How Do You Classify Adhesive Failure Modes on a Broken Bond?

What do you need before you classify adhesive failure modes?

Before you classify adhesive failure modes, you need both halves of the broken bond with their fracture surfaces untouched, even lighting, a scale for photos, and the record of what the joint was: adherends, adhesive, surface preparation, cure and the load at failure. A single half shows only where adhesive is present on that side; the classification depends on comparing the two faces.

Keep the fracture surfaces as they came apart. Do not wipe, solvent-clean or fit the halves back together, because any residue removed or added changes what the surface shows. If the bond broke before full cure or has uncured adhesive on it, handle it with the protective equipment named in the adhesive's SDS and dispose of residues as the SDS and local waste rules require.

Have the classification scheme on hand before you look. ISO 10365:2022 specifies designations for the main failure patterns of bonded assemblies and "applies to all mechanical tests performed on a bonded assembly, regardless of the nature of the adherends and adhesive" (ISO 10365:2022). For bonded fiber-reinforced plastic (FRP) joints, ASTM D5573-99(2019), the active edition, gives a parallel set of definitions that its scope offers as a guide "for contracts, drawings, product specifications, and product performance" (ASTM D5573-99(2019)). Pick one scheme per report, so every record reads the same way.

Classifying adhesive failure modes on a broken bond, step by step

Classifying adhesive failure modes on a broken bond follows four steps, each producing one field of the record.

1. Inspect both fracture surfaces. Lay the halves side by side in the orientation they were bonded, under the same light, and photograph both with a scale. Note where adhesive sits on each face and whether it is glossy, dull, whitened or thin enough to show the adherend through it.

2. Name each area with an ISO 10365 code. Match each region to a pattern: adhesion failure (AF), cohesion failure (CF), special cohesion failure (SCF), stress whitening failure (SWCF), substrate failure (SF), cohesive substrate failure (CSF), delamination failure (DF), adhesion and cohesion failure with peel (ACFP) or bondline corrosion (COR) (ISO 10365:2022).

3. Estimate each area percentage. Judge the share of the bonded area each pattern covers as an approximate percentage, which ISO 10365:2022 requires when more than one pattern occurs; shares total 100 (ISO 10365:2022).

4. Record the split. Write the codes with their percentages, then the failure load and the joint record. ISO 10365:2022 gives the format "AF (50 %) + CF (50 %), OSC" for an oscillating failure of two patterns (ISO 10365:2022).

ASTM D5573 bases each classification "solely on a visual observation of the failure surface without the aid of a microscope" (ASTM D5573-99(2019)), so the first pass needs no instrument. Use magnification or surface analysis only when the visual call is in doubt, and say in the record which method decided it.

What do adhesive, cohesive and substrate failure look like on a broken bond?

On a broken bond, adhesive failure leaves one adherend bare, cohesive failure leaves adhesive on both faces, and substrate failure leaves the adherend itself broken or torn. ISO 10365:2022 defines adhesion failure as a rupture "in which the separation appears visually to be at the adhesive/adherend interface" and cohesion failure as one "in which the separation appears visually to be in the adhesive or the adherend" (ISO 10365:2022).

The table matches what the fracture surface shows to a mode and the first cause to check.

The key row is the first: an apparently clean adherend with a glossy adhesive surface on the other face is adhesion failure, and it sends the investigation to the surface and its preparation, not to the adhesive. When a coating tears off the adherend, ISO 10365:2022 requires the delamination designation (DF) to be followed by the nature of the coating, such as primer, paint or phosphatization (ISO 10365:2022).

What is thin-layer cohesive failure and how is it told apart from adhesive failure?

Thin-layer cohesive failure is a break inside the adhesive so close to one interface that only a thin film of adhesive stays on that adherend, and it is told apart from adhesive failure by looking for that film. ASTM D5573 lists thin-layer cohesive failure as its own mode, with its own figure (ASTM D5573-99(2019)). Sika's structural bonding guideline describes the corresponding ISO pattern, SCF, as one where "a thin layer of adhesive lays on one adherend" (Sika, 2022).

The difference matters because the two point in opposite directions. Adhesive failure says the interface was the weakest link; thin-layer cohesive failure says the interface held and a thin zone of adhesive gave way.

Visual checks come first. Tilt the bare-looking face under raking light: a thin film often shows a sheen, color or texture that the clean adherend does not. Compare it with an unbonded piece of the same adherend. Sika notes that in a classic adhesive failure the adhesive surface is glossy, with no discoloration, and darker than in cohesive or white failure (Sika, 2022).

When the visual call stays uncertain, surface analysis settles it. NPL describes XPS as surface-sensitive with "a maximum operational depth of less than 10 nm" and frequently used for quantitative elemental analysis of fracture surfaces (NPL, 2023). Elements from the adhesive on the apparently bare face indicate a thin cohesive layer rather than a true interface failure.

How do you estimate and record mixed failure as area percentages?

You estimate mixed failure by judging the share of the bonded area each pattern covers on the matched fracture surfaces, and you record it as each code followed by its percentage. ISO 10365:2022 states that "if more than one type of failure occurs, an approximate percentage for each type of failure pattern shall be given after each designation" (ISO 10365:2022). ASTM D5573 illustrates the same practice with mixed failures such as 32 % adhesive failure and 68 % fiber-tear failure (ASTM D5573-99(2019)).

A practical way to estimate is a grid on the photo: overlay ten by ten squares on the bonded area and count the squares each pattern fills. A joint with 70 squares of adhesive on both faces and 30 squares of bare adherend is recorded as CF (70 %) + AF (30 %) (illustrative).

The table checks the record before it goes into a report.

For a visit report that holds these fields in the same place every time, see How do you make every customer visit report follow one template?. In Lubinpla, fracture surface photos can be attached to a work log entry and the failure mode tagged with a hashtag, so past cases at the same customer can be found by that tag.

What does each adhesive failure mode point to as a cause?

Each adhesive failure mode points to the layer that gave way first: adhesion failure to the interface, cohesive failure to the adhesive, and substrate failure to the adherend. These are directions for the investigation, not conclusions, and the failure load decides how much each one matters.

Adhesion failure frequently signals inadequate surface energy, contamination or improper surface preparation, according to one adhesive maker's guide (Incure, 2026). Check the surface preparation record, the time between preparation and bonding, and any release agent, oil or plasticizer on the adherend.

Cohesive failure indicates that the bond to the substrate is stronger than the adhesive's internal strength (Incure, 2026). When it occurs at a low load, look at the adhesive: mix ratio, cure time and temperature, voids, and service temperature against the glass transition temperature.

Special cohesion failure suggests a weak zone in the adhesive close to one interface, such as a primer layer or a partly wetted surface; this is an inference that needs the surface checks above.

Substrate failure signifies that both bond strength and adhesive internal strength exceed