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ScienceDirect Procedia Engineering 63 (2013) 287 – 294

The Manufacturing Engineering Society International Conference, MESIC 2013

Degradation of adhesive joints for joining composite material with aluminum under immersion in water and motor oil J.M. Arenasa, *, C. Alíaa, R. Ocañaa, J.J. Narbóna a

Universidad Politécnica de Madrid. C/ Ronda de Valencia 3, Madrid 28012, Spain

Abstract Structural adhesive joints are adequate for joining aluminum with composite material (automotive, aeronautic, etc.). However, in many occasions, these adhesive joints are subjected to very adverse service conditions. This way, depending where they are, they must resist great humidity, oil projections, high temperatures, etc. For this cause, an appropriate operation of these adhesive joints requires know its behavior under these conditions. In line with this objective, present work analyzes the degradation that adhesive joints of composite material and aluminum experiences when they are immersed in water or motor oil. Also, the loss of mechanical properties originated by this degradation has been evaluated quantitatively by flexion tests. © © 2013 2013 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. Selection and and peer-review peer-review under under responsibility responsibility of of Universidad Universidad de de Zaragoza, Zaragoza, Dpto Dpto Ing Ing Diseño Diseño yy Fabricacion. Fabricacion Selection Keywords: Structural adhesives; degradation of adhesives;water; motor oil; flexion tests.

1. Introduction Currently in many sectors of transport, new materials are investigated that are able to meet the requirements relating to the design and manufacture of structures becoming lighter and stronger that achieve greater speed with less energy consumption. In this sense, K.K. Charla (2001), N. Rastogi (2004), D.H. Cho et al. (1997) and D.R. Grant et al. (2009) consider that the polymer matrix composites reinforced with carbon fiber are a good alternative to make certain

* Corresponding author. Tel.: +34 913367694; fax: +34 913367677. E-mail address: [email protected]

1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of Universidad de Zaragoza, Dpto Ing Diseño y Fabricacion doi:10.1016/j.proeng.2013.08.218

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structural components of a vehicle. However, vehicles there are also other components and parts made of aluminum alloys to be joined to the composite. R. D. Adams et al. (1997) indicate that the use of structural adhesives in such unions (instead of traditional systems) provides, among other advantages, reduced weight, uniform stress distribution, sealed, and elimination galvanic corrosion. Arenas et al. (2013) have shown the feasibility of using these applications in structural adhesive joints for joining aluminum with composite material. However, in many cases, these adhesive joints are subjected to extremely adverse operating conditions. Thus, depending on the area where they are, they must withstand high humidity, motor oil projections, high temperatures, etc. When these conditions occur, the adhesive joints often suffer degradation and its mechanical properties are modified. Thus, when the adhesive joints are used in areas of the vehicle with condensation or oil projections, it required ensuring the adequate mechanical behavior in such conditions. In line with this objective, the present work analyzes the degradation of adhesive in joints of aluminum and composite when they subjected to immersion in water and oil. Likewise, the loss of mechanical properties caused by this degradation has been evaluated quantitatively by flexural testing. The immersion in the fluid (water or oil) is a procedure that allows knowing the long term degradation of the adhesive joint. In our work, concentration ratios in water and motor oil have been calculated by gravimetric tests (samples of bulk adhesive and independent immersion times up to 128 days). Also, end-notched flexure tests (ENF) have been performed in order to evaluate quantitatively the loss of mechanical properties that the degradation causes in the adhesive. ENF test is the most used method for the experimental assessment of the shear resistance. This test allows calculating the value of the fracture energy at the beginning of the inter-laminar crack by shear effort. The ENF test was initially proposed by Carlsson (1986) and consists of a three-point bending test in which the pre-existing interlaminar crack is forced to propagate by shear stresses that appear on both surfaces of the crack. These shears are generated when the sample is flexurally loaded at three points and a relative movement between both surfaces of the crack occurs, causing its mode II propagation. Chen et al. (2010) and Arrese et al. (2010) have used numerical methods to predict the mode II, while De Moura (2009) uses data reduction schemes. Todo et al. (2002) have also performed simulations using the finite element method. In summary, the goal of this study is analyzing the degradation of aluminum-composite adhesive joints under the action of water and motor oil. For this purpose, we have performed an accelerated aging of the adhesive joint by immersion in water and motor oil. Likewise, we have evaluated (with ENF test) the loss of mechanical properties that aging causes in the adhesive joint. 2. Methodology 2.1 Adherends properties. The composite samples are made of carbon fiber fabric, and epoxy matrix. They were prepared using the handmoulding technique. The carbon fiber is a reinforced, high-performance fabric (HexForce® 46301/1000/50% 6K HR), and the matrix is an epoxy resin (Resoltech® 1050/1056). The composite used in the tests consists of five layers of fabric and was cured at room temperature (pressure is 400 Pa). The other adherend is an aluminum alloy (series 6160) that is widely used in the automobile industry for its light weight and adequate mechanic behaviour. 2.2 Adhesives and surface treatments considered Arenas et al. (2013) have studied the viability of structural adhesives for joining aluminum with composite. They have concluded that the most suitable adhesives for this type of bond are epoxies (for static mechanical loads that require rigid adhesives) and polyurethane (for dynamic mechanical loads that require flexible adhesives). The tensile tests performed on single lap joints of composite and aluminum were used as references for a technicaleconomic analysis. This analysis concluded that the best results within the bond are achieved when the aluminum is subjected to machining processes (sanding for polyurethane and sand-blasting for epoxy), as well as an additional coating of peel ply for the carbon fiber. The peel ply treatment consisted of adding a final 80 g/m2, 0.1 mm thick nylon layer to the composite. Once

