Maximizing the Efficiency of Adhesive Joint Designs and Improving Joint Strength

This article was originally published in 2005 and updated in 2022.

The design of the adhesive joint will play a significant factor in determining how it will survive service loads. Although it may be tempting to use joints intended for other fastening methods, adhesives require joints of a special design for optimum properties.

The practice of using joints designed for some other method of assembly and altering them for adhesives can lead to unfavorable results.

As with most fundamental processes involving adhesives, joint design cannot be complete without consideration of many factors. The article aims to make the reader accustomed to the fact that the joint strength is not the same or even proportional to the intrinsic strength of the adhesive. Whereas non-uniform stresses within the adhesive joint can significantly reduce the maximum strength of the joint. Non-uniform stress distributions cannot be eliminated, but they can be reduced through proper joint design and the selection of certain design variables.


Let's discuss in detail about the variables effecting stress distribution on different types of joints.

Common Types of Stress in Adhesives

A uniform stress pattern in an adhesive joint is seldom produced by the application of an external force. Rather, the non-uniform stress distribution is the norm. Since fracture initiates when and where local stress exceeds local strength, stress concentrations have a large influence on the breaking strength of a joint. External loads produce local stresses that may be many times the average stress. These high-stress concentrations are often unexpected but must be considered in joint design for optimum efficiency.

Four basic types of loading stress are common to adhesives:

• Tensile,
• Shear,
• Cleavage, and
• Peel.

Any combination or variations of these stresses may be encountered in practice.

Tensile Vs. Compressive Strength

Tensile stress develops when forces acting perpendicular to the plane of the joint are distributed over the bonded area. However, the stresses are never truly uniform, and the adhesive develops high-stress regions at the outer edge. Those edges then support a disproportionate amount of the load. The first small crack that occurs at the weakest area of one of the highly stressed edges will propagate swiftly and lead to failure.

Proper design requires that the joint have parallel substrate surfaces and axial loads. Unfortunately, in practical applications, bond thickness is difficult to control, and loads are rarely axial. Undesirable cleavage or peel stresses (explained below) then tend to develop. Tensile joints should, therefore, be avoided, but if necessary, they should be designed with physical restraints to ensure continual axial loading. The adherends must also have sufficient rigidity so that the stress is distributed evenly over the entire bonded area.

Compression loads are the opposite of tensile. As with tensile loads, it is important to keep the load aligned so that the adhesive will see mostly pure compressive stress. An adhesive loaded in compression has high strength, although it may crack at weak spots due to localized stress. A joint in "pure" and only compression hardly needs bonding of any sort. If the compressive force is high enough and there is no movement of the parts, the parts will generally stay in position relative to one another unless the adhesive fails cohesively.

Shear Stress

Shear stress results when forces acting in the plane of the adhesive try to separate the adherends. Joints that are dependent on the adhesive's shear strength are relatively easy to make and are commonly preferred in practice. Adhesives are generally strongest when stressed in shear because all of the bonded area contributes to the strength of the joint and the substrates are relatively easy to keep aligned.

The lap shear joint, shown in Figure 1 left, represents a common joint design in adhesive bonding. Shear stress is measured similarly to tensile stress, as force per bonded area. By overlapping the substrates, one places the load-bearing area in shear. However, note that most of the stress is localized at the ends of the overlap. The center of the lap joint contributes little to joint strength.

Depending on the joint geometry and physical properties of the adhesive and adherend, two small bands of adhesive at each end of the overlap may provide the same bond strength as when the entire overlap area is continuously bonded with the adhesive (Figure 1).


Figure 1: Stress distribution on an adhesive based on geometry¹

Cleavage and Peel Stresses

Cleavage and peel stresses are generally undesirable for adhesives. Cleavage is the stress occurring when forces at one end of a rigid bonded assembly act to pry the adherends apart. Peel stress is similar to cleavage, but it applies to a joint where one or both of the adherends are flexible.

Joints loaded in the peel or cleavage offer lower strength than joints loaded in shear because the stress is concentrated at only a very small area of the total bond area. The stress distribution of an adhesive in cleavage is shown in Figure 1 right. All of the stress is localized at the end of the bond that is bearing the load.

