Drilling in carbon/epoxy composites: Experimental investigations and finite element implementation

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Abstract

Drilling carbon fibre reinforced plastics (CFRPs) is typically cumbersome due to high structural stiffness of the composite and low thermal conductivity of plastics. Resin-rich areas between neighbouring plies in a laminate are prone to drilling-induced delamination that compromises structural integrity. Appropriate selection of drilling parameters is believed to mitigate damage in CFRPs. In this context, we study the effect of cutting parameters on drilling thrust force and torque during the machining process both experimentally and numerically. A unique three-dimensional (3D) finite element model of drilling in a composite laminate, accounting for complex kinematics at the drill-workpiece interface is developed. Cohesive zone elements are used to simulate interply delamination in a composite. Experimental quantification of drilling-induced damage is performed by means of X-ray micro computed tomography. The developed numerical model is shown to agree reasonably well with the experiments. The model is used to predict optimal drilling parameters in carbon/epoxy composites.

Introduction

Composite materials offer excellent strength-to-weight ratio, damage tolerance, fatigue and corrosion resistance, which make them good candidates for replacement of conventional materials for structural applications. As a result, advanced composite materials make about 50% of the structural weight of Boeing 787 and Airbus A350XWB [1]. Generally, parts made of composites are produced to a near-net shape, but additional machining operations are often required to facilitate component assembly. For example, joining of composite components to a structure often requires manufacturing holes in them in order to place bolts or rivets. To manufacture these holes, drilling is a commonly used machining process. In it a rigid tool, typically a twist drill, cuts out the required area of the composite workpiece. During this process the tool encounters alternatively matrix and reinforcement materials, of which response to machining can be completely different. The process implies destruction of fibre continuity with generation of large stress concentration in the material and delamination at the hole entry and exit [2], [3], [4], [5], [6], [7]. The damage caused can significantly reduce the fatigue strength of the component, thus degrading the long-term performance of composite laminates [8], [9]. Previous studies have shown that machining fibre-reinforced polymer (FRP) composites materials differs significantly in many aspects from machining conventional metals and alloys primarily due to the underlying heterogeneity and anisotropy of FRP materials [2], [3], [4], [5], [6], [7], [8], [9].

In the literature, experimental, analytical and numerical modelling techniques have been used to study cutting mechanisms in FRP machining; excellent reviews on composite machining can be found in [5], [6], [10], [11], [12], [13], [14]. Experimental findings, though useful, provide limited information on underlying mechanics of composite deformation and damage propagation. Recently, numerical modelling has been used as a tool for a better understanding of machining of these composites. These studies typically focus on 2D models of cutting [15], [16], [17], which cannot account realistically for actual complex three-dimensional shape of cutting tool and the kinematics of the drilling process. However, advances in computational power led to the development of modelling tools and numerical strategies, which cover a wide range of temporal and geometrical length scales, as well as higher dimensionality.

This paper deals with drilling of CFRP composites and is arranged as follows: In Section 2, a comprehensive overview of drilling experiments and an X-ray micro computed tomography (μCT) scanning procedure is provided followed by discussion of experimental results. In Section 3, a detailed strategy used to develop a 3D finite-element model of drilling in a CFRP laminate is discussed. In Section 4, we present the results of finite-element simulations along with an optimisation study focussed on determination of an appropriate combination of machining parameters in order to mitigate drilling-induced damage.

Section snippets

Machine setup

The drilling experiments were conducted on a Harrison M-300 lathe machine with 2.24 kW spindle power and a maximum speed of 2500 rpm. A Jobber carbide TiN-coated twist drill bit with diameter 3 mm was mounted in its three-jaw universal chuck. The experimental setup and the drill bit are shown in Fig. 1.

A dynamometer was placed on the cross-slide of the lathe using an angle plate. The two-channel Kistler™ (Model number 9271 A) dynamometer was used to acquire thrust force and torque data. The

Finite element model of drilling in carbon/epoxy composites

In case of composite laminates, to increase drilling efficiency along with damage mitigation, it is imperative to understand the effect of machining parameters on CFRP. Various experimental studies were carried out in the past [4], [7], [14], [20], [21], [22], [23], [24] to optimise the machining parameters in order to obtain better performance in drilling of CFRP composites. Several analytical models [25], [26], [27], [28] were also developed to determine the critical thrust force and torque

Conclusions

In this paper the effect of discrete machining parameters on thrust force and torque in drilling of a cross-ply T300/LTM45-EL composite laminate was investigated both experimentally and numerically. Drilling-induced delamination, being one of the critical modes of damage in CFRP, was quantified experimentally from micro-tomography images after appropriate image processing. A 3D FE model of drilling in CFRP was developed. The underlying user-defined material model accounts for an orthotropic

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