Effect of oxidation and surface roughness on the shear strength of single-lap-joint adhesively bonded metal specimens by tension loading
© The Author(s) 2016
Received: 26 October 2016
Accepted: 29 November 2016
Published: 12 December 2016
An experimental investigation was performed to study the effect of surface roughness and oxidation on the shear strength of single-lap-joints of AA6061, AA7075 aluminum alloys and an AISI 1080 steel alloy bonded with two different epoxy adhesives. An optimum surface roughness that provided the maximum shear strength was obtained for all the alloys at room temperature. The variation of the shear stress due to the different heat treatment temperatures revealed that the oxides developed on the alloy surfaces adversely affected the adhesive shear strength. The bulk concentration of magnesium of the aluminum alloys were also observed to affect the adhesive shear strength of the joints.
Adhesives are known to have been extensively used for several decades in the aerospace industry for joining of structural load-bearing components [1, 2]. Their application to the automotive industry has been inspired by the desire for cheaper and lighter products . The consideration of sealants and high strength adhesives as replacements for welding, which has been the traditional major automotive joining method to bond load bearing structural components, is also due to the limits observed with welding (such as in the joining of aluminum) and their high joint stiffness and superior fatigue performance [1–3]. Although adhesive bonding is particularly effective in joining thin metals and composites, the structural performance of the adhesive can be judged by its tendency to debond or fracture . Debonding is caused by high interface adhesive stresses existing at free or terminating ends of adhesive layers, while fracture in the adhesive or the adhesive-adherend interface has been linked to the presence of shear and peel stresses in the adhesive layer . However, a strong bonding joint is produced by the adhesive layer when it is thin and continuous despite the strength of the adhesive been much lower than the adhered metals . The nature of the adhesive bond is dependent on the atomic arrangement, chemical properties, constitution and diffusivity of the constituent elements . Therefore the properties of the interface between the different adherends must possess specific properties to achieve acceptable joints . Good adhesion between adherends, i.e., the shear strength of the adhesive bond, would therefore be heavily dependent on the pre-treatment of the adhered surfaces, the choice of adhesives, the design of the joint and the service condition as well as the adhesive application process [3, 5, 6]. Surface pre-treatment involves the removing of contaminants as well as chemically activating and modifying the surfaces to facilitate the chemical bond between the surfaces, initiate resistance to hydration and other environmental deterioration agents [3, 6].
A typical test used to assess the shear strength of adhesive bonds is the single-lap shear tests [5, 7]. The shear strength of these bonds is also known to be altered by environmental changes, changes in metal specimens and adhesive, and changes in the joints in any manner. Early research by Brewis et al.  on the effect of humility on the durability of aluminum-epoxide joints revealed that joints formed with this adhesive were stable up to 1008 h at 50 °C and relative humidity’s between 23 and 100%, after which time, some weakening was observed. Similarly, Butkus  reported the reduction in strength and toughness of epoxy adhesives on exposure to high temperature and humidity. However, Su et al.  observed that some adhesives were resistant to environmental effects. They suggested that exposure to the natural environment represented the severest test conditions, and that the performance of these adhesives were reduced significantly in the natural environment as compared to other environmental conditions, such as high humidity . They also proposed that the fatigue performance of some adhesives could possibly be improved with age . Datla et al.  studied the mixed-mode fatigue behaviour of degraded toughened epoxy-aluminum adhesive joints and revealed that the fatigue threshold strain energy release rate was decreased, while the crack growth rate increased with aging time in constant humidity environments. Also, they observed the degradation of joints and crack growth rates increased in relation to the aging temperature increase. The adhesive modulus is another factor that has been reported to possess considerable influence on the shear stress. It has been observed that the maximum peel stress, shear stress, and longitudinal stress in a single-lap-joint is directly proportional to the adhesive modulus, however, these parameters are inversely proportional to the adherend modulus .
