Drilling Damage in Laminated Polymer Matrix Composites Considering Thermal Efefcts: Experimental and Numerical Analysis

Abstract

In the present work, drilling induced damage in multi-directional carbon fiber reinforced polymer (MD-CFRP) laminates has been investigated by experimental and numerical approaches. Exit-ply delamination during drilling is known to be the most detrimental form of drillinginduced damage in FRPs that results in significant loss of structural integrity of the component. Initially, a finite element (FE) model using surface based cohesive zone model (CZM) has been adopted to simulate the push-out delamination considering thermal effects. Comparison with experimental push-out data yielded a good match. To investigate the temperatures generated during drilling comprehensively, a novel inverted drilling setup has been developed that allows in-situ cutting temperature measurement using fiber Bragg grating (FBG) sensors embedded in the stationary drill bit mounted on a dynamometer. Such a setup yields in-situ temperatures generated during drilling that are synced with the cutting forces/torques. Thus, a rich machining data has been obtained that provides insight into the relationship between cutting temperatures, tool wear and machining parameters used for drilling MD-CFRP laminates. Additionally, drilled MD-CFRP samples and drill bits have been characterized to evaluate machining-induced damage in the composite laminates and tool wear in the drill bits. Finally, a coupled thermo-mechanical transient FE framework has been developed to simulate drilling of MD-CFRP laminates. The laminate has been modelled ply-by-ply as an equivalent homogeneous material using temperature dependent properties. A modified Hashin stressbased criterion has been implemented via a user material model for element deletion that delineates the specific damage modes occurring during drilling. This novel proposed damage model allows for inclusion of out of plane damage behaviour along tool feed direction. Additionally, a surface based CZM approach has been included to simulate delamination onset during drilling. The highlight of the proposed numerical approach is the inclusion of frictional heat generation and appropriate thermo-mechanical damage model to capture damage processes specific to drilling of MD-CFRP. The numerical model predictions show a good agreement with CFRP drilling experiments for thrust force, delamination damage and in-situ cutting temperatures.