The Influences of Overlap Length, Bond
Line Thickness and Pretreatmant on the
Mechanical Properties of Adhesives:
Focussing on Bonding Glass
Janneke Vervloed, Adrie Kwakernaak, Hans Poulis
The Adhesion Institute, Delft University of Technology, The Netherlands, info@hechtingsinstituut.nl, www.hechtingsinstituut .nl
This paper focuses on the influences of overlap length, bond line thickness and pretreatment on the mechanical properties of adhesive bonds. In order to determine the bond strength, lap shear tests were performed. The researched adhesives are a 2 component epoxy and MS polymer. The smallest overlap length of the epoxy adhesive results in the highest maximum bond stress. However, there is no significant difference in maximum bond stresses due to different overlap lengths of the MS polymer. When the bond line thickness of the MS polymer is increased, the maximum stress of the adhesive bond decreases considerably. In addition of these tests some examples of our experience in bonding glass are presented. Keywords: Adhesion, Epoxy, MS polymer, Pretreatment, Glass
1.Introduction
The Adhesion Institute is a university institute specialized in validation of research by direct applied research on structural adhesive bonding. Often designers get their information on adhesive bonding from the adhesive retailers. It may be a risk to rely on their advice only, since they are not neutral and often they do not have enough
knowledge of the application, mechanical properties and durability of the adhesive bond.
Adhesive bonding as a joining technology has been applied since ancient times using natural materials. The first known application of adhesive in a structural application is the use of bitumen about 36,000 years ago [1], to bond wooden tool handles to flint stone. In early aircraft design, adhesive bonding was used to build up wooden structures for frames, contoured ribs and spars. Wooden parts were generally laminated with various fiber directions to create sufficient strength. The first large-scale application of structural adhesive bonding was in the fuselage (stiffened panels) of the first jet airliner, the De Havilland Comet. Since these early applications of adhesive bonding, technology has developed much further over the years. In the Airbus A380 (Figure 1) GLARE® is applied. GLARE® is a fiber metal laminate; glass fibers embedded in an epoxy resin (adhesive) are bonded to thin aluminum sheets (Figure 2). The development of this hybrid material took place in the laboratory of Delft
Figure 1: The application of GLARE® in the Airbus A380
Figure 2: A schematic presentation of the fiber metal laminate, GLARE®. The grey layers are aluminum sheets and the green layers are high strength glass fibers in an epoxy resin.
Papers on adhesive bonding usually focus on one particular aspect of bonding such as one particular substrate with one type of adhesive system. This paper focuses on some general, basic, aspects of adhesive bonding. In order to show the influences of overlap length, bond line thickness and pretreatment of a material on the mechanical properties of adhesives, lap shear tests were performed. Two kinds of adhesive were used for these tests: a 2 component epoxy and MS polymer. The test results show that the smallest overlap length of the epoxy adhesive results in the highest maximum bond stress. However, there is no significant difference in maximum bond stresses due to different overlap lengths of the MS polymer, however, when the bond line thickness of the MS polymer was increased, the maximum stress and strain of the adhesive bond decreased considerably. The strength of a bonded joint does not only depend on the geometry of the joint and the adhesive used. The surface treatment of the substrate can have a large influence on the strength, especially when joints are exposed to the environment.
The question of which pretreatment to use depends on the material or surface to bond to, the type of application, as well as the adhesive used. Only degreasing and abrasion of aluminum resulted in a 50% higher bond strength of the adhesive bond. Section 2 of this paper describes the experimental procedure, Section 3 presents the results and discussion, Section 4 gives examples of bonding glass and Section 5 presents the conclusions.
2.Experimental procedure
This section describes how the specimens were manufactured and tests performed.
2.1.Substrate and pretreatment
The specimens were made out of Aluminum 6061-T6 and degreased and/or abraded.
2.2.Adhesive materials
Two types of adhesive material were used to bond the substrates: a 2-component epoxy adhesive (3M EC 9323) and a MS polymer (MS 930).
2.3.Specimens
The dimensions of the substrates were: 25 x 100 x 1.5 mm. The overlap lengths varied from 6 to 80 mm and the bond line thicknesses varied from 0.15 to 2.5 mm (Figure 3).
Figure 3: Specimen for lap shear tests
2.4.Test specifications
The tests were performed according to the ASTM D1002 test standard.
