Study On The Interface Bonding Mechanism Of Repair Of Civil Aircraft Composite Structure

There will be a lot of damage during the use of civil aircraft composite structure,some damage is beyond the tolerance of aircraft structure damage.In order to restore the structural integrity and structure function,ensure the safety flight of the aircraft,these structures should be repaired or replaced.Bonding repair is an important method to repair the composite structure for its advantages such as small gaining rate,corrosion resistance and short time consuming.However,due to the complex physical and chemical processes during the bonding process,there are many factors that affect the bonding performance,which makes the bonding strength appear larger discrete difference.There are many bonding theory in bonding process,the key of each theory is the understanding of bonding interface.These theories can effectively guide bonding repair in design and analysis.Based on the above,the microscopic properties of different surface are tested and characterized.Combining with macro micro mechanical verification,the contribution of interfacial bonding mechanism to bonding strength and the internal relationship between surface repair technology and maintenance effect are studied.Bonding theories mainly includes mechanical interlocking theory,adsorption theory,chemical bond theory.In this paper,the bonding mechanism of bonding interface by analysis and experiment has been studied.First of all,for decomposing the bonding effect,four kinds of different surface properties samples are prepared,100 mesh grinding sample(A),100 mesh grinding + non-reactivity surface treatment sample(B),polishing sample(C),polishing +non-reactivity surface treatment sample(D).Then,using laser confocal microscopy,contact angle measurement and X ray photoelectron spectroscopy to characterize A,B,C,D surface properties,including the surface morphology,roughness,micro channel surface free energy and surface active functional group concentration of surface information.After that,the macro mechanical properties of bonding interface are test by universal test machine.Finally,according to the four kinds of fracture failure mode(surface damage,mixed failure adhesive and cohesive failure is viscoelastic cohesive failure,interface,bonding interface),the paper use nano-indentation test micro mechanical properties of bonding interface to explain the failure mode of specimen fracture.The contribution of bonding mechanism to the bonding strength and effect was explored based on the results of micro characterization and mechanical testing.Bonding repair strength mainly compose of three kinds of function,mechanical interlock(X),adsorption(Y)and chemical bond(Z).Combing with micro characterization andmechanical test,the results show that the surface roughness of C is greatly reduced,and the surface morphology and roughness of B and D are not changed.The surface free energy of A is higher than C,but the surface free energy of Band Dare very low.There are more active functional groups on the surface of C,the content of A surface active group is less,while the content of Band D was the least.According to the bonding theory,A surface has X,Y,Z.B surface only has Y,C surface only has Z,D surface lose X,Y,Z.The results of macro mechanical tests show that the bonding strength is from large to small order: A> C>B>D.The fracture modes of A,B,C and D were respectively: mixed failure,cohesive failure of adhesive,cohesive failure of adhesive,interfacial failure.The results of macro mechanics are in agreement with the results of surface micro test.The results show that the mechanical properties of the interface are poor(modulus and hardness were significantly lower than other areas),which is consistent with the fracture failure mode of the D surface.Based on the above results,the conclusion is as follows.(1)For mechanical interlocking mechanism,increasing roughness in a certain range can significantly enhance the mechanical interlock,so that the final bonding strength can be increased.(2)For adsorption mechanism:the contribution of adsorption to bonding strength is big,and the increase of surface free energy can increase the final bonding strength.(3)For chemical bond mechanism,the contribution of chemical bond to bonding strength is the biggest,and the concentration of active functional group can increase the final bonding strength obviously.