Da Silva et al.  studied the effect of material, geometry, surface treatment, and environment as variables that would influence the strength of single-lap-joints in long term. They concluded that lap shear strength was directly proportional to the overlap, the adherend yield strength, and the adhesive toughness at relatively lower applied stresses and inversely proportional to the adhesive thickness, the adherend thickness, and the adhesive toughness at higher stresses . Subsequent studies on the effect of adhesive thickness and toughness on the lap shear strength revealed that decreasing the adhesive thickness and increasing the adhesive toughness increased the lap shear strength . Studies on the influence of some geometrical parameters on the stress concentrations of the single-lap-joints revealed that a significant concentration of stress can exist throughout the thickness of the adhesive . Therefore, detailed analysis of the stress distribution in the adhesive was suggested as being preferable to the simplified analysis of the average stress state through the joint, as it would prove a more ideal analysis . Shorter bond lengths, and stiffer laminate adherends have been observed to improve the fatigue strength of bonded composites . However, hot and wet environmental exposure has been reported to adversely affect the fracture toughness of double cantilever beam (DCB) and end-notched flexure (ENF) aluminum/FM 73/boron-epoxy bonded joints . While oxidation has been shown to induce only superficial damage in constant temperature environments, when the temperature is varied cracks can developed on the sample edges, eventually propagating towards the core .
As adhesive bonding becomes one of the main joining methods for aluminum alloys with its introduction into the aerospace and transport industries, research into the adhesive bonded aluminum joints continues to grow [6, 19]. It has been shown that the surface roughness induced on the aluminum alloy by abrasive grinding has a pronounced effect on the adhesive strength . The complex relationship between adhesion and the surface roughness of the adherend has been noted by several researchers, who have reported an optimum surface roughness and surface conditions for maximum adhesive strength [19, 20]. While oxides formed on the aluminum alloy surface due to magnesium diffusion have been reported to be detrimental to the bonding of adhesives, certain concentrations of magnesium in aluminum alloys have been reported to increase the bond strength [21–24]. The effect of the bulk concentration of magnesium in Al–Mg alloys on the adhesion strength has also been another topic of concern for researchers .
As the surface quality of the adherend has a strong influence on the adhesive strength, it is therefore practical that oxidation and the surface roughness of specimens are two factors that can influence the behaviour of single-lap-joint adhesively bonded metal. In this paper, the effect of these two factors, oxidation and roughness of the specimens, on shear strength of single-lap-joint adhesively bonded metal specimens by tension loading are investigated, and the obtained results from different cases are studied and compared. While similar work has been carried out, this paper concentrates on the performance of automobile alloys, in particular magnesium containing aluminum alloys. The aim was to investigate the influence of the adhesive type and alloy composition on the optimum surface condition for maximum bond strength. The effect of bulk magnesium concentration of aluminum alloys at various stages of oxidation and the influence of oxides produced on the alloy surface on bond strength were also of interest. A steel alloy was used as a baseline to determine dependency of the alloy type on the effect of surface roughness and surface oxidation.
Mechanical properties of the A1 and A2 adhesives
Cure temperature (℃)
Cure time (min)
Tensile strength (Mpa)
Average surface roughness (Ra) values of the alloys ground with the corresponding SiC type emery papers
Grinding 60 grit SiC
1.88 ± 0.52
1.88 ± 0.52
1.45 ± 0.52
Grinding 120 grit SiC
1.01 ± 0.15
1.01 ± 0.15
0.86 ± 0.07
Grinding 240 grit SiC
0.83 ± 0.05
0.83 ± 0.05
0.62 ± 0.09
Grinding 320 grit SiC
0.6 ± 0.07
0.6 ± 0.07
0.47 ± 0.05
Tensile tests performed on the bonded single joint sample was carried out with a MTS universal testing machine at a test speed of 0.01 mm/sec. The specimens were place in the jaws of MTS Universal testing machine and carefully tightened in order to ensure that the specimen stayed perfectly vertical as any inclination would lead to the bending moment on the specimen and the pure shear strength would be compromised. The tensile test was performed until the adhesive or cohesive fracture was achieved. All the experimental procedures including cleaning and preparing the specimens and application of the adhesives were performed at room temperature and air moisture content of 17 g/m3. The specimen surfaces were then examined using an FEI Quanta 200 FEG environmental scanning electron microscope (SEM) under high vacuum.
