Polymer Testing 20 (2001) 179–189www.elsevier.nl/locate/polytest

Material Behaviour

The effects of hole locations and hole sizes on damagedbehaviour of woven thermoplastic composites

M. Mariatti, M. Nasir, H. Ismail*

School of Industrial Technology, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia

Received 7 July 1999; accepted 1 March 2000

Abstract

Woven thermoplastic composites are slowly emerging as potential materials in many engineering applications suchas transportation, construction, automotive, etc. This is due to their unique properties, for example better damagetolerance, ease of handling and product design, apart from the usual light weight and recyclability. The commonapproach to realizing such a system involves a wet process or utilizing an unimpregnated woven fabric. Prepreg thermo-sets and thermoplastics are now available and commercially used. The only difference is the use of COFIT or impreg-nated tape as the precursor of the woven system. In short, the impregnation is done in situ during shaping. Here, adifferent technique of using preimpregnated tapes or prepreg is advocated. It is generally found that this approach iseasier with better control of the composites’ properties since it is a dry process. This paper is an extension of otherswhich advocates the potential of COFIT based woven systems. The effect of location and size of holes were investigated.In general, the effect of damage in COFIT woven thermoplastic composite is clearly dependent on the location andsize of the hole. 2000 Elsevier Science Ltd. All rights reserved.

Keywords:Woven thermoplastic; Composites; Damaged behaviour; Hole location; Hole size

1. Introduction

In our previous reports [1,2], it was clearly shown thatCOFIT woven systems have been realized with uniqueproperties and advantages in which the properties are gov-erned by numerous process variables, such as the natureof towpregs, the weave geometry and the ply effect. Thedifference in behavior as compared to typical non-wovensystems can be easily attributed to the additional effectscoming from weave characteristics such as interlaces, floats,fiber crimp, etc. and to some extent processing.

It is well known that all sorts of damage can be furtherintroduced during any composite handling and service[3,4]. Some are with purpose while others are accidental.The effect of damage is important because it can often

* Corresponding author. Tel.:+60-04-6577888-2214; fax:+60-04-657-3678.

E-mail address:ihanafi@usm.my (H. Ismail).

0142-9418/01/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.PII: S0142 -9418(00 )00021-0

lead to premature or subsequent failure. In view of thissituation, it was felt necessary to examine the effect ofdamage on this woven system, although limited to anopen hole type damage alone and a plain weave system.The effects of the location and size of holes were investi-gated also.

2. Experimental

2.1. Materials

Acrylonitrile Butadiene Styrene (ABS) resin of gen-eral-purpose Polylac PA757 grade from Chi-Mei (M)Ltd. was made into 30% w/w solution using MethylEthyl Ketone (MEK) as a solvent. The continuous Eglass fiber manufactured by Central Glass Ltd., Japanand supplied locally by EuroChemo-Pharma (M) SdnBhd was utilized throughout. High impact polystyrene(HIPS) of grade Poly-Star HT-50 was bought from alocal producer, PetroChemical (M) Sdn Bhd.

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Fig. 1. A schematic diagram showing the two holes’ positions in a single ply plain weave.

Fig. 2. General phenomena of damages around hole [7].

2.2. Preparation of COFIT prepreg

Both ABS and HIPS COFIT prepregs or towpregswere prepared under similar molding conditions usingthe SIRNA prepregger system, viz. Dryer temperature80°C; die temperature 220°C and pulling rate 0.35m/min. ABS towpregs of 4, 7 and 12 tows and also that

Table 1Influence of holes’ positions on the properties of woven system

Hole positions Warp Interlace point Undamaged

Wf 0.83 0.84 0.84Voids (%) 7.9 10.8 5.40° TS (MPa) 171.8 155.7 187

of 12 tow with HIPS as a matrix were prepared. All pre-pregs were then characterized for their properties such asdimensions, specific gravity (S.G) and tensile properties.

