Delamination analysis of carbon fibre reinforced laminates: Evaluation of a special step drill

Abstract

Drilling of carbon fibre/epoxy laminates is usually carried out using standard drills. However, it is necessary to adapt the processes and/or tooling as the risk of delamination, or other damages, is high. These problems can affect mechanical properties of produced parts, therefore, lower reliability. In this paper, four different drills – three commercial and a special step (prototype) – are compared in terms of thrust force during drilling and delamination. In order to evaluate damage, enhanced radiography is applied. The resulting images were then computational processed using a previously developed image processing and analysis platform. Results show that the prototype drill had encouraging results in terms of maximum thrust force and delamination reduction. Furthermore, it is possible to state that a correct choice of drill geometry, particularly the use of a pilot hole, a conservative cutting speed – 53 m/min – and a low feed rate – 0.025 mm/rev – can help to prevent delamination.

Introduction

The use, in primary structures, of fibre reinforced plastics has increased in the last decades due to their unique properties. Advantages are related with their low weight, high strength and stiffness. Although the development of these materials have been mostly related with aerospace and aeronautical applications, recent years have seen the spread of their use in many other industries, like automotive, where high production rates are required.

The use of composite laminates in dynamic structures had enabled a considerable weight reduction and, consequently, an improvement in their characteristics. Although composite components are produced to near-net shape, machining is often needed, as it turns out necessary to fulfil requirements related with tolerances or assembly needs. Machining operations in composites can be carried out using conventional machinery with adaptations. Among machining processes, drilling is one of the most frequently used to make holes for screws, rivets and bolts. However, drilling is a complex process which is characterized by the existence of extrusion by the drill chisel edge and cut by the rotating cutting lips.

As composites are neither homogeneous nor isotropic, drilling raises specific problems that can be related with subsequent damage in the region around the hole. The most frequent defects caused by drilling are delamination, fibre pull-out, interlaminar cracking or thermal damages [1], [2]. These defects can affect not only the load carrying capacity of laminated parts but also reliability [3]. Rapid tool wear, as a result of material abrasiveness, can be an important factor in damage occurrence too [4] and increases the need of frequent tool changes that affects the production cycle.

When drilling of a composite part is considered, results are more dependent on fibre nature than on matrix material [5].

The importance of tool geometry in delamination reduction is evidenced by several published works on the subject. For example, Piquet et al. [6] suggested the use of a great number of cutting edges, from 3 to 6, in order to increase the contact length between tool and part, a point angle of 118° and a small rake angle. Chisel edge should be as reduced as possible. Park et al. [7] applied the helical-feed method to avoid fuzzing and delamination. The use of helical feed allow for an efficient completion of the drilling operation avoiding depth limitation. Stone and Krishnamurthy [8] studied the implementation of a neural network thrust force controller. The control scheme can minimize delamination by varying feed rate in order to control thrust force. Persson et al. [3] presented an orbital drilling method, where the hole is machined both axially and radially. This method eliminates the stationary tool centre, thus reducing the axial force, reduces the risk of tool clogging and allows for the use of one tool diameter to machine several hole diameters. Davim and Reis [9] studied the effect of cutting parameters on specific cutting pressure, delamination and cutting power in carbon fibre reinforced plastics. They concluded that feed rate has the greater influence on thrust force, so damage increases with feed. Tsao and Hocheng [10] analyzed the effect of a backup plate on delamination, in order to understand and explain the advantage of its use in composite laminate drilling. Results show that the use of a backup plate causes an increase in the critical thrust force, allowing for higher feed rates. In another work [11], Tsao and Hocheng conducted several practical experiences to prove the benefit of using special drill when compared to commercially available tools, like twist drill. The effect of their use on delamination was evaluated. Delamination extent was determined with the help of ultrasonic C-scan. At the end, it was possible to conclude that thrust force varies with drill geometry and with feed rate. That enables for the use of higher feed rates if adequate drill geometry is selected. More recently, Tsao and Hocheng [12] have presented the advantages of a core drill. The influence of spindle speed was relatively insignificant. Fernandes and Cook [13] investigated the thrust force during drilling with “one shot” drill bit. Their objective was to extend tool life and improve hole quality. For that, a mathematical model leading to the calculation of feed in order to keep thrust force under a critical threshold was developed. Finally, Tsao [14], [15] evaluated the importance of pilot hole on delamination reduction when using core drills and saw drills. Pilot hole eliminates the chisel edge effect, reducing delamination hazard. The ratio of pre-drilled hole to drill diameter must be controlled in order to drill with higher feed rate without delamination. In his experimental work, Tsao has found an optimal ratio of 0.85 for the use of the largest feed rate – 0.012 mm/rev.

Dharan and Won [16] conducted a series of machining experiments in order to propose an intelligent machining system that avoids delamination by peel-up at entrance and by push-out at exit. Such a system should be able to limit feeds when the tool contacts the plate and before drill breakthrough, avoiding delamination onset.

Recently, statistical tools had been used in the search for optimal cutting conditions or tool design. Tsao [17] applied the Taguchi method and analysis of variance to recommend a step angle of 100° and a step ratio equal to 0.4 mm/mm. Both feed rate and spindle speed were conservative. Gaitonde et al. [18] developed a model based on response surface methodology for the study of the effects of cutting speed, feed rate and point angle on delamination. Results show a decrease in delamination tendency when cutting speed increases and also advise the use of low feed rates combined with adequate tool point angle. A multi-objective optimization of cutting parameters using genetic algorithms was presented by Sardiñas et al. [19].

Delamination is not an exclusive problem of carbon reinforced laminates, as there are other published works with the same objective of delamination reduction for glass fibre reinforced plastics [20], [21], a literature review on the theme [22] or even medium-density fibreboard (MDF) delamination assessment [23]. In all these works a relation between drilling parameters, tool geometry and delamination damage is studied.

As carbon fibre/epoxy laminates are opaque some of these defects are not visible in a visual inspection, so it is needed to establish non-destructive testing (NDT), like enhanced radiography, in order to determine the extension of internal damages, such as delamination. Digital images are also used to assess this type of damage after drilling [24].

Advantages in the use of composite plates are growing every day, therefore an increase in parts production and need for higher quality machining and dedicated tooling is to be expected. In this work, a different drill geometry is suggested and some comparison regarding thrust forces and delamination damage with different tools is established.