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The solid particle erosion of polymer matrix composites
149
Wear, 171 (1994) 149-161
The solid particle erosion of polymer matrix composites Manish &fence
Roy Met~Ilu~*c~i Research
Lo~mto~,
~anchonbagh
P.O., Hyder~bod,
500 258 (India)
P.U., Hyderabad,
500 258 (India)
B. Vishvvanathan Aeronautical
Defence
Establishment,
Bangalore
(India)
G. Sundararajan &fence
Metaliu~ai
(Received
Research
Labomtory,
May 18, 1993; accepted
K~ncha~ba~
September
9, 1993)
Abstract The solid particle erosion behaviour of four different types of polymer matrix composites reinforced with glass fibres have been characterized. The erosion rates of these composites have been evaluated at two impact angles (90’ and 30”) and two impact velocities (38 and 45 m s-l). The erosion response, erosion efficiency and the erosion micromechanisms of these composites are presented and discussed in detail and also compared with the available data in the literature on similar materials.
1. Introduction Polymers are finding an ever increasing application as structural materials in various components and engineering systems. The high specific strength and stiffness of polymers are primarily responsible for their popularity. However, the resistance of polymers to solid particle erosion has been found to be very poor [l], and in fact it is two or three orders of magnitude lower than metallic materials [2]. One possible way to overcome such a shortcoming is to introduce a hard second phase in the polymer to form polymer matrix composites (PMCs). A number of investigators [l-9] have evaluated the resistance of various types of PMCs to solid particle erosion. Table 1 provides a compilation of such data. Tilly [l] and Tilly and Sage [3] tested Nylon and epoxy reinforced with various fibres such as graphite, glass and steel and concluded that the reinforcement can either increase or decrease the erosion resistance depending on the type of fibres. Zahavi and Schmitt [2] tested a number of PMCs for erosion resistance and concluded that glass-reinforced epoxy composite had a particularly good erosion resistance. Pool et al. [4], conducted erosion tests on four PMCs and inferred that wee-handled, ductile fibres in a the~oplastic matrix should exhibit the lowest erosion rates. The above study was extended further by Tsiang [5]. He carried out sand erosion tests on a wide range of
~3-1~~4/$07.~ 0 1994 Elsevier Sequoia. Afl rights reserved SSDI ~043-1648(93)06347-7
thermoset and thermoplastic PMCs having glass, graphite and Kevlar fibres in the forms of tape, fabric and chopped mat as reinforcements. Kevlar fibres in an epoxy resin provided the best erosion resistance. In a recent study, Mathias et al. f6] and also Karasek et al. [S] have evaluated the erosion behaviour of a graphitefibre-reinforced bismaleimide polymer composite. These investigators observed the erosion rates of the PMC to be higher than the unreinfor~d polymer. Many of the investigators [2--6,9] also consistently noted that the erosion rates of the PMCs were considerably larger than those obtained in metallic materials. In addition, PMCs with a thermosetting matrix invariably exhibited a maximum erosion rate at normal impact angles (i.e. a brittle erosion response) while for the thermoplastic PMCs the erosion rate reached a maximum at an intermediate impact angle in the range 40”-5O”, signifying a semiductile erosion response. The present work is concerned with the characterization of the solid particle erosion behaviour of four types of glass-fibre-reinforced polymer matrix composites. Apart from enlarging the existing database on the erosion behaviour of PMCs, the other objectives of the present study are to examine the interrelationship between the mechanical properties of the PMCs and their erosion resistance and to enhance our understanding of the micromechanisms leading to erosion in such PMCs.
Mar&
150
TABLE
1. Compilation
of erosion
Hay et ul. i Solid particle erosion of po&ner
experiments
matrix conpxirec
carried out with polymer and polymer matrix composites
by
various
mvestigators
__-
Material
tested
Nylon 66, graphite-Nylon
66
Quartz-polymide, glass cloth-epoxy quartz polybutadiene Nylon and epoxy reinforced glass and steel
and
Investigators
Test conditions
G.P. Tilly (I]
v= 104 m s -1, Erodent, Quartz
J. Zahavi and G.F. Schmitt [3]
a= 30”, 45”, 60”, 75” and 90” Erodent, Sand
with carbon,
G.P. Tilly and W. Sage [2]
Graphite-reinforced polymide, epoxy and thermoplastic. PMCs and aramid-fibrereinforced epoxy
K.V. Pool, C.K.H. Dharan I. Finnie [4]
Epoxy, bismaleimide, polymide, polyphenylene sulfide, polyetheretherketone and Nylon-reinforced glass, graphite and kevlar
Tseng-Hau [51
BMI and BMI reinforced
P.J. Mathias, W. Wu, K.C. Goretta, J.L. Routbort, D.P. Groppi and K.R. Karasek [6]
1/=20, 40 and 60 m S-I (Y=3p, 90 Erodent, alumina (63, 130 and 390 pm) at RT
A.M. Latifi [7]
V=66 m s-‘, o=30”, 45”, 60” and 90” Erodent, alumina (142 pm)
K.R. Karasek, K.C. Goretta, D.A. Helberg and J.L. Routbort [8]
V=20-120 m s-’ (Y=lY to 90 Erodent, alumina (63, 143 and 390 pm) at RT
Glass-reinforced
with carbon fihre
epoxy system
BMI and BMI reinforced fibre
with graphite
V, impact velocity (m s-l);
a, impact angle (degree);
Tsiang
2.1. Material In the present investigation, four different types of polymer matrix composites namely, glass epoxy resin, glass phenolic resin (modified), glass phenolic resin (unmodified) and glass polyester resin were used. For the purpose of reinforcment E-glass plain weave woven roving fabric, obtained from Fibre Glass Pilkington India Ltd. was used. The physical characteristics of these fibres are given in Table 2 [lo]. The matrix resin solution was prepared by dissolving commercially available resin powder in ethyl alcohol. For glass epoxy and glass polyester composites, the required size fibres were coated uniformly with respective matrix resins. The 2. Characteristics
of plain weave woven roving glass
Nominal thickness (nun) Average weight (f 10%) (g m-‘) Ends for 10 cm Picks per 10 cm ( *2.5%) Nominal count of warp yarn (Tex.) Nominal count of weft yarn (Tex.)
V=31 m s-l CX=15”, 30”, 45”, 60” and 90” Erodent, silica sand (100-200
mesh) at RT
1/=143 m s-’ cu=30”, 45”, 60” and 90” Erodent, aluminium oxide and garnet (16-74 urn and 74-130 pm) at RT
RT, room temperature.
2. Experimental details
TABLE fabric
and
0.33 363 61 305 305
resin-coated plies were then stacked in a steel mould. While stacking, the orientation of warp and weft fibres were maintained in all the plies. These plies were subsequently moulded in a hydraulic press at a pressure of 0.3 MPa at room temperature. In the case of glass phenolic composite, the reinforcing fibres were coated with matrix resin. For unmodified and modified composites, unmodified and modified resin powders were used respectively. The resin was modified so as to improve the mechanical properties of the composites. The exact manner in which the resin has been modified has not been revealed by the manufacturer of the resin powder. This was followed by preparation of prepregs in a temperature-controlled oven. Plies of required size were made from prepregs. Plies were stacked in a steel mould keeping the orientation of warp and weft fibres unaltered. These plies were then pressed hydraulically at a pressure of 13.5 MPa and at a temperature of 160 “C. 2.2. Mechanical testing Various important mechanical properties of the PMCs, such as the tensile, flexural and interlaminar shear strengths were determined using a screw-driven Tinius Olsen universal testing machine at a cross head
151
Manish Roy et al. / Solid particle erosion of polymer matrix composites
speed of 3 mm min-‘. The impact resistance of the PMCs was obtained using a Dynatup instrumented impact system. Determination of the coefficient of restitution was carried out in a gravity drop system. The detailed description of the system and the procedure for determining the coefficient of restitution are given elsewhere [ll]. 2.3. Erosion testing The room temperature erosion test facility used in the present investigation is illustrated schematically in Fig. 1. The facility consists of an air compressor, a particle feeder, an air particle mixing and accelerating chamber. The compressed air (dried using an air dryer) is allowed to mix with the particles, fed at a constant rate (using the conveyor-belt-type feeder) in the mixing chamber. These fluidized particles are then accelerated by passing the mixture through a WC converging nozzle. These accelerated particles impact the specimen held at certain angle with respect to the impacting particle with the help of a sample holder. The feeding rate of the particles can be controlled by monitoring the distance between the particle feeding hopper and the belt drive carrying the particles to the fluidizing chamber. The impact velocities of the particles can be varied by varying the pressure of the compressed air. The impact angle can be varied by changing the orientation of the sample holder. Samples of 30 mm X30 mmX 5 mm were cut from the PMC laminates for the erosion tests. The conditions under which the erosion tests were carried out are listed in Table 3. The standard test procedure is described in a separate publication [12]. The velocity of the eroding particles was determined using a rotating disc method, originally devised by Ruff and Ives [13].
