Mechanical and abrasive wear characterization of bidirectional and chopped E-glass fiber reinforced composite materials

Materials and Design 35 (2012) 467–479

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Mechanical and abrasive wear characterization of bidirectional and chopped E-glass fiber reinforced composite materials Siddhartha ⇑, Kuldeep Gupta 1 Department of Mechanical Engineering, National Institute of Technology, Room No. 207, Hamirpur, H.P., India

a r t i c l e

i n f o

Article history: Received 17 July 2011 Accepted 4 September 2011 Available online 14 September 2011

a b s t r a c t Bi-directional and chopped E-glass fiber reinforced epoxy composites are fabricated in five different (15, 20, 25, 30 and 35) wt% in an epoxy resin matrix. The mechanical characterization of these composites is performed. The three body abrasive wear behavior of fabricated composites has been assessed under different operating conditions. Abrasive wear characteristics of these composites are successfully analysed using Taguchi’s experimental design scheme and analysis of variance (ANOVA). The results obtained from these experiments are also validated against existing microscopic models of Ratner-Lancaster and Wang. It is observed that quite good linear relationships is held between specific wear rate and reciprocal of ultimate strength and strain at tensile fracture of these composites which is an indicative that the experimental results are in fair agreement with these existing models. Out of all composites fabricated it is found that tensile strength of bi-directional E-glass fiber reinforced composites increases because of interface strength enhancement. Chopped glass fiber reinforced composites are observed to perform better than bi-directional glass fiber reinforced composites under abrasive wear situations. The morphology of worn composite specimens has been examined by scanning electron microscopy (SEM) to understand about dominant wear mechanisms. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Composites are advanced materials constituting of two or more chemically distinct constituents on a macro-scale, having a distinct interface separating them. One or more discontinuous phases are, therefore, embedded in a continuous phase to form a composite. In most of the situations the discontinuous phase is usually harder and stronger than the continuous phase and is called the reinforcement, whereas, the continuous phase is termed as the matrix. The matrix material can be metallic, polymeric or can even be ceramic. When the matrix is a polymer, the composite is called polymer matrix composite (PMC). The reinforcing phase can either be fibrous or non-fibrous (particulates) in nature. The fiber reinforced polymers (FRP) consist of fibers of high strength and modulus embedded in or bonded to a matrix with distinct interface between them. In this form, both fibers and matrix retain their physical and chemical identities. In general, fibers are the principal load bearing members while the matrix places them at the desired location and orientation, acts as a load transfer medium between them, and protects them from environmental damages [1]. ⇑ Corresponding author. Tel.: +91 1972 254744, mobile: +91 9816016194; fax: +91 1972 223834. E-mail addresses: [email protected], [email protected] ( Siddhartha). 1 Tel.: +91 9457964939. 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.09.010

In past years, because of fairly good strength, low density, and high performance/cost ratios with rapid clean processing, tremendous growth in the developments and applications of fiber reinforced thermo-setting polymer composites such as epoxy, polyester and vinyl ester have been observed. Polymer and their composites are used in variety of industrial applications such as bearing material, rollers, seals, gears, cams, wheels, clutches and transmission belts [2–5]. Therefore, the mechanical and tribological behavior of these materials should be studied systematically. In the situations where the wear performance in nonlubricated conditions is a key parameter for material selection, polymer composites are used in mechanical components [6,7] and these components are used in various types of wear situations. Among various types of wear, abrasive wear situation occurs in numerous equipments such as vanes and gears, in pumps handling industrial fluids, sewage and abrasive-contaminated water, chute liners abraded by coke, coal and mineral ores; bushes and seals in agricultural and mining equipment, thus have been received increasing attention [8]. Carbon, glass, aramide and graphite fibers are most common fibers used for reinforcement in polymer matrix composites [9–11]. It is evident from the literature that in general, the short fiber reinforcement led to the deterioration in the abrasive wear resistance of the matrix [12] while on the other hand reinforcement of the fabric improved the abrasion resistance of the polymers [13]. That is why the bi-directional fabric reinforcement

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offers a unique solution to the advanced materials in terms of better performance and ease in processing [14]. Three body abrasive wear behavior of polymer composites have been reported by many researchers [15–18]. Stachowiak and Stachowiak [19] while studying the effects of particle characteristics specially shape and hardness on three body abrasive wear of metallic samples, concluded that rounded particles generated round craters and smooth grooves while angular particles produce sharp indents and narrow cutting grooves. The present work is undertaken for assessing the wear behavior of bi-directional E-glass fiber and chopped E-glass fiber reinforced epoxy composites under abrasive situations. The mechanical characterization of these composites is also performed so as to have an insight about this aspect. An economical and viable experimental strategy based on Taguchi’s parameter design has been used to analyse the effect of various parameters and their interactions. This experimental procedure has been successfully applied earlier for solid particle erosion behavior and dry sliding characteristics of polymer–matrix composites [20,21].

2. Experimental details

2.2. Abrasive wear test To evaluate the performance of composites under three body abrasion conditions, wear tests are carried out as per ASTM G 65 [22] using the dry abrasion test rig (TR-50) supplied by DUCOM Ltd. The dry sand/rubber wheel (Dia 228.6 mm, Hardness durometer A-60) abrasion test involves the abrading of test specimen with a grit of controlled size and composition. The abrasive is introduced between test specimen and a rotating wheel with a chlorobutyl rubber tyre. The test specimen is pressed against a rotating wheel at a specified force by means of a lever arm while a controlled flow of grit abrades the test surface. The test duration and force applied by the lever arm is varied. The specimens are weighed before and after the test and loss in mass is recorded. Due to wide difference in material density abrasion are reported on volume loss basis as.

W s ¼ DM=q  L  F N

ð1Þ

where DM is the mass loss in the test duration in grams (gm), q is the density of the composite (gm/cm3), L is the sliding distance (m) and FN is the normal load (N). The specific wear rate is defined as the volume loss of the specimen per unit sliding distance per unit applied normal load.

