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A study of the surface texture of composite drilled holes
Journal of Materials Processing Technology, 37 (1993) 373 389 Elsevier
373
A study of the surface texture of composite drilled holes C.W. Wern, M. R a m u l u and K. Colligan Department of Mechanical Engineering, FU-10, University of Washington, Seattle, WA, USA
Industrial S u m m a r y An investigation of the surface texture of composite drilled holes was conducted by drilling experiments at constant speed with two different PCD tipped drill geometries at varying feed rates. Surface profilometry was used to measure the surface roughness profiles of machined graphite/epoxy holes and roughness characteristics were evaluated using average roughness heights, statistical methods, and random process methods. It was found that the width of damage in a composite material is a function of the drill geometry and the feed rate. All the surfaces studied were found to be non-Gaussian, negatively skewed, leptokurtic, and produced a good bearing surface. The use of maximal roughness height parameters, power spectral density function, and cumulative height density plotted on a probit scale allows characterization of the texture of the surfaces of the drilled holes.
1. Introduction In recent years, polymer composites have gained considerable attention in the aircraft and automobile industries due to their light weight and high strength. Even t h o u g h most of the polymer composite structures are molded into near-net shapes, m a c h i n i n g of the components is always necessary to attain the required dimensional tolerance, surface roughness, and complex geometry. In the past two decades, research efforts have been mainly on composite materials' m e c h a n i c a l properties and very few studies have dealt with the m a c h i n i n g of composites and their machined surfaces. It is pertinent to understand the effects of m a c h i n i n g processes on graphite/epoxy because machined surface quality determines the fatigue strength of the c o m p o n e n t ' s service life. In the s e a r c h for an optimal c u t t i n g tool material for m a c h i n i n g graphite/epoxy, poly-crystalline d i a m o n d (PCD) inserts have been s h o w n to be more abrasive r e s i s t a n t t h a n carbide (C6 grade) inserts [1,2]. Studies have also been
Correspondence to: M. Ramulu, Department of Mechanical Engineering, FU-10, University of Washington, Seattle, WA, USA. 0924-0136/93/$06.00 ((~ 1993 Elsevier Science Publishers B.V. All rights reserved.
374
C.W. Wern et al./Surface texture of" composite drilled holes
conducted to evaluate the toot geometry in machining graphite/epoxy laminates [3]. Surface roughness heights are always used as a gauge to evaluat~ the amount of damage on the machined surface to assess the quality of surfaces produced by machining processes or machining parameters. Average surface roughness heights such as average roughness (R~) and maximum peak-tovalley heights (Rt) are used to describe the amount of micro-geometric v a r i ation of the machined metal surface. However, these parameters may no~ t)e~ sufficient for an inhomogeneous material such as graphite/epoxy which ha,~ a rougher surthce compared to metals under the same machining condition~ [4]. Recently, Konig and Graft [5] studied the surface texture of holes and analyzed the drilling of fiber reinforced thermosets (carbon fiber, glass fibe~ and aramid fiber) by quantifying the amount of machining damage t~:dng ten-point height (R~) and width of the damage zone, However, the ,~m'face roughness characteristics of high modulus graphite/epoxy composite drilled holes have not been studied. This paper presents the results of a study of drilled graphite/epoxy holes produced by two different drill point geometries. The feed rate in the drilling process was varied to determine its effects on the surface roughness profile~ and damage width. Machined graphite/epoxy surface textures were evaluated by inspecting the effectiveness of average roughness parameters, statistical methods (e.g. skewness, kurtosis, cumulative height distribution or bearing length), and random process methods (e.g., power spectral density function, G((~)), auto-correlation function, R(r)) in describing surface roughness profiles. To augment the results obtained from surface profile studies, scanning electror~ microscopy was used to observe the damage of the machined surfaces
2. Experimental procedure An experimental study was conducted to assess the surface quality of drilled graphite/epoxy composites. Test panels used in this study were approximatel5 25.4 mm (1 in) thick, and were made up of IM-6 graphite fibers (6 pm diameter~ embedded in 3501-6 thermoset epoxy resin. The lay-up of the test pandits is shown schematically in Fig. 1. On each panel, the outer plies were made ~i bias-weaved fabric while the internal plies were unidirectional tape of t h i c k ness 0.2 mm, laminated in a _+45 alternating pattern. The fabric plies wer~ used to suppress delamination on the entrance and exit of the drill. H o w e v e r the main interest of this study was the unidirectional tape portion in th~ middle of the test panels. Two different PCD tipped drills were used to generate the holes. Drill A was 12.7 mm (0.5 in) in diameter with an axial rake angle of 27, while drill B had a diameter of 13.9 mm (0.5494 in) and an axial rake angle of 7'. Apart from the, difference in axial rake angle and drill size, the two drills have essentially tiat~ same geometry. A Mazak 3-axis CNC milling machine was used for drilling the holes at a constant speed of 4550 rpm. Holes were drilled at feed rates of
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Fig. 1. Test panel and position of the cutting edge with respect to the fiber orientations: the drill rotates clockwise.
from 0.0254 m m / r e v (0.001 in/rev) to 0.254 m m / r e v (0.01 in/rev) in i n c r e m e n t s of 0.0254 m m / r e v (0.001 in/rev). D u r i n g the drilling process, the holes were flooded with D a r a c o o l 706LF, a water-soluble s y n t h e t i c coolant. S u r f a c e r o u g h n e s s profiles of the hole s u r f a c e s were o b t a i n e d using the F e d e r a l S u r f a n a l y z e r System 4000 w i t h a n EPT-01049 d i a m o n d stylus probe. The 2.54 pm d i a m e t e r stylus was chosen for m e a s u r i n g the r o u g h n e s s profile of the s u r f a c e since the fiber d i a m e t e r is a p p r o x i m a t e l y 6 pm. Profile c o o r d i n a t e s were stored for f u r t h e r p r o c e s s i n g t h r o u g h the i n t e r f a c e to a c o m p u t e r . The s u r f a c e r o u g h n e s s c h a r a c t e r i s t i c s were e v a l u a t e d using a cutoff length of 0.8 m m and r o u g h n e s s h e i g h t m e a s u r e m e n t i n t e r v a l s of 1.25 pro. F o u r roughness r e a d i n g s were t a k e n c i r c u m f e r e n t i a l l y , a p p r o x i m a t e l y 9 0 a p a r t on the u n i d i r e c t i o n a l plies m a c h i n e d surface, as s h o w n in Fig. 2. T h e four positions w e r e c h o s e n a l o n g l o c a t i o n s of high m i c r o s c o p i c i r r e g u l a r i t i e s t h a t were e s t a b l i s h e d by visual and optical i n s p e c t i o n of the hole surfaces. T h e m e a s u r e m e n t l e n g t h (or t r a v e r s e length) was a p p r o x i m a t e l y 4 mm, w h i c h c o r r e s p o n d s to a s a m p l e size of a b o u t 3200 r o u g h n e s s h e i g h t m e a s u r e m e n t s . The surface r o u g h n e s s h e i g h t d e n s i t y and s t a t i s t i c a l p a r a m e t e r s were o b t a i n e d u s i n g class
376
C.W. Wern et al./ Surface texture of composite drilled holes
Fiber orientation
Direction of drill rotation"
\ Area of the hole wilh extremely rough surface
Fig. 2. Severe surface damage zone in a unidirectional ply and locations o~' roughn~,~ measurements.
