Research On CFRP Drilling Damage Mechanism And Drill Structure Modification

Composite materials have gained a wide application in aeronautics and astronautics industries since the high specific strength,high specific modulus and anticorrosion properties.Carbon fiber-reinforced polymer(CFRP),which is a kind of advanced composite materials,has already been used as important components in aircraft structures.During assembling,a huge number of holes associated with CFRP parts needed to be drilled.The hole quality will affect the strength and service life to a large extent.CFRP laminates is generally cured using unidirectional prepregs which are paved according to specific stacking sequence.The anisotropic,brittle,high hardness and non-homogeneous properties make drilling of CFRP an extremely difficult procedure.Many damages,such as fiber pull-out,burr,delamination,micro-cracks and matrix burn,often occur.The drilling associated damages are still not able to solve effectively.The most common method is drilling parameters optimizing,which is considered affected greatly by worker’s experiences.Hence,the drilling quality cannot be improved thoroughly,leading the increasing of manufacturing costs.To solve the aforementioned problems,the CFRP drilling damage mechanism and a drill modification strategy is studied in this thesis.Firstly,the fibers and matrix failure mode is investigated with respect to single edge CFRP cutting.Finite element method(FEM)and experiments are combined to study the relation between micro-cutting behavior and macro-cutting responses.Secondly,strategies are proposed in order to reduce the sub-surface damage,burr and delamination during drilling or cutting.Thirdly,thrust force distribution model is built to analyze how the thrust force affect the location of the damages along the hole-wall.Lastly,the minor cutting edge and chisel edge of a drill are modified to minimizing damages during CFRP drilling.The major work are presented as follows:(1)The single edge CFRP cutting mechanism is investigated using FEM and experimental method.A micro-cutting simulation model is built first and then the effects of rake angle,cutting edge radius on chip formation is analyzed.A double rake angle design is presented based on the analysis to improve the chip evacuation and wear resistance.Also,a macro-cutting simulation model is established to reveal the deformation transformation in cutting area.In order to verify the simulation and to analyze the cutting responses,experiments are conducted to measure CFRP cutting forces,chip formation process and cutting area deformation.In the end,the micro and macro cutting simulation and the experimental results are analyzed to specify the cutting process from micro to macro level.(2)Strategies are proposed to minimize sub-surface damage,burr and delamination.An “orthogonal stacking” method is presented with respect to reduce the sub-surface damage.The method is verified by macro-cutting simulation and cutting experiment.After analyzing the interaction between cutting edge and fibers,fibers and fibers,a large inclination angle cutting method is proposed to reduce burr and sub-surface damage.The method is verified by comparing orthogonal and oblique cutting experiments.The reason of delamination during drilling is analyzed and large fiber angle cutting and fiber “early broken” cutting method are presented to reduce the drilling delamination.(3)The thrust force distribution and damage occurrence location is investigated.A unidirectional CFRP drilling thrust force distribution model is built.Based on angles transformation and drilling timing converting,the model is used to calculate multidirectional CFRP drilling thrust force.Then the thrust force variation in a single drilling cycle is analyzed.The thrust force experiences two cycles as drill rotates once which brings the location of drilling damages also occurs periodically.(4)Stacking sequence and drill structure optimizing strategies are proposed.The effects of thickness of each ply on sub-surface damage is verified.It is concluded that small ply thickness(0.125mm)yields less sub-surface damage compare to large ply thickness(0.25mm).The minor cutting edge and chisel edge are modified based on the damage minimizing strategy presented previously.The efficiency of transverse cutting edge and modified chisel edge is analyzed.Experiments are conducted to verify the modified drill generates less damages compare to the unmodified drill.