Types of Stress in Adhesive Joints

One of the main purposes of an adhesive joint is to transfer force from one adherend to another adherend through a relatively thin layer of adhesive. How successfully an adhesive joint manages its stress load depends not only on the strength of the adhesive but also on how the joint converts the external load into stress within the adhesive, at the interfaces, and around the joint itself.

Five main loading modes are common in adhesive bonding: shear, tension, compression, peel, and cleavage. These modes all impact the bond in different ways. Adhesive joints perform best when the load is transferred through shear, tension, or compression. Peel and cleavage joints do not hold up well with adhesives because they concentrate stress near the edges.

Joints that are intended to carry stress through shear and tension may resort to developing peel and cleavage stress if the joint is not properly configured. For example, a butt joint positioned in tension may develop cleavage if the force is slightly misaligned. Likewise, a single-lap joint positioned in shear may develop peel from adherend bending. Knowing that these types of interactions can impact bond integrity is important to design the joint around actual stress distribution rather than relying on nominal adhesive strength.

What is Stress in an Adhesive Joint?

Stress can be described as an internal force acting over a defined area of a material. In its simplest equation:

Stress = Force ÷ Area

Stress can be quantified in pascals, megapascals, pounds per square inch, or similar units. The correct equation is identified based on the direction of the force. A force that acts perpendicular to the bond produces normal stress while a force that acts parallel to the bond produces shear stress.

Stress is not the same as load. A load is the external force or movement applied to an assembly. Stress is the internal response as the joint assembly transfers that load. Stress is internally produced in the adhesive, adherends, and the interfaces of the assembly. The same external load can create varying stress distributions conditional on the geometry, bonding area, material stiffness, and alignment of the joint.

For instance, a centered tensile load that is placed upon a broad, symmetrical joint can produce a relatively uniform normal stress. If the same load is positioned away from the center line, the load can bend the adherends and concentrate tensile stress along the edge. The magnitude of the stress has not changed but the positioning changes how the joint handles it.

Several components of an adhesive joint influence this stress response:

The adhesive is the bonding substance that transfers force between the two adherends
The adherends are the components being joined
The bond line is where the adhesive sits between the two adherends
The interfaces are the boundaries where the adhesive contacts each adherend.

Bond failure can occur at any of these locations so that tells us that stress is not confined to the adhesive layer.

Dividing the force by the area provides us with average stress but it does not show what happens in concentration at specific locations. Dividing the applied force by the area suggests an equally distributed stress profile but in practical terms stress can concentrate at certain points. Factors such as adherend deformation, joint eccentricity, abrupt edges, voids, thickness variation, and material differences can create non-uniform stress fields. A bond with a lower overall stress value can still failure if higher local stress develops near an overlap end, corner, crack, or defect. Therefore, it is important to pay attention to the nominal stress and local peak stress.

The Five Primary Loading Modes in Adhesive Joints

The five primary loading modes have been classified according to the direction and distribution of the forces acting on the joint:

Shear force acts parallel to the bond and attempts to slide the adherends past each other.
Tension acts perpendicular to the bond and pulls the adherends directly apart.
Compression acts perpendicular to the bond and pushes the adherends together.
Peel slowly lifts at least one flexible adherend away from another.
Cleavage pries two rigid adherends apart from one end.

While these are useful theoretical models, it does not mean that every joint is loaded using only one specific loading mode. Local concentrations of stress may vary, and a single joint can combine multiple loading modes. For example, single lap joints combine shear with peel because their load paths are offset. The dominant loading mode should guide joint design, but the secondary stresses should not be ignored. This is especially important when you are considering the lifecycle of the joint.

Shear Stress

Shear stress occurs when two parallel adherends attempt to slide past each other. A typical example of a joint that produces shear stress is a lap joint.

How Shear Stress Acts

Shear stress acts parallel to the plane of the adhesive layer. It attempts to slide one adherend past another while the adhesive resists the sliding movement.

A single lap joint can experience shear stress when the adherends are pushed past each other in opposite directions.

The nominal average shear stress can be calculated by dividing the force by the bonded area:

Average Shear Stress = Parallel Force ÷ Bonding Area

This calculation is a start to estimate the type and amount of stress the joint could experience. It does not mean that shear stress is equal at every point. As the adherends stretch, bend, and transfer load into the adhesive, new stress concentrations can develop at varying magnitudes.

Adhesive Joints Commonly Loaded in Shear

In this section, we list a number of common joint types that are loaded in shear.

