Polimery i kompozyty konstrukcyjne 1998

Ashraf I. HASSAN, Jan KOSMOL, Jerzy BURSA

Katedra Budowy Maszyn, Politechnika Śląska, Gliwice

RECENT ADVANCES IN MACHINING OF POLYMERIC

COMPOSITE MATERIALS BY AWJM

Abstract. Conventional machining of polymeric composite materials is often both

technically and economically less effective because the structural characteristics

inherent in fibre reinforcement promote excessive tool wear. In recent years, abrasive

waterjet machining (AWJM) has been proving to be successful in the machining of such

materials. This paper presents a state of the art review of research in AWJM of

polymeric composite materials. Among the main topics discussed are mechanisms of

material removal, productivity and surface quality.

1. INTRODUCTION

Polymeric composite materials, or fibre reinforced plastics (FRP), are increasingly being to

be rapidly developing commercial materials for use in applications which require both high

strength and light weight. Their applications extend in industrial fields such as aircraft,

aerospace, automobile, construction, corrosion resistant, electrical and marine [1]. As

composite materials become commonplace, the search for efficient machining processes

becomes more significant. However, conventional machining of polymeric composite materials

is often not effective because the structural characteristics inherent in fibre reinforcement

promote excessive tool wear [2]. Also the material is subject to severe damage including inter

laminar delamination, fibre pullout and poor surface quality that may adversely affect the part

performance under service loading conditions.

Abrasive waterjet machining (AWJM) has been successfully applied in the machining of

different types of composite materials such as metal matrix composites [3,4,5] and ceramic

matrix composites [3,4,6] where conventional machining is often not technically or

economically feasible. Figure 1 shows the principle of AWJM, where water at high pressure

(P), up to 400 MPa, exits from a sapphire nozzle, whose diameter is (dn), and it is mixed with

fine abrasives, which are fed from an abrasive feed hooper at a rate of ( &m ), to form a high

velocity, high pressure coherent jet exiting from a mixing tube nozzle of inside diameter (dm),

and length (lm). This jet is able to cut any type of material of any contour by a feed motion

Ashraf I. HASSAN, Jan KOSMOL, Jerzy BURSA

called the traverse rate (u). The mixing tube is usually displaced from the workpiece surface

by a small stand off distance (S).

Fig. 1 Principle of AWJM

Compared with conventional and other non conventional machining of polymeric

composite materials, AWJM has the following advantages [6, 7, 8]:

Suitable for many machining operations such as turning, drilling or milling

Able to machine both very soft and very hard composites

Able to machine multi layered composites

Can cut stacks of different composites

Omnidirectional machining

No thermal stresses

The real benefit of AWJM lies in the last item of the advantages i.e. the non thermal machining

of high cost heat sensitive materials such as polymeric composite materials to justify the slower

traverse rates than plasma arc machining whose principal limitation is the thermal damage of

the workpiece surface [9].

Recent advances in machining of polymeric composite materials by AWJM

The following limitations are relevant to AWJM of composite materials [10]: high capital

investments are required, high cutting power is required, delamination occasionally occurs, the

jet has only a limited stability perpendicular to its own axis, and the process is noisy and

produces a great deal of spray.

Two recent reviews conducted by Hassan and Kosmol [11] and Hoogstrate et al [12]

provided a state of the art review of AWJM technology. An early study on the practical

applications of AWJM [10] provided a comprehensive list of potential composite materials that

were successfully machined.

This paper is a review of recent developments in AWJM of polymeric composite materials.

We will not deal with other types of composite materials such as metal matrix and ceramic

matrix. Much of the research, although scarce, has dealt with the mechanisms of material

removal, productivity and surface quality. Historically, plain waterjet machining (WJM) was

initially applied on composite materials at the late seventies and the beginning of the eighties.

Table 1 shows the different machining parameters used in the early research works of WJM

studies when the waterjet alone was used in trial tests to machine polymeric composite

materials or fibre reinforced plastics (FRP) such as glass fibre reinforced plastics (GFRP),

carbon fibre reinforced plastics (CFRP), aramid fibre reinforced plastics (AFRP) and sheet

molding compounds (SMC)..

Table 1

Operating parameters used in studies of WJM of polymeric composites

Type of

FRP

P

(MPa)

u

(mm/s)

S

(mm)

dn

(mm)

Authors

GFRP 350

200 - 500

350

200, 300,

400

up to 70

1.1

4, 16, 20

1.1

up to 40

0 - 40

up to 100

0.15 - 0.25

0.18

0.12, 0.25

König et al [13]

Oweinah [14]

König & Wulf [15]

Engemann [16]

CFRP 350

300 & 400

200, 300,

400

up to 70

up to 33

1.1

up to 40

0-20

up to 100

0.15-0.25

0.1-0.3

0.12, 0.25

König et al [13]

König &Wulf [17]

Engemann [16]

AFRP 350

200, 300,

400

up to 70

1.1

up to 40

up to 100

0.15-0.25

0.12, 0.25

König et al [13]

