Materials Science Research International, Vol.6, No.3 pp. 186-192 (2000)

General paper

Effect of Fiber Surface Treatment on the Interface and

Bending Strength of Aramid Short Fiber Composite

A.K.M. MASUD*, Kiyoshi ISOGIMI** and Jippei SUZUKI*** Dept. of Material Sci. Engng, Bangladesh Univ. of Engng. Technol., Dahka, Bangladesh

** Dept. of Mechanical Engng, Mie Univ., Kamihama-cho 1515, Tsu, 514-8507, Mie, Japan

Abstract: Composite materials used in the present study are fabricated by Aramid short fibers and Unsaturated Poly-ester resin. The bending strength and energy absorption properties of the composite are studied in this paper. From the single-fiber pull-out test it is found that 1mm fiber is not long enough to reinforce the polyester, thus the strength of the composite made of 1mm fiber does not increase when compared to that of the resin. But the strength of the composite made of 3mm fiber starts to increase when compared to that of the resin. The strength and absorbed en-ergy always increases with the length and content of the fiber. Some fiber surface treatments are done in order to in-crease the bonding between the fiber and the matrix. It is found that these treatments affect the interfacial shear strength and fiber strength simultaneously. As strength is an important property of a new material, the effect of these treatments on the strength is discussed in this paper. The bending strength of the composite is assumed to depend not only on the interfacial shear strength but also on the fiber strength, depending upon the length of the short fibers.

Key words: Composite material, Bending strength, Pull-out test, Interfacial shear stress, Surface treatment

1. INTRODUCTION

Short fiber composites can be widely used because of

its easy fabrication and sophisticated applications. Ex-

perimental studies have been carried out to investigate

the bending property of Aramid short fiber composite.

Fiber/matrix interface is a crucial region in the com-

posite and is usually considered to be responsible for

transmitting the load from the matrix to the fiber. In case

of Aramid fiber and the treatments employed here, it is

found that the interfacial shear strength and fiber strength

vary simultaneously with the treatments. So far, the

change of the fiber strength due to fiber treatment is often

neglected in many studies on composite interface. The

change of mechanical properties of the composite may

not only depend on the interfacial property but it may

also depend on the fiber strength. In this research the ef-

fect of fiber treatment on the bending strength of the

Aramid short fiber composite has been studied.

2. EXPERIMENTAL

Aramid (Kevlar 29 of type T950 and T965A, made

by DuPont Toray Inc.) short fibers and unsaturated poly-

ester (8285AP type, made by Takeda Yakuhin Co., Ltd.)

resin are used as the filler and the matrix, respectively.

Permek-N (Nihon Yushi Co., Ltd.) is used as a hardener

for the polyester resin. The short fibers are 1 and 3mm in

length. The fabrication process of the short fiber compos-

ite is similar to that employed in our previous paper [1, 2],

which is named as •gPressurized Molding Process.•h

A three point bending test [1, 2] was carried out to in-

vestigate the static property of the composite. The thick-

ness and the width of the specimens are 5 and 10mm,

respectively. The span in between the supports is 35mm

and the cross-head speed was kept constant at 0.027

mm/s during the test.

2.1. Pull-out TestPreparation of the specimen for the single-fiber pull-

out test is described in our previous paper [1]. An aramid single fiber is set on a construction paper and one end of the fiber is embedded into the resin and the other end is firmly fixed by a strong adhesive (cyano-acrylate). After hardening of the resin, embedded length L is measured by using the optical microscope. Pt is the maximum load required to pull-out the fiber from the resin. At a certain embedded length, load Pt exceeds the fracturing load of the fiber and this value is defined as the critical embed-ded length Lc. Most of the fibers are found to be fractured out when the embedded length is greater than the critical one.

2.2. Fiber TreatmentsH2SO4 and cyano-acrylate gas treatments [2] were

found to be ineffective in developing the interfacial prop-erty and the mechanical property of the short fiber com-posite. Here, two other types of treatments are carried out. The first one is an epoxy treatment, where two kinds of solutions are prepared by mixing epoxy into water and toluene solvent where the amount of the epoxy is 1% of the solution. Two types of Kevlar 29 fibers (T950 and T965A type) are immersed into the solutions and dried at 513K. The second one is done by heating Kevlar 29 (T950 type) fiber at 353K for 60 days in an oven. These treatments are characterized as shown in Table 1. Here, Kevlar 29 fiber T950 and T965A are the specifications of

Table 1. Identification of the fiber treatments.

