Combine experimental and FEM analysis of adhesive bonded single lap joint with Al-alloy flat adherends and pre-embedded artificial defects
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Adhesive bonded lap joint is considered to be better substitute to riveted or welded joints for aerospace, marine and structural applications. It is due to the ability to develop relatively lower stress concentration and fatigue severity in highly dynamic environment. In real practice, there are defects present in the joint interface that decide the strength and durability of the component. Hence, it is required to understand the joint strength with different defect geometries. In this paper, a combined experimental and finite element method is conducted to evaluate the strength of lap joint of Al-alloy flat plate, pre-embedded with defects of various geometries: square, rectangle, circular and elliptical. The results of experimental and FEM analysis are converging, and indicate the clear variation of strength due to different type of defects. It encourages further analysis.
KeywordsAdhesive bond Lap joint Von Mises stress Strain Deformation
Now-a-days, adhesive bonded joints are applied to components used in various industries like automobile, aeronautical and other production industries. Reason being, it has the number of influential properties over the other traditional joints. Single lap joint is the simplest joining of two materials through an overlap arrangement of similar or dissimilar materials. It may be adhesively bonded, riveted or welded to form the joint. Among these, adhesively bonded joints are those joints in which adhesive are placed in the interface of strap and lap adherend. Such joints are suitable for small stress concentration in adherends, shows excellent fatigue properties, sealed against corrosion, relatively light weight and more efficient in load transfer [1, 2]. Further, its manufacturing cost is very low and load is distributed over a large area. Again, joint efficiency is high (strength/weight ratio), no holes in the joint area to develop unnecessary stress concentration. It is better than riveted joint in aerodynamic applications. Above all, it can be retained in a high level of residual stress after initial cracking, if designed properly.
Further, there is difficulty in producing a defect free lap joint in real practice. But defect may be minimized to the lowest level by proper handling. The defect in a joint arises due to presence of voids, porosity and micro cracking in the lap area which causes a disbond and doesn’t transfer load. As a result, stress increases at the other load transferring region but the micro level defect doesn’t affect significantly to the joint strength. Heslehurst  worked on the anatomical response of an adhesive bond line defects and generalized the defects in adhesive bonded joints of debond/weak bond. He proposed that poor bonding affects the load transferring potential of the joint mainly due to the decrease in stiffness. The defects alter stress distribution and diminish the joint strength.
With increasing demand of composite material application, carbon fiber gained popularity due to many improved properties. Moura et al.  worked with carbon epoxy single lap bonded joint with strip defect and studied it’s strength using an adhesive of limited ductility. It is predicted that the joint strength is not affected due to presence of defect at the center of the overlap. While, presence of the same at the end reduces the joint strength significantly.
Karachalios and Adams  worked with high strength steel adherend. They estimated the joints up to overlap 25 mm on application of load fails due to global yielding, while that more than 25 mm overlap if loaded, gives rise to shear strain along the load plane interface. Grant and Adams  worked with toughened epoxy adhesive and mild steel adherend with bond line up to 3 mm thick. They found the increase in joint strength due to reduction in the bond line thickness resulted out of the bending moment at the end of the overlap.
Lang and Mallick  investigated the effect of adhesive spew geometry on stress fringe and the peak stresses of adhesively bonded single lap joint. The finite element technique is used in this case to evaluate the stresses for full triangular and full rounded spew. The τxy, σyy, and σxx are reduced by 50%, 73% and 28% in case of triangular and 37%, 42%, and 20% in case of rounded spew. Due to provision of a fillet to the full rounded spew, the σyy, and σxx is almost doubled. Circular arc (radius = 6 mm) spew display the highest percent reduction in τxy, σyy, and σxx i.e. 60%, 87% and 35%. So full triangular spew is advisable.
Neim et al.  worked on the effect of surface roughness on the joint strength and found that the mechanical interlocking between adherend and base drastically increases due to roughening of the surface in contact. In case of aluminum adherend used in a lap joint, due to the formation of disband  at the middle strength remains unaffected but the tensile loading is resisted by the end of the joint.
Karachalios and Adams  carried out combined numerical and experimental studies of low strength steel adherends in transferring load for total overlap. Due to longer overlap, failure is out of high local adhesive strains. Further to such studies, Ashrafi et al.  worked on lap joint of sinusoidal bonding surfaces and flat interface using fiber reinforced epoxy composite adherends. Mechanical behaviour and strength of bonded joints are greatly influenced due to non-flatness of interface. In some cases, its use is limited to thin adherends.
Tsuey et al.  studied the defect of lap joint in tension and its effect of shear strength using aluminum adherend and brittle adhesive. Bak et al.  worked on the effect of bonded lap thickness area on the tensile strength of lap and found that maximum stress occurred at the corner section of the joint whereas minimum stress occurred at the displacement 0.2 mm. Significant decrease in the stress distribution occurs throughout the joint, due to enhanced adhesive thickness area of the overlap.
Xiaocong  studied the adhesively bonded single lap joint and found that due to loading, there occurs stress concentration near free ends of the interfaces, while there is almost zero stress at the center. Razavi et al.  worked on joint strength based on various substrate surface geometries. They correlated stress distribution in adhesive layer with the overall strength of the joint. Belingardi et al.  characterized adhesively bonded joint and suggested two harmful conditions; one due to offset of two adherends and subsequent bending moment and other variable stress distribution along adhesive layer resulted peak stress at ends.
