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Acoustic emission during orthogonal metal cutting
Int. I. Mech. Sci. Vol. 22, pp. 285-296 Pergamon Press Ltd., 1980. Printedin Great Britain
ACOUSTIC EMISSION ORTHOGONAL METAL
DURING CUTTING
D. A. DORNFELDand E. KANNATEY-AsIBU Department of Mechanical Engineering, University of California, Berkeley, CA 94720, U.S.A.
(Received 9 July 1979; in revised form 30 November 1979) Summary--The theory of acoustic emission and the analysis of emission signals is reviewed as it applies to generation of acoustic emission in metal cutting. Based upon the mechanics of the orthogonal cutting operation a relationship is developed between the root mean square (RMS) voltage of the acoustic emission and fundamental cutting parameters. The validity of this relationship is evaluated by a series of tests varying cutting speed, feed and rake angle for orthogonal machining. Strong dependence of the RMS voltage of the emission on both strain rate and cutting velocity was observed. The sources of acoustic emission in metal cutting are discussed and areas of additional work in the study of acoustic emission from metal cutting are identified. NOTATION b magnitude of burgers vector Ci, C2 constants E energy of acoustic emission rate of energy generation f feed k maximum shear stress, rmax I average distance between dislocations rc chip thickness ratio As shear plane deformation average dislocation velocity t time T~ uncut chip thickness T2 chip thickness U volume of participating material 1. INTRODUCTION
Acoustic emission analysis has steadily grown as a sophisticated non-destructive testing technique over the past 25 yr. It has been applied to a variety of situations including weld flaw detection, fracture and crack propagation in pressure vessels and mechanical equipment and material property evaluation during tensile testing. A recent application of acoustic emission to the monitoring of manufacturing operations is the work of Iwata and Moriwaki[1]. Their work relates acoustic emission signal characteristics to tool flank wear during metal cutting. Acoustic emission analysis is ideally suited for applications in which process changes such as tool wear must be detected because the emissions from the operation can be directly related to the mechanics of the process. Thus, the potential for increased use of acoustic emission for monitoring of manufacturing processes such as punching and forming, drawing, extrusion, welding and metal cutting is great. To be able to apply acoustic emission analysis to any one of these processes requires that the relationship between the mechanics of the process and the acoustic emission generated be understood. The objectives of this paper are threefold; (1) to review the theory of acoustic emission as it applies to metal cutting, (2) to propose a relationship between the RMS voltage level of the acoustic emission signal and basic metal cutting parameters, and (3) to evaluate this relationship by experimental investigation. A review of the theory of acoustic emission and the analysis of acoustic emission signals is presented in Section 2. A relationship between acoustic emission signal characteristics and metal cutting parameters is developed in Section 3, and Section 4 discusses the experimental procedure used and presents the data from those experiments. Analysis of the data and conclusions are presented in Sections 5 and 6, respectively. MS Vol. 22, No. 5--B
285
286
D.A. DORNFELD and E. KANNATEY-ASIBU 2. ACOUSTIC EMISSION THEORY AND DATA ANALYSIS
Acoustic emission can be defined as the transient elastic energy spontaneously released in materials undergoing deformation, fracture, or both [2]. It is, therefore, dependent on basic deformation mechanisms, such as dislocation motion, twinning, grain boundary sliding, and vacancy coalescence. Dislocation motion is the major mechanism of plastic deformation in most crystalline materials and it depends on the microstructure of a material. Thus, acoustic emission can be strongly related to the grain size, dislocation density and distribution of second phase particles in materials. However, since it is also observed during deformation of noncrystalline materials, its generation cannot be attributed solely to the above mentioned mechanisms. Other proposed sources of acoustic emission in metals include fracture and decohesion of inclusions, realignment or growth of magnetic domains, and phase transformations. Neither a comprehensive explanation of the sources of acoustic emission nor a complete analytical description of the stress waves generated by an emission source have been presented to date. The emission signal is usually detected by an instrumentation system using sensors (transducers) which, when stimulated by stress waves, transform mechanical excitations into electrical signals which are then amplified and transmitted to an oscilloscope, counter, RMS (root mean square) voltmeter, recorder, or spectrum analyzer, depending on the type of analysis required. The most commonly used transducer type is a piezoelectric ceramic element, lead zirconate titanate. As will be seen, the initial output voltage of a transducer is proportional to the square root of the energy released during a given deformation process. There are two distinctive types of acoustic emissions: (a) the high amplitude, somewhat erratic, low-frequency type called the burst emission which is generally associated with surface events, such as slip line formation and surface microcracks, and (b) the lower amplitude, steady, and high-frequency type called the continuous emission that is generally associated with internal mechanism activity, being normally observed during tests of tensile specimens. Acoustic emission in metal cutting is considered to be continuous. That is, the signal characteristics are such that the average time between emissions of similar amplitude is less than or comparable to the duration of the emission[3]. The energy contained in an acoustic emission and the rate at which it is dissipated are strongly dependent on the rate of deformation (strain rate), the applied stress, and the volume of the participating material. Thus, a process can be monitored using acoustic emission if it can be directly related to one or more of the above parameters. Changes in the process parameters can then be correlated with changes in the emission observed. For example, changes in the RMS voltage level and its characteristics of an observed signal can be related to the strain rate and volume. This follows from the understanding that in crystalline materials, in which most of the deformation processes are mainly a result of dislocation motion, the work of plastic deformation is normally made up of two parts: (a) an elastic part that is partially or fully recoverable on unloading, and (b) the plastic work of deformation for which a small portion is associated with an increase in the number of dislocations, the major part appearing as thermal energy. This thermal energy can be considered as a potential power source for acoustic emissions since it is the cumulative effect of a large number of phonons of dislocations generated as they pass through the crystal lattice [4]. The phonon can be looked upon as the elastic strain that is suddenly released to produce a vibrational wave in the lattice as a dislocation moves from one minimum energy position to another. In the general case, for an element of volume d U subject to stresses ~i which cause plastic strain increments de,i, the energy increment per unit volume of plastic deformation (dW) is given by;
(I)
d W = o'ii d~ 0 d U .
Over the entire volume, then, the rate of energy dissipation is given by;
g,
= fu
(2)
d U.
Acoustic emission from dislocation generation is easily seen in Fig. 1, illustrating the AE activity during the testing of a low carbon steel tensile specimen[5]. At the beginning of the test (in the elastic region) most dislocations are either dormant or move with very little velocity. The signals observed at the beginning are emitted from local volumes where plastic processes are occurring. Thus, the rate of energy dissipation and consequently the emission rate are very small. The irreversibility of the acoustic emission process supports the contention that plastic processes are involved. As the stress increases, the average dislocation velocity lOxiO3 ACOUSTIC EMISSION
8 ~7
56 ~s ~4 )-
~2 c)
0.02
0.04
~- '. '.~" L-! ~, 1 I 0.06 0.08 0.10 0.12
STRAIN,
in/in.
(ram/ram)
FIG. 1. Acoustic emission and stress as a function of strain f o r a mild steel tensile specimen [5].
Acoustic emission during orthogonal metal cutting
287
increases as does the emission rate until yielding occurs. Then the majority of the dormant dislocations suddenly begin to move. It is this sudden mass mobilization involving the tearing off of pinned dislocations that causes the sudden jump in emission activity at yield. With strain hardening, an increase in the dislocation density follows; this results in a reduction in average dislocation motion, causing a much lower emission activity. The greatest problem encountered in the application of acoustic emission is the analysis or interpretation of the emission signals obtained due to the randomness of the acoustic emission process. An emission signal is nonperiodic, contains many frequencies and cannot be described by an explicit mathematical relationship [6]. One of the primary methods for quantitatively presenting acoustic emission data is by measuring the energy of the AE signal. The RMS voltage of a continuous AE signal can be used to make this energy measurement. This method of monitoring the AE signal is known to have several advantages over the traditional count and count rate techniques[7]. Among these are; (a) the smoothing of the acoustic emission data which facilitates the modeling of the data with analytical functions, (b) the disappearance of the extreme sensitivity of the count rate technique to small changes in the threshold level, (c) the reduced sensitivity of the RMS values to small changes in the system electronic gain or in the transducer coupling efficiency, and (d) the possible ease of relating the RMS acoustic emission data to the energy contained in the signal. From the definition of the RMS voltage level of the emission, the energy, E, contained in the emission can be derived. The energy of the emission signal can be expressed as AE ac (RMS)2At.
(3)
when AE is the energy expenditure during the interval At. 3. ACOUSTIC EMISSION AND METAL CUTTING The basic cutting process can be represented schematically as shown in Fig. 2. Here it is assumed that a continuous chip without a built-up edge is formed principally by a shear deformation (referred to as primary deformation), caused by the motion of the tool parallel to the surface of the work piece. The location of this shearing action is defined as a straight line in Fig. 2, extending from the point of intersection of the chip with the uncut surface to the tip of the tool and separating the uncut material in the work piece from the cut material in the chip. The length of this line multiplied by the depth of the cut (or width of the tool where the tool is totally engaged) determines the shear area at an angle 4), the shear angle, to the surface of the work. After separation from the work piece, the chip slides over the rake face of the cutting tool and may experience some additional deformation (referred to as secondary deformation, see Fig. 2) before losing contact with the tool face. The cutting process is termed orthogonal when the tip of the tool formed by the intersection of the rake surface, assumed a plane, and the flank surface of the tool is perpendicular to the direction of tool motion. If the width of the chip is large compared to the thickness of the chip, then, neglecting end conditions, a uniform stress distribution can be assumed in any section taken through the chip perpendicular to the cutting edge of the tool, and the process can be analyzed as one of plane strain. Knowing the chip thickness ratio, rc = T~]T2, the shear angle 4) can be calculated by geometry as
(k = tan -~ rc cos l - rc sin a
PRI SHEARzONE MARY ~
---
~
~
T
(4)
/SECONDARY SHEAR ZONE
~
TOOL Vw
WORKPIECEa, rake angle 8, clearance angle (k, shear angle Vw, cutting velocity Vc, chip velocity Vs, shear velocity TI , uncut chip thickness T2, chip thickness FIG. 2. Schematic of orthogonal cutting.
288
D.A. DORNFELD and E. KANNATEY-ASIBU
i
/
v---.
/ :y
/
TOOL ~b, IS#E~#TTSI~q~AIN
~y &$
B~
c
o SHEAR STRAIN ~N
D
A
GENERAL
FIG. 3. Determination of shear strain in orthogonal cutting. where a is the tool rake angle. If the shear mechanism along the shear plane is assumed to act as illustrated in Fig. 3 [8], i.e. similar to a sliding deck of cards, then the shear stress is maximum at the shear plane, which, as shown in Fig. 3, is a line in two dimensions coincident with the slip line. The straight line representation of the shear plane in two dimensions is imposed by continuity requirements for the motion of a particle of material passing from the uncut work piece material through the shear zone and exiting as part of the chip/9]. The shear strain in cutting, y, in the primary deformation zone can then be defined as (refer to Fig. 3), As Ay
A D DB' CD ~--~--~-=tan~,-c~j+cos~o'l, ~
(5)
where Ay is the spacing of successive shear planes. The shear strain specified by (5) is a finite quantity and a measure of the large plastic shear deformation occurring in the shear zone[10]. Taking into consideration the time AT for the metal to move a distance As along the shear plane, the shear strain rate during cutting, y, is As 1
V~,
Ay At
Ay
(6)
where the shear velocity along the shear plane, V,, is c o s ot
Vs
c o s ( d ~ - a ) Vw
(7)
and from (6) and (7) COS a
~/
Vw
cos (~ - a) Ay
(8)
where Ay, the shear plane spacing, is taken generally as 10-3 > Ay > 10-4 in (0.025 > Ay > 0.0025 mm) for carbon steel, and Vw is the cutting speed. Continuing, strain rate can be represented as r~ c o s ot Vw
sin d~ Ay
(9)
knowing that r~
sin 4~ cos (d~ - a)"
The parameters upon which the acoustic emission from metal cutting is dependent are strain rate, applied stress, and volume of participating material; these are in turn a function of the following basic cutting parameters in orthogonal machining; (a) rake angle, a, degrees, (b) clearance angle, 8, degrees, (c) feed, f, inches/revolution (mm/rev) and (d) cutting velocity, Vw, feet/minute (m/s). It is then possible to relate these basic cutting parameters directly to the emission signal characteristics. From (9) it is seen that the strain rate is a function of cutting speed. Further, the rate of dislocation generation is a function of the strain rate and the instantaneous dislocation density. The shear strain can be represented as a function of the average distance between dislocations, I, as, ~, = bpl
(10)
Acoustic emission during orthogonal metal cutting
289
where O = dislocation density, that is, number of mobile dislocations in a unit area of the material section, and b = magnitude of the burgers vector. Then the strain rate -~ is [11]
~, = ~t = bl -~tt + bP -di dl Since dislocation density is defined on a per unit area basis, p =
(i/12), or I = p .2
"~=~bp ,/2_~
(11)
dp = ~_ pl/2. ~-
(12)
or the rate of dislocation generation, t~, is
Here the effects of fixed dislocations and loss of dislocation mobility through interaction are not accounted for. Thus, equation (12) estimates the rate of dislocation generation for a uniform array of mobile dislocations and this rate of generation (and therefore the level of acoustic emission activity) is proportional to the externally influenced strain rate in metal cutting. The average strain rate in metal cutting can also be represented as a function of p, b and the average dislocation velocity[12], as
~'a~ bpS
(13)
=
where S = average dislocation velocity, and the energy rate due to the dislocation generation as
E'= T,~.,,
(14)
where zs is the shear stress in the primary deformation zone. Thus, the energy in the signal over the entire volume of material can be related to material deformation characteristics by equating (3) to (2) and replacing the stress tr by the shear stress, zs, and the strain rate ~ by the average shear strain rate, ~,,o, yielding (RMS) 2 0c
d E I| r~%,, d U. -dT = J u
(15)
Then, for material shear strength k = ~'s and strain rate ~/(15) becomes (RMS) 2 =
CtkS, U
(16)
where U = volume of participating material, in3(mm3). The volume of participating material, U, must include material undergoing deformation in both the primary and secondary shear zones. A suitable approximation for U is the volume of the slipline field for orthogonal cutting without built up edge of Lee and Shaffer [13], Fig. 4. The slipline field theory is based on plain strain conditions which hold true for this case. Here deformation occurs instantaneously across the shear plane AC, where AC represents a direction of maximum shear stress in the material being cut when the constant maximum shear stress ~'m~ = k is exceeded. This assumes that the material behaves as an ideal plastic (rigid-perfectly plastic) but does not account for the work-hardening that occurs during machining. Such an assumption is reasonable in metal cutting. Under normal machining conditions, strains of an order of magnitude as high as 1 occur. Since for most materials the elastic yield point strain is of the order 10 3, this can be neglected when analyzing large plastic deformation and the material can be treated as a rigid plastic. The material is considered rigid to the left of the shear plane AC and in the chip beyond AB, being an ideal plastic in the region ABC after the instantaneous shearing along AC and subjected to a uniform state of stress.
,.~ ~ , .
,-;-~ ~ - ~ /
i
Tool
c
WORKPIECE
FIG. 4. Slip-line field configuration with no built up edge.
290
D.A. DORNFELD and E. KANNATEY-ASIBU
~
Top LocatingFrame
FI ' I # ~ /EpoxyWear Plate Transducer-~_ J ELi ~ t - - ~ ~ B o t t o r n Locating Frame Cutting TooI~
(
J ~
~Rubber Bond Hold Down
FIG. 5. Cutting tool with possible transducer locations.
A comprehensive expression relating the RMS value of the acoustic emission signal to process and material parameters can be written from (9) and (16) as; (RMS)2 = C1k rc c-~°sa Vw U sm ~
Ay
or
(17)
/ rcCOSa V, XJ/2 R M S = C z ~ k s i - - ~ -~yU ) .
4. EXPERIMENTAL INVESTIGATION OF METAL CUTTING ACOUSTIC EMISSION To evaluate the dependence of acoustic emission on metal cutting conditions, orthogonal cutting tests were conducted with low carbon steel under stable cutting conditions without lubricant. Using a twocomponent cutting force dynamometer, Rex 95 high speed steel (HSS) cutting tools and SAE 1015 steel tubing as the work piece, cutting tests were performed over a range of cutting speeds and feeds of 33 ft/min (fpm)-239fpm (0-168m/s-l.214m/s) and 0.00257in./rev. (ipr)-0.0108ipr (0.0653mm/rev-0.2743mm/rev), respectively. Tool rake angles of 0 and 20 with 3° clearance angles were used. The acoustic emission signal was detected via a transducer mounted on a specially adapted cutting tool, Fig. 5. The tool is constructed of a conventional HSS orthogonal cutting tool to which a cube of similar material has been silver brazed. This provides a large enough surface for the mounting of the acoustic emission transducer using a locating frame and a rubber band to prevent movement of the transducer, Fig. 6. A highly viscous resin couplant is used to promote efficient transmission of the acoustic emission sign',d to the transducer. The combination of the couplant, locating frame and rubber band insure that the transducer is adequately affixed to the tool. As seen in Fig. 5, the tool is designed to allow location of the transducer at any one of three positions, A, B, or C. Preliminary tests indicated that there is no significant variation in the acoustic emission RMS voltage from any of the transducer locations under similar cutting conditions. For the tests described here, position A was used. The frequency response of the transducer was 200-300 kHz which was found to be suitable for the experiments. The acoustic emission signal from the transducer was recorded on a videotape recorder for further analysis. A schematic of the test equipment is shown in Fig. 7. A typical acoustic emission signal during metal cutting of the 1015 material is shown in Fig. 8. Although not necessarily obvious from the photograph, due to the lack of resolution, the acoustic emission is continuous. Table 1 lists the machining conditions for the 0 and 20° rake angle tools along with calculated values of strain rate, j,, based on (9). The strain rate is dependent upon Ay, the shear zone thickness, in addition to the cutting speed and shear and rake angles. For the purposes of this calculation, the shear zone thickness was approximated based on the data of Kececioglu[14] who observed a range of the average shear zone thickness of 0.0007-0.007 in. (0.018--0.18 ram) for dry machining of SAE 1015 seamless tubing. The actual shear zone thickness, Ay, is A y = Ay. cos (~b - a).
Here Ay, is the shear zone thickness measured in a plane perpendicular to the cutting edge and in the direction parallel to the tool rake face. A value of Ay, = 0.0039 in. (0-099 mm), the mid range of shear zone thickness observed by Kececioglu was used for strain rate calculations. The square of the RMS voltage of the acoustic emission generated during cutting under the conditions listed in Table I are shown in Fig. 9 as a function of calculated strain rate, "~, Fig. 10 as a function of cutting speed, V,, and Fig. 11 as a function of feed at different cutting speeds. Data for both 0 and 20° rake angle tools is included.
RakeFace , ~ . ~ A
/
)
\
Flank Face
B
FIG. 6. Schematic of transducer mounting and locating technique.
Acoustic emission during orthogonal metal cutting Work Two. Component
~
--
/Tool/Cultln9 Force
:22Z '+,
['re Amo"*'e*I +
Monitoring ~ Osc Iloscope I
II
Recorder I
+
I Am°'''+er I +
Threshold Level
~
Bond Poss Filte
~n,er+ r
I i
Chort Recorder
~ J
RMS [ Vottmeter
I
IJ
FIG. 7. Schematic of test equipment.
8I 7
_o 5
o • 0° RAKE 0 20° RAKE
--
2
3
4
6
5
7
9
8
I0
i2
al
13
STRAIN RATE,~ (103/sec)
FIG. 9. (RMS) 2 vs "~.
8
+i 'o
•
0° RAKE
•
20 = RAKE
x
i .I i,
0
,~4~I
• i,
50 (254)
t
i
I,
I00 (.508)
i,
i
I
FI
,
150 (.762)
,
I,
200 (I.016)
+ J,
I
250 0.270)
Vw ,fpm (m/s)
Fro. 10. (RMS)2 vs cutting speed.
292
D . A . DORNFELD and E. KANNATEY-ASIBU
TABLE I. TEST CONDITIONS
Test A2
Tool Rake Angle, deg. 0
Cutting Speed, fpm
(m/s) 64 (.325} 64 (.325) 33 (.168)
B2
0
C2
0
E2
0
F2
0
H2
0
L2
0
M2
0
64 (.325)
XAA
20
XBB
20
239 (1.214) 120
148 (.752) 95 (.483) 239 (1.214) 64 (.325)
(.610)
XCC
20
XDD
20
XEE
20
XFF
2O
XGG
20
XHH
2O
XII
2O
XJJ
2O
XKK
20
XLL
2O
120 (.610) 41 (.208) 41 (.208) 64 (.325) 120 (.610) 239 (1.214) 64 (.325) 38 (.193) 239 (1.214) 64 (.325)
Feed, ipr (mm/rev)
rc
.0077 (.1956)
.453
24.4
.00351 (.0892)
3997.
.00257 (.0653) .00474 (.1204) .00343 (.0871) .00343 (.0871) .00257 (.0653) .00617 (.1567) .00343 (.0871)
.234
13.2
3537.
.396
21.6
.113
6.5
.286
16.0
.123
7.0
.385
21.1
.342
18.9
.00375 (.0953) .00358 (.0909) .00383 (.0973) .00370 (.0940) .00382 (.0970) .00359 (.0912) .00364 (.0925)
.00257 (.0653)
.342
20.0
.00385 (.0978)
I1686.
.00617 (.1567)
.404
25.6
.00383 (.0973)
5981,
.00257 (.0653) .00257 (.0653) .00617 (.1567)
.382
22.4
5857.
.332
19.4
.433
25.5
.00385 (.0978) .00385 (.0978) .00383 (.0973)
.00257 (.0653)
.359
21.0
.00385 (.0978)
3115.
.0108 (.2743)
.475
28.1
.00381 (.0968)
5955.
.0108 (.2743)
.476
28.1
.00381 (.0968)
I1934.
.00617 (.1567) .0108 (.2743) .00617 (.1567) .0108 (.2743)
.442
26.1
.00383
3140.
¢, deg
.451
26.6
.451
26.6
.435
25.7
AX, in (mm)
(.0973) .00382 (.0970) .00382 (.0970) .00382 (.0970)
l/sec
1966. 7861. 5353. 12658. 3793. 3741.
1981, 2004.
1909. I1863. 3135.
Acoustic emission during orthogonal metal cutting
FIG. 8. Typical acoustic emission signal during metal cutting.