Results and discussion
Effect of surface roughness
An optimum surface roughness for achieving maximum adhesive strength was observed for all the alloys tested regardless of the adhesive used. A similar observation of optimum surface roughness for maximum bonding strength was observed by Budhe et al. , who compared the effect of surface roughness on bonding strength for wood and aluminum AA6061. It can be observed that for the aluminum alloys the maximum adhesive bonding strength was observed at a surface roughness (Ra) of 0.83 µm for both adhesives, which would imply that for this particular alloys, the adhesive type does not have much of an influence on the optimum surface roughness for maximum adhesive shear strength. The maximum adhesive strength of the steel alloy was observed to occur at different surface roughness for the A2 (Ra = 0.47 µm) and A1 (Ra = 0.62 µm) adhesives, implying that the adhesive type has an influence on adherend surface roughness that yields the maximum bond strength. It is interesting to note that for the A1 adhesive the maximum adhesive strengths were observed at surface roughness values achieved from grinding with the 240 grit SiC emery paper.
An evaluation of the adhesive shear strengths also revealed the effect of the adherend material on the adhesive bond strength. Similar observations have been previously made, with researchers noting that the stiffer, stronger adhrends possessed stronger bond strengths. The A1 displayed higher adhesive bond strengths for steel in comparison to both aluminum alloys, with adhesive shear strengths for the steel alloy being at least twice those of the aluminum alloys. The difference in adhesive shear strengths with adherend material was not as distinct with the A2 adhesive. There was not much disparity observed between the adhesive shear strengths for the steel and aluminum alloys, except for adhesive strenghts observed at surface roughness values obtained with the 320 grit SiC emery papers. Although the maximum adhesive shear strength was observed with the steel alloy, the lowest shear strength was also observed with this alloy. Therefore, the type of adhesive would also influence the extent the adherend material impacts the bond strength.
A comparison of shear strength values for both adhesives revealed higher shear stress using the A2 adhesive with the aluminum alloys at all surface roughness values. The shear strength values of the AISI 1080 steel were within similar ranges with only a slight variation in shear strengths observed at surface roughness (Ra) of 0.47 µm where A2 possessed almost twice the shear stress and 0.62 µm where A1 was distinctly higher. However, the trends for each of the adhesives were distinctly different and while the peak shear stresses were observed at similar surface roughness values for the aluminum alloys, they were observed at different surface roughness values (0.47 µm for A2 and 0.62 µm for A1) for the steel alloy. The lowest shear stresses were observed at different surface roughness values with both adhesives for all the alloys.
In summary, an optimum surface roughness was observed to induce a maximum bond strength, regardless of the adhesive type or alloy. However, while increasing the surface roughness of a material would result in increasing the adhesive strength, this action becomes detrimental after a critical surface roughness, which is dependent on the adherend material. The bond strength was also observed to be dependent on the adherend material and the adhesive type.
Effect of surface oxidation
The trend for the graphs of the AISI 1080 steel (Fig. 4b) and the AA7075 aluminum alloy (Fig. 4a) were rather similar, displaying a fluctuation of the shear stresses with heat treatment temperature increase after an initial decrease from 25 °C. The trend for the AA6061 alloy (Fig. 4a) though displays a continuous drop from 25 to 200 °C after which a slight increase is observed at 300 °C. Again the steel sample displayed the highest shear stresses at each temperature while the AA7075 alloy displayed the lowest shear stress at each temperature except at 200 °C where the AA6061 alloy displayed the lowest shear stress.
These results display the influence of the adherend surface condition on the bond strength between the metal and the adhesive. The results above indicate that as the heat treatment temperature rises, the increase in the oxide content on the surface of the specimen will result in the reduction of the adhesive shear strength. It can be seen that the shear strength of the joint was maximum at room temperature, were the oxide content of the alloy surfaces would ideally be at a minimum and would have good adherence to the aluminum alloy surface. However, as can be seen in Fig. 4, shear strength of the specimens slightly increases when the heat treatment temperature changes from 100 to 200 °C. Therefore, it can be suggested that the adhesives are not resistant to the environmental effects to the adherend surfaces. The impact that oxidation of the adherend surface has on adhesive strength was dependent on the alloy type, as a higher percentage reduction in shear stress between 25 and 100 °C was observed for steel (54%), while for the aluminum alloys it was an average of 34%.