Engemann [16]

SMC 350 up to 70 up to 40 0.15-0.25 König [13]

Due to the limited energy used in WJM, larger thicknesses of FRP were not able to be cut

effectively. Afterwards, the introduction of abrasive particles into the waterjet stream at the

beginning of the eighties opened a new era for the effective machining of FRP. But up till now,

Ashraf I. HASSAN, Jan KOSMOL, Jerzy BURSA

only a limited number of research studies have been conducted in this field. Table 2 shows the

different machining conditions employed in AWJM studies of polymeric composite materials

such as carbon fibre/epoxy (C/Ep), graphite/epoxy (Gr/Ep), sheet molding compounds (SMC),

glass/epoxy (G/Ep) and quartz/FRP.

Table 2

Operating parameters in AWJM research works of polymeric composites

Type

of

FRP

P

MPa

u

mm/s

S

mm

m

g/s

Abr. Mesh

No

dn

mm

dm

mm

lm

mm

Ref

C/Ep 300 8-33 5 5 garnet 80 0.225 [18]

Gr/Ep

SMC

G/Ep

quartz

/ FRP

100-

240

100-

240

-

345-

370

300

-

1.6-

3.9

1.6-

3.9

-

1-105

8-33

-

1-4

1-4

-

5

-

10

5,10

-

5

-

garnet

garnet

-

garnet

sand/

air

garnet

sand/

air

80,

100,

150

50,80,

100,

150

-

80

120

80

-

0.3

0.3

-

0.3

0.225

-

1

1

-

0.67

5

-

44

-

300

-

[19]

[2]

[20]

[6]

[21]

[18]

[22]

2. MECHANISMS OF MATERIAL REMOVAL

Several studies to develop an understanding of the mechanisms of erosive wear of isotropic

materials such as metals have been conducted. On the other hand, There is still a lack of

understanding of the erosion mechanisms of anisotropic materials such as composite materials.

One of the main drawbacks of WJM of composite materials is that if the jet hits an obstacle,

such as a fibre, it will be deflected and will erode the soft components of the composite first.

Therefore, FRP surfaces cut by WJM showed that typical appearance of washed out matrix and

protruding fibres. The glass fibre compound with the non-oriented fibres (SMC) has a tendency

for chipping at the exit side even at low traverse rates. Due to the low resin content and the

low interior support between fibres, comparatively large pieces of matrix and fibres chip off. In

aramid fibre composites, damage occurs as wide spread delamination between fibre and matrix.

This damage is partially caused by the different displacement and the spring back of the single

Recent advances in machining of polymeric composite materials by AWJM

fibre layers, and by some part caused by water pressed in between the laminate layers. This

latter effect is the main cause of damage for CFRP [13].

After switching to AWJM in the early eighties, several studies concerning the erosion

process had been evolved. In an early study of sand blasting of polymeric composite materials,

Tilly and Sage [23] found that composite materials generally behave in an ideally brittle fashion

i.e. maximum erosion rate occurs at normal impact, i.e. at 90°. Fibre reinforcement may

improve or worsen the resistance to erosion, depending on the type of fibres used. In addition,

the erosion rate in composites increases with the particle size. It was found that the erosion

rate of fibre reinforced plastics is greater than that of steel. Zahavi and schmitt [22] found out

that three polymeric composite materials, quartz-polyimide, glass-epoxy and quartz-

polybutadiene, showed maximum erosion at normal impact i.e. 75 -90. This is consistent with

their properties as thermosetting resins and inorganic fibres. The material removal mechanism

in fibre reinforced plastics (FRP), as described by them can be characterized as follow:

1. There is a local removal from the resin material from the impacted surface which results in

exposure of the fibres to the erosion environment.

2. Abrasive particles impact on the fibres and cause fibres to break because of the formation of

cracks perpendicular to their length due to fibre bending stresses.

3. Further damage results when the interfaces between the broken fibres and matrix resin are

degraded until the fibres are removed to subsequent impact.

This mechanism was later verified by Pool et al [21] using scanning electron microscopy for

three different composite materials, graphite/epoxy, graphite/polyphenylene sulfide and

aramid/epoxy.

In an early study on the application of AWJM to composite materials, Hashish [10] showed

for the first time the possibility of AWJM of composite materials such as kevlar/fiberglass/steel

wire with reasonable traverse rates and less delamination than other processes. Ramulu and