TABLE
3. Erosion
test conditions
Test parameters Erodent Erodent size (pm) Erodent shape Impact angle Impact velocity (m s-l) Erodent feed rate (g min-‘) Test temperature Nozzle to sample distance (mm)
Silica sand 200*50 Angular 90” and 30” 38*5, 45rt5 3.65 RT 10
FLUX
m
SIDE A
m
SIDE B SIDE C
Fig. 2. A schematic diagram describing three differen: for the three-dimensional microstructure of PMCs.
surfaces
2.4. Characterization of eroded samples To characterize the morphology of as-received and eroded surfaces and to understand the mode of material removal, the eroded samples were observed under the scanning electron microscope. Each sample was coated with carbon using a vacuum evaporation technique before being observed.
3. Results 3.1. Microstructure
AIR -
NOZZLE SYSTEM
SAMPLE HOLDER
l--l
n
Fig. 1. A schematic
diagram
of the erosion
rig.
In the case of composites, a full appreciation of their three-dimensional microstructure requires observation of their two-dimensional structure in three directions, i.e. on the surfaces marked A, B and C in Fig. 2. The microstructures of glass epoxy, glass polyester, glass phenonolic (unmodified) and glass phenonolic (modified) composites, as observed on surfaces A, B and C, are illustrated in Figs. 3-6. Glass phenolic composites (modified and unmodified) (Figs. 3 and 4) exhibit microstructures representative of unidirectional glass fibres in a phenolic matrix. The glass epoxy composite (Fig. 5) represents the other extreme and has glass fibres laid bidirectionally in an epoxy matrix. The glass polyester composite (Fig. 6) represents an intermediate case in that the glass fibres are present neither in a fully one-directional or in a bi-directional manner in the polyester matrix.
Fig. 3. A photomicrograph of (modified) glass phenolic (a) surface A, (b) surface B and (c) surface C.
composite.
3.2. Mechanical properties Tensile strength, tensile modulus, flexural strength and flexural modulus of all the composites, as determined from mechanical tests, are illustrated in the form of bar diagrams in Fig. 7. It can be seen clearly that glass phenonolic resin (modified) exhibits the highest tensile strength, flexural strength and flexural modulus
Fig. 4. A photomicrograph of (unmodified) glass phenolic positr. (a) surface A, (b) surface B and (c) surface C.
com-
among the four composites. In contrast, the highest tensile modulus is obtained in glass epoxy resin composite. The interlaminar shear strength and charpy impact strengths of these composites were presented in the form of bar diagrams in Fig. 8. With respect to
153
Manish Roy et al. / Solid particle erosion of polymer matrix composites
Fig. 5. A photomicrograph of glass epoxy composite. A, (b) surface B and (c) surface C.
(a) surface
both the properties, glass phenolic resin (modified) has the highest value. In short, glass phenolic resin (modified) possesses the best combination of strength and toughness. The coefhcient of restitution determined using the gravity drop system is also shown in Fig. 8. Glass epoxy resin and glass phenolic resin (unmodified) exhibit the highest coefficient of restitution.
Fig. 6. A photomicrograph of glass polyester surface A, (b) surface B and (c) surface C.
composite.
(a)
It has been demonstrated by Tirupataiah et al. [ll] that the coefficient of restitution of a material is related to its hardness by 1.9PB C.?= Ee,lR PblBVf4 which can be rearranged
0) to give
Fig. 7. A bar diagram showing the tens&z strength,
Fig. 8. A bar diagram to e&bit
the interfaminar
tensile modulus, flexural strength
and fiexuraf modulus of the composites.
shear strength, charpy impact strength and e+c&Esient of restitution
‘In eqns. (1) and CZ),e is the coefficient of restitution, N is the dynamic hardness of the target material, pb is the density of the impacting ball, Y is the velocity of impact, and ~5,~is the effective modulus of the target material-ball material combination and is given by
where E, and E, are the modulus of elasticity and V, and JJ*are Poisson’s ratio of the impacting hall and the target material respectively+ The values of the as calculated using eqns. (2) and hardness and &, (3) for these composites, are presented in Table 4 along with their coeficient of restitution and modulus of eIaseicity. It can be seen clearly that glass epoxy resin is the hardest among the composites. It is also interesting to note that although unmodified @ass phenolic resin has a low hardness, its coefficient of restitution is significantly higher than other composites.
nf the composites.
The variation in t.he incremental erosion rates as a function of the ~~~~ulati~e weight of the impinging paraides is shown in Figs_ 9-12 fur grass pofyester resin, glass epoxy resin, glass phenolic resin (modified) and glass phenofic resin (unmodified) composites respectively. For each composite, the erosion data pertaining to two impact angles (30” and W) and two impact velocities (38 and 45 rn s - ‘) have been presented. Figures 9-12 indicate that for glass epoxy resin, glass polyester resin and for glass phenolic resin ~unrn~i~~d~~ the incremental erosion rate either initiafiy increases from a Iow value to a constant value or initialIy increases from a low value to a high value and then decreases ta a constant value, at all impact angles and impact velocities, On the contrary for glass phenolic resin ~rnud~ed~ the increme.ntaZ erosion rate initially decreases from a high value to a constant value for normal impact and initially increases for a low value fa a constant value for oblique impact at all impact velocities (Fig_ 12). These constant values af the erosion rate henceforth will be called the erosion rate (E) of the material. It is evident from Fig. 11, that the incremental erosion sate of glass phenolic (modified) resin composite
Manish Roy et al. / Solid particle erosion of po&ner
TABLE 4. Coefficient
of restitution,
modulus
Material
of elasticity
mati
155
composites
and effective modulus of elasticity
and hardness
Coefficient of restitution
Modulus of elasticity (GPa)
Effective modulus of elasticity (GPa)
of test material Dynamic hardness (MPa)
Glass polyester
resin
0.67
40.0
41.0
770
Glass phenolic
resin (modified)
0.65
43.0
43.0
777
resin (unmodified)
0.75
21.0
22.0
556
0.76
45.0
46.0
1029
Glass phenolic
Glass epoxy resin
G \
100
.?
GLASS
POLYESTER
RESIN
0
0
0
NORMAL
IHPACT
V=45m/r
.
l
.
NORMAL
IMPACT
V=3Bm/s
d,
I
,t
OBLIQUE
IHPACT
V=Gm/s
A
A
A
OBLIQUE
IMPACT
V=3Bm/r
rsllO.0. \
cn
GLASS PHENOLIC IMOOlFlEOl RESIN
01
1
IO 0
I
20 0
30 0
40 0
50 0
60 0
I
I
I
70 0
SO.0
90.0
CUMULATIVE WEIGHT OF THE IMPINGING PARTICLES (gml
10
Fig. 9. The variation in the incremental erosion rate of the glass polyester resin composite with cumulative weight of impinging particles.
GLASS EPOXY 0 0 0 . A A A A
0 0 A A
NORMAL NORMAL OBLIQUE OBLIOUE
20
30
‘0
IMPACT IMPACT IMPACT IHPACT
-. \ z
0
I 10
20
I
I
30
40
CUMULATIVE IMPINGING
-t
.
1
60
70
0
I
I SO
60
9c
Vd,Sr/s V:lOm/r V.&%./r VdBr,,
-G
r
70
RESIN
GLASS
12.0
2.0 -
60
Fig. 11. The variation in the incremental erosion rate of glass phenolic resin (modified) composite as a function of cumulative weight of the impinging particles.
14.0
E
50
CUMULATIVE WEIGHT OF THE IMPINGING PARTICLES lg)
BO
‘;
I
Y
100
c
PHENOLIC
RESIN
IUNMOOIFIEOI
0
0
0
NORMAL
IMPACT
V = 45m/r
0 B
0
0
NORHAL
IMPACT
A
A
OBLIQUE
IHPACT
V = 3Bm/r V = 45m/s
A
A
A
OBLIOUE
IHPACT
V = 3Bm/r
1
0
60
v
90
WEIGHT OF THE PARTICLES fgl
Fig. 10. The variation in the incremental erosion rate of glass epoxy resin composite with cumulative weight of impinging particles. 01
at normal impact (impact velocity of 45 m s-l) continuously decreases. This is probably because of the fact that the incremental erosion rate of the said material under the above-mentioned test condition does not reach steady state even after such long exposure. How-
I-
10
I
I
I
20
30
40
I
so
60
I
70
I
80
90
CUMULATIVE WEIGHT OF THE IMPINGING PARTICLES fgl
Fig. 12. The variation in the incremental erosion rate of glass phenolic resin (unmodified) composite as a function of cumulative weight of the impinging particles.
