| Carbon fiber/epoxy resin composites (C/E composites) are of considerable interest due to their excellent properties such as high strength-to-weight ratios, high stiffness-to-weight ratios, corrosion resistance, etc. They have been widely used in a diverse range of industries and applications including aerospace, defence and military, energy, transportation and so on. The percentage of C/E composites in aircraft has been an important symbol of advancement and competitiveness. Also, the scope of their application will change from minor to primary load bearing structures. On the large-scale aircraft composite structures, a large number of bolted or riveted holes need to be produced in order to meet the assembly requirements. Hole-making quality becomes the critical factor directly affects the strength and fatigue life of C/E composite structures. However, due to their anisotropic, non-homogeneous, low interlaminar strength and high heat-sensitive characteristics, many kinds of damages in multi-scale and multi-form are easy to appear during machining, such as burr, spalling and delamination, making them difficult-to-machine materials. Large amount of researches on the hole-making tool and process technology have been carried out. However, most researches are experimental investigations and many of the experimental phenomena have not been thoroughly explained. Great differences between carbon fiber, matrix and interface make the action mechanism between tool and material extremely complex. Deep understanding of damage mechanism is an important technical foundation to realize the high-quality and high-reliability machining of C/E composites.Aims at the above problems, this paper adopt theoretical analysis, finite element simulation and simulation experiment to thoroughly and systematically analyze the mechanical behavior between tool and C/E composites during hole-making process. Material removal and damage formation mechanisms have been revealed, which provide theoretical guidances for tool design and choosing reasonable process parameters. The main research contents and results are as follows:(1) An analytical cutting force model is proposed. In the model, contact theory is applied to the chisel edge cutting zone. Oblique cutting is introduced in the cutting lips cutting zone that is considered to have three distinct regions. Through the model, mechanical behavior of drilling C/E composites by twist drill is clarified. Influencing parameters of cutting force are discussed including drilling parameter, tool geometry and fiber orientation. Results show that the thrust force contributed by the chisel edge is between50percent and80percent, while the torque contributed by the chisel edge generally does not exceed8percent of the total torque value. High spindle speed and low feed rate are recommended because of the lower generated cutting forces. For the same tool diameter, the thrust force increases with the tool nose radius, point angle and web thickness. In addition, the anisotropy of C/E composites is a significant factor that influences the cutting force. The curves of thrust force and torque show strong oscillations with the periodically changing rotation angle.(2) By the study of mechanical behavior of abrasive drilling C/E composites by electroplated diamond abrasive trepanning tool, a meso-scale finite element model is established which effectively revealed the material removal mechanism for C/E composites cut by single grain. When single grain scratches the C/E composites, they bend and failure mainly by local fiber tensile damage. Based on the theory of beam deformation, fracture mechanics and conservation of energy, a mechanical model is established to predict the cutting force of single grain. Results show that the cutting force of single grain significantly increase with cutting depth, grain size and strain energy release rate. Also, mechanical model of C/E composites drilled by electroplated diamond abrasive trepanning tool is proposed. Thrust force and torque increase with the spindle speed and decrease with the feed rate. Model predictions are in agreement with experimental results.(3) In order to investigate the formation mechanism of delamination, a macro-scale finite element model is established. Different constitutive relationships and failure modes of inner-laminar and inter-laminar C/E composite materials are considered respectively. Cohesive zone model is introduced in the interface to simulate the delamination formation and propagation. With this developed model, the influence factors on the delamination damage are studied including fiber orientation, uncut thickness and tool type. It shows that delamination morphology of unidirectional C/E composites is elliptical. For the electroplated diamond abrasive trepanning tool, the ratio of the major axis and the minor axis is1.68; for the twist drill, that value is1.85. Failure mode of inner-lamina material is mainly resin tensile fracture. With the decrease of uncut thickness, degree of delamination damage increases. The damage degree of multi-directional C/E composites is weaker than unidirectional C/E composites. Compared with electroplated diamond abrasive trepanning tool, twist drill tends to produce delamination.(4) In order to investigate the material anisotropy on the formation mechanisms of burr and spalling damage, scratching experiment is adopted to study the fracture and removal process of C/E composites cutting in different orientations. Results show that when scratching direction is perpendicular to fiber orientation, scratching force is the largest and spalling is prone to appear. When the angle between scratching direction and fiber orientation is30°,45°and60°, the fibers on the left side of groove yield towards the left and tend to form burr damage. While the fibers on the right side of groove bend in a large deformation and fracture homogenously, which are not easy to form burr. Theoretical analyses and scratching experimental results show:burr is prone to appear in the region where the included angle between fiber orientation and cutting speed direction is acute; hole exit edge is smooth in the region where the included angle is obtuse; spalling is prone to appear where fiber orientation is perpendicular to cutting speed direction.(5) Aims at the singleness and limitation of the existing evaluation method of composite machining damage, a comprehensive damage evaluation method is proposed taking consideration of burr, spalling and delamination. The evaluation method is based on the relative index in statistical method. Coefficients are determined through tool wear experiments. The method is verified by ultrasonic detection. Under experimental conditions of the thesis, when the coefficients a, b, c, a and β are respectively chosen as1,1.5,4,0.2and0.8, the variance of damage factor is very similar to tool flank wear curve which’has three stages. The critical damage factor is0.615. When damage factor k is bigger than0.615, tool has been in rapid wear stage and the machined hole is unacceptable for use.The above research findings provide reliable theoretical and technological bases for the development of series of diamond abrasive tools, the successful completion of the relevant national research and the solvement of the C/E composite structures hole-making problems encountered during the development and production of a number of aerospace products. |
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