2.1. Panel preparation 2.3. Mechanical characterization Bi-directional E-glass fiber and chopped E-glass fibers are reinforced separately in epoxy resin to prepare the fiber reinforced composites B1–B5 and C1–C5. The composition and designation of the composites prepared for this study are listed in Table 1. The fiber material is mixed with Epoxy LY 556 resin in five different percentages (15 wt%, 20 wt%, 25 wt%, 30 wt% and 35 wt%). The fabrication of the composite slabs is done by conventional hand-layup technique followed by light compression molding technique. The fibers are mixed thoroughly in the epoxy resin. The low temperature curing epoxy resin and corresponding hardener (HY951) are mixed in a ratio of 10:1 by weight as recommended. The epoxy resin and the hardener are supplied by Ciba Geigy India Ltd. The bidirectional E-glass fiber, chopped E-glass fiber and the epoxy resin possess Young’s modulus of 72.5 GPa, 72.5 GPa and 3.42 GPa respectively and a density of 2600 kg m3, 2500 kg m3 and 1200 kg m3 respectively. Each ply of fiber is of dimension 200  200 mm2. A wooden mold having dimensions of 210  210  40 mm3 is used. A releasing agent (Silicon spray) is used to facilitate easy removal of composites from the mold after curing. The cast of each composite is cured under a load of about 50 kg for 24 h before it is removed from the mold. After this the cast is post cured in the air for another 24 h after removing out of the mold. Specimens of suitable dimensions are cut using a diamond cutter for physical/mechanical characterization and abrasive wear testing. Utmost care has been taken to maintain uniformity and homogeneity of the composites.

Table 1 Designations and detailed compositions of the composites. Designation

Composition

B1 B2 B3 B4 B5 C1 C2 C3 C4 C5

Epoxy Epoxy Epoxy Epoxy Epoxy Epoxy Epoxy Epoxy Epoxy Epoxy

(85 wt%) + bi-directional E-glass fiber (15 wt%) (80 wt%) + bi-directional E-glass fiber (20 wt%) (75 wt%) + bi-directional E-glass fiber (25 wt%) (70 wt%) + bi-directional E-glass fiber (30 wt%) (65 wt%) + bi-directional E-glass fiber (35 wt%) (85 wt%) + chopped E-glass fiber (15 wt%) (80 wt%) + chopped E-glass fiber (20 wt%) (75 wt%) + chopped E-glass fiber (25 wt%) (70 wt%) + chopped E-glass fiber (30 wt%) (65 wt%) + chopped E-glass fiber (35 wt%)

The experimental density of the composites is obtained by the Archimedes principle of weighing small pieces cut from the large composite panel first in air and then in water. Theoretical density of composite is calculated and compared with experimental density in order to calculate void fraction of the composites. Hardness measurement is done using a Rockwell-hardness tester equipped with a steel ball indenter (1/1600 ) indenter by applying a load of 50 Kgf. The tensile test is performed on flat dog-bone shaped composite specimens as per ASTM D 3039-76 [23] test standards on universal testing machine (UTM) Hounsfield H25KS. The flexural and inter laminar shear strength test is conducted as per ASTM standard D2344-84 [24] using the same UTM. The low velocity instrumented impact tests are carried out on composite specimens. The tests are done as per ASTM D 256 [25] using an impact tester. At the last, the worn surfaces of some selected samples are examined by scanning electron microscope Carl Zeiss NTS GmbH, SUPRA 40VP. 2.4. Experimental design The Taguchi method is a commonly adopted approach for optimizing design parameters. Taguchi method provides the designer with a systematic and efficient approach for experimentation to determine near optimum settings of design parameters for performance, quality and cost [26–29]. Since experimental procedures are generally expensive and time consuming, the need to satisfy the design objectives with the least number of tests is clearly an important requirement. Exhaustive literature review reveals that parameters viz., RPM, fiber loading, normal load, sliding distance, and abrasive size largely influence the abrasive wear characteristics of polymer composites. Thus, the impact of five parameters are studied using L25 (56) orthogonal design [30]. The control factors and the parameter settings for wear test (given in Tables 2 and 3) present the selected levels for various control factors. The array chosen in this work is the L25 (56) which has 25 rows corresponding to the number of tests (24 degrees of freedom) with six columns at five levels, the factors are assigned to the columns. The plan of the experiments is as follows: the first column is assigned to RPM (A), the second column to Fiber loading (B), the third column to Normal load (C), fourth column to Sliding distance

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Table 4 Composite designations and their experimental and theoretical densities.

Control factors

Symbols

Sliding speed Fiber loading Normal load Sliding distance Abrasive size

Factor Factor Factor Factor Factor

Composite designation

A B C D E

B1

B2

A: Sliding speed B: Fiber loading C: Normal load D: Sliding distance E: Abrasive size

Levels

B3

I

II

III

IV

V

Units

0.48 15 2.5 50 125

0.72 20 5 60 210

0.96 25 7.5 70 355

1.20 30 10 80 420

1.44 35 12.5 90 600

m/S % Kgf M lm

B4

B5

(D) and the fifth and column is assigned to Abrasive size (E) respectively and the remaining columns are used to estimate experimental errors. In practice, these factors can be assigned arbitrarily to any of the arrays columns, provided that all combinations are included. After assigning appropriate level settings, the S/N analysis (S/N: signal-to-noise ratio) is needed to evaluate experiment results. In S/N analysis, the greater the S/N, the better the experimental results:

g ¼ 10 logðM:S:D:Þ

ð2Þ

where M.S.D. is the mean-square deviation for the output characteristic (specific wear rate). As mentioned earlier, there are three categories of quality characteristics, i.e. lower-the-better, higher-the-better, and nominalthe-better. To obtain optimal performance, lower-the-better characteristic for wear rate must be taken. The mean-square deviation (M.S.D.) for the lower-the-better characteristic can be expressed as [30]:

S 1 X 2  ¼ 10 log y N n

Experimental density (de) (gm/cm3)

Theoretical density (dt) (gm/cm3)

Void fraction (%) e V f ¼ dt dd t

Table 3 Levels for various control factors. Control factor

Composites compositions

ð3Þ

where n the number of observations, y the observed data. Further ANOVA is performed to find out the significant process parameters. With the help of S/N ratio and ANOVA analysis, the optimal combination of the process parameters can be predicted. At the end, a confirmation experiment is conducted to verify the optimal process parameters obtained from the parameter design. 3. Results and discussion 3.1. Mechanical properties In the present research work, the theoretical and measured densities of Bidirectional E-glass fiber–epoxy and chopped E-glass– epoxy composites, along with the corresponding volume fraction of voids are presented in Table 4. It is found that the composite density values calculated theoretically from weight fractions are not equal to the experimentally measured values, as expected. It is evident from Table 4 that the density of Bidirectional E-glass fiber–epoxy composites increase with the fiber content. Similar trends are noticed for the chopped E-glass–epoxy composites as well. The increase in density indicates that particle breakage may not have any significant influence on the composites. It is believed to achieve an improvement of the bonding between the particle