2.3. Preparation of woven system

In ensuring the quality of the woven composite, onlyprepregs having acceptable properties of dimensions,density and fiber fraction (Wf) were chosen. They werewoven manually into a single ply laminate with plainweave. The single ply laminate was molded by com-pression molding at 220°C for 15 min preheating andsubsequently pressure of 12 MPa for 5 min. A plainweave was selected for simplicity in preparation and datainterpretation. To prevent sticking and to assist demold-ing, they were placed between TEFLON sheets. No

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Fig. 3. The stress-strain deformation behaviors of damaged and undamaged specimens of single ply plain weave. Damaged causedby different hole positions at warp/weft and interlace.

spacer or mold was used, i.e. an open mold was usedthroughout. Finally, they were removed from the pressand allowed to cool to room temperature under pressureso as to prevent warping. The laminates were cut intosuitable specimen sizes in order to characterize thephysical properties of the system such as density, voidcontent, and fiber weight fraction (wf). For the tensiletest, the coupons were cut from the laminated system,

followed by a drilling process. The experimentalmethods for hole damage can be divided into two parts.The first was to determine the influence of hole locationin which holes of 2.5 mm were drilled at warp/weft andinterlace locations. The second was to examine the effectof hole sizes of 2.5, 4.5, 6.5 and 8.5 mm diameter madeat the warp location. For both cases, 12 tow ABS plainweave was prepared under similar molding conditions

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Fig. 4. SEM photomicrographs of single ply plain weave due to hole damage and tensile mode failure for certain cases such as (a)Hole damage at warp location (300× and 600×); (b) Hole damage at interlace point (300× and 600×).

and test coupons were based on Type 1, 0° with 120 mmgage length. The specimens were then tested in tensionin accordance with ASTM D638.

3. Results and discussion

3.1. Hole locations

In woven composite structural applications there areseveral methods to join or assemble components

together, either drilling a hole for riveting or boltedjoints, or using adhesives, welding, etc. [3, 5:145–151].Mechanical fastening on any woven composite isimportant since it would weaken the overall inherentproperties.

Weave consists of two sets of interlacing towpregs oryarns, the warp and the weft that interlace at a certainangle. Thus, from this definition, basically in any wovencomposites there are three different locations which needto be considered when choosing the hole location [6].These are warp, weft and interlace points. Unlike fabric

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Fig. 4. (continued)

based systems, here the choice of hole location is obvi-ous since towpregs are large. Fig. 1 shows a schematicposition of the two holes, in which the warp/weft is at themiddle of a towpreg while the other is at the interlaces.

It is obvious since the plain weave is symmetrical, thatthe warp hole location is similar to the weft. Thus, thestudy of warp hole location can be assumed similar to weftlocation. Beside the location, another parameter which hasto be considered is the distance of hole from the edge.

When the composites were subjected to hole forma-tion by drilling, all sorts of defects might be created inthe system. These can be delamination, chip-out of

fiber/matrix, degradation of matrix due to overheating,etc. [4,5]. The hole results in discontinuity of fibers thusaffecting the stress distribution along the fiber orien-tation. The hole damage also gives rise to localized stressconcentration near to the hole boundary [4]. As the resultof stress concentration, the discontinuous fiber ends getseparated from the matrix at a very small load, and thusproduce a microcrack in the material [6]. Hence, thepresence of a hole results in additional macro and microvoids in the specimen. Table 1 compares the variousproperties of damaged and undamaged specimens of sin-gle ply plain weaves.

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Fig. 5. The stress-strain deformation behaviors of different hole sizes of single ply plain weave.

It is obvious that a hole basically gives rise toincreased void content. The higher voids are due to thehole itself, which can be considered as a macrovoidcoupled with possible micro voids. This increased voidcontent in the damaged sample accounts for the low ten-

sile strengths obtained. Also as expected, the interlaceposition exhibits the highest void content and lower ten-sile strengths than the warp/weft position. The hole atthe interlace might weaken even the bonding betweentowpregs combined with more fiber failure. Hence, hole

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Fig. 6. Effect of different hole sizes on percentage of voids compared to those of undamaged specimens.

position plays a significant role, with the interlace regionbeing more sensitive.

