DESIGN AND DEVELOPMENT OF DRILL POINT GEOMETRY FOR DAMAGE FREE HOLES IN FRP LAMINATES

Abstract

The Fiber Reinforced Plastics (FRPs) due to their superior mechanical properties like strength to weight ratio, high stiffness to weight ratio, better impact characteristics, corrosion resistance and design flexibility are used for many applications such as space, transport, electronics etc. Usually FRPs products are made to net shape by primary manufacturing processes like hand layup process and filament winding. As the product complexity increases, the products are produced in parts and are then finally assembled. The secondary manufacturing in terms of machining in general and drilling in particular thus becomes unavoidable in order to facilitate the assembly operations. However this leads to drilling induced damage which results in high rejection rate. Delamination is recognized as one of the most critical defects that can result from the machining of composites. Delamination is a major problem associated with drilling of fiber reinforced composite materials that, in addition to reducing the structural integrity of the material, also results in poor assembly tolerance and has the potential for long-term performance deterioration. In the present research work, the objective was to design and develop modified drill point geometries in order to minimize the drilling induced damage and compare the performance of these geometries with the already established drill geometries for FRPs. The thrust force and torque were measured at different machining parameters. The piezoelectric dynamometer has been used for measuring thrust force and torque. It has been found that the U shape drill and trepanning tool results in lower thrust force and torque and subsequently leads to lesser exit delamination around the drilled hole. A Finite Element (FE) model was also developed as a part of the investigation to understand the drilling behaviour of FRPs. The results of the FE analysis were found to be in close agreement with the experimentally established results.