C1

C2

C3

C4

C5

Epoxy + bidirectional Eglass fiber (15 wt%) Epoxy + bidirectional Eglass fiber (20 wt%) Epoxy + bidirectional Eglass fiber (25 wt%) Epoxy + bidirectional Eglass fiber (30 wt%) Epoxy + bidirectional Eglass fiber (35 wt%) Epoxy + chopped E-glass fiber (15 wt%) Epoxy + chopped E-glass fiber (20 wt%) Epoxy + chopped E-glass fiber (25 wt%) Epoxy + chopped E-glass fiber (30 wt%) Epoxy + chopped E-glass fiber (35 wt%)

1.3055

1.3071

0.1224

1.3230

1.3586

2.6203

1.3453

1.3869

2.9994

1.4054

1.4356

2.1036

1.4459

1.4814

2.3963

1.1801

1.3020

9.3625

1.1913

1.3333

10.6502

1.2444

1.3793

9.7803

1.2566

1.4224

11.6563

1.2058

1.4684

17.8834

and matrix. The porosities of composites are evaluated from the difference between the expected and the observed density of each sample. The variations of porosity level in these composites are also shown in Table 4. This table indicates that increasing amount of porosity is observed with increasing the volume fraction, especially for low particle sizes of composites, because of the decrease in the inner-fiber spacing. In this work, bidirectional E-glass fiber reinforced composites show lesser void fraction than that of chopped E-glass fiber reinforced composites, as void fraction also depends on type of fiber used. The variation in hardness of bidirectional E-glass fiber reinforced epoxy and chopped E-glass fiber reinforced epoxy composites with fiber content is represented in Fig. 1a. It can be observed from the Fig. 1a that the hardness of the bi-directional E-glass fiber–epoxy composites is improved and this improvement is a function of the fiber content. This trend of improvement of hardness with fiber content is also observed in case of the chopped E-glass–epoxy composites up to 30 wt% fiber content but for 35 wt% of fiber content in case of chopped E-glass fiber a slight dip in the value of hardness is observed. The reason behind such behavior is of two fold. This dip in the hardness value at 35 wt% of chopped E-glass fiber occurred due to improper distribution of fiber into the matrix and presence of large void fraction in 35 wt% chopped E-glass fiber–epoxy composite (Table 4). This improper distribution of chopped E-glass fiber composites can be contributed to anisotropic nature of chopped E-glass fiber. As far as the comparison between the composites with bidirectional E-glass fiber and chopped E-glass fiber reinforcement is concerned,

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65

Glass Fiber Chopped

Tensile Modulus (GPa)

55 50

Hardness (HRB)

Glass Fiber Bi directional

1.6

Glass fiber Bi Directional Glass Fiber Chopped

60

45 40 35 30 25 20

1.4 1.2 1.0 0.8 0.6

15 10

0.4

5

15

0

20

25

30

35

Fiber Loading (wt %) 10

15

20

25

30

35

40 Fig. 1c. Variation of tensile modulus of the composites with fiber loading.

Fiber Loading (wt%) Fig. 1a. Variation of hardness of the composites with fiber loading.

140

Tensile strength (MPa)

50

Glass Fiber Bi directional Glass Fiber Chopped

Flexural strength (MPa)

55 45 40 35 30 25 20

Glass Fiber Bi Directional Glass Fiber Chopped

120 100 80 60 40

15

20

10 5 15

20

25

30

35

Fiber Loading (wt %)

15

20

25

30

35

Fiber Loading (wt %)

Fig. 1b. Variation of tensile strength of composites with fiber loading.

Fig. 1d. Variation of flexural strength of composites with fiber loading.

the bidirectional E-glass–epoxy composites exhibit superior hardness values. Among all the composites under this investigation, the maximum hardness value 59.67 HRB is recorded for bidirectional E-glass fiber–epoxy composite at 35 wt%. The variation of tensile strength of both the bidirectional E-glass–epoxy and chopped E-glass–epoxy composites with fiber content is presented in Fig. 1b. Little improvement in tensile strength for the bidirectional E-glass fiber–epoxy composites is observed when the fiber content was increased from 15 wt% to 20 wt%. However, with the incorporation of 25 wt% and 30 wt% of the fiber a significant enhancement of 41.94% and 25.10% in tensile strength is observed with respect to sequential enhancement of fiber loading. On incorporation of 35 wt% fiber again a marginal improvement is observed. Similar behavior is also reported by Li [31] in case of carbon fibers, in their research they reported that on addition of fibers mechanical properties are greatly improved but when the content of carbon fiber is greater than 30% there is marginal improvement which is in fair accordance with the observations recorded during this research work. Such a peculiar behavior of bidirectional E-glass fiber–epoxy composites may be attributed to the reason that the strength of glass fiber is strongly dependent on surface defects [32]. Once a surface flow is covered with a surroundings epoxy matrix the stress concentration causing the flow is reduced. Incorporation of fiber into epoxy matrix creates this condition, i.e. the bonding between the fiber surface and epoxy matrix protects against stress concentration at the surface defect and thus increases the strength. Again nominal improvement at higher fiber loading in tensile strength may be because

of insufficient wetting of epoxy into fiber and poor fiber matrix adhesion. At higher volume fraction many fiber ends in unit volume which leads to higher stress concentration which in turn results into crack propagation at finite localized region and the region is not able to sustain the applied tensile stress. The similar observations have been reported by Thomason et al. [33]. Contrary to this, in chopped E-glass fiber–epoxy composites an unorderly distribution in the tensile strength can be attributed to the nonuniform and anisotropic nature of glass fiber in the composite. Similar observations were reported by other researchers [34,35] and the decline in strength may be attributed to two reasons: a first possibility is that the due to the presence of pores at the interface between the fiber and the matrix, the interfacial adhesion may be too weak to facilitate significant stress transfer mechanism; the other is that the corner points of the irregular shaped chopped glass fiber composites act as sites of high stress concentration. Fig. 1c shows the mean tensile modulus and the mean strain at break as a function of the fiber loading. The tensile modulus is found to be an increasing function of fiber content after 20 wt% of fiber content in bidirectional E-glass fiber–epoxy composite although in case of chopped E-glass fiber composite, the trend of Tensile modulus is quite scatter. This is clear cut indication of poorer fiber distribution and impregnation. Fig. 1d shows the variation of flexural strength of both the bidirectional E-glass fiber–epoxy and chopped E-glass fiber–epoxy composites with different fiber loading. Under a flexural loading situation, a gradual improvement in flexural strength with the fiber weight fraction is noticed in both the bidirectional E-glass