Munish Roy et al. I Solid particle erosion
1%
ever, for the purpose of comparison, the average of the last five incremental erosion rates data points have been taken as the steady state erosion rate. The erosion resistance which is the reciprocal of the erosion rates (l/E) obtained from Figs. 9-12 for all these composites are presented in the form of histogram in Fig. 13. It is obvious from Fig. 13 that glass epoxy resin exhibits the highest erosion resistance at all impact angles and impact velocity, while the glass phenolic resin (modified) composite exhibits the least erosion resistance. The influence of the impact angles on the erosion rates of the composites are shown in Fig. 14. For each test material and impact velocity, erosion rate has been measured only at two impact angles (30” and 907. Although there is a possibility that the erosion rate may go through a maximum at some intermediate impact angles, the two data points have been joined by a straight line only to help the reader identify the behaviour of each test material. It is to be noted that glass epoxy resin and glass phenolic resin (unmodified) exhibit brittle erosion response, i.e. maximum erosion rate at normal impact at all impact velocities. In contrast glass polyester resin behaves in ductile way (having maximum erosion rate at oblique impact) at all impact velocities. It is also interesting to note that glass phenolic
of polymer matrix composite5
-_o__o-
GLASS P"iNALlC
RESIN ,nOOlFlEO, Y = 45m/r
GLASS
PHENAIIC
RESIN lM0DlFIEDl " = Plm/r
GLASS
EPOXY
mm.-.--
RESIN AT
Y=LSm/r
:: w so+
3s:
r 2 40t i30i
_____-
20 _
ok-4cx OF PlPAC6; (“1
IMPACT
ANGLE=30'
IMPACT
YELOCITY=4Sm/s
IMPACT
VELOCITY=38m/s
m K 5
60.
GLASS
EPOXY
RESIN
RESIN
of
Impact angle
Ka
P’
resin
90” 30”
8.40~ lo-‘* 11.6x10-‘”
4.79 3.50
Glass phenolic (modified)
resin
90” 30”
33.8x lO_” 18.87x lo-*’
4.53 4.69
Glass phenolic (unmodified)
resin
90 30”
5.75 x lo-‘* 4.54x 10-r*
4.99 4.91
90” 30”
2.71 x lo-‘* 12x lo-=
4.97 5.74
“E =WP PHENOLIC
the velocity dependence
Glass polyester
Glass epoxy resin
,,UUU GLASS
90
Fig. 14. The effect of the impact angle on the erosion rates of the composites.
Material
ANGLE=30"C
- --
1OL
TABLE 5. Parameters characterising erosion rate of the test materials
IMPACT
___o-
____o____---------
(units: K, (scm-‘)-p).
COMPOSITES
COMPOSITES
IMPACT
ANGLE='?,,'
IMPACT
ANGLE
IMPACT
"ELOCITY=4Sm,s
IMPACT
YELOCITY
i 90' i 38m/s
Fig. 13. A histogram illustrating the erosion resistance of various composites at hvo different impact angles and two different impact velocities.
resin (modified) indicates brittle erosion behaviour at lower velocity (38 m s-l) and exhibit ductile erosion at higher velocity (45 m s-l). The dependence of erosion rate on the velocity of impact is characterized by the exponent p (where E=kVP) [14,15]. The values of p and k for all the composites are listed in Table 5 for normal and oblique impact. The strong velocity dependence of erosion rates of composites is evident as indicated by the values in the range. The high velocity exponent of glass epoxy resin composite at all impact angles is particularly noteworthy.
Manish Roy et al. I Solid particle erosion of polymer matrix composites
3.4. Morphology of eroded sugaces The scanning electron micrographs of the eroded surfaces of the various composites are presented in Figs. 15 and 16. Figure 15 corresponds to the surface eroded at normal impact angle (velocity, 38 m s-l) while Fig. 16 is pertinent to the surface eroded at oblique impact angle (30”, V=38 m s-l). The eroded surfaces of a given composite appear very similar at normal and oblique impact angles and thus can be discussed together. At one extreme, in the case of glassepoxy and glass-polyester composites, both the glass fibres and the matrix material are present in appropriate proportions on the eroded surface, indicating that the erosion rate of the matrix controls the overall erosion rate. In contrast, in the case of glass-phenolic (modified or unmodified) composites, the entire eroded surface is almost composed of fibres protruding out of the matrix phase. Thus, it appears that the erosion rate of the phenolic matrix in these composites is very high.
157
4. Discussion
4.1. Comparison with literature In this section the erosion data of these polymer matrix composites will be compared with the erosion data of various polymers and PMCs reported by other investigators. Towards that purpose it is necessary to compare and contrast the conditions under which tests were conducted. The erosion experiments carried out by other investigators with different polymer composites are listed in Table 1 alongwith experimental conditions. The table clearly shows that the tests have been carried out under a wide variety of conditions, with regard to the impact velocities, impact angles and erodents. Thus, only a qualitative comparison between the present data and literature data is possible. Most of the observations clearly demonstrated that the erosion rate of the polymer matrix composites are an order of magnitude higher than that of metallic
Fig. 15. Scanning electron micrographs of the eroded surfaces of the composites, Impact angle equal to 90” and at impact velocity 38 m s-‘. (a) Glass polyester resin composite, (b) glass phenolic resin composite (unmodified), (c) glass phenolic resin composite (modified) and (d) glass epoxy resin composite.
Fig. 16. Scanning electron micrographs of the eroded surfaces of the composites. Impact angle equal to 30”. (a) Glass epoxy resin composite, (b) glass phenolic resin composite (modified), (c) glass phenolic resin composite (unmodified) and (d) glass polyester resin
composite.
materials (AISI 1018 steel 141, Al, Ti, stainless steel [5], PH stainless steel [12], Cu, Cu-Zn, Cu-Al [16]). In the present work also a similar conclusion can be reached since earlier work by us on the erosion of a number of metallic materials using the same erosion rig and under similar test conditions indicates that the erosion rates were in the range 3 x 1O-5-3O~ 10d5 g g-’ [12,16], i.e. an order of magnitude lower than that of the PMCs. In the present work, glass-reinforced epoxy composite has the best erosion resistance and modified phenolic resin composites have lowest erosion resistance. Zahavi and Schmitt [2] also obtained the best erosion resistance in glass-reinforced epoxy composites, as in the present study, although Zahavi tested the materials for five different impact angles (30”, 45”, 60”, 75” and 900). According to Pool et al. [4], for polymeric material behaving in a ductile manner, the velocity exponent p will vary in the range 2-3 while for polymer composites
behaving in brittle fashion the value ofp should be in the range 3-5. Tilly and Sage [3] using Nylon and epoxy, reinforced with carbon, glass and steel, reported the velocity exponent to be 2.3. In the present work, the velocity exponents obtained lie in the range of 3.5 to 6 irrespective of the erosion response of the composites. Thus, for composites exhibiting brittle response, the velocity exponents obtained in the present work are in conformity with Pool et al. [4]. The reasons for the high velocity exponent of the PMCs is related to their high coefficient of restitution as discussed below. For materials having high coefficient of restitution, the equation of the type E =K,(V-- Vreb)” (where Vrebis the rebound velocity) is more appropriate than the conventional equation E =KVP. Assuming the average exponent n for composites to be 2.5 [14,15] and fitting it to the equation of the typeE=K,(V’Vrc,,)” over the experimental velocity range 38-45 m s- ‘, the average rebound velocity (l/ret,) of the composites can
159
Manish Roy et al. / Solid particle erosion of polymer matrix composites
This efficiency parameter
be computed. The calculated I/reb values of the four composites lie in the range 18-21 m SK’ and more importantly increase in the same manner as the coefficient of restitution (e). The above Vrcb values correspond to an e value of around 0.5 for impact velocities in the range 3&45 m s-l and are quite consistent with the higher e values obtained at lower impact velocities in the present study (Fig. 8) [ll]. It has been noted by various investigators that the erosion rate of composites may increase or decrease with increase in impact angle. According to Tsiang [5], composites having a thermoset matrix behave in a brittle manner, while composites having a thermoplastic matrix exhibit semi-ductile erosion behaviour. In the present study, consistent with the above observations, a therm0 setting matrix (epoxy, and phenolic) composites show a brittle behaviour while the therm0 plastic matrix (polyester) composite indicates a ductile behaviour. The only exception, is the modified glass phenolic resin composite. This thermosetting matrix composite possesses brittle erosion response at low impact velocity and ductile erosion response at high impact velocity. Reasons for such behaviour are not clear.
2EH 77= pv2
4.3. Erosion efficiency To describe the nature and the mechanism of erosion a new parameter, erosion efficiency, 77 was proposed by Sundararajan et al. [17]. This parameter indicates the efficiency with which the volume that is displaced by the impacting erodent particle is actually removed.
Serial number
of erosion
Correlation
rate with various parameters
parameters
Correlation Normal V=45
1
2 3 4 5
E E E E E
vs. vs. vs. vs. vs.
tensile strength charpy impact energy interlaminar shear strength coefficient of restitution dynamic hardness
E, steady state erosion
rate.