Arola [2] carried out a scanning electron microscopic study on Graphite/Epoxy composite and

suggested that for 90° fibre orientation to the direction of the traverse rate, the dominant

modes of material removal are shearing and abrasive micro machining. Fibre pullout and the

degree of fibre-matrix delamination is limited due to the high interfacial bond strength. For 00

fibre orientation, the abrasives account for the dominant portions for material removal and the

matrix remains intact in the machined surface. Shallow abrasive wear tracks are observed on

the surface of the machined specimens. The process of erosion by solid particle impact has

been investigated by several notable researchers since 1960. The early work of Finnie [24] is

still regarded as the leading work, since then a number of researchers in AWJM have

developed several cutting models on erosion. Later, an intensive work in the field of AWJ

erosion was conducted by Hashish [25,26-28]. A simplified model, based on Finnie's model, for

cutting of ductile materials was suggested [26]. Two mechanisms have been identified as the

dominant modes of material removal. These are the cutting wear mode and the deformation

wear mode. The surface produced by the first mode is relatively smooth due to cutting wear at

a shallow angle of impact and it exists at the upper portion of the cutting kerf. The surface

Ashraf I. HASSAN, Jan KOSMOL, Jerzy BURSA

produced by the other mode is striated due to deformation wear mode at large angles of impact

which is characterized by material removal due to excessive plastic deformation.This wear

mode results in an unsteady penetration process. Later, these two mechanisms were found to

apply also to brittle materials such as glass [27]. The relative contribution of each mechanism

significantly depends on the process parameters. However, the deformation wear mode

becomes more dominant in brittle materials. This is concluded from the lower ratios of cutting

wear depth to deformation wear depth obtained for brittle materials.

Recent studies in AWJM of graphite/epoxy composites pointed out the existence of a small

initial damage zone near the jet entrance, which is evident from the curvature at the top of the

kerf, in addition to the previous two modes which are characteristic for AWJM [2,19]. The

initial damage zone at the top of the kerf is due to jet expansion prior to impingement as shown

in Fig. 2. Workpieces machined with a low stand off distance exhibited the smallest initial

damage zone whose width may reach 500 µm and its depth 600 µm in some cases. The effect

of other AWJM parameters were negligible. Regression models were developed with the aid of

Fig. 2 Initial damage zone in AWJM [19]

measurements of the width and depth of the initial damage zones. The length of the cutting

wear zone was found to be most affected by abrasive grain size, pressure and the traverse rate.

On the other hand, stand off distance has a little effect on this length. The mechanism of

material removal in FRP includes broken fibres, pullout in all the three zones and rather large

removal pockets in the initial damage zone. Therefore, changes in kerf geometry and kerf wall

characteristics with cutting depth are solely a product of the inherent energy of the jet.

Recent advances in machining of polymeric composite materials by AWJM

3. PRODUCTIVITY

A limited experimental as well as analytical work on productivity has been reported. Early

attempts for WJM of polymeric composites started two decades ago and included

boron/epoxy, boron/polyester, Aramid/epoxy (up to 12.5 mm), glass/epoxy, glass/phenolic (12

mm thickness), glass/polyimide and graphite/epoxy (up to 16 mm) [29]. Table 3 shows the

traverse rates that were attainable at that time for some types of composites. Another early

Table 3

Traverse rates for WJM of some composites [29]

Type of FRP

Thickness

mm

dn

mm

P

MPa

u

mm/s

Fiberglass epoxy 3.55 0.25 412 2.5

Graphite epoxy 1.7 0.2 380 25

Aramid epoxy

3.2

0.25

380

12.50

study on WJM of polymeric composite materials was conducted by Engemann [16] who found

out that a stand off distance of 10-20 mm, which is the range for the core zone of the free jet,

is optimum for obtaining the maximum depth of cut. Any increase in this distance will lead to

an increase in the width of cut, waviness and surface roughness. Increases in jet pressure and

nozzle diameter led to an increased depth of cut. GFRP could be cut up to 10 mm thickness

without delamination at low traverse rates while this value for CFRP is 5 mm. Oweinah [14]

showed that the maximum WJ erosion in glass fibre composite occurs at a stand off distance

around 20 mm. It was possible to penetrate this type of composite up to a depth of cut of 25

mm using pressure of 500 MPa. Figure 3 shows some early attempts of WJM of glass fibre

reinforced polyester as compared to different metals and alloys. It is obvious that the volume

removal for GFRP is much larger than aluminum, brass and mild steel for the same pressure.

This is conforming with the results obtained from sand blasting of three different composite

materials, graphite/epoxy, graphite/polyphenylene sulfide and aramid/epoxy which showed that

the erosive wear rates in these materials are at least an order of magnitude greater than that of

low carbon steel. Also the factors influencing the erosion rate were found to be the brittlness of

the fibres, type of matrix, either thermosetting or thermoplastic, and the interfacial bond

strength between the fibres and the matrix. Thus well-bonded ductile fibres in a thermoplastic

matrix should exhibit the lowest erosion rates [21]. The volume removal rate for AWJ milling

of Gr/Ep composite could be in the range 100-300 mm3/s [25]. Fibre orientation has a strong

effect at the tool exit side compared to the direction of feed. Early spalling of fibres in the

transverse direction to traverse rate limit the usable traverse rate to only 10% of the maximum

value. An increase in pressure results in higher traverse rates and better surface quality. It was

pointed out that pressures used must exceed 300 MPa for thicknesses over 1 mm [13].