Fibers T950 and T965A are the non-treated (virgin) fi-bers.

Received January 6, 2000Accepted July 17, 2000

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Fiber Treatment and Bending Strength of Short-ArFRP

(a) (b)

Fig. 1. Load-displacement curves of the (a) matrix and (b) composite made of 3mm fiber (about 29vol%).

Fig. 2. Bending strength of the composite made of virgin fiber T950.

DuPont Toray Inc. These two fibers have almost the similar mechanical properties, but they have different ability of absorbing the epoxy radical in the surface of the fiber.

3. EXPERIMENTAL RESULTS

3.1. Results of Bending TestThe load-displacement curves of the matrix and the

composite are shown in Fig. 1. Strength at load Pk is de-fined as the bending strength. The strength of the com-

posite made of 1mm fiber does not increase compared to that of the resin and the strength of the composite made of 3mm fiber increases a little compared to that of the resin as shown in Fig. 2. Further, the area under the load-displacement curve is used to calculate the absorbed en-ergy and it increases dramatically as compared to that of the resin, moreover, it increases with the length and con-tent of the fiber, as shown in Fig. 3.

3.2. Effect of the Treatments on the Fiber and Fi-ber/Matrix Interface

Figure 4 shows the results of the pull-out test of the

Fig. 3. Energy absorbed by the composite made of virgin fiber T950, during bending.

virgin and treated fibers and found a transient region

where some fibers are torn and some are pulled out and

this region is around the critical length Lc. As the pull-out

load is assumed to increase linearly with the embedded

length, the data of the pull-out region are fitted by the

method of the least squares and the line is defined as the

pull-out load-line. The mean value (Pf) of the load re-

quired to fracture the fiber is also shown in Fig. 4 and this line is defined as the •gfiber-fracture load-line•h.

Critical embedded length Lc is defined as the embed-

ded length at which the pull-out load-line and the fiber-

fracture load-line intersect each other. As this critical

embedded length depends both on the pull-out and fiber

fracture property, this length is considered to be an im-

portant parameter in defining the fiber treatments.Interfacial shear strength for each treatment is calcu-

lated using the following Eq. (1), with the assumption

that shear stress is distributed uniformly in the part of the

fiber/matrix interface.

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A.K.M. MASUD, Kiyoshi ISOGIMI and Jippei SUZUKI

where, Ąi and Da are defined as the interfacial shear

strength and the average diameter of each type of fiber,

Table 2. Fiber fracture and interfacial property of the

virgin and treated fibers.

respectively. In Eq. (1) Pt/L is an inclination of the pull-

out load-lines. The average fiber-fracture load is divided

by a cross sectional area of the fiber to calculate the aver-

age fiber strength ƒÐf. The result obtained from the pull-

out test is summarized in Table 2. The standard deviation

of diameters (Da) and tensile force (Pf) mentioned in Ta-

ble 2 are obtained as 0.9•`1.2ƒÊm and 0.04•`0.07N,

respectively. The critical embedded length Lc depends on

the inclination of the pull-out load line as well as the in-

terfacial shear strength and the fiber strength, for all the

cases of the fiber treatments. Moreover, it can be con-

cluded that the decrease of the fiber strength reduces the

critical embedded length and the increase of that strength

makes the length longer, and the increase of the interfa-

cial shear strength reduces the critical embedded length

Fig. 4. Results of single- fiber pull-out test of virgin (T-950) and treated Aramid fibers .

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Fiber Treatment and Bending Strength of Short-ArFRP

and the decrease of that strength makes the length longer . However, H-T950 treatment seems somewhat different .

