Computerized tomography and C-Scan for measuring delamination in the drilling of composite materials using various drills

Transcription

1 International Journal of Machine Tools & Manufacture 45 (2005) Computerized tomography and C-Scan for measuring delamination in the ing of composite materials using various s C.C. Tsao a, *, H. Hocheng b a Department of Automatic Engineering, Ta-Hua Institute of technology, Hsinchu 307, Taiwan, ROC b Department of Power Mechanical Engineering, National Tsing-Hua University, Hsinchu 300, Taiwan, ROC Received 27 September 2004; accepted 13 January 2005 Available online 7 March 2005 Abstract Whilst ing is the most frequently employed operation of secondary machining for structure joining, delamination is a very serious defect during ing of fiber-reinforced composite materials,. The evaluation of the delamination damage in the material is important but rather difficult, particularly for carbon fiber-based composites, because their colour makes visual inspection difficult. A special technique using medical equipment for computerized tomography is presented in this paper. It is compared to techniques using ultrasonic and is demonstrated as a feasible and an effective tool for the evaluation of ing-induced delamination. q 2005 Elsevier Ltd. All rights reserved. Keywords: Drilling; Dlamination; Composite material; Computerized tomography; Ultrasonic C-Scan 1. Introduction Carbon fiber-reinforced composites are well recognized for their superior mechanical properties and are advantageous for use in aerospace, defense and transportation structures. Composite materials have particular characteristics that govern their machining behaviour, and the mechanism of machining of composite materials has been recognized as a process fundamentally different from that of homogeneous metal removal [1]. Among all machining operations for composites, ing by twist s is the most common method, and poor hole quality accounts for an estimated 60% of all part rejection [2]. Furthermore, since holes are ed in finished products, part rejections due to poor hole quality are very costly. The mechanics of ing composite materials has been examined along with the quality of the hole and the effect of tool design parameters. The fibrils or fuzz caused by conventional tools, which cut the holes from the centre and force chips against walls, can be significantly reduced. For example, the influence of tool wear and the resulting increase of thrust have been discussed [3 6]; * Corresponding author. Tel.: C ; fax: C address: aetcc@msdb.thit.edu.tw (C.C. Tsao). Koenig et al. investigated the effect of processing variables on ing damage [1,7]; and there is also general overview of the various possibilities for composites machining [8]. Drilling-induced delamination occurs both at the entrance and the exit planes of the workpiece. Investigators have studied analytically and experimentally the cases in which delamination in ing have been correlated to the thrust force during exit of the [9,10]. Experiments indicate that there is a thrust force threshold, below which no delamination occurs. In ing, a twist induces a high thrust force, so this high thrust force between the and the composite materials may cause separation of plies at the exit. Delamination at the exit side can be improved if the ing-induced thrust with distributed circular load is used. The saw and candle stick have the merit of a better-distributed circular load than twist [10]. The saw and candle stick both punches through the last plies over a smaller area than the twist, thus a smaller section of the last laminate is subjected to a bending force from the center. Though the delamination is a very serious defect during ing of fiber-reinforced composite materials, the evaluation of the ing-induced delamination damage in the material is rather difficult. Particularly for carbon /$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi: /j.ijmachtools