(4)
where E is the erosion rate, H is the hardness, p is the density of the target material and V is the velocity of impact. This parameter can be used to identify the brittle and ductile erosion response of various materials. For example, ideal microploughing involving just the displacement of material from the crater without any fracture (and hence no erosion) will have zero erosion efficiency. Alternatively, in the case of ideal microcutting, q will be unity. In case erosion occurs by the formation of a lip and its subsequent fracture, erosion efficiency will be in the range O-l. In contrast, as happens with brittle material, if the erosion takes place by spalling and removal of large chunks of material by interlinking of lateral or radial cracks, then the erosion efficiency is expected to be even greater than 100%. The values of the erosion efficiencies of these composites calculated using eqn. (4) are summarized in Table 7 alongwith their hardness. The erosion efficiencies of these composites vary from 18% to 43% for impact velocities of 45 m SK’ and from 14% to 33% for impact velocities of 38 m s-‘. Thus it can be concluded that erosion takes place by microploughing and microcutting. The erosion efficiency map which indicates the effect of hardness of various materials on their erosion efficiency is shown in Fig. 17. The regimes which correspond to erosion efficiency of polymers and polymer matrix composites are also indicated in the figure. In Fig. 17, the polymer regime has been identified using the literature erosion data [ 181, while the present results on PMCs have been utilized to delineate the PMC regime. The lower erosion efficiencies of the PMCs, as compared with the polymers, can be attributed to the resistance offered by the hard reinforcing fibres to the crack propagation. These reinforcing fibres reduce the erosion efficiency also by inhibiting the spread of deformation.
4.2. Correlation with mechanical properties Effort has been made to correlate the erosion rates of various composites with their mechanical properties. Various correlating parameters along with correlation coefficients under different test conditions are summarized in Table 6. It is clear from Table 6 that the erosion rates of the composites do not correlate well with their mechanical properties. Among various parameters, the coefficient of restitution appears to correlate best with the erosion rate.
TABLE 6. Correlation
(77) can be obtained from
0.09 0.04 0.43 0.69 0.69
coefficient
impact m s-’
Oblique
impact
V=35 m s-’
V=4_5 m s-’
V=35 m SK’
0.04 0.20 0.47 0.62 0.62
0.12 0.20 0.44 0.91 0.36
0.31 0.24 0.07 0.95 0.43
Manish Roy et al.
160 TABLE
7. Erosion
efficiencies
of various
Material
test
I Solid pu&Ae erosion
materials
at normal
Density (kg m -j)
Static (Ml%)
of polynrr
nmtrix
cornpxitcs
Impact
hardness
Erosion efficiency at V=45 m s ~’ (“ro)
Glass
polyester
resin
Glass
phenolic
resin
(modified)
resin
(unmodified)
Glass
phenolic
Glass
epoxy
resin
1787
488
19
l?i
1929
806
43
2x
1892
806
35
21
1782
1097
27
16
. IMPACT VELOCITY : 45m/r b IMPACT VELOCITY = 38m/s Oil
i
I
10
100
1000
HARDNESS (HI’) Fig. 17. The erosion efficiency hardness corresponding to metals, metallic alloys, mers and polymer matrix composites.
Erosion efficiency at V=38 171s-r ( ‘X )
I
10000
map indicating regimes ceramics, coatings, poly-
4.4. Erosion micromechanisms
The dependence of the erosion rates of the four polymer composites on impact velocity is very strong at both impact angles, as indicated by the velocity exponents in the range 3.5-6.0 (Table 5). In contrast, in the case of metallic materials, the velocity exponent lies in the range 2-3.5. Similarly, the erosion efficiencies of the four PMCs investigated in the present study (17%-40%) are substantially higher than those for the metals (l%-10%). However, compared with polymers (erosion efficiencies around 100%) and glasses (efficiencies up to 1000%) the erosion efficiencies of PMCs are considerably lower. Thus, the erosion behaviour of the present PMCs can be broadly classified as semiductile. The fact that the erosion rates of each of the four PMCs at the impact angles of 30” and 90” are not dramatically different also supports the above con-
tention, since either purely ductile or purely brittle erosion response causes the erosion rates at 30” and 90” to be widely different [19]. PMCs represent a twophase structure namely, the glass fibres and the matrix phase which is epoxy, polyester or phenolic resin. In principle, the overall erosion rate of the PMCs can be controlled either by the erosion rate of the glass fibre or by the erosion rate of the matrix. Glass behaves in an ideally brittle manner under erosion conditions [20]. Thus, if erosion of the PMC is controlled by the glass fibre, a high velocity exponent, considerably higher erosion rate at normal impact angles and erosion efficiences of the order of 100% and above are to be expected. Since the present experimental results are inconsistent with the above expectations (except for high velocity exponents), it can be concluded that the erosion rates of the PMCs are most probably controlled by the matrix phase. The erosion behaviour of the polymer phase depends substantially on whether they belong to the thermoplastic or thermosetting category. Thermosetting polymers, such as epoxy and phenolic resins, are characterized by a brittle erosion response while in the case of thermoplastic polymers (such as polyester) the erosion response is closer to ductile (i.e. maximum erosion rate at intermediate impact angles). However, the velocity exponents are generally high for both types of polymers [4,5,7]. Thus, the experimental results obtained in the present study are generally consistent with the postulate that the erosion rate in PMCs is controlled by the erosion of the matrix phase. The erosion rates of PMCs (investigated by us and also by others [l-5,7,9]) are generally lower than that of the polymers. Thus it is clear that in the presence of glass fibres, the erosion rate of the polymer matrix is reduced. As noted earlier, this could arise because the glass fibres are capable of limiting the size of the damaged zone beneath the impacting erodent particle by both arresting the cracks as well as halting the spread of the deformation. The lower erosion efficiency exhibited by the PMCs as compared with polymers (see Fig. 17) is a direct reflection of this inhibiting effect of the glass fibres. However, in some cases [6,8,18]
Manish Roy et al. 1 Solid particle erosion of polymer matrix composites
PMCs exhibit a lower erosion resistance than the polymers. This can be attributed to the weak bonding between the matrix and the fibres. In such cases not only the interface acts as sites for nucleation of cracks but also the fibre is unable to share its full load. As a result the erosion rates of such weakly bonded composites are higher than the erosion rates of the matrix polymer. Among the four PMCs tested in the present investigation, the epoxy resin matrix provides the best erosion resistance and hence the epoxy resin matrix glass fibre composite has the best erosion resistance at all impact angles and velocities (see Fig. 13). In contrast, the phenolic resin (modified) glass fibre composite, has the least erosion resistance reflecting the high erosion rate of the phenolic (modified) resin matrix. In addition, a good bonding between the glass fibre and the epoxy matrix could have also contributed to the excellent erosion resistance exhibited by the glass-~bre-reinforced epoxy composite.
(5) The erosion behaviour of the composites is largely controlled by the erosion behaviour of the matrix.
The authors wish to express their gratitude to Shri S.L. N. Acharyulu, Director, Defence Metallurgical Research Laboratory for permission to publish this paper. References
6 7 8
Conclusions 9 10
(1) The glass-reinforced epoxy resin composite exhibits the lowest erosion rate and bass-reinforced phenolic resin (modified) shows the highest erosion rate (at ar=30” and 90”, for V=38 and 45 m s-‘). The erosion rates of glass-polyester resin and glass(unmodified) phenolic resin exhibit intermediate values. (2) Composites having thermoset matrix (epoxy and phenolic) behave in a brittle way while the composites with therm0 plastic matrix (polyester) respond in a ductile manner. (3) The erosion rates of the PMCs investigated in the present study do not correlate well with their mechanical properties except for the coefficient of restitution. (4) Reinforcing fibres reduces the erosion efficiencies and hence the erosion rates of the composites most probably by arresting the crack and controlling the spread of deformation.
161
11 12 13 14
15 16 17 18 19 20
G.P. Tilty, Wear, 16 (1969) 63. J. Zahavi and G.F. Schmitt, Wear; 71 (1981) 179. G.P. Tilly and W. Sage, Wear, 16 (1970) 447. K.V. Pool, C.K.H. Dharan and I. Finnie, Wear, 107 (1986) 1. Tseng-Hau Tsiang, Test methods for design allowables for fibrous composites, Vol. 2, ASTM STP I003 (CC. Chamis ed.), American Society for Testing andMaterials, Philadelphia, PA, p. 55. P-J. Math&s, W. Wu, K.C. Goretta, L. Routbort, D.P. Groppi and K.R. Karasek, Wear, I35 (1989) 161. A.M. Latifi, Masters Thesis, Wichita State University, 1987. K.R. Karasek, K.C. Goretta, D.A. Helberg and J.L. Routbort, J. Mater. Sci. Lett., II (1992) 1143. G.P. TiIIy, Wear, 14 (1969) 241. B. Vishwanath, A.P. Verma and C.V.S.K. Rao, Composites, 21 (1990) 531. Y. Ti~pataiah, B. Venkataraman and G. Sundararajan, Mater. Sci. Eng., A124 (1990) 133. Manish Roy, M. Subramaniyam and G. Sundararajan, TriboZ. Int., 25 (4) (1992) 271. A.W. Ruff and L.K. Ives, Wear, 35 (1975) 195. I.M. Hutchings, Proc. Conf: on Corrosion/Erosion of Coal Conversion System Materials, NACE, Houston, TX, 1979, p. 393. G. Sundararajan and P.G. Shewmon, Wear, 84 (1983) 237. Manish Roy, Y. Tirupataiah and G. Sundararajan, Mater. Sci. Eng., AI65 (1993) 51. G. Sundararajan, M. Roy and B. Venkataraman, Wear, I40 (1990) 369. A. Brandstadter, K.C. Goretta, J.L. Routbort, D.P. Groppi and K.R. Karasek, Wear, 147 (1991) 155. P.G. Shewmon and G. Sundararajan, Ann. Rev. Mater. Sci., I3 (1983) 303. G.A. Sargent, P.K. Mehrotra and H. Conrad, Erosion prevention and useful application, ASTM STP 664 (W.F. Adler ed.), America1 Society for Testing and Materials, 1979, p.