3.3. Effect of the Treatments on the Bending Strength of the Composite

Figure 5 shows the bending strength of the composite of the five kinds of treated fibers mentioned in Table 1. In the case of 1mm fiber, at low fiber content, treatments hardly affect the strength and at high fiber content , the strength of the composite of EW-T950, ET-T950 and H-T950 fibers increases compared to that of the virgin fiber . In the case of 3mm fiber, at low fiber content , the strength of the composite is hardly affected by the treat-ments except the H-T950 fiber. At a high fiber content , the strength of the composite of all the treatments in-

creases as compared to that of the fresh fiber. However , the strength of the composite made of 3mm ET-T950 fiber has the highest value .

4. DISCUSSION

From Fig. 1 (a), it can be concluded that the matrix possesses brittle-like fracture behaviour, because the load suddenly drops to zero after it linearly increases up to the peak load Pk. It is found that the specimen is divided into two portions at the load just exceeding the peak load. But the composite gradually fractures beyond the peak load Pk and the specimen does not divide into two parts, even when the load becomes very small.

From the results of the single-fiber pull-out test it can be concluded that minimum length of the short fiber should be at least more than double the critical embedded length, that is, 1.86mm to reinforce the polyester. Be-cause the strength starts to increase, compared to that of the resin, when the length of the fiber is 3mm, so in or-der to investigate the cause of this fact , the fractured sur-face of the composite, made of 1 and 3mm fibers, is ob-

served by a Scanning Electron Microscope (SEM) and is

(a) (b)

Fig. 5. Comparison of the bending strength of the composite made of treated fibers with that of the virgin fibers .

(a) Composite made of 1mm fiber. 100ƒÊm (b) Composite made of 3mm fiber. 100ƒÊm

Fig. 6. Fractured surface of the bending test specimen.

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A.K.M. MASUD, Kiyoshi ISOGIMI and Jippei SUZUKI

(a) (b)

Fig. 7. Typical appearance of the fibers on an imaginary plane where micro cracks start during the fracturing of

(a) composite made of 1mm fiber and (b) composite made of 3mm fiber.

shown in Fig. 6. In Fig. 6 (a), the presence of some holes and fibers can be distinctly observed on the fractured surface of the composite made of 1mm fiber, which are not fibrillated. As we know that the Aramid fibers are used to fibril out during its fracture, so the appeared fi-bers on the fractured surface of the composites are as-sumed to pull out from the matrix and the holes are left on to pull out the fiber. But, in the case where the com-posite is made of 3mm fiber, some holes, fibrillated and non-fibrillated fibers are observed on the fractured sur-face as shown in Fig. 6 (b). Thus, we assumed that some fibers are pulled out from the matrix and some are frac-tured out during the fracturing process of the composite made of 3mm fiber. This above-mentioned phenomenon can be clearly defined below.

Let us consider a composite under bending force. The maximum tensile force acts on a very thin layer on the bottom surface and the direction of the tensile force is along the neutral plane. So, we can assume that the maximum stress induced by load Pk occurs in the thin layer of the bottom surface of the specimen and it is con-sidered that the tensile stress at this thin layer would act parallel to the neutral plane. We have assumed an imagi-nary plane S that is perpendicular to the neutral plane and it is expected that the composite would start to fracture along this plane. This plane would divide each fiber that appeared on the plane into two parts as shown in Fig. 7 and the smaller part of either side of the plane would tend to be pulled out from the matrix. In the case of the com-posite made of 1mm fiber, the smaller length of either side of the plane would be less than 1mm and the critical embedded length is about 1mm that is found from the pull out test. Hence, all the fibers would pull out from the matrix during the fracturing process of the composite fiber, thus the strength does not increase compared to that of the matrix. But, in the case of the composite made of 3mm fiber, we could think of two types of situations where the smaller length in either side of the plane might be less than 1mm in one situation and larger in the other situa-tion as shown in Fig. 7. From the results of the pull out test, we could conclude that the fiber would be pulled out in the former situation while the fiber would fracture in the later situation. As the probability of the later situation occurring is very low, the strength of the composite in-

creases a little as compared to that of the matrix, espe-

cially in the case of a high fiber content. Then, high fiber

content increases the occurrence of both of the situations,

so the strength always increases with the content of the

fiber.

