1、Effects of special drill bits on drilling-induced delamination of composite materials H. Hocheng and C.C. TsaoAbstract：Drilling is the most frequently employed operation of secondary machining for fiber-reinforced materials owing to the need for joining structures. Delamination is among the serious
2、concerns during drilling. Practical experience proves the advantage of using such special drills as saw drill, candle stick drill, core drill and step drill. The experimental investigation described in this paper examines the theoretical predictions of critical thrust force at the onset of delaminat
3、ion, and compares the effects of these different drill bits. The results confirm the analytical findings and are consistent with the industrial experience. Ultrasonic scanning is used to evaluate the extent of drilling-induced delamination. The advantage of these special drills is illustrated mathem
4、atically as well as experimentally, that their thrust force is distributed toward the drill periphery instead of being concentrated at the center. The allowable feed rate without causing delamination is also increased. The analysis can be extended to examine the effects of other future innovative dr
5、ill bits. Keywords: Drilling; Delamination; Special drill; Ultrasonic C-Scan; Composite material 1. Introduction The peculiar behavior of composite materials during machining has been widely observed experimentally. A proper choice of cutting conditions is difficult due to the coexistence of hard ab
6、rasive fibers and a soft matrix. Based on the experimental observations, little plastic deformation of composite materials occurs during cutting, and the fracture resistance is 10100 times lower than that of common steels. Also, the extent of delamination is well correlated with the drilling thrust
7、force. In drilling operations, the center of the twist drill induces a large thrust force, which can cause separation of plies at the exit as the interlaminar bonding yields. Delamination at the exit side can be reduced if a drill with a small chisel edge is used. The candle stick drill has a smalle
8、r center than a twist drill. It punches through the last pliesover a smaller area compared with the twist drill. Thus a smaller width of the last laminate is subjected to a bending force from the center. Saw drills and core drills eliminate the chisel, while the step drill cuts a hole mainly at the
9、second stage after its chisel is through. This paper first presents the experimental correlation between the drilling-induced delamination and the thrust force in use of special drills, and secondly examines the predictions based on theoretical models. 2. Experimental setup 2.1. Specimen preparation
10、 Composite laminates were made of woven WFC200 fabric carbon fiber prepregs with the stacking sequence of 0/9012S. The laminates were cured in an autoclave at 150C and 600KPa. The plates were cut into coupon specimens of 60mm60mm. Twenty-four lamina make a plate thickness at 6mm. The fiber volume fr
11、action was 0.55, the Youngs modulus was 18.4GPa, Poissons ratio was 0.3 and strain energy release rate was 140J/m2 . 2.2. Drilling test Drilling tests were carried out on a vertical machining center. The mean thrust forces at the exit of the drill bits during drilling were measured with a Kistler 92
12、73 four-component piezoelectric dynamometer and Kistler 5007 charge amplifiers, and were stored on a TEAC DR-F1 digital recorder. The amplifiers have to stabilize for at least an hour. The twist drill, saw drill, candle stick drill and step drill made of high speed steel (10mm in diameter) were used
13、. The drill diameter ratio () of step drills was 0.2. The core drills are 10mm in diameter plated with diamond of #60 grit size at the front end. All tests were run at the spindle speed of 900 and 1000rpm and feed rates of 0.003, 0.005, 0.008, 0.0088, 0.009, 0.01, 0.011, 0.0111, 0.012, 0.0122 and 0.
14、0133mm/rev. The drilling is run dry except in use of the core drill, for which water cooling is applied to ensure that burning does not occur during drilling. 2.3. Ultrasonic C-Scan and image acquisition To determine the extent of delamination produced by various drills, the specimens were examined
15、by the ultrasonic C-Scan technique to evaluate drilling-induced delamination. The ultrasonic C-Scan equipment was an AIT-5112 unit. The specimen was placed between the sender and receiver. When the sender emits ultrasonic waves, the attenuation through the receiver is recorded in a computer. A large
16、 number of high-contrast images, each consisting of 200200 pixels, were obtained from each scanning. Each delamination image is represented by an array of gray scale values (0255) corresponding to the differences in lamination density. To obtain the processed image, the pixel value of the drilled hole was set to 0 (black) because it is less than the threshold value, while the delamination zone was set to 255 (white). The proper threshold value