Cleavage and peel forces are measured as force per linear length of bond (e.g., g/cm or pounds/inch).
Brittle adhesives are particularly weak in peel because the stress is localized at only a very thin line at the edge of the bond. The stiffness of the adhesive does not allow the distribution of stress over an area much larger than the thickness of the bond. On the other hand, tough and flexible adhesives distribute peeling stress over a wider bond area and show greater peeling forces (see Figure 2).


Figure 2: Stress distribution in brittle and tough adhesive²

Variables that Affect Stress Distribution

Non-uniform Stress occurs when an adhesive joint is subjected to shear or cleavage-type loading.¹ The effect of non-uniform stress distribution is that the average stress (i.e., the joint load divided by bond area) is always lower than the maximum stress at localized areas within the joint. Uniform stress occurs only in cases where near-uniform stress occurs does the average stress approach the maximum stress.

The following important variables affect stress distribution, even in the most common joint designs:

• Adhesive Properties,
• Adhesive Thickness,
• Geometry of the Bond Area, and
• Adherend Properties,

Failure in the bond always begins at the maximum stress points. It is, therefore, advantageous to reduce the maximum stress levels that occur within a joint. The remainder of this article will attempt to show how this can be accomplished.

Adhesive Properties

The stress distribution in an adhesive joint is very much dependent on the rheological characteristics of the adhesive.

Stress Distribution in Tough, flexible Vs. Brittle Adhesives

Tough, flexible adhesives provide joints with a more uniform stress distribution and less of a difference between average and maximum stress. If high-stress distributions are expected in service either because of external loading, such as peel, or cleavage, or from internal stress, such as from thermal expansion differences or shrinkage, then tough, flexible adhesives and sealants are usually preferred over more brittle ones. Figure 2 illustrates how tough, flexible adhesives distribute peel stress over a larger area, whereas brittle adhesives result in a concentration of stress at line in the joint interface. Similarly, the sensitivity to crack propagation is greater with brittle adhesives, and fatigue life is reduced.

Since adhesives with high elongation typically have lower cohesive strength than more rigid adhesives, the advantage of the flexibility and high elongation is usually compromised. Rigid adhesives provide greater tensile shear strength by their greater degree of crosslinking. Flexible adhesives have better peel and impact strength by virtue of their ability to reduce stress concentration. It is difficult for an adhesive to achieve both high shear strength and high peel strength as shown in Figure 3.


Figure 3: The effect of adhesive modulus on T-peel, shear, permeability, and chemical stability³

Usually a tough, flexible adhesive or sealant is preferred over a brittle, stiff one. With these adhesives, it is much easier to have a joint with uniformly distributed stress. However, there are disadvantages in using tough, flexible adhesives that must be overcome. They usually have lower shear strength, and their cohesive strength is lower as well. Since the forces that hold the internal molecules together are lower, the temperature capability and environmental resistance also suffer.

Applications of Tough, Flexible and Rigid, Strong Adhesives

Tough, flexible adhesives are commonly used on substrates such as plastics and elastomers where the environmental service conditions are not extreme, and the physical properties of the adhesive closely match those of the substrates. More rigid, cohesively strong adhesives are usually employed in structural applications that are likely to see elevated temperatures and aggressive environmental conditioning.

One of the more difficult compromises is when an application requires both high-temperature resistance and high peel strength. Usually, the more heat-resistant polymers are also more densely crosslinked and brittle. Flexibilized polymers have reduced heat resistance but better resistance to peel, impact, and fatigue-type forces. However, modern adhesive formulations, notably temperature-resistant thermoset base resins that are toughened with a discrete elastomer phase, have come a long way in solving this impasse.⁴

Adhesive Thickness

The most important aspects of adhesive thickness are:

• Magnitude: Generally, one tries to have as thin a bond layer as possible without any chance of bond starvation. In practice, this translates into bond Line thicknesses ranging from 0.002 in. to 0.010 inches. Adhesive strength does not vary significantly in this range. Many laboratory experimentalists will try to achieve a constant bond-line thickness of 0.005 inches.
  
  With thicker adhesive bond lines, one runs the risk of incorporating higher void concentrations into the joint. Stresses at the corner of the adhesive-adherend also tend to be larger because it is difficult to keep the loads in alignment with a very thick bond line.
  