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fiber–epoxy and chopped E-glass–epoxy composites. In case of chopped E-glass fiber–epoxy composite a little drop at 35 wt% is observed, this is because of improper wetting of epoxy resin in fiber and poor fiber matrix adhesion. While in the bidirectional E-glass fiber–epoxy composite an abrupt change of 229% in flexural strength of composites is observed upon the incorporation of 20 wt% fiber, this may be attributed to the presence of microcracks which in turns results in the release of stress given by flexural loading over a large area and hence increase in flexural strength. Fig. 1e shows the flexural modulus as a function of the fiber loading. The flexural modulus is found to be an increasing function of fiber

Flexural Modulus (GPa)

6.0 5.5

Glass Fiber Bi directional Glass Fiber Chopped

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 15

20

25

30

35

Fiber Loading (wt %) Fig. 1e. Variation of flexural modulus of the composites with fiber loading.

60 Glass Fiber Bi Directional Glass Fiber Chopped

55 50

content for both of bidirectional E-glass fiber–epoxy and chopped E-glass fiber–epoxy although in case of bidirectional E-glass fiber composite, the trend of flexural modulus is followed up to 30 wt%. Bijwe et al. [36] in case of aramide fabric reinforced polyethersulfone composites have reported similar observations. It is well known that bearing failure occurs in the material immediately adjacent to the contact area between the fastener and the laminate and is caused primarily by compressive stresses acting on the surface. In the present work short beam shear test is carried out on the composites with different fiber loading to determine the inter-laminar shear strength (ILSS). The variation of ILSS of bidirectional E-glass fiber–epoxy and chopped E-glass fiber–epoxy composites with fiber content is presented in Fig. 1f an improvement in ILSS values of both the bidirectional and chopped E-glass fiber–epoxy composites with increase in fiber content is recorded. Similar observations have been reported by Satapathy et al. [37] in case of jute fiber epoxy composites. Fig. 1g illustrates the measured impact strength values of the various composites under this investigation. It is seen from this figure that the impact strength of chopped E-glass fiber–epoxy composites increase gradually with increase in fiber loading, while in case of bidirectional E-glass fiber–epoxy composite little dip in impact strength is observed at the 20 wt% fiber content and then impact strength gradually increases with increase in fiber loading. This decline in impact energy indicates the lack of proper mixing of matrix and fiber and fiber agglomeration was more which enhances the tendency of fiber to act as stress concentrators within the matrix and the crack initiation sites at 20 wt%. This reason may also be attributed to the small impact strength values of chopped E-glass fiber–epoxy composites as compared to bidirectional E-glass fiber–epoxy composites.

3.2. 2. Steady state specific wear

I.L.S.S (MPa)

45 40 35 30 25 20 15 10 5 15

20

25

30

35

Fiber Loading (wt%)

14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Specific Wear Rate (mm3/Nm)

Impact Strength (KJ/m2)

Fig. 1f. Variation of inter-laminar shear strength with fiber loading.

3.2.1. Variation of specific wear rate with normal load under steady state condition for bidirectional E-glass fiber reinforced–epoxy composites Each standard test was conducted for 50 m of sliding distance, 0.48 m/s of sliding velocity and 125 lm of abrasive size. In this way easily measurable wear was obtained. Fig. 2a shows the variation of specific wear rate with normal load under steady state conditions for bidirectional E-glass fiber reinforced–epoxy composites. It is observed that at low loads 15 wt% bidirectional glass fiber reinforced composites exhibit highest wear and this is also the highest wear among all applied loads. No regular trends are observed in wear rate when the normal load is increased which is in equal accordance with established literature. The lowest wear

Glass fiber Bi Directional Glass Fiber Chopped

4.00E-02

Ws(15%) Ws(20%) Ws (25%) Ws(30%) Ws(35%)

3.50E-02 3.00E-02 2.50E-02 2.00E-02 1.50E-02 1.00E-02 5.00E-03 0.00E+00 2.5

5

7.5

10

12.5

Normal Load (Kgf) 15

20

25

30

35

Fiber Loading (wt%) Fig. 1g. Variation of impact strength of the composites with fiber loading.

Fig. 2a. Variation of specific wear rate with normal load under steady state condition for bidirectional E-glass fiber reinforced–epoxy composites (Sliding speed = 0.48 m/s, Sliding distance = 50 m, Abrasive size = 125 lm).

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rate is shown by 35 wt% bidirectional E-glass fiber reinforced composites at a normal load of 5 kgf; there is significant variation in trend of specific wear rate. 15 wt% bidirectional E-glass fiber– epoxy composite exhibit highest wear which imply that at this loading there are less fibers to support the matrix and therefore abrasive particles create large depth grooves and sever cutting mode of abrasive wear is probably a dominant wear mechanism. Contrary to it 35 wt% bidirectional E-glass fiber reinforced composites represent lowest wear which indicates that probable wear mechanism could be ploughing or wedge formation. At extreme loading conditions of 12.5 Kgf it is found that 20 wt% bidirectional E-glass fiber reinforced composites exhibit higher wear rate while again 35 wt% bidirectional E-glass fiber reinforced composites exhibit lowest wear rate. It is suggested that during abrasive wear condition 35 wt% fiber loading is quite good and composites of such composition perform better at low as well as high loading conditions. The reason that at high fiber loading wear rate is less can be attributed to the fact that there is comparatively good adhesion between the fiber and matrix. The type of reinforced fiber also affects the wear response of composites. The wear mechanisms observed for the unidirectional composite do not simply add up when going to the case of two dimensional fibers. The simultaneous occurrence of different orientation on the fiber surface seems to lead to a synergetic behavior that is wear protective performance. The phenomenon of wear debris entrapment is a vital mechanism in mitigating wear in case of bidirectional fiber reinforced composites. Nevertheless the wear debris, besides imparting the lubricating effect, also serves two other beneficial causes. On the one hand, it cushions the composite surface and reduces the load on the fiber in the normal direction of Plane of abrasion and hence the bending stresses on them. This in turn, reduces the tendency for fracture. Secondly by occupying the regions between the fibers in the normal direction, it actually protects these regions from direct wear and frictional effects [14].

