©Freund Publishing House Ltd. Science and Engineering of Composite Materials 17, 199-212 (2010)

The Effects of the Butterfly Joints on Failure Loads and Fatigue Performance of Composite Structures

G rkan Altan*, Muzaffer Τορφυ, Hasan Qalhoglu

Faculty of Engineering, Department of Mechanical Engineering, Pamnkkale University, Kinikli 20070 Denizli, Turkey

ABSTRACT

The present study was designed to examine experimentally the effects of butterfly fitting clearances and different adhesives on the load carrying abilities of the composite structures joined via a butterfly-shaped joining component and the fatigue performance of the specimens joined by the best joint style obtained. Experiment specimens and butterfly- shaped locking components were cut from a composite plate by means of water jet. Fatigue experiments were conducted at the constant load ratio and at different maximum fatigue loads. To compare the fatigue performance, the fatigue experiments of the butt joints were also carried out. According to the experiment results obtained, it was determined that fatigue strengths of adhesively-bonded butterfly joints have a longer life span than those of the bonded butt joints under the same circumstances. The present study has made it possible that with the use of butterfly joining component, the earliest damage should occur on the butterfly and should have a longer service life with the repairs of the butterfly joining component.

Keywords: Butterfly joint; Composite; Fatigue life; Failure load.

1. INTRODUCTION

Composite materials are especially preferred in the aviation, navigation and automobile industries for their lightness. Large structures made from composite materials are generally used under static and dynamic loading conditions. Each large composite structure consists of one or more joints because of their production and design circumstances. Composite structures are usually joined via mechanical and adhesive joints. While industrial adhesive is used for the adhesive joints, such joining components as bolts and pins are preferred for mechanical joints. The main purpose here is to conduct the transfer of strength from the main structure through the combination of two or more materials. The most important problem with composite structures is the weakness in the joint areas. Load-carrying behavior of the joints and their advantages and disadvantages over each other were analyzed numerically and experimentally by many researchers and the researches are still going on. It is seen in the studies on mechanical joints that three types of damage modes are observed on the joining areas; net-tension, shear-out and bearing. The damage by net-tension and shear-out are more dangerous than the one by bearing /I, 21. Adhesive joint of the composite structures can be carried out by various designs /3-7/. Whether the

' Corresponding author. Tel: +90 258 296 31 63; Fax: +90 258 296 32 62 E-mail address: [email protected] (G rkan Altan).

199 Vol. Ð, Ëá 3, 2010 The Effects of the Butterfly Joints on Failure Loads and FatiguePerformance of Composite Structures adhesive to be used here adapts well with the composite material, the type of the joint geometry and the thickness of adhesion are factors that affect the capacity of carrying the load. Kim et al. /8/ presented a methodology for the failure prediction of the composite single lap bonded joints considering both the composite adherent and the bond line failures. In this methodology, they used an elastic-perfectly plastic model of the adhesive and a delamination failure criterion. They verified their suggested technique with numerical investigation. Alex and Wang /9/ optimized the profile of the .between dissimilar modulus adherents with an analytical method. Chen /10/ examined the effects of hygrothermal cycling upon the performance of a bolted composite joint. He determined that the bolt torque relaxed as the number of environmental cycles increased. Avila and Bueno /I I/ carried out a performance study on a new design of single-lap bonded joint, so called wavy- for laminate composites. Τορςõ et al. /12/ investigated damage forces formed on glass-fiber laminated composite plates that were joined with a component in the shape of butterfly with experimental method. Choi and Chun /13/ investigated a failure area method to predict failure loads of mechanically-fastened composite joints under stress condition. Herrington and Sabbaghian /14/ investigated the effects of a number of parameters; applied stress level, orientation of the outer layer reinforcing filaments and the bolt torque level on the fatigue life on the fatigue characteristics of a bolted graphite/epoxy composite laminate. Joining of the composite structures is commonly made by the single or double lap techniques. The increase in thickness of composite structure in the single or double lap joints affects the strength of the whole joined composite structure negatively. In such joints, the increase in the value axial width of the joined composite plates leads to high stresses on the adhering surface and a decrease in the strength of the structure /15/. In the laminated plates joined in this way, the damage usually takes place on the uppermost layer. In other words, with the increase in the amount of the width, the capacity of loading decreases. In mechanical joints, however, the increase in the amount of the width leads to damage in such joining elements as bolts and pins. Therefore, is often preferred in thick composite structures. In butt joint, bonding joints are used. In such joints, scarf or stepped-overlapping joints are preferred instead of butt joints in order to avoid peeling stresses. Though joint forms change in the bonded butt joints, peeling stresses affect the lifespan of the joint negatively /16-18/. Because of these reasons, butterfly-shaped mechanical butt joints were used in this study to minimize the negative effects. Mechanical butt joints were used to lock two semi specimens through their foreheads in shape. In the composite specimens joined with a butterfly-shaped joining component, the effects of the composite material fiber orientations, the butterfly fitting clearances and different adhesives on the load-carrying capacities were analyzed experimentally, and so have the fatigue performances of the specimens joined with the best joining style obtained; then their results were presented.