2 C.C. Tsao, H. Hocheng / International Journal of Machine Tools & Manufacture 45 (2005) fiber-based composites, because their colour makes visual inspection difficult. With increasing application of composite structures, visualization and assessment of the internal delamination is becoming a vital and challenging task. Nondestructive examination of the composite materials to obtain the size, shape and location of delamination is highly desirable, and ultrasonic C-Scan has been widely employed for this purpose. However, due to the heterogeneity of composite materials, ultrasonic waves suffer from large acoustic attenuation and scattering, making data interpretation difficult. The evaluation of delamination requires experience and a good knowledge of the structure of composite materials. These difficulties are often dealt with by the proper selection of probe and parameter settings, such as pulse width, pulse amplitude, pulse repetition rate, delay, blanking, gain, and data processing. Here, a special technique using medical computerized tomography is presented in this paper. This method of evaluating internal delamination is considered very advanced among the available diagnostic tools for the composite materials research community. X-ray computerized tomography (CT) has been invaluable for medical diagnosis since the first commercial tomography was introduced in The ability of this technique to show sample cross-sections in a nondestructive way has also made it successful in industry. Recent advances in X or g-ray technologies have allowed development of computerized tomography methods for the rapid nondestructive, three-dimensional (3D) analysis of intact biological tissue [11]. With X or g-ray CT, the intensity of a collimate X or g-ray beams passing through an object is measured by an array of detectors located opposite to the X or g-ray source. This is proposed as a feasible and an effective tool for the evaluation of ing-induced delamination in the current study. This paper presents the use of CT for the evaluation of delamination produced by ing. In the experiment, the correlation between thrust force and the measured delamination using various s are illustrated and compared with the known ultrasonic C-Scan technique. 2. Experimental set-up 2.1. Specimen preparation A carbon/epoxy composite material is ed in this study. Composite laminates were prepared from woven WFC200 fabric carbon fiber prepregs. The stacking sequence of all the laminates is [0/90] 12S, and 24 lamina produce a plate thickness of 6 mm. The fiber volume fraction is 0.55, the modulus of elasticity (E 1 ) is 18.4 GPa, the energy release rate (G IC ) is 140 J/m 2 and the Poisson ratio (n) is Drilling test A of 10 mm diameter with high speed steel was used. Drilling tests were carried out on a LEADWELL MCV- 610AP vertical machining center. The thrust forces induced during ing were measured with a Kistler 9273 piezoelectric dynamometer. Meanwhile, the ing and thrust forces signals were transmitted to Kistler 5019 charge amplifiers and then stored on a TEAC DR-F1 digital recorder. The amplifier was allowed to stabilize for at least an hour. All tests were run without coolant at spindle speeds of 900 and 1000 rpm and feed rates of 3, 5, 8, 9, 10, 11 and 12 mm/min. 3. Measurement of delamination To determine the extent of ing delamination produced by ing, two non-destructive examination (NDE) methods were studied. The specimens were examined by computerized tomography and ultrasonic C-Scan. Fig. 1. Schematic of X-ray computerized tomography.

3 1284 C.C. Tsao, H. Hocheng / International Journal of Machine Tools & Manufacture 45 (2005) Fig. 2. Schematic of ultrasonic C-Scan Computerized tomography The schematic of X-ray computed tomography is shown in Fig. 1. In this method, X or g-rays from a finely collimated source provide the cross-sectional imaging of an object using either transmission or reflection data that are collected by illuminating a slice of the object or patient from many different directions. Basically a CT scanner consists of an X or g-ray source, from which an X or g-ray beams are transmitted through an object and are detected and manipulated electronically, after which they are stored in a computer (Fig. 1). These transmitted radiation beams have a modulated intensity dependent on the overall linear attenuation characteristics of the intervening material. This modulated or varying intensity with respect to distance is referred to a profile. This profile information is then manipulated to produce a reconstructed image, which is then displayed on a viewing monitor for evaluation and interpretation. The computer tomography for the carbon fiber-reinforced composites used the Siemens Somatom AR high performance X-ray medical computer tomography provided with an MCT141 CT X-ray tube, whose acceleration potential can be selected between 110 and 130 kv. By CT, it is possible to determine damage through the thickness of the laminate. During one scan, the specimen is irradiated with x-rays. A detector on the opposite side detects the attenuation caused by the object and converts the signal to an optical image, which is recorded with a video camera. The X-ray is rotated incrementally 1808 around the axis perpendicular to the specimen feed direction. The scan is then visualized on a computer screen, which shows a slice of the specimen perpendicular to the direction of the X-rays. A schematic of X-ray computed tomography of the attenuation and detection of radiation is shown in Fig. 1. The image reconstruction was performed by a FUJIX Medical Image Processor MF-300S, which ensures fast reconstruction within 6 14 s. The reconstructed images were displayed on a high definition monitor, from which they were copied on a film. All the images presented in this report are positive copies on photographic paper obtained from the negatives. To illustrate the fine delamination of the carbon fiberreinforced 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 Fig. 3. Ultrasonic C-Scan and CT scan show the extent of ing-induced delamination for various s (spindle speedz1000 rpm, feedz mm/rev)

