Delamination of Fibre Metal Laminates Due to Drilling: Experimental Study and Fracture Mechanics-Based Modelling

Fibre metal laminates (FML) are significantly adopted in the aviation industry due to their convenient combination of specific strength, impact resistance and ductility. Drilling of such materials is a regular pre-requisite which enables assembly operations, typically through rivet joining. However, the hole-making operation is of increased complexity due to the dissimilarity of the involved materials, often resulting in defects (i.e., material interface delamination), which can significantly compromise the otherwise excellent fatigue strength. This work explores the potential of three different drill geometries, operating under variable cutting speeds and feeds on CFRP-AA laminates. In addition, the usage of sacrificial back support is investigated and cutting load, surface roughness and delamination extension are examined. In order to predict delamination occurrence, ADCB tests are performed, enabling the calculation of fracture energy threshold. Drill geometry presents a very significant influence on delamination occurrence. The usage of specific step-tools with secondary cutting edge showed superior performance. Despite its simplicity, the applied critical force threshold model was able to successfully predict interface delamination with good accuracy.