NSCC2009

Effects of geometrical modifications on behavior of adhesive joints used to bond CFRP laminates to steel members - experimental investigation

R. Haghani, M. Al-Emrani, R. Kliger

Department of Civil and Environmental Engineering, Division of Structural Engineering, Chalmers University of Technology, SE- 412 96, Göteborg, Sweden

ABSTRACT: One major problem when using FRP laminates to strengthen and repair flexural members is the stress concentration in an area close to the end of laminate which might govern the failure of the strengthening. A method that has been suggested to reduce the stress concentration in this area is to modify the geometry of the adhesive joint by tapering the laminate end and or adding an adhesive fillet. This paper presents the results of a comprehensive study on effects of geometrical modifications on beha-vior of adhesive joints carried out at Chalmers University of Technology. The focus of this paper is on the strain distribution in adhesive joints with untapered and normal-tapered laminates since the results from normal tapering of the laminate showed nega-tive effect of tapering on strength of joints. An optic measurement system, ARAMIS, was used to monitor the strain field in the adhesive layer. The results indicated that normal tapering of the laminate did not affect the shear and principal strain compo-nents, while it increased the maximum peeling strain in the joint for the tapering length examined in this study.

1 INTRODUCTION

One major problem when using bonded laminates to strengthen flexural members is the stress con-centration in terms of high shear and peeling stresses in the adhesive layer near the ends of the la-minate which might govern the failure of the strengthening in form of debonding, delamination or cohesive failure. To avoid the mentioned unfavorable failure modes, which are mostly governed by the transverse, i.e. through-thickness, failure of the joint components, special attention should be paid to areas with high shear and peeling stresses, i.e. the end of the laminate, when designing the adhesive joints. A suggested method to reduce the stress concentration at the area close to the end of the laminate is to modify the geometry of the joint in this area to provide a smooth path to transfer the force from the substrate to laminate (see for example Cadei et al. 2004). Geometrical modifications usually in-clude tapering of the laminate end and or adding an adhesive fillet at the end of the joint. The idea behind this method is to obtain a more gradual force transfer from structural member to strengthen-ing element. Two different configurations could be made using tapered laminates. If the use of a tapered lami-nate results in a joint with constant adhesive thickness, the schedule is called “normal tapering”, while, if the thickness of the adhesive layer increases towards the end of the joint, it is called “re-verse tapering”. These configurations are illustrated in Figure 1b and c.

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Figure 1. Different joint configurations: (a) basic, (b) normal tapering and (c) reverse tapering

A number of studies have been devoted to investigating the effect of tapering on the stress distribu-tion and strength of adhesive joints and the results from the literature indicate that a significant in-crease in the strength of the joint may be achieved through using tapered laminates – see, for exam-ple (da Silva and Adams, 2007; Belingardi et al., 2002; Hildebrand, 1994). However, there is some experimental evidence in the literature indicating that using normal-tapered laminates in adhesive joints might result in a reduction in the strength of the adhesive joint. Vallee and Keller (2006), Deng and Lee (2007) and Wendel and Luke (2007) carried out experi-mental investigations on effect of normal tapering on strength of adhesive joints. Vallee and Keller (2006) concluded that tapering of the laminate did not improve the strength of the joint even though it reduced the peak stresses along the joint. Results from the study carried out by Deng and Lee (2007) indicated that normal tapering of the CFRP laminate used for strengthening of steel beams might reduce the strength of the reinforcement. Wendel and Luke (2007) concluded that load bear-ing capacity of steel member strengthened with normal tapered strips was less than that of the spe-cimen with an untapered joint. The common point between these articles is that all authors have attributed their observations to joint defects such as non-uniform thickness of the adhesive layer in the joints, defects in the adhe-sive layer and etc. because the concept of tapering as a method to reduce the stress concentration and increase the strength of the joint has been accepted. Although geometrical modification of adhesive joints has been suggested by some design guidelines (see Cadei et al., 2004; Schnerch et al., 2007) as a method to reduce stress concentration in the ad-hesive layer and consequently increase the joint strength, they do not recommend any values for type of tapering, length of tapering or practical issues involved. A comprehensive study on effects of geometrical modifications on behavior of adhesive joints was carried out by Haghani (2008). The author investigated the effects of tapering of the laminate end and adding an adhesive fillet on strength and stress/strain distribution of adhesive joints used to bond CFRP laminates to steel mem-bers using numerical and experimental approaches. This paper presents some of the most important results and conclusions of the abovementioned study.

