Drilling Force Modeling And Delamination Suppression Strategy Analysis Of CFRP-metal Stacks

The upgrading of the aeronautics and astronautics products and the rising of the performance requirements are calling for advanced structural materials with better synthetical performance. Composite materials, specifically the carbon fiber reinforced plastics(CFRP), are utilized more and more widely in aviation, aerospace and automotive industries due to its high specific strength, high specific modulus and good corrosion resistance. As structural materials, it could not be avoided that joining FRP to other metal materials, such as titanium alloy and aluminum alloy, forming the so-called metal-FRP stacks. Since the mechanical joint methods, such as bolting and riveting, are most commonly used in assembly, the hole quality of the metal-FRP stack is an important factor that affects the assembly quality. In mechanical manufacturing field, conventional drilling with twist drill is the most frequently employed operation to machine holes prepared for bolting and riveting. Unlike metals, FRPs are inhomogeneous and their interaction with the cutting tool during drilling is a complex phenomenon that is not well understood. Drilling of metal-FRP stacks is a challenging task because of different machining properties of the two kinds of materials. Machining of CFRP can destruct of fiber continuity and cause several damages such as delamination, fiber pull-out, matrix thermal degradation, and among these damages push-down delamination is regarded as the most critical one owing to the fact that it can reduce considerably the load carrying capacity of the joint and the fatigue life. In allusion to the problems mentioned above, some research on drilling force modeling and delamination suppression strategy of CFRP-metal stacks are carried out. The main works of this study are as follows:Firstly, the mechanism of the orthogonal cutting of unidirectional CFRP in micro-scale is discussed and several three-dimensional multiphase finite element models are established to simulate the orthogonal cutting behavior of carbon fiber reinforced plastic/polymer(CFRP) and acquire a more in-depth understanding of the cutting mechanism of CFRP. The tool nose is scanned and measured, and the geometric models are set up according to the geometric information of the fiber, the matrix and the tool. The constitutive relationship, involving elastic-plastic relationship, failure criterion and damage evolution, is described by VUMAT and various CFRP cutting simulations with different fiber orientations are conducted. The results of the simulations are verified by the orthogonal cutting experiments. The failure model, the chip formation and the sub-surface damage in different cutting situations are displayed visually. The relationship between fiber orientation and cutting forces is revealed by the outputted data of different cutting situations, and the finite element models are validated by the experimental results.Secondly, a force prediction model for CFRP orthogonal cutting is established. The deflection of the Representative Volume Element(RVE), comprised of a single fiber and the surrounding matrix, is analyzed considering the effect of the surrounding materials based on the Minimum Potential Energy Principle(MPEP). The critical force in the cutting edge that causes fracture of the RVE is obtained according to the bending deflection expression of the RVE. In addition, by taking slipping, peeling, and bounding mechanism in three different deformation areas into consideration, a force prediction model of CFRP orthogonal cutting is established for fiber orientation ranging from 0° to γα+90°. Several experiments have been conducted, and the results comparison shows that the model, though approximately, has gotten acceptable agreement with the experimental results, which proves the effectiveness of the analysis method.Thirdly, a five stages force predicting model for full life-cycle CMSs drilling is establised based on infinitesimal method. The CMSs drilling process is deconstructed and analyzed, as a result, the embodiment of the primary parameters in micro-scale element cutting is revealed. According to the cutting characteristics and states, the hole-machining process is divided into several stages. The combination and superposition of the cutting force in each stage are analyzed. By studying the characteristics of each stage, the dynamic cutting force models of three processing operations are employed to calculate the cutting force of the corresponding stage. Meanwhile, a prediction model of the cutting force for the chisel edge is proposed by theory of contact mechanics and finally the model of cutting force on CMSs drilling is established.Lastly, predicting models for critical thrust force predicting during CMSs drilling is established, and some damage control strategy is suggested. Taking the tapered drill point into account, a qualitative finite element(FE) analysis of push-out delamination is carried out to ascertain the location where the delamination firstly occurs. The delamination during CMSs drilling is discussed with the effect of the metal part taken into account. Two cases were studied, case(a): drilling from metal to FRP and case(b): drilling from FRP to metal. In case(a), a mechanical model for predicting the critical thrust force was established based on linear elastic fracture mechanics, classical bending plate theory and the mechanics of composites. In case(b), plate shape and edge conditions were taken into consideration to analyze the deformation of the metal plate. The superposition principle was applied to calculate the effect of the deformation of the metal plate and FRP plate on the CTF, and a mechanical model was established based on the analysis in case(a). Several finite element models for critical thrust force predicting at delamination onset are devoted. 3D FE models are developed for predicting the CTF of drilling CMSs at different stages by using traction-separation law to represent the interlaminar property. To validate the rationality of the theoretical models and FE models, the results of the presented models are compared with a series of experiments works from the previous literature and acceptable agreements are reached.