| Owing to superior performances such as low weight,high strength and outstanding designability,Carbon Fiber Reinforced Plastics(CFRP)has become an excellent structural material for decreasing additional weight of high-end equipments.In order to meet the connecting or assembling requirements of CFRP components,edge trimming and drilling are always essential in the manufacturing of CFRP components.However,since CFRP has characteristic of multi-layer,belonging to typical difficult-to-cut material,in the machining process,the surface layers of CFRP are difficult to be effectively removed due to insufficient constraints.That results in frequent occurrence of surface damages including cracks and burrs,which not only decreases the assembly precision of CFRP components,but severely decreases their service life.Therefore,effectively suppressing the surface damages in edge trimming/drilling of CFRP,has become a key issue that needs immediate settlement in practical engineering.Since in edge trimming/drilling of CFRP,the surface damages are all formed in the multiple material removal processes under the tool,the structure of the tool used,as well as the process parameters determining the material removal processes,are key factors determining the formation of surface damages.Hence,for effectively suppressing the surface damages in edge trimming/drilling of CFRP,it is essential to improve such key factors.However,CFRP is composed of the fiber and the resin(including the interface)at micro level,and there are significant differences on the properties between them.Under the effects of the tool,the surface fibers are vulnerable to in-plane and out-of-plane deformation,leading to the coexistence of the two removal modes:cut off and bending fracture.Moreover,CFRP is anisotropic at macro level,with the tool rotating in edge trimming/drilling processes,at different instants,the corresponding material removal states of surface layers are different,and the surface damages formed at each instant will be influenced by the subsequent removal processes.These characteristics make the influences of tool structure and process parameters on the formation of surface damages very complicated,which makes it difficult to reasonably improve the tool and the process,hindering further improvement of CFRP components machining qualities.Focusing on this issue,this dissertation conducts researches on the influences of tool structure and process parameters on the formation of surface damages in edge trimming/drilling of CFRP,and improve the tool and the process accordingly,so that the surface damages can be suppressed more effectively.The outline of the main research works is as follows:(1)In order to study the influences of tool structure on the formation of surface damages in edge trimming of CFRP,for the characteristic that the surface fibers are likely to be removed in the two different modes including cut off and bending fracture under the effects of the tool,the stress criteria considering fiber's failure states of cut off and bending fracture are proposed.Comparing with conventional maximum stress criterion or maximum principal stress criterion,in the same model the calculation precision of main cutting force is improved obviously under the proposed criteria.On basis of that,three dimensional cutting models of surface fibers in edge trimming of CFRP are established,and through which the deformation and removal processes of surface fibers under the effects of the tools with different structures are calculated.Results suggested that using the right-handed cutting edge is beneficial to suppress the deformation of surface fibers in the bottom layer,but that will in turn aggravate the deformation of surface fibers in the top layer.When using the left-handed cutting edge,the situation is just on the contrary.In other words,using any tool mentioned above alone cannot effectively suppress the surface damages in edge trimming of CFRP.Such results provide theoretical foundations for the structural improvement of the edge trimming tool.(2)In order to suppress surface damages in edge trimming of CFRP,for a kind of tool which has advantages in cutting CFRP,the multitooth cutter,based on the influences of tool structure on the formation of surface damages,the structural improvement method for alternatively arranging right-handed cutting edges and left-handed cutting edges in circumferential direction is proposed.Such method can effectively avoid the formation of excessively large deformation of surface fibers in both the top layer and the bottom layer,which is induced by the continuous cutting effects of the cutting edges with the same spiral direction in edge trimming process,so that the formation of surface damages can be suppressed.Upon the method,series of improved multitooth cutters are fabricated.Through the verification on edge trimming results,the results suggest that comparing with the tool before improvement,using the improved multitooth cutter can suppress the surface damages better.Specifically,the damage area ratio(surface damages area/removed material area)can be controlled within 2%.(3)In addition to the tool structure,the process parameters are also the key factors determining the formation of surface damages in edge trimming of CFRP.For studying the influences of process parameters on the formation of surface damages,the relations between the surface damages formation process and the material removal process of the surface layer determined by process parameters are analyzed.Upon that,the prediction method on surface damages in edge trimming of CFRP is proposed,and the extent and distribution of surface damages under different process parameters are predicted accordingly.The prediction results suggest that:the process parameters related to the changing range of the fiber cutting angle have more obvious influences on the formation of surface damages in edge trimming of CFRP.According to the result,through reasonably controlling the changing range of the fiber cutting angle,the edge trimming process of CFRP with 135° surface fiber angle in which the surface damages are more likely to form is improved,and thereby the surface damages are suppressed.Moreover,the improved edge trimming tool and process have been applied into an engineering site,which effectively suppress the surface damages and thereby meet the high-quality edge trimming requirements of a CFRP specimen.(4)Based on the research method of surface damages suppression in edge trimming of CFRP,for the issue of suppressing the hole-exit cracks in drilling of CFRP,three dimensional cutting models of surface fibers at hole-exit in drilling of CFRP are established.Through the calculation under the proposed fiber stress criteria,the influences of drill structure on the formation of hole-exit cracks are analyzed:decreasing the cutting edge angle is beneficial for suppressing the deformation of surface fibers,which can further suppress the formation of hole-exit cracks.Besides,the influences of drilling process parameters on the formation of hole-exit cracks are also analyzed:On the premise that effectively controlling the hole-exit temperature,decreasing the feed rate,and meanwhile increasing the spindle speed are beneficial to decrease the deformation of surface fibers,which can also suppress the formation of hole-exit cracks.On basis of that,the structure of micro-edged drill,as well as reversed-cooling process are improved.Through the verification on drilling results,the results suggest that using the micro-edged drill of which the cutting edge angle is decreased,and the reversed-cooling process of which the feed rate is decreased and also the spindle speed is increased,can suppress the hole-exit cracks in drilling of CFRP better.Through applying the improved drilling tool and process into engineering sites,the hole-exit cracks are effectively suppressed,and thereby the low-damage drilling requirements of large CFRP specimens are met.The research works of this dissertation not only have important scientific significance on enriching theories of CFRP machining damages suppression,but also have obvious application values of facilitating the development of advanced tools and processes,as well as accelerating the engineering application process of CFRP. |
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