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Title: Development of Improved Hybrid Joints for Composite Structures Authors: C.T. Sun, Bhawesh Kumar, School of Aeronautics and Astronautics, Purdue University, W. Lafayette, Indiana 4790 and P. Wang, R. Sterkenburg, Aviation Technology, Purdue University, West Lafayette. Indiana 47907 Abstract: For structural joints in composites, the use of mechanical fastener is not a preferred joining method as drilling holes in composites reduce the strength due to fiber breakage. Adhesive bonding is found to be more promising as compared to mechanical fastening, because bonding two surfaces of composites does not need to break continuous fibers. However, bonded joints have disadvantages in controlling the thickness of the adhesive layer and some other quality control issues associated with the uncertainty of the joint strength. In order to provide a fail-safe design, the common practice is to use hybrid joints (bonded/bolted) in composites structures. In the existing designs of hybrid joints, the adhesive bond supports the entire load of the joint as long as the adhesive is intact and bolts do not have any contribution in load transfer before the failure of bonded surfaces. Therefore, the existing hybrid joint design has the same strength as the bonded joint. In the present work, a new hybrid joint design was proposed for composite lap joints, which use a small flat piece of composite laminate attachment to create an alternate load path to transfer part of the load from the adherend to the bolt. Experimental investigations revealed that the strength of the new hybrid joint was significantly greater than that of the conventional hybrid joint design. Two-dimensional finite element analyses were performed to provide the explanation of this strength enhancement. 2
INTRODUCTION Composites have been joined using mechanical fasteners or structural adhesives in various structural applications. Since aircraft structures are, in general, an assembly of many small or large parts, joining composite parts is an important technology. In metallic structural joints, mechanical fastening has been the most commonly used jointing technique. In composite joints, mechanical fasteners may reduce the strength of composites due to the fiber breakage required in drilling holes to accommodate fasteners. On the other hand, fiber breakage and stress concentration around the bolt location are avoided if adhesive bonding is used. There are other advantages in using adhesive bonding in composite structural joints including less weight penalty. However, controlling the quality of bonded joints as well as predicting their strength is quite difficult if not impossible. Therefore, a fail-safe design approach is commonly adopted where mechanical fasteners are used in addition to the adhesive bond. The combination of bonded and mechanical fastening is usually named as a hybrid joint or bolted/bonded joint. The hybrid joint technique has been studied by several researchers [1-9]. An analytical investigation was conducted by Hart-Smith [1] on a hybrid joint with stepped lap joints between titanium and carbon fiber reinforced plastic adherends. The strength of hybrid joints was found to be the same as well-designed bonded joints. In the peripheral of this problem, many researchers [2-5] proposed different numerical approaches and analytical solutions to analyze hybrid joints. Chan and Vedhgiri [3] conducted experiments with composite joints as well as a parametric study using finite element analysis to study the stacking sequence effect on joint strength. In that work, it was also found that bolts do not take an active role in load transfer before the initiation of failure in bonding, which was also noted by Hart-Smith [1]. Barut and Madenci [4] developed a semi-analytical solution method for stress analysis of a hybrid joint, and found that most of the load is transferred through the adhesive, even though it has low modulus as compared to the bolt. Kweon et al. [7] observed a similar phenomenon in their experiments, where a double lap hybrid joint was considered using composite and aluminum adherends. A sketch of the conventional hybrid joint is shown in Figure 1. 3