intervals of 1.25 }am so t h a t there were more t h a n 20 class intervals fo~' the r o u g h n e s s h e i g h t s studies. The class interval size of 1.25 pm was chosen to minimize a n y r e d u c t i o n in r e s o l u t i o n with an increase in the n u m b e r of class i n t e r v a l s used. A Jeol JSM-T330A s c a n n i n g electron m i c r o s c o p e (SEM) was used to assess and v a l i d a t e the surface r o u g h n e s s m e a s u r e m e n t s and describe q u a l i t a t i v e l } the d a m a g e observed due to the m a c h i n i n g process. M i c r o g r a p h s were t a k e n at v a r i o u s l o c a t i o n s on the m a c h i n e d surface both at low and high magnifications. Low m a g n i f i c a t i o n of 35 × was used to examine the global d a m a g e while localized d a m a g e was examined at a h i g h e r m a g n i f i c a t i o n of 200 x~
3. R e s u l t s a n d d i s c u s s i o n F i g u r e 3 shows a typical hole surface both at the e n t r a n c e and exit regions of the drill. The use of bias-weaved plies on the surface of the test panels enables a hole to be drilled w i t h o u t a n y d e l a m i n a t i o n . A typical low m a g n i f i c a t i o n m i c r o g r a p h of the sectioned hole surface is s h o w n in Fig. 4(a). Two m a c h i n e d surface zones were g e n e r a l l y observed on the surface of the holes g e n e r a t e d by b o t h drill geometries. P a r t s of the hole surface showed severe pitting and g r o o v i n g on a l t e r n a t e plies, while o t h e r p a r t s of the hole surface were s m o o t h and were covered with m a t r i x and s h e a r e d fibers. F i g u r e 4(b) shows a magnified view of the pitted surface zone. It was observed t h a t bundles of r e o r i e n t e d
C.W. Wern et al./ Surface texture of composite drilled holes
entrance surface
(a)
exit surface
377
entrance surface
exit surface
(b)
Fig. 3. Entrance and exit regions of the holes machined using: (a) drill A; and (b) drill 13. (feed rate = 0.254 mm/rev)
fibers coupled w i t h fiber pull-out leads to an e x t r e m e l y r o u g h surface. Since the s u r f a c e r o u g h n e s s of the s m o o t h s u r f a c e zone does not h a v e a significant v a r i a t i o n in surface texture, t h a t p o r t i o n of the hole s u r f a c e was not included in this study. The surface d a m a g e g e n e r a t e d in m a c h i n i n g g r a p h i t e / e p o x y is d e p e n d e n t on the r e l a t i v e a n g l e b e t w e e n the fiber o r i e n t a t i o n and the d i r e c t i o n of c u t t i n g m o t i o n [6,7]. Fiber pull-out, which caused the p i t t i n g p h e n o m e n a in the r o u g h s u r f a c e zone, o c c u r s w h e n the fiber is at a n e g a t i v e a n g l e to the c u t t i n g direction. W h e n the c u t t i n g edge is at a positive a n g l e with the fiber orientation (fibers inclined a w a y from the c u t t i n g edge), the surface p r o d u c e d tends to be s m o o t h a n d is u s u a l l y covered with m a t r i x s m e a r i n g and c r u s h e d fibers. In the drilling of u n i d i r e c t i o n a l tape, a c u t t i n g edge will first a p p r o a c h fibers at a n e g a t i v e a n g l e (with the fibers being inclined t o w a r d s the c u t t i n g edge), t h e n p e r p e n d i c u l a r to the fibers and finally at a positive a n g l e to the fibers as the c u t t i n g edge r o t a t e s from 0 ~' to 180, as s h o w n in Fig. 1. T h u s the m a c h i n e d s u r f a c e is e x p e c t e d to be r o u g h a l o n g O" 9 0 and 180 ~' 270" regions as fiber pull-out and fiber r e o r i e n t a t i o n are the d o m i n a n t m a c h i n i n g m e c h a n i s m s for these areas. Along the c i r c u m f e r e n t i a l arcs of 90 ° to 180" and 2 7 0 to 0 ~, the
378
C. W. Wern et al./ Surface texture of composite drilled holes
area witl no visibl pits
reoriented fiber bundles
Fig. 4. Typical SEM pictures of the drilled graphite/epoxy hole surfaces: (a) low magnitica.. tion picture of the sectioned hole surface: and (b) high magnification picture of the severe1? damaged zone. m a c h i n i n g m e c h a n i s m is due m a i n l y to s h e a r i n g of the fiber and m a t r i x which p r o d u c e s a s m o o t h surface. As a consequence, pitting or fiber pull-out o c c u r s on one ply a r o u n d positions 1 and 3 as s h o w n in Fig. 2, while positions 2 and 4 of the ply surface are s m o o t h and covered with matrix. As the u n i d i r e c t i o n a l l a m i n a t e plies are a r r a n g e d in a _+45" o r i e n t a t i o n , while one ply is at a negative angle to the c u t t i n g edge, its a d j a c e n t plies are at a positive angle to the c u t t i n g direction. The a l t e r n a t i n g _+45° plies used in c o n s t r u c t i n g the test
C.W. Wern et al./ Surface texture of composite drilled holes
379
panels will thus lead to severe damage along all the four positions shown in Fig. 2. Thus the generated surface is not uniform, and the variation in the measured roughness profile height is large. When the cutting edge is at an angle of greater than - 5 ° or less than - 8 0 ~' with the fiber orientation, pitting is not observed on the surface. This phenomenon produces a hole that had different surfaces along the periphery. Bare fibers in the micrograph of Fig. 4(a) show clearly that when the cutting edge is at 0':' to the fiber orientation (fibers tangential to the hole), the chip-formation mechanism is dominated by breakage of the matrix-fiber bond. In plies adjacent to the bare fibers, matrix smearing was dominant on the machined surface. Thus, any surface roughness measurement along a 0~'/90 orientation will produce a smooth surface when compared to the measurements along a + 45" orientation. As mentioned previously, it is the intention of this study of characterize the surface along the highest roughness variation, thus roughhess height measurements were not taken along 0°/90 orientation. Optical microscopy was used to evaluate the width of the damage zone (defined in Fig. 2) for both drills at varying feed rates. The width of the damage zone is shown in Fig. 5 as a function of the feed rate for both drill geometries. The effect of drill geometry on the generated hole surface was very pronounced. Drill A, with an axial rake angle of 27 ~', produced a surface that had a narrower damage width than that for drill B. While high rake angle caused the fiber to fracture near the cutting edge producing a surface with less fiber pull-out, the low axial rake angle drill generates a surface by pulling out relatively large portions of material. Analysis of the surface roughness characteristics of the machined hole was made circumferentially at four different locations. Figure 6 shows the surface roughness profiles of a hole machined with drill B at a feed rate of 0.0254 mm/rev. It is noteworthy that the profiles did not vary much for the four measurements. The standard deviation of the arithmetic average roughness heights and maximum peak-to-valley heights for the four profiles are 0.98 gm and 4.89 gm respectively. Due to the similarity in the profiles measured at different locations of the hole, the characteristics of these profiles are also expected to be similar. Since the surface profiles and the calculated characteristics of the profiles are based inherently upon the microstructure of several adjacent plies, and since the microstructures are predominantly a function of the ply orientation relative to the direction of cutting edge travel, one would expect that profiles composed of plies with the same mixture of fiber orientations relative to the cutting edge should have identical characteristics. Shown in Fig. 7 is an analysis of a typical surface roughness profile in terms of roughness height density, cumulative height distribution, auto-correlation function, R(r), and power spectral density function, G(~), plotted for surfaces generated by drills A and B, using a feed rate of 0.0762 mm/rev. Figure 7(a) shows the surface profile of a hole generated by drill A, while Fig. 7(b) shows the significantly rougher surface produced by drill B at the same feed rate. The surface roughness height densities in Fig. 7(c) for the profiles corresponding to
380
