EXPERIMENTAL INVESTIGATION OF ADHESIVE STRENGTHS OF ADHESIVELY BONDED JOINTS

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1 EXPERIMENTAL INVESTIGATION OF ADHESIVE STRENGTHS OF ADHESIVELY BONDED JOINTS Ki-Yeob Kang, Myung-Hyun Kim, Dong-Hyun Moon, Jae-Myung Lee Department of Naval Architecture and Ocean Engineering Pusan National University Jangjeon-dong, Geumjeong-gu, Busan, Republic of Korea Abstract: - In this study, the joining strengths of adhesively bonded joints are experimentally investigated. A series of lap-shear tests are conducted using single lap type adhesive joints. In order to analyze the joint fabrication factors that affected the joining strength, a parametric test is conducted by varying the adhesive thickness, surface roughness and fillet of adhesive. The quantitative results of the strength analysis are summarized, and some proposals are made on setting up testing standards for adhesively bonded joints. Key-Words: - Single lap joint, Adhesively bonded joints, Adhesive thickness, Surface roughness, Spew of fillet, lap shear test, 1 Introduction Structures for special purposes are designed by combining specific materials and structural units. Hence, a combination of components is essential in structural design. Recently, with the development of up-to-date technology, the magnitude of industrial structures has become increasingly larger. In addtion, these structures have more components and substructures, enabling them to perform diverse functions. However, this combination of components is one of the factors that increases the weight of a structure. Abnormal joints are directly or indirectly caused by this increased structural weight, which leads to fatigue fractures and economic losses. Because of this, the optimum combination of structural components and substructures must be found to decrease the weight of a structure and ensure structural safety. In general, steel materials are structurally joined using rivet and welding methods. However, it is difficult to join composite type structures and metal-composite hybrid type structures using these methods. In addition, these methods occur a stress concentration phenomenon and needlessly increase the structure s weight. In contrast, the stress concentration phenomenon hardly occurs with the adhesive bonding method, which can produce a lightweight structure [1]. In the shipbuilding industry, adhesives are used to manufacture hermetic bondings for boat hulls, resulting in a lower weight and higher floatability [2]. However, in order to obtain the considerable advantages of an adhesives, its application in a manufacturing process requires a specific adhesive joint design to enhance its performance and reduce its limitations such as its strength vulnerabilities, resistance to peeling efforts, etc. Hence, the importance of adhesive bonding has recently been spotlighted, and adhesive bonding-related researches are being carried out by engineers and scientists. Adhesive bonding systems show more vulnerabilities than welding system and rivets from the strength aspect. Therefore, adhesive bonding systems require structural reinforcement to be applied in an actual industry. Hence, adhesive bonding systems have been studied with the goal of reducing their strength vulnerabilities. In addition, a feasibility study is needed to apply in adhesives in structural units. In present study, a series of experiments was performed to reinforce the strength of an adhesive bonding system, and verify its suitability for actual industrial application. To achieve this, three experiment variables were selected as essential factors in adhesive bonding, namely, the adhesive thickness, surface roughness of the adherends, and the presence of spew fillet. Pereira et al. [3] investigated the strength variation according to the effect of the adherend surface treatment. You M et al. [4] reported an increased experimental shear strength after embedding a couple of metal components into the fillets. Lucas [5] considered diverse influence factors such as the adherend, geometric condition, surface treatment, environment variable, and investigated their effect Corresponding author: (Tel) , (Fax) , ( ) jaemlee@pusan.ac.kr ISBN:

