16TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS DELAMINATION ANALYSIS OF CARBON FIBRE REINFORCED LAMINATES António T. Marques*, Luís M. Durão**, António G. Magalhães**, João M. Tavares* [A. T. Marques]: [email protected] *DEMEGI/FEUP, Univ. Porto,Portugal, **DEM/ISEP, Polytech. Inst. Porto, Portugal Keywords: composites, drilling, tool geometry, delamination and computational vision Abstract machining is often needed, as it turns out necessary to fulfill requirements related with tolerances or Drilling of carbon/epoxy laminates is normally assembly needs. Machining operations in carried out using standard drills, like twist or Brad composites can be carried out in conventional drills. However, it is necessary to bear in mind the machinery with some adaptations. Among the usual need to adapt the processes and/or tooling as the machining processes, drilling is one of the most risk of delamination, or other damages, is high. frequently used to make holes for screws, rivets and These problems can affect the mechanical properties bolts. Drilling is a complex process which is of the produced parts, hence, lower reliability. characterized by the existence of extrusion and cut Production of higher quality holes, with damage mechanisms. The former is caused by the drill chisel minimization, is a challenge to everyone related with edge that has null or very small linear speed and the composites industry. latter by the existence of rotating cutting lips at a In this paper, four different drills are certain speed. compared in terms of thrust force during drilling As composites are neither homogeneous nor and delamination caused by this machining isotropic, drilling raises specific problems that can operation. In order to evaluate delamination be related with subsequent damage in the region damage, enhanced radiography is applied. The around the hole. The most frequent defects caused resulting images were then processed using a by drilling are delamination, fibre pull-out, previously developed image analysis and processing interlaminar cracking or thermal damages [1]. These platform. Results show that a correct choice of drill defects can affect not only the load carrying capacity geometry, cutting speed or feed rate can reduce of laminated parts but also reliability [2]. Rapid tool delamination. wear, as a result of material abrasiveness, can also be an important factor in damage occurrence [3]. Koplev et al. [4] examined the cutting process 1 Introduction of unidirectional carbon fibre reinforced plastics and The use of fibre reinforced plastics has stated that the machining of CFRP consists in a increased in the last decades due to their unique series of fractures, each creating a chip. The properties. Advantages are related with their low examination of these chips revealed that they are not weight, high strength and stiffness. Although the subjected to large plastic deformation as is normally development of these materials has been related with found in metal chips. aerospace and aeronautical applications, recent years The importance of tool geometry in have seen the spread of their use in many other delamination reduction is evidenced by several industries like automotive, railway, naval, sport and published papers on the subject. Piquet et al. [5] many others. suggested the use of a great number of cutting edges, The use of composite laminates in dynamic from three to six, in order to increase the contact structures had enabled a considerable weight length between tool and part, a point angle of 118º reduction and, consequently, an improvement in for the main cutting edges and a small rake angle. their characteristics. Although composite Chisel edge should be as reduced as possible. The components are produced to near-net shape, use of pre-drilling neutralizes the chisel edge effect. A low feed rate can reduce delamination, but a better 1 António T. MARQUES*, LUÍS M. DURÃO improvement can be accomplished by applying a ratio of 0.85 for the use of the largest feed rate – variable feed rate. This requires the use of CNC 0.012 mm/rev. drilling machines. Murphy et al. [13] compared the performance Park et al. [6] applied the helical-feed method of three different types of tungsten carbide drills, to avoid fuzzing and delamination. Stone and TiN (titanium nitride) and DLC (diamond like Krishnamurthy [7] studied the implementation of a carbon) coated and an uncoated drill. The use of neural network thrust force controller. The control coatings was found to be of no benefit when scheme can minimize delamination varying feed rate machining carbon/epoxy laminates, reducing neither in order to control thrust force. Persson et al. [2] tool wear nor composite damage. presented an orbital drilling method, where the hole Dharan and Won [14] conducted a series of is machined both axially and radially. This method machining experiments in order to propose an eliminates the stationary