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Figs. 7(a) and 7(b) are n e g a t i v e l y skewed and a r e not G a u s s i a n distributed. It was n o t e d f u r t h e r t h a t the h e i g h t density of the s u r f a c e p r o d u c e d by drill B had m o r e s p r e a d t h a n t h a t for drill A, s h o w i n g t h a t the s u r f a c e h e i g h t v a r i a t i o n is g r e a t e r for s u r f a c e s p r o d u c e d by drill B. The s u r f a c e t e x t u r e of the drilled holes was c h a r a c t e r i z e d by o b t a i n i n g the s t r a t a of the s u r f a c e profile by p l o t t i n g the c u m u l a t i v e h e i g h t d i s t r i b u t i o n on t h e p r o b i t scale, as s h o w n in Fig. 7(d). If" the s u r f a c e r o u g h n e s s profile is G a u s s i a n distributed, only one s t r a t u m is seen on the c u m u l a t i v e h e i g h t d i s t r i b u t i o n function: w h e n two or m o r e distinct s t r a t a are present, the s u r f a c e is not G a u s s i a n distributed. T h e plot s h o w n in Fig. 7(d) h a s two curves, which c a n e a c h be a p p r o x i m a t e d as two s t r a i g h t lines, which define the different s t r a t a in the s u r f a c e profiles. I n t e r s e c t i o n of the two lines o c c u r s at p r o b a b i l i t y of a b o u t 30 50%. U s i n g the s u r f a c e t e x t u r e classification p r o p o s e d by Zipin [8], all the s u r f a c e s in this s t u d y c a n be described as h a v i n g a n RS t o p o g r a p h y . An RS s u r f a c e h a s a s t e e p e r slope on the lower s t r a t u m t h a n on t h e u p p e r s t r a t u m , while a n SR s u r f a c e will h a v e a s t e e p e r u p p e r s t r a t u m RS s u r f a c e t e x t u r e m a k e s a good b e a r i n g surface, but will h a v e deep g r o o v e s t h a t a c t as stress r a i s e r s and h a v e a n e g a t i v e l y skewed h e i g h t distribution. As the s t r a t a i n t e r s e c t at a b o u t 30-50% of the c u m u l a t i v e h e i g h t d i s t r i b u t i o n , up to h a l f of the s u r f a c e in the severe s u r f a c e d a m a g e zone has deep v a l l e y s t h a t c o r r e s p o n d to g r o o v e s seen on the m i c r o g r a p h s . In the use of r a n d o m process m e t h o d s to c h a r a c t e r i z e the m a c h i n e d surface, the a u t o - c o r r e l a t i o n f u n c t i o n of t h e s u r f a c e profiles, s h o w n in Fig. 7(e), has a w a v e l e n g t h of a b o u t 390 pm for b o t h surfaces. Similarly, the p o w e r spectra]
C.W. Wern et al./ Surface texture of composite drilled holes 0
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Fig. 7. Showing for measurements along position 1 for both drills: (a) drill A surface profiles: (b) drill B surface profiles; (c) roughness height probability density; (d) cumulative height distribution; (e) auto-correlation function; and (f) power spectral density. (feed r a t ( = 0.0762 mm/rev)
C.W. Wern et aI./ Surface texture of composite drilled holes 0
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384
C.W. Wern et al./ Surface texture of composite drilled holes
density f u n c t i o n of the r o u g h n e s s profiles, shown in Fig. 7(f), shows that: two p r o m i n e n t w a v e l e n g t h s of 390 and 190 gm exist for the surfaces produced by both drills. N o t e t h a t drill B produces a much h i g h e r spike in the power spectral function, which corresponds to the deeper valleys of the surface profiles. The longer w a v e l e n g t h of 390 gm is a p p r o x i m a t e l y equal to the thickness of two u n i d i r e c t i o n a l plies. To e v a l u a t e the feed-rate effects on surface roughness, surface rough~ess profiles m e a s u r e d along position I for holes g e n e r a t e d using drill A and drill B are shown in Figs. 8 and 9 respectively. Surface r o u g h n e s s heights were observed to decrease with an increase in feed rate over the feed r a n g e of fr(m~ 0.0254 to 0.1778 mm/rev. However, above 0.1778 mm/rev, the surface roughness increased with feed. S u m m a r i e s of the m e a s u r e d surface r o u g h n e s s heights :~ a function of the feed rates are shown in Fig. 10. F i g u r e 10(a) shows that: the. m a x i m u m peak-to-valley height, Rt, increases significantly for feed rates a b o v ( 0.1778 mm/rev for surface produced by drill A, while o t h e r r o u g h n e s s i~eight p a r a m e t e r s such as a v e r a g e roughness, R~, root-mean-squared height~ R~ ten-point height, R~, did not v a r y significantly. The r a n g e of feed rates us~d ir~ drilling with drill B did not seem to affect the surface r o u g h n e s s heights (Fig. 10(b)) as the a r i t h m e t i c a v e r a g e r o u g h n e s s height varied from 15 t(; ~: im~ over the r a n g e of feed rates used, while R~ varied from 75 to 90 gin. 'Uht~s ir would seem t h a t the optimal feed r a t e for both drills, based on m m i m m ~ surface r o u g h n e s s of the m a c h i n e d surface, is a b o u t 0.1778 mm/rev. Furthe~ study into the chip-formation m e c h a n i s m in drilling must be conducted be~b~'~.~ any c o n c l u s i o n can be drawn on the r e l a t i o n s h i p between d a m a g e width ~)(i feed rate. Figures 11 and 12 show the surface r o u g h n e s s profile c h a r a c t e r i s t i c s , ~ r c ~ ponding to the profiles shown in Figs. 8 and 9. The r o u g h n e s s height ctet~sit:, function, shown in Fig. 1 l(a), reveals t h a t the surfaces of holes m a c h i n e d wiih drill A are n e g a t i v e l y skewed. From the c u m u l a t i v e h e i g h t d i s t r i b u t i o n t}:~" i n t e r s e c t i o n of the profile's s t r a t a occurs a r o u n d 3 0 40%,, showing t h a t ~p ~ 40% of the surface profiles m e a s u r e d are covered with deep valleys. F o r ~11 fi~d r a t e s studied, the a u t o - c o r r e l a t i o n function shows t h a t the domin~nt w a v e l e n g t h is a b o u t 390 ~tm, while power spectral density d e m o n s t r a t e s i hat a n o t h e r p r o m i n e n t w a v e l e n g t h of a r o u n d 190 gm is present also. However, drill B does not have the same effect on the m a c h i n e d hole surfaces. Various plots i~ Fig. 12 show t h a t an increase in feed r a t e does not have any significant affect on the surface texture, which r e i n f o r c e the o b s e r v a t i o n s made in Fig. 10(b). h~ all cases, the profile s t r a t a i n t e r s e c t at a r o u n d 40 50% of the r o u g h n e s s h e i g h t probability. The two p r o m i n e n t w a v e l e n g t h s observed in Fig. 1 l(d) were seen also in Fig. 12(d). In the analysis on the effects of feed rate on hole surface texture, it was evident t h a t the significant c h a n g e in the r o u g h n e s s h e i g h t was associated with an i n c r e a s e in feed r a t e r e s u l t i n g in a drastic c h a n g e in the surface texture. Surfaces produced by drill A were affected by the r a n g e of feed rates used, while surfaces m a c h i n e d by drill B were less d e p e n d e n t on the feed ratt~s.
C.W. Wern et al./ Surface texture of composite drilled holes 0
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It was observed also that both drills produced surfaces that are non-Gaussian, negatively skewed, and highly periodic for the range of feed rates studied. The skewness of the profile height density of the surfaces is highly dependent on
386
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the depth of the valleys. For all the surfaces studied, the height densities are n e g a t i v e l y s k e w e d a n d t h e s k e w n e s s o f t h e p r o f i l e s o f t e n e x c e e d - 1.0. A l l t h e m a c h i n e d s u r f a c e s a r e l e p t o k u r t i c w i t h k u r t o s i s g r e a t e r t h a n 3. H o w e v e r .
C.W. Wern et al./ Surface texture of composite drilled holes
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a considerable a m o u n t of s c a t t e r was observed in the computed kurtosis and skewness of all the surface h e i g h t distributions. The h e i g h t density of surface profiles m e a s u r e d at four different locations of the same hole had a kurtosis value t h a t r a n g e d from 3.2 to 4.3 as shown in Fig. 6. This s c a t t e r in kurtosis and skewness values does not tell clearly w h e t h e r the given value is a f u n c t i o n a l l y significant p r o p e r t y of the m e a s u r e d surface profiles, or w h e t h e r it is a mere artifact of the sampling process [9]. F r o m Figs. 6, 8 and 9, one observes t h a t the average r o u g h n e s s height, Ra, does not describe the e x t e n t of surface profile variation, while maximal roughness heights, Rz and Rt, quantify the m a x i m u m groove depth t h a t was seen in both SEM m i c r o g r a p h s and the surface profile. Even t h o u g h the auto-correlation function, R(r), consistently identifies the long w a v e l e n g t h (390 I~m) of the surface r o u g h n e s s profiles, it did not reveal the observed short wavelength.