4 C.C. Tsao, H. Hocheng / International Journal of Machine Tools & Manufacture 45 (2005) Fig. 4. Experimental correlation between thrust force and delamination for various s. the sample. The intrinsic X-ray absorption density scale of the CT is in Housfield units (HU) Ultrasonic C-Scan The ultrasonic C-Scan test was made on an AIT-5112 unit. The specimen was placed between the sender and receiver and scanned at normal incidence in throughtransmission mode by means of a focused broadband transducer (9.5 mm in diameter) with a center frequency of 5 MHz. The testing device consists of a mm resolution scanning bridge, an AIT-2230 ultrasonic pulser/receiver and a digital oscilloscope used for radio frequency echo signal acquisition. During scanning, the sender emits ultrasonic waves, while the attenuation through the receiver is recorded at each point, stored on the internal buffer of the oscilloscope, and transferred to the computer hard disk once the buffer is filled. A database representing the internal delamination of the specimen is built, which allows post-processing of data

5 1286 C.C. Tsao, H. Hocheng / International Journal of Machine Tools & Manufacture 45 (2005) for reconstruction of delamination by selecting the appropriate gate location and width. A schematic of the ultrasonic C-Scan is shown in Fig. 2. In Fig. 3, the ultrasonic C-Scan and CT scan for the extent of the ing defects (delamination) are presented. Around the hole in the specimen, the damage was evident at the edge of the hole. Though delamination is a very common defect during ing, its detection using radiological evaluation is rare. The present study presents a comparison between the CT and C-Scan findings, using 12 specimens with delamination that were sent to detailed ultrasonic C-Scan and CT for examination Digitization of delamination extent Commercial software (Photo Impact 6.0) was used to extract the computerized tomography and the ultrasonic C-Scan image data during scanning for measuring the delamination. A large number of high-contrast images, each consisting of 200!200 resolution (pixels), were obtained from each scanning. The extent of delamination was an average taken from six measurements along the perimeter of the delamination. 4. Experimental results The delamination photograph from the ultrasonic C-Scan and CT is shown in Fig. 3. Using the two NDE methods, the shape of delamination was found to be slightly different. Among the three bits, the delamination from twist at the same feed rate is the highest, followed by saw, while the candle stick is the lowest. This confirms that the twist is more susceptible to causing delamination damage in ing. The different geometry results in different level of thrust during ing and different value of critical thrust force causing delamination as well. It can be seen that the traditional twist offers a relatively high threshold of the thrust force in delamination compared to the saw and candle stick. The correlation between the theoretical analysis and the practical experience for twist and other special bits is further explored as follows. According to the basic principles of fracture mechanics, first postulated by Griffith in 1920, the thrust force from the workpiece as ing proceeds supplies the energy required for incremental extent of delamination during ing, plus the increased strain energy of the body as a result of the crack propagation. Hence a positive relationship between thrust force and delamination is expected, as shown in Fig. 4. The critical thrust force at the onset of delamination can be thus identified as the intersection of the regression line and the horizontal axis shown in the figure. The experimental correlation between thrust force and delamination for various s by C-Scan and CT are also listed in Table 1. Based on the analytical push-out model [9 10], the critical thrust force of various s at onset of inginduced delamination are p F A Z p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 32G IC M for twist, sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 32G F S Z p IC M ð1 K2s 2 Cs 4 Þ Z p 8G ICEh 3 1=2 3ð1 Kn 2 (1) Þ for saw, and sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 32G F C Z pð1 CaÞ IC M 1 Ca 2 ð1 K2s 2 Cs 4 Þ for candle stick. Table 1 Experimental results of thrust force and delamination measured by C-Scan and CT for various s Drill bit Feed rate (mm/rev) Thrust force (N) (2) (3) Delamination extent (mm) C-Scan CT Twist Saw Candle stick