2 on the adhesive strength. These studies were simple quantitative analyses to determine the factor affecting the material properties of an adhesive. In this study, to expand the application range of adhesive in structural bonding, experiments were conducted to find a better bonding condition than the strength performance of general adhesive bonding. The results can be utilized as an applicable database for lightweight structure design. 2 Experimental Apparatue A series of lap-shear tests was performed to compare the adhesive bonding strength. Fig. 1 shows the universal testing machine (SHIMADZU UH-1000KNI) used for the lap shear test. All of the tests were carried out under room temperature (293K) to consider the general functional environment of adhesively bonded joints. Fig 2. Photograph of test specimen of adhesively bonded joints A specimen of aluminum steel, which is used as a light-weight structural material, was produced with a single lap joint based on ASTM and GTT [10,11]. Fig 3, 4 show the lap-shear test specimens, along with a schematic of the specimens used in this study. Fig. 1 Photograph of universal testing machine and control box The loading speed was controlled at a rate of 5.0mm/min using the crosshead of the UTM. Prior to conducting lap-shear tests, all of the specimens were cured for the same period and at the same temperature. We categorized the tests by the adhesive layer thickness, surface roughness and spew fillet dependent tests. 2.1 Test Specimen The adhesive used in this study was DP-460 from 3M company. DP-460 is epoxy type adhesive and is the most commonly used adhesive in several industrial fields such as the pipe, pressure vessel, and aircraft composite fields[1, 6]. In addition, the most widely used type of adhesive bonding joints is single lap joint. A single lap joint is more economical than other methods and the manufacturing procedure is also relatively simple [7]. Hence this method is commonly used in aerospace and automobile industries [8, 9]. Fig. 2 shows a representative test specimen of the adhesively bonded lap shear joints. Fig. 3. Photograph of adhesive lap-shear specimen and specimen of presence of spew fillet Fig. 4. Standard of GTT lap-shear test specimen 2.2 Adhesive Lap-Shear Tests In this study, three kinds of experimental variables were selected: the adhesive layer thickness, surface roughness, presence of spew fillet effect. These variables have a direct effect on the adhesive bonding strength. The adhesive thicknesses were 0.5, 1.0 and 2.0 mm. Arenas et al. [12] investigated the effect of the adhesive thickness on the bonding strength. In their test results, adhesive thicknesses of less than 0.4mm showed the highest shear strength but had a high typical deviation. However, when the adhesive thickness was 0.5mm, the test results showed the highest reliability and a comparatively ISBN:

3 high shear strength value. Hence, in the present study, 0.5mm was selected as an adhesive thickness, along with other adhesive thickness with relatively large differences in order to investigate the bonding strength in relation to the adhesive thickness. In addition, surface roughness value of 0.0, 0.4, and 0.8mm were selected because 0.4mm and 0.8mm are the most widely used diameters for the steel balls used in the shot blasting method, with zero representing a non-treated surface. The adhesive fiillet represents the presence of spews. As known, the presence of spews can reduce peak stresses and therefore increase the joint strength. However, the reduction in peak stresses is related, not only to the presence of a spew, but also to the shape and size of the spew. In present study, how the adhesive fillet affects an adhesively bonded single lap joints was investigated. In addition a fixed strain rate was adopted as the test scenario in order to focus on a method for improving the strength performance. Table 1 summarizes the adhesive lap-shear test scenario. In order to ascertain the repeatability of the test results, the tests in each case were performed five times. Table 1 Condition for the adhesive lap-shear tests Group Surface Fillet Layer Treatment Thickness (mm) A B C D E F Non treatment Shot blasting (D= 0.4mm) Shot blasting (D= 0.8mm) Non treatment Shot blasting (D= 0.4mm) Shot blasting (D= 0.8mm) With Fillet Without Fillet Case No. 0.5 A-1 1 A-2 2 A B-1 1 B-2 2 B C-1 1 C-2 2 C D-1 1 D-2 2 D E-1 1 E-2 2 E F-1 1 F-2 2 F Roughness Measurement Fig. 5. Surface roughness measuring equipment Fig 5 shows a measuring equipment of surface roughness. Measure the depth of tiny hole on the surface using the pin treated by heating on the edge. Change what is moving on the edge of pin according to the height of the moving need to a current signal. In addition, measure the roughness, calculating the detected signal by computer. In this study, using the unit of R a. R a is the distance from the mean line to the center line on roughness curve is applied to mean roughness of the center line. This is the most frequently used unit for surface profile measurement when indicating the roughness. The measured profiles on the aluminium sheet surfaces resulted in the following roughness value. No treatment - Ra = 0.625μm Shot blasting ( diameter: 0.4mm ) - Ra = 8.72μm Shot blasting ( diameter: 0.8mm ) - Ra = 14.2μm 3 Test Results The obtained test results for each case are summarized in Table 2. In addition, Fig. 6, 7 show the load-displacement relationship obtained from the lap-shear tests in each case. Fig. 6 shows the test results for the specimen with the presence of an adhesive fillet, while Fig. 7 shows the test results for a specimen with no adhesive fillet. The results show that the presence of a fillet has a significant impact on the fracture force. In addition, in terms of the adhesive layer thickness, a thinner adhesive layer represents a higher load value. In this study, the adhesive layer thickness of 0.5mm represented the highest load value. No clear relationship was observed between the surface roughness and load. When the steel ball diameter was 0.4mm, the load had the highest value, while the non-surface treatment specimens obviously showed the lowest load value. Through these results, it can be concluded that an optimum surface roughness exists for the shear strength. In this study, when the diameter of the abrasive material was 0.4mm, the surface roughness was optimal. ISBN:

4 Table 2 Test result from adhesive lap-shear tests Group Maximum Maximum Case load Displacement No. (kn) (mm) A A A A B B B B C C C C D D D D E E E E F F F F (C) Fig 6. Relationship between fracture force and displacement, with fillet (A), (B), (C), (D) (A) (E) (B) ISBN:

5 (F) Fig 7. Relationship between fracture force and displacement, without fillet (D), (E), (F), 4 Conclusion In this study, the bonding strength of an adhesive was evaluated while the surface roughness, adhesive layer thickness, and the presence of fillet were varied systematically. In order to investigated these parameters, a series of lap-shear tests was performed under diverse test condition. The conclusion of this study are summarized below. The strength of the adhesive lap-shear specimens increased as the adhesive layer thickness decreased. This was because a thicker adhesive layer has numerous flaws such as voids and microcracks, which are considered to be potential strength reduction factors. Specimens with surface treatment had higher failure load values than the non-treated specimen. Hence, the surface treatment was considered to improve the adhesive strength. However, no clear relationship was observed between the bonding strength and surface roughness. An additional study is necessary to determint the optimal surface roughness. The presence of fillet had an effect on the failure load and displacement. Different welding techniques and welding joint types are utilized based on the structural type. In a comparison with a welded joint, it was concluded that a structural joint using adhesive bonding has a significantly high strength level. Hence, when structural joints with relatively small loads are designed, the adhesive bonding method could be applied, which would be a more cost-effective method. If the test assessment technique for the different scenarios examined in this study is used, in regard to the production of a structural joint using adhesive, it will be possible to suggest a method for a performance test and conduct a quantitative performance analysis. It is essential to carry out diverse systematic research on suggestions for a standard test method for a surface roughness analysis, a standard method for determining the thickness of the applied adhesive, and so on. Acknowledgment This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) through GCRC-SOP (Grant No ). This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (Grant No ). This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) through GCRC-SOP (Grant No ). References: [1] Regal plastic supply company, a division of regal supply company, plastics reference handbook, [2] SIKA Industry. Prontuario de productos. Madrid; [3] A. M. Pereira, J. M. Ferreira, F. V. Antunes and P. J. Bartolo, Analysis of manufacturing parameters on the shear strength of aluminum adhesive single-lap joints, Journal of Materials Processing Technology, Vol.210, No.4, 2010, pp [4] You M, Zheng Y, Zheng XL, et al. Effect of metal as part of fillet on the tensile shear strength of adhesively bonded single lap joint, International Journal of Adhesion & Adhesives, Vol.23, No.5, pp [5] Lucas. F. M. da silva, R. J. C. Carbas, G. W. Critchlow, M. A. V. Figueiredo and K. Brown, Effect of material, geometry, surface treatment and environment on the shear strength of single lap joints, International Journal of Adhesion & Adhesives, Vol. 29, No.6, 2009, pp [6] Adams RD, Comyn J, Wake WC. Structural adhesive joints in engineering. 2nd edition. Cornwall: Springer; ISBN:

6 [7] Kahraman R, Sunar M, Yilbas B. Influence of adhesive thickness and filler content on the mechanical performance of aluminum single-lap joints bonded with aluminum powder filled epoxy adhesive, Journal of Materials Processing Technology, Vol.205, No.1-3, 2008, pp [8] Higgins A, Adhesive bonding of aircraft structures, International Journal of Adhesion & Adhesives, Vol.20, No.5, 2000, pp [9] Grant. LDR, Adams RD, da Silva LFM. Experimental and numerical analysis of single-lap joints for the automotive industry, International Journal of Adhesion & Adhesives, Vol.29, No.4, 2009, pp [10] ASTM. Standard Test Method for Lap Shear Strength of Sealants, ASTM C961-06;2011. [11] Gaztransport & Technigaz S.A.s 1, Route de Versailles Saint Remy-les-Chevreuse, Procedure for gluing of supports for shearing samples, 2004 [12] Arenas, Jose M. Narbon, Julian J. & Alia, C., Optimum adhesive thickness in structural adhesives joints using statistical techniques based on weibull distribution. International Journal of Adhesion & Adhesives, Vol.30, No.3, 2010, pp ISBN: