Micromechanics of the Polymeric Matrix Composites Cutting Processes
A. Tarasyuk1 , A. Symonova2 , N. Verezub3
1
Department of Cutting Equip ment and Transport Technologies , Ukrain ian Engineering and Pedagogical
Academy, 16 Universitetska St., Kharkov, Ukraine, 61003
2
Department of Manufacturing Engineering, Kremenchuk Mykhailo Ostrohradskyi National Un iversity,
20 Pershotravneva St., Kremenchuk, Ukraine, 39600
3
Department of Integrated Technologies in Mechanical Engineering , National Technical University
(“Kharkov Polytechnic Institute”), 21 Frunze St., Kharkov, Ukraine, 61002
Abstract
Based on the present investigations, it has been possible to define the rules and
explain the mechanism of the direct mechanical breakdown of material in the removing
excess of material when high resistant fibre reinforced plastic was machined. The
effective cutting speeds for fibre reinforced plastic machining have been calculated
based on the experimental and theoretical results. The peculiarities of the mechanics of
the fibre reinforced plastic mechanical breakdown considered in this paper formed the
basis for the theoretical development of the process of fibre reinforced plastic cutting.
1. Introduction
Nowadays composite materia ls are wide ly used in ind ustry as their higher
specific properties (properties per unit weight) o f s trength and stiffness.
However, being non- homo geneous, anisotropic and reinforced by very abrasive
components, these mater ia ls are difficult to machine.
Conventiona l mac hining processes, such as turn ing, dr illing a nd milling,
are wide ly app lied to the machining of composite materia ls [1- 3]. However, the
major defic ie ncy o f numero us investigations o f fibre reinforced plastic
machining cons ists o f stra ight tra ns fer of the pr incip les o f mecha nics of meta l
cutting into the process of fibre reinforced plastic machining.
Fibre reinforced plastic (FRP) mater ia ls are for med from two or more
mater ia ls producing properties that could not be obtained fro m any other
mater ia l. One of the constituent mater ia ls acts as a matr ix and at least another
one constituent mater ia l acts as the reinforce ment in the composite. The
properties of a composite mater ia l depend on the nature of the re inforce ment
and the matr ix, the form of the re inforcement and the re lative content of
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reinforcement a nd matr ix. The most commo n types of re inforceme nt used for
fibre reinforced plastics are strong and br ittle fibres incorporated into a soft and
ductile polymer ic matr ix. FRP can be re inforced by glass (G) , carbon (C) a nd
ararnid (A) fibres. Capital letters G, C and A are p laced before the acronym
FRP to specify the nature of the re inforc ing fibres. The co mmon matr ix
mater ia ls for FRP compos ites are polyester, epoxy, polya mide, peek, etc. FRP
is used commonly for aerospace applicatio ns, militar y equip me nt, satellite
antennae, sports equipme nt, medica l prostheses, etc.
In this paper, experimenta l a nd theoretical ap proaches for studying
micro mechanics o f FRP machining are presented. Theoretical exp lanatio n of the
method of FRP vibration mac hining is one o f the exa mp les of the obtained
results on micromecha nics of machining that show the way for develop ment
new techno logica l processes for FRP machining.
2. Expe rime ntal Des ign
The investigations ha ve been carried out o n the specia l data acquis itio n
system that can store data of cutting process into a file (Fig. 1).
Figure 1: Schematic d iagra m of data acquis itio n syste m in machining o f FRP
Cutting tool (1) has been fixed in specia lly darkened rotationa l impact
testing mac hine (2) and has been shot o ff in the mo me nt when given c utting
speed has been reached.
In the mo me nt of s ignal re gistration, an e xcess of mater ia l (3) with the
give n c ut cross is re moved by the tool. A micro metr ic device (4) a llows to
2
obtain essentia l parameters of cut with the accuracy o f 0.01 mm. A
sync hronization
of
the
beginning
of
machining
process
and
mecha no luminescence registration has been provided by the contact unit. Flow
of photons §mitted during the mecha nical breakdown of FRP in the cutting zone
has been registered by means o f photoeleetronic multip lier (5), cutting force has
been measured by pie zoelectr ic dyna mo meter (6). Signa ls o f photoeleetronic
multip lier and p iezoe lectronic dyna mo meter ha ve been received on the
electronic re gisters o f s ingle impulses a nd ha ve been processed us ing co mputer
(7). Turning on re gisters has been imp le mented according to externa l synehro im
pulses in the mome nt o f approaching o f the cutting too l to the sa mp le of
mater ia l. The time inter va l between separate points o f s igna ls was within the
limits of 0.25·10-6 ÷ 1.0·10-5 s.
3. Res ult and Dis cuss ion
When c utting tool starts inter ferenc ing with work piece mater ia l, the d irect
mecha nica l breakdown in the re moving excess of mater ia ls occurs. The result of
breaking o f the polymer che mica l bonds of FRP leads to appearance of free
radicals with the ir further reco mb inatio n, as we ll as or igin, growth a nd
combination o f micro- cracks with major cracks when a new mac hined sur face is
for med.
The FRP breakdown leads to appearance of luminescence that is
characterized by emiss ion of photons with parameters corresponding to
condition o f initia l points of mater ial luminescence. Mechanica l lum inescence
(photoemiss ion) method is a comparative ly new method of FRP mecha nica l
breakdown
process
investigation.
It
was
fo und
t hat
emiss io n
of
mecha no luminescence photons is directly connected with e leme ntary acts of
structure c hange o f FRP. Thus, e miss ion with a wa ve length in a vis ib le fie ld of
a spectrum that occurs d ur ing the de formatio n process of FRP is a conseque nce
of the structure defect origin in the workpiece materia l and is cons idered to be
an objective index of FRP mecha nical breakdown. There fore, registratio n of
intens ity a nd character of mecha nolur ninescence ma y be used as a method for
experime nta l investigat io n of microprocesses being ava ilab le in case of directed
mecha nica l breakdown of FRP during the cutting action [4].
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The Ad vanta ges of the present method for the study o f micro mechanics o f
FRP machining are as follows :
1. The infor mat ion about micromec hanica l breakdown is registered by devices
almost witho ut inertia that allows to use this method dur ing the cutting action.
2. The in initia l carr ier of the FRP deformatio n, called as a flow of p hotons, is a
product of micromec hanica l breakdown process of FRP.
3. The process of the signa l register ratio n exists on the micro leve l.
Figure 2: Graphica l interpretation of investigation results
( V=2 m / s ; d = 0.05mm; λ 1 = 0°)
Figure 2 shows the experime nta l results of fibre reinforced plastic
machining. The time inter va l o f the machining process is d ivided into a few
regions. The linear increases in c utting forces with s lightly increases in the
va lue of luminescence are observed in regio n I when c utting too l starts
inter ferenc ing with workpiece mater ial. This leads to elastic stra in of FRF.
Region II is characterised by the rapid increases in cutting forces and
mecha no luminescence. It might be assumed that the practica l comb ining of
arise micro- and macro- cracks in re moving excess o f mater ia l and the ir sta rt
gives r ise to direct mec hanica l breakdown of FRP. This confir ms that the
deformatio n model o f FRP is deve loped early. The va lues of c utting force in
region II reach the ir limit va lues that are suffic ient in order to remo ve the
excess of mater ia l.
The re gions I and II s how the process of or igin, cracks growing and the
conditions of start o f cracks witho ut generation o f new machined sur face. The
time inter va l o f these regio ns depends on the directio n o f c utting speed and a
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position o f fibres in p lasties with the respect to the cutting ed ge. This
relations hip is character ised by the orientatio n of re inforce ment mater ia l λ1 .
When mac hining is conducted at a n angle λ1 = 0 to the fibre or ientatio n
macro- mechanica l breakdown of the re moving excess of mater ia l occurs a lo ng
fibres in plastic. In addition, most comprehe ns ive stresses effect fibres of
machining mater ia l a nd lead to active mechanica l breakdown of a sur face layer
of FRP. The time o f mec hanica l breakdown, the cutting forces and the intens ity
of mecha noluminescence in this case can be identified as minima l λ1 the same
time under this cond itio n, time duration o f the first two regio ns was decreased
in 1.5- 3 times.
When λ1 = 90º growth and combining o f micro- and macro- cracks during
the cutting actio n occur perpendicular to the reinforced materia l that lead to
increases in time o f mecha nical breakdown of re mo ving e xcess o f mater ia l, as
well as increases in cutting forces and in stresses, that have a breakdown
influence on machined sur face of FRP. Growth o f photo n quantity is another
evidence of the above condition.
The beginning of regio n III is characterised by the rapid decrease in cutting,
forces and the inte ns ive increase in the q uantity o f p hotons, followed by the
move me nt o f major and minor cracks as well as the cracks branching. These
pheno mena lead to the new surface for mation in the process of machining. The
decrease in the intens ity o f mecha no luminescence up to a certain le ve l a nd
gradua l increase in cutting forces occur when the too l re moves a n e xcess of
mater ia l.
The above machining process is characterised by taking tur ns of cr itica l
va lues of the intens ity of mechano luminescence and the cutting forces with the
certain freque ncy. This freque ncy depends on the d irectio n of fibres in FRP and
the ir relations hip with the cutting speed. The fluctuatio n va lues of force
amp litude and the photon quantity occur mostly in antip hase.
The number o f e xperime nta l results s hows that the maximums o f the
photoemiss ion inte ns ity occur just before d rops of the cutting forces (Fig. 2). It
might be exp la ined d ue to the mechano luminescence inte ns ity that re flects
immed iate ly the process of mechanica l breakdown. Accumulated defor mation
leads to critica l va lues that were registered by the p iezoe lectr ic ind icator ( Fig.
2).
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The frequenc y of consequitive cyc les can be well observed when λ 1 = - 90°
and it can be defined by c ut o ff a verage time o f fibres in FRP. The d iameter of
the reinforced mater ia l of around 300- 400 μm leads to fluctuations of freque ncy
va lue fro m 23- 12 mks when the cutting speed is 2- 20 m/s. The freq uency of
consecutive cyc les increases in 1.6 and 1.3 times whe n λ 1 =0°and λ1 = 45°
respective ly a nd whe n the c utting speed is paralle led or inc l ined at the angle
45° to the fibres.
The cyc le of c hanges of the mec hano lumiscehce and the cutting forces is
characterised by the quas i- statio nary process of FRP machining when the cut
off factor o f a new s ur face for mation is the or igin of comb ining micro- cracks
into macro- cracks. Under the limit stresses in the zone of inter ference o f the
tool and workpiece materia l, the major cracks and the system of Branc hed
cracks are formed. The cyc ling process of mecha noiumiscencc a nd cha nges in
the cutting forces determine the impact character of the mecha nical breakdown
of FRP.
Based on the investigatio ns there is possib le to define the r ules and exp la in
the mecha nis m o f the d irect mecha nical breakdown of mater ia l in the remo ving
excess of mater ia l when a high res istant FRP is machined. If has been found
that the cutting process of FRP has a cyclic, impact- dyna mic character. The
mecha nis m of a cycle is as fo llows : start, growth and interactio n of sub - micro
and micro- cracks in the machining mater ia l, take p lace under the cutting forces
in FRP. When the stresses in the contact zone reach the ir ma ximum, e ither a
start of a major crack or a start and branching of a s yste m of cracks occurs. The
cutting forces and the inte ns ity o f mecha no luminescence are cha ngin g fro m
the ir minima l to ma ximum. With the beginning of the breakdown, the quick
growth o f crack syste m in the re mo ving e xcess of mater ia l occurs and it leads
to decreases in mechano luminescence, cutting forces and c ut o ff the remo ving
excess of mater ia l. Then a nother cyc le is ready to start.
The duration o f the breakdown cycle its a mp litude and frequenc y depend on
the cutting speed, the cut thickness, the type o f work piece mater ia l a nd the
position o f the cutt ing edge with the respect to reinforced mater ia ls in p lastic.
Thus, the machining process of FRP is dyna mic, because of the rapid increases
in cutting forces up to the cr itica l that lead to start of macro- cracks and their
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further s upercritica l deve lop ment is a functio n of c utting speed and time of
contact.
GFRP, CFRP, AFRP possess the dominant res istant properties [5 - 7]. From
the point of the mecha nics of breakdown, they can be cons idered idea lly br ittle
with extre me ly sma ll zone of fluctuatio n at the top of a crack.
Moreover, the experime nta l results with GFRP ha ve shown that with
increases in the cutting forces, the crack life (that is the cr itical coeffic ient of
intens ity o f stresses (CIS) of a crack start) decreases. This means the decrease
in the viscosity properties in co mpar ison with e lastic. It is a lso necessary to
me ntion that this pheno meno n occurs when the cutting speed V ≈ 0.13·C2 ,
where C2 - is the speed of Rayle igh's waves.
At the same time under rapid ly applying cutting forces the above materia ls
can change the mec hanica l breakdown that might be expla ined by the quantity
of the k inetic energy a nd the interactio n o f the top of the moving crack and the
elastic waver reflected fro m the border of removing excess of mater ia l. The
consequence of the multip le wave reflectio n of stresses from borders is an
intense growth of micro- cracks, their branching and crushing of the remo v ing
excess of mater ia l whe n the brittle mater ia ls are machined.
From the position o f the mechanica l breakdown, let's assess the influe nce of
cutting speed on type o f FRP breakdown. When the c utting speed is a n interva l
fro m sma ll to medium, the developed k inetic ener gy is not enough for the
effective process of breakdown. This qua ntity of e nergy in the cutting zone
leads to the growth a nd interactio n o f micro - cracks and micro- feet sa nd the ir
combining into a major crack. The existence of b ig: pieces of chip in the
breakdown zone are the evidence that crush of the removing excess of materia l
occurs when micro- cracks in the presence of a major crack are moving [1].
Increases in the cutting speed lead to decrease in the critica l CIS start and
the critica l CIS crack life. More over the increases in the quantity o f mo ving
micro- cracks in the remo ving e xcess of mater ia l, stabilization of the c utting
forces and inte ns ity of mechano luminescence, increases in supercritica l growth
of cracks lead to crash of the chip. For GFRP the cutting speed varies fro m 15
to 18 m/s.
The further increases in the cutting speed lead to increases in the CIS start
crack, and the speed of the supercritical growth of crack riches its ma ximum.
7
However, the last finding does not depend on t he ma gnitude o f c utting forces
but depends only on the structure of compos ite. The ma ximum speed value is
connected with a crack branching and a crack system. Further more this k inetic
energy is e nough not only for moving a major crack, but also for combining a nd
moving a head branched cracks. Surplus o f e nergy goes towards the growth of
quantity of branched cracks. Parton and Boriskovskij [5] have shown that
increase in the speed of defor mation that c hanges strained state of mater ia l up
to the limits and higher, leads to increase of the crack speed gradient up to the
limits and as a consequence, the redundance of the way where the process of a
crack branching starts. The present exper iments ha ve proved that discontinuous
process of micro- cracks development exis ts, first o f all in the re moving excess
of mater ia l because this is a zo ne of the minima l energy breakdown. The
inc linatio n angle of cracks branching is 60° when λ 1 = 90° and 15- 20° when λ 1 =
0°.
In order to define the re lationship between the cutting speed and the
distr ibutio n speed of a crack it is necessary to calculate maximum speed of
crack moveme nt in FRP. It was shown above that the re lations hip between the
distr ibutio n speed of a crack Vmax and the d istr ibutio n speed of cross- waves C2
(Rayle igh's waves) is give n by
V max  0 . 13  C 2

C2 

(1)
(2)
where  - shear modulus;  - density of the mater ia l.
The va lue of shear modulus is given by
 
E
2  (1   )
(3)
where E - elastic modulus;  - Poisson's ratio.
Based on the investigation res ults o f micro mechanics o f machining us ing
the method of mecha no lumiuescence, the ma gnitude of the speed of cracks
distr ibutio n in the remo ving excess o f mater ia l can be establis hed. The
ma gnitude of the speed of cracks move ment is calc ulated in the region of the
extreme drop of the mechano luminescence intens ity a nd the cutting forces due
to the rapid growth o f cracks in the re moving excess of materia l. For GFRP the
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distr ibutio n speed of a crack is 705 m/s while for AFRP - 375 m/s. Tinte of
macro- cracks moveme nt in the re moving excess of mater ia l before the
mecha nica l breakdown is defined by the time interva l between maximum a nd
minimum va lues o f the mechano luminescence and the c utting forces. The
mecha nica l breakdown of FRP occurs a long the d iv is io n line o f the mater ia l
where the maximum stress and the maximum for this c onditio ns CIS are
establis hed. Thus, the start of a crack happens at the line of cut d ue to the
ma ximum stress in this area. The angle of a crack turn that was defined
experime nta lly is in the range o f 20-60° (λ1 = 90°, the turn angle is 45- 50°).