2. DESIGN OF THE BUTTERFLY JOINT

In this study, a new design of butterfly joint was used instead of bonded butt joints. As the thickness of the composite structure increases, the thickness of the butterfly-shaped joint component increases. This case leads to an increase in the cross section areas affecting the joint strength. Therefore, a fixed thickness was selected for this study and other variables affecting the joint strength were taken into consideration. Butterfly-shaped joining components were used to connect the composite plates from their forehead by the tight fitting method. As seen in Figure 1, specimens were cut out from the composite plate joined mechanically on forehead and then experiments were carried out. The effects of the change of the geometric parameters of butterfly-shaped joining components on the load-carrying capacities were analyzed in detail in Ref 12. In this study, the primary dimensions (b, L, t) of the specimen joined in mechanic butt joint were kept as fixed and width (w), middle width (x) and semi-length (y) of the joint lock in the shape of butterfly were changed. To see the effects of geometric parameters of the butterfly joints on the failure loads,

200 G. Altan. M. Τορςα. If. frllioglu Science and Engineering of Composite Materials

t i h T Ku t \fi i, L _\ X

Fig. 1: Type of butterfly joint. changes were made in the ratio of butterfly end width to specimen width (w/b), butterfly middle width to butterfly end width (w/x) and butterfly semi-length to specimen width (y/b) and a series of experiments were carried out. According to the result of these experiments, maximum joint load was determined when the butterfly semi-length is y=16 mm, the end width of butterfly joining component is w=16mm and butterfly middle width isx= 3.2 mm I\2I. The dimensions of the butterfly joining component were taken with these values in this study and experiments were carried out. If the joint between the A and B specimens shown in Figure 2 is made by the butterfly joining component only, the load applied from the A specimen is transferred to the B specimen through the edge surfaces shown by 3 via the butterfly joining component. During the transfer of the load, compression and shearing loads are formed on the edge surfaces shown by 3. If the butt joint is made both by the butterfly joining component and adhesive, the load transferred is sent to the B specimen via all the bonded edge surfaces and via the butterfly joining component. In this case, while the surfaces 1 and 2 (thick-lined areas) are exposed to the tension loads, the bonded surfaces 3 (thin-lined areas) are exposed to compression and shearing loads.

Â

t Figure 2: Load transfer of butterfly joint.

201 Vol. 17, No. 3, 2010 The Effects of the Butterfly Joints on Failure Loads and Fatigue Performance of Composite Structures