293
Acoustic emission during orthogonal metal cutting 8
O'RAKE fpm (m/s) 0
7
• 0 •
ro i
64 (.33) 240(1.22)
20 ° RAKE fpm (m/s) • 64(.33) [ ] 120(.61) • 240(I 22)
5
0
x
295
4
rr 3 []
I
I
[]
I
I
I
I
I
.005 (.127)
I
I
I
I
.010 (.254)
FEED, ipr (ram/r)
FIG. 11. (RMS) 2 vs feed. 5. ANALYSIS OF DATA It is evident from the data presented in Section 4 that metal cutting is a good source of acoustic emission. A close investigation of the tool-chip-work piece interaction during machining reveals that acoustic emission can originate from five different sources; (a) material deformation in the shear zone during chip formation, (b) chip motion, sliding and sticking, along the tool rake face, (c) chip breaking or fracture, (d) impact of broken chips on tool or work piece or entanglement of continuous chips with the tool or work piece, and (e) tool-work rubbing, friction on the flank face. Considering the stress and strain-rates involved, the first two sources, a and b, constitute the major sources of acoustic emission during cutting. The contribution of emission from sources c and d is relatively minor. During the experimental investigation the effect of chip breaking and entanglement was insignificant in comparison to the overall emission RMS voltage or nonexistent. The acoustic emission due to tool-work piece friction on the flank surface, e, was minimized by the use of sharp tools and adequate flank clearance during the tests. Thus, the RMS level of the acoustic emission signals generated reflects the shear zone deformation and chip-tool rubbing/sticking during metal cutting for the tests conducted. The data in Fig. 9 substantiate the proportional relationship between calculated strain rate and the square of the RMS voltage of the acoustic emission signal as proposed in (17). The effect of the tool rake angle on the acoustic emission is not seen for primarily two reasons. First, the accuracy with which the thickness of the shear zone is approximated in the strain rate calculation is poor. The thickness, Ay, will decrease at higher rake angles and cutting velocities and, thus, yield a higher strain rate than that calculated based on a constant Ay. Observations of Kececioglu[14] indicate that the shear zone thickness should decrease approximately 7% for a 5° increase in rake angle and 11% for a 100 fpm (30.5 m/s) increase in cutting velocity. Decreasing feed also caused a decrease in shear zone thickness as well. In the absence of exact measurements of Ay for the cutting conditions used, the values of Ay listed in Table 1 are used to approximate the strain rate, but do not incorporate the true variation of Ay with cutting conditions. Second, the effect of tool rake angle is also masked by the contribution of acoustic emission from chip-tool friction to the total acoustic emission signal as previously discussed. Since the strain rate is influenced by the cutting speed, a strong dependence of the RMS voltage on speed is expected. This is evident in Fig. 10. The effect of tool rake angle is again masked by the changes in shear zone thickness and chip-tool interaction. The effect of feed on acoustic emission activity is shown in Fig. 11 for several cutting speeds. From Kececioglu's observations[14] an increase in strain-rate of at least 50% for a 0.004ipr (0.102 mm/rev) decrease in feed is expected. Thus, a decrease in the RMS voltage of the acoustic emission should occur with increasing feed at constant velocity. As evident in Fig. 11, however, this does not occur for all cutting speeds and RMS voltage is essentially constant with respect to feed at the lowest velocity. This can be explained in part by the observation that at constant rake angle and cutting speed, for an increase in feed, the length of the tool-chip contact also increases (see, for example,J15]). Since the increased tool-chip rubbing evidenced by increased contact length generates additional acoustic emission, the effect of increases in the shear zone at higher feeds is nullified. In addition, whereas a slight decrease in the RMS level with feed is seen at higher speeds, it is possible that the presence of a built up edge at lower speeds contributes to additional acoustic emission activity at higher feed rates. Since acoustic emission activity is related to dislocation movement and the volume of material in which this movement occurs, the strongest dependency is expected to occur between the acoustic emission energy and the metal strain rate and participating volume combined. This dependency has been demonstrated in uniaxial tension tests in which both strain rate and participating volumes are known but is impossible to verify here for metal cutting. Several areas for additional work with respect to acoustic emission generated
296
D.A. DORNFELDand E. KANNATEY-ASIBU
in metal cutting that would aid in the verification of this dependency can be identified based upon this investigation; (a) accurate measurement of shear zone thickness, Ay, to allow calculation of realistic strain rates. In addition, the present model assumes no strain hardening occurs during cutting. In actuality, strain hardening will reduce the level of acoustic emission activity during cutting, (b) estimation of the volume of participating material, U, and (c) isolation of the contributions to the acoustic emission signal from deformation in the primary and secondary zones, respectively. The "sliding cards" representation of shear zone deformation for strain rate calculation neglects deformation in other zones during cutting. For more accurate estimation of acoustic emission activity, deformation in these other zones, as well as effects due to a built-up edge must be included. The present model (17) will underestimate the level of acoustic emission activity by exclusion of these contributions. 6. CONCLUSIONS T h e g e n e r a t i o n of a c o u s t i c e m i s s i o n d u r i n g m e t a l c u t t i n g b a s e d u p o n the m e c h a n i c s of the c u t t i n g p r o c e s s has b e e n a n a l y z e d a n d a r e l a t i o n s h i p p r o p o s e d c o r r e l a t i n g the e n e r g y of the a c o u s t i c e m i s s i o n signal as m e a s u r e d b y the R M S voltage of the signal with m a c h i n i n g p a r a m e t e r s . A c o u s t i c e m i s s i o n d a t a r e c o r d e d d u r i n g o r t h o g o n a i c u t t i n g tests h a v e verified the d e p e n d e n c y of e m i s s i o n e n e r g y o n the c u t t i n g speed a n d strain rate. A r e a s of a d d i t i o n a l w o r k n e c e s s a r y to fully u n d e r s t a n d the g e n e r a t i o n of a c o u s t i c e m i s s i o n in m e t a l c u t t i n g w e r e identified. Acknowledgements--The authors wish to thank Mrs. Carol Chiang for typing the manuscript. REFERENCES I. K. IWATAand T. MORlWAKI,An application of acoustic emission measurement to in-process sensing of
tool wear. Ann. CIRP 26, 1 (1977). 2. J. C. SPANNER,Acoustic emission-applications and trends. Elastic Waves and Non-Destructive Testing of Materials. ASME AMD29 (1978). 3. U. S. Acoustic Emission Working Group Subcommittee Report, (Chairman: J. R. Frederick). Dept. of Mechanical Engineering, Univ. of Michigan (1974). 4. P. P. GILLIS,Dislocation motions and acoustic emissions. Acoustic Emission, ASTM STP 505, pp. 20-29. American Society for Testing and Materials (1972). 5. H. L. DUNEGANand A. T. GREEN, Factors affecting acoustic emission response from materials. Acoustic Emission, ASTM STP 505, p. 201. American Society for Testing and Materials (1972). 6. K. ONO,R. STERNand M. LONG,JR., Application of correlation analysis to acoustic emission. Acoustic Emission, ASTM STP 505, pp. 152-163. American Society for Testing and Materials (1972). 7. M. A. HAMSTAD,On energy measurement of continuous acoustic emission. Lawrence Livermore Laboratory, Report no. UCRL-76286 (1974). 8. M. C. SHAW,Metal Cutting Principles. 3rd Edn. MIT Press, Cambridge (1956). 9. B. W. SCHAFFER,The mechanics of the simple shearing process during orthogonal machining. Trans. ASME, 77, 331 (1955). 10. E. G. THOMSEN,S. KOBAYASHIand C. T. YANG,Mechanics of Plastic Deformation in Metal Processing. p. 451. Macmillan, New York (1967). 11. B. F. YON TURKOVICH,Dislocation theory of shear stress and strain rate in metal cutting. Proc, 8th LM.T.D.R. Conf. p. 534.(1967). 12. A. H. COTrRELL, The Mechanical Properties of Matter. p. 269. Wiley, New York (1964). 13. E. H. LEE, B. W. SHAFFER,The theory of plasticity applied to a problem of machining. J. App. Mech. Trans. ASME 73, 405 (1951). 14. D. KECECIOGLU,Shear zone size, compressive stress and shear strain in metal cutting and their effects on mean shear flow stress. 3. Engng Indust. ASME Trans. 82, 79-86 (1960). 15. J. H. CREVLING,T. F. JORDANand E. G. THOMSEN,Some studies of angle relationships in metal cutting. Trans. ASME, 79, 132 (1957).
