Experimental Studies On Damage Reduction For Trimming Process Of NOMEX Honeycomb Core

In the 21st century,cost-effectiveness is the main concern for the modern aviation.For the ultimate lightweight which is beneficial for cost-effectiveness,the sandwich structure has been extensively used in the aviation,such as aircraft floors and wing flaps.To date,aramid honeycomb core,a combination of honeycomb structure and aramid fiber reinforced composites,has been one of the most ideal candidate of the core for sandwich structure in the aviation.A large number of NOMEX honeycomb cores(a kind of meta-aramid honeycomb core)produced by DuPont were used in A380.But more excellent physical,mechanical and chemical properties also mean more difficulty in machining.The outstanding weak rigidity of NOMEX honeycomb core in the W-L plane will lead to serious machining defect in trimming NOMEX honeycomb core,such as collapsed cell wall,residual cell wall and other machining damage like burrs and tears.For saving above problem,researchers have made a lot of important investigations of the technology for high-efficiency machining and rigidity compensation of NOMEX honeycomb core,such as ultrasonic assisted machining technology,electric-magnetic fixture and ice fixture.However,the material removal mechanism,damage formation and strategy for damage reduction during the trimming process of NOMEX honeycomb core were unsolved and the structural characteristics of NOMEX honeycomb core in macro/meso scale level were not considered in detail.Based on the research status as above,the rigidity characteristic,damage formation,cutting process and cutting angle of NOMEX honeycomb core was studied with considering the structural characteristics of NOMEX honeycomb core in macro/meso scale level.This research received the support of the National Natural Science Foundation of China(NSFC).At last,for achieving low-damage trimming of NOMEX honeycomb core,some reasonable strategies for damage reduction were proposed and a self-design multi-tooth disc cutter was made.In summary,the main research contents of this research are as follows:(1)In order to reveal the origin of difficulties in trimming NOMEX honeycomb core,the machinability of those materials was systematically studied with considering their physical and mechanical properties in macro/meso scale level.Firstly,the rigidity characteristics of NOMEX honeycomb core were studied to understand the machinability in macro scale level.Characteristics of machining damages in different cell wall locations were predicted by theoretical analysis,and the results were verified by cutting experiments.In addition,a cutting model of a single cell wall was established with considering its meso-structure to understand the causes of damage formation and the machinability in meso scale level.(2)For establishing the inexpensive strategies for damage reduction of NOMEX honeycomb core,the cutting process was systematically studied on the basis of tool geometry,cutter location and periodic thin-wall cell geometry of honeycomb core.Firstly,the key cutting angles in cutting process(engagement angle/entrance angle)were calculated by geometric analysis.The effects of above cutting angles on machining damage were discussed in detail,and those effects were originated from the changing of stress states due to the changing of the contact and motion relationship between cutter tooth and cell wall.And then,some strategies for damage reduction by cutting angle adjustment were proposed.In addition,based on the analysis results of cutting angles,the differences between intermittent cutting and continuous cutting of disc cutters were compared and the origin of tears damage using traditional disc cutters without tooth was revealed.(3)For the further application of above strategies for damage reduction of NOMEX honeycomb core,a self-design multi-tooth disc cutter was developed based on the cutting mechanism of composites.Optimum wedge angle?~*and optimum tool cutting edge inclination angle?_?~*were determined through the experiments of tool geometry.Due to the conformity of intermittent cutting process and multi-micro-cutting theory,cell wall in NOMEX honeycomb core can be cut off effectively by the self-design multi-tooth disc cutter.With the strategies for damage reduction by cutting angle adjustment,low-damage requirements during the trimming process of the core for sandwich structure in the aviation can be obtained by the self-design multi-tooth disc cutter.The machining surface of NOMEX honeycomb core is clean and tidy without tears damage,and other machining damages such as uncut fibers and burrs also have been remarkably reduced.