Adhesive (Main Interface) Bolt Adherend Figure 1. Conventional hybrid (bonded/bolted) joint Matsuzaki et al. [8] used a different approach to prepare hybrid joint using a bolted/cocured technique in order to avoid drilling holes in the fabricated composite parts. The resulting single lap hybrid joints of GFRP and aluminum were tested with an expectation that the hybrid joint would have a higher strength as compared to the co-cured (bonded) joint. The improvement in the hybrid joint strength was found to be rather insignificant. In the force-displacement curve of the hybrid joint, there are two segments; one corresponds very closely to the bonded joint and the other to that of the bolted joint. Therefore, even manufacturing techniques can not improve the performance of a hybrid joint over the bonded joint, except for achieving fail-safe design due to the presence of bolts. In all existing designs of hybrid joints, bolts do not participate in taking loads until almost after the complete failure of the bondline. In other words, bolts stay idle until the failure in the bonded part of the joint occurs. It is evident that to improve the hybrid joint a new design is needed that would involve the bolts in contributing to the load bearing at the joint before the complete failure of the adhesive. Other forms of improving adhesively bonded joints have been pursued. Qian and Sun [10] proposed an idea of using attachments to provide additional load transfer paths at a double strap joint. In this proposed design, in addition to additional load transfer paths, the localized interfacial stress concentrations near the joint edges are reduced. The strength of the joint with attachments is 20-30% greater than the conventional double strap joint. A similar approach was employed by Turaga and Sun [11] to improve single lap bonded joints. In [11], experiments were conducted with aluminum as well as composite adherends. Failure loads for conventional single lap joints and single lap joints with 4
attachments were compared. It was found that there was an increase of 59% in the bonded joint strength by using attachments. Recently, Kumar et al. [12] proposed a new hybrid (bonded/bolted) joint with bent attachments. All the experiments in [12] were conducted on joints with metallic adherends, Al-alloy 2024-T3. The increase in joint strength of the new hybrid joint with attachments was found to be approximately 80% higher as compared to the conventional hybrid joint. A sketch of the hybrid joint with bent attachments is shown in Figure 2 below. Adhesive (Main Interface) Bolt Attachment Adherend Figure 2. Hybrid joint with angular attachments [12] In this study, a new hybrid joint design for composites was proposed. Instead of bent attachments, straight composite laminate attachments were used. A sketch of the new hybrid joint is shown in Figure 3 below. In the proposed hybrid joint design, the bolt contributes to load bearing as soon as the joint is loaded. Adhesive (Main Interface) Bolt Attachment Adherend Figure 3. New hybrid joint with L-shaped attachments EXPERIMENTAL DETAILS 5
Experiments were conducted on five different joint configurations, bonded joint, bolted joint, conventional hybrid joint and hybrid joint with attachments. Hybrid joint with attachments had two subtypes, L-shaped and stepped, to be described in the following section. Specimen Preparation The AS4/3501-6 carbon/epoxy prepreg tape was used to fabricate a 20-plied laminate with a stacking sequence of [0 2 /90 2 /0 3 /90 2 /0] s. The stacked laminates were cured in an autoclave and then a water jet cutting-machine was used to obtain the desired dimensions of the adherends. The width of the specimens was kept as 38mm. The adherend thickness was maintained the same for all joint configurations. For the new hybrid joints (hybrid joint with attachments), two types of attachments were prepared. L-SHAPED ATTACHMENT L-shaped attachments were composed of two flat pieces of composite laminates (Fig. 3). The long flat piece was prepared with the same prepreg material with a stacking sequence of [0 2 /90 2 /0] s which is half of the total thickness of the adherend. The smaller piece was cut from the same panel for the adherends These two flat pieces of attachments were bonded together using structural paste adhesive Hysol EA 9394@Henkel Co. STEPPED ATTACHMENT To prepare the stepped attachments, four 0-deg AS4/3501-6 plies were stacked together and cured in the autoclave. Rectangular pieces of dimension 25mm x 38mm were cut using water jet cutting machine. Four of these small rectangular pieces were glued together in a stepped fashion. The top part of this stepped attachment was attached using Hysol to the top flat attachment as in the L-shaped attachment. 6