C.W. Wern et al./Surface texture of composite drilled holes
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Fig. 12. As for Fig. 11, for drill B. The p o w e r spectral density f u n c t i o n will thus be a m o r e effective t()oi i~) a n a l y z i n g the w a v e l e n g t h distribution of the surface profile. On the basis of the p r e l i m i n a r y results of h o l e surface e v a l u a t i o n , a c o m b i n a t i o n of p a r a m e t e r s is required to describe fully the drilled g r a p h i t e / e p o x y surfaces w h i c h is consistent w i t h the findings of a p r e v i o u s study on m a c h i n e d u n i d i r e c t i o n a l l a m i n a t e s u r f a c e s [4]. The surface describing p a r a m e t e r s for c o m p o s i t e surface evalua t i o n s s h o u l d i n c l u d e the c u m u l a t i v e h e i g h t distribution of the surface plotted on a probit scale, the p o w e r spectral density f u n c t i o n of the surface profile, and the use of m a x i m a l r o u g h n e s s h e i g h t p a r a m e t e r s such as Rz or R , 4. S u m m a r y
and conclusions
G r a p h i t e / e p o x y test panels were drilled u s i n g two different P C D tipped drills at a c o n s t a n t speed of 4550 rpm, for a feed r a n g e from 0.0254 to 0.254 mm/rev in i n c r e m e n t s of 0.0254 mm/rev. S c a n n i n g and optical m i c r o s c o p y were used to
C.W. Wern et al./ Surface texture of composite drilled holes
389
e v a l u a t e the surface defects and damage. Profilometry was used to study the surface textures. The surface t e x t u r e s of the holes were e v a l u a t e d by analyzing the a v e r a g e r o u g h n e s s heights, statistical methods, and r a n d o m process methods of describing the m e a s u r e d surface r o u g h n e s s profiles. The surface produced by drill geometry A was superior to t h a t produced by drill g e o m e t r y B due to t h r e e reasons. First, the surface produced by drill B is almost four times r o u g h e r t h a n t h a t produced by drill A w h e n the feed rate is low. Drill B also produces a surface with a wider surface damage zone. Lastly, the probability of the o c c u r r e n c e of deep valleys in surfaces produced by drill B is g r e a t e r t h a n t h a t in surfaces produced by drill A. Feed rates h a v e a definite effect on the m a c h i n e d surfaces. Between 0.0254 and 0.1778 mm/rev, the surface r o u g h n e s s decreased with an increase in feed rate. At feed rates above 0.1778 mm/rev, the surface r o u g h n e s s increases with an increased feed. Average r o u g h n e s s height, R~, did not describe the e x t e n t of the surface r o u g h n e s s height v a r i a t i o n observed on all the profiles studies, but maximal r o u g h n e s s heights such as R~ and Rz will q u a n t i f y the depth of the valleys or the m a c h i n i n g damage in the drilled composites. P l o t t i n g the c u m u l a t i v e h e i g h t distribution on the probit scale showed successfully the existence of different s t r a t a associated with peaks and valleys of the m a c h i n e d graphite/epoxy surface profiles. Power spectral density f u n c t i o n analysis gave a better u n d e r s t a n d i n g of the w a v e l e n g t h distribution of the surface roughness profiles of m a c h i n e d g r a p h i t e / e p o x y surfaces.
Acknowledgments The a u t h o r s would like to t h a n k Mr. Paul Russel, supervisor in Boeing MR&D group, for e n c o u r a g e m e n t and financial support, and the W a s h i n g t o n T e c h n o l o g y C e n t e r for providing the n e c e s s a r y facilities as part of this res e a r c h at the U n i v e r s i t y of Washington.
References [1] M. Ramulu, M. Faridnia, J.L. Garbini and J.E. Jorgenson, J. Eng. Mater. Technol.. 112 (1991) 430 436. [2] K. Colligan and M. Ramulu, Manuf. Rev., 5 (1992). [3] M. Ramulu and J. Park, Mechanical Engineering Technical Report to Boeing, T.R. No. ME/91/2, August, 1991. [4] C.W. Wern, M.S. Thesis, Department of Mechanical Engineering, University of Washington, Seattle, Washington, August, 1991. [5] W. Konig and P. Grag, Ann. CIRP, 38(1) (1989) 119 124. [6] A. Koplev, A. Lystrup and T. Worm, Composites, 14, (1983) 371 376. [7] D.-H. Wang, M. Ramulu and C.W. Wern, Trans. NAMRI/SME, 20 (1992) 159 165. [8] R.B. Zipin, Appl. Surf. Sci. 15 (1983) 334 358. [9] T.R. Thomas, Rough Surfaces, Longman Group Limited, New York, 1982.
373
A study of the surface texture of composite drilled holes C.W. Wern, M. R a m u l u and K. Colligan Department of Mechanical Engineering, FU-10, University of Washington, Seattle, WA, USA
Industrial S u m m a r y An investigation of the surface texture of composite drilled holes was conducted by drilling experiments at constant speed with two different PCD tipped drill geometries at varying feed rates. Surface profilometry was used to measure the surface roughness profiles of machined graphite/epoxy holes and roughness characteristics were evaluated using average roughness heights, statistical methods, and random process methods. It was found that the width of damage in a composite material is a function of the drill geometry and the feed rate. All the surfaces studied were found to be non-Gaussian, negatively skewed, leptokurtic, and produced a good bearing surface. The use of maximal roughness height parameters, power spectral density function, and cumulative height density plotted on a probit scale allows characterization of the texture of the surfaces of the drilled holes.
1. Introduction In recent years, polymer composites have gained considerable attention in the aircraft and automobile industries due to their light weight and high strength. Even t h o u g h most of the polymer composite structures are molded into near-net shapes, m a c h i n i n g of the components is always necessary to attain the required dimensional tolerance, surface roughness, and complex geometry. In the past two decades, research efforts have been mainly on composite materials' m e c h a n i c a l properties and very few studies have dealt with the m a c h i n i n g of composites and their machined surfaces. It is pertinent to understand the effects of m a c h i n i n g processes on graphite/epoxy because machined surface quality determines the fatigue strength of the c o m p o n e n t ' s service life. In the s e a r c h for an optimal c u t t i n g tool material for m a c h i n i n g graphite/epoxy, poly-crystalline d i a m o n d (PCD) inserts have been s h o w n to be more abrasive r e s i s t a n t t h a n carbide (C6 grade) inserts [1,2]. Studies have also been
Correspondence to: M. Ramulu, Department of Mechanical Engineering, FU-10, University of Washington, Seattle, WA, USA. 0924-0136/93/$06.00 ((~ 1993 Elsevier Science Publishers B.V. All rights reserved.
