Experimental Study On Cutting Force And Heat Variation And Surface Quality Evlaluation Based On Anisotropy Of Composites

With the rapid development of the global aerospace industry, the military aircrafts with emphasis on flight performance and the civil aircrafts with emphasis on safety, economy, environmental protection and comfort, both have the strong trend to employ composite materials with lighter and stronger performance. In the past few decades, carbon fiber reinforced polymer(CFRP) has been rapidly developed into the main structural materials for aircraft components. The machining process of CFRP materials is involved in great challenges, due to the extremely obvious differences between CFRP and traditional isotropic metals, and the main reason is anisotropy of the CFRP materials. Therefore, it is of great importance to establish a new theoretical system based on anisotropy to explain the cutting process of CFRP materials, which has great meaning to apply CFRP materials with high level and amount for China’s aerospace industry with broad prospects.Firstly, study was focused on cutting fracture behaviors of the long carbon fiber with high strength and hardness which is the essential cause for remarkable anisotropy of CFRP composite materials. By application of "subtraction" principle, the fracture behavior of the monolayer material of CFRP materials which were taken as prepregs in cutting was studied separately. The cutting force together with the macro fracture and micro defect morphology of carbon fiber under different fiber orientations was analyzed. Based on the analysis, the mechanisms of the surface defects on entrance and exit of CFRP unidirectional laminates were obtained. By AE cluster analysis, the AE cracking features of six typical fracture behaviors of CFRP materials were determined. And, the model based on AE features to predict splintering defects in CFRP materials online was established, in which three counting AE parameters of hits, energy and counts were taken as inputs.Secondly, force and thermal variations of CFRP unidirectional laminates in cutting were studied:(1) The cutting force variations of CFRP unidirectional laminates are of extremely obvious anisotropy, and under different fiber orientations the specific energy results in cutting are of great difference. Hereby, the specific energy map of monolayer material of CFRP laminates was proposed in circumferential cutting. The significance of anisotropy in cutting force behaviors was discussed under different cutting parameters. The cutting edge radius of tools in cutting CFRP materials was optimized. The material removal mechanism of CFRP materials under parallel, vertical, along and against cutting relationships was analyzed, and the respective material fracture mechanisms were given. Based on experimental results of the specific energy under 0°, 90° and 135°, the universal specific energy formula of CFRP unidirectional laminates was derived, and based on which the two-dimensional cutting force model was established with consideration of cutting speed and fiber orientation.(2) The cutting thermal variations of CFRP unidirectional laminates are of extremely obvious anisotropy, and under different fiber orientations the cutting temperatures are of great difference. CFRP materials produce serious rebound in cutting and the thrust force is of high level. Therefore, the contact friction between workpiece and the flank of tool generates a lot of cutting heats in the third zone, which directly causes cutting temperature rise in both workpiece and tool, and the rise range is proportional to thrust force. The influencing factors are in turn cutting speed, thrust force and tool wear. The significance of anisotropy in cutting thermal behaviors was discussed under different cutting parameters and tool geometry parameters, and it was found that the cutting temperature in parallel direction was more easily affected than that in vertical direction. By considering the friction heats in the III zone, the cutting temperature prediction model with fiber orientation involved by the universal specific energy formula was established.Thirdly, the surface characteristics of CFRP unidirectional laminates were studied based on the anisotropy of force and thermal behaviors. The suitable evaluation methods of surface roughness profile and surface defects for CFRP unidirectional laminates were discussed. The surface forming mechanism of CFRP materials under parallel, vertical, along and against cutting relationships was analyzed. In order to describe the special surface roughness profile with coarse surface base and cyclical peak valley, Ra or Rq was proposed to evaluate the base fluctuation of the profile and Rz was advised to be added to evaluate the cyclical gap in surface. Based on ultrasonic scanning microscope(SAM), the three-dimensional feature of structural delamination was observed and a new evaluation factor, three-dimensional volume delamination factor, was proposed.Finally, taken making hole in CFRP unidirectional laminates by drilling process as application, variation of cutting force and heat together with drilling defects in machining were analyzed. Anisotropy of CFRP unidirectional laminates in three material structural levels were considered, and a new theory system was established to explain surface defects of CFRP unidirectional laminates such as delamination, splintering, burr, spalling, burning, fiber pull-out and fiber fragmentation. The mechanism including the occurrence conditions and the locations and regions in making hole of CFRP unidirectional laminates were overall concluded.