Ashraf I. HASSAN, Jan KOSMOL, Jerzy BURSA

Pressure (P) MPa

Fig. 3 Effect of pressure on the depth of cut in WJM of GFRP [29]

Table 4 summarizes the results of traverse rates for different thichnesses obtained by Hashish

[6]. Observe that the maximum traverse rate is primarily controlled by the matrix material.

Table 4

Typical traverse rates for AWJM of selected composite materials [6]

Type of

FRP

t = 0.79

1.6

3.18

Thick (t)

mm

6.36

12.7

19.1

50.8

plastics&

FRP

u = 53 38 29 21 15 10 2.5

G/Ep u = 105 95 76 42 17 12 5

Gr/Ep u = 74 63 52 40 15 10 4.2

Recent advances in machining of polymeric composite materials by AWJM

Table 5 shows the optimum traverse rates for cutting of two different thicknesses of CFRP

based on optimum edge quality which results in a smooth edge with a minimum kerf taper. The

optimum traverse rate in terms of edge quality is about two thirds that of the maximum

possible traverse rate.

Table 5

Optimum traverse rates for CFRP [9]

P = 690 Mpa S = 2 mm dm = 3 mm Abrasive : Zirconia &m = 33 g/s

Workpiece thickness

mm

Traverse rate mm/s

2 17

8 5.8

It must be noted that the depth of cut is closely connected with the hardness of the abrasive as

well as the workpiece hardness as shown in Fig. 4. It is clear from the figure that the depth of

cut increases with increasing the abrasive grain hardness. The more brittle the workpiece is, the

greater the depth of cut. Beyond 2000 HV, no significant increase in the depth of cut could be

Abrasive grain hardness (HV)

Fig. 4 Effect of abrasive grain hardness on the depth of cut [30]

obtained [30].

4. SURFACE QUALITY

Surface quality of AWJ machined composites has drawn greater attention from most of the

researchers although they are few. Hashish [6] conducted a comprehensive pioneering research

Ashraf I. HASSAN, Jan KOSMOL, Jerzy BURSA

work on the surface quality and productivity of some composite materials including fibre

reinforced plastics, such as carbon epoxy and fibre reinforced thermoplastic composites. It was

found that the matrix material is more influential on the surface quality than the reinforcement

material to the contrary of machining using conventional processes. Removal rates were found

to be larger than many of the other machining processes, but the dimensional accuracy was still

low. Table 6 lists the different parameters representing the surface quality which were reported

in different studies.

Table 6

Surface quality of AWJ machined workpieces

Surface

feature

Material Type of analysis Authors

kerf taper Gr/Ep experimental

experimental

Arola & Ramulu [19]

Ramulu & Arola [2]

kerf width Gr/Ep

CFRP

Theor/exp

experimental

Arola & Ramulu [19]

König & Wulf [17]

surface

roughness

Gr/Ep

CFRP

experimental

experimental

Ramulu & Arola [2]

König & Wulf [17]

delamination Gr/Ep theoretical Ho-Cheng [20]

4.1 Kerf width and taper

A preliminary experimental work on WJM of CFRP was conducted by König and Wulf

[17]. It was shown that the stand off distance has a great influence on the interance width of

cut. As the stand off distance increases, the entrance width of cut also increases. On the other

hand, it has little influence on the exit width of cut. Increasing the pressure from 300 MPa to

400 MPa has a slight effect. A similar effect was found by the same authors [15] to apply to a

hybrid laminate of AFRP/GFRP.

A characteristic feature associated with AWJM of composite materials is the taper of the

kerf from jet entrance to the jet exit. Kerf geometry may not only limit the ability to meet

component tolerances, affecting lay-up and fastening of joints, but may also adversely influence

the strength and service life of component parts [19]. Figure 5 shows the kerf geometry as it is

produced by AWJM. A model for predicting the kerf taper (TR), which is defined as the ratio

of the entrance kerf width (Wt) to the exit kerf width (Wb), in graphite/epoxy composite, which

is the most attracting type of composites for researchers, has been developed by Ramulu and

Arola [2] and can be used for determining cutting parameters for tailored surface quality. Kerf

widths were measured using an optical microscope. Afterwards, this model was extended for

Recent advances in machining of polymeric composite materials by AWJM

Fig. 5 Kerf Geometry schematic [19]

kerf profile prediction and the effect of cutting parameters on kerf characteristics could be

obtained using analysis of variance techniques [19]. The effect of AWJM conditions on the kerf

taper is shown in Fig. 6. Higher pressures significantly reduce the kerf taper over the whole

depth of cut, Fig. 6 (a). It is recommended to use pressures in excess of 250 MPa for

machining of thick laminates. Large abrasives increase the initial damage zone which results in

a wider entrance kerf and in turn, a larger taper ratio, Fig. 6 (b). Increasing the traverse rate

results in a lower kerf taper, Fig. 6 (c), due to the reduction in kerf entrance width due to the

decrease in exposure time. It must be noted that the increase in the traverse rate at lower jet

pressures result in higher kerf taper ratios. It is apparent from Fig. 6 (d) that as the stand off

distance increases beyond 2 mm, the kerf taper begins to decrease due to the reduced effects of

jet expansion. Generally when higher jet pressures are used, the kerf taper increases with an

increase in the stand off distance.