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the composite material was cured, the nylon layer was removed, leaving a rough but uniform surface. Sanding was completed manually with sandpaper (320 μm) and yielded uniform machined substrates. Sand blasting is a deepabrasion substrate treatment and was conducted using Guyson sand blasting equipment, model Euroblast 2SF. Artificial corundum with a 120 μm grain was used as an abrasive, and one pass was made at a distance of 10 cm. In this study, the adhesives used were a very-high-strength epoxy (Loctite® 9466 - Henkel) and a twocomponent polyurethane (Teromix®6700 - Henkel). The properties of these adhesives are shown in Table 1. Table 1. Properties of the adhesives used. PROPERTIES Shear Strength Rest time ISO 527 (h) (MPa)

Brookfield viscosity mPa·s (cP)

Curing time (h)

Epoxy Loctite® 9466

15000 a 50000

32

3

72

Polyurethane Teromix® 6700

-

13

0.5

48

2.3 Degradation in water and motor oil. For this study, bulk adhesive specimens were manufactured with the dimensions that the standard EN ISO ASTM D5229 proposes (1992) (Figure 1). Tests have been performed with specimens without introducing in water and motor oil. Also, tests have been performed with specimens immersed in water and motor oil during independent periods of time in order to study the diffusion of water and motor oil into the adhesive.

Fig 1. Dimensions of bulk adhesive specimen.

Specimens were immersed in water (pH: 6.35; conductivity: 95 ± 0.01 ms/cm; temperature: 20±0.01ºC) and motor oil (Castrol Power 1 Racing 4t 10W-50), for durations of 1, 2, 4, 8, 16, 32, 64 and 128 days. The amount of water absorbed was determined through gravimetric analysis. Three tests were performed for each immersion period. When the percentage of fluid absorbed by the polymer is small, Dillard (2010) proposes that Fick’s second law can be applied to calculate the diffusion coefficient of fluid in the adhesive:

dC dt

D

d 2C dx

2

where C is the concentration of fluid absorbed (%) t is the immersion time (s) D is the diffusion coefficient of fluid in the adhesive (m2s-1) x is the depth of fluid penetration (m)

(1)

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Absorbed fluid concentration C (t) is usually expressed as the relative difference between the wet and dry mass of the specimen. That is: C (%) = ((wet mass-dry mass) / dry mass) x 100

(2)

