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MaterialsJournal of Composite

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DOI: 10.1177/002199838902300403

1989 23: 337Journal of Composite Materials

J.H. Lee and S. MallStrength of Composite Laminate with Reinforced Hole

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Strength of Composite Laminate withReinforced Hole

J. H. LEE*AND S. MALL**

Department ofAeronautics andAstronauticsAir Force Institute of Technology

Wright-PattersonAir Force Base, OH 45433

(ReceivedApril 13, 1988)(RevisedAugust 24, 1988)

ABSTRACT

Tension and compression tests of quasi-isotropic [0/ 45/90]2s, graphite/epoxy lami-nates containing a circular hole with reinforcement were conducted. Two types of rein-forcement were investigated; adhesively bonded plug reinforcement or snug-fit unbonded

plug in the hole. For each case of reinforcement, four different sizes of hole diameter andthree types of reinforcing material (aluminum, plexiglass, steel) were employed for in-

vestigation. The experiments were mainly focused on the evaluation of ultimate strength

of laminate with reinforced hole in comparison to its counterpart with the open hole.

INTRODUCTION

HEUSE OF advanced composite materials has grown in recent years in aero-

space and other structures. The maintenance and repair of these structures is

always the concern of the users as well as the manufacturers. Various repair con-

cepts of composites are available which include a wide range of approaches from

highly refined and structurally efficient but expensive flush patch repairs to theexternal

mechanicallyattached metal

patch.In all these

repairmethods, one of

the several concerns is the prediction of the strength of the repaired laminates.Out of various kinds of repairing methods, the one selected for this study is anidealized case which simulates a situation where a damaged laminate has beenrepaired by drilling a hole and thereafter plugging the hole with reinforcement.Therefore, the present study involves the investigation of strength of compositelaminate containing a circular hole with reinforcement. This is a first steptowards the investigation of more sophisticated repair methods.The problem of notched strength of a composite laminate containing a circular

hole has been considered

byseveral

investigators (e.g.,see Reference [1]). How-

ever, very few studies have been reported for the evaluation of strength of a com-

* Former Graduate Student.** Professor.

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posite laminate containing circular hole with reinforcement. Kocher and Cross[2] investigated reinforced cutouts in composite laminates. These reinforcementswere cocured with the composite laminate so as to form an integral part of thestructure. ONeill [3], Pickett and Sullivan [4] studied the reinforcement of a holein a

very large composite plate (66cm

longand 25 cm

wide). They employedthe

reinforcing technique where additional layers of the same composite materials ofcircular shape were cocured on the sides of laminates with a hole. Recently, Tanand Tsai [5] have developed a closed form analysis which provides the stress dis-tribution near a reinforced hole in a composite laminate. This analysis has shownthat the strength of a notched laminate can be drastically increased with the

proper selection of reinforcement. However, no experimental verification of this

analysis [5] has been reported so far.The objective of the present study was, therefore, to investigate experimentally

the

strengthand failure mechanism of a

compositelaminate with a reinforced

hole. Two types of reinforcements were studied: (1) a bonded plug reinforcementand (2) snug-fit plug reinforcement. Three materials-steel, aluminum, and plexi-glass-were used as plug reinforcements. To investigate the hole size effect, fourhole diameters were selected. Two types of loading, i.e. tension and compres-sion, were included in this study. The composite material for this study was a

graphite/epoxy with a quasi-isotropic lay-up of [0/=i= 45/90~.

EXPERIMENTAL PROCEDURE

A test program was designed to evaluate the effect of the following factors onthe strength of composite laminates with a reinforced hole.

1. Type of reinforcement-(a) adhesively bonded plug and (b) snug-fit (fingerpress fit) plug.

2. Material of reinforcement-(a) steel, (b) aluminum and (c) plexiglass.3. Hole sizes-four different diameters - 2.54, 5.08, 10.16 and 15.24 mm.4. Loading type-(a) tension and (b) compression.Further, the test program included the investigation of failure initiation and

progression in these composite laminates with reinforced holes.All tests wereconducted at room temperature.

SPECIMEN PREPARATION

The material selected for the present study wasAS4/3501-6 graphite/epoxycomposite. Three panels were prepared from prepreg in an autoclave accordingto the manufacturers recommended procedure. To make sure that these were freefrom defects, each panel was inspected by ultrasonic C-scan. These panels wereof quasi-isotropic lay-up of [C/~45/90~. The nominal thickness was 2 mm.