77.
Wear, 171 (1994) 149-161
The solid particle erosion of polymer matrix composites Manish &fence
Roy Met~Ilu~*c~i Research
Lo~mto~,
~anchonbagh
P.O., Hyder~bod,
500 258 (India)
P.U., Hyderabad,
500 258 (India)
B. Vishvvanathan Aeronautical
Defence
Establishment,
Bangalore
(India)
G. Sundararajan &fence
Metaliu~ai
(Received
Research
Labomtory,
May 18, 1993; accepted
K~ncha~ba~
September
9, 1993)
Abstract The solid particle erosion behaviour of four different types of polymer matrix composites reinforced with glass fibres have been characterized. The erosion rates of these composites have been evaluated at two impact angles (90’ and 30”) and two impact velocities (38 and 45 m s-l). The erosion response, erosion efficiency and the erosion micromechanisms of these composites are presented and discussed in detail and also compared with the available data in the literature on similar materials.
1. Introduction Polymers are finding an ever increasing application as structural materials in various components and engineering systems. The high specific strength and stiffness of polymers are primarily responsible for their popularity. However, the resistance of polymers to solid particle erosion has been found to be very poor [l], and in fact it is two or three orders of magnitude lower than metallic materials [2]. One possible way to overcome such a shortcoming is to introduce a hard second phase in the polymer to form polymer matrix composites (PMCs). A number of investigators [l-9] have evaluated the resistance of various types of PMCs to solid particle erosion. Table 1 provides a compilation of such data. Tilly [l] and Tilly and Sage [3] tested Nylon and epoxy reinforced with various fibres such as graphite, glass and steel and concluded that the reinforcement can either increase or decrease the erosion resistance depending on the type of fibres. Zahavi and Schmitt [2] tested a number of PMCs for erosion resistance and concluded that glass-reinforced epoxy composite had a particularly good erosion resistance. Pool et al. [4], conducted erosion tests on four PMCs and inferred that wee-handled, ductile fibres in a the~oplastic matrix should exhibit the lowest erosion rates. The above study was extended further by Tsiang [5]. He carried out sand erosion tests on a wide range of
~3-1~~4/$07.~ 0 1994 Elsevier Sequoia. Afl rights reserved SSDI ~043-1648(93)06347-7
thermoset and thermoplastic PMCs having glass, graphite and Kevlar fibres in the forms of tape, fabric and chopped mat as reinforcements. Kevlar fibres in an epoxy resin provided the best erosion resistance. In a recent study, Mathias et al. f6] and also Karasek et al. [S] have evaluated the erosion behaviour of a graphitefibre-reinforced bismaleimide polymer composite. These investigators observed the erosion rates of the PMC to be higher than the unreinfor~d polymer. Many of the investigators [2--6,9] also consistently noted that the erosion rates of the PMCs were considerably larger than those obtained in metallic materials. In addition, PMCs with a thermosetting matrix invariably exhibited a maximum erosion rate at normal impact angles (i.e. a brittle erosion response) while for the thermoplastic PMCs the erosion rate reached a maximum at an intermediate impact angle in the range 40”-5O”, signifying a semiductile erosion response. The present work is concerned with the characterization of the solid particle erosion behaviour of four types of glass-fibre-reinforced polymer matrix composites. Apart from enlarging the existing database on the erosion behaviour of PMCs, the other objectives of the present study are to examine the interrelationship between the mechanical properties of the PMCs and their erosion resistance and to enhance our understanding of the micromechanisms leading to erosion in such PMCs.
Mar&
150
TABLE
1. Compilation
of erosion
Hay et ul. i Solid particle erosion of po&ner
experiments
matrix conpxirec
carried out with polymer and polymer matrix composites
by
various
mvestigators
__-
Material
tested
Nylon 66, graphite-Nylon
66
Quartz-polymide, glass cloth-epoxy quartz polybutadiene Nylon and epoxy reinforced glass and steel
and
Investigators
Test conditions
G.P. Tilly (I]
v= 104 m s -1, Erodent, Quartz
J. Zahavi and G.F. Schmitt [3]
a= 30”, 45”, 60”, 75” and 90” Erodent, Sand
with carbon,
G.P. Tilly and W. Sage [2]
Graphite-reinforced polymide, epoxy and thermoplastic. PMCs and aramid-fibrereinforced epoxy
K.V. Pool, C.K.H. Dharan I. Finnie [4]
Epoxy, bismaleimide, polymide, polyphenylene sulfide, polyetheretherketone and Nylon-reinforced glass, graphite and kevlar
Tseng-Hau [51
BMI and BMI reinforced
P.J. Mathias, W. Wu, K.C. Goretta, J.L. Routbort, D.P. Groppi and K.R. Karasek [6]
1/=20, 40 and 60 m S-I (Y=3p, 90 Erodent, alumina (63, 130 and 390 pm) at RT
A.M. Latifi [7]
V=66 m s-‘, o=30”, 45”, 60” and 90” Erodent, alumina (142 pm)
K.R. Karasek, K.C. Goretta, D.A. Helberg and J.L. Routbort [8]
V=20-120 m s-’ (Y=lY to 90 Erodent, alumina (63, 143 and 390 pm) at RT
Glass-reinforced
with carbon fihre
epoxy system
BMI and BMI reinforced fibre
with graphite
V, impact velocity (m s-l);
a, impact angle (degree);
Tsiang
2.1. Material In the present investigation, four different types of polymer matrix composites namely, glass epoxy resin, glass phenolic resin (modified), glass phenolic resin (unmodified) and glass polyester resin were used. For the purpose of reinforcment E-glass plain weave woven roving fabric, obtained from Fibre Glass Pilkington India Ltd. was used. The physical characteristics of these fibres are given in Table 2 [lo]. The matrix resin solution was prepared by dissolving commercially available resin powder in ethyl alcohol. For glass epoxy and glass polyester composites, the required size fibres were coated uniformly with respective matrix resins. The 2. Characteristics
of plain weave woven roving glass
Nominal thickness (nun) Average weight (f 10%) (g m-‘) Ends for 10 cm Picks per 10 cm ( *2.5%) Nominal count of warp yarn (Tex.) Nominal count of weft yarn (Tex.)
V=31 m s-l CX=15”, 30”, 45”, 60” and 90” Erodent, silica sand (100-200
mesh) at RT
1/=143 m s-’ cu=30”, 45”, 60” and 90” Erodent, aluminium oxide and garnet (16-74 urn and 74-130 pm) at RT
RT, room temperature.
2. Experimental details
TABLE fabric
and
0.33 363 61 305 305
resin-coated plies were then stacked in a steel mould. While stacking, the orientation of warp and weft fibres were maintained in all the plies. These plies were subsequently moulded in a hydraulic press at a pressure of 0.3 MPa at room temperature. In the case of glass phenolic composite, the reinforcing fibres were coated with matrix resin. For unmodified and modified composites, unmodified and modified resin powders were used respectively. The resin was modified so as to improve the mechanical properties of the composites. The exact manner in which the resin has been modified has not been revealed by the manufacturer of the resin powder. This was followed by preparation of prepregs in a temperature-controlled oven. Plies of required size were made from prepregs. Plies were stacked in a steel mould keeping the orientation of warp and weft fibres unaltered. These plies were then pressed hydraulically at a pressure of 13.5 MPa and at a temperature of 160 “C. 2.2. Mechanical testing Various important mechanical properties of the PMCs, such as the tensile, flexural and interlaminar shear strengths were determined using a screw-driven Tinius Olsen universal testing machine at a cross head
151
Manish Roy et al. / Solid particle erosion of polymer matrix composites
speed of 3 mm min-‘. The impact resistance of the PMCs was obtained using a Dynatup instrumented impact system. Determination of the coefficient of restitution was carried out in a gravity drop system. The detailed description of the system and the procedure for determining the coefficient of restitution are given elsewhere [ll]. 2.3. Erosion testing The room temperature erosion test facility used in the present investigation is illustrated schematically in Fig. 1. The facility consists of an air compressor, a particle feeder, an air particle mixing and accelerating chamber. The compressed air (dried using an air dryer) is allowed to mix with the particles, fed at a constant rate (using the conveyor-belt-type feeder) in the mixing chamber. These fluidized particles are then accelerated by passing the mixture through a WC converging nozzle. These accelerated particles impact the specimen held at certain angle with respect to the impacting particle with the help of a sample holder. The feeding rate of the particles can be controlled by monitoring the distance between the particle feeding hopper and the belt drive carrying the particles to the fluidizing chamber. The impact velocities of the particles can be varied by varying the pressure of the compressed air. The impact angle can be varied by changing the orientation of the sample holder. Samples of 30 mm X30 mmX 5 mm were cut from the PMC laminates for the erosion tests. The conditions under which the erosion tests were carried out are listed in Table 3. The standard test procedure is described in a separate publication [12]. The velocity of the eroding particles was determined using a rotating disc method, originally devised by Ruff and Ives [13].
