Study On Thermal Curing Of Resin Matrix Composite Laminates During Bonding Repair

Because of excellent properties such as high specific strength,high specific modulus,fatigue resistance and corrosion resistance,resin matrix composites have been widely used in aerospace structural components.With the massive application of advanced composite materials,the defects or damage of composite structures are more common.For the damaged structure,it is necessary to repair the corresponding damaged structure in order to enhance its safety and reliability,ensure its normal service in the life and restore its use function and integrity.Adhesively bonded repair is a common repair method for the repair of composite structural parts.The composite structure repaired by adhesively bonded repair has light weight and smooth surface,which can restore the structural strength to a greater extent.In this thesis,AS4/3501-6 composite laminates were taken as the research object,and the thermal curing of resin matrix composite laminates during adhesively bonded repair are studied by computational and experimental research.The main research contents and conclusions are as follows:(1)The AS4/3501-6 composites were taken as the research object,and the transient heat conduction equation,Arrhenius formula,heat generation rate formula and curing rate equation involved in the thermal curing process of the composite were adjusted and established.The curing kinetic parameters and thermophysical parameters of resin matrix composites were tested and determined.(2)Combined with the theoretical model,the subroutines program that can calculate temperature,curing degree and residual stress during curing process of composite materials were compiled,including the subroutine HETVAL for calculating curing heat generation,the subroutine UMATHT for calculating thermal physical property parameters that change with temperature and curing degree,and the subroutine UEXPAN for calculating strain increment.UMAT subroutine for calculating physical performance parameters and residual thermal stress.(3)Reaction heat of AS4/3501-6 prepreg in curing process and glass transition temperature of laminates after curing were measured by DSC experiment,and the test results were compared with reaction heat of AS4/3501-6 prepreg and glass transition temperature of laminates in references.The reaction heat and glass transition temperature used in numerical simulation were determined to be 209.2J/g and 202.3℃ respetively.The resin mass content in prepreg was predicted to be 30.28% by TG test,and the volume fraction of composite fiber used in numerical simulation was 62.06%.Compression experiments were carried out on bonded reparing composite laminates under different scarf modes.By comparing the compressive strength,circular platform scarf repair was determined as the research object.(4)The temperature,curing degree and residual stress in the curing process of composite laminates were calculated,and the calculated results were compared with those in the literature to verify the correctness of the calculation model and calculation method for the thermal curing process of composite laminates.(5)The temperature field and curing degree field of resin matrix composite laminates with different thicknesses were simulated.The results show that the peak value of heat release increases with the increase of thickness.The time for completing curing of the patch and adhesive with different thickness is close.The temperature gradient of the bonding surface near the upper surface of the adhesively bonded repair structure was larger at the beginning and end of curing,and the temperature gradient of the bonding surface near the lower surface of the adhesively bonded repair structure was larger during curing.At the moment of the second temperature peak value,the temperature gradient in the adhesively bonded repaired structure is the largest,1.6K/mm,and the curing inhomogeneity is up to 13%.At the other moment,the curing degree of each point in the patch is close,and each point can be cured almost at the same time.(6)The temperature field and curing degree field in the curing process of adhesive repair structure under different thermal environment were simulated numerically.The results show that the higher the heating rate is,the higher the temperature peak value is,and the earlier the exothermic peak appears in both upper and lower surface heating.The higher the heating rate is,the earlier the curing starts and complet.During the curing process,the temperature of the points inside the patch is higher than that at the edge,and the curing time is earlier.The temperature peak values in the bonded repairing structure cured by heating the lower surface were slightly higher than those obtained by heating the upper surface,and the time for completing curing is shorter.(7)The residual stress during the curing process of the bonded repairing structure with different fiber volume fraction and different expansion coefficient was simulated numerically.The results show that the maximum residual stress is obtained when the fiber volume fraction is close to 64%.The residual stress of the patch is distributed symmetrically in the fiber direction.The maximum residual stress in the adhesive layer occurs when the fiber volume fraction is close to 60%.Under a certain fiber volume fraction,the stress is mainly concentrated in the adhesive layer,and the residual stress is largest at the center of the mother plate.The variable expansion coefficient Considering the influence of temperature,curing degree and fiber volume fraction can describe the residual stress distribution more accurately.(8)The residual stress during the curing process of the bonded repaired structure under different constraints and pressures was simulated numerically.The results show that the stress of the patch is concentrated in the center of the bottom of the patch,and the residual stress on the patch is 5-6 times larger than that under other constraint methods when all sides are fixed longitudinally.The stress of the adhesive layer concentrates at the position close to the lower surface under the constraint methods of fixing the lower surface,fixing the upper surface and fixing the longitudinal surface around and the lower surface.In the case of longitudinal surface fixation,the stress of the adhesive layer concentrates near the lower surface and the upper surface,and the maximum stress of the adhesive layer is 2-3 times larger than that of other constraint methods.The maximum residual stress in the patch decreases with the increase of curing pressure.Under any pressure conditions,the maximum residual stress of adhesive layer is always larger than the maximum residual stress in the patch,the former is 6-7 times of the latter.The influences of the patch thickness,fiber volume fraction,material thermal expansion coefficient,heating mode,restraint mode and curing pressure on the structural properties of the patch were obtained,which provided a reference for the optimization of the bonded reparing process.