374
C.W. Wern et al./Surface texture of" composite drilled holes
conducted to evaluate the toot geometry in machining graphite/epoxy laminates [3]. Surface roughness heights are always used as a gauge to evaluat~ the amount of damage on the machined surface to assess the quality of surfaces produced by machining processes or machining parameters. Average surface roughness heights such as average roughness (R~) and maximum peak-tovalley heights (Rt) are used to describe the amount of micro-geometric v a r i ation of the machined metal surface. However, these parameters may no~ t)e~ sufficient for an inhomogeneous material such as graphite/epoxy which ha,~ a rougher surthce compared to metals under the same machining condition~ [4]. Recently, Konig and Graft [5] studied the surface texture of holes and analyzed the drilling of fiber reinforced thermosets (carbon fiber, glass fibe~ and aramid fiber) by quantifying the amount of machining damage t~:dng ten-point height (R~) and width of the damage zone, However, the ,~m'face roughness characteristics of high modulus graphite/epoxy composite drilled holes have not been studied. This paper presents the results of a study of drilled graphite/epoxy holes produced by two different drill point geometries. The feed rate in the drilling process was varied to determine its effects on the surface roughness profile~ and damage width. Machined graphite/epoxy surface textures were evaluated by inspecting the effectiveness of average roughness parameters, statistical methods (e.g. skewness, kurtosis, cumulative height distribution or bearing length), and random process methods (e.g., power spectral density function, G((~)), auto-correlation function, R(r)) in describing surface roughness profiles. To augment the results obtained from surface profile studies, scanning electror~ microscopy was used to observe the damage of the machined surfaces
2. Experimental procedure An experimental study was conducted to assess the surface quality of drilled graphite/epoxy composites. Test panels used in this study were approximatel5 25.4 mm (1 in) thick, and were made up of IM-6 graphite fibers (6 pm diameter~ embedded in 3501-6 thermoset epoxy resin. The lay-up of the test pandits is shown schematically in Fig. 1. On each panel, the outer plies were made ~i bias-weaved fabric while the internal plies were unidirectional tape of t h i c k ness 0.2 mm, laminated in a _+45 alternating pattern. The fabric plies wer~ used to suppress delamination on the entrance and exit of the drill. H o w e v e r the main interest of this study was the unidirectional tape portion in th~ middle of the test panels. Two different PCD tipped drills were used to generate the holes. Drill A was 12.7 mm (0.5 in) in diameter with an axial rake angle of 27, while drill B had a diameter of 13.9 mm (0.5494 in) and an axial rake angle of 7'. Apart from the, difference in axial rake angle and drill size, the two drills have essentially tiat~ same geometry. A Mazak 3-axis CNC milling machine was used for drilling the holes at a constant speed of 4550 rpm. Holes were drilled at feed rates of
C.W.
Wern
et al./ Surface
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texture
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375
PCD tipped drill
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Bias weaved fabric
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.............. ..............
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..............
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Fig. 1. Test panel and position of the cutting edge with respect to the fiber orientations: the drill rotates clockwise.
from 0.0254 m m / r e v (0.001 in/rev) to 0.254 m m / r e v (0.01 in/rev) in i n c r e m e n t s of 0.0254 m m / r e v (0.001 in/rev). D u r i n g the drilling process, the holes were flooded with D a r a c o o l 706LF, a water-soluble s y n t h e t i c coolant. S u r f a c e r o u g h n e s s profiles of the hole s u r f a c e s were o b t a i n e d using the F e d e r a l S u r f a n a l y z e r System 4000 w i t h a n EPT-01049 d i a m o n d stylus probe. The 2.54 pm d i a m e t e r stylus was chosen for m e a s u r i n g the r o u g h n e s s profile of the s u r f a c e since the fiber d i a m e t e r is a p p r o x i m a t e l y 6 pm. Profile c o o r d i n a t e s were stored for f u r t h e r p r o c e s s i n g t h r o u g h the i n t e r f a c e to a c o m p u t e r . The s u r f a c e r o u g h n e s s c h a r a c t e r i s t i c s were e v a l u a t e d using a cutoff length of 0.8 m m and r o u g h n e s s h e i g h t m e a s u r e m e n t i n t e r v a l s of 1.25 pro. F o u r roughness r e a d i n g s were t a k e n c i r c u m f e r e n t i a l l y , a p p r o x i m a t e l y 9 0 a p a r t on the u n i d i r e c t i o n a l plies m a c h i n e d surface, as s h o w n in Fig. 2. T h e four positions w e r e c h o s e n a l o n g l o c a t i o n s of high m i c r o s c o p i c i r r e g u l a r i t i e s t h a t were e s t a b l i s h e d by visual and optical i n s p e c t i o n of the hole surfaces. T h e m e a s u r e m e n t l e n g t h (or t r a v e r s e length) was a p p r o x i m a t e l y 4 mm, w h i c h c o r r e s p o n d s to a s a m p l e size of a b o u t 3200 r o u g h n e s s h e i g h t m e a s u r e m e n t s . The surface r o u g h n e s s h e i g h t d e n s i t y and s t a t i s t i c a l p a r a m e t e r s were o b t a i n e d u s i n g class
376
C.W. Wern et al./ Surface texture of composite drilled holes
Fiber orientation
Direction of drill rotation"
\ Area of the hole wilh extremely rough surface
Fig. 2. Severe surface damage zone in a unidirectional ply and locations o~' roughn~,~ measurements.
intervals of 1.25 }am so t h a t there were more t h a n 20 class intervals fo~' the r o u g h n e s s h e i g h t s studies. The class interval size of 1.25 pm was chosen to minimize a n y r e d u c t i o n in r e s o l u t i o n with an increase in the n u m b e r of class i n t e r v a l s used. A Jeol JSM-T330A s c a n n i n g electron m i c r o s c o p e (SEM) was used to assess and v a l i d a t e the surface r o u g h n e s s m e a s u r e m e n t s and describe q u a l i t a t i v e l } the d a m a g e observed due to the m a c h i n i n g process. M i c r o g r a p h s were t a k e n at v a r i o u s l o c a t i o n s on the m a c h i n e d surface both at low and high magnifications. Low m a g n i f i c a t i o n of 35 × was used to examine the global d a m a g e while localized d a m a g e was examined at a h i g h e r m a g n i f i c a t i o n of 200 x~
3. R e s u l t s a n d d i s c u s s i o n F i g u r e 3 shows a typical hole surface both at the e n t r a n c e and exit regions of the drill. The use of bias-weaved plies on the surface of the test panels enables a hole to be drilled w i t h o u t a n y d e l a m i n a t i o n . A typical low m a g n i f i c a t i o n m i c r o g r a p h of the sectioned hole surface is s h o w n in Fig. 4(a). Two m a c h i n e d surface zones were g e n e r a l l y observed on the surface of the holes g e n e r a t e d by b o t h drill geometries. P a r t s of the hole surface showed severe pitting and g r o o v i n g on a l t e r n a t e plies, while o t h e r p a r t s of the hole surface were s m o o t h and were covered with m a t r i x and s h e a r e d fibers. F i g u r e 4(b) shows a magnified view of the pitted surface zone. It was observed t h a t bundles of r e o r i e n t e d
C.W. Wern et al./ Surface texture of composite drilled holes
entrance surface
(a)
exit surface
377
entrance surface
exit surface
(b)
Fig. 3. Entrance and exit regions of the holes machined using: (a) drill A; and (b) drill 13. (feed rate = 0.254 mm/rev)
fibers coupled w i t h fiber pull-out leads to an e x t r e m e l y r o u g h surface. Since the s u r f a c e r o u g h n e s s of the s m o o t h s u r f a c e zone does not h a v e a significant v a r i a t i o n in surface texture, t h a t p o r t i o n of the hole s u r f a c e was not included in this study. The surface d a m a g e g e n e r a t e d in m a c h i n i n g g r a p h i t e / e p o x y is d e p e n d e n t on the r e l a t i v e a n g l e b e t w e e n the fiber o r i e n t a t i o n and the d i r e c t i o n of c u t t i n g m o t i o n [6,7]. Fiber pull-out, which caused the p i t t i n g p h e n o m e n a in the r o u g h s u r f a c e zone, o c c u r s w h e n the fiber is at a n e g a t i v e a n g l e to the c u t t i n g direction. W h e n the c u t t i n g edge is at a positive a n g l e with the fiber orientation (fibers inclined a w a y from the c u t t i n g edge), the surface p r o d u c e d tends to be s m o o t h a n d is u s u a l l y covered with m a t r i x s m e a r i n g and c r u s h e d fibers. In the drilling of u n i d i r e c t i o n a l tape, a c u t t i n g edge will first a p p r o a c h fibers at a n e g a t i v e a n g l e (with the fibers being inclined t o w a r d s the c u t t i n g edge), t h e n p e r p e n d i c u l a r to the fibers and finally at a positive a n g l e to the fibers as the c u t t i n g edge r o t a t e s from 0 ~' to 180, as s h o w n in Fig. 1. T h u s the m a c h i n e d s u r f a c e is e x p e c t e d to be r o u g h a l o n g O" 9 0 and 180 ~' 270" regions as fiber pull-out and fiber r e o r i e n t a t i o n are the d o m i n a n t m a c h i n i n g m e c h a n i s m s for these areas. Along the c i r c u m f e r e n t i a l arcs of 90 ° to 180" and 2 7 0 to 0 ~, the