Ashraf I. HASSAN, Jan KOSMOL, Jerzy BURSA

Fig. 6 Effect of AWJM parameters on kerf taper [2]

4.2 Surface texture

The surface texture that may be associated with WJM include: surface waviness, burr

formation, surface finish and lay. König and Wulf [17] found that increasing the pressure from

300 MPa to 400 MPa in WJM of CFRP would improve the surface finish of the machined

surfaces. Yet the values of surface roughness were comparatively high (up to Rt = 100 µm). As

the jet penetrates into the workpiece material, the loss of the WJ velocity causes a decrease in

the surface quality towards the exit side, Fig. 7. The variations of measured roughness values in

FRP are significantly higher than in metals due to the inhomogenious material structure. The

dependency of roughness on the angle between the fibre and feed direction in WJM is less

pronounced than other machining processes. The surface texture machined by AWJM exhibits

two distinct contributions from the process: roughness occurring at the upper portion of the

kerf, due to the micro effects of each impacting particle and waviness or striation, occurring at

the lower portion of the kerf, due to jet penetration and loss of stability as the cutting depth

Recent advances in machining of polymeric composite materials by AWJM

increases. König et al [18] carried out a comparison between WJM and AWJM, for two FRP

Traverse rate u (mm/s)

Fig. 7 Surface roughness of kerf wall cut by WJM [13]

Stand off distance S (mm)

Fig. 8 Comparison between WJM and AWJM regarding surface roughness [18]

materials i.e. carbon fibre/epoxy and SMC, regarding the surface roughness for the same

cutting conditions. It is apparent from Fig. 8 that the surface roughness decreases significantly

in case of AWJM than in WJM due to the high cutting ability of the the entrained abrasives.

Ashraf I. HASSAN, Jan KOSMOL, Jerzy BURSA

Also high traverse rates, 20 times higher than WJM in Fig. 8, could be used and hence, higher

productivity is available.

It must be noted that the influence of the cutting parameters on the surface roughness

changes as a function of the cutting depth. Figure 9 shows the effect of AWJ machining

parameters on the surface roughness obtained as a result of regression models. As shown in fig.

9 (a), the pressure has a more predominant influence on the surface roughness in the

deformation wear zone than in the cutting wear zone. It must be noted that higher pressures

provide high energy to the abrasives and prolong their capacity for material removal and hence,

the surface roughness decreases. Increasing the supply pressure would also extend the cutting

wear region. Under a supply pressure of 240 MPa, a 12 mm thick laminate could be

successfully machined within 5 µm of surface roughness. The effect of abrasive grain size on

the surface roughness is shown in Fig. 9 (b). The grain size in general shows a predominant

influence on the surface roughness throughout the cutting depth. Wear tracks resulting from

stray particles on the perimeter of the jet increase in depth and width with larger grain sizes,

hence reducing the surface quality. It is apparent that the surface roughness could be minimized

at low depths of cut by decreasing the abrasive grain size. However, smaller abrasives lose

Fig. 9 Effect of AWJM parameters on the surface roughness [2]

quickly the ability of material removal with penetration depth due to lower kinetic energy of

the smaller mass abrasives. The traverse rate has little effect on both the initial damage zone

Recent advances in machining of polymeric composite materials by AWJM

and the cutting wear zone, Fig. 9 (c). Its effect becomes more prominent in the deformation

wear zone due to the formation of the waviness patterns which is attributed to severe jet

deflection as the traverse rate increases. The effect of stand off distance is more predominant in

the initial damage zone, Fig. 9 (d), due to its effect on the coherency of the jet prior to

impingement on the workpiece surface. Increasing the stand off distance results in increasing

the jet diameter and in turn reduces the energy density of the jet. The minimum surface

roughness appears to occur at a stand off distance of 2 mm.

It was observed that AWJM of FRP produces a striated surface just as the surfaces

produced in ductile metals. This waviness increases with the depth of cut and the largest degree

of waviness in the deformation wear zone is observed in workpieces machined with a

combination of low pressure, small abrasive grain size and high traverse rate [2]. Later, Arola

and Ramulu [19] used regression techniques in developing mathematical models for the initial

damage zone, and for waviness height on the kerf wall within deformation wear zone. Table 7

shows the results of surface waviness for some composites.

