Adhesive Bonding: How It Works, Joint Stresses and Failure Modes

Short answer

Adhesive bonding joins two parts by placing a substance between them that holds to both surfaces and then develops strength, usually by curing. ASTM D907 defines an adhesive as “a substance capable of holding materials together by surface attachment”. A bonded joint is only as good as its weakest link: the adhesive-to-surface interface, the adhesive layer itself, or the parts being joined. Reading which of these gave way is the most practical way to diagnose a failed bond.

Definition and scope

The vocabulary of the field is standardised in ASTM D907, Standard Terminology of Adhesives (current edition D907-15(2023)), which describes itself as a compilation of definitions used in the science and technology of the adhesives industry. Four of its definitions do most of the work when you describe a joint or a failure:

• Adhesive: “a substance capable of holding materials together by surface attachment”.
• Adherend: “a body held to another body by an adhesive”.
• Adhesion: “the state in which two surfaces are held together by interphase forces”.
• Cohesion: “the state in which the constituents of a mass of material are held together by chemical and physical forces”.

D907 also defines an adhesive joint as the “location at which two adherends are held together with adhesive”, and cure as “to develop the strength properties of an adhesive by chemical reaction”. The distinction between adhesion (between the adhesive and a surface) and cohesion (within the adhesive itself) is what makes failure-mode diagnosis possible, covered below.

Henkel’s Loctite bonding manual, a manufacturer document, describes adhesive bonding as an efficient, economical and durable method for assembling dissimilar materials, capable of joining almost all types of materials to themselves and to each other, whereas welding and riveting are limited by material characteristics, joint geometry and application requirements. The same manual notes that an adhesive layer can fill gaps and tolerances, seal against liquids and gases, and, with organic polymers, provide electrical insulation; silver-filled adhesives are used where conductivity is needed, and some adhesives are modified for heat transfer (see thermal interface materials).

This page is about the general concept in industrial and laboratory settings. Dental adhesives are a separate product class; see dental bonding agents.

How adhesive bonding works

The manual lists four classical theories of adhesion. In practice more than one applies to a given joint.

• Mechanical interlocking: liquid adhesive flows into undercuts and surface roughness and then hardens, and roughness also adds contact area.
• Electrostatic: an electrical double layer forms at the interface.
• Diffusion: for long-chain polymers, molecules diffuse across the interface.
• Wetting (adsorption): adhesion arises from the surface forces of molecular contact, which requires the liquid adhesive to spread over the surface.

Wetting and surface energy

Wetting is governed by surface energy, expressed in mN/m (the older unit is dyn/cm). The manual groups substrates into bands of high (above 30 mN/m), medium (25 to 30 mN/m) and low (below 25 mN/m) surface energy. The ideal combination is a high-surface-energy substrate with a low-surface-tension adhesive, so that the adhesive spreads rather than beads up. Low-energy plastics are therefore the hard case.

Surface energy can be approximated with an ink test, or measured more exactly through contact angle. The manual states that plasma, corona or flame treatment can raise the surface energy of plastics but that the effect is often temporary, so parts should be bonded promptly after treatment. Primers can also raise the surface energy of low-energy plastics.

Surface preparation: rougher is not automatically stronger

Roughening adds contact area and mechanical keying, but the manual warns that extreme roughness needs more adhesive and a longer cure, and that thin-layer adhesives work less well on very textured surfaces. It also states: “The cohesive strength of the adhesive layer will not be increased by roughening the substrate, so assemblies suffering cohesive failure will not benefit from surface preparation.”

Types of adhesive and how joints are loaded

Adhesive families

The Henkel manual distinguishes three broad families by their molecular behaviour:

• Thermoplastic: not crosslinked and able to be re-melted (hot melts are the example). Service temperature is limited, they can creep, and solvent and chemical resistance is lower.
• Thermosetting: crosslinked during cure.
• Elastomeric: large elongation and energy absorption, useful for sealing and vibration damping, resistant to peel and cleavage, but less suited to structural bonding.