378
C. W. Wern et al./ Surface texture of composite drilled holes
area witl no visibl pits
reoriented fiber bundles
Fig. 4. Typical SEM pictures of the drilled graphite/epoxy hole surfaces: (a) low magnitica.. tion picture of the sectioned hole surface: and (b) high magnification picture of the severe1? damaged zone. m a c h i n i n g m e c h a n i s m is due m a i n l y to s h e a r i n g of the fiber and m a t r i x which p r o d u c e s a s m o o t h surface. As a consequence, pitting or fiber pull-out o c c u r s on one ply a r o u n d positions 1 and 3 as s h o w n in Fig. 2, while positions 2 and 4 of the ply surface are s m o o t h and covered with matrix. As the u n i d i r e c t i o n a l l a m i n a t e plies are a r r a n g e d in a _+45" o r i e n t a t i o n , while one ply is at a negative angle to the c u t t i n g edge, its a d j a c e n t plies are at a positive angle to the c u t t i n g direction. The a l t e r n a t i n g _+45° plies used in c o n s t r u c t i n g the test
C.W. Wern et al./ Surface texture of composite drilled holes
379
panels will thus lead to severe damage along all the four positions shown in Fig. 2. Thus the generated surface is not uniform, and the variation in the measured roughness profile height is large. When the cutting edge is at an angle of greater than - 5 ° or less than - 8 0 ~' with the fiber orientation, pitting is not observed on the surface. This phenomenon produces a hole that had different surfaces along the periphery. Bare fibers in the micrograph of Fig. 4(a) show clearly that when the cutting edge is at 0':' to the fiber orientation (fibers tangential to the hole), the chip-formation mechanism is dominated by breakage of the matrix-fiber bond. In plies adjacent to the bare fibers, matrix smearing was dominant on the machined surface. Thus, any surface roughness measurement along a 0~'/90 orientation will produce a smooth surface when compared to the measurements along a + 45" orientation. As mentioned previously, it is the intention of this study of characterize the surface along the highest roughness variation, thus roughhess height measurements were not taken along 0°/90 orientation. Optical microscopy was used to evaluate the width of the damage zone (defined in Fig. 2) for both drills at varying feed rates. The width of the damage zone is shown in Fig. 5 as a function of the feed rate for both drill geometries. The effect of drill geometry on the generated hole surface was very pronounced. Drill A, with an axial rake angle of 27 ~', produced a surface that had a narrower damage width than that for drill B. While high rake angle caused the fiber to fracture near the cutting edge producing a surface with less fiber pull-out, the low axial rake angle drill generates a surface by pulling out relatively large portions of material. Analysis of the surface roughness characteristics of the machined hole was made circumferentially at four different locations. Figure 6 shows the surface roughness profiles of a hole machined with drill B at a feed rate of 0.0254 mm/rev. It is noteworthy that the profiles did not vary much for the four measurements. The standard deviation of the arithmetic average roughness heights and maximum peak-to-valley heights for the four profiles are 0.98 gm and 4.89 gm respectively. Due to the similarity in the profiles measured at different locations of the hole, the characteristics of these profiles are also expected to be similar. Since the surface profiles and the calculated characteristics of the profiles are based inherently upon the microstructure of several adjacent plies, and since the microstructures are predominantly a function of the ply orientation relative to the direction of cutting edge travel, one would expect that profiles composed of plies with the same mixture of fiber orientations relative to the cutting edge should have identical characteristics. Shown in Fig. 7 is an analysis of a typical surface roughness profile in terms of roughness height density, cumulative height distribution, auto-correlation function, R(r), and power spectral density function, G(~), plotted for surfaces generated by drills A and B, using a feed rate of 0.0762 mm/rev. Figure 7(a) shows the surface profile of a hole generated by drill A, while Fig. 7(b) shows the significantly rougher surface produced by drill B at the same feed rate. The surface roughness height densities in Fig. 7(c) for the profiles corresponding to
380
(7. W. Wern et al./ Surface texture of composite drilled holes F. . . . . . . . . . . . . . . . . . . . . , [
•
•
l)rill A Drill B
g
0
0.05
O.1
O.15
(I.2
0.25
(I.3
Feed rate (mm/rev) Fig. 5. Plot of the width of the severely damaged zone w.~rsus feed rate.
Figs. 7(a) and 7(b) are n e g a t i v e l y skewed and a r e not G a u s s i a n distributed. It was n o t e d f u r t h e r t h a t the h e i g h t density of the s u r f a c e p r o d u c e d by drill B had m o r e s p r e a d t h a n t h a t for drill A, s h o w i n g t h a t the s u r f a c e h e i g h t v a r i a t i o n is g r e a t e r for s u r f a c e s p r o d u c e d by drill B. The s u r f a c e t e x t u r e of the drilled holes was c h a r a c t e r i z e d by o b t a i n i n g the s t r a t a of the s u r f a c e profile by p l o t t i n g the c u m u l a t i v e h e i g h t d i s t r i b u t i o n on t h e p r o b i t scale, as s h o w n in Fig. 7(d). If" the s u r f a c e r o u g h n e s s profile is G a u s s i a n distributed, only one s t r a t u m is seen on the c u m u l a t i v e h e i g h t d i s t r i b u t i o n function: w h e n two or m o r e distinct s t r a t a are present, the s u r f a c e is not G a u s s i a n distributed. T h e plot s h o w n in Fig. 7(d) h a s two curves, which c a n e a c h be a p p r o x i m a t e d as two s t r a i g h t lines, which define the different s t r a t a in the s u r f a c e profiles. I n t e r s e c t i o n of the two lines o c c u r s at p r o b a b i l i t y of a b o u t 30 50%. U s i n g the s u r f a c e t e x t u r e classification p r o p o s e d by Zipin [8], all the s u r f a c e s in this s t u d y c a n be described as h a v i n g a n RS t o p o g r a p h y . An RS s u r f a c e h a s a s t e e p e r slope on the lower s t r a t u m t h a n on t h e u p p e r s t r a t u m , while a n SR s u r f a c e will h a v e a s t e e p e r u p p e r s t r a t u m RS s u r f a c e t e x t u r e m a k e s a good b e a r i n g surface, but will h a v e deep g r o o v e s t h a t a c t as stress r a i s e r s and h a v e a n e g a t i v e l y skewed h e i g h t distribution. As the s t r a t a i n t e r s e c t at a b o u t 30-50% of the c u m u l a t i v e h e i g h t d i s t r i b u t i o n , up to h a l f of the s u r f a c e in the severe s u r f a c e d a m a g e zone has deep v a l l e y s t h a t c o r r e s p o n d to g r o o v e s seen on the m i c r o g r a p h s . In the use of r a n d o m process m e t h o d s to c h a r a c t e r i z e the m a c h i n e d surface, the a u t o - c o r r e l a t i o n f u n c t i o n of t h e s u r f a c e profiles, s h o w n in Fig. 7(e), has a w a v e l e n g t h of a b o u t 390 pm for b o t h surfaces. Similarly, the p o w e r spectra]
C.W. Wern et al./ Surface texture of composite drilled holes 0
I000
2000
30~
381
4('00
40
0
Ra = Rq = Rt = Rz = Kurtosis = Skewness =
-20 40 60
16.4/~m 20.8 p m 87.4 ~um 29.9 p m 3.2 - 1.16
-gO 100
(a) P o s i t i o n 1
Ra = Rq = Rt = Rz = Kurtosis = Skewness =
15.3 p m 20.5 p m 88.8/~m 29.8 ,um 4.3 -1.46
Ra= Rq = Rt = Rz = Kurtosis = Skewness =
15.4 p m 19.9 p m 91.4 p m 28.7 p m 3.5 -1.27
(b) P o s t i o n 2
(c) P o s t i o n 3
Ra = Rq = Rt = Rz = Kurtosis = Skewness =
14.0/~m 18.5/~m 80.0 t~m 28.93 p m 3.8 - 1.34
(d) Position 4 F i g . 6. S u r f a c e 0.0254 m m / r e v )
profiles
measured
along
positions
1 4
using
drill
B. ( f e e d
rate =
C.W. Wern et al./Surface texture of composite drilled holes
382 411
~
20
~ •~
-20
•~
2o o 20
,f i~i ,
'i
I 1000
2000
30~0
0
5000
4000
I000
2000
3000
40@i
N'~O0
Traverse length (ym)
Traverse length (ym)
(b)
(a)
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o
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[
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1
5 tO 2030 50 7080 9095
99
5~)9~-z~1
Percent
%//~m (d)
(c)
i l)f~I B I j ........... J
;
05
0
iI l'x
{
/
I',I
I
I
v q).5
• 1.5
V
U ,
,
,
\/
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v a_ . . . .