The distance of the hole from the edge might also havecontributed to the different tensile strength properties.This can be explained by referring to the damaged areasnear the holes [7] shown in Fig. 2. As seen, there is highpeak of tension stresses in the fibers adjacent to the hole

and the stresses become less with the distance away fromthe hole.

Applying the concept to the illustrated damaged speci-mens in Fig. 1, it is clear that the warp/weft hole (X1)is a shorter distance than the interlace (X2) from theedge. The shorter distance means less fiber between thehole and the edge to prevent the tension stress transfer-

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Fig. 7. A schematic diagram highlighting the effect of hole sizes on single ply plain weave.

ring to the fiber adjacent to hole. This would then resultin poorer damage resistance of the material and the lowertensile properties observed. However, in this case theeffect seems to be marginal since the hole at thewarp/weft location exhibits higher tensile strength andbetter damage resistance than that of a hole at the inter-lace point. Perhaps other factors are significant here suchas void content, fiber-matrix interface, etc.

Fig. 3 shows the overall stress-strain deformationbehaviors of different hole positions. It is obvious here,that the hole at the warp/weft position exhibits betterdamage resistance than that at the interlace point. Forexample, the stiffness or secant modulus and toughnesstaken at 3% strain show higher properties for warp/weftover interlace positions. Toughness values are indicatedby the area under the curves. However, the different intensile stress is marginal. Judging from the stress-strainbehavior of damaged and undamaged cases, the damageresistance or behavior of this woven system is dependenton the position of the hole. The poor tensile propertiesfor the interlace position perhaps can be explained bymany factors such as higher void content, the presenceof the hole at intraply bonding, etc. This observation oftensile properties is similar to that in a previous study[6] where it was noted that different hole positions wouldaffect the drop in tensile properties, although the systemwas a symmetrical fabric plain weave.

The SEM photomicrographs of the hole fracture sur-faces for both warp and interlace positions are given inFig. 4. It must be noted that there are two types of dam-age present in the samples, one is hole drilling damageand the second is damage due to the tensile deformationtest itself. Judging from void content after hole damage,

the sample with the hole at the interlace is severelyweakened as compared to that with the hole at the warp.Thus, when a second deformation, for example a tensiletest, was performed the resistance of the system isalready weakened. This explains the large drop in tensileproperty (155.7 MPa compared to 171.8 MPa atwarp/weft hole location).

From the photomicrographs taken at the damage area,it is apparent that the interlace hole location shows anobvious and distinct fracture appearance as compared tothose of the warp/weft location. For example, more fiberdisorientation or misalignment and fiber fracture whichmight result in void formation and fiber pull-out areobserved in the specimen of the interlace hole location.This indirectly explains the observed drop in tensilestrength of the hole at the interlace location.

From the experiment carried out, it is apparent thatthe warp/weft hole position gives better hole damageresistance than at the interlaces. This can be easilyjudged from the increased voids and reduced tensileproperties, which were further evidenced from SEMphotomicrographs of the fractured specimens.

3.2. Hole sizes

Apart from hole position, in composite assembly ormechanical fastening, the need to use holes of varyingsizes is obvious. Previous studies on hole size on com-posite materials have been reported by several workers[4,8]. It was reported by Waddoups et al. [9], that, notsurprisingly, for tension specimens containing varioushole sizes, larger holes normally cause greater strengthreductions than smaller holes.

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Fig. 8. Effect of hole distance from the edge (x-r) on damage/undamaged tensile strength.

Bailie et al. [10] have studied the effect of holes ongraphite/epoxy woven fabric and UD composite tensilestrength. They found that the notched to unnotchedstrength ratio (residual strength ratio) of woven fabriccomposites was greater than that for UD tape com-posites. Chang et al. [11] working on notched strengthof woven fabric composite with molded-in holes have

noted that specimens with a molded hole imparts higherfailure strength than those of drilled specimens. Hence,many works on woven system have been done, yet theunderstanding of failure behavior with and without holesis still inadequate, particularly when working on a totallynew system.