3.2.2. Variation of specific wear rate with normal load under steady state condition for chopped E-glass fiber reinforced–epoxy composites Fig. 2b shows the variation of specific wear rate with normal load under steady state conditions for chopped E-glass fiber reinforced–epoxy composites. It is observed that at low loads 35 wt% chopped E-glass fiber reinforced composites exhibit highest wear and this is also the highest wear among all applied loads. No regular trends are observed in wear rate when the normal load is increased. The lowest wear rate is shown by 15 wt% chopped E-glass fiber reinforced composites at a normal load of 2.5 kgf; there is significant variation in trend of specific wear rate.

35 wt% chopped E-glass fiber–epoxy composite exhibit highest wear which may be attributed to enhancement of brittleness of composites because of inherent nature of chopped E-glass fiber and fracture mode of abrasive wear is occurring Contrary to it 35 wt% chopped E-glass fiber reinforced composites exhibit low wear at high load this trend in wear rate can be attributed to the fact that there is comparatively good adhesion between the fiber and matrix which indicates that probable wear mechanism could be combination of cutting and ploughing or wedge formation. At low loading conditions 15 wt% and 25 wt% chopped E-glass fiber reinforced composites exhibits lowest wear at 2.5 Kgf and 5 Kgf respectively. This trend occurred mainly because of wear debris entrapment mechanism. On the other hand 20 wt% chopped Eglass fiber reinforced exhibit almost linear behavior with respect to load .Specific wear rate increased almost linearly with load which is as per expected trend. Similar observations have been reported by Mishra and Acharya [38]. As normal load on abrasive wear particles is increased, the load distributed over all asperities and each asperity penetrate deeper into the surface. That means a large depth of grooves was occurred and deeper grooving caused higher wear rate. This resulted in more material removal by a sever plastic deformation. As observed from Figs. 2a and 2b 15 wt% bidirectional glass fiber reinforced composites exhibit highest wear and lowest wear rate is shown by 35 wt% bidirectional E-glass fiber reinforced composites at a normal load of 5 kgf while in case of Chopped E-glass fiber reinforced composites highest wear exhibited by 35 wt% chopped E-glass fiber reinforced composites and the lowest wear rate is shown by 15 wt% chopped E-glass fiber reinforced composites at a normal load of 2.5 kgf. One of the most important observation that can be concluded from Figs. 2a and 2b is that up to 30 wt% of fiber loading at all loading condition starting from low load to high load chopped E-glass fiber reinforced composites almost shows better wear properties than that of bidirectional E-glass fiber reinforced composites but if we go for fiber loading more than 30 wt% the bidirectional E-glass fiber reinforced composites exhibit lower specific wear or higher wear resistance rate than that of chopped E-glass fiber reinforced composites. This behavior can be contributed to the fact that as we go for higher wt% of fiber the chopped E-glass fiber reinforced composites become more brittle because of inherent brittle nature of chopped E-glass fiber, which indicates that probable wear mechanism could be fracture that in turn results in higher wear rate. Therefore it is suggested that during abrasive wear condition of high fiber loading bidirectional E-glass fiber reinforced composites are quite good and composites of such composition perform better at low as well as high loading conditions.

Specific Wear rate (mm3/NM)

3.3. Taguchi experimental analysis 7.00E-02 6.00E-02

Ws(15%) Ws(20%) Ws (25%) Ws(30%) Ws(35%)

5.00E-02 4.00E-02 3.00E-02 2.00E-02 1.00E-02 0.00E+00 2.5

5

7.5

10

12.5

Normal Load (Kgf) Fig. 2b. Variation of specific wear rate with normal load under steady state condition for chopped E-glass fiber reinforced–epoxy composites (Sliding speed = 0.48 m/s, Sliding distance = 50 m, Abrasive size = 125 lm).

In Table 5, the 8th and 10th column represents S/N ratio of the wear rate which is actually the average of two replications of bidirectional E-glass fiber–epoxy and chopped e-glass fiber–epoxy composites respectively. The overall mean for the S/N ratio of the wear rate is found to be 11.1802 db for bidirectional E-glass fiber–epoxy composites and 11.8121 db for the chopped E-glass fiber–epoxy composites. Before any attempt is made to use this simple model as a predictor for the measure of performance, the possible interactions between the control factors must be considered but in this particular case no interaction among the control factors are allowed. Analysis of the result leads to the conclusion that factor combination of A5 (Sliding speed 1.44 m/s), B3 (Fiber content 25%), C1 (Normal load 2.5 Kgf), D5 (Sliding distance 90 m) and E1 (Abrasive size 125 lm) gives minimum wear rate (Fig. 3a) for bidirectional E-glass fiber–epoxy composites and for chopped E-glass fiber–epoxy composites the factor combination of A3

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Table 5 Comparison of specific wear rate of bidirectional E-glass fiber–epoxy composites with those of chopped E-glass fiber–epoxy composites under different test conditions as per L25 orthogonal array. Runs Sliding speed (A) (m/s)

Fiber content (B) (%)

Normal load (C) (Kgf)

Sliding distance (D) (m)

Abrasive size (E) (lm)

WsB (102  mm3/ S/N ratio N-m) (db)

WsC (102  mm3/ S/N ratio N-m) (db)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

15 20 25 30 35 15 20 25 30 35 15 20 25 30 35 15 20 25 30 35 15 20 25 30 35

2.5 5.0 7.5 10.0 12.5 5.0 7.5 10.0 12.5 2.5 7.5 10.0 12.5 2.5 5.0 10.0 12.5 2.5 5.0 7.5 12.5 2.5 5.0 7.5 10.0