2 TEST SPECIMEN AND SET-UP

A total number of 14 specimens with different end details were prepared as illustrated in Figure 2 and Table 1. Each specimen consisted of a steel plate and two CFRP laminates bonded on both sides of the steel plate with epoxy adhesive. Figure 3 shows a schematic drawing of the test speci-mens and nominal dimensions. In tapered specimens, Figure 2c, d and e, laminates were tapered for 16 mm, equivalent to four times the thickness of the laminate and the adhesive fillets, Figure 2b and e, had an angle of 45 degrees. The E-modulus of laminate and adhesive used was 383 GPa and 7 GPa, respectively. The thickness of the laminate was 4 mm. Before bonding the laminates, the steel plates were first sandblasted on both sides and then cleaned with acetone as recommended by Cadei et al. (2004). A thin layer of primer was then applied to the sandblasted surfaces to protect them from corrosion and enhance the bond strength. Great attention was paid to the details at the ends of the laminates during preparation and bonding. Teflon plates were fixed on the steel plates at these locations in order to ensure the good alignment of the laminates at both sides, see Figure 3. Two parallel spacers were used on the edges of the specimen to ensure a uniform adhesive thickness of 2 mm. In order to prevent any sliding or shift in the laminate during the production of the test specimens, bonding was first performed on one side of the steel plate and the adhesive was left to cure for around five hours before bonding the other side.

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Figure 2. Different joint configurations studied, a)basic, b) filleted, c)normal-tapered, d)reverse-tapered and e)reverse-tapered with fillet

Figure 3. Configuration and dimensions of the specimen

Tabel 1. Specifications of test specimens.

Series Number of specimens

Notation (Figure 2)

SpecificationNormal taper Reverse taper Fillet

B-R-40 3 a B-NT-40 3 c xB-RT-40 3 d xB-RT-F-40 3 e x xB-F-40 2 b x

To fabricate specimens with tapered laminates, the laminates were sanded through the thickness us-ing a roller sander machine. Testing was carried out in a universal uniaxial testing machine in dis-placement control mode at a rate of 0.35 mm/min. One of the major challenges, when experimentally studying the adhesive joints, is the measurement of strains in adhesive layer due to the relatively little thickness. Recent advances in optic measure-ment techniques have provided new opportunities for the high-precision measurement of deforma-tions and displacements in many engineering applications. In this study a full-field commercial non-contact optic deformation measurement system ARAMIS TM 4M by GOM was used. The system uses a measurement technique based on Digital Image Cor-relation (DIC) with a stereoscopic camera setup, consisting of two CCD cameras with resolution of 4 Megapixels (2048×2048 pixels). The basic idea behind DIC is to measure the displacement of the specimen under testing by tracking the deformation of a natural occurring or applied surface speckle pattern in a series of digital images acquired during the loading. The experimental setup of the system is rather simple, see Figure 5. The cameras are placed in front of the specimen. In this study, the system was calibrated for a measurement area of approximately 35×35 mm2. The surface of the specimen should have a characteristic speckle pattern with high contrast which can deform together with the specimen during loading. In this study, the pattern was achieved by first applying white retro reflective paint as a background and then application of black stains on top of the white paint, Figure 4. Prior to painting, the surface of the specimen was carefully cleaned by acetone to ensure a good bond with paint. Image pairs at a sampling rate of one per second were taken by digital cameras. In addition, the sig-nals of the load and displacements obtained from the testing machine were also recorded in ARA-MIS system at the time of recording the images and therefore it was possible to synchronize the im-ages and the corresponding post-processed results with the load history of the specimen. The system

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used in this study was capable of measuring strains in range of 0.01 up to 100% with an accuracy of 0.01%.

Figure 4. Painted specimen and the area of strain measurement

Figure 5. Test set-up

3 RESULTS

3.1 Strength of joints and failure modes Tabel 2 presents the average value of measured strengths of specimens in each series with different configurations. Figures 6 to 9 present the failure modes of specimens. It should be mentioned that all specimens in the each series experienced the same failure mode. Tabel 2. Strength and corresponding failure mode in test specimens.