The surfaces of all the heat treated aluminum alloys (Figs. 5, 6) were observed to be covered with magnesium and oxygen. The magnesium distribution on the heat treated surfaces were predominantly coincident with the oxygen distribution indicating that the oxygen on the alloy surfaces was due to the formation of a magnesium-rich oxide, likely MgO, on heating. The aluminum alloy surfaces, AA6061 (Fig. 5a) and AA7075 (Fig. 6a) without heat treatment possessed a lower concentration of magnesium and oxygen on their surfaces. The magnesium under this condition was not observed to be coincident with oxygen, indicating that the oxide was most likely Al2O3, which has good adhesion to the aluminum subsurface and is known to acts as a protective layer . The oxides on the heat treated AA7075 alloy (Fig. 6b–d) were observed to predominantly occur within the grooves created during the grinding of the alloy surfaces, while for the AA6061 alloy (Fig. 5b–d) they covered the surface at 100 °C but at higher temperatures they occurred within grooves and as MgO islands. It should be noted that while magnesium diffusion in aluminum alloys is enhanced by surface defects and voids in the alloy and oxide layer, the disparity of concentration of magnesium at the surface is also associated to the roughness of the sample, which in this case was induced by grinding . The local enrichment of MgO oxide at the metal/adhesive interface thus reduced the stability of the adhesive joints .
The increase in the oxygen concentration on the AISI 1080 steel surfaces (Fig. 8b) was also coincident with the reduction of the adhesion shear stress. The oxides, most likely Fe3O4 and Fe2O3, their thickness and composition are dependent on the temperature and composition of the steel alloy . Fe3O4 is reported as the dominant oxide at 200 °C, and has been observed to have good oxide adherence with the steel substrate [27, 28]. At 300 °C, a change in the oxidation rate and the dominant oxide begins [28, 29]. The formation of Fe2O3 with increasing temperatures, from 300 °C, has been reported to be accompanied by oxide separation from the metal surface [28, 29]. These oxides were observed to occur predominately within the grinding grooves of the steel. Although the oxygen concentration on the steel surfaces was much higher than that of both aluminum alloys (Fig. 8), the adhesive strength of the steel was still higher than both aluminum alloys regardless of the heat treatment (Fig. 4). The fluctuation of the adhesive strength with the increase of the heat treatment temperature for both the aluminum and steel alloy surfaces could be a result of either or a combination of the difference in oxide induced roughness, oxide thickness and composition and the oxide adhesion to the alloy surface on these alloys at each of the heat treatment temperatures.
Linear regression analysis
Values of constants in equation for the A1 adhesive
AISI 1080 steel
Values of constants in equation for the A2 adhesive
AISI 1080 steel
The optimum surface roughness value which has been surmised to provide a maximum adhesive bond strength is dependent on the adherend material and the type of adhesive employed.
The shear strength of the adhesive joint is influenced by the concentration and distribution of oxides covering the adherend surface due to heat treatments. In the case of aluminum alloys, the distribution and concentration of oxides on the alloy surface is determined by the heat treatment temperature and the bulk magnesium concentration of the alloy.
The nature of the oxides formed on the adherend material surface and its adhesiveness to the adherend material significantly influence the shear strength of the adhesive joint. In the case of aluminum and steel alloys, the formation of a porous MgO, and descaling Fe2O3 oxides respectively, reduced the adhesive bond strength.
MHK and OAG were responsible for completing the article. MHK, ARR, and AE performed the experimental work from the preparation of the samples to the testing of the specimens. OAG carried out further surface analysis of the sample. ARR and AE were involved in the discussion and significantly contributed to making the final draft. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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