The failure mode of joint changes from peel to shear due to tapering of adherends . It is due to decrease in peel stresses in adhesive layer and subsequent increase in joint strength. The peak values of peel stresses at both ends of the bond line will be effectively reduced when using external tapers with 30° fillets. Adams et al.  experimented on square ended joint without fillet and with fillet and found joint with 45° fillets can bear around 5 kN more load than without fillet and by making a comparison between 30° and 45° fillet found that 30° fillet is better. Karachalios et al.  worked with three different adherends; Mild steel, Gauge steel, Hard steel and two adhesives; one is brittle and other is ductile in nature.
Banea et al.  described the adhesive bonding to be material joining process that involves melting and solidification bonding materials between adherends. Segerlind  gave out the same conclusion that as the lap length increases the stresses are in general reduced, but the stress maxima at the lap ends are increased.
Karachalios et al.  suggested the size of damage and location as dependable on parameters such as interface material plasticity, overlap length and loading type (Either tension and compression). Further adherend thickness and geometry of joint also influence the failure. Hunston et al.  predicted the nature of crack propagation in the brittle adhesive material in the joint. Further, Haghani et al.  made a parametric study on the effect of tapering length and the material properties of joint constituents on stress distribution in adhesive joints.
Grant et al.  worked on adhesively bonded lap joint and effect of temperature on it. They found that when the working temperature is more the joint becomes more ductile and load resistance capacity is less for which after applying a small amount of load to joint, it develops elongation. Brewis et al.  worked on adhesive bonded lap joint with aluminum alloy adherend and effect of moisture on it and found that the joint performance is better in dry condition than the higher humidity for this cause etched and anodized process is not preferable. Elhannani et al. [26, 27] made a probabilistic assessment of the defects present on the single lap bonded joints. Prior to this they also studied the effect of presence of number and shape of bonding defects on shear stress distribution on the joint.
From this broad literature review some gap can be observed which inspired for the present work. A lot of work has been done on adhesive bonded lap joint but little work has been focused on joints with defects. Although the joint strength under circular and rectangular defect has been investigated but aluminum alloy with such defects has not been experimented. The comparison of joint strength under different types of defect with different sizes has not been studied, which is the main goal of this research.
2 Material and experimental methods
The model deals with the joining of two Al-alloy plates with defects. The defects are created by giving Teflon coating in the center area of the joining surface. Araldite AW 106 resin/Hardener HV 953U epoxy adhesive is a multi-purpose, two components, room temperature curing, and viscous material of high strength and toughness that is suitable for bonding a variety of materials including metal, ceramic, wood, rubber glass, rigid plastic. The electrically insulating adhesive is easy to apply either manually by spatula and stiff brush or mechanically with meter/mix and coating equipment.
2.1 Description of specimen
2.2 Description of defects
Circular, rectangular and square defects and geometry
Area of defects (mm2)
Circular (R in mm)
Rectangular (a*b in mm2)
Square (a in mm)
R = 7.04 (C1)
R = 9.965 (C2)
R = 11.5 (C3)
R = 12.2 (C4)
R = 12.48 (C5)
Elliptical defects table
2.3 Testing method
3 Finite element methods for defect joint strength
For the defect joint creation, two flat plates of Al-alloy are joined through the solid model. The Fig. 5a, b persents the solid model and the mesh model of the joint respectively. The material properties of all constituents such as adherends, epoxy are defined. Finite element technique is a most widely used study to evaluate stress, strain and deformations in component.
Design of defect
No. of elements
No. of nodes
Minimum edge length
Young’s modulus—7.1E + 10 Pa
Tensile yield strength—2.8E+08 Pa
Compressive yield strength—2.8E+08 Pa
Coefficient of thermal conductivity—2.3E−05 C−1
Young’s modulus—5E + 08 Pa
Tensile yield strength—3900 psi
Compressive yield strength—3500 psi
Coefficient of thermal conductivity—7.5 F−1
Young’s modulus—3000 MPa
Tensile yield strength—17 MPa
Coefficient of thermal conductivity—0/0001 K−1
4 Results and discussion
4.1 Experimental result analysis
When the defect size exceeds 75% of lap area then strength of joint reduces and the rigidity increases to a greater extent. The strength curve is similar with the rectangular defects but extension of the joint is unpredictable. There is not a single defect curve which exceeds elastic limits all fails before it.
Design of defect
Stress (in MPa)
Deformation (in mm)
The objective of this work was to study the effect of different shape of defects on the strength of lap joint one adhesive and aluminium alloy adherend were used to manufacture single lap joint with different types and different dimensions of artificial defects. Comparison of strengths made between same overlap area in the Figs. 4 and 6.
When an adhesive is used for lap joint, joint strength can be achieved with different amount depending on size of the defect. In all cases strength of joint non-linearly decreases with increase in defect size. When defect size is small, exerts less effect on strength but with increase in defect area strength of joint decreases apparently. It is due to less stress carrying area of joints. It is observed after a deliberate test that the defect size affect the joint strength.
By observing rectangular, circular and square defect when the defect is middle part of overlap, affect joint strength less and defects nearer to fillet end of joint affect more to joint strength. If defect is nearer to fillet end and there is sufficient contact area from other sides it will not affect to joint strength up to a certain extent which is observed by comparing 25% and 50% of circular defect and 25% and 50% of elliptical defects joint respectively.
When the defect spreads equally to each side of overlap area, it affects to strength greatly which is observed from circular and square defects joints. As the aluminum alloy used here has less strain and few joints are less rigid so some case those joints may be the substitude of aluminum alloy material.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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