Investigation of fracture mechanism evolution model for UD-CFRP and MD-CFRP during the milling process

Carbon fiber reinforced plastic (CFRP) is widely used in the aerospace field because of its excellent mechanical properties. To ensure that the structure has a certain bearing capacity in multiple directions, MD-CFRP is generally used in engineering applications, which makes the anisotropy and inhomogeneity more complex. Therefore, it is easy to produce defects when machining CFRP. Additionally, MD-CFRP is recognized as the superposition of UD-CFRP, but this has not been verified. To solve these problems, the fracture mechanism evolution model for milling CFRP is established and verified by a series of experiments. The results show that the fiber cutting angle is an important factor for the fracture mechanism in orthogonal cutting. Upon increasing the fiber cutting angle from 0 degrees to 180 degrees, the fracture mechanism changes from interface separation to shear fracture, and then to bending fracture. Because the instantaneous fiber cutting angle of the surface is equal to the fiber orientation angle, the fiber orientation angle is the decisive factor in milling CFRP. After the slot milling of UD-CFRP and two types of MD-CFRP, the cutting force, surface morphology, and surface roughness are analyzed in detail. The results also show that each layer of MD-CFRP maintains the mechanical properties and fracture mechanism of UD-CFRP. Therefore, some of the existing research results of UD-CFRP can be directly applied when designing and processing MD-CFRP with different laying orders. This provides an important basis for the research on the machining mechanism, damage mechanism, and engineering application of MD-CFRP.