For the final joint preparation, the adherends were bonded together with Hysol first and then the attachment was bonded to the adherend as shown in Fig. 3. Subsequently, a diamond drill bit was used to drill a hole at the center of the overlapped region of the joint and a titanium bolt was placed in place and fastened. In the last step of the specimen preparation, tabs were attached to the adherends to mainly minimize load eccentricity in the single lap joint. Final specimens of joint configurations before testing are shown in Figure 4 below. A magnified view of the hybrid joint with attachments, L-shaped and stepped, is also shown in Figure 5. Bonded joint Bolted joint Conventional hybrid joint Hybrid joint with L-shaped attachments Figure 4. Joint specimens before testing 7
Hybrid joint with Stepped attachments Hybrid joint with L-shaped attachments Figure 5. Magnified view of the L-shaped and stepped attachments RESULTS AND DISCUSSION The experiments were conducted on an MTS 22Kips machine at a crosshead displacement rate of 0.01mm/Sec. Bonded and Bolted Joint Typical load-displacement curves for the bonded joint and bolted joint are shown in Figure 6 and Figure 7, respectively. 8
10 8 Load (KN) 6 4 2 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Displacement (mm) Figure 6. Load-displacement curve for the composite bonded joint 10 8 Shear-out failure begins (Bolt does not fail) Load (KN) 6 4 2 0 0 0.5 1 1.5 2 2.5 Displacement (mm) Figure 7. Load-displacement curve for the composite bolted joint 9
The failure of the bonded joint was catastrophic and was similar to that with metallic adherends as observed in [12]. The load-displacement curve varies linearly before failure. The failure load for the bonded joint was 8.5 KN. In the case of bolted joint, the peak load was observed to be 7.3 KN. At this load, shear-out initiated in the composite adherends. As shear-out progressed, the adherends became weaker and load decreased as shown in Figure 7. Unlike metallic joints, there was no bolt-failure in the composite bolted joint. A typical bearing failure around the bolt-hole location is shown in Figure 8. Magnified view Shear-out failure 90-Deg Plies 0-Deg Plies Figure 8. A typical shear-out failure pattern around the bolt Conventional Hybrid Joint The conventional hybrid joints were tested under the same test condition. A typical load-displacement curve for is shown in Figure 9. The failure process involves two steps in this case. The first failure occurs as complete debonding similar to the bonded joint followed by the failure similar to the bolted joint. The first failure of the conventional hybrid 10
joint occurred at a load slightly higher than the failure load of the bonded joint. The average initial failure load of the hybrid joint was found to be 11 KN. New Hybrid Joint with Attachments The new hybrid joint with attachments were tested under the same test conditions. Three specimens of each attachment type, L-shaped or stepped, were tested. The characteristics of the typical load-displacement curve were similar to that of the conventional hybrid joint except for the level of the failure load. 12 10 Bond failure Shear-out failure begins (Bolt does not fail) 8 Load (KN) 6 4 2 0 0 0.5 1 1.5 2 2.5 Displacement (mm) Figure 9. Load-displacement curve for the conventional hybrid joint HYBRID JOINT WITH L-SHAPED ATTACHMENT In the hybrid joint with L-shaped attachments, the first failure occurred at the main interface between the adherends as well as at the interface between the bottom part of the attachment and the adherend. After this failure, only the bolt carried the applied load, and the load-displacement behavior is shown in the second segment of the curve in 11
Figure 10. The average strength of this joint configuration was found to be 19 KN, which is around 75 % more than the conventional hybrid joint. HYBRID JOINT WITH STEPPED ATTACHMENT For the hybrid joint with stepped attachments, the load-displacement curve is shown in Figure 10. The first failure occurred at the main interface between the adherends while the stepped attachments remained bonded to the adherend. As the load increased, shearout started as in the bolted joint case. This implies that the stepped attachment performs better than the L-shaped attachment. The average failure load for this joint configuration was found to be 23.5 KN, which is 115% more than the conventional hybrid joint. Comparison of all joint configurations For comparison purposes, the load-displacement curves for the bonded joint, the conventional hybrid joint and the new hybrid joints are placed together in Figure 11. It can be clearly seen that the hybrid joint with stepped attachments yields the best strength among all these joint configurations. Figure 12 presents the strengths in bar chart. The failure load for the hybrid joint with stepped attachment is 160%, 120% and 20% higher than the bonded joint, the conventional hybrid joint and the new hybrid joint with L-shaped attachments, respectively 12