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12(20
v i
1600
L
H
i
g,ooo,,F :
//
/
!
.... 2003
0
0004
(e)
0008
0012
09t6
'302
~o/2~
x (ym)
(0
Fig. 7. Showing for measurements along position 1 for both drills: (a) drill A surface profiles: (b) drill B surface profiles; (c) roughness height probability density; (d) cumulative height distribution; (e) auto-correlation function; and (f) power spectral density. (feed r a t ( = 0.0762 mm/rev)
C.W. Wern et aI./ Surface texture of composite drilled holes 0
10~
2000
3000
383
4000
20 10
Ra = Rq = Rt = Rz = Kurtosis = Skewness =
0 10 20 30
4.3 /~m 5.5 t~m 23.7/~m 9.7 y m 3.7 -1.05
40
(a)
Ra = Rq = Rt = Rz = Kurtosis = Skewness =
4.9/~m 5.8 y m 22.5/Am 8.6 ,um 3.1 -0.87
Ra= Rq = Rt = Rz = Kurtosis = Skewness =
4.1 /~m 5.4 •m 24.5 y m 8.9 y m 4.2 - 1.42
Ra= Rq = Rt = Rz= Kurtosis = Skewness =
4.6 y m 6.2 y m 23.6 y m 9.1 /~m 6.0 - 1.74
(b)
(c)
(d) F i g . 8. S u r f a c e p r o f i l e s m e a s u r e d a l o n g p o s i t i o n 2 u s i n g d r i l l A a t f e e d 0 . 0 2 5 4 m m / r e v ; (b) 0 . 0 7 6 2 m m / r e v ; (c) 0 . 1 7 7 8 m m / r e v ; a n d (d) 0 . 2 5 4 m m / r e v .
rates
of: ( a )
384
C.W. Wern et al./ Surface texture of composite drilled holes
density f u n c t i o n of the r o u g h n e s s profiles, shown in Fig. 7(f), shows that: two p r o m i n e n t w a v e l e n g t h s of 390 and 190 gm exist for the surfaces produced by both drills. N o t e t h a t drill B produces a much h i g h e r spike in the power spectral function, which corresponds to the deeper valleys of the surface profiles. The longer w a v e l e n g t h of 390 gm is a p p r o x i m a t e l y equal to the thickness of two u n i d i r e c t i o n a l plies. To e v a l u a t e the feed-rate effects on surface roughness, surface rough~ess profiles m e a s u r e d along position I for holes g e n e r a t e d using drill A and drill B are shown in Figs. 8 and 9 respectively. Surface r o u g h n e s s heights were observed to decrease with an increase in feed rate over the feed r a n g e of fr(m~ 0.0254 to 0.1778 mm/rev. However, above 0.1778 mm/rev, the surface roughness increased with feed. S u m m a r i e s of the m e a s u r e d surface r o u g h n e s s heights :~ a function of the feed rates are shown in Fig. 10. F i g u r e 10(a) shows that: the. m a x i m u m peak-to-valley height, Rt, increases significantly for feed rates a b o v ( 0.1778 mm/rev for surface produced by drill A, while o t h e r r o u g h n e s s i~eight p a r a m e t e r s such as a v e r a g e roughness, R~, root-mean-squared height~ R~ ten-point height, R~, did not v a r y significantly. The r a n g e of feed rates us~d ir~ drilling with drill B did not seem to affect the surface r o u g h n e s s heights (Fig. 10(b)) as the a r i t h m e t i c a v e r a g e r o u g h n e s s height varied from 15 t(; ~: im~ over the r a n g e of feed rates used, while R~ varied from 75 to 90 gin. 'Uht~s ir would seem t h a t the optimal feed r a t e for both drills, based on m m i m m ~ surface r o u g h n e s s of the m a c h i n e d surface, is a b o u t 0.1778 mm/rev. Furthe~ study into the chip-formation m e c h a n i s m in drilling must be conducted be~b~'~.~ any c o n c l u s i o n can be drawn on the r e l a t i o n s h i p between d a m a g e width ~)(i feed rate. Figures 11 and 12 show the surface r o u g h n e s s profile c h a r a c t e r i s t i c s , ~ r c ~ ponding to the profiles shown in Figs. 8 and 9. The r o u g h n e s s height ctet~sit:, function, shown in Fig. 1 l(a), reveals t h a t the surfaces of holes m a c h i n e d wiih drill A are n e g a t i v e l y skewed. From the c u m u l a t i v e h e i g h t d i s t r i b u t i o n t}:~" i n t e r s e c t i o n of the profile's s t r a t a occurs a r o u n d 3 0 40%,, showing t h a t ~p ~ 40% of the surface profiles m e a s u r e d are covered with deep valleys. F o r ~11 fi~d r a t e s studied, the a u t o - c o r r e l a t i o n function shows t h a t the domin~nt w a v e l e n g t h is a b o u t 390 ~tm, while power spectral density d e m o n s t r a t e s i hat a n o t h e r p r o m i n e n t w a v e l e n g t h of a r o u n d 190 gm is present also. However, drill B does not have the same effect on the m a c h i n e d hole surfaces. Various plots i~ Fig. 12 show t h a t an increase in feed r a t e does not have any significant affect on the surface texture, which r e i n f o r c e the o b s e r v a t i o n s made in Fig. 10(b). h~ all cases, the profile s t r a t a i n t e r s e c t at a r o u n d 40 50% of the r o u g h n e s s h e i g h t probability. The two p r o m i n e n t w a v e l e n g t h s observed in Fig. 1 l(d) were seen also in Fig. 12(d). In the analysis on the effects of feed rate on hole surface texture, it was evident t h a t the significant c h a n g e in the r o u g h n e s s h e i g h t was associated with an i n c r e a s e in feed r a t e r e s u l t i n g in a drastic c h a n g e in the surface texture. Surfaces produced by drill A were affected by the r a n g e of feed rates used, while surfaces m a c h i n e d by drill B were less d e p e n d e n t on the feed ratt~s.
C.W. Wern et al./ Surface texture of composite drilled holes 0
1000
2000
3000
385
41100
411 20 0
Ra Rq Rt Rz Kurtosis Skewness
20 40 60
= = = = = =
16.4/~m 20.8/~m 87.4/~m 29.9/~m 3.2 - 1.16
80
(a)
Ra Rq Rt Rz Kurtosis Skewness
= = = = = =
17.6/~m 22.1 /~m 93.3 /~m 32.9/~m 3.6 -1.28
Ra Rq Rt Rz Kurtosis Skewness
= = = = = =
13.4 p m 17.9/~m 76.6/~m 23.4 u m 5.4 - 1.70
Ra Rq Rt Rz
= = = =
18.4/~m 22.7 ,urn 88.0 ,urn 32.0 u m
(b)
(c)
Kurtosis = Skewness =
3.4 -1.09
(d) F i g . 9. A s f o r F i g . 8, f o r d r i l l B .
It was observed also that both drills produced surfaces that are non-Gaussian, negatively skewed, and highly periodic for the range of feed rates studied. The skewness of the profile height density of the surfaces is highly dependent on
386
C.W. Wern et al./Surface texture of composite drilled holes 50 [I - ' -
I
ill
1
,~, 40
~)
30
r~
2o
~
,,i
---i,, ......
;": .... -~
"~:.....
4
i
" .......
4 . . . .
0
i
. . . .
0.05
i
. . . .
0.1
(a)
. . . .
0.15
0.2
2~
J
. . . .
