Computerized Tomography and C-scan for Measuring Drilling-Induced Delamination in Composite Material Using Twist Drill and Core Drill

Delamination is one of the most concern defects in drilling of composite material. Delamination depends on the factors such as feed rate, tool geometry and wear. The mechanics of drilling composite materials has been examined along with the quality of the hole and the effect of tool design parameters. The capacity of computerized tomography (CT) showing sample cross-section in a nondestructive way made it successful in measuring the drilling-induced delamination. In the experiment, the correlation between thrust force and the measured delamination extent in use of twist drill and core drill is illustrated and compared with the known ultrasonic C-scan. It is compared with the ultrasonic technique and is demonstrated a feasible and an effective tool for the evaluation of drilling-induced delamination. Introduction The mechanical properties of composite materials are well recognized as a different from that of metal for their high specific strength, high specific stiffness and corrosion resistance. In composites, due to the inherent nonhomogenerous and orthotropic nature of the material, the characteristic of machining is different from that of homogeneous metal removal [1]. Failure mechanisms of composite materials are quite complex in machining. Delamination is one of the most concern defects in drilling of composite material. Delamination varies from depending on several factors such as feed rate, tool geometry and wear [1-5]. The mechanics of drilling composite materials has been examined along with the quality of the hole and the effect of tool design parameters. It is well known that drill point geometry has a significant effect of the thrust force of a twist drill [6-8]. Many types of drill bits have been tested on glass-epoxy laminates [9]. Koenig et al. investigated the effect of drilling thrust on delamination damage [1,10]. The point geometry of the multifacet drill (MFD) has the obvious advantages in reducing thrust force and in distributing the cutting temperature. Wu used multifaceted drills to reduce the thrust force up to 70 percent as compared with a conventional twist drill has been achieved [11]. Haggerty and Ernst found that “spiral” point drills performed much better than the conventional ones [12]. Ko et al. used a step drill to minimize the burr size in drilling [13]. Jain and Yang designed a hollow grinding drill and tested [14]. This tool resulted in a much smaller thrust and much better hole quality compared with the twist drill. Delamination at the exit side can be improved if the drilling-induced thrust is distributed to circular load. The saw drill, candle stick drill, core drill and step drill have the merit of a better distributed circular load than twist drill [15]. Delamination in drilling processing was usually investigated by ultrasonic C-scan [16-17]. Generally, due to the heterogeneity of composite material, conventional C-scan is unable to provide information regarding the details of damage. The normal practice in evaluation of delamination through ultrasonic C-scan is to obtain a projected area. However, delamination at various interfaces distributed through the thickness. The capacity of CT showing sample cross-section in a nondestructive way made it successful in industry. There are few works using computerized tomography for the evaluation of delamination produced by drilling. It is proposed a feasible and effective tool for the evaluation of drilling-induced delamination extent in the current study. In the Key Engineering Materials Vol. 339 (2007) pp 16-20 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland Online available since 2007/May/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 140.114.58.102-11/02/09,07:45:44) experiment, the correlation between thrust force and the measured delamination in use of twist drill and core drill are illustrated and compared with the known ultrasonic C-scan. Experimental Setup The carbon/epoxy composite materials for drilling were fabricated from the toughened woven carbon/epoxy of Amoco T300 fibers in 934 epoxy matrix using autoclave molding. The stacking sequence of the laminates was . Specimens of size 60 mm S 12 ] 90 / 0 [ ×60 mm were cut on a water cooled diamond table saw. Twenty-four lamina made the plate thickness 3.6 mm. The fiber volume fraction is 0.63, the modulus of elasticity ( ) is 189 Gpa, the energy release rate ( ) is 240 J/m 1 E IC G , and Poisson’s ratio (ν ) is 0.3. The twist drills of 10 mm diameter with high speed steel was used. The diameter of the core drill is 10 mm with one end of the tube coated with diamond and its length and thickness are 12 mm and 1 mm, respectively. Drilling tests were carried out on a LEADWELL MCV-610AP vertical machining center. The mean thrust forces at the exit of the drill bits during drilling were measured with a Kistler 9273 piezoelectric dynamometer. Meanwhile, the drilling and thrust forces signals were transmitted to Kistler 5019 charge amplifiers and stored in a TEAC DR-F1 digital recorder subsequently. The amplifier has to stabilize for at least an hour. All tests were run with coolant at spindle speed of 900 and 1000 rpm and feed rates of 10, 11 and 12 mm/min, respectively. Nondestructive Evaluation Methods In X-ray computed tomography, a beam of radiation passes through the object and exposes a film in a light-tight packet. The resulting image is a high resolution projection of the object. Computed tomography is the reconstruction of a cross-sectional image of an object from its X-ray projection. In other words, it is a coherent superposition of projections obtained using a scanner to reconstruct a pictorial representation of the object. The schematic of X-ray computed tomography is shown in Fig. 1. The computer tomography for the carbon fiber-reinforced composites was made by means of the Siemens Somatom AR high performance X-ray medical computer tomography provided with an MCT141 CT X-ray tube, of which the acceleration potential can be selected between 110 and 130 kV. The X-ray is rotated incrementally 180 around the axis perpendicular to the specimen feed direction. The schematic of X-ray computed tomography of the attenuation and detection of radiation is shown in Fig. 1. To illustrate the fine delamination of the carbon fiber-reinforced composites, the serial CT slides were taken by optimal window width and center. These pictures correlate the X-ray absorption density to the mechanical density of the sample. D / A Converter A / D Converter Line Printer