Analysis and prediction of axial force and exit damage in drilling of composites with delamination damage

In aircraft maintenance, the delaminated composites are commonly repaired by riveting to inhibit delamination propagation and restore load-bearing performance. Making connecting holes in damaged areas is inevitable and may cause damage aggravation. This paper focused on the axial force and exit delamination damage in the drilling of delaminated composites. The influence factors including spindle speeds, feeds, drilling positions, and delamination locations and widths were considered for the drilling test, in which the axial force was collected to analyze the interaction mechanism between the tool and workpiece. The equivalent delamination factor was used to evaluate exit damage. The quadratic nonlinear regression (QNR) and Support Vector Regression (SVR) models were built to predict the cutting force curve and exit damage, respectively. The results revealed that the cutting force curves of the delaminated laminates produced obvious concavity when the drill bit reached the position of delamination. The optimal cutting conditions were 10,000 r/min and 100 mm/min for both intact and delaminated laminates in the drilling test. It was better to make holes in the inside of the damaged area to obtain small equivalent delamination factor and the increase of delamination width would aggravate exit damage. The errors of the QNR model were controlled within 13.40 % and 11.07 % for the intact and delaminated laminates, respectively. The maximum error of SVR models was 3.79 %. The results of QNR and SVR models had been proven to be accurate.