25 Hybrid Joint with L-Shaped Attachment Hybrid Joint with Stepped Attachment 20 Load (KN) 15 10 5 0 0 0.5 1 1.5 2 2.5 Displacement (mm) Figure 10. Load-displacement curves for hybrid joint with attachments 25 20 Bonded Joint Conventional Hybrid Joint Hybrid Joint with Rectangular Attachment Hybrid Joint with Stepped Attachment Load (KN) 15 10 5 0 0 0.5 1 1.5 2 2.5 Displacement (mm) Figure 11. Load-displacement curves for all joints 13
25 20 Failure load (KN) 15 10 5 0 Bonded Joint Conventional hybrid joint Hybrid joint with L-shaped attachment Hybrid joint with stepped attachment Figure 12. Comparison of failure loads for four joint configurations FINITE ELEMENT ANALYSIS Plane strain finite element analysis was performed using commercial code Abaqus 6.8-1[13] to find the effect of attachments on the hybrid joint. Since initial failure in all types of hybrid joints is due to debonding, a two dimensional FE analysis should give a reasonable estimation of the effect of attachments. Three cases are analyzed, bonded joint, hybrid joints with L-shaped attachments and stepped attachments. Dimensions of these FE models are similar to those of the actual specimens. A typical stress pattern for all the three cases are shown in the Figure 13. 14
Bonded joint Joint with L-shaped attachment Joint with stepped attachment Figure 13. A typical stress pattern for three joint configurations Peel stress distribution For the aforementioned three cases, peel stress distributions along the main interface, are shown in Figure 14. The peel stress distributions are calculated for the same applied load. It can be seen that the bonded joint has a higher peel stress compared to other two hybrid joints with attachments. The peel stress variations along the length of the main interface of the hybrid joints with L-shaped and stepped attachments are very similar, but are significantly lower than of the peel stress of the bonded joint. This explains why the hybrid joints with attachments are stronger than the bonded joint. Peel stress distributions along the interface between the adherend and the attachment of the hybrid joints with attachments. It is seen that the peel stress at the end of the L- shaped attachment is greater than that of the stepped attachment. This explains why the hybrid joint with L-shaped attachments is lower than that of the hybrid joint with steppedattachments. It is noted that the peel stress is oscillatory due to the stepping of the attachment. 15
Peel Stress (MPa) 7 6 5 4 3 2 1 0-1 Bonded Joint Bonded Joint with L-shaped Attachment Bonded Joint with Stepped Attachment 0 5 10 15 20 Distance along the main interface (mm) Figure 14. Peel stress distribution along the main interface Peel Stress (MPa) 7 6 5 4 3 2 1 Bonded Joint with L-shaped attachment Bonded Joint with Stepped Attachment 0-1 0 5 10 15 20 25 Distance along the interface between adherend and attachment (mm) Figure 15. Peel stress distribution along the interface of the attachment and the adherend 16
Load distribution For the hybrid joint with attachments, distributions of the load transfer were also calculated. It was found that 22% of the applied load was transferred to the attachment and the remaining 78% was transferred to the other adherend through the main interface. The optimization of the shape of the attachment can be done to transfer more loads through the attachment thereby reducing the load on the main interface. C D C 22% 78% Figure 16. Load ditribution in the hybrid joint with L-shaped attachment D CONCLUSIONS Five composite single lap joint configurations were tested. Bondline thicknesses were kept same for all cases, wherever bonding was needed. It was found that hybrid joint with attachments perform better than any conventional joint configurations, i.e., bonded or hybrid joints. Hybrid joints with stepped attachments performed better than hybrid joints with L-shaped attachments. It was found that the enhancements in the joint strength were 75% and 115% for the L-shaped and stepped attachments, respectively, over the conventional hybrid joint. Finite element analysis result indicates the hybrid joint with stepped attachments experiences the lowest peel stress among all the joint configurations considered. 17
ACKNOWLEDGMENT This research was supported by a grant from Federal Agency of Aviation for the JAMS Center of Excellence. 18
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