In this lap shear test, tensile loads are applied on each side of the bonded specimen. The test stops when the bond of the specimen fails. This test is most commonly used for testing adhesive bonding on shear stresses, since the preparation of the specimens is easy and fast. For almost every application of adhesive bonds, a test standard has been developed.
Overlap length
Substrates Adhesive
3.Results and discussion
3.1.Influence of the overlap length on the maximum shear stresses
Figure 4 shows the influence of the overlap length on the maximum shear stresses. Influence of overlap length on the maximum shear stresses
0 5 10 15 20 25 30 35 40 0 10 20 30 40 50 60 70 Overlap length [mm] M a xi m u m s h e a r st re ss [ M p a ] Al, 2c epoxy, t=1.6mm Al, MS polymer, t=1.6mm
Figure 4: Influence of overlap length on maximum shear stresses
Surprisingly the maximum shear stress of the 2c epoxy decreases when the overlap length increases. An explanation for this effect can be found in the deviation of stresses that occur in the epoxy adhesive. The shear lag theory by Volkersen [2] shows that the shear stress distribution is highly non-linear in the direction of the overlap. High peak stresses are found at the overlap edges, whereas in the middle of the bond line the stress levels are low, as shown in Figure 5. Peel stresses, which are perpendicular to the substrate surface, are also highly non-linear (first addressed by Goland and Reissner [3]). From this it follows that in a bonded (overlap) joint almost all load is carried by the first few mm near the edges [4].
When assuming the adhesive to behave linear elastic, the bonded joint fails when the peak stresses at the ends of the overlap become equal to the adhesive shear strength. Due to the non-uniform distribution, the average shear stress is significantly lower, which has an unfavorable effect on the efficiency of the bonded joint. The joint strength is not only dependent on the strength of the adhesive used, but also on the elastic modulus of the adherend materials as well as the joint geometry (adherend thickness and length of the overlap).
Shear Stresses 0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 0.00 2.50 5.00 7.50 10.00 12.50 Overlap Dis tance (m m )
T x y ( M P a ) ta=0.05mm ta=0.15mm ta=0.25mm ta=0.35mm ta=0.45mm Peeling Stresses -10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 0.00 2.50 5.00 7.50 10.00 12.50 Overlap Distance (mm) S y ( M P a ) ta=0.05mm ta=0.15mm ta=0.25mm ta=0.35mm ta=0.45mm Shear Stresses 0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 0.00 2.50 5.00 7.50 10.00 12.50 Overlap Dis tance (m m )
T x y ( M P a ) ta=0.05mm ta=0.15mm ta=0.25mm ta=0.35mm ta=0.45mm Peeling Stresses -10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 0.00 2.50 5.00 7.50 10.00 12.50 Overlap Distance (mm) S y ( M P a ) ta=0.05mm ta=0.15mm ta=0.25mm ta=0.35mm ta=0.45mm
Figure 5: Influence of overlap length on shear stresses of an epoxy adhesive
As stated before, the stresses in an adhesive bond are generally not evenly distributed along the bond line. Thus, the influence of the overlap length on the bond strength will depend on both the adhesive properties and the design of the bond. At a certain point, the middle of the overlap length will not encounter any stresses. In that case a long overlap length is not preferable; the failure of the specimen is than fully dependant on the peak stresses at the overlap edges. However, there are no significant differences between the maximum shear stresses of the MS polymer for each overlap length. Unlike the rigid epoxy adhesive in Figure 6, the MS polymer is a much more flexible adhesive, meaning that the load transfer is proportional to the overlap length (Figure 7).
Figure 6: 2c epoxy adhesive: Peek stresses at the edges the overlap length
Figure 7: MS polymer: Stresses constant to the overlap length
The stress distribution depends largely on the mechanical characteristics of the adhesive. Flexible adhesives provide joints with a much more uniform distribution and less difference between the average and maximum stress. These adhesives distribute peel and shear stresses over a larger area thereby improving the joint efficiency. However, since adhesives with high flexibility and elongation typically show lower cohesive strength than more rigid adhesives, the advantage of flexibility and high elongation is usually compromised. In order to transfer the same load, a much larger overlap is needed, as shown in Figure 8.