TABLE
3. Erosion
test conditions
Test parameters Erodent Erodent size (pm) Erodent shape Impact angle Impact velocity (m s-l) Erodent feed rate (g min-‘) Test temperature Nozzle to sample distance (mm)
Silica sand 200*50 Angular 90” and 30” 38*5, 45rt5 3.65 RT 10
FLUX
m
SIDE A
m
SIDE B SIDE C
Fig. 2. A schematic diagram describing three differen: for the three-dimensional microstructure of PMCs.
surfaces
2.4. Characterization of eroded samples To characterize the morphology of as-received and eroded surfaces and to understand the mode of material removal, the eroded samples were observed under the scanning electron microscope. Each sample was coated with carbon using a vacuum evaporation technique before being observed.
3. Results 3.1. Microstructure
AIR -
NOZZLE SYSTEM
SAMPLE HOLDER
l--l
n
Fig. 1. A schematic
diagram
of the erosion
rig.
In the case of composites, a full appreciation of their three-dimensional microstructure requires observation of their two-dimensional structure in three directions, i.e. on the surfaces marked A, B and C in Fig. 2. The microstructures of glass epoxy, glass polyester, glass phenonolic (unmodified) and glass phenonolic (modified) composites, as observed on surfaces A, B and C, are illustrated in Figs. 3-6. Glass phenolic composites (modified and unmodified) (Figs. 3 and 4) exhibit microstructures representative of unidirectional glass fibres in a phenolic matrix. The glass epoxy composite (Fig. 5) represents the other extreme and has glass fibres laid bidirectionally in an epoxy matrix. The glass polyester composite (Fig. 6) represents an intermediate case in that the glass fibres are present neither in a fully one-directional or in a bi-directional manner in the polyester matrix.
Fig. 3. A photomicrograph of (modified) glass phenolic (a) surface A, (b) surface B and (c) surface C.
composite.
3.2. Mechanical properties Tensile strength, tensile modulus, flexural strength and flexural modulus of all the composites, as determined from mechanical tests, are illustrated in the form of bar diagrams in Fig. 7. It can be seen clearly that glass phenonolic resin (modified) exhibits the highest tensile strength, flexural strength and flexural modulus
Fig. 4. A photomicrograph of (unmodified) glass phenolic positr. (a) surface A, (b) surface B and (c) surface C.
com-
among the four composites. In contrast, the highest tensile modulus is obtained in glass epoxy resin composite. The interlaminar shear strength and charpy impact strengths of these composites were presented in the form of bar diagrams in Fig. 8. With respect to
153
Manish Roy et al. / Solid particle erosion of polymer matrix composites
Fig. 5. A photomicrograph of glass epoxy composite. A, (b) surface B and (c) surface C.
(a) surface
both the properties, glass phenolic resin (modified) has the highest value. In short, glass phenolic resin (modified) possesses the best combination of strength and toughness. The coefhcient of restitution determined using the gravity drop system is also shown in Fig. 8. Glass epoxy resin and glass phenolic resin (unmodified) exhibit the highest coefficient of restitution.
Fig. 6. A photomicrograph of glass polyester surface A, (b) surface B and (c) surface C.
composite.
(a)
It has been demonstrated by Tirupataiah et al. [ll] that the coefficient of restitution of a material is related to its hardness by 1.9PB C.?= Ee,lR PblBVf4 which can be rearranged
0) to give
Fig. 7. A bar diagram showing the tens&z strength,
Fig. 8. A bar diagram to e&bit
the interfaminar
tensile modulus, flexural strength
and fiexuraf modulus of the composites.
shear strength, charpy impact strength and e+c&Esient of restitution
‘In eqns. (1) and CZ),e is the coefficient of restitution, N is the dynamic hardness of the target material, pb is the density of the impacting ball, Y is the velocity of impact, and ~5,~is the effective modulus of the target material-ball material combination and is given by
where E, and E, are the modulus of elasticity and V, and JJ*are Poisson’s ratio of the impacting hall and the target material respectively+ The values of the as calculated using eqns. (2) and hardness and &, (3) for these composites, are presented in Table 4 along with their coeficient of restitution and modulus of eIaseicity. It can be seen clearly that glass epoxy resin is the hardest among the composites. It is also interesting to note that although unmodified @ass phenolic resin has a low hardness, its coefficient of restitution is significantly higher than other composites.
nf the composites.
The variation in t.he incremental erosion rates as a function of the ~~~~ulati~e weight of the impinging paraides is shown in Figs_ 9-12 fur grass pofyester resin, glass epoxy resin, glass phenolic resin (modified) and glass phenofic resin (unmodified) composites respectively. For each composite, the erosion data pertaining to two impact angles (30” and W) and two impact velocities (38 and 45 rn s - ‘) have been presented. Figures 9-12 indicate that for glass epoxy resin, glass polyester resin and for glass phenolic resin ~unrn~i~~d~~ the incremental erosion rate either initiafiy increases from a Iow value to a constant value or initialIy increases from a low value to a high value and then decreases ta a constant value, at all impact angles and impact velocities, On the contrary for glass phenolic resin ~rnud~ed~ the increme.ntaZ erosion rate initially decreases from a high value to a constant value for normal impact and initially increases for a low value fa a constant value for oblique impact at all impact velocities (Fig_ 12). These constant values af the erosion rate henceforth will be called the erosion rate (E) of the material. It is evident from Fig. 11, that the incremental erosion sate of glass phenolic (modified) resin composite
Manish Roy et al. / Solid particle erosion of po&ner
TABLE 4. Coefficient
of restitution,
modulus
Material
of elasticity
mati
155
composites
and effective modulus of elasticity
and hardness
Coefficient of restitution
Modulus of elasticity (GPa)
Effective modulus of elasticity (GPa)
of test material Dynamic hardness (MPa)
Glass polyester
resin
0.67
40.0
41.0
770
Glass phenolic
resin (modified)
0.65
43.0
43.0
777
resin (unmodified)
0.75
21.0
22.0
556
0.76
45.0
46.0
1029
Glass phenolic
Glass epoxy resin
G \
100
.?
GLASS
POLYESTER
RESIN
0
0
0
NORMAL
IHPACT
V=45m/r
.
l
.
NORMAL
IMPACT
V=3Bm/s
d,
I
,t
OBLIQUE
IHPACT
V=Gm/s
A
A
A
OBLIQUE
IMPACT
V=3Bm/r
rsllO.0. \
cn
GLASS PHENOLIC IMOOlFlEOl RESIN
01
1
IO 0
I
20 0
30 0
40 0
50 0
60 0
I
I
I
70 0
SO.0
90.0
CUMULATIVE WEIGHT OF THE IMPINGING PARTICLES (gml
10
Fig. 9. The variation in the incremental erosion rate of the glass polyester resin composite with cumulative weight of impinging particles.
GLASS EPOXY 0 0 0 . A A A A
0 0 A A
NORMAL NORMAL OBLIQUE OBLIOUE
20
30
‘0
IMPACT IMPACT IMPACT IHPACT
-. \ z
0
I 10
20
I
I
30
40
CUMULATIVE IMPINGING
-t
.
1
60
70
0
I
I SO
60
9c
Vd,Sr/s V:lOm/r V.&%./r VdBr,,
-G
r
70
RESIN
GLASS
12.0
2.0 -
60
Fig. 11. The variation in the incremental erosion rate of glass phenolic resin (modified) composite as a function of cumulative weight of the impinging particles.