Table 7

Surface waviness and corresponding traverse rates for graphite/epoxy composite [6]

P = 370 MPa dn = 0.299 mm dm = 0.762 mm Abrasive: garnet Mesh No : 80

Type of

FRP

W = 1.9

2.5

Waviness

(W) m

3.8

5

6

8

Gr/Ep

t =3.18 mm

u = 4 8 12 20 30

Gr/Ep

t=18.5 mm

u = 0.6 0.85 1.7 2.5 3.4 4.25

4.3 Surface integrity

Delamination is usually observed when machining FRP using AWJM. It only occurs if the

deformation wear mode of erosion exists. The impact of the jet on a step formed at a certain

depth will result in water deflection which, if with sufficient pressure, will laterally penetrate

between layers to cause delamination. This mechanism of delamination occurs if the abrasive

flow rate is interrupted during cutting [31]. Ramulu and Arola [2] found out that fibre pullout

and fibre/matrix delamination are limited due to high interfacial bond strength coupled with the

localized cutting forces associated with AWJ. Nearly all the matrix remains intact in its

supportive position adjacent to the fibres after AWJM. The mechanism of delamination in

WJM was studied in detail by Ho-Cheng [20] using fracture mechanics and the optimum

waterjet pressure for no delamination is now predictable. This pressure could be applied in a

control strategy for maximizing productivity. In waterjet drilling of composite laminates, there

is a thrust jet force acting perpendicularly to the laminate that show flexural bending in

Ashraf I. HASSAN, Jan KOSMOL, Jerzy BURSA

response and the laminate tends to be pushed away. At a certain point the jet force causes such

a deformation that the interlaminar bond strength can no longer hold and delamination around

the exit hole occurs. This happens before the laminate is completely pierced by the waterjet as

shown in Fig. 10. As soon as an interlaminar crack is created, water is pressed into the opening

Fig. 10 Waterjet-induced delamination at exit [24]

causing damage around the hole. Nozzle diameter and pressure affect delamination by changing

the magnitude of the jet force. The waterjet pressure should not exceed a critical value to avoid

delamination. Many idealizations of HoCheng's model are the main reason for significant

inaccuracies of the results. Hashish [6] found that the pressure plays an important role in the

delamination process while drilling Gr/Ep composite using the piercing method. When reducing

pressure from 345 MPa to 240 MPa, hole delamination at the bottom was significantly

reduced. This is not always the proper strategy, but the pressure must be selected so that the

lateral flow does not cause delamination.

5. TYPES OF AWJM PROCESSES

Most of researchers have been working in AWJ slotting [2,19]. This could be performed

either in linear cutting or contour cutting using a robot or a CNC control. A limited number of

attempts were made in an effort to apply AWJ to other types of machining processes. An early

attempt to apply AWJ to milling of FRP was conducted by Hashish [25]. The preliminary

results were encouraging in providing reasonable metal removal rates. But on the other hand,

controlling the depth of cut was a difficult problem and an optimum machining strategy was

needed to maximize metal removal rates and minimize the surface texture effects. Small

diameter holes in the range of 0.5 mm could be drilled [10]. Another attempts [6,20] were

aiming at applying AWJ to drilling. While a limited success was observed for AWJ turning of

metal matrix composites [32,33]. However, no data were reported for its application to FRP.

Table 8 shows the different machining processes used in the AWJM process.

Recent advances in machining of polymeric composite materials by AWJM

Table 8

Different types of AWJM processes

AWJM process Material Limitations Remarks Authors

Slot cutting Arola & Ramulu

[19]

Ramulu & Arola

[2]

Drilling

Gr/Ep

Gr/Ep

fracture,

cracking or

delamination at

high pressures

(200-400) MPa

burr formation

or chipping at

the bottom of

hole at high

pressures e.g.

350 MPa

AWJ piercing

AWJ piercing

Hocheng [20]

Hashish [6]

Hashish [6]

Milling kerfs had

irregular depths,

delamination

depth of cut

controlled by u,

S and jet angle

Hashish [6]

6. NOZZLE WEAR

A new gradually contracting conical WJ nozzle was designed by König and Wulf [34]. This

new design was found to produce better kerf width and kerf taper than the conventional

cylindrical nozzle while machining of glass/kevlar reinforced epoxy. Also the dynamic cutting

forces were reduced. No data has been reported in the literature about the nozzle wear in case

of AWJM of composite materials. This area needs to be explored if more economical

machining is required.

7. COMPARISON WITH ALTERNATIVE MACHINING PROCESSES

In any comparison with other machining processes, the required post-cutting finishing

operations, often encountered in thermal cutting, are an additional cost item that may not be

needed if AWJM is used [25]. Although it is faster, laser cutting of composite materials does

Ashraf I. HASSAN, Jan KOSMOL, Jerzy BURSA

not prove advantageous. For example, laser cutting of CFRP causes the material to burn as

well as delamination may occur as a result of the difference in thermal conductivity between the

fibre and the resin. GFRP could not also be machined by laser unless very low traverse rates

are utilized [12]. A comparison between AWJM and different types of non conventional

machining processes, such as plasma arc machining (PAM), laser beam machining (LBM),

electro chemical machining (ECM), electrical discharge machining (EDM), ultrasonic

machining (USM), abrasive jet machining (AJM) and finally electron beam machining (EBM),

is shown in Table 9.