3.2.Influence of the bond line thickness on the maximum shear stresses
Figure 9 shows the influence of bond line thickness on the maximum shear stresses for a MS polymer adhesive.
Figure 9: Influence of overlap length on maximum shear stresses of MS polymer to aluminum bonds
Thicker adhesive bond lines with flexible adhesives, such as a MS polymer, allow more elongation. Adhesives with high flexibility and elongation have an advantage when bonding substrates of different materials, where the flexibility can overcome differences in thermal expansion.
3.3.Influence of pretreatment on the maximum shear stresses
Effective bonding is not only the question of choosing the most suitable adhesive for the application, but also combining it with the appropriate pretreatment. Pretreatment in this case exists of cleaning and roughening of the surface to bond to. Half of the here discussed and tested aluminum substrates were only degreased before bonding with a 2c epoxy adhesive and the other half of them were degreased and abraded with Scotch-Brite. The old oxide layer was removed by abrasion and the fresh oxide layer surface was thus enlarged and roughened to improve the adhesion.
Test result show that the maximum shear stresses of the aluminum specimens that were only degreased were lower than the maximum stresses of the specimens that were just abraded. The tested specimens that were only degreased before bonding showed a fracture between the substrate and the adhesive material (adhesive fracture), though the specimens that were degreased and abraded showed a fracture within the adhesive material (cohesive fracture). A cohesive fracture indicates that at least the bond between the adhesive and substrate was stronger than the cohesive strength of the adhesive itself. Half of the aluminum substrates that were bonded with MS Polymer were degreased, and the other half was degreased and abraded with Scotch-Brite. The specimens that were only degreased showed an adhesive failure and the specimens that were degreased and abraded showed a 100% cohesive fracture, but the maximum shear
Influence of bond line thickness on shear stress and elongation
0 0.2 0.4 0.6 1 0.5 1 1.5 2 2.5 3
Bond line thickness [mm]
S h e a r S tr e ss [ M P a ] 0 2 4 6 8 10 12 14 S tr a in [ m m /m m ] Shear stress Elongation at failure Strain at failure E lo n g a ti o n [ m m ] 1.2 0.8 1.4
stresses were the same. This indicates that the cohesive strength of the flexible adhesive is the weakest part of the bond.
3.4.Pretreatment and durability of an adhesive bond
Figure 10 presents the influence of pretreatment on the durability of an adhesive bond on aluminum. The pretreatment is critical when an adhesive bond is to be exposed in a corrosive environment. The higher the temperature during such a test, the more pronounced the effect.
Figure 10: Influence of pretreatment on the durability of an adhesive bond on aluminum [5]
Generally speaking, the design principles and rules found for adhesive bonding in the aircraft industry also hold for other types of industry, such as bonding glass. Glass also needs the right pretreatment in order to obtain high performance and durable bonds. Some examples are given in the next section.
4.Bonding Glass
In addition to the lap shear tests that were performed to show the most important principles of adhesive bonding, this section focuses on bonding glass.
4.1.Pretreatment of glass
The strength of the adhesive bond on glass is very critical and highly dependant on the pretreatment: the cleaning of the glass surface. Glass is most clean after a pretreatment with UV/Ozone or plasma. The second best pretreatment is degreasing the surface and applying a silane primer. Of course, for each application of bonded glass, testing should be done to determine the most suitable adhesive and pretreatment in relation to the initial strength and durability.
4.2.Bonding glass to aluminum with UV curing adhesives
For a company that builds greenhouses on a large scale, tests were performed to determine the effect of moisture exposition to adhesive bonding of glass to aluminum with UV adhesives (Loctite, Dymax, Photobond). The pretreatments applied to the glass were degreasing, UV/ozone treatment and a silane primer. Half of the specimens were exposed to moisture (3 weeks at 50°C and 85% RH) and the other half of them were not exposed. All specimens, lap joints, were tested under compression load to determine the maximum shear stresses before failure.
In Figure 11 presents the maximum shear stresses of the adhesive bonds with UV adhesives. Specimens 4 and 5 were bonded with the same adhesive, but pretreated differently. Specimens 4 were primed and specimens 5 were degreased only. The effect of moisture in combination with these types of pretreatment is shown. Clearly, the adhesive bond is highly influenced by moisture ingress.