ACOUSTIC EMISSION ORTHOGONAL METAL
DURING CUTTING
D. A. DORNFELDand E. KANNATEY-AsIBU Department of Mechanical Engineering, University of California, Berkeley, CA 94720, U.S.A.
(Received 9 July 1979; in revised form 30 November 1979) Summary--The theory of acoustic emission and the analysis of emission signals is reviewed as it applies to generation of acoustic emission in metal cutting. Based upon the mechanics of the orthogonal cutting operation a relationship is developed between the root mean square (RMS) voltage of the acoustic emission and fundamental cutting parameters. The validity of this relationship is evaluated by a series of tests varying cutting speed, feed and rake angle for orthogonal machining. Strong dependence of the RMS voltage of the emission on both strain rate and cutting velocity was observed. The sources of acoustic emission in metal cutting are discussed and areas of additional work in the study of acoustic emission from metal cutting are identified. NOTATION b magnitude of burgers vector Ci, C2 constants E energy of acoustic emission rate of energy generation f feed k maximum shear stress, rmax I average distance between dislocations rc chip thickness ratio As shear plane deformation average dislocation velocity t time T~ uncut chip thickness T2 chip thickness U volume of participating material 1. INTRODUCTION
Acoustic emission analysis has steadily grown as a sophisticated non-destructive testing technique over the past 25 yr. It has been applied to a variety of situations including weld flaw detection, fracture and crack propagation in pressure vessels and mechanical equipment and material property evaluation during tensile testing. A recent application of acoustic emission to the monitoring of manufacturing operations is the work of Iwata and Moriwaki[1]. Their work relates acoustic emission signal characteristics to tool flank wear during metal cutting. Acoustic emission analysis is ideally suited for applications in which process changes such as tool wear must be detected because the emissions from the operation can be directly related to the mechanics of the process. Thus, the potential for increased use of acoustic emission for monitoring of manufacturing processes such as punching and forming, drawing, extrusion, welding and metal cutting is great. To be able to apply acoustic emission analysis to any one of these processes requires that the relationship between the mechanics of the process and the acoustic emission generated be understood. The objectives of this paper are threefold; (1) to review the theory of acoustic emission as it applies to metal cutting, (2) to propose a relationship between the RMS voltage level of the acoustic emission signal and basic metal cutting parameters, and (3) to evaluate this relationship by experimental investigation. A review of the theory of acoustic emission and the analysis of acoustic emission signals is presented in Section 2. A relationship between acoustic emission signal characteristics and metal cutting parameters is developed in Section 3, and Section 4 discusses the experimental procedure used and presents the data from those experiments. Analysis of the data and conclusions are presented in Sections 5 and 6, respectively. MS Vol. 22, No. 5--B
285
286
D.A. DORNFELD and E. KANNATEY-ASIBU 2. ACOUSTIC EMISSION THEORY AND DATA ANALYSIS
Acoustic emission can be defined as the transient elastic energy spontaneously released in materials undergoing deformation, fracture, or both [2]. It is, therefore, dependent on basic deformation mechanisms, such as dislocation motion, twinning, grain boundary sliding, and vacancy coalescence. Dislocation motion is the major mechanism of plastic deformation in most crystalline materials and it depends on the microstructure of a material. Thus, acoustic emission can be strongly related to the grain size, dislocation density and distribution of second phase particles in materials. However, since it is also observed during deformation of noncrystalline materials, its generation cannot be attributed solely to the above mentioned mechanisms. Other proposed sources of acoustic emission in metals include fracture and decohesion of inclusions, realignment or growth of magnetic domains, and phase transformations. Neither a comprehensive explanation of the sources of acoustic emission nor a complete analytical description of the stress waves generated by an emission source have been presented to date. The emission signal is usually detected by an instrumentation system using sensors (transducers) which, when stimulated by stress waves, transform mechanical excitations into electrical signals which are then amplified and transmitted to an oscilloscope, counter, RMS (root mean square) voltmeter, recorder, or spectrum analyzer, depending on the type of analysis required. The most commonly used transducer type is a piezoelectric ceramic element, lead zirconate titanate. As will be seen, the initial output voltage of a transducer is proportional to the square root of the energy released during a given deformation process. There are two distinctive types of acoustic emissions: (a) the high amplitude, somewhat erratic, low-frequency type called the burst emission which is generally associated with surface events, such as slip line formation and surface microcracks, and (b) the lower amplitude, steady, and high-frequency type called the continuous emission that is generally associated with internal mechanism activity, being normally observed during tests of tensile specimens. Acoustic emission in metal cutting is considered to be continuous. That is, the signal characteristics are such that the average time between emissions of similar amplitude is less than or comparable to the duration of the emission[3]. The energy contained in an acoustic emission and the rate at which it is dissipated are strongly dependent on the rate of deformation (strain rate), the applied stress, and the volume of the participating material. Thus, a process can be monitored using acoustic emission if it can be directly related to one or more of the above parameters. Changes in the process parameters can then be correlated with changes in the emission observed. For example, changes in the RMS voltage level and its characteristics of an observed signal can be related to the strain rate and volume. This follows from the understanding that in crystalline materials, in which most of the deformation processes are mainly a result of dislocation motion, the work of plastic deformation is normally made up of two parts: (a) an elastic part that is partially or fully recoverable on unloading, and (b) the plastic work of deformation for which a small portion is associated with an increase in the number of dislocations, the major part appearing as thermal energy. This thermal energy can be considered as a potential power source for acoustic emissions since it is the cumulative effect of a large number of phonons of dislocations generated as they pass through the crystal lattice [4]. The phonon can be looked upon as the elastic strain that is suddenly released to produce a vibrational wave in the lattice as a dislocation moves from one minimum energy position to another. In the general case, for an element of volume d U subject to stresses ~i which cause plastic strain increments de,i, the energy increment per unit volume of plastic deformation (dW) is given by;
(I)
d W = o'ii d~ 0 d U .
Over the entire volume, then, the rate of energy dissipation is given by;
g,
= fu
(2)
d U.
Acoustic emission from dislocation generation is easily seen in Fig. 1, illustrating the AE activity during the testing of a low carbon steel tensile specimen[5]. At the beginning of the test (in the elastic region) most dislocations are either dormant or move with very little velocity. The signals observed at the beginning are emitted from local volumes where plastic processes are occurring. Thus, the rate of energy dissipation and consequently the emission rate are very small. The irreversibility of the acoustic emission process supports the contention that plastic processes are involved. As the stress increases, the average dislocation velocity lOxiO3 ACOUSTIC EMISSION
8 ~7
56 ~s ~4 )-
~2 c)
0.02
0.04
~- '. '.~" L-! ~, 1 I 0.06 0.08 0.10 0.12
STRAIN,
in/in.
(ram/ram)
FIG. 1. Acoustic emission and stress as a function of strain f o r a mild steel tensile specimen [5].
Acoustic emission during orthogonal metal cutting
287
increases as does the emission rate until yielding occurs. Then the majority of the dormant dislocations suddenly begin to move. It is this sudden mass mobilization involving the tearing off of pinned dislocations that causes the sudden jump in emission activity at yield. With strain hardening, an increase in the dislocation density follows; this results in a reduction in average dislocation motion, causing a much lower emission activity. The greatest problem encountered in the application of acoustic emission is the analysis or interpretation of the emission signals obtained due to the randomness of the acoustic emission process. An emission signal is nonperiodic, contains many frequencies and cannot be described by an explicit mathematical relationship [6]. One of the primary methods for quantitatively presenting acoustic emission data is by measuring the energy of the AE signal. The RMS voltage of a continuous AE signal can be used to make this energy measurement. This method of monitoring the AE signal is known to have several advantages over the traditional count and count rate techniques[7]. Among these are; (a) the smoothing of the acoustic emission data which facilitates the modeling of the data with analytical functions, (b) the disappearance of the extreme sensitivity of the count rate technique to small changes in the threshold level, (c) the reduced sensitivity of the RMS values to small changes in the system electronic gain or in the transducer coupling efficiency, and (d) the possible ease of relating the RMS acoustic emission data to the energy contained in the signal. From the definition of the RMS voltage level of the emission, the energy, E, contained in the emission can be derived. The energy of the emission signal can be expressed as AE ac (RMS)2At.