Table 9

Comparison of AWJM with other machining processes [25]

Machining

process

Surface

roughness

(m)

Volume

removal

rate

(mm3/s)

Power requirements

(KW)

AWJM 1-10 50-200 10-70

PAM NA 1000 200

LBM 0.8-6 NA 4000V& 3000J

ECM 0.4-6 35/1000

Amp

30V&10000Amp

EDM

USM

AJM

EBM

3-12

0.3-0.8

NA

0.8-6

7

4-60

up to 1

up to 5

20 Amp & 400V

NA

2-5

130 V &5000 Amp

While EDM and ECM are only suitable for electrically conductive materials and require high

tooling costs, EDM is characterized by its slow volume removal rates and ECM is

characterized by its high capital costs. On the other hand, USM, AJM and LBM are

characterized by low volume removal rates. While, PAM and LBM are characterized by

thermal distortion of the machined surfaces.

8. APPLICATIONS OF AWJM OF FRP IN THE POLISH INDUSTRY

With the ability to cut multi layered composites, AWJM offers a considerable industrial

potential for the future for many sectors in the Polish industry. Table 10 shows the potential

applications of AWJM for different industrial components made from composites.

Table 10

Applications of AWJM in the Polish industry

Industrial sector Examples of componets

Recent advances in machining of polymeric composite materials by AWJM

Automobile body panels, sopiler,

chassis, leaf springs

Marine recreational boats

Aircraft landing gear leg,

helicopter roofs, airplane

body

CONCLUSIONS

AWJM has been successfully applied in the machining of polymeric composite materials

whenever several conventional and non conventional machining processes are not technically

or economically feasible due to some reasons. As a result of the present review, the following

conclusions could be drawn:

1. AWJM is significantly more dependent on the matrix material than on the reinforcing fibres

to the contrary of conventional machining processes in which it depends on fibres. The

material removal mechanism in Graphite/Epoxy through the whole depth of kerf occurs by

brittle fracture in the form of abrasive induced shear loading and micro machining. Three

distinct cutting zones are found on AWJM surfaces, initial damage zone, cutting wear zone

and deformation wear zone. This is similar to AWJM of ductile materials.

2. AWJM is faster than many machining processes but the dimensional accuracy is still low.

3. Abrasive grain size and stand off distance are the most influential parameters affecting

surface roughness at shallow depth of cut. whereas the effects of abrasive grain size,

traverse rate and pressure are more predominant in the deformation wear zone. Kerf width

and taper for thin laminates, less than 5 mm, is mainly dependent on stand off distance.

Waviness or striation could be reduced with combinations of high pressures, large abrasive

particle size and moderate traverse rates. Generally, for obtaining better surface quality,

regarding surface finish and kerf taper, it is recommended to use pressures over 250 MPa. It

must be noted that the proper choice of machining parameters for optimizing the surface

quality is more dependent on the depth of cut. There are optimal range of parameters for

machining polymer matrix composites to avoid shredding, splintering and delamination.

REFERENCES

1. Mallick, P. K., Newman, S.: Composite Materials Technology, first edition, Germany,

Hanser Publishers, 1990.

Ashraf I. HASSAN, Jan KOSMOL, Jerzy BURSA

2. Ramulu M., Arola D.: The influence of Abrasive Waterjet Cutting Conditions on the

Surface Quality of Graphite/Epoxy Laminates, International Journal of Machine Tools and

Manufacture, No. 3, Vol. 34, 1994, pp. 295.

3. Savrun E., Taya M.: Surface Characterization of SiC Whisker/2124 Aluminum and Al2O3

Composites Machined by Abrasive Waterjet, Journal of Materials Science, Vol. 23, 1988,

pp. 1453.

4. Hamatani G., Ramulu M.: Machinability of High Temperature Composites by Abrasive

Waterjet, Transactions of the ASME, Journal of Engineering Materials and Technology,

Vol. 112, Oct. 1990, pp. 381.

5. Ramulu M., Raju S. P., Inove H., Zeng J.: Hydro-Abrasive Erosion Characteristics of 30

vol % SiC/6061-T6 Al Composite at Shallow Impact Angles, Wear, No. 1, Vol. 166, 1993,

pp. 55.

6. Hashish M.: Machining of Advanced Composites with Abrasive-Waterjets, Winter Annual

Meeting of the American Society of Mechanical Engineers, PED-Vol. 35, MD-Vol. 12,

1988, pp. 1.

7. Kulpa, C.:Hochdruck-Wasserstrahlschneiden von Kuntostoffen: Souveräne Technologie,

Schweitz Maschinenmarkt, No. 6, 1995, pp.12.

8. Henning A., Anders S.: Integration der Wasserstrahltechnik in die Fertigung, VDI-Z, No.

3/4, Vol. 140, 1998, pp. 54.

