The Study Of Parameters Optimization And Performance For Composite Bonding Repair

As composite material plays a leading role in aircraft,composite bonding repair has extensive application.Among composite bonding repairs,the scarf bonding repair is widely adopted and has high repair efficiency.Available literatures related to composite bonding repair have the shortages that existed theoretical methods have relative low accuracy and the research on the impact resistance for repair structure is defective.This thesis has four researching contents as follows: preliminary design method based on semi-analytical method;FEM optimization design method;in-plane load-bearing capacity,damage mechanisms and verifications of semi-analytical method based on experiments and simulations;and the damage mechanism of low velocity impact and tensile carrying capacity after impact.Firstly,preliminary design method based on MAM(Modified Semi-analytical Method)was studied.For composite scarf bonding structures,most reseachers use FEM to simulate stress concentration of adhesive,but seldom using analytical method and semi-analytical method.Based on Harman’s new governing equations,this thesis introduced local micro units that get stress by stiffness allocation critiron,finite difference method,and gave MAM method which could solve Harman’s shortages.Comparing to Harman method,MAM had some advantages of reducing the error of shear stress peak of 0°plies with FEM,being able to obtain all of the shear stress peak,and solvinging the problem of shear stress distribution asymmetry caused by Harman method.Secondly,optimizing plies angles were proposed to reduce parent material removing.Usually,decreasing scarf angle would be adopted to reduce stress concentration in engineering.But this method could lead to remove more material as the composite plate thickness increases.Sensibilities and interactions of scarf angle,adhesive thickness and patch plies angles were analyzed by Isight and Abaqus.Genetic algorithm was adopted to optimize patch plies angles to reduce the stress concentration of adhesive.Based on the results of optimizations,robustness analysis was carried out by using Mont-Calos descriptive sampling method,which got the optimized solution of plies angles.Keeping the scarf angle constant,the specimen was optimized and adhesive stress concentration was reduced obviously.Thirdly,carrying capacity and damage mechanisms of composite scarf structures were studied by experiments and simulations.The damage mode,damage mechanisms and benchmark of carrying capacity were investigated by experiments of in-plane tensile load.Then the damage propagations and carrying capacity under tensile and compressional loads were studied by simulations.Competing sequences of all kinds of materials damage such as 90°,±45 ° matrix crack,delamination,adhesive failure in structures were found from the simulations.Furthermore,Modified Semi-analytical Method(MAM)proposed in this thesis was validated by experimental results.Fourthly,experiments of low velocity impact and related simulations were investigated for scarf bonding repair of composite structures considering different impact energy and locations.It was found that smaller than the critical energy(23J)the major damage mode was delamination,and more damage modes appear upon critical energy including adhesive cohesive damage and adhesion damage which propagated from back tip to inside of adhesive.Impact procedure of composite scarf repair can be divided into 4 phases.In phase Ⅰ,impact load increased linearly and structure has no damage.In phase Ⅱ,impact load decreased steeply and the composite damage propagated rapidly.In phase Ⅲ,impact load increased again and the composite damage propagated sequentially.In phase Ⅳ,impact decreased nolinearly and no damge occurred.Greater than 23 J impact energy,impact load decreased steeply at the beginning of phase Ⅳ in the graph of impact load and deflection,which was caused by adhesive damage.In five typical impact locations,impact damage upon the center location was biggest,and impact sensitivity of center location was highest.Fifthly,carrying capacity and failure mechanism of tension after impact(TAI)were investigated by experiments and related simulations.Smaller than critical impact energy(23J),composite damage exists only,and TAI doesn’t decrease obviously.Upon the critical impact energy,TAI decreased about 29% to 40% comparing with undamaged composite scarf repairs,and adhesive damage area is biggest.From the research of impact locations,it was found that the most sensitive location is central point of adhesive bonding area.The difference of tensile and compressional mechanism between TAI and no damage specimen is that,90°,±45° matrix crack of TAI reduce;Compression after impact(CAI)causes a little delamination propagation of impact.Furthermore,some empirical formulas were given as design reference from the experimental carrying capacity of the tension after impact.