For gravimetric analysis, an electronic scale, model COBOS JC, with a precision of 0.01 g was used. Mass of fluid has been measured before and after of each independent period of immersion in order to calculate concentration coefficients. Water or motor oil that polymer absorbs can be partially eliminated by subjecting the samples to a vacuum drying (thereby recovering the initial mechanical properties). However, Suarez (2011) indicates that water or motor oil chemically bonded to the polar groups remains bound and is responsible for the adhesive long term degradation (with loss of the mechanical properties). 2.4 Preparation of the ENF test Figure 2 illustrates the arrangement and parameters that are characteristic of the adhesive bond used in the ENF tests. To produce the adhesive bond arrangements, a polyethylene tool was manufactured. This tool ensures the reproducibility of the experiments by maintaining the geometric parameters of the adhesive bond (the correct adhesive thickness and substrate alignment). Teflon was placed on a 25 mm extension corresponding to the precrack dimension to generate the notch on the end of the adhesive bond setup. To have the same adhesive thickness on all joints, 0.5 mm buffers were positioned on the ends of the aluminum adherent. A 125 g weight was placed on the upper substrate to homogenize the adhesive layer across the joint. Subsequently, the bond setups were introduced into a chamber (with controlled temperature and humidity) during the curing time of the adhesive.

Fig. 2. Dimensions (in mm) of the samples for the ENF test of aluminum-composite joints.

After this curing time had elapsed, the bonded materials were removed from the chamber, and a dimensional verification was carried out with a digital gauge. For the ENF test, a computer-operated, motorised TN-MD (HOYTOM) was used. Its capacity was 200 kN, its stroke was 125 mm, and its speed was set at 2 mm/min. A MotionMeter high-speed video camera (Redlake MASD) recorded the ENF tests and, with image correlation, measured the shear slippage of the crack tip. Crack length is measure at time intervals using an image analyzer. Three tests were performed for each of the selected adhesive bonds. Figure 3 illustrates a schematic layout of the

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ENF test.

Fig. 3. Schematic layout of the ENF test.

3. Results and discussion 3.1 Concentrations of water and motor oil. Figure 4 shows the relative concentrations of the fluid (oil and water) into the polyurethane as a function of immersion time. Figure 5 shows the relative concentrations of the fluid (oil and water) into the epoxy as a function of immersion time. In both fluids, there is a first section where the concentration increases. This first section is used to calculate the diffusion coefficient (critical level). From this maximum, the behavior of adhesives that have been immersed in water and oil are different. The concentration rises continuously with water. The concentration decreases sharply for oil and tends to stabilize at 0.5%.

Fig. 4. Concentrations of the fluid into the polyurethane as a function of immersion time.

Fig. 5. Concentrations of the fluid f into the epoxy as a function of immersion time.

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Thus, the oil concentration in the adhesive is lower than in water. Water concentration increases in the adhesive while that oil concentration decreases and stabilizes. Therefore, it follows that water will cause more degradation in the adhesive than motor oil. Additionally, comparison of Figures 4 and 5 leads to the conclusion that water spreads more into epoxy that into polyurethane (nearly doubles the concentration of water in the long term). It is also observed that the motor oil concentration into polyurethane during early periods of immersion is lower than into epoxy. However, both concentrations tend to be similar in the long time. 3.2 Results of ENF tests. Figures 6 and 7 show the variation of the failure stress as a function of immersion time for each adhesive joint. Adhesive degradation causes a progressive decrease in the failure stress of the joint (for both fluids).

Fig. 6. Failure stress curves for adhesive joint with epoxy.

Fig. 7. Failure stress curves for adhesive joint with polyurethane.

The analysis of Figures 6 and 7 yielded the following conclusions: - Adhesives degradation leads to a loss of mechanical properties in the adhesive joints. Considering the relative percentage, the reduction of failure stress in the epoxy is higher than in the polyurethane. With immersion in oil, the reduction is 20% for the polyurethane and 30'7% for the epoxy. With immersion in water, the reduction is 54.4% for epoxy and 23% for polyurethane. - The failure stress with immersion is higher in oil than in water (for polyurethane and epoxy). At end of 128 days of immersion and with relative percentage, the failure stress of polyurethane adhesive joint