3. PRODUCTION OF THE MATERIAL AND ITS MECHANICAL PROPERTIES

The glass fiber-epoxy composite material used in this study was produced by the method of hot pressing at Izoreel Composite Isolate Materials. Glass fiber design was chosen as one-sided and two-sided. The epoxy resin used as the matrix material was formed with the mixture of CY224 epoxy of the ratio 100/80 and HY225 hardener. The mixture of epoxy resin and hardener was applied to each glass fiber layer in a mold and a 16-layer wet composite material was obtained. The wet glass fiber obtained in a mold was put under hot press in order that the epoxy composite material will be cured and reduced to minimum thickness. The glass fiber-epoxy material was cured under hot press at 120°C and pressure of 14 MPa for 2 hours. After these production procedures, the 16-layer composite plate was removed from the press and left to cool down at the room temperature. The thickness of the composite plate was measured as 3.5 mm after the process of trimming. The mechanical properties of glass fiber-epoxy composite material were characterized under tension, compression and in plane shear by using three specimens for each mechanical property and the average properties were determined. The standard deviation of the results is approximately 1.8%. The mechanical properties of composite material are

tabulated in Table l /19/. Tensile properties such as longitudinal modulus (£/), transverse modulus (£2), Poisson's ratio

(v/2), longitudinal tension strength (X,) and transverse tension strength (Y,) were measured by static tension according to the ASTM D3039-76 standard. Compressive properties such as longitudinal compression strength (JQ and transverse

compression strength (Yt) were measured by static compression on unidirectional specimens according to the ASTM D3410-75 standard. An important problem in the measurement of shear strength properties is to obtain a pure shear stress in the section of specimens. losipescu shear test method was used to define the shear strength (S) according to the ASTM D5379 standard. Shear modulus (G/>) was determined by a specimen whose principal axis on 45° according to the ASTM D3518-76 standard. Shear modulus was calculated by measurement of the strain in the tensile direction /20/.

Table 1 Mechanical properties of glass fiber-epoxy composite material

E, E-, G,2 X, Y, Xc Yc S Vi MPa MPa MPa 2 MPa MPa MPa MPa MPa

44150 12300 4096 0.20 775 130 305 80 95

Specimens used in the experiments were cut off with the water jet from the composite plates produced according to the geometric parameters at the firm of Zümrüt Glass. The material of the butterfly joining component used in the experiments is the same as that of the composite plate.

4. EFFECT OF THE FIBER ORIENTATION ANGLES

The present study, which is concerned with the joints made with the butterfly joining component, was designed to analyze the effects of the fiber orientation angles of the composite specimens and composite butterfly. To see the effect of the orientation angle changes, the fiber angles of the composite butterfly were taken to be the same as the specimen fiber angles. The specimens were joined by tight fitting the butterfly joining component and without using adhesive.

202 G. Alton. M. Topfu. H. Calhoglu Science and Engineering of Composite Materials

Figure 3 shows the damage states if the reinforcement fibers are 0° (longitudinal), 90° (transverse) and 0°/90° (longitudinal/transverse). As seen in Figure 3(a) and (b), the first damage was observed to be formed on the specimen and as matrix crack along the (S) line on the butterfly edges because of lateral loads of the butterfly in the longitudinally- and transversely-reinforced composite joints. With increase in the static tension load, the damage was again seen as matrix crack in the (Bl) and (B2) areas of the butterfly joining component. When the reinforcement fibers were 0°/90° (longitudinal / transverse) (Figure 3(c)), the damage was usually observed on the butterfly joining component (B3) rather than on the specimen. Because the composite butterflies with 0°/90° reinforced prevented the matrix cracks in the composite butterflies with 0° and 90° reinforced, they were seen to be carrying more loads. Moreover, thanks to the longitudinal and transverse reinforcement of the composite specimens with 0°/90° reinforced, it was determined that longitudinal and transverse cracks on the specimens were prevented and accordingly, there was a higher increase in the load-carrying ability.

(a) (b) (c)

Fig. 3: (a) [0]!6 (b) [90]i6 and (c) [(0/90)8]s; damage states at laminated composite specimens.

Static load-displacements relationships of the joined composite specimens are given in Figure 4. As seen in the

Figure, static tensile strength (F=2740 N) of the specimen joined with the reinforcement of [(0/90)8]s is higher than that of the other specimen [90]!6 joined with the transverse reinforcement (F=//#0 N) and that of the specimen [0]i6with the longitudinal reinforcement (F=2380 N). In addition, load versus displacements of the specimens joined with the reinforcement of 0° and 90° is smaller than that of the specimens joined with the reinforcement of ΟΎ900 because of the marix cracks on the specimens. Because the specimens joined with the reinforcement of 0°/90° have higher load-carrying displacements and capacity, they allow the longer period of working and quick repair of the joining components in case of any damage. Because much of the damage takes place on the butterfly, service life may be lengthened with the change of the butterfly joining component. For these reasons, composite specimens joined with the reinforcement of 0°/90° (longitudinal/transverse) were dealt with in the following studies.