  It should also be remembered that adhesives are generally formulated to cure in thin sections. Thicker sections could change the curing properties (e.g., increased exotherm) and result in increased internal stresses and different physical properties than optimal.
  
  
• Uniformity: The substrates should be as parallel as possible, thus, requiring uniformity in adhesive thickness across the bonded area. If the substrates were not parallel, the loading would not remain aligned, and this condition could translate into cleavage stresses on the joint.

Methods to Maintain Adhesive Thickness

There are several methods used for maintaining a constant, predetermined adhesive thickness. These methods include:

• adjusting the viscosity of the adhesive,
• application of a precalculated amount of pressure during cure,
• using fixturing that is specifically designed for the application, and
• application of a shim or insert within the bond line so that a uniform, predetermined thickness can be maintained.

Effects of Bond Area Joints

For a given adhesive and adherend, the strength of a joint stressed in shear depends primarily on:

• the width and depth of the overlap and
• the thickness of the adherend.

Adhesive shear strength is directly proportional to the width of the joint.
Increasing the overlap depth can sometimes increase strength, but the relationship is not linear. Since the ends of the bonded joint carry a higher proportion of the load than the interior area, the most efficient way of increasing joint strength is by increasing the width of the joint.

A plot of failure load versus overlap length for brittle and ductile adhesives is shown in Figure 4. Increasing overlap length increases the joint strength to a point where a further increase in bond length does not result in increases in load-carrying ability. This curve is the result of the more uniform stress distribution in ductile adhesive systems than in brittle systems, as explained above.


Figure 4: Effect of overlap length on failure load using ductile and brittle adhesives

Since the stress distribution across the bonded area is not uniform and depends on joint geometry, the failure load of one specimen cannot be used to predict the failure load of another specimen having a different joint geometry. The results of a particular shear test pertain only to joints that are exact duplicates. This means that the results of laboratory tests on lap shear specimens cannot be directly converted to more complex joint geometries.

To compare different, simple joint geometries, curves showing the ratios of overlap length to adherend thickness, l/t, are sometimes used. Figure 5 shows an example of the effect of l/t ratio on aluminum joints bonded with a nitrile rubber adhesive. From such a chart the design engineer can determine the overlap length required for a given adherend thickness to obtain a specific joint strength.


Figure 5: The effect of ratio of length of overlap to adherend thickness (l/t) on adhesive strength at three test temperatures for aluminum joints bonded with a nitrile adhesive⁵

Adherend Properties

The properties of the substrates being joined by adhesives have a major influence on the shear stress distribution in the joint. Of primary importance is the stiffness of the substrate. Non-uniform shear stress distribution can be caused by the relative displacement of the adherend due to the strain. The greater the relative strain, the greater will be this displacement differential. Since the adhesive must accept this displacement differential, the flexibility of the adhesive is, therefore, also involved as described above.

The stiffness of the substrates is characterized by the product of the Young's modulus, E, and the adherend thickness, t
Et of each adherend becomes an important factor in the shear stress distribution. As the product Et becomes large, the shear stress distribution becomes more uniform.

Distortion Caused by Loading

In a shear joint made from thin, relatively flexible adherends, there is a tendency for the bonded area to distort because of eccentricity of the applied load. This distortion causes cleavage stress on the ends of the joint, and the joint strength may be considerably impaired. Thicker adherends are more rigid, and the distortion is not as much a problem as with thin-gauge adherends.

Figure 6 shows the general interrelationship between failure load, depth of overlap, and adherend thickness for a specific metallic adhesive joint. As the adherend thickness (i.e., the relationship Et) increases, the failure load increases for identical overlap lengths. For constant adherend thicknesses or constant Et, the failure load increases with increasing overlap length up to a certain point. Beyond that overlap distance, the failure load remains constant. In this region, the entire load is supported by the edge region of the overlap. The central section of adhesive is not contributing to the strength of the joint.