Specimen Strength (kN) Failure mode Notation (Figure 2) Failure B-R-40 93.0 A* a Figure 6 B-NT-40 80.7 A c Figure 8 B-RT-40 105.0 A d Figure 7 B-RT-F-40 >130.0 Yielding in steel eB-F-40 121.0 B* b Figure 9

*A: debonding at adhesive-steel interface, B: failure in adhesive fillet Considering the strengths of joints with different end details provides some useful information. It is seen that using reverse tapering of laminate (B-RT-40) or adding adhesive fillets (B-F-40) resulted in higher strengths of joints with regard to reference configuration (B-R-40). This is mainly due to reductive effect of these modifications on stress concentration. It is observed that effect of adhesive fillet in reduction of stress concentration and therefore increasing the joint strength is more pro-nounced in comparison to reverse tapering alone. Combination of reverse tapering and adhesive fil-lets can enhance the strength of joint even more. Surprisingly, it is seen that using normal tapering of laminate did not make any improvement in strength of the joint. This observation is in contradic-tion with recommendations made in some design guidelines (see for example Cadei et al., 2004) to use this configuration in order to increase the joint strength. In a previous study carried out by the authors of this paper (Haghani et al., 2009) the stress distribution in adhesive joints with different configurations including normal-tapered joints was investigated using the FE method. The results indicated that normal tapering may actually increase the magnitude of peeling stress at the mid-thickness of the adhesive layer at an area close to the end of laminate, while it does not affect the shear and principal stress components significantly for the examined tapering length (i.e. equal to the thickness of the laminate).

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Two distinct failure modes were observed in specimens. In all specimens without adhesive fillets, the failure took place at steel-adhesive interface. This failure mode was expected since the stress concentration is the highest at steel-adhesive corner and failure of the joint usually starts at this lo-cation. However, in filleted joint (Figure 2b), presence of adhesive fillet reduced the stress concen-tration and failure took place in the adhesive fillet. Specimens with combination of reverse taper and adhesive fillet did not experience failure before yielding in steel plates.

Figure 6. Failure in basic configuration Figure 7. Failure in reverse-tapered specimen

Figure 8. Failure in normal-tapered specimen Figure 9. Failure in filleted specimen

Figure 10 shows the principal strain fields in two specimens including basic (Figure 2a) and filleted configurations (Figure 2b). This Figure shows the strain concentration locations in adhesive layer in the two configurations from which the failure of joints started. It is seen that the highest strain con-centration took place at steel-adhesive corner in the absence of an adhesive fillet. The crack started at the very end of the bond line and propagated along the joint. When an adhesive fillet was present at the end of joint, the maximum strains formed around the sharp corner of laminate. In this case, the crack started from this point and propagated in two opposite directions towards the surface of the fillet and steel-adhesive interface (see Figure 9).

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Figure 10. Principal strain fields in (a)basic and (b)filleted joint configurations

3.2 Strain distribution in the adhesive layer Since the focus of this study is on comparison of the strain field between specimens with untapered and normal tapered specimens, only comparative results are presented. To investigate the effect of tapering on strain distribution, shear and peeling strain distributions for untapered and normal-tapered joints at two load levels of 30 and 70 kN are plotted together in Fig-ures 11 and 12 respectively. It is worth to mention that the ultimate strength of the specimens was around 80 kN. It can be seen that higher peeling strains exist close to the end of the adhesive joint when a tapered laminate is used. Figure 12 shows the corresponding shear strain distribution for the two joints. It can be seen that normal tapering of the laminate does not affect the shear strains in terms of either distribution or the peak value. These results are consistent with the findings in the previous numerical study conducted by the authors (Haghani et al., 2009). The strain distributions at the steel-adhesive interface are also compared in two specimens. Figures 13a and b show the prin-cipal strain distribution at the mid-thickness and steel-adhesive interface for untapered and normal-tapered joints respectively. It can be seen that the normal tapering of the laminate at either location did not cause any reduction in the peak strain value. This observation might be a plausible explanation for the fact that normal tapering of the laminate does not improve the strength of the joint.

