How to test an adhesive bond? Test methods and failure mode analysis

How to test an adhesive bond? Test methods

After choosing the right adhesive, preparing the surface, selecting the application method, and finally bonding the parts, you may still wonder whether the joint will meet the required performance. Will it be strong enough, durable, and resistant to different factors? In this article we explain how to verify an adhesive bond and which common test methods can be used to evaluate bonded joints.

The use of adhesive bonding across many industries is possible thanks to well-established quality testing methodologies. These are largely based on mechanical strength testing. Multiple methods have been developed to test adhesive joints under conditions similar to their real service loads—shear, peel, and tensile stresses—as well as cohesive failure within the adhesive layer.

Adhesive joint testing procedures

The most common approaches include:

• shear test,
• roller peel test,
• adhesive strip test,
• ageing tests.

Shear test

Shear is the most common type of load acting on bonded joints. Shear strength is often treated as a key mechanical property of an adhesive. The shear test is defined in DIN EN 1465 and is widely used. This method is applied when the bonded parts are made of specified materials (e.g., steel, wood, glass, composites) and have defined dimensions.

Specimen geometry (DIN EN 1465)

• specimen length: 100 mm,
• width: 25 mm,
• thickness: 1.6 mm,
• overlap length: 12.5 mm.

Fig. 1 – specimen prepared for the shear test according to DIN EN 1465

Two prepared specimens bonded together (a simple single-lap joint) are pulled in the plane of the adhesive layer. In this method it is important to apply an appropriate adhesive layer thickness. Shear strength typically decreases as the bondline thickness increases, largely due to changes in the stress state within the joint. The load is applied parallel to the bonded surface and increased continuously until failure.

Failure mode analysis

After testing, the fracture surface is evaluated. Common failure modes include:

• adhesive failure – the adhesive separates from the substrate (often linked to incorrect adhesive selection, inadequate surface preparation, or process conditions),
• cohesive failure – internal failure within the adhesive (often desired: adhesive remains on both parts),
• substrate failure – failure of the bonded part itself (may be desired, but does not provide information about adhesive strength),
• mixed failure – a combination of different failure modes.

Fig. 2 – fracture appearance of an elastic adhesive specimen (aluminium and screen-printed glass)

Other methods related to shear loading

• anaerobic adhesive for coaxial fits (e.g., bearing retention on a shaft) – compression/shear test DIN 54452, torsion/shear test DIN 54455
  
• anaerobic threadlocking – strength testing of bonded threaded joints DIN EN ISO 10964
  
• thick adherend shear test – when standard specimen thickness cannot be met: DIN 54451
  
• long-term strength – creep limit testing: DIN EN 15336
  

Roller peel test

This method evaluates peel resistance by creating a situation where one bonded part is peeled away from the other, generating tensile stress in the adhesive layer. The roller peel test is defined in DIN EN 1464 and is applied depending on the material type and specimen dimensions.

Specimen geometry (DIN EN 1464)

• rigid part length: 300 mm,
• flexible part length: 250 mm,
• width: 25 mm,
• rigid part thickness: 2.5 mm,
• flexible part thickness: 0.5 mm,
• overlap length: 200 mm.

Fig. 3 – tape specimen before the roller peel test

The prepared specimens are subjected to a force nearly perpendicular to the bonded surface. As the force increases continuously, an initial tear forms and peel propagation follows. The difference between shear and peel is that shear loads the entire bonded area, whereas peel loads the bondline in a more localized, line-like manner.

Peel testing can be used for peeling two rigid adherends apart or peeling a flexible adherend from a rigid substrate. This method is common for testing different types of tapes. In joint design, peel loads should be minimized because adhesives are generally least resistant to peel; designs are usually optimized so shear loads dominate.

Other peel-related methods

• Peel load test DIN EN ISO 11339
  
• Wedge test DIN 65448
  
• Bending/peel test DIN 54461
  

Adhesive strip test

The adhesive strip test is defined in DIN 54457. It involves applying an elastomer strip (elastic adhesive) onto a defined surface and then peeling the strip while systematically cutting it. This creates additional opportunities for the strip to tear.

Fig. 4 – adhesive strip test prepared according to DIN 54457

Ageing tests

To assess adhesive bond durability in an optimal way, mechanical tests are usually performed first, then ageing tests are conducted, and finally mechanical tests are repeated. This cycle provides insight into how ageing affects a specific bonded joint.

Ageing is defined as time-dependent changes occurring in a bonded joint. Adhesive joints can age due to multiple stressors, which can reduce strength and durability. The key factors influencing ageing include:

1. Temperature

At elevated temperatures, adhesive cohesion can decrease, chemical reactions accelerate, and ageing processes intensify (e.g., water ingress). Low temperatures may contribute to bond brittleness.

2. Moisture/chemicals

If a bonded joint is exposed to high humidity or chemicals, it may experience reduced adhesion (loss of physical interactions), decreased cohesion (softening), or hydrolysis of polymer chains. Not only the adhesive joint but also the substrates can be affected (e.g., corrosion of metals, softening of plastics).

3. Radiation (most commonly UV)

Excessive exposure can reduce adhesion or cause complete loss of adhesion, introduce brittleness, or lead to discoloration through fading.

4. Mechanical loads

Mechanically stressed joints age faster, and above certain load levels may experience plastic deformation and microcracking.

5. Fungi/microorganisms

Some fungi under suitable conditions can feed on adhesive components and affect bond strength.

6. Migration of modifying additives

Plastics may contain fillers, plasticizers, and anti-adhesive agents which can migrate to the surface over time and weaken the bond.

Typical ageing test types

• constant climate test – specimens stored under constant conditions, e.g., 85⁰C / 85% humidity / 2000 h,
• variable climate test – cyclic climate conditions (common in automotive),
• immersion test – specimens immersed in a medium at a set temperature,
• salt spray test – spraying specimens with a 5% sodium chloride solution,
• cataplasma test – specimen wrapped in a wet compress in a sealed plastic bag with deionized water, conditioned at 70⁰C for 7 or 14 days, then cooled to -20⁰C and held for 15 h.

Fig. 5 – adhesive strip test specimen (DIN 54457) after the cataplasma test

Adhesive joint testing – service offer

Melkib, as a company with long-standing experience in industrial adhesives, can arrange adhesive joint testing in a certified laboratory according to applicable standards. Most commonly, the cataplasma test is performed together with the DIN EN 1465 shear test and the DIN 54457 adhesive strip test.

What is the cost of testing?

The cost depends on the number of specimens and the scope of testing. Details are available here: Contact.

FAQ – adhesive bond testing

How do I choose a test method that matches real joint loads?

Start by identifying the dominant load type in your application (shear, peel, tensile). Then select the test method and specimen geometry that best reflect those conditions.

What does adhesive failure mean and is it a problem?

Adhesive failure means the adhesive separates from the substrate. In practice it can indicate an issue with adhesive selection, surface preparation, or process conditions.

Does bondline thickness affect shear strength results?

Yes. In many cases, shear strength decreases as the adhesive layer becomes thicker due to changes in stress distribution within the bondline.

Why are ageing tests performed “before and after” mechanical tests?

Comparing mechanical results before and after ageing helps evaluate how environment and time influence joint durability.

Which factors most often accelerate adhesive joint degradation?

Most commonly: temperature, moisture/chemicals, UV radiation, mechanical loads, and material-related effects such as additive migration in plastics.

Author

Marcin Filipczyk – long-time specialist in adhesive bonding.