50 60 70 80 90 70 80 90 50 60 90 50 60 70 80 60 70 80 90 50 80 90 50 60 70

125 210 355 420 600 420 600 125 210 355 210 355 420 600 125 600 125 210 355 420 355 420 600 125 210

3.16 5.12 5.96 7.45 7.84 3.53 4.9 1.1 7.49 3.17 2.54 4.09 6.98 2.84 2.12 6.78 3.4 1.23 2.24 5.47 5.39 1.64 6.42 2.73 1.58

1.70 3.70 2.58 6.37 6.99 2.14 3.93 3.31 8.76 3.28 3.11 5.24 4.80 1.22 1.50 11.00 7.36 1.41 2.27 6.43 9.96 2.15 7.13 4.55 6.08

0.48 0.48 0.48 0.48 0.48 0.72 0.72 0.72 0.72 0.72 0.96 0.96 0.96 0.96 0.96 1.20 1.20 1.20 1.20 1.20 1.44 1.44 1.44 1.44 1.44

(Sliding speed 0.72 m/s), B3 (Fiber content 25 wt%), C1 (Normal load 2.5 Kgf), D3 (Sliding distance 70 m) and E1 (Abrasive size 125 lm) gives minimum wear rate (Fig. 3b). Table 5 presents a comparison of specific wear rates of bidirectional E-glass fiber– epoxy composites and compares them with the similar set of chopped E-glass fiber–epoxy composites. It is concluded under similar test conditions Bidirectional E-glass fiber–epoxy composites exhibit superior wear resistance than Chopped E-glass fiber– epoxy composites.

9.99374 14.1854 15.5049 17.4431 17.8863 10.9555 13.8039 0.8279 17.4896 10.0212 8.0967 12.2345 16.8771 9.0664 6.5267 16.6246 10.6296 1.7981 7.0050 14.7597 14.6318 4.2969 16.1507 8.7233 3.9731

4.6090 11.3640 8.2324 16.0828 16.8895 6.6083 11.8879 10.3966 18.8501 10.3175 9.8552 14.3866 13.6248 1.7272 3.5218 20.8279 17.3376 2.9844 7.1205 16.1642 19.9652 6.6488 17.0618 13.1602 15.6781

3.4. Surface morphology To characterize the morphology and to find out predominant wear mechanisms, worn surfaces of materials were examined by SEM. Abrasive wear occurs generally by three different mechanisms, viz. microploughing, microcutting and microcracking (brittle fracture) [39]. The examination of the wear scars indicated that the damage morphologies for all samples were similar, consisting of three zones, a short entrance and exit area and the main central

Main Effects Plot for SN ratios Data Means Sliding Speed (A)

Fiber Content (B)

Normal Load (C)

-8 -10

Mean of SN ratios

-12 -14 -16 0.48

0.72

0.96

1.20

1.44

15

Sliding Distance (D)

20

25

30

35

2.5

5.0

7.5

Abrasive Size (E)

-8 -10 -12 -14 -16 50

60

70

80

90

125

210

355

420

600

Signal-to-noise: Smaller is better Fig. 3a. Effect of control factors on wear rate (bidirectional E-glass fiber composites).

10.0

12.5

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Siddhartha, K. Gupta / Materials and Design 35 (2012) 467–479

Main Effects Plot for SN ratios Data Means Sliding Speed (A)

Fiber Loading (B)

Normal Load (C)

-6 -9

Mean of SN ratios

-12 -15 -18 0.48

0.72 0.96 1.20 Sliding Distance (D)

1.44

15

20 25 30 Abrasive Size (E)

90

125

210

35

2.5

5.0

7.5

10.0

12.5

-6 -9 -12 -15 -18 50

60

70

80

355

420

600

Signal-to-noise: Smaller is better Fig. 3b. Effect of control factors on wear rate (chopped E-glass fiber composites).

Fig. 4. Typical appearance of wear scars on specimens.

wear zone. A typical wear scar obtained at different loading conditions is shown in Fig. 4. Figs. 5 and 6 presents the SEM micrographs of Bidirectional E-glass fiber reinforced composites and Chopped E-glass fiber reinforced composites. Fig. 5a shows the SEM micrograph of 15 wt% bidirectional E-glass fiber reinforced Composite at a load of 5 Kgf. As observed from Fig. 2a it exhibit highest wear among all applied loads due to the less fiber to support matrix and sever cutting mode of abrasive wear is occurred which results in deep grooves that are clearly visible in micrograph. Fig. 5b and c represents the micrographs of 20 wt% and 25 wt% of bidirectional E-glass fiber reinforced composites at a load of 12.5 Kgf respectively and both these composites shows moderate wear (Fig. 2a) which results in breakage of fiber at distinct places due to the combination of wedge formation and ploughing mechanism of abrasive wear. This also confirmed form Fig. 5b. A wear track is clearly visible in micrograph (Fig. 5c). Fig. 5d represents the SEM micrograph of 35 wt% of bidirectional E-glass fiber reinforced composite at a load of 5 Kgf. It is

clear from the micrograph that the only mechanism that causes wear at this condition is wedge formation mode of abrasive wear, this can be attributed to the fact that there is comparatively good adhesion between the fiber and matrix which in turn results into lowest wear rate. This fact is also supported by Fig. 2a. Fig. 6a shows the SEM micrograph of 15 wt% chopped E-glass fiber reinforced composites at 2.5 Kgf. Surface morphology reveals the fact that the wear take place due to ploughing and debris entrapment mechanism, This results in mitigating the wear resulting in lowest wear rate (Fig. 2b) .Wear tracks are also clearly visible in Fig. 6a. Fig. 6b and c represents the micrographs of 20 wt% and 25 wt% of chopped E-glass fiber reinforced composites at a load of 7.5 Kgf and 2.5 Kgf respectively. Micrographs show the existence of ploughing and wedge formation which is characterized by wear due to plastic deformation and results into moderate wear rate. Fig. 6d represents the micrographs of 35 wt% of chopped E-glass fiber reinforced composite at a load of 5 Kgf. Micrographs show the matrix damage and subsequent matrix removal due to the formation and propagation of microcracks as well as macrocracks at the surfaces. The absence of any plastic deformation or grooves formation is due to the inherent brittle nature of chopped E-glass fiber hence the contributing wear mechanism should be fracture of surface.