Cure mechanisms listed in the manual are solvent evaporation, cooling, a mixing reaction, heat, moisture, light, and dual cure. Which one applies determines what the user must control: mixing ratio, temperature, ambient humidity, light exposure or time.

Stress types in a bonded joint

The manual identifies five ways a joint can be loaded and names a standard test for several of them.

Stress type	How the load acts (per the manual)	Standard test named in the manual
Shear	Load parallel to the bonded surfaces	Single lap shear: ASTM D1002 / ISO 4587
Tensile	Load pulling the surfaces apart, as in a butt joint	None named
Compressive	Load pressing the surfaces together	ASTM D695
Peel	A combination of shear and tensile stress	T-peel: ISO 11339
Cleavage	Rigid substrates opened at one end	None named

The manual calls single lap shear a common quality-control test but a poor way to compare different adhesive and substrate combinations and a limited predictor of real geometries, and it concludes that testing on real-world assemblies under real-world conditions is essential. For the lap-shear method itself, see ASTM D1002.

Where it matters for labs and production

Reading a failed joint

The manual describes four failure modes: adhesive, cohesive, substrate, and mixed adhesive/cohesive. Because each leaves a different pattern on the broken parts, the failure surface tells you which element to change. The data table below the article sets out each mode, what is seen, what it implies and what to change.

Two points from the manual are worth keeping in mind when interpreting results. Cohesive failure is common when the adhesive is uncured or insufficiently cured, so a cohesive failure in a new batch is a prompt to check cure conditions before changing the surface preparation. And for service life, heat and humidity are usually the most damaging environmental factors, and cyclic (slow) stresses are more damaging than steady ones.

Standards for classifying failures

Failure classification has its own standardisation. ASTM D5573, Standard Practice for Classifying Failure Modes in Fiber-Reinforced-Plastic (FRP) Joints, defines seven primary failure-mode classes, but it applies only to FRP joints and is used with the corresponding FRP joint test methods; it is not a general scheme for all adhesive joints.

Common misunderstandings

• “Rougher surfaces always bond better.” Roughness helps adhesion up to a point, but extreme texture demands more adhesive and longer cure, and it cannot raise the strength of the adhesive layer itself.
• “A good lap-shear number predicts a good part.” The manual describes lap shear as a screening and quality-control test that is poor at ranking different adhesive and substrate pairs and limited at predicting real geometries.
• “Adhesion and cohesion are the same thing.” ASTM D907 separates them: adhesion is between surfaces, cohesion is within a material. Treating them as one hides which side of the joint failed.
• “A treated plastic stays treated.” Plasma, corona and flame effects are often temporary.
• “Substrate failure means a bad adhesive.” The opposite: when the substrate yields, the adhesive was not the weak point.

Frequently asked questions

What is the difference between adhesive and cohesive failure?

In adhesive failure the joint separates at the interface between the adhesive and a surface, leaving residue mainly on one part. In cohesive failure the fracture runs through the adhesive layer, leaving adhesive on both parts (Henkel bonding manual, 2022).

Can adhesive bonding replace welding or riveting?

The manufacturer’s manual presents bonding as capable of joining almost all materials, including dissimilar ones, where welding and riveting are limited by material, geometry and application. Whether it suits a particular load and environment must be shown by testing real assemblies.

Why does the adhesive not wet some plastics?

Those plastics have low surface energy (below 25 mN/m in the manual’s banding). Surface treatment or primers can raise it, and treated surfaces should be bonded soon after treatment.

Sources

• Henkel Loctite, How to Create Reliable and Effective Bonded Joints (ACM Bonding Manual), 7 August 2022, manufacturer guidance. Henkel document library. Read 2026-11-12.
• ASTM D907-15(2023), Standard Terminology of Adhesives. Catalogue entry. Read 2026-11-12.
• ASTM D5573-99(2019), Standard Practice for Classifying Failure Modes in Fiber-Reinforced-Plastic (FRP) Joints, catalogue description. Read 2026-11-12.