Thus, the present study has attempted to analyze the

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Fig. 9. The average tensile strength of different hole sizes of single ply plain weave.

effects of different hole sizes on towpreg derived wovencomposite properties. The studies were carried out usingtensile strength properties of a 12 tow, ABS single plyplain weave. Here, holes with 2.5, 4.5, 6.5 and 8.5 mmdiameter were drilled at the warp position of Type I, 0°single ply plain weave specimens. The warp direction

was chosen as it showed better reliability and damageresistance in the earlier study.

Fig. 5 shows the stress-strain deformation behaviorwith different hole sizes in the woven composite system.In general, the damage resistance decreases with increas-ing the hole diameter. For example the 2.5 mm hole has

189M. Mariatti et al. / Polymer Testing 20 (2001) 179–189

the least effect while the 8.5 mm hole has the highest.However, an anomaly was observed in the case of the6.5 mm hole. Here, the stiffness was found to be higherthan that of 4.5 mm hole size, but the tensile strength ofthese two systems is more or less similar. This perhapsmight be due to an error during the preparation, i.e. cut-ting and hole drilling, of the 4.5 mm hole size. Asexpected, the specimen with 8.5 mm hole exhibits thelowest tensile properties. The drop of tensile propertieswith hole size is generally similar to the trend with theprevious studies [9,12], where the failure stresses andstrain at maximum stress are reduced as the hole diam-eter is increased. Theoretically, the presence of a hole ormacrovoids also leads to an increase in stress concen-tration effects near to the hole boundary. This will theninfluence the stress to be transferred from one point toanother point through the discontinuous fiber. Increase ofhole size implies more discontinuous fiber, higher stressconcentration and void content, thus resulting in a largerdrop in tensile strength. Fig. 6 shows the comparison invoid content for undamaged and damage specimens withvarying hole sizes.

Another factor that might contribute to this tensilestrength reduction is the distance of the hole to the edgeas shown in Fig. 7. In general, increasing the size ofholes which in return decreases the distance of the holefrom the edge, represented by the notations, “a” and “b”respectively causes a corresponding reduction in tensilestrength. This is due to the presence of less fibers nearto the edge that can prevent the tension stress adjacentto the hole. To realize this effect of the distance (x-r) isplotted against the damaged/undamaged tensile strengthor residual tensile strength, given in Fig. 8. Apparently,with increased distance from the edge to the hole bound-ary (x-r), the residual tensile strength increases.

Fig. 9 shows the effects of varying hole sizes on theaverage tensile strengths of the woven system. In gen-eral, a large hole causes bigger reduction in tensilestrength, at the same time attributing to poorer reliabilityor scatter in the properties as compared with small holes.This situation could be due to several possible reasonswhich amongst others include defects during drilling ofthe hole, the distance of the hole from the edge, fiber-matrix interface flaw and so forth.

4. Conclusions

From the experiment carried out, although still pre-liminary in nature, some conclusions can be made,which are:

1. Hole damage in a typical single ply plain towpregwoven thermoplastic system is controlled by the nat-ure of holes, with those at interlace positions showingthe most profound drop in tensile properties.

2. The damage behavior of single ply plain weave isalso controlled by the hole size, where large holesresult in lower damage resistance in the woven lami-nate system.

3. Position and size of holes are an important aspect inmaking a hole during assembly for a single ply woventhermoplastic composite.

References

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[2] M. Mariatti, M. Nasir, H. Ismail, Int. J. Polym. Mater.,in press.

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[4] J.M. Whitney, I.M. Daniel, R.B. Pipes, Experimentalmechanics of fiber reinforced composite materials, in: TheSociety in Experimental Mechanics, Prentice Hall, NewJersey, 1982 Brookfield Center, Connecticut, rev. ed.

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