Damage Of Interface Region And Hole Exit In Drilling Of CFRP And Ti Stacks

Carbon fiber reinforced plastic(CFRP)and titanium alloy(Ti)stacks have been widely used in modern aerospace industry due to their superior mechanical/physical properties.Because of the numerous bolting and riveting connections,drilling process is inevitable in the assembly stage of these structures.As the key load-bearing components in the aircraft structure.CFRP and Ti stacks generally suffer from complicated mechanical-thermal loads,in which the assembly holes with high quality are demanded to guarantee the excellent connections of stacks.Compared to the traditional hole making method,the drilling of stacks in single-shot time can reduce the position error of assembly holes and further improve the connection quality.However,CFRP and Ti own significantly different mechanical behaviors and poor machinability,therefore the drilling of stacks in single-shot time still faces challenges such as poor applicability of tool and machining parameters.Severe hole damages are caused eventually,especially in the interface region and hole exit of stacks.In view of the above problems,this paper makes an in-depth study and discussion on the generating mechanism,influence factors and suppression method of the interface and hole-exit damage of CFRP and Ti stacks.The main research contents are as bellows:(1)The load characteris in the interface region of stacks are analyzed in the drilling process.A micro-scale mechanical model of single fiber considering the constraints of surrounding material and Ti is established based on the load characters and the theory of elastic foundation.The effects of Ti on fiber deformation and fracture are obtained through the proposed mechanical model.Furtherly,simulation models of oblique cutting stacks are established based on the assumption of macroscopic anisotropy of equivalent homogenization,combined with the effect of Ti on the formation of sub-surface damage.According to the proposed model,a continuous process of cutting materials in the interface region of stacks is available.Besides,the influences of fiber direction,cutting parameters,stacking sequence and constraint degree of Ti on the depth of sub-surface damage are also obtained based on the proposed model.The results indicate that reducing the squeezeing or improving the support of Ti is beneficial to reduce the sub-surface damage of CFRP in the interface region,which provides the theoretical basis for the reduction of the sub-surface damage of CFRP in the interface region.(2)In order to reveal the thermal-induced damage of CFRP in the interface region in drilling of stacks,the characters of each stage in drilling of stacks are studied and the variations of thrust force and drilling temperature at different stages are analyzed.The results indicate that the drilling temperature of CFRP layer reaches the peak value as drilling of the interface region and exceeds the glass transition temperature of resin matrix.Furtherly,a temperature field prediction model based on the finite difference method is developed considering the effect of anisotropy of CFRP and introducing the temperature conductive coefficient of interface region.Drilling temperature of interface region in drilling of stack is predicted with the maximum calculated error of 10.3%according to the developed model.Moreover,the experiment results indicate that the resin color changes and the elastic modulus of resin in the changing region decreases by more than 20%after the CFRP layer suffering thermal-induced damage,therefore the region of resin color change can be defined as thermal damage region.On this basis,a novel extraction approach of thermal damage is developed according to the greyscale of digital image and the proportion of thermal damage area,namely thermal damage factor,is utilized to quantitively represent thermal damage.Finally-combining the calculated results of temperature and thermal damage factor,the relation between the thermal damage factor and maximum predicted temperature of CFRP in the interface region is fitted,The thermal damage significantly aggravates after the maximum temperature of CFRP in the interface region reaching 410?.which could be utilized to predict and evaluate the thermal damage.(3)Aiming at the frequently-occurred delamination at the exit of CFRP in drilling of stacks,the prediction models of critical thrust force are established under two stacking sequences based on the force characters at the exit of CFRP layer,the influence of cutting heat on properties of CFRP,the classical bending plate theory and virtual work principle.The critical conditions of delamination are acquired according to the proposed model.Meanwhile,the finite element models for delamination of CFRP under the squeezing of the tool are developed.Cohesive elements are introduced to simulate the process of delamination initiation and propagation and the shape of delamination region.Besides,an experimental method is proposed to verify the rationality of the analytical model and numerical model through the value of critical thrust force and the shape of delamination region.The experimental results validate the assumption of elliptical delamination region in the proposed theoretical model and the calculated error of the proposed theoretical model is 9.4%.Furthermore,the influences of the number of uncut plies,stacking sequence and drilling temperature on the critical thrust force on the critical thrust force of delamination are obtained through the proposed model.The results indicate that decreasing the exit temperature and increasing the support of remaining material can increase the critical thrust force and then suppress the initiation of delamination,which provides a theoretical basis to inhibit the delamination of CFRP in drilling of stacks.(4)The results of the influencing factors of interface and hole-exit damage reveal that the method of decreasing thrust force and drilling temperature can be used to decrease the interface damage and hole-exit damage.According to this method,a thrust force model considering tool geometry is presented,and then a method for tool diameter optimization of step drill is proposed combining the results of critical thrust force.On this basis,the effects of tool diameter proportion of step drill on the exit burrs of Ti and interface damage of stacks are analyzed and the tool diameter proportion are optimized to restrict the delamination of CFRP and ensure the drilling quality of Ti.Besides,the influence of point angle of step drill on interface damage and hole-exit damage are studied.The results show that the increase of first point angle can be suit to remove Ti layer,and the second point angle of step drill should be suit to reduce the drilling damage of bottom layer of the stacks.Furtherly,the influences of cutting parameters and pecking drilling process on drilling induced damage of stacks are studied.The results show that processing with variable parameters can simultaneously guarantee the drilling quality of CFRP and Ti,and peck drilling can further reduce thermal damage but aggravate delamination.A processing method based on the optimized step drill and combined with variable parameters and peck drilling can be used to decrease the drilling damage of stacks significantly,as ensuring that whole Ti layer is processed with cutting parameters suitable for Ti layer.Through the above researches,the formation and influencing factors of interface damage and hole-exit damage in drilling of stacks are revealed.Based on the results,a method for geometry optimization of step drill and a processing method combined with variable parameters and peck drilling are conducted according to the principle of controlling thrust force and decreasing drilling temperature.The research can be a new method for reducing interface damage and hole-exit damage in drilling of stacks.