Common examples include:

Single lap joint
Double-lap joints
Step joints
Scarf joints
Sleeve joints
Strap joints
Shaft-to-hub joints
Socket-and-spigot joints
Coaxial tube joints
Skin-to-stiffener joints
External patch joints
Face-to-face laminations

Shear-Stress Distribution

The shear stress in a bonded overlap is usually not evenly distributed. In a typical lap joint, stress concentrates near the ends of the overlap because this is where the force enters and leaves the adhesive layer. The center may be more stable and carry a smaller portion of the total force.

The non-uniformity is partially caused by the difference in lengthwise pulling force carried by each adherend. At the beginning of the overlap, the first adherend carries most of the lengthwise load. As the adhesive gradually transfers the load into the second adherend, the force carried by the first adherend decreases while the force carried by the second adherend increases. As a result, the force concentrates near the ends of the overlap, and the center bears less stress.

Joint eccentricity can further increase the local stress. Joint eccentricity means that the stress does not travel along straight lines. This offset makes the joint bend or rotate while being pulled, creating additional peel stress at its edges.

In a single lap joint, increasing the overlap length can reduce the average sheer stress, but it does not reduce the stress at the edges in direct proportion.

Advantages and Limitations of Shear Loading

Shear is compatible with adhesives because it allows force to be transferred across a large bonding area, and adhesives generally favor continuous load distribution. Correctly configured lap, scarf, and cylindrical joints can support substantial loads without the need to drill holes through the adherends.

However, shear does have its limitations:

Stress concentrates near overlap ends and edges
Peel stress can be caused by eccentric loading
Creep can develop under a sustained load
Adherend may bend or rotate
Fatigue can occur under repeated loading
Reduced performance at hot temperatures

The optimal shear-loaded designs align the force path, use appropriate overlap proportions, and provide gradual or tapered load transfer. Double-lap, scarf, stepped, or tapered geometries are effective at reducing joint eccentricity and softer stress transition.

Tensile Stress

Tensile stress acts perpendicular to the bond and pulls the adherends directly apart across the entire bonding area.

How Tensile Stress Acts

Tensile stress acts perpendicular to the plane of the adhesive layer and pulls the adherends directly away from each other. In a perfect scenario, the force in the tensile joint remains centered, the adherends remain parallel, and the stress is distributed evenly across the entire bonding area.

The nominal average tensile stress can be calculated using the following formula:

Average tensile stress = Perpendicular force ÷ Bonding area

This relationship assumes that the load is distributed evenly. If the force is offset, the adherends have different levels of stiffness, or one edge begins to separate, the stress will no long be uniform and may begin to resemble cleavage or peel.

Adhesive Joints Commonly Loaded in Tension

These types of adhesive joints can experience tensile stress under the specified conditions:

Face-to-face joints pulled directly apart
Butt joint where the load is distributed along a common center line or axis
Stud-to-panel joints
Insert joint
Skin-to-core joints under through-thickness loading
Sandwich-panel skins subject to local uplift

A butt joint can technically carry uniform tension when the adherends have the same cross-sections, and the force passes along a common axis. However, in practice, small alignment errors can cause bending or shifting of the load toward one side.

Tensile-Stress Distribution

Tensile stress is most uniform when:

The load is perpendicular to the bond.
The force passes through a common axis
The adherends have near-identical stiffness
The bonding surfaces are flat and parallel.
The adhesive layer has a consistent viscosity and thickness.
The joint is free from voids and edge defects.

Deviations from these conditions create concentrations of local stress. For example, a tilted load can cause one side of the bond to experience greater tension than the other. Additionally, a flexible adherend can bend and lift from an edge. Likewise, a highly thick adhesive layer can deform differently from the surrounding bond line.

Local stress concentrations can develop around holes, corners, inclusions, or areas where the adherend thickness changes. Therefore, it is important not only to look at the nominal tensile stress because smaller areas with higher stress concentrations may alone experience failure.

Advantages and Limitations of Tensile Loading

Uniform tensile loading is preferable to peel or cleavage because the force is not intentionally concentrated along one edge. Instead, the load is distributed throughout the bonding area.

However, it does have limitations, which include:

Sensitivity to load misalignment
Bending caused by uneven configurations or adherends
Lifting of edge in flexible materials
Local stress developing around defects
Through-thickness weakness in composite or sandwich cores
Difficulty maintaining uniformity of load distribution.

Tension stress should not be mistaken for peel because tension pulls the entire bonded area apart while peel progressively separates the joint from an edge. If one adherend is flexible, peel stress may develop at the edge.

Compressive Stress

Compressive stress acts perpendicular to the bond and presses the adherends together.

How Compressive Stress Acts

Compressive stress acts perpendicular to the bond and presses the adherends toward one another and is the opposite directional loading mode to tensile stress.

The adhesive resists deformation and helps transfer the load between the adherends. Because the adherends are being pressed together, compression stress does not initiate separation like peel or cleavage.