203 Vol. 17, No. 3, 2010 The Effects of the Butterfly Joints on Failure Loads and FatiguePerformance of Composite Structures

5. EFFECT OF THE BUTTERFLY CLEARANCE AND ADHESIVE

Fitting clearances were formed on the butterfly without making any change in the dimensions of the specimen but by changing the dimensions of butterfly. Butterfly joining components with the fitting clearance were placed in their place on the specimen and then the clearances were filled with adhesives and then bonded. In Figure 5, the dimensions v>.^ of the butterfly are shown as (w) butterfly end width, (x) butterfly middle width and fy) butterfly semi-length.

3000

01 3456 Butterfly Displacement (mm)

Fig. 4: Failure loads of [0],6, [90],6 and [(0/90)8]s laminated composite specimens.

w

Fig. 5: Dimension of butterfly.

204 G. Alton, M. Τορςη. H. Calhoglu Science and Engineering of Composite Materials

In the butterfly joints, the effect of five different fitting clearance amounts on the damage* loads was analyzed. The amounts of newly-formed clearance were formed by reducing the dimensions of the tightly fitting butterfly joining component (x,w and y) by 0.1, 0.2,0.3 and 0.4 mm. Three types of adhesive were used in the joints made with five different fitting clearance formed. These are Loctite Hysol 3421, 3450 and 9464 epoxy adhesive. Hysol 3421 is a medium-viscosity epoxy adhesive for general purposes. Hysol 3450 is an epoxy adhesive that can be used in the GRP composite and cured quickly. Hysol 9464 is a strengthened epoxy adhesive that can be applied to the composite or different materials. The effects of the amounts of the butterfly fitting clearances on the damage loads are shown in Figure 6. Here, butterfly joints with 0.05 mm fitting clearance is called tight fitting butterfly joint. According to this, the amount of butterfly fitting clearance changes between 0.05 mm and 0.45 mm. The specimens with different fitting clearances and joined with epoxy adhesives were loaded at the speed of 1 mm per minute under the same conditions and the damages to them were observed. At least three specimens were prepared for each experiment. As seen in Figure 6, the more the fitting clearance increases, the more the load-carrying ability of the joint decreases. At the end of the experiments, the effect of the adhesive and butterfly fitting clearance were observed and the load-carrying capacity of the specimen with tight fitting and Hysol 9464 adhesive is better than that of the other spaced-fitting and adhesives.

3500

3000 -

2500 -

£ 2000 - _

e 1500 -

1000 -

500 ·

0.05 0.15 0.25 0.35 0.45 Quantity of Butterfly Clearance (nun)

Fig. 6: The effects of butterfly fitting clearance and adhesives on failure loads.

The relationship between the load-carrying capacities and displacements of the composite butterfly joints with different clearances and strengthened with Hysol 9464 epoxy adhesive is given in detail in Figure 7. The amounts of fitting clearance of the butterfly joining components are shown. Here the fitting clearance of 0.05 shows that the butterfly was fitted tightly and the one between 0.15 and 0.45 shows that the amounts of clearance increase. The load- carrying capacities of the butterfly joints strengthened with adhesive were analyzed in three parts. In the first part,

205 Vol. 17. No. 3. 2010 The Effects of the Butterfly Joints on Failure Loads and FatiguePerformance of Composite Structures bonded flat surfaces appeared to separate after the maximum load was attained in the inserted joint that is composed of an adhesive and butterfly. In the second part, the adhesive remained only on the bevel surfaces of the butterfly (Fig. 3). Some fluctuations were observed in the second part due to the breaking off the adhesive that came out on the side surfaces with the adhesive under the compression and shearing loads. In the third part, the adhesive lost its effect and the load was only met by the butterfly component and much later did some permanent damage occur.

