameters such as elastic modules and shear modulus in different directions of the laminate were obtained by the self-consistent model of composites.The above parameters were used to construct the composite stiffness matrix to form a residual stress model,which was combined with the heat transfer-curing model to form a threedimensional finite element model of coupled thermal-force-chemical multi-physical fields.The influence of thermal process parameters and configuration process parameters on the history of residual stress changes during curing were numerically analyzed.The study results show that:increasing the heating rate or the convective heat transfer coefficient will increase the residual stress;under the same curing temperature regime,the residual stress under the autoclave curing method is lower than that under the heat-blanket curing method;residual shear stress concentration of the adhesive layer will be aggravated by increasing the scarf angle and the thickness of the adhesive layer;reducing the scarf angle can effectively reduce the residual shear stress of the adhesive layer and the average shear stress gradient along the radial direction of the adhesive layer;when the thickness of the adhesive layer is reduced,the residual shear stress of the adhesive layer decreases,while the average shear stress gradient increases.Therefore,the residual stress can be reduced by reducing the design parameters such as heating rate,convective heat transfer coefficient,scarfing angle and adhesive thickness.(3)Based on Hashin criterion and secondary stress criterion,the progressive damage analysis models were established for composite laminates and adhesive layers respectively.And the effects of configuration process parameters and over-laminating ply parameters on the tensile strength of the repaired laminate were numerically analyzed.The study shows that: at a6° scarf angle,the load capacity of the repaired laminate is the highest;if the thickness of the adhesive layer is too large or too small,the tensile strength of the repaired laminate will be reduced and 0.9mm is the most preferred thickness of the adhesive layer in the current repair scheme;the over-laminating ply has the function of transferring stress between different parts and improving stress distribution,at the same time,the fiber orientation angle of the overlaminating ply will affect the evolution direction of the base plate damage.The residual strength of the repaired laminate can be improved by increasing the number,size,number of edges of the over-laminating ply and adjusting the fiber orientation angle of the over-laminating ply. |
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