14.0
E
50
CUMULATIVE WEIGHT OF THE IMPINGING PARTICLES lg)
BO
‘;
I
Y
100
c
PHENOLIC
RESIN
IUNMOOIFIEOI
0
0
0
NORMAL
IMPACT
V = 45m/r
0 B
0
0
NORHAL
IMPACT
A
A
OBLIQUE
IHPACT
V = 3Bm/r V = 45m/s
A
A
A
OBLIOUE
IHPACT
V = 3Bm/r
1
0
60
v
90
WEIGHT OF THE PARTICLES fgl
Fig. 10. The variation in the incremental erosion rate of glass epoxy resin composite with cumulative weight of impinging particles. 01
at normal impact (impact velocity of 45 m s-l) continuously decreases. This is probably because of the fact that the incremental erosion rate of the said material under the above-mentioned test condition does not reach steady state even after such long exposure. How-
I-
10
I
I
I
20
30
40
I
so
60
I
70
I
80
90
CUMULATIVE WEIGHT OF THE IMPINGING PARTICLES fgl
Fig. 12. The variation in the incremental erosion rate of glass phenolic resin (unmodified) composite as a function of cumulative weight of the impinging particles.
Munish Roy et al. I Solid particle erosion
1%
ever, for the purpose of comparison, the average of the last five incremental erosion rates data points have been taken as the steady state erosion rate. The erosion resistance which is the reciprocal of the erosion rates (l/E) obtained from Figs. 9-12 for all these composites are presented in the form of histogram in Fig. 13. It is obvious from Fig. 13 that glass epoxy resin exhibits the highest erosion resistance at all impact angles and impact velocity, while the glass phenolic resin (modified) composite exhibits the least erosion resistance. The influence of the impact angles on the erosion rates of the composites are shown in Fig. 14. For each test material and impact velocity, erosion rate has been measured only at two impact angles (30” and 907. Although there is a possibility that the erosion rate may go through a maximum at some intermediate impact angles, the two data points have been joined by a straight line only to help the reader identify the behaviour of each test material. It is to be noted that glass epoxy resin and glass phenolic resin (unmodified) exhibit brittle erosion response, i.e. maximum erosion rate at normal impact at all impact velocities. In contrast glass polyester resin behaves in ductile way (having maximum erosion rate at oblique impact) at all impact velocities. It is also interesting to note that glass phenolic
of polymer matrix composite5
-_o__o-
GLASS P"iNALlC
RESIN ,nOOlFlEO, Y = 45m/r
GLASS
PHENAIIC
RESIN lM0DlFIEDl " = Plm/r
GLASS
EPOXY
mm.-.--
RESIN AT
Y=LSm/r
:: w so+
3s:
r 2 40t i30i
_____-
20 _
ok-4cx OF PlPAC6; (“1
IMPACT
ANGLE=30'
IMPACT
YELOCITY=4Sm/s
IMPACT
VELOCITY=38m/s
m K 5
60.
GLASS
EPOXY
RESIN
RESIN
of
Impact angle
Ka
P’
resin
90” 30”
8.40~ lo-‘* 11.6x10-‘”
4.79 3.50
Glass phenolic (modified)
resin
90” 30”
33.8x lO_” 18.87x lo-*’
4.53 4.69
Glass phenolic (unmodified)
resin
90 30”
5.75 x lo-‘* 4.54x 10-r*
4.99 4.91
90” 30”
2.71 x lo-‘* 12x lo-=
4.97 5.74
“E =WP PHENOLIC
the velocity dependence
Glass polyester
Glass epoxy resin
,,UUU GLASS
90
Fig. 14. The effect of the impact angle on the erosion rates of the composites.
Material
ANGLE=30"C
- --
1OL
TABLE 5. Parameters characterising erosion rate of the test materials
IMPACT
___o-
____o____---------
(units: K, (scm-‘)-p).
COMPOSITES
COMPOSITES
IMPACT
ANGLE='?,,'
IMPACT
ANGLE
IMPACT
"ELOCITY=4Sm,s
IMPACT
YELOCITY
i 90' i 38m/s
Fig. 13. A histogram illustrating the erosion resistance of various composites at hvo different impact angles and two different impact velocities.
resin (modified) indicates brittle erosion behaviour at lower velocity (38 m s-l) and exhibit ductile erosion at higher velocity (45 m s-l). The dependence of erosion rate on the velocity of impact is characterized by the exponent p (where E=kVP) [14,15]. The values of p and k for all the composites are listed in Table 5 for normal and oblique impact. The strong velocity dependence of erosion rates of composites is evident as indicated by the values in the range. The high velocity exponent of glass epoxy resin composite at all impact angles is particularly noteworthy.
Manish Roy et al. I Solid particle erosion of polymer matrix composites
3.4. Morphology of eroded sugaces The scanning electron micrographs of the eroded surfaces of the various composites are presented in Figs. 15 and 16. Figure 15 corresponds to the surface eroded at normal impact angle (velocity, 38 m s-l) while Fig. 16 is pertinent to the surface eroded at oblique impact angle (30”, V=38 m s-l). The eroded surfaces of a given composite appear very similar at normal and oblique impact angles and thus can be discussed together. At one extreme, in the case of glassepoxy and glass-polyester composites, both the glass fibres and the matrix material are present in appropriate proportions on the eroded surface, indicating that the erosion rate of the matrix controls the overall erosion rate. In contrast, in the case of glass-phenolic (modified or unmodified) composites, the entire eroded surface is almost composed of fibres protruding out of the matrix phase. Thus, it appears that the erosion rate of the phenolic matrix in these composites is very high.
157
4. Discussion
4.1. Comparison with literature In this section the erosion data of these polymer matrix composites will be compared with the erosion data of various polymers and PMCs reported by other investigators. Towards that purpose it is necessary to compare and contrast the conditions under which tests were conducted. The erosion experiments carried out by other investigators with different polymer composites are listed in Table 1 alongwith experimental conditions. The table clearly shows that the tests have been carried out under a wide variety of conditions, with regard to the impact velocities, impact angles and erodents. Thus, only a qualitative comparison between the present data and literature data is possible. Most of the observations clearly demonstrated that the erosion rate of the polymer matrix composites are an order of magnitude higher than that of metallic
Fig. 15. Scanning electron micrographs of the eroded surfaces of the composites, Impact angle equal to 90” and at impact velocity 38 m s-‘. (a) Glass polyester resin composite, (b) glass phenolic resin composite (unmodified), (c) glass phenolic resin composite (modified) and (d) glass epoxy resin composite.
Fig. 16. Scanning electron micrographs of the eroded surfaces of the composites. Impact angle equal to 30”. (a) Glass epoxy resin composite, (b) glass phenolic resin composite (modified), (c) glass phenolic resin composite (unmodified) and (d) glass polyester resin
composite.
materials (AISI 1018 steel 141, Al, Ti, stainless steel [5], PH stainless steel [12], Cu, Cu-Zn, Cu-Al [16]). In the present work also a similar conclusion can be reached since earlier work by us on the erosion of a number of metallic materials using the same erosion rig and under similar test conditions indicates that the erosion rates were in the range 3 x 1O-5-3O~ 10d5 g g-’ [12,16], i.e. an order of magnitude lower than that of the PMCs. In the present work, glass-reinforced epoxy composite has the best erosion resistance and modified phenolic resin composites have lowest erosion resistance. Zahavi and Schmitt [2] also obtained the best erosion resistance in glass-reinforced epoxy composites, as in the present study, although Zahavi tested the materials for five different impact angles (30”, 45”, 60”, 75” and 900). According to Pool et al. [4], for polymeric material behaving in a ductile manner, the velocity exponent p will vary in the range 2-3 while for polymer composites
behaving in brittle fashion the value ofp should be in the range 3-5. Tilly and Sage [3] using Nylon and epoxy, reinforced with carbon, glass and steel, reported the velocity exponent to be 2.3. In the present work, the velocity exponents obtained lie in the range of 3.5 to 6 irrespective of the erosion response of the composites. Thus, for composites exhibiting brittle response, the velocity exponents obtained in the present work are in conformity with Pool et al. [4]. The reasons for the high velocity exponent of the PMCs is related to their high coefficient of restitution as discussed below. For materials having high coefficient of restitution, the equation of the type E =K,(V-- Vreb)” (where Vrebis the rebound velocity) is more appropriate than the conventional equation E =KVP. Assuming the average exponent n for composites to be 2.5 [14,15] and fitting it to the equation of the typeE=K,(V’Vrc,,)” over the experimental velocity range 38-45 m s- ‘, the average rebound velocity (l/ret,) of the composites can
159
Manish Roy et al. / Solid particle erosion of polymer matrix composites
This efficiency parameter
be computed. The calculated I/reb values of the four composites lie in the range 18-21 m SK’ and more importantly increase in the same manner as the coefficient of restitution (e). The above Vrcb values correspond to an e value of around 0.5 for impact velocities in the range 3&45 m s-l and are quite consistent with the higher e values obtained at lower impact velocities in the present study (Fig. 8) [ll]. It has been noted by various investigators that the erosion rate of composites may increase or decrease with increase in impact angle. According to Tsiang [5], composites having a thermoset matrix behave in a brittle manner, while composites having a thermoplastic matrix exhibit semi-ductile erosion behaviour. In the present study, consistent with the above observations, a therm0 setting matrix (epoxy, and phenolic) composites show a brittle behaviour while the therm0 plastic matrix (polyester) composite indicates a ductile behaviour. The only exception, is the modified glass phenolic resin composite. This thermosetting matrix composite possesses brittle erosion response at low impact velocity and ductile erosion response at high impact velocity. Reasons for such behaviour are not clear.