The risk from compression stress is lateral deformation, especially if the adhesive layer is thick and soft.

Adhesive Joints Commonly Loaded in Compression

Several instances of adhesive joint geometry can lead to the development of compression stress such as:

Face-to-face joints beneath clamping or bearing loads
Laminated members carrying compressive stress
Bonded blocks or pads
Sandwich panels under localized pressure
Bonded joints in columns or supports
Sealed flange joint
Mortise and tenon under bearing
Adhesive layers underneath machine bases or inserts
Joint regions compressed by mechanical fasteners

Compression stress may only develop during assembly, during service, or during both stages. Clamps and presses can add or cause a joint to experience compressive stress.

Compression, Buckling, and Adhesive Squeeze-Out

Compression stress can be generally favorable to the adhesive because it reinforces the bond by pushing the adherends together. However, thin or slender adherends may buckle before the adhesive achieves its compressive limit. Foam, honeycomb, wood, or other low-density materials can crack or crush under concentrated pressure. Likewise, plastics may deform, creep, or develop permanent indentation.

Excessive clamping pressure during assembly can cause uncured adhesive to squeeze out from the joint’s bond line. This produces a bond line with insufficient adhesive to fill surface irregularities or sustain continuous contact. High pressure can cause the components to shift, distort, or get crushed.

On the other hand, if there is not enough compressive pressure, the adhesive may not wet both surfaces and may avoid making contact with gaps or voids, which would remain without treatment. As a result, you need to ensure that there is sufficient compressive pressure for the adhesive to make contact between adherends along the bond line.

Advantages and Limitations of Compression Loading

Compression is generally favorable for the adhesive, but the adherends can still buckle, crush, deform, or force adhesive from the joint during assembly. Compressive stress is probably the least concerning type of stress among the five major loading modes because it encourages contact between the bonded adherends.

However, there are still limitations from compressive stress that can arise from:

Core or surface crushing
Adhesive buckling
Local indentation
Plastic deformation
Long-term creep
Adhesive leakage from unconfined surface area
Uneven bearing pressure

A joint should not be considered completely safe just because it is under compressive stress, but it should be regularly inspected for instability, crushing, deformation, or for movement under continuous loading.

Peel Stress

Peel stress applies where force lifts one flexible adherend away from the bonded surface.

How Peel Stress Acts

Peel stress acts when an adherend is flexible enough to bend and progressively lift away from the other adherend.

The adherend is not separated completely at once, but the load progresses along the joint from an exposed edge. The peel force is generally concentrated at a narrow point near the separation area. Only a small portion of the total bonding area resists the separation at a given moment. As the edge separates the joint, the load concentration moves into the next section of the bond.

This behavior is similar to removing adhesive tape from a surface. The total bonding area of the tape to the adherend may be significant, but the peel force of pulling away at the tape is sufficient to eventually remove the whole strip of tape.

The peel angle affects the loading condition, with steep angles generally carrying more stress concentration.

Why Peel Stress Is Damaging

Peel is particularly dangerous because adhesives are less capable of carrying load over a highly concentrated area rather than an evenly distributed load in shear or compression. The local stress concentration near the peel can be much greater than the nominal force divided by the total bonded area.

As separation progresses, the total bonding area diminishes. The remaining edge carries the load, allowing failure to peel away through the joint. Surface defects, incomplete wetting, contamination, and weak boundary layers can trigger peel stress to develop.

Adhesive Joints Susceptible to Peel

Peel stress can develop in situations involving:

Adhesive tapes
Flexible sheet seams
Single-sided patch joints
Labels and films
Thin metal or plastic lap joints
Flexible laminates
Membranes and fabrics
Skin-to-core bonds
Bonded trim
Joints with lifted or unsupported edges.

Thinner adherends are more likely to develop peel stress because they bend more easily. Likewise, thermal expansion, warping, residual manufacturing stress, impact damage, or an external object can cause peel stress to develop.

Methods for Reducing Peel Stress

Peel can be reduced by changing the geometry of the joint, stiffening the adherends, protecting the edges, or using a more suitable adhesive. Techniques you can use to reduce the likelihood of developing peel stress include:

Apply loads in the plane of the bond.

Increase adherend stiffness near the joint.
Avoid pulling directly on a flexible edge.
Use a double-lap or double-sided configuration.
Taper patch, strap, or doubler edges.
Add an adhesive fillet at the overlap end.
Use rounded terminations rather than sharp ones.
Extend or fold a flexible adherend to allow gradual transition of force.
Protect exposed edges from impact and lifting.
Use mechanical fastening where unavoidable peel loads are significant.
Select an adhesive with better toughness and peel resistance.
Ensure adequate wetting and correct surface preparation.