To investigate the effect of reinforcement material, three materials having dif-ferent stiffnesses were used for inclusion as mentioned before. Steel has a much

higher stiffness than the base laminate. On the other hand, the stiffness of alumi-num is almost the same as the quasi-isotropic graphite/epoxy composite used and

plexiglass has much lower stiffness than the composite.



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Figure 1. Specimen configuration (W = 25.4 mm for hole diameters 2.54 and 5.08 mm, andW = 38.1 mm for hole diameters 10.16 and 15.24 mm).

All

specimenswere cut

bya diamond saw, and end tabs were attached.

Gagelength of the tension specimen was selected as 15.24 cm to exclude edge effect,but for compression it was selected as 10.16 cm to prevent buckling of specimen.Nominal specimen dimensions (both tension and compression) are shown in

Figure 1. Holes were drilled in the specimen center with a carbide tipped drill.Drilling the hole in each specimen was started by using a small drill, with an ad-ditional aluminum plate attachment on both top and bottom surface of specimento prevent burrs. The hole was then carefully enlarged to its final dimensions byusing the proper size drill.The reinforcing plug was made by using an automatic turning machine with a

ceramic tipped cutting tool by setting low feeding and high turning speed (to geta very fine and uniform surface). The laminates in the present study were in astate of plane stress and hence out-of-plane stresses are very nearly zero. The

length of plug should have, thus, very little effect. This was verified by conduct-

ing several preliminary tests with unbonded plug. However, proper bonding ofthese thin plugs in holes was a very difficult task. The plug length was, therefore,selected equal to 4 times the thickness of laminate for both tension and compres-sion test specimens to obtain maximum contact between plug and hole, and forconvenience of handling (especially during bonding).For the case of bonding, 0.1 mm clearance between plug and hole was selected

to obtain a good bondline between plug and composite. But for the unbondedcase, the plug was made almost the same size (less than 0.025 mm clearance) asthe hole, so it can be fit with hard finger pressure, and have the maximum contactwith hole but not damage the inside wall of the hole. The structural adhesive EA



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9302 (Hysol) was employed to bond the reinforcement in the hole. The bondingprocedure involved the standard steps of surface preparation and curing at roomtemperature for at least five days.

Testing Procedure

Both tension and compression tests were conducted in an Instron test machineat cross-head speed of 0.5 mm per minute. Several specimens were strain gagedto measure the strain-to-failure for the laminate. During the test, the stress-straincurves as well as load-crosshead displacement relations were recorded.A typicaltest involved the testing of specimen until complete failure occurred. However,several specimens were loaded incrementally up to failure at a regular interval,and specimens were taken out of the test machine after increasing each intervalof load to examine the

progressionof failure

by usingan

X-ray technique. Duringcompression tests, an antibuckling fixture was used to prevent the buckling of the

specimen. The anti-buckling fixture consisted of two support plates with holeswhich were clamped on the specimen by two C-clamps. The effectiveness of thisfixture was verified by observing the failed specimens which were actually due to

compression and not due to some combination of buckling and bending.

RESULTSAND DISCUSSIONS

In the first series of experiments, tension and compression tests of unnotchedand open hole specimens were conducted to develop the database for comparisonwith the results of reinforced hole specimens. Thereafter, tests with unbondedand bonded reinforcements were conducted, respectively. These will be dis-cussed separately in the following.

Unbonded Reinforcement

The detailed experimental results for both tension and compression tests forunbonded reinforcement will be discussed in the following.

TENSION TEST RESULTS

During tension loading of the reinforced composite laminates, it was veryimportant to maintain the maximum contact between plug and surrounding holethroughout the whole load history. To do this it is necessary to have the same sizeof plug as hole if possible, but not damage the inside of the hole when insertingthe plug. By the experience of several preliminary tests, it turned out that snug-fitclearance showed generally the maximum increase of ultimate strength. Heresnug-fit means the condition that the plug can be fitted with hard finger pressure

and can be moved inside the hole by finger pressure. The results of all these ten-sion tests are shown in Figure 2 in terms of Strength Reduction Factor, SRFwhich is defined as:



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Figure 2. Comparison of SRF among all unbonded inclusions and open hole (tension).

