Prediction of critical thrust force generated at the onset of delamination in machining carbon reinforced composites

In aerospace manufacturing, drilling carbon reinforced composites is one of the dominant processes. The exit-ply delamination, directly related to the drilling thrust force, occurs more often despite all precautions. In this paper, a new analytical model is presented to predict the critical thrust force at which delamination will start. The model takes into account the thermo-mechanical loads and considers a mixed mode of fracture occurring in the delamination area. This model explains how the delamination area starts with a circular cross-sectional area in the entrance layer and grows into an ellipsoid, in the same direction of the fibers, in the exit layer. Besides, the distribution of the load in the delamination area includes a center point load for the chisel edge of the drill bit and a linearly distributed load for the cutting lips. The results of the proposed model are compared with previous analytical models using three different sets of experimental data and the comparison showed that the presented model is more accurate.