tool centre, thus reducing intelligent machining scheme that avoids the axial force. Davim and Reis [8] studied the effect delamination by peel-up at entrance and by push-out of cutting parameters on specific cutting pressure, at exit. delamination and cutting power in carbon fibre As some of these defects are not visible in a reinforced plastics. The authors concluded that feed visual inspection, it is needed to establish non- rate has the greater influence on thrust force, so destructive testing (NDT) in order to be able to damage increases with feed. Tsao and Hocheng [9] determine the existence of internal damages, like analyzed the effect of a backup plate on delamination, between the laminate plies. delamination, in order to understand and explain the Carbon/epoxy laminates are opaque, so radiography advantage of its use in composite laminate drilling. is needed for plate damage evaluation after drilling. Two drill geometries were used – saw drill and core A good deal of information about composite drill. Results show that the use of a backup plate quality can be obtained as long as the damages are causes an increase in the critical thrust force, orientated perpendicular to radiation beam. In order allowing for higher feed rates. In another paper [10], to detect delamination, it is necessary to use a the same authors conducted a series of practical penetrating fluid, so the method used is known as experiences to prove the benefit of using special drill enhanced radiography. when compared to commercially available tools, like Advantages in the use of composite plates are twist drill. The effect of their use on delamination growing everyday, therefore an increase in parts was evaluated. Based on experimental observation, production and need for higher quality machining it is possible to state that fracture resistance is lower and dedicated tooling is to be expected. than that of common steels and that almost no plastic deformation occurs during cutting. Delamination 2 Composites Delamination extent was determined with the help of ultrasonic C- scan. Mathematical models to the determination of 2.1 Damage Models critical thrust force for each tool geometry were presented. At the end, it was possible to conclude When delamination is considered, it is that thrust force varies with drill geometry and with necessary to tell from two types of damage that are feed rate. That enables for the use of higher feed different in their causes and effects: peel-up rates if adequate drill geometry is selected. delamination and push-down delamination. Fernandes and Cook [11] investigate the thrust force Peel-up delamination is a consequence of the during drilling with ‘one shot’ drill bit. Their cutting force pushing the abraded and cut materials objective was to extend tool life and improve hole to the flute surface – figure 1. The material spirals quality. For that, a mathematical model leading to up the drill flute before it is completely machined. the calculation of feed in order to keep thrust force A peeling force pointing upwards is introduced that under a critical threshold was developed. Finally, tend to separate the upper laminas of the uncut Tsao [12] evaluated the importance of pilot hole on portion held by the downward acting thrust force. delamination reduction when using core drills. Pilot Normally a reduction in feed rate can reduce this hole eliminates the chisel edge effect, reducing delamination. delamination significantly. 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 2 DELAMINATION ANALYSIS OF CARBON FIBER REINFORCED LAMINATES Lachaud et al. [16] considered that a normal stress is applied to the ply surface and modeled the uncut plate under the drill as a thin orthotropic plate clamped on the laminate surface. This model is valid only if a small number of uncut plies are involved. In a distributed load model the resultant critical thrust force is 1 2 (2) ⎡ GIc D ⎤ Fcrit = 8π ⎢ ' ⎥ ⎣()1 3 − ()D 8D ⎦ Fig. 1. Peel-up delamination mechanism [15]. and in a point load model hypothesis Push-out delamination is a damage that occurs 1 (3) in interlaminar regions, so it depends not only on ⎡ 2G D ⎤ 2 F = 8π Ic fibre nature but also on resin type and respective crit ⎢1− (D' 8D)⎥ properties. This damage is a consequence of the ⎣ ⎦ compressive thrust force that the drill tip always exerts on the uncut laminate plies. There is a where D and D’ are calculated using relations certain point at which the loading exceeds the of laminates theory. interlaminar bond strength and delamination occurs Tsao and Chen [17] started from equation 1, – figure 2. and tried to predict the location of delamination, The reduction of this delamination type is the determining a value of h corresponding to critical focus of this paper. thrust force – hisotropic. The authors also stated that, in fact, each ply is highly anisotropic and a more realistic shape of delamination should be elliptical.