Figure 6: Distortion caused by loading can introduce cleavage stress and must be considered in the joint design⁶

Types of Joints for Flat Adherends

Butt Joint

Plain Butt Joint is the simplest joint. Butt joints cannot withstand bending forces because the adhesive would experience cleavage stress. If the adherends are too thick to design simple overlap-type joints, the butt joint can be improved by redesigning in several ways, as shown in Figure 7a. All the modified butt joints reduce the cleavage effect caused by side loading.

Types of Modified Butt Joints

• Tongue-and-groove joints have an advantage in that they are self-aligning and act as a reservoir for the adhesive.
• The scarf joint keeps the axis of loading in line with the joint and does not require a major machining operation.

Figure 7: (a) Interrelation of failure loads, depth of lap, and adherend thickness for lap joints with a specific adhesive and adherend⁷

Lap Joints

Lap joints are the most commonly used adhesive joint because they are simple to fabricate, applicable to thin adherends, and stress the adhesive in shear. However, the adherends in the simple lap joint are offset, and the shear forces are not in-line, as illustrated in Figure 7. Misaligned loads on the lap shear specimen result in cleavage stress at the ends of the joint, which seriously impairs its efficiency.

Modifications of Lap-joint Design

1. Redesigning the joint to bring the load on the adherends in-line.
2. Making the adherends more rigid (thicker) near the bond area (see Figure 7b).
3. Making the edges of the bonded area more flexible for better conformance, thus minimizing peel (i.e., tapering the edges of the adherend).

Figure 7 (b): Butt (left), lap (center), and strap (right) joint designs

Types of Modified Lap Joints

• The joggle-lap-joint design is the easiest method of bringing loads into alignment. The joggle lap can be made by bending the adherends. It also provides a surface to which it is easy to apply pressure.
• The double-lap joint has a balanced construction that is subjected to bending only if loads on the double side of the lap are not balanced.
• The beveled lap joint is also more efficient than the plain lap joint. The beveled edges, made by tapering the ends of the adherends, allow conformance of the adherends during loading. This reduces cleavage stress on the ends of the joint.

Tapering or beveling the edges of the joint greatly improves the load-bearing capacity, because it permits those sections to bend and, thus, to distribute the stress down the length of the bonded area to some degree. Figure 8 shows the advantage that can be gained by tapering the ends of the standard lap shear specimen.


Figure 8: Effect of beveled and plain lap joint design on breaking load⁸

Strap Joints

Strap joints keep the operating loads aligned and are generally used where overlap joints are impractical because of adherend thickness. Strap-joint designs are shown in Figure 7b. Like the lap joint, the single strap is subjected to cleavage stress under bending forces.

Types of Modified Strap Joints

• The double strap joint is more desirable when bending stresses are encountered.
• The beveled double strap and recessed double strap are the best joint designs to resist bending forces. Unfortunately, they both require expensive machining.

When thin members are bonded to thicker sheets, operating loads generally tend to peel the thin member from its base, as shown in Figure 9 (top). The subsequent illustrations show what can be done to decrease peeling tendencies in simple joints.

Figure 9: Minimizing peel in adhesive joints⁹

Stiffening Joints

Often thin sheets of a material are made more rigid by bonding stiffening members to the sheet. When such sheets are flexed, the bonded joints are subjected to cleavage stress. Some design methods for reducing cleavage stress on stiffening joints are illustrated in Figure 10. Resistance of stiffening members to bending forces is increased by extending the bond area, providing greater flange flexibility, and increasing the stiffness of the base sheet.


Figure 10: Stiffening assembly designs⁹

Cylindrical Joints

Several recommended designs for rod and tube joints are illustrated in Figure 11. These designs should be used instead of the simpler butt joint. Their resistance to bending forces and subsequent cleavage is much better, and the bonded area is larger. Unfortunately, most of these joint designs require a machining operation.

Figure 11: Designs for rod (a) and tube (b) joints¹⁰

Angle and Corner Joints

The type of angle joints described below are shown in Figure 12:

• A butt angle joint is the simplest method of bonding two surfaces that meet at an odd angle. Although the butt joint has good resistance to pure tension and compression, its bending strength is very poor.
• Dado, L, and T angle joints offer greatly improved properties.
• The T design is the preferable angle joint because of its large bonding area and good strength in all directions.