3500

3000

-10123456 Displacement (mm)

Fig. 7: The effect of I lysol 9464 adhesive on failure load-displacement.

The picture of the formation of the first damage of the bonded butterfly joint is shown in Figure 8. Here, the first damage, which arose during the transition from the 1s' (I) area to the 2nd (II) as mentioned in Figure 7, is shown. After reaching the maximum load, separation of adhesive was observed on the flat surfaces of the joint (discontinuous lines). Afterwards, the adhesive remaining on the side bevel surfaces of the butterfly (thick-lined areas) lost its effect by cracking through the compression and shear stresses that arose during the process of loading and then the load was carried by means of the butterfly.

Fig. 8: The first failure of bonded butterfly joint.

206 G. Altan, M. Τορςιι, H. Qalhoglu Science and Engineering of Composite Materials

6. COMPARISON BETWEEN THE BUTTERFLY AND BUTT JOINTS IN TERMS OF DAMAGE LOADS

Figure 9 gives the comparison between the butterfly joints and butt joints under the same conditions in terms of the load-carrying capacities and load-carrying displacements. Hysol 9464 epoxy adhesive was used in the study. Butterfly joints were fitted tightly. The primary dimensions (b, L, t) of the specimens are respectively 40 mm, 180 mm and 3.5 mm. In the butt joints, however, bonding thickness was made equivalent to the butterfly tight-fitting clearance. Static damage experiments were repeated at least three times under the same conditions. As seen in Figure 9, load-carrying capacities of the butt joints are lower than those of the butterfly joints. The displacements of the butt joints are also lower. The biggest damage occurs in the butt joints with the sudden breaking of the adhesive. Load-carrying lifespan of the butterfly joint is longer. As seen in Figure 9, the rupture suddenly occurs after the formation of linear deformation under the load in butt joints. In the bonded butterfly connections, first linear deformation appeared and then non-linear deformation did; and finally, rupture occurred.

3500

0.5 1 1.5 2 2.5 3 3.5 4.5 Displacement (mm) Fig. 9: Displacements with respect to the failure loads of butt and butterfly joint.

Fig. 10: Adhesive failure in butt joint.

207 Vol. 17. No. 3, 2010 The Effects of the Butterfly Joints on Failure Loads and FatiguePerformance of Composite Structures

The damage of separation of adhesive in the butt joint is shown in Figure 10. In butt joints, irreparable damages may be caused by the sudden ruptures. If the joint is strengthened with a mechanical attachment in the shape of a butterfly, sudden and irreparable damages may be prevented and the service life spans of the composite structures may be lengthened. Figure 11 shows the comparison of damage load of different joint types. Here also the primary dimensions (b, L, t) of the specimens are respectively 40 mm, 180 mm and 3.5 mm. According to this comparison, the least load-carrying capacity was formed in butt joints. It was also observed that maximum damage loads of the butterfly joints strengthened with the adhesive are higher than those of the tightly-fitted butterfly joints. Error interval of the experiments was observed to increase always in the presence of the adhesive. As the error interval is influenced by the quality of the bonding workmanship, the cleanliness of bonding surfaces, the quality of the adhesive, the period of curing and environmental conditions for the process, it may be thought that the error interval increases.

4000 • Butterfly joint with 9464 Î Butt joint with 9464 Butterfly joint

Joint Type

Fig. 11: Comparison of different joint types.

7. COMPARISON OF THE FATIGUE PERFORMANCES UNDER DIFFERENT LOAD LEVELS

Fatigue experiments were carried out under a load ratio of 0.1 and a frequency of 10 Hz and under tension- tension load of constant amplitude in the shape of a sinus curve. As seen in Figure 12, the fatigue performances of the joints

under different Fmca loads were analyzed. To compare the fatigue performances, the same loads were applied to all the joints. Fatigue experiments were made under the room temperature of 23°C and relative humidity of 50%. Fmia fatigue load was taken at 60%, 50%, 40% and 30% values of the average static failure load of the tightly-fitted butterfly joint without adhesive. An average 60% static load of the tightly-fitted butterfly joint is almost equal to 50% of the average static load of butterfly joint strengthened with an adhesive, so fatigue performances of the butterfly joints with the adhesive were examined until 80% of their average static loads. The changes of the fatigue life spans of the butterfly and butt joints with the maximum fatigue loads are shown in Figure 13. According to the experiment results obtained, fatigue performances of the butterfly joints with adhesive are better than those of the others. Fatigue performances of the tightly-fitted butterfly joints, in contrast to the static loading,