Increasing the bonding area may not be sufficient if the load remains concentrated. Changing the design to alter how the force reaches the bond may be necessary.

Cleavage Stress

Cleavage stress develops where one rigid adherend is pried away from another.

How Cleavage Stress Acts

Cleavage occurs when two rigid adherends are pried apart from one end of the joint. This force creates a strong tensile-stress concentration near the opening edge.

This action resembles opening a rigid lid or separating two interlocked plates with a wedge. The adherends do not separate in a continuous flexible strip like with peel, but the joint still opens progressively from one side.

Cleavage converts an otherwise broad bonded area into an edge-dominated load path. Therefore, a small angular misalignment can prove more damaging than a properly centered tension load.

Adhesive Joints Susceptible to Cleavage

Peel and cleavage follow similar mechanisms, but peel occurs with flexible adherends while cleavage occurs with rigid adherends.

Cleavage can occur in:

Butt joints with off-center loads.
Corner joints.
Rigid lap joints prone to prying.
Flange joints.
Bonded brackets.
Stiff external patches.
T-joints under overturning loads.
Rigid covers or panels lifted from one side.
Joints susceptible to bending movements.
Assemblies contain gaps or uneven supports.

Methods for Reducing Cleavage Stress

Cleavage opens a relatively rigid joint from one end, producing a strong stress concentration near the opening edge. There are several methods you can use to reduce cleavage by improving load alignment and making the transfer of force more gradual:

Align the applied force with the center of the bonded area
Avoid unsupported prying loads
Use balanced or symmetrical joint arrangements
Add gussets, flanges, straps, or doublers where possible.
Taper stiff reinforcements
Use scarfed or stepped transitions
Increase the joint radius at corners
Add adhesive fillets where permitted
Reduce gaps and angular misalignment.
Select an adhesive with cleavage resistance.
Use mechanical fasteners where appropriate

The objective is to redirect force from concentrated areas to a more even distribution of the load.

Frequently Asked Questions

This section covers frequently asked question related to stress in adhesive joints.

What is the best type of stress for an adhesive joint?

Shear and compression are usually the best types of loading modes for adhesive bonding. Shear provides a large bonding area to transfer force, while compression pushes the adherends toward the adhesive. Uniform tensile stress may also be favorable if the force is centered and distributed across a complete bond. The best loading mode will also depend further on the adhesive, adherends, geometry, environment, and duration of loading.

What is the worst type of stress for an adhesive joint?

Peel and cleavage are generally the worst types of stress for an adhesive joint because they concentrate stress along an edge. Once the separation begins, the effective bonding area becomes smaller, and failure starts to occur. The severity of peel or cleavage stress depends on the adherend flexibility, joint stiffness, adhesive toughness, loading angle, and edge condition.

Is shear stronger than tension in an adhesive joint?

Not necessarily, because shear strength and tensile strength cannot be compared without the appropriate test methods or geometric considerations. Joint designers typically prefer shear because the load is easier to distribute over a large area. Alignment is the key factor that determines the feasibility of a shear or tensile loading mode because misaligned adherends can begin to develop peel or cleavage stress.

What is the difference between peel and cleavage?

The difference between peel and cleavage stress is mainly that peel involves flexible adherends, while cleavage involves rigid adherends. Either way, both begin separating at the edge.

Can an adhesive joint experience several stresses at once?

Yes, adhesive joints may experience multiple stresses at one time, usually from different angles. A single lap-joint can carry shear and peel while a bonded bracket may experience shear, tension, and cleavage. The dominant loading mode is useful for describing the adhesive joint, while the secondary stresses indicate where failure may begin to occur.

Does increasing the bonding area always increase strength?

Not necessarily, because increasing the bonding area may result in lower average stress, but concentrations can still develop at failure points, resulting in reduced strength for the overall joint. Whether increasing the bonding area will effectively increase strength depends on the geometry, stiffness, load alignment, and adhesive properties.

Conclusion

The five main loading modes in adhesive joints are shear, tension, compression, peel, and cleavage. These loading modes were discussed in this publication and represent the various ways stress can affect adhesive joint geometry. Generally speaking, shear and compression are the most favorable loading modes for adhesive joints, while tension is a moderate one, and peel and cleavage should be avoided. We also discussed that the average stress is calculated by dividing the applied force by the total bonding area, but this does not exclude local stress concentrations that may develop in certain locations. Local stress concentrations are a major concern because over time they may result in bond failure. A reliable adhesive joint should be designed to distribute force gradually, keep loads aligned, minimize bending, safeguard bond edges, and avoid peel and cleavage stress.

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