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with immersion in oil is 16% higher than in water. Similarly, the failure stress of the epoxy adhesive joint immersed in oil is 26'6% higher than in water. - This difference between failure stress of both fluids increases with immersion time. Considering the case of epoxy, the relative difference increases from 35% (1 day of immersion) to 55.17% (128 days of immersion). Similarly, case of the polyurethane, this relative difference increases from 7.69% (1 day of immersion) to 24% (128 days of immersion). 4. Conclusions This work analyzes the degradation of aluminum-composite adhesive joints under the action of water and motor oil. For this purpose, we have been performed an accelerated aging of the adhesive joint by immersion in water and motor oil. Likewise, we have evaluated the loss of mechanical properties that aging causes in the adhesive joint (with ENF tests). Results of gravimetric tests show that motor oil concentration in the adhesive is lower than water concentration (except during the first days of immersion). Water concentration increases in the adhesive while that oil concentration decreases and stabilizes (0.5%). Additionally, tests have shown that the water spreads more into epoxy that into polyurethane (nearly doubles the concentration of water in the long term). It is also observed that the motor oil concentration into polyurethane during early periods of immersion is lower than into epoxy. However, both concentrations tend to be similar in the long time. ENF tests (end-notched flexure tests) show the loss of mechanical properties that aging causes in the adhesive joint. This loss of mechanical properties has been evaluated by the percentage reduction of failure load of the adhesive. Considering the relative percentage, the reduction of failure stress in the epoxy is higher than in the polyurethane. With immersion in oil, the reduction is 20% for the polyurethane and 30'7% for the epoxy. With immersion in water, the reduction is 54.4% for epoxy and 23% for polyurethane. The failure stress with immersion in oil is higher than in water (for polyurethane and epoxy). At end of 128 days of immersion and considering relative percentage, the failure of polyurethane adhesive joint with immersion in oil is 16% higher than in water. Similarly, the failure stress of the epoxy adhesive joint immersed in oil is 26'6% higher than in water. In summary, water degrades the adhesive more than motor oil. Additionally and under the action of water or motor oil, polyurethane adhesive joints retain their mechanical properties (failure load) better than epoxy adhesive joints. Acknowledgements The authors wish to thank Antonio Conesa of Henkel-Loctite for providing the adhesives and dispensers needed to perform the tests, as well as the Technical University of Madrid for funding this project (Q105605177). References Adams, R.D., Comyn, J., Wake, W.C., 1997. Structural adhesive joints in engineering. Chapman & Hall, London (U.K.). Arenas, J.M., Alía, C., Narbón, J.J., Ocaña, R., González, C., 2013. Considerations for the industrial application of structural adhesive joints in the aluminium–composite material bonding. Composites: Part B, 44, 417– 423. Arrese, A., Carbajal, N., Vargas, G., Mujika, F., 2010. A new method for determining mode II R-curve by the End-Notched Flexure test. Engineering Fracture Mechanics, 77, 51-70. ASTM D5229/D5229M-92, 1992. Standard test method for moisture absorption properties and equilibrium. American society for testing and materials. Carlsson, L.A., Gillespie, J.W., Pipes, R.B., 1986. On the Analysis and Design of End Notch Flexure (ENF) for Mode II Testing. Journal of Composite Materials, 20, 594-604. Charla, K.K., 2001. Composite Materials Science and Technology. Springer, New York (EEUU). Chen, C.C., Linzell, D.G., 2010. Modeling end notched flexure tests to establish cohesive element Mode II fracture parameters. Engineering Fracture Mechanics, 77, 1338-1347. Cho, D.H., Lee, D.G., 1997. Manufacture of one-piece automotive drive shafts with aluminum and composite materials. Journal Composite Structructural, 38, 309 – 319. De Moura, M.F.S.F., Campilho, R.D.S.G., Gonçalves, J.P.M., 2009. Pure mode II fracture characterization of composite bonded joints. International Journal of Solids and Structures, 46, 1589-195. Dillard, D.A., 2010. Advances in structural adhesive bonding, Woodhead Publishing Limited, Virginia (EEUU).

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Grant, D.R., Adams, R.D., da Silva, L.F.M., 2009. Experimental and Numerical Analysis of Clinch (Hemflange) Joints Used in the Automotive Industry. Journal of Adhesion Science and Technology, 23, 405-413. Rastogi, N., 2004. Design of composite drive shafts for automotive applications. In: SAE 2004 World Congress & Exhibition, Technical paper series, 2004-01-0485. Detroit (EEUU) Suárez, J.C., Alía, C., Pinilla, P., Biezma, M.V., 2011. Degradación a largo plazo de uniones adhesivas estructurales: ensayos en modo mixto de probetas DCB. Anales de Mecánica de Fractura, 28, 663-668. Todo, M., Potter, K.D., Wisnom, M.R., Adams, R.D., 2002. Initiation of a mode-II interlaminar crack from an insert film in the end-notched flexure composite specimen. Composites Science and Technology, 60, 263-272.