Based on the average value of the cut thickness and the tur n angle of a
crack, it is possib le to define the way passed b y the crack in the remo ving
excess of FRP from a cut line till a machining sur face. For each, cutti ng speed
it is necessary to calc ulate the speed of a crack move ment, which can be
defined as a d istance passed by a crack by the time o f its mo ve ment based on a
graphic interpretation of micro mecha nics process. The interactio n of speed of
cracks distributio n for differe nt cutting speeds and types of FRP is s hown in
Figure 3.
Figure 3: Interaction of speed of cracks distrib ution and cutting speed for FRP
It is obvious that for GFRP whe n the cutting speed is 20 m/s, the speed of
cracks distribut ion reaches its maximum va lues (over 700 m/s). For AFRP the
critica l speed of cracks distr ibutio n (over 350 m/s) co mes when the c utting
speed is 1.4-1.6 m/s. It can be argued that the effective cutting speed is 20 m/s
for GFRP and 14- 16 m/s for AFRP. For GFRP this sp eed is 15- 18 m/s.
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Increases in the cutting speed to the level higher tha n critica l do not
influence the mechanica l break down of the re moving excess o f the mater ial but
lead to intens ive cracks branching. Decreases in the area o f c ut, first o f all, cut
thickness
as
was
shown
above,
lead
to
decreases
in
inte ns ity
of
mecha no luminescence and c utting forces that is an essentia l part of the
effective mechanica l breakdown of FRP.
4. Conclusions
Micro mechanics of the FRP machining have been studied experime nta lly
based on the intens ity o f mecha noiumilice nce. The investigations have been
carried out on the special data acquis itio n syste m that can store data of cutting
process into a file.
Based on the investigations it is possib le to define the rules and e xp lain the
mecha nis m o f the d irect mecha nical breakdown of mater ia l in the remo ving
excess of materia l when high resista nt FRP is machined. It has been found that
the cutting process of FRP has a cyclic, impact dyna mic character. Mechanis m
of a cyc le works as fo llows : start, growth and interaction o f s ub micro - and
micro- cracks in the mac hining mater ia l take place undercutting forces in FRP.
When the stresses in the contact zone reach the ir ma ximum, either a start of a
major crack or a start and branching o f a s ys te m o f cracks occurs. Cutting
forces and inte ns ity o f mecha noluminescence are changing fro m the ir minima l
to ma ximum. Whe n the breakdown begins, the q uick growt h o f crack syste m in
the removing excess of mater ial occurs and it leads to decreases in
mecha no luminescence, cutting forces and cut off the remo ving excess of
mater ia l.
The duration o f the breakdown cycle, its amp litude and freque ncy depend on
the cutting speed, the cut thickness, and the type of workpiece mater ia l and the
position o f the cutting edge with the respect to reinforced mater ia ls in p lastic.
Thus, the machining process of FRP is dynamic because of the rapid increases
in cutting forces up to the critica l that lead to start, of macro - cracks and their
further s upercritica l deve lop ment is a functio n of c utting speed and time of
contact.
The effective cutting speeds for FRP machining ha ve been calculated, based
on the experime nta l and theoretica l results.
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Re fe re nces
1. R. Teti, “Machining of Composite Materials”, Annals of CIRP, vol. 51(2),
pp. 527-551, 2002.
2. W Konig, C. Wulf, P. Grab and H. Willerscheid, “Machining of Fibre
Reinforced Plastics”, Annals of CIRP, vol. 34(2), pp. 537-548, 1985.
3. G. Spur, “Turning of Fibre Reinforced Plastics”, In S. Jahanmir, M. Ramuiu and
P. Koshy (eds.) Machining of Ceramics and Composites, pp. 209-248, 1999.
4. N. Verezub, “The basis of the processes of composite materials machining”,
Kharkov (in Russian), 1995.
5. V. Z. Parton, V. G. Boriskovskiy, “Dynamics of brittle mechanical breakdown”,
Moscow: Mashinostroenie (in Russian), 1998.
6. G. P. Cherepanov, “Mechanics of brittle mechanical breakdown”, Moscow:
Nayka (in Russian), 1974.
7. S. G. Lechnickij, “Theory of elasticity for anisotropic material”, Moscow:
Navka (in Russian), 1977.
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