4 CONCLUSIONS

Effects of geometrical modifications including tapering of laminate end and adding adhesive fillet on strength of adhesive joints were investigated. It should be mentioned that the following conclu-sions are strictly applicable to examined tapering length, i.e. four times the thickness of laminate.

1. The normal tapering of laminate increases the peeling strain at the mid-thickness of the ad-hesive layer, whereas it does not cause a considerable reduction in shear strains for the in-vestigated tapering length, i.e. 4 times the laminate thickness.

2. A comparison of the results indicates that normal tapering of the laminate does not result in any reduction in maximum principal strain at the mid-thickness of the adhesive layer or steel-adhesive interface.

3. The normal tapering of the laminate did not enhance the strength of the adhesive joint for the examined tapering length. This is believed to be mainly due to an increase in peeling stress/strain in the adhesive layer.

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Figure 11. Comparison between peeling strains for untapered and normal-tapered joints at (a) 30 kN and (b) 70 kN

Figure 12. Comparison between shear strains for unta-pered and normal-tapered joints at (a) 30 kN and (b) 70 kN

Figure 13. Principal strain distribution at a load level of 50 kN at (a) mid-thickness and (b) steel-adhesive interface

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4. The optic measurement system used in this study was found to be a powerful tool to monitor

the strain field in adhesive joints. Using this technique, it is possible to investigate the strain field over the thickness of the adhesive layer and around points of stress concentration.

5. The most effective configuration to increase the joint strength was found to be reverse-tapered with adhesive fillet followed by filleted and reverse-tapered configurations respec-tively.

6. Normal tapering of laminate is not recommended to be used as a method to reduce the stress concentration or increase the joint strength.

It should be noted that the results presented here are strictly applicable to the material properties and geometric configurations investigated in this study. The tests were performed on adhesive joints as-sumed to be used to strengthen steel beams with FRP laminates. The tests were performed in a la-boratory environment and long-term effects such as creep in the joint were not considered.

REFERENCES

Belingardi, G., Goglio, L., Tarditi, A., 2002, Investigating the effect of spew and chamfer size on the stresses in metal/plastics adhesive joints, International Journal of Adhesion and Adhesives, 22, 2002, 273-282. Cadei, J. M. C., Stratford, T. J., Hollaway, L. C., Duckett, W. G., 2004, Strengthening metallic structures us-ing externally bonded fiber-reinforced polymers, CIRIA 2004, ISBN 0-86017-595-2. da Sliva, L., F., M., Adams, R.D., 2007, Techniques to reduce the peel stresses in adhesive joints with com-posites, International Journal of Adhesion and Adhesives, 27, 2007, 227-235. Deng, J., Lee, M.K. 2007. Behaviour under static loading of metallic beams reinforced with a bonded CFRP plate. Composite Structures, Vol. 78, 2007: 232-242. Haghani, R., 2008, Analysis of geometrically modified adhesive joints in steel beams strengthened with composite laminates, Licentiate thesis, Chalmers University of Technology, Gothenburg, Sweden. Haghani, R., Al-Emrani, M., Kliger, R., 2009, Interfacial stress analysis of geometrically modified adhesive joints in steel beams strengthened with FRP laminates, Construction and Building Materials, V.23, No. 3, 1413-1422.

Hildebrand, M., 1994, Non-linear analysis and optimization of adhesively bonded single lap joints between fiber-reinforced plastics and metals, International Journal of Adhesion and Adhesives, 1994, V.14, N.4, 261-267. Schnerch, D., Dawood, M., Rizkalla, S., 2007, Design guidelines for the use HM strips: strengthening of steel concrete composite bridges with high modulus carbon fiber reinforced polymer (CFRP) strips, technical report No. IS-06-02. Vallee, T., Keller, T., 2006, Adhesively bonded lap joints from pultruded GFRP profiles. Part 3:Effects of chamfers, Composites part B, 37, 2006, 328-336. Wendel, S., Luke, S., 2007, Interface Failure Mechanics of Elastically (Advanced Composite) Reinforced Steel Members, Journal of Structural Engineering, ASCE, May 2007, 683-694.

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