Corner joints for relatively flexible adherends such as sheet metal should be designed with reinforcements for support. Various corner-joint designs are shown in Figure 12. With very thin adherends, angle joints offer low strengths because of high peel concentrations. A design consisting of right-angle corner plates or slip joints offers the most satisfactory performance. Thick, rigid members such as rectangular bars and wood may be bonded with an end lap joint, but greater strengths can be obtained with mortise and tenon. Hollow members, such as extrusions, fasten together best with mitered joints and inner splines.



Figure 12: Angle joint (top) and corner joint (bottom) designs

Plastic and Elastomer Joints

Design of joints for plastics and elastomers generally follows the same practice as for metal. However, the designer should be aware of certain characteristics for these materials that require special consideration. Such characteristics include:

• flexibility,
• low modulus,
• high thermal expansion coefficients,
• thin section availability, and
• anisotropy.

These characteristics tend to produce significant non-uniform stress distribution in the joint. Thus, tough, flexible adhesives are usually recommended to bond to plastic or elastomer substrates.

Joint Design for Thin, Flexible and Rigid substrates

Thin or flexible polymeric substrates may be joined using a simple or modified lap joint. The double strap joint is best, but also the most time-consuming to fabricate. The strap material should be made out of the same material as the parts to be joined, or at least have approximately equivalent strength, flexibility, and thickness. The adhesive should have the same degree of flexibility as the adherends. If the sections to be bonded are relatively thick, a scarf joint is acceptable. The length of the scarf should be at least four times the thickness; sometimes larger scarfs may be needed.

When bonding elastic material, forces on the elastomer during cure of the adhesive should be carefully controlled, since excess pressure will cause residual stresses at the bond interface. Stress concentrations may also be minimized in rubber-to-metal joints by eliminating sharp corners and using metal thick enough to prevent peel stresses that may arise with thinner-gauge metals.

As with all joint designs, flexible plastic and elastomeric joints should avoid peel stress. Figure 13 illustrates methods of bonding flexible substrates so that the adhesive will be stressed in its strongest direction.


Figure 13: Joint designs for flexible substrates⁹

Rigid plastic joints can be designed like any other rigid substrate. However, special consideration needs to be given to reinforced plastics since they are often anisotropic. This means their strength properties are directional. Joints made from anisotropic substrates should be designed to stress both the adhesive and adherend in the direction of greatest strength. Plastic laminates, for example, should be stressed parallel to the laminations. Stresses normal to the laminate may cause the substrate to delaminate. Single and joggle lap joints are more likely to cause delamination than scarf or beveled lap joints. The strap-joint variations are useful when bending loads may be imposed on the joint.

General Adhesive Joint Design Rules

The joint designer should take into consideration the following rules in the design of adhesive or sealant joints. These rules will provide the basis for successful joints.

1. Keep the stress on the bond-line to a minimum.
2. Whenever possible, design the joint so that the operating loads will stress the adhesive in shear.
3. Peel and cleavage stresses should be minimized.
4. Distribute the stress as uniformly as possible over the entire bonded area.
5. Adhesive strength is directly proportional to bond width. Increasing width will always increase bond strength; increasing the depth of overlap does not always increase strength.
6. Generally, rigid adhesives are better in shear, and flexible adhesives are better in peel.
7. Although typically a stronger adhesive may produce a stronger joint, a high elongation adhesive with a lower strength could produce a stronger joint in applications where the stress is non-uniform.
8. The stiffness of the adherends and adhesive influence the strength of a joint. In general, the stiffer the adherend with respect to the adhesive, the more uniform the stress distribution in the joint and the higher the bond strength.
9. The higher the Et (modulus x thickness) of the adherend, the less likely the deformation during load, and this usually results in a stronger joint.
10. The adhesive bond-line thickness has not a strong influence on the strength of the joint. More important characteristics are a uniform and void-free adhesive layer.

The application of these rules is evident in the preferred adhesive joint designs. The ideal adhesive-bonded joint is one in which under all practical loading conditions the adhesive is stressed in the direction in which it most resists failure. A favorable stress can be applied to the bond by using proper joint design. Some joint designs may be impractical, expensive to make, or hard to align. The design engineer will often have to weigh these factors against optimum adhesive performance.


Methods to Measure Impact Strength of Adhesive Joints