Each data in this figure is the average value of at least three test results.For the aluminum reinforced case, there was about 5 to 12 % improvement in

the strength from the open hole case for the hole diameter greater than 5 mm.

There was no such improvement in the case of 2.54mm. This may be due to a

very small amount of interaction between plug and composite laminate duringtension loading.The results of plexiglass reinforcement showed no increase. Because the stiff-

ness of plexiglass is too low in comparison to laminate, it did not respond to theload interaction inside the hole. In most cases the plexiglass reinforcement frac-tured when the laminate failed.

In the case of steel inclusion, the results for 2.54 and 5 mm showed similar

response as in the case of aluminum, but for the case of a larger hole, it showed

less increase than with aluminum. Since the stiffness of steel is much higher thanthat of the composite, it did not follow the deformed shape of laminate hole withincrease of the load.

In general, the aluminum reinforcement showed little improvement in ultimatestrength relative to open hole, while the steel and plexiglass inclusion showed noincrease. This may be attributed to the difference in moduli of laminate and rein-



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forcement.As mentioned previously, aluminum has the same in-plane Youngsmodulus as the composite, plexiglass has far less than the composite, and steelhas greater than composite. Thus it can be concluded that the reinforcementmaterial should have the same Youngs modulus as that of the base laminate for

the proper reinforcement.

COMPRESSION TEST RESULTS

The next series of experiments included compression tests with unbonded rein-forcement.As in the case of tension test, every plug was fitted with snug-fit clear-ance. The complete test results are shown in Figure 3 along with open hole datafor comparison.During the compression tests, as in the tension case, aluminum showed the

largest amount of improvement. For 2.54 mm diameter, as mentioned before, the

reinforcing area was very small, thus the improvement was relatively smallerthan other cases. For the larger hole sizes, the increase was significantly higher(e.g. 48% increase for 15 mm diameter).

In case of plexiglass reinforcement the improvement was relatively small.However, it was higher than tension case because the compressive load caused

Figure 3. Comparison of SRF among all unbonded inclusions and open hole (compression).



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more interaction between plug and hole. The results of steel inclusion alsoshowed a large amount of improvement as in aluminum case. This phenomenonis different than in the tension case. This can be attributed again to increasedinteraction between plug and hole during compression.

Bonded Reinforcement

Several preliminary tests were conducted involving different adhesives andclearances between hole and plug. These tests showed that EA 9302 adhesive wasbetter in comparison to other adhesives and a clearance of 0.1 mm was the ap-propriate clearance from bonding consideration as well as in getting the maxi-mum improvement in strength.The results of all tension tests with bonded plug are compared with the open

hole data in Figure 4. In this figure each symbol indicates the average value ofthree test results. These results, as given in Figure 4, clearly show that there wasno improvement in strength due to bonded plug for any hole size and for any plugmaterial in comparison to open hole. This was contrary to the expected increasein strength. To investigate the reason for this, the initiation and progression of

Figure 4. Comparison of SRF among all bonded inclusions and open hole (tension).



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Figure 5. Comparison of SRF among all bonded inclusions and open hole (compression).

damage in these tests were examined and is discussed later.The next series of experiments involved compression tests with bonded rein-

forcement. These tests were run with twotypes

of

reinforcement,i.e. steel and

aluminum. These results are shown in Figure 5. This clearly shows the improve-ment in strength due to reinforcement in comparison to open or unreinforced holewhich is very similar to its counterpart with unbonded reinforcement under com-

parison. In general, the increase in strength was comparatively more in case ofbonded reinforcement than unbonded reinforcement for both aluminum andsteel. However, bonded aluminum of larger diameter showed significant improve-ment in strength in comparison to the open hole (e.g. 68% increase for 15 mmdiameter) .