208 G. Allan, M. Τορςα. Ç. Calhoglu Science and Engineering of Composite Materials were obtained as even worse than butt joints because of the different fatigue cross-sectional areas. In tightly-fitted butterfly joints, fatigue failure formed only in butterfly middle cross-section, but in the butt joints, fatigue failure formed along the bonded specimen width. Because of the difference of these cross-sectional areas, the fatigue performances of tightly-fitted butterfly joints were obtained lower than butt joints. The experiments were ended after 1800000 cycles without observing the damage on the specimens.

3500

3000

2500

2000

1500

1000

0.5 1.5 2 2.5 3 3.5 4.5 Displacement (mm)

Fig. 12: The values of maximum fatigue loads.

3000 Butt Joint Butterfly Joint 2500 Bonded Butterfly Joint

0 200000 400000 600000 800000 1000000 1200000 1400000 1600000 18000002000000 Number of Cycles Fig. 13: Fatigue performances of the different joints.

209 Vol. 17. No. 3, 2010 The Effects of the Butterfly Joints on Failure Loads and Fatigue Performance of Composite Structures

The pictures of the butterfly joints' fatigue damages are given in Figure 14. As fatigue damages always occur in the butterfly middle width in the joints with tight fitting (Figure 14(a)), it was found out that their fatigue life spans are rather short. In the butterfly joints strengthened with the adhesive (Figure 14(b)), the fatigue performances were found to be pretty high because of the mechanical effects and bonding properties. Fatigue failure cycles of the butterfly joint and bonded butterfly joint exposed to the same fatigue loads were determined respectively 17860 and 1356357.

(a) Tightly-fitted butterfly joint.

(b) Butterfly joint reinforced with the adhesive.

Fig. 14: Fatigue failures of mechanic and mechanic-adhesive joints.

8. CONCLUSIONS

This study was designed to analyze the load-carrying capacities and fatigue performances of the butterfly joints both bonded and not bonded and the butt joints experimentally. The glass fiber-epoxy composite material used in the experiments was manufactured with the hot pressing method. The experiment specimens were cut out of this composite material by the water jet. Cuttings were carried out at a proper cutting speed and pressure so as to eliminate the cutting

210 G. Altan, M. Topcu. Ç. Calhoglu Science and Engineering of Composite Materials irregularity and delamination that may emerge along the cross section of cutting during the cutting by water jet. Below are the results obtained from this experimental study: • It was determined that if the composite butterflies are reinforced by 0°/90°, they can transfer more load as they can prevent the matrix cracks that occurs in the composite butterflies reinforced with 0° or 90°. • It was also found out that load-carrying capacity of the specimens with tight fitting and Hysol 9464 adhesive is better than the other spaced- fittings and adhesives. • Load-carrying capacities and displacements of the butterfly joints are higher than those of the butt joints. • In the butt joints, some irreparable damages may arise out of the sudden ruptures. If the joint is strengthened by a mechanical connection such as the butterfly joining component, sudden or irreparable damages may be prevented and services life spans of the composite structures may be lengthened. • Fatigue performances of the bonded butterfly joints are better than those of the others. In the butterfly joints strengthened with an adhesive, the joined specimens were found out to have a very high fatigue performance as they were mechanical and worked as the bonded joint. • By changing the shapes of the butterfly-shaped joining components, different joint load-carrying capacities and fatigue performances may be attained.

ACKNOWLEDGEMENTS

The authors would like to express their appreciation to the TUBITAK, Turkey, Project No: 106M113 for providing financial support for this study.

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