0.25
0.3
Feed rate (mm/rev)
100 1 ...................
80
: ...................
[ ..............
.--,,...:
..................
: _...=.:.,.. ..........
o O0 00
o
~ .................
.... 40
~ 2o
......................................................
r .......................
........ • - ' - - = i
~
0
(b)
......... ~ ......... ~. . . . . . . . . . . . .
r
-
7
~
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i
i
i
i
0.05
O.I
0.15
0.2
-
R, I1
i .......
~
.............
0.25
0.3
F e e d rate ( m m / r e v )
Fig. 10. Average roughness heights versus feed rate for: (a) drill A; (b) drill B.
the depth of the valleys. For all the surfaces studied, the height densities are n e g a t i v e l y s k e w e d a n d t h e s k e w n e s s o f t h e p r o f i l e s o f t e n e x c e e d - 1.0. A l l t h e m a c h i n e d s u r f a c e s a r e l e p t o k u r t i c w i t h k u r t o s i s g r e a t e r t h a n 3. H o w e v e r .
C.W. Wern et al./ Surface texture of composite drilled holes
387
2O 2O
i::
io o
j
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.
"
-10
-10
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-20
o
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0.0254 mmlrCv 0.0762 s m i t e r - - 0.1778 s m / r e v -0.254 mm/rcv
-
- -- -
0.0254 m m / t c v 0.0762 m m / r e v --0.1778 mm/rev - 0.25.4 m m / r e v
-50 -50
4
8
12
16
.01
20
.1
1
5 10 2 0 3 0
50
7080
9095
99
99.99999
Percent
%Ipm (a)
(b)
4000 I04 15
~
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T
,
,
•
,
•
.
,
,
--0.0254
mm/rev
.... 00762 mm/r~ - - - - 0 1778 m l a / t e v -- 0 2 5 4 mm/rev
I
3OOO
10 4
- -
0.5 v
3
Iooo l o 4
-0.5
0
-I
400
8(]0
1200
1600
2000
0 0254 m m / , ~ - 00'762 mmlwev - - 0 1778 m m / r c v - 0 2 3 4 mm/r©v
2.000 l o 4
0
I
--
000
104
,
,
,
,
0.004
,
,
,
i
0008
,
,
,
E
0012
,
0016
0.02
"~(,/am) (c)
(d)
Fig. 11. Showing for measurements along position 1 for drill A: (a) roughness height probability density; (b) cumulative height distribution; (c) auto-correlation function; and (d) power spectral density.
a considerable a m o u n t of s c a t t e r was observed in the computed kurtosis and skewness of all the surface h e i g h t distributions. The h e i g h t density of surface profiles m e a s u r e d at four different locations of the same hole had a kurtosis value t h a t r a n g e d from 3.2 to 4.3 as shown in Fig. 6. This s c a t t e r in kurtosis and skewness values does not tell clearly w h e t h e r the given value is a f u n c t i o n a l l y significant p r o p e r t y of the m e a s u r e d surface profiles, or w h e t h e r it is a mere artifact of the sampling process [9]. F r o m Figs. 6, 8 and 9, one observes t h a t the average r o u g h n e s s height, Ra, does not describe the e x t e n t of surface profile variation, while maximal roughness heights, Rz and Rt, quantify the m a x i m u m groove depth t h a t was seen in both SEM m i c r o g r a p h s and the surface profile. Even t h o u g h the auto-correlation function, R(r), consistently identifies the long w a v e l e n g t h (390 I~m) of the surface r o u g h n e s s profiles, it did not reveal the observed short wavelength.
C.W. Wern et al./Surface texture of composite drilled holes
388
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Fig. 12. As for Fig. 11, for drill B. The p o w e r spectral density f u n c t i o n will thus be a m o r e effective t()oi i~) a n a l y z i n g the w a v e l e n g t h distribution of the surface profile. On the basis of the p r e l i m i n a r y results of h o l e surface e v a l u a t i o n , a c o m b i n a t i o n of p a r a m e t e r s is required to describe fully the drilled g r a p h i t e / e p o x y surfaces w h i c h is consistent w i t h the findings of a p r e v i o u s study on m a c h i n e d u n i d i r e c t i o n a l l a m i n a t e s u r f a c e s [4]. The surface describing p a r a m e t e r s for c o m p o s i t e surface evalua t i o n s s h o u l d i n c l u d e the c u m u l a t i v e h e i g h t distribution of the surface plotted on a probit scale, the p o w e r spectral density f u n c t i o n of the surface profile, and the use of m a x i m a l r o u g h n e s s h e i g h t p a r a m e t e r s such as Rz or R , 4. S u m m a r y
and conclusions
G r a p h i t e / e p o x y test panels were drilled u s i n g two different P C D tipped drills at a c o n s t a n t speed of 4550 rpm, for a feed r a n g e from 0.0254 to 0.254 mm/rev in i n c r e m e n t s of 0.0254 mm/rev. S c a n n i n g and optical m i c r o s c o p y were used to
C.W. Wern et al./ Surface texture of composite drilled holes
389
e v a l u a t e the surface defects and damage. Profilometry was used to study the surface textures. The surface t e x t u r e s of the holes were e v a l u a t e d by analyzing the a v e r a g e r o u g h n e s s heights, statistical methods, and r a n d o m process methods of describing the m e a s u r e d surface r o u g h n e s s profiles. The surface produced by drill geometry A was superior to t h a t produced by drill g e o m e t r y B due to t h r e e reasons. First, the surface produced by drill B is almost four times r o u g h e r t h a n t h a t produced by drill A w h e n the feed rate is low. Drill B also produces a surface with a wider surface damage zone. Lastly, the probability of the o c c u r r e n c e of deep valleys in surfaces produced by drill B is g r e a t e r t h a n t h a t in surfaces produced by drill A. Feed rates h a v e a definite effect on the m a c h i n e d surfaces. Between 0.0254 and 0.1778 mm/rev, the surface r o u g h n e s s decreased with an increase in feed rate. At feed rates above 0.1778 mm/rev, the surface r o u g h n e s s increases with an increased feed. Average r o u g h n e s s height, R~, did not describe the e x t e n t of the surface r o u g h n e s s height v a r i a t i o n observed on all the profiles studies, but maximal r o u g h n e s s heights such as R~ and Rz will q u a n t i f y the depth of the valleys or the m a c h i n i n g damage in the drilled composites. P l o t t i n g the c u m u l a t i v e h e i g h t distribution on the probit scale showed successfully the existence of different s t r a t a associated with peaks and valleys of the m a c h i n e d graphite/epoxy surface profiles. Power spectral density f u n c t i o n analysis gave a better u n d e r s t a n d i n g of the w a v e l e n g t h distribution of the surface roughness profiles of m a c h i n e d g r a p h i t e / e p o x y surfaces.
Acknowledgments The a u t h o r s would like to t h a n k Mr. Paul Russel, supervisor in Boeing MR&D group, for e n c o u r a g e m e n t and financial support, and the W a s h i n g t o n T e c h n o l o g y C e n t e r for providing the n e c e s s a r y facilities as part of this res e a r c h at the U n i v e r s i t y of Washington.
References [1] M. Ramulu, M. Faridnia, J.L. Garbini and J.E. Jorgenson, J. Eng. Mater. Technol.. 112 (1991) 430 436. [2] K. Colligan and M. Ramulu, Manuf. Rev., 5 (1992). [3] M. Ramulu and J. Park, Mechanical Engineering Technical Report to Boeing, T.R. No. ME/91/2, August, 1991. [4] C.W. Wern, M.S. Thesis, Department of Mechanical Engineering, University of Washington, Seattle, Washington, August, 1991. [5] W. Konig and P. Grag, Ann. CIRP, 38(1) (1989) 119 124. [6] A. Koplev, A. Lystrup and T. Worm, Composites, 14, (1983) 371 376. [7] D.-H. Wang, M. Ramulu and C.W. Wern, Trans. NAMRI/SME, 20 (1992) 159 165. [8] R.B. Zipin, Appl. Surf. Sci. 15 (1983) 334 358. [9] T.R. Thomas, Rough Surfaces, Longman Group Limited, New York, 1982.