Failure MechanismAnalysis

To investigate the initiation and progression of failure mechanism, a few speci-mens with bonded reinforcement were loaded and unloaded at certain incrementsof failure load.At each interval, these specimens were inspected by microscopicand X-ray techniques. The initiation of crack in bondline was clearly seen by



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using a microscope, at approximately 90 to 95 percent of the failure load. Inalmost all the cases, failure initiated at the interface, either between adhesive andinclusion or between adhesive and composite laminate. Once the crack initiated,it grew criss-crossing from one interface to the other when the load was increased

further, and from that moment the plug was separated from the hole. Once theseparation occurred, the reinforcement was no longer more useful. Thereafter,the catastrophic failure occurred immediately. Obviously the adhesive is veryweak in tension, hence bonded reinforcement under tension failed without im-

proving the strength; rather they acted similar to unbonded reinforcement. On theother hand, bonded reinforcement showed significant improvement under com-pression even though failure initiated at the bondline. It can therefore be con-cluded that the full benefit of bonded reinforcement cannot be realized until a

proper bonding can be achieved.Authors devoted a great amount of effort in this

direction, but it was a very difficult task due to limited space between hole andreinforcement.The typical failed specimens for both tension and compression are shown in

Figures 6 and 7, respectively. Generally the tension failure occurred approx-imately at a 45 degree direction with the main load axis. However, the com-pression failure occurred at almost a 90 degree direction with respect to the load

Figure 6. Typical failure mode of tension test (left: 15.24 mm aluminum bonded, right: 5.08mm aluminum unbonded).



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Figure 7. Typical failure mode of compressive test (left 5.08 mm aluminum unbonded, right:15.24 mm plexiglass unbonded).

axis. Similar failure characteristics were observed with both types of reinforce-ment (i.e. bonded and unbonded).

General

As mentioned previously, Tan and Tsai [5] have developed an analysis whichprovides the stress distribution near a reinforced hole in a composite laminate.Using this analysis, the strength of a laminate with an inclusion can be predicted.These predicted strengths are given in Reference [5]. This study has shown thatthe maximum strength could be achieved by using inclusion that has similarelastic properties as that of base laminate since it minimizes the stress concentra-tion in the base laminate. Hence, an aluminum inclusion should provide the max-imum improvement in strength of the tested quasi-isotropic graphite/epoxy lami-

nate i.e. almost 100 percent recovery of strength (or SRF=

1) in comparison tosteel or plexiglass. However, it should be mentioned that this analysis [5] is basedon the assumption that there is perfect bonding between the base laminate and in-clusion. The present experimental investigation is, in a general way, in agreementwith the analysis of Tan and Tsai [5] if this assumption of perfect bondingbetween inclusion and base laminate could be realized as elaborated above.



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CONCLUSIONS

The following conclusions can be drawn from the present study:

1. In general, the composite laminate with a reinforced hole showed improve-

ment of ultimate strength relative to the laminate with open hole.2. The aluminum reinforcement showed the largest improvement out of all rein-forcing materials and especially under compression loading. This improve-ment depended on the size of reinforcement. In general, the largest size ofreinforcement showed more improvement.

3. With plexiglass reinforcement, the improvement of ultimate strength was verysmall because of the maximum mismatch in stiffness with base laminate.

4. The steel reinforcement showed almost the same amount of increase in ulti-mate strength as aluminum in compression but less improvement in tension.

5. The composite laminate with bonded reinforcement under tension showedpractically no increase in ultimate strength because of adhesive failure. Thus,if a strong bondline between reinforcement and plug could be developed, theultimate strength is expected to be better than unbonded reinforcment.

ACKNOWLEDGEMENTS

The authors would like to express their thanks to Dr. S. W. Tsai who sponsoredthis project, and to Drs. S. C. Tan and R. Y. Kim for their valuable help and sug-gestions during the course of this investigation.

REFERENCES

1.Awerbuch, J. and M. S. Madhukar. "Notched Strength of Composite Laminates; Prediction andExperiments-A Review," Journal of Reinforced Plastics and Composites, 4 (1985).

2. Kocher, L. H. and S. L. Cross. "Reinforced Cutouts in Graphite Composite Structures," Com-posite Materials (2nd Conference)ASTM STP 497, p. 382 (1972).

3. ONeill, S. S.Asymmetric Reinforcement of a Quasi-Isotropic Gr/Ep Plates Containing a CircularHole. MS thesis, Naval Post Graduate School, Monterey, CA (1982).

4. Pickett, D. H. and P. D. Sullivan.Analysis of Symmetrical Reinforcement of Quasi-Isotropic Gr/EpPlates with a Circular Cutout under Uniaxial Tension Loading. MS thesis, NPGS, Monterey, CA(December 1983).

5. Tan, S. C. and S. W. Tsai. Notched Strength. Composites Design, 3rd Edition (1987).


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