9. Harris I. D. : Abrasive Waterjet Cutting and Its Applications at TWI (UK), Welding in the

World, No. 4, Vol. 33, 1994, pp. 277.

10.Hashish M.: Cutting with Abrasive Waterjets, Mechanical Engineering, Vol. 106, March

1984, pp. 60.

11.Hassan A. I., Kosmol J.: An Overview of Abrasive Waterjet Machining (AWJM), Prace

Naukowe Katedry Budowy Maszyn, Politechnika Slaska, Gliwice, No. 2, 1997, pp. 181.

12.Hoogstrate A. M., et al : Opportunities in Abrasive Water-Jet Machining, Annals of the

CIRP, No. 2, Vol. 46, 1997, pp. 697.

13.König W., Wulf Ch., Grab P., Wilershied H.: Machining of Fibre Reinforced Plastics,

Annals of the CIRP, No. II, Vol. 34, 1985, pp. 537.

14.Oweinah H.: Hochgeschwindigkeits-Wasserstrahlen (Fluid-Jet-Cutting), Industrie Anzeiger,

No. 35, Vol. 106, 1984, pp. 47.

15.König W., Wulf Ch.: Ein sauberes Trennverfahren für Faserverbundwerkstoffe, Industrie

Anzeiger, No. 56, Vol. 105, 1983, pp. 32.

16.Engemann B. K. : Water Jet Cutting of Fibre Reinforced Composite Materials, Industrial &

Production Engineering, No. 3, 1981, pp. 162.

17.König W., Wulf Ch.: Kolenstoffaserverstärkte Kunststoffe mit Hochdruck Wasserstrahl

bearbeiten, Industrie Anzeiger, No. 49, Vol. 104, 1982, pp. 84.

18.König W., Trasser, Fr.-J., Schmelzer M.: Bearbeitung fasserverstärkter Kunststoffe mit

Wasser und Laserstrahl, VDI-Z, No. 11, Vol. 129, 1987, pp. 6.

Recent advances in machining of polymeric composite materials by AWJM

19.Arola D., Ramulu M.: A study of Kerf Characteristics in Abrasive Waterjet Machining of

Graphite/Epoxy Composite, Transactions of the ASME, Journal of Engineering Materials

and Technology, Vol. 118, Apr 1996, pp. 256.

20.Ho-Cheng H.: A Failure Analysis of Water Jet Drilling in Composite Laminates,

International Journal of Machine Tools and Manufacture, No. 3, Vol. 30, 1990, pp. 423.

21.Pool K.V., Dharan C.K., Finnie I.: Erosive Wear of Composite Materials", Wear, Vol. 107,

1986, pp.1.

22.Zahavi J., Schmitt Jr., G. F.: Solid Particle Erosion of Reinforced Composite Materials,

Wear, Vol 71, 1981, pp. 179.

23.Tilly G.P., Sage W.: The interaction of Particle and Material Behavior in Erosion Processes,

Wear, Vol. 16, 1970, pp.447.

24.Finnie I.: Erosion of Surfaces by Solid Particles, Wear, No. 3, Vol. 3, 1960, pp. 87.

25.Hashish M.: An Investigation of Milling with Abrasive-Waterjets, Transactions of the

ASME, Journal of Engineering for Industry, Vol. 111, May 1989, pp. 158.

26.Hashish M.: A Modeling Study of Metal Cutting with Abrasive Waterjets, Transactions of

the ASME, Journal of Engineering Materials and Technology, No. 1, Vol. 106, 1984, pp.

88.

27.Hashish M.: Visualization of the Abrasive-Waterjet Cutting Process, Experimental

Mechanics, June 1988, pp. 159.

28.Hashish M.: A Model for Abrasive-Waterjet (AWJ) Machining, Transactions of the ASME,

Journal of Engineering Materials and Technology, Vol. 111, Apr 1989, pp. 154.

29.Machinability Data Handbook, Vol II, Third edition, Cincinnatti, USA, Machinability Data

Center, 1980.

30.Faber K., Oweinah H.: Influence of Process Parameters on Blasting Performance with the

Abrasive-Jet, Proceedings of the 10th International Conference on Jet Cutting Technology,

1991, pp. 365.

31.Hashish M. : Characteristics of Surfaces Machined with Abrasive-Waterjets, Transactions

of the ASME, Journal of Engineering Materials and Technology, Vol. 113, July 1991, pp.

355.

32.Ansari A. I., Hashish M.: Effect of Abrasive Waterjet Parameters on Volume Removal

Trends in Turning, Transactions of the ASME, Journal of Engineering for Industry, Vol.

117, Nov 1995, pp. 475.

33.Hashish M.: Turning with Abrasive Waterjets- A First Investigation, Transactions of the

ASME, Journal of Engineering for Industry, Vol. 109, Nov. 1987, pp. 281.

34.König W., Wulf Ch.: Düsen mit stetiger Geometrie: Wasserstrahlschneiden, Industrie

Anzeiger, No. 92, Vol. 106, 1984, pp. 3.