6 C.C. Tsao, H. Hocheng / International Journal of Machine Tools & Manufacture 45 (2005) Table 2 Critical thrust force of various s Drill bit Twist Saw Candle stick Experimental critical thrust force (N) obtained by CT In the results, M is flexural rigidity of the plate, E is the modulus of elasticity, G IC is the energy release rate, h is the uncut depth under the tool, n is the Poisson ratio, s is the ratio between the radius of (c) and the radius of delamination (a), and a is the ratio between the central concentrated force (p 1 ) and the peripheral circular force (p 2 ). The theoretical thrust force of various s was calculated by Eqs. (1) (3) and the extent of delamination (represented by s) was determined by the ultrasonic C-Scan and CT. Table 2 shows that the theoretical analysis can fairly predict the ing-induced delamination. Fig. 4 and Table 2 illustrate that CT is an effective tool, comparable to C-Scan for the evaluation of delamination in composite materials. Table 2 shows that the errors of saw and candle stick are higher than twist, which can be attributed to the following. The precision of manufacture for these bits is less well controlled due to the more complicated tool geometry. On the other hand, the tool-work contact is in fact an equally spaced discrete loading rather than the uniform circular load adopted in the mathematic analysis. 5. Conclusions Experimental critical thrust force (N) obtained by C-Scan Theoretical critical thrust force (N) ABS. error of CT (%) ABS. error of C-Scan (%) This paper shows that the ing-induced delamination of composite materials can be visualized and measured by the ultrasonic C-Scan, as well as the X-ray computerized tomography. The obtained results indicated that both ultrasonic C-Scan and CT perform similarly. Both can reveal experimentally the critical thrust force of various bits at the onset of delamination. CT is demonstrated to be a feasible and effective tool for the evaluation of ing-induced delamination. Furthemore, the recognized correlation between the thrust force and delamination, as well as the proportionality between feed rate and delamination, can be successfully demonstrated by the proposed technique. Acknowledgements This work is partially supported by National Science Council, Taiwan, ROC, under contract NSC E The authors appreciate the assistance of Mrs C.F. Chu at WMH for the CT operation. References [1] W. Koenig, C. Wulf, P. Grass, H. Willerscheid, Machining of fiber reinforced plastics, Annals of the CIRP 34 (2) (1985) [2] T.L. Wong, S.M. Wu, G.M. Croy, An analysis of delamination in ing composite materials, Proceedings 14th SAMPLE Technology Conference, Atlanta, GA, USA 1982; [3] J.H. Doran, C.R. Maikish, Machining boron composite in: B.R. Noton (Ed.),, Composite Materials in Engineering Design, ASM Press, 1973, pp [4] A. Koplev, A. Lystrup, P. Vorm, The cutting process, chips and cutting forces in machining CFRP, Composites 14 (4) (1983) [5] K. Sakuma, Y. Yokoo, M. Seto, Study on ing of reinforced plastics- relation between tool material and wear behavior, Bulletin of JSME 27 (228) (1984) [6] F. Veniali, A. Llio, V. Tagliaferri, An experimental study of the ing of aramid composites, Transaction of ASME Journal of Energy Resources Technology 117 (1995) [7] W. Koenig, P. Grass, A. Heintze, F. Okcu, C. Schmitz-justin, Developments in ing, contouring composites containing Kevlar, Production Engineer 9 (1984) [8] R. Komanduri, Machining fiber-reinforced plastics, ASME Mechanical Engineering 4 (1993) [9] H. Hocheng, C.K.H. Dharan, Delamination during ing in composite laminates, Transaction of ASME Journal of Engineering for Industry 112 (1990) [10] H. Hocheng, C.C. Tsao, Comprehensive analysis of delamination in ing of composite materials with various bits, Journal of Materials Processing Technology 140 (2003) [11] G.N. Hounsfield, Computerized trans-axial scanning (tomography) I: description of system, British Journal of Radiology 46 (1973)