0 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 Specimens M a x im u m s h e a r s tr e s s [ M P a ] not exposed exposed
Figure 11: Influence of moisture on maximum shear stresses of bonding glass to aluminum with UV adhesives. 1: Loctite 3494/40secUV. 2: Loctite 352+activator 7075/20secUV. 3: Dymax 6-620. 4: Photobond
4468 +silane primer. 5: Photobond 4468+degreasing.
4.3.Bonding glass to stainless steel with MS polymer
Tests were performed to determine the effect of moisture exposition to adhesive bonding of glass to stainless steel with MS polymer. Half of the specimens were exposed to moisture (1 week in water at 55°C) and the other half were not exposed. All
specimens were tested under shear compression load to determine the maximum shear stresses before failure. In Figure 12 presents the maximum shear stresses of the MS polymer adhesive bonds. All failures occurred in the adhesive layer. The fractures were thus 100% cohesive, meaning that the cohesive strength of the MS polymer is the weakest part of the bond.
Figure 12: Influence of moisture on maximum shear stresses of bonding glass to stainless steel with MS polymer
4.4.Bonding glass to stainless steel with 2c methacrylate and 2c epoxy adhesive
Tests were performed to determine the influence of moisture exposure to the adhesive bond of glass to stainless steel with a 2c methacrylate and a 2c epoxy adhesive. Bonding without moisture exposure showed good results. However, after 6 weeks exposure to moisture (50°C, 95% RH), different results between bonds without pretreatment and bonds with pretreatment were obtained. Especially the 2c methacrylate without pretreatment could not stand the exposure to moisture. The pretreatment applied was UV/Ozone cleaning. After 12 weeks of exposure to moisture (50°C, 95% RH), the bond strength of the 2C methacrylate adhesive systems was reduced with 100% for both pretreated and not pretreated specimens. The bond strength of the 2C epoxy adhesive system, however, was only reduced with 50% for both pretreated and unpretreated specimens.
Conclusions
The influences of overlap length and bond line thickness on the mechanical properties of adhesive bonded joints differ between rigid adhesives, like a 2c epoxy adhesive, and much more flexible adhesives, like MS polymers due to the deviation of stresses in the adhesive bond. The smallest overlap length of the epoxy adhesive results in the highest maximum bond stress. However, there is no significant difference in maximum bond stresses due to different overlap lengths of the MS polymer and large elongation is allowed.
When the bond line thickness of the MS polymer is increased, the maximum stress of the adhesive bond decreases considerably. The influence of the pretreatment is also related to the type of adhesive: flexible or rigid. The epoxy adhesive bond shows higher bond strength after a pretreatment with degreasing and abrasion. The bond strength of the MS polymer shows no significant difference between pretreated and not pretreated specimens. Though, the surface of the fracture shows a different failure mode. Not pretreated specimens show an adhesive fracture and pretreated specimens a cohesive fracture. The bond strength of the flexible adhesive is limited by its own cohesive strength.
Critical in bonding structural applications of glass is the resistance to moisture of the adhesive bond. For this reason durability tests should be done with bonded specimens exposed to moisture. The strength of the adhesive bond to glass is very critical and found to be highly dependant on the cleaning of the glass surface. Glass is most clean after a surface treatment with UV/Ozone or plasma. The second best pretreatment is degreasing the surface and applying a silane primer. Of course, almost each application is different (in loads, materials to bond, etc.) and testing should be done to determine the most suitable adhesive, and pretreatment and bond design.
6. References
[1] E. Boëda, et. al., Bitumen as hafting material on Middle Palaeolithic artefacts, Nature, 380, pp. 336-338 (1996).
[2] O. Volkersen, Die Nietkraftverteilung in zugbeanspruchten Nietverbindungen mit konstanten
Laschenquerschnitten, Luftfahrtforschung, 15, No. 1/2, pp. 41–47 (1938).
[3] M. Goland and E. Reissner, The Stresses in Cemented Joints, Journal of Applied Mechanics, 11, (1), pp. A17–A27 (1944)
[4] A. Vlot, S. Verhoeven and P.J.M. Nijssen, Bonded Repairs for Aircraft Fuselages, Series 07 Aerospace Materials, TU Delft (1998).
[5] A.J. Kinloch, Durability of Structural Adhesives, Applied Science Publishers Ltd. (1983)