Energy would be absorbed during the pull out and the

fiber fracture process and this absorbed energy depends

on the fiber content. The content controls the incidences

of fiber being pulled out or fractured. Furthermore, from

the result which indicated that the energy absorbed in-

creased with the length of the fiber, we could think that

energy absorbed during the fracture of the fiber is higher

than the energy absorbed during the pull out because the

probability of fiber fracture increases and the fiber pull

out decreases with an increase in length of the fiber in the

composite.

From the results obtained from Fig. 5 to Fig. 7 and

Table 2, we can summarize the characteristic changes of

fiber strength ƒÐf and interfacial strength ƒÑI . It is found that

the interfacial shear strength and fiber strength vary si-

multaneously with the treatments employed in this re-

search. So far, the change of the fiber strength due to fi-

ber treatment is often neglected in many studies on com-

posite interface. Therefore, a conclusion on the effect of

the interface on the mechanical properties of composite

cannot be correctly derived, and thus a complete under-

standing of the effect of fiber treatment is not possible.

As all the fibers are pulled out from the matrix in cases of

composites made of 1mm fiber, the property would de-

pend on the interface only. As the interfacial shear

strength is maximum in the cases of EW-T950 and ET-

T950, the strength of the composite made of these treated

1mm fibers increases when compared to that of the vir-

gin fiber only in the high fiber content because the effect

of the interface is high enough for a high content. How-

ever the strength of the composite made of H-T950 in-

creases as compared to that of the virgin fiber. On the

other hand, the bending property of the composite made

of 3mm would depends on the interfacial property

and the fiber strength as well, because some fibers are

pulled out and some are fractured out during the fracture

process. Although the interfacial shear strength of the

EW-T950 and ET-T950 treated fiber increases but the

fiber strength decreases consequently when compared to

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Fiber Treatment and Bending Strength of Short-ArFRP

(a) Virgin T950 fiber (b) EW-T950 fiber

(c) EW-T965A fiber (d) ET-T950 fiber

(e) ET-T965A fiber (f) H-T950 fiber

Fig. 8. Surface appearance of the virgin aramid fiber and the treated fibers.

that of the virgin fiber. Therefore, the strength of the composite of all the treated 3mm fibers increases com-

pared to that of the virgin fiber especially at a high fiber content because both the effects of the fiber pull-out and the fiber-fracture are high for high contents.

Furthermore, from the appearances of the treated fi-ber surfaces shown in Fig. 8, it can be seen that the ups

and downs are fairly visible in case of EW-T950 and ET-T950, and the presence of ups and downs are con-firmed by tracing the surface at the high magnification, one of these examples is shown in Fig. 9. Here, we may also think that these changes of the surface affect the in-terface and the interfacial shear strength of the fiber and matrix. Thus, the interfacial shear strengths of these fiber

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A.K.M. MASUD, Kiyoshi ISOGIMI and Jippei SUZUKI

Fig. 9. Example of surface profile of EW-T950 fiber.

and matrix are higher compared to those of the cases of other fibers.

To understand clearly the effect of the fiber treat-ments on the bending strength of the composite, all the data of the composite of virgin and treated fibers are summarised in Fig. 10. It can be concluded that these fiber treatments develop the strength of the composite as much as 5% compared to that of the virgin fiber, except in cases of the composite of 1mm fiber and about 12vol%, as shown in Fig. 10.

5. CONCLUSIONS

Because the minimum length of the short fiber should be greater than 1.86mm for reinforcement, the strength of the composite made of 1mm fiber does not increase compared to that of the resin and it starts to in-crease when the length is 3mm. Thus, the results of the

pull-out test coincide with the strength property of the composite.

Fig. 10. Bending strength of the composite of virgin and treated fibers.

The strength and energy absorbed were always found to increase with the length and content of the fiber.

The fiber-surface treatment employed in this study changed the interfacial shear strength and the strength of the fiber itself, simultaneously. It is found that both of the strengths are important to explaining the strength of the composite.

REFERENCES1. A.K.M. Masud, K. Isogimi and J. Suzuki, JSME Interna-

tional Journal, Series A, 43 (2000) 94.2. A.K.M. Masud and K. Isogimi, Proc. of 11th Int. Conf. on

Experimental Mechanics (ed. By I.M. Allison), British Soc. Strain Measurement, Oxford (1998) 1417.

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