(3)
when AE is the energy expenditure during the interval At. 3. ACOUSTIC EMISSION AND METAL CUTTING The basic cutting process can be represented schematically as shown in Fig. 2. Here it is assumed that a continuous chip without a built-up edge is formed principally by a shear deformation (referred to as primary deformation), caused by the motion of the tool parallel to the surface of the work piece. The location of this shearing action is defined as a straight line in Fig. 2, extending from the point of intersection of the chip with the uncut surface to the tip of the tool and separating the uncut material in the work piece from the cut material in the chip. The length of this line multiplied by the depth of the cut (or width of the tool where the tool is totally engaged) determines the shear area at an angle 4), the shear angle, to the surface of the work. After separation from the work piece, the chip slides over the rake face of the cutting tool and may experience some additional deformation (referred to as secondary deformation, see Fig. 2) before losing contact with the tool face. The cutting process is termed orthogonal when the tip of the tool formed by the intersection of the rake surface, assumed a plane, and the flank surface of the tool is perpendicular to the direction of tool motion. If the width of the chip is large compared to the thickness of the chip, then, neglecting end conditions, a uniform stress distribution can be assumed in any section taken through the chip perpendicular to the cutting edge of the tool, and the process can be analyzed as one of plane strain. Knowing the chip thickness ratio, rc = T~]T2, the shear angle 4) can be calculated by geometry as
(k = tan -~ rc cos l - rc sin a
PRI SHEARzONE MARY ~
---
~
~
T
(4)
/SECONDARY SHEAR ZONE
~
TOOL Vw
WORKPIECEa, rake angle 8, clearance angle (k, shear angle Vw, cutting velocity Vc, chip velocity Vs, shear velocity TI , uncut chip thickness T2, chip thickness FIG. 2. Schematic of orthogonal cutting.
288
D.A. DORNFELD and E. KANNATEY-ASIBU
i
/
v---.
/ :y
/
TOOL ~b, IS#E~#TTSI~q~AIN
~y &$
B~
c
o SHEAR STRAIN ~N
D
A
GENERAL
FIG. 3. Determination of shear strain in orthogonal cutting. where a is the tool rake angle. If the shear mechanism along the shear plane is assumed to act as illustrated in Fig. 3 [8], i.e. similar to a sliding deck of cards, then the shear stress is maximum at the shear plane, which, as shown in Fig. 3, is a line in two dimensions coincident with the slip line. The straight line representation of the shear plane in two dimensions is imposed by continuity requirements for the motion of a particle of material passing from the uncut work piece material through the shear zone and exiting as part of the chip/9]. The shear strain in cutting, y, in the primary deformation zone can then be defined as (refer to Fig. 3), As Ay
A D DB' CD ~--~--~-=tan~,-c~j+cos~o'l, ~
(5)
where Ay is the spacing of successive shear planes. The shear strain specified by (5) is a finite quantity and a measure of the large plastic shear deformation occurring in the shear zone[10]. Taking into consideration the time AT for the metal to move a distance As along the shear plane, the shear strain rate during cutting, y, is As 1
V~,
Ay At
Ay
(6)
where the shear velocity along the shear plane, V,, is c o s ot
Vs
c o s ( d ~ - a ) Vw
(7)
and from (6) and (7) COS a
~/
Vw
cos (~ - a) Ay
(8)
where Ay, the shear plane spacing, is taken generally as 10-3 > Ay > 10-4 in (0.025 > Ay > 0.0025 mm) for carbon steel, and Vw is the cutting speed. Continuing, strain rate can be represented as r~ c o s ot Vw
sin d~ Ay
(9)
knowing that r~
sin 4~ cos (d~ - a)"
The parameters upon which the acoustic emission from metal cutting is dependent are strain rate, applied stress, and volume of participating material; these are in turn a function of the following basic cutting parameters in orthogonal machining; (a) rake angle, a, degrees, (b) clearance angle, 8, degrees, (c) feed, f, inches/revolution (mm/rev) and (d) cutting velocity, Vw, feet/minute (m/s). It is then possible to relate these basic cutting parameters directly to the emission signal characteristics. From (9) it is seen that the strain rate is a function of cutting speed. Further, the rate of dislocation generation is a function of the strain rate and the instantaneous dislocation density. The shear strain can be represented as a function of the average distance between dislocations, I, as, ~, = bpl
(10)
Acoustic emission during orthogonal metal cutting
289
where O = dislocation density, that is, number of mobile dislocations in a unit area of the material section, and b = magnitude of the burgers vector. Then the strain rate -~ is [11]
~, = ~t = bl -~tt + bP -di dl Since dislocation density is defined on a per unit area basis, p =
(i/12), or I = p .2
"~=~bp ,/2_~
(11)
dp = ~_ pl/2. ~-
(12)
or the rate of dislocation generation, t~, is
Here the effects of fixed dislocations and loss of dislocation mobility through interaction are not accounted for. Thus, equation (12) estimates the rate of dislocation generation for a uniform array of mobile dislocations and this rate of generation (and therefore the level of acoustic emission activity) is proportional to the externally influenced strain rate in metal cutting. The average strain rate in metal cutting can also be represented as a function of p, b and the average dislocation velocity[12], as
~'a~ bpS
(13)
=
where S = average dislocation velocity, and the energy rate due to the dislocation generation as
E'= T,~.,,
(14)
where zs is the shear stress in the primary deformation zone. Thus, the energy in the signal over the entire volume of material can be related to material deformation characteristics by equating (3) to (2) and replacing the stress tr by the shear stress, zs, and the strain rate ~ by the average shear strain rate, ~,,o, yielding (RMS) 2 0c
d E I| r~%,, d U. -dT = J u
(15)
Then, for material shear strength k = ~'s and strain rate ~/(15) becomes (RMS) 2 =
CtkS, U
(16)
where U = volume of participating material, in3(mm3). The volume of participating material, U, must include material undergoing deformation in both the primary and secondary shear zones. A suitable approximation for U is the volume of the slipline field for orthogonal cutting without built up edge of Lee and Shaffer [13], Fig. 4. The slipline field theory is based on plain strain conditions which hold true for this case. Here deformation occurs instantaneously across the shear plane AC, where AC represents a direction of maximum shear stress in the material being cut when the constant maximum shear stress ~'m~ = k is exceeded. This assumes that the material behaves as an ideal plastic (rigid-perfectly plastic) but does not account for the work-hardening that occurs during machining. Such an assumption is reasonable in metal cutting. Under normal machining conditions, strains of an order of magnitude as high as 1 occur. Since for most materials the elastic yield point strain is of the order 10 3, this can be neglected when analyzing large plastic deformation and the material can be treated as a rigid plastic. The material is considered rigid to the left of the shear plane AC and in the chip beyond AB, being an ideal plastic in the region ABC after the instantaneous shearing along AC and subjected to a uniform state of stress.
,.~ ~ , .
,-;-~ ~ - ~ /
i
Tool
c
WORKPIECE
FIG. 4. Slip-line field configuration with no built up edge.
290
D.A. DORNFELD and E. KANNATEY-ASIBU
~
Top LocatingFrame
FI ' I # ~ /EpoxyWear Plate Transducer-~_ J ELi ~ t - - ~ ~ B o t t o r n Locating Frame Cutting TooI~
(
J ~
~Rubber Bond Hold Down
FIG. 5. Cutting tool with possible transducer locations.