2EH 77= pv2
4.3. Erosion efficiency To describe the nature and the mechanism of erosion a new parameter, erosion efficiency, 77 was proposed by Sundararajan et al. [17]. This parameter indicates the efficiency with which the volume that is displaced by the impacting erodent particle is actually removed.
Serial number
of erosion
Correlation
rate with various parameters
parameters
Correlation Normal V=45
1
2 3 4 5
E E E E E
vs. vs. vs. vs. vs.
tensile strength charpy impact energy interlaminar shear strength coefficient of restitution dynamic hardness
E, steady state erosion
rate.
(4)
where E is the erosion rate, H is the hardness, p is the density of the target material and V is the velocity of impact. This parameter can be used to identify the brittle and ductile erosion response of various materials. For example, ideal microploughing involving just the displacement of material from the crater without any fracture (and hence no erosion) will have zero erosion efficiency. Alternatively, in the case of ideal microcutting, q will be unity. In case erosion occurs by the formation of a lip and its subsequent fracture, erosion efficiency will be in the range O-l. In contrast, as happens with brittle material, if the erosion takes place by spalling and removal of large chunks of material by interlinking of lateral or radial cracks, then the erosion efficiency is expected to be even greater than 100%. The values of the erosion efficiencies of these composites calculated using eqn. (4) are summarized in Table 7 alongwith their hardness. The erosion efficiencies of these composites vary from 18% to 43% for impact velocities of 45 m SK’ and from 14% to 33% for impact velocities of 38 m s-‘. Thus it can be concluded that erosion takes place by microploughing and microcutting. The erosion efficiency map which indicates the effect of hardness of various materials on their erosion efficiency is shown in Fig. 17. The regimes which correspond to erosion efficiency of polymers and polymer matrix composites are also indicated in the figure. In Fig. 17, the polymer regime has been identified using the literature erosion data [ 181, while the present results on PMCs have been utilized to delineate the PMC regime. The lower erosion efficiencies of the PMCs, as compared with the polymers, can be attributed to the resistance offered by the hard reinforcing fibres to the crack propagation. These reinforcing fibres reduce the erosion efficiency also by inhibiting the spread of deformation.
4.2. Correlation with mechanical properties Effort has been made to correlate the erosion rates of various composites with their mechanical properties. Various correlating parameters along with correlation coefficients under different test conditions are summarized in Table 6. It is clear from Table 6 that the erosion rates of the composites do not correlate well with their mechanical properties. Among various parameters, the coefficient of restitution appears to correlate best with the erosion rate.
TABLE 6. Correlation
(77) can be obtained from
0.09 0.04 0.43 0.69 0.69
coefficient
impact m s-’
Oblique
impact
V=35 m s-’
V=4_5 m s-’
V=35 m SK’
0.04 0.20 0.47 0.62 0.62
0.12 0.20 0.44 0.91 0.36
0.31 0.24 0.07 0.95 0.43
Manish Roy et al.
160 TABLE
7. Erosion
efficiencies
of various
Material
test
I Solid pu&Ae erosion
materials
at normal
Density (kg m -j)
Static (Ml%)
of polynrr
nmtrix
cornpxitcs
Impact
hardness
Erosion efficiency at V=45 m s ~’ (“ro)
Glass
polyester
resin
Glass
phenolic
resin
(modified)
resin
(unmodified)
Glass
phenolic
Glass
epoxy
resin
1787
488
19
l?i
1929
806
43
2x
1892
806
35
21
1782
1097
27
16
. IMPACT VELOCITY : 45m/r b IMPACT VELOCITY = 38m/s Oil
i
I
10
100
1000
HARDNESS (HI’) Fig. 17. The erosion efficiency hardness corresponding to metals, metallic alloys, mers and polymer matrix composites.
Erosion efficiency at V=38 171s-r ( ‘X )
I
10000
map indicating regimes ceramics, coatings, poly-
4.4. Erosion micromechanisms
The dependence of the erosion rates of the four polymer composites on impact velocity is very strong at both impact angles, as indicated by the velocity exponents in the range 3.5-6.0 (Table 5). In contrast, in the case of metallic materials, the velocity exponent lies in the range 2-3.5. Similarly, the erosion efficiencies of the four PMCs investigated in the present study (17%-40%) are substantially higher than those for the metals (l%-10%). However, compared with polymers (erosion efficiencies around 100%) and glasses (efficiencies up to 1000%) the erosion efficiencies of PMCs are considerably lower. Thus, the erosion behaviour of the present PMCs can be broadly classified as semiductile. The fact that the erosion rates of each of the four PMCs at the impact angles of 30” and 90” are not dramatically different also supports the above con-
tention, since either purely ductile or purely brittle erosion response causes the erosion rates at 30” and 90” to be widely different [19]. PMCs represent a twophase structure namely, the glass fibres and the matrix phase which is epoxy, polyester or phenolic resin. In principle, the overall erosion rate of the PMCs can be controlled either by the erosion rate of the glass fibre or by the erosion rate of the matrix. Glass behaves in an ideally brittle manner under erosion conditions [20]. Thus, if erosion of the PMC is controlled by the glass fibre, a high velocity exponent, considerably higher erosion rate at normal impact angles and erosion efficiences of the order of 100% and above are to be expected. Since the present experimental results are inconsistent with the above expectations (except for high velocity exponents), it can be concluded that the erosion rates of the PMCs are most probably controlled by the matrix phase. The erosion behaviour of the polymer phase depends substantially on whether they belong to the thermoplastic or thermosetting category. Thermosetting polymers, such as epoxy and phenolic resins, are characterized by a brittle erosion response while in the case of thermoplastic polymers (such as polyester) the erosion response is closer to ductile (i.e. maximum erosion rate at intermediate impact angles). However, the velocity exponents are generally high for both types of polymers [4,5,7]. Thus, the experimental results obtained in the present study are generally consistent with the postulate that the erosion rate in PMCs is controlled by the erosion of the matrix phase. The erosion rates of PMCs (investigated by us and also by others [l-5,7,9]) are generally lower than that of the polymers. Thus it is clear that in the presence of glass fibres, the erosion rate of the polymer matrix is reduced. As noted earlier, this could arise because the glass fibres are capable of limiting the size of the damaged zone beneath the impacting erodent particle by both arresting the cracks as well as halting the spread of the deformation. The lower erosion efficiency exhibited by the PMCs as compared with polymers (see Fig. 17) is a direct reflection of this inhibiting effect of the glass fibres. However, in some cases [6,8,18]
Manish Roy et al. 1 Solid particle erosion of polymer matrix composites
PMCs exhibit a lower erosion resistance than the polymers. This can be attributed to the weak bonding between the matrix and the fibres. In such cases not only the interface acts as sites for nucleation of cracks but also the fibre is unable to share its full load. As a result the erosion rates of such weakly bonded composites are higher than the erosion rates of the matrix polymer. Among the four PMCs tested in the present investigation, the epoxy resin matrix provides the best erosion resistance and hence the epoxy resin matrix glass fibre composite has the best erosion resistance at all impact angles and velocities (see Fig. 13). In contrast, the phenolic resin (modified) glass fibre composite, has the least erosion resistance reflecting the high erosion rate of the phenolic (modified) resin matrix. In addition, a good bonding between the glass fibre and the epoxy matrix could have also contributed to the excellent erosion resistance exhibited by the glass-~bre-reinforced epoxy composite.
(5) The erosion behaviour of the composites is largely controlled by the erosion behaviour of the matrix.
The authors wish to express their gratitude to Shri S.L. N. Acharyulu, Director, Defence Metallurgical Research Laboratory for permission to publish this paper. References
6 7 8
Conclusions 9 10
(1) The glass-reinforced epoxy resin composite exhibits the lowest erosion rate and bass-reinforced phenolic resin (modified) shows the highest erosion rate (at ar=30” and 90”, for V=38 and 45 m s-‘). The erosion rates of glass-polyester resin and glass(unmodified) phenolic resin exhibit intermediate values. (2) Composites having thermoset matrix (epoxy and phenolic) behave in a brittle way while the composites with therm0 plastic matrix (polyester) respond in a ductile manner. (3) The erosion rates of the PMCs investigated in the present study do not correlate well with their mechanical properties except for the coefficient of restitution. (4) Reinforcing fibres reduces the erosion efficiencies and hence the erosion rates of the composites most probably by arresting the crack and controlling the spread of deformation.
161
11 12 13 14
15 16 17 18 19 20
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