3.5. ANOVA and the effects of factors In order to find out statistical significance of various factors like Sliding speed (A), Fiber content (B), Normal load (C), Sliding distance (D) and Abrasive size (E) on specific wear rate, analysis of variance (ANOVA) is performed on experimental data. Tables 6 and 7 show the results of the ANOVA for the specific wear rate of bidirectional E-glass fiber–epoxy composites and chopped E-glass fiber–epoxy composites respectively. The last column of the table indicates percentage contribution of the control factors on the performance output i.e. specific wear rate [40]. From Table 6, one can observe that Fiber content (B) (P = 64.2517%) and Normal load (C) (P = 19.2824%) have great influence on specific wear rate of bidirectional E-glass fiber composites which is in appropriate accordance with the fact reported

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Breakage of fibers

Large Depth Grooves Cutting Mode of Abrasive Wear

(a)

(b)

Wear Tracks Wedge Formation

Broken fibers

(c)

(d)

Fig. 5. SEM micrographs of the abraded bidirectional E-glass fiber–epoxy composites at different loading conditions (Sliding speed = 0.48 m/s, Sliding distance = 50 m, Abrasive size = 125 lm).

by Mohan et al. [41] in their research that abrasive wear volume loss increases with increase in applied load but the factor Sliding speed (A) (P = 8.1983%), sliding distance (D) (P = 4.6711%) and Abrasive size (E) (P = 3.0505%) have less significant contribution on wear rate of bidirectional E-glass fiber composites. For short glass fibers other researchers [16,42] found that increasing abrasive particle size and sliding distance deteriorates the wear resistance characteristics of composites this in turn leads to the fact that abrasive size and sliding distance play major role in influencing the wear characteristics of short glass fiber composites which is in harmony with the experimental results of this research work: in case of chopped E-glass fiber composites (Table 7) the Fiber content (B) (P = 45.051%), Abrasive size (E) (P = 28.8654%)and Sliding distance (D) (P = 13.2925%) have significant contribution on wear rate of chopped E-glass fiber composites but Sliding speed (A) (P = 11.8465%) and Normal load (C) (P = 0.9455%) has less contribution. Similar observations have been reported by Yousif and his

co-researchers [43]. Moreover, it can be seen from Tables 6 and 7 that factor like sliding velocity has lesser contribution on specific wear rate but in confirmation experiment this factor cannot be neglected because sliding velocity has major contribution on specific wear rate in collaboration with other factors. 3.6. Confirmation experiment The optimal combination of control factors has been explored in the previous section. However, the final step in any design of experiment approach is to predict and verify improvements in observed values through the use of the optimal combination level of control factors. The confirmation experiment was performed by taking an arbitrary set of factor combination A2 B5 C3 D1 E2, but factor A, D and E have also least contribution on specific wear rate for bidirectional E-glass fiber composites as evident from Table 6. In the similar fashion, for chopped E-glass fiber composites the

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Siddhartha, K. Gupta / Materials and Design 35 (2012) 467–479

Wear Debris Entrapment Wear Tracks

Wedge Formation

Ploughing Fiber breakage due to ploughing

(a)

(b)

Wedge Formation Fracture due to brittleness

(c)

(d)

Fig. 6. SEM micrographs of the abraded chopped E-glass fiber–epoxy composites at different loading conditions (Sliding speed = 0.48 m/s, Sliding distance = 50 m, Abrasive size = 125 lm).

Table 7 ANOVA table for wear rate (chopped E-glass fiber composites).

Table 6 ANOVA table for wear rate (bidirectional E-glass fiber composites). Source A B C D E Error Total

DF 4 4 4 4 4 4 24

Seq SS 92.924 12.908 189.142 136.080 174.017 20.915 627.986

Adj SS 92.924 12.908 189.142 136.080 174.017 20.915

Adj MS 23.731 3.227 47.286 34.020 43.504 5.229

F 4.54 0.62 9.04 6.51 8.32

P%

Source

DF

Seq SS

Adj SS

Adj MS

F

P%

8.1983 64.2517 19.8284 4.6711 3.0505

A B C D E Error Total

4 4 4 4 4 4 24

93.732 15.393 470.148 85.313 37.773 39.743 742.103

93.732 15.393 470.148 85.313 37.773 39.743

23.433 3.848 117.537 21.328 9.443 9.936

2.36 0.39 11.83 2.15 0.95

11.8465 45.0501 0.9455 13.2925 28.8654

100

P: percent contribution.

arbitrary set of factor combination A2 B5 C3 D1 E2, but factor C has least effect on specific wear rate (Table 7). The estimated S/N ratio for specific wear rates can be calculated with the help of following predictive equation:

100

P: percent contribution.

g^ Bidirectional ¼ T þ ðA2  TÞ þ ðB5  TÞ þ ðC 3  TÞ þ ðD1  TÞ þ ðE2  TÞ

ð4Þ

g^ Chopped ¼ T þ ðA2  TÞ þ ðB5  TÞ þ ðC 3  TÞ þ ðD1  TÞ þ ðE2  TÞ

ð5Þ

477

Siddhartha, K. Gupta / Materials and Design 35 (2012) 467–479 Table 8 Results of the confirmation experiments for wear rate.

3.5

0.48 m/s 1.20 m/s

Optimal control parameters

Level S/N ratio for wear rate (db) (bidirectional E-glass fiber composites) Level S/N ratio for wear rate (db) (chopped Eglass fiber composites)

Prediction

Experimental

Error

A2 B5 C3 D1 E2 11.9440

A2 B5 C3 D1 E2

(%)

12.5586

5.15

Ko (×10-11)

3

0.72 m/s 1.44 m/s

0.98 m/s

2.5 2 1.5 1 0.5

A2 B5 C3 D1 E2 12.8869

0

A2 B5 C3 D1 E2

(%)

13.6986

6.30

11.6

6.11

5.84

5.42

5.29

(σu u)-1(×10-4) Fig. 7a. Variation of specific wear rate with (ruu)1 (for bidirectional E-glass fiber).

3.5

glass fiber–epoxy and chopped e-glass fiber–epoxy composites, T the overall experimental average and A2 ; B5 ; C 3 ; D1 and E2 is the mean response for factors at designated levels. By combining like terms, the equation reduces to

3

ð6Þ

g^ Bidirectional ¼ A2 þ B5 þ C 3 þ D1 þ E2  4T

ð7Þ

A new combination of factor levels is used to predict deposition rate through prediction equation and it is found to be gBidirectional = 11.9440 db and gChopped = 12.8869 db. For each performance measure, an experiment is conducted for a different factors combination and compared with the result obtained from the predictive equation as shown in Table 8. Actual runs were performed to verify if the results obtained by above equations are acceptable. It is found than when actual runs have been performed on above factor settings an error of 5.15% (bidirectional E-glass fiber composites) and 6.30% (chopped E-glass fiber) has occurred which is well within the reasonable limits. The error can be reduced if the number of runs is enhanced. This verifies that the predicted values are reliable and testifies the validity of this predictive model for predicting the performance output on the basis of input characteristics.

0.48 m/s

0.72 m/s

0.96 m/s

1.20 m/s

1.44 m/s

2.5 2 1.5 1 0.5 0 29.16

18.08

24.47

(

u)

28.46

28.19

-1

Fig. 7b. Variation of specific wear rate with (u)1 (for bidirectional E-glass fiber).

3.5 0.48 m/s

0.72 m/s

0.96 m/s

1.20 m/s

1.44 m/s

3

Ko (×10-11)

g^ Bidirectional ¼ A2 þ B5 þ C 3 þ D1 þ E2  4T

Ko (×10-11)

g Bidirectional and g Chopped is the predicted average for bidirectional E-

2.5 2 1.5 1 0.5

3.7. Theoretical prediction of specific wear rate

0

K o aðru Þ

3=2

ðu Þ

1

ð8Þ

and by Ratner Lancaster Correlation

K o aðru u Þ

1

0.96

2.75

1.88

(σu

u)

2.21

-1

1

-5

(×10 )

Fig. 7c. Variation of specific wear rate with (ruu)1 (for chopped E-glass fiber).

3.5 3

Ko (×10-11)

Mechanical properties of polymers and polymer composites are quite dependent on temperature, the wear rates, hardness and elastic moduli. Ratner et al. [44] considered that in abrasive wear situations the wear rates are very much dependent on the magnitude of elongation to break. Since the product of ruu represents the work required for detaching a particle from the wearing surface by tensile failure, this relationship emphasizes the role of plastic deformation in the wear process. Wang et al. [45] considered that in abrasive wear, the scale of plastic deformation is limited to the sites of intimate micro-asperity contacts and wear rate is defined by a critical strain criterion. In case of Wang model [45]

0.48 m/s

0.72 m/s

0.96 m/s

1.20 m/s

1.44 m/s

2.5 2 1.5 1

ð9Þ

0.5

where Ko is Specific wear rate (mm3 per N-m), ru the ultimate tensile strength (MPa) and u is the elongation at tensile fracture of test sample. In case of experimental variables to calculate specific wear rate, the Wang’s model and Ratner’s correlation both insist on the importance of ultimate strength and strain at tensile fracture. In

0 70.42

163.93

115.34

(

u)

128.2

69.93

-1

Fig. 7d. Variation of specific wear rate with (u)1 (for chopped E-glass fiber).

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Siddhartha, K. Gupta / Materials and Design 35 (2012) 467–479

3.5 0.48 m/s

0.72 m/s

0.98 m/s

1.20 m/s

1.44 m/s

Ko (×10-11)

3 2.5 2 1.5 1 0.5 0 2.31

1.12

(σu)-1.5(

0.9

0.75

0.71

-1(×10-10)

u)

Fig. 7e. Variation of specific wear rate with (ru)1.5(u)1 (for bidirectional E-glass fiber).

4. The flexural modulus is found to be an increasing function of fiber content for both bidirectional E-glass fiber–epoxy and chopped E-glass fiber–epoxy although in case of bidirectional E-glass fiber composite, the trend of flexural modulus is followed up to 30 wt% . 5. The impact energies of chopped E-glass fiber–epoxy composites increase gradually with increase in fiber loading, while in case of bidirectional E-glass fiber–epoxy composite little dip in impact energy is observed at the 20 wt% fiber content and then impact energies gradually increase with increase in fiber loading. 6. Abrasive wear characteristics of these composites are successfully analyzed using Taguchi experimental design scheme, ANOVA and affect of each control factor on abrasive wear characteristics is investigated. Chopped glass fiber reinforced composites perform better than bi-directional glass fiber reinforced composites under abrasive wear situations.

3.5 0.48 m/s

0.72 m/s

0.96 m/s

1.20 m/s

1.44 m/s

Ko (×10-11)

3 2.5 2 1.5 1 0.5 0 3.51

11.3

(σu)-1.5(

7.6

9.23

3.78

-1 -9 u) (×10 )

Fig. 7f. Variation of specific wear rate with (ru)1.5(u)1 (for choppedl E-glass fiber).

this research work the test results are validated against these existing models. It is observed from Figs. 7a–7f that specific wear rate and the reciprocal of toughness show the excellent linear relationship which supports the fact the experimental results are in harmony with the microscopic wear models proposed by researchers [44,45]. 4. Conclusion An experimental study has been carried out for bidirectional Eglass fiber and chopped E-glass fiber reinforced epoxy composites and following conclusions are drawn: 1. Bidirectional E-glass fiber reinforced-epoxy composite exhibit most superior average hardness of 59.67 HRB at 35 wt% while chopped E-glass fiber–epoxy composites 30 wt% composites show highest hardness of 45.36 HRB. 2. On increase in addition of fiber in resin, tensile strength of bidirectional E-glass fiber reinforced composites increases because of interface strength enhancement. In case of chopped E-glass fiber–epoxy composites a scatter in the tensile strength values is observed because of the nonuniform and anisotropic nature of glass fiber. 3. Under flexural loading and inter laminar shear loading situations, bidirectional E-glass fiber–epoxy performed better than chopped E-glass fiber–epoxy composites. The tensile modulus is found to be an increasing function of fiber content after 20 wt% of fiber in bidirectional E-glass fiber–epoxy composite although in case of chopped E-glass fiber composite, the trend of tensile modulus is quite scattered. Because of probable poorer fiber distribution and impregnation.

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