A comprehensive expression relating the RMS value of the acoustic emission signal to process and material parameters can be written from (9) and (16) as; (RMS)2 = C1k rc c-~°sa Vw U sm ~
Ay
or
(17)
/ rcCOSa V, XJ/2 R M S = C z ~ k s i - - ~ -~yU ) .
4. EXPERIMENTAL INVESTIGATION OF METAL CUTTING ACOUSTIC EMISSION To evaluate the dependence of acoustic emission on metal cutting conditions, orthogonal cutting tests were conducted with low carbon steel under stable cutting conditions without lubricant. Using a twocomponent cutting force dynamometer, Rex 95 high speed steel (HSS) cutting tools and SAE 1015 steel tubing as the work piece, cutting tests were performed over a range of cutting speeds and feeds of 33 ft/min (fpm)-239fpm (0-168m/s-l.214m/s) and 0.00257in./rev. (ipr)-0.0108ipr (0.0653mm/rev-0.2743mm/rev), respectively. Tool rake angles of 0 and 20 with 3° clearance angles were used. The acoustic emission signal was detected via a transducer mounted on a specially adapted cutting tool, Fig. 5. The tool is constructed of a conventional HSS orthogonal cutting tool to which a cube of similar material has been silver brazed. This provides a large enough surface for the mounting of the acoustic emission transducer using a locating frame and a rubber band to prevent movement of the transducer, Fig. 6. A highly viscous resin couplant is used to promote efficient transmission of the acoustic emission sign',d to the transducer. The combination of the couplant, locating frame and rubber band insure that the transducer is adequately affixed to the tool. As seen in Fig. 5, the tool is designed to allow location of the transducer at any one of three positions, A, B, or C. Preliminary tests indicated that there is no significant variation in the acoustic emission RMS voltage from any of the transducer locations under similar cutting conditions. For the tests described here, position A was used. The frequency response of the transducer was 200-300 kHz which was found to be suitable for the experiments. The acoustic emission signal from the transducer was recorded on a videotape recorder for further analysis. A schematic of the test equipment is shown in Fig. 7. A typical acoustic emission signal during metal cutting of the 1015 material is shown in Fig. 8. Although not necessarily obvious from the photograph, due to the lack of resolution, the acoustic emission is continuous. Table 1 lists the machining conditions for the 0 and 20° rake angle tools along with calculated values of strain rate, j,, based on (9). The strain rate is dependent upon Ay, the shear zone thickness, in addition to the cutting speed and shear and rake angles. For the purposes of this calculation, the shear zone thickness was approximated based on the data of Kececioglu[14] who observed a range of the average shear zone thickness of 0.0007-0.007 in. (0.018--0.18 ram) for dry machining of SAE 1015 seamless tubing. The actual shear zone thickness, Ay, is A y = Ay. cos (~b - a).
Here Ay, is the shear zone thickness measured in a plane perpendicular to the cutting edge and in the direction parallel to the tool rake face. A value of Ay, = 0.0039 in. (0-099 mm), the mid range of shear zone thickness observed by Kececioglu was used for strain rate calculations. The square of the RMS voltage of the acoustic emission generated during cutting under the conditions listed in Table I are shown in Fig. 9 as a function of calculated strain rate, "~, Fig. 10 as a function of cutting speed, V,, and Fig. 11 as a function of feed at different cutting speeds. Data for both 0 and 20° rake angle tools is included.
RakeFace , ~ . ~ A
/
)
\
Flank Face
B
FIG. 6. Schematic of transducer mounting and locating technique.
Acoustic emission during orthogonal metal cutting Work Two. Component
~
--
/Tool/Cultln9 Force
:22Z '+,
['re Amo"*'e*I +
Monitoring ~ Osc Iloscope I
II
Recorder I
+
I Am°'''+er I +
Threshold Level
~
Bond Poss Filte
~n,er+ r
I i
Chort Recorder
~ J
RMS [ Vottmeter
I
IJ
FIG. 7. Schematic of test equipment.
8I 7
_o 5
o • 0° RAKE 0 20° RAKE
--
2
3
4
6
5
7
9
8
I0
i2
al
13
STRAIN RATE,~ (103/sec)
FIG. 9. (RMS) 2 vs "~.
8
+i 'o
•
0° RAKE
•
20 = RAKE
x
i .I i,
0
,~4~I
• i,
50 (254)
t
i
I,
I00 (.508)
i,
i
I
FI
,
150 (.762)
,
I,
200 (I.016)
+ J,
I
250 0.270)
Vw ,fpm (m/s)
Fro. 10. (RMS)2 vs cutting speed.
292
D . A . DORNFELD and E. KANNATEY-ASIBU
TABLE I. TEST CONDITIONS
Test A2
Tool Rake Angle, deg. 0
Cutting Speed, fpm
(m/s) 64 (.325} 64 (.325) 33 (.168)
B2
0
C2
0
E2
0
F2
0
H2
0
L2
0
M2
0
64 (.325)
XAA
20
XBB
20
239 (1.214) 120
148 (.752) 95 (.483) 239 (1.214) 64 (.325)
(.610)
XCC
20
XDD
20
XEE
20
XFF
2O
XGG
20
XHH
2O
XII
2O
XJJ
2O
XKK
20
XLL
2O
120 (.610) 41 (.208) 41 (.208) 64 (.325) 120 (.610) 239 (1.214) 64 (.325) 38 (.193) 239 (1.214) 64 (.325)
Feed, ipr (mm/rev)
rc
.0077 (.1956)
.453
24.4
.00351 (.0892)
3997.
.00257 (.0653) .00474 (.1204) .00343 (.0871) .00343 (.0871) .00257 (.0653) .00617 (.1567) .00343 (.0871)
.234
13.2
3537.
.396
21.6
.113
6.5
.286
16.0
.123
7.0
.385
21.1
.342
18.9
.00375 (.0953) .00358 (.0909) .00383 (.0973) .00370 (.0940) .00382 (.0970) .00359 (.0912) .00364 (.0925)
.00257 (.0653)
.342
20.0
.00385 (.0978)
I1686.
.00617 (.1567)
.404
25.6
.00383 (.0973)
5981,
.00257 (.0653) .00257 (.0653) .00617 (.1567)
.382
22.4
5857.
.332
19.4
.433
25.5
.00385 (.0978) .00385 (.0978) .00383 (.0973)
.00257 (.0653)
.359
21.0
.00385 (.0978)
3115.
.0108 (.2743)
.475
28.1
.00381 (.0968)
5955.
.0108 (.2743)
.476
28.1
.00381 (.0968)
I1934.
.00617 (.1567) .0108 (.2743) .00617 (.1567) .0108 (.2743)
.442
26.1
.00383
3140.
¢, deg
.451
26.6
.451
26.6
.435
25.7
AX, in (mm)
(.0973) .00382 (.0970) .00382 (.0970) .00382 (.0970)
l/sec
1966. 7861. 5353. 12658. 3793. 3741.
1981, 2004.
1909. I1863. 3135.
Acoustic emission during orthogonal metal cutting
FIG. 8. Typical acoustic emission signal during metal cutting.
293
Acoustic emission during orthogonal metal cutting 8
O'RAKE fpm (m/s) 0
7
• 0 •
ro i
64 (.33) 240(1.22)
20 ° RAKE fpm (m/s) • 64(.33) [ ] 120(.61) • 240(I 22)
5
0
x
295
4
rr 3 []
I
I
[]
I
I
I
I
I
.005 (.127)
I
I
I
I
.010 (.254)
FEED, ipr (ram/r)
FIG. 11. (RMS) 2 vs feed. 5. ANALYSIS OF DATA It is evident from the data presented in Section 4 that metal cutting is a good source of acoustic emission. A close investigation of the tool-chip-work piece interaction during machining reveals that acoustic emission can originate from five different sources; (a) material deformation in the shear zone during chip formation, (b) chip motion, sliding and sticking, along the tool rake face, (c) chip breaking or fracture, (d) impact of broken chips on tool or work piece or entanglement of continuous chips with the tool or work piece, and (e) tool-work rubbing, friction on the flank face. Considering the stress and strain-rates involved, the first two sources, a and b, constitute the major sources of acoustic emission during cutting. The contribution of emission from sources c and d is relatively minor. During the experimental investigation the effect of chip breaking and entanglement was insignificant in comparison to the overall emission RMS voltage or nonexistent. The acoustic emission due to tool-work piece friction on the flank surface, e, was minimized by the use of sharp tools and adequate flank clearance during the tests. Thus, the RMS level of the acoustic emission signals generated reflects the shear zone deformation and chip-tool rubbing/sticking during metal cutting for the tests conducted. The data in Fig. 9 substantiate the proportional relationship between calculated strain rate and the square of the RMS voltage of the acoustic emission signal as proposed in (17). The effect of the tool rake angle on the acoustic emission is not seen for primarily two reasons. First, the accuracy with which the thickness of the shear zone is approximated in the strain rate calculation is poor. The thickness, Ay, will decrease at higher rake angles and cutting velocities and, thus, yield a higher strain rate than that calculated based on a constant Ay. Observations of Kececioglu[14] indicate that the shear zone thickness should decrease approximately 7% for a 5° increase in rake angle and 11% for a 100 fpm (30.5 m/s) increase in cutting velocity. Decreasing feed also caused a decrease in shear zone thickness as well. In the absence of exact measurements of Ay for the cutting conditions used, the values of Ay listed in Table 1 are used to approximate the strain rate, but do not incorporate the true variation of Ay with cutting conditions. Second, the effect of tool rake angle is also masked by the contribution of acoustic emission from chip-tool friction to the total acoustic emission signal as previously discussed. Since the strain rate is influenced by the cutting speed, a strong dependence of the RMS voltage on speed is expected. This is evident in Fig. 10. The effect of tool rake angle is again masked by the changes in shear zone thickness and chip-tool interaction. The effect of feed on acoustic emission activity is shown in Fig. 11 for several cutting speeds. From Kececioglu's observations[14] an increase in strain-rate of at least 50% for a 0.004ipr (0.102 mm/rev) decrease in feed is expected. Thus, a decrease in the RMS voltage of the acoustic emission should occur with increasing feed at constant velocity. As evident in Fig. 11, however, this does not occur for all cutting speeds and RMS voltage is essentially constant with respect to feed at the lowest velocity. This can be explained in part by the observation that at constant rake angle and cutting speed, for an increase in feed, the length of the tool-chip contact also increases (see, for example,J15]). Since the increased tool-chip rubbing evidenced by increased contact length generates additional acoustic emission, the effect of increases in the shear zone at higher feeds is nullified. In addition, whereas a slight decrease in the RMS level with feed is seen at higher speeds, it is possible that the presence of a built up edge at lower speeds contributes to additional acoustic emission activity at higher feed rates. Since acoustic emission activity is related to dislocation movement and the volume of material in which this movement occurs, the strongest dependency is expected to occur between the acoustic emission energy and the metal strain rate and participating volume combined. This dependency has been demonstrated in uniaxial tension tests in which both strain rate and participating volumes are known but is impossible to verify here for metal cutting. Several areas for additional work with respect to acoustic emission generated
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in metal cutting that would aid in the verification of this dependency can be identified based upon this investigation; (a) accurate measurement of shear zone thickness, Ay, to allow calculation of realistic strain rates. In addition, the present model assumes no strain hardening occurs during cutting. In actuality, strain hardening will reduce the level of acoustic emission activity during cutting, (b) estimation of the volume of participating material, U, and (c) isolation of the contributions to the acoustic emission signal from deformation in the primary and secondary zones, respectively. The "sliding cards" representation of shear zone deformation for strain rate calculation neglects deformation in other zones during cutting. For more accurate estimation of acoustic emission activity, deformation in these other zones, as well as effects due to a built-up edge must be included. The present model (17) will underestimate the level of acoustic emission activity by exclusion of these contributions. 6. CONCLUSIONS T h e g e n e r a t i o n of a c o u s t i c e m i s s i o n d u r i n g m e t a l c u t t i n g b a s e d u p o n the m e c h a n i c s of the c u t t i n g p r o c e s s has b e e n a n a l y z e d a n d a r e l a t i o n s h i p p r o p o s e d c o r r e l a t i n g the e n e r g y of the a c o u s t i c e m i s s i o n signal as m e a s u r e d b y the R M S voltage of the signal with m a c h i n i n g p a r a m e t e r s . A c o u s t i c e m i s s i o n d a t a r e c o r d e d d u r i n g o r t h o g o n a i c u t t i n g tests h a v e verified the d e p e n d e n c y of e m i s s i o n e n e r g y o n the c u t t i n g speed a n d strain rate. A r e a s of a d d i t i o n a l w o r k n e c e s s a r y to fully u n d e r s t a n d the g e n e r a t i o n of a c o u s t i c e m i s s i o n in m e t a l c u t t i n g w e r e identified. Acknowledgements--The authors wish to thank Mrs. Carol Chiang for typing the manuscript. REFERENCES I. K. IWATAand T. MORlWAKI,An application of acoustic emission measurement to in-process sensing of
tool wear. Ann. CIRP 26, 1 (1977). 2. J. C. SPANNER,Acoustic emission-applications and trends. Elastic Waves and Non-Destructive Testing of Materials. ASME AMD29 (1978). 3. U. S. Acoustic Emission Working Group Subcommittee Report, (Chairman: J. R. Frederick). Dept. of Mechanical Engineering, Univ. of Michigan (1974). 4. P. P. GILLIS,Dislocation motions and acoustic emissions. Acoustic Emission, ASTM STP 505, pp. 20-29. American Society for Testing and Materials (1972). 5. H. L. DUNEGANand A. T. GREEN, Factors affecting acoustic emission response from materials. Acoustic Emission, ASTM STP 505, p. 201. American Society for Testing and Materials (1972). 6. K. ONO,R. STERNand M. LONG,JR., Application of correlation analysis to acoustic emission. Acoustic Emission, ASTM STP 505, pp. 152-163. American Society for Testing and Materials (1972). 7. M. A. HAMSTAD,On energy measurement of continuous acoustic emission. Lawrence Livermore Laboratory, Report no. UCRL-76286 (1974). 8. M. C. SHAW,Metal Cutting Principles. 3rd Edn. MIT Press, Cambridge (1956). 9. B. W. SCHAFFER,The mechanics of the simple shearing process during orthogonal machining. Trans. ASME, 77, 331 (1955). 10. E. G. THOMSEN,S. KOBAYASHIand C. T. YANG,Mechanics of Plastic Deformation in Metal Processing. p. 451. Macmillan, New York (1967). 11. B. F. YON TURKOVICH,Dislocation theory of shear stress and strain rate in metal cutting. Proc, 8th LM.T.D.R. Conf. p. 534.(1967). 12. A. H. COTrRELL, The Mechanical Properties of Matter. p. 269. Wiley, New York (1964). 13. E. H. LEE, B. W. SHAFFER,The theory of plasticity applied to a problem of machining. J. App. Mech. Trans. ASME 73, 405 (1951). 14. D. KECECIOGLU,Shear zone size, compressive stress and shear strain in metal cutting and their effects on mean shear flow stress. 3. Engng Indust. ASME Trans. 82, 79-86 (1960). 15. J. H. CREVLING,T. F. JORDANand E. G. THOMSEN,Some studies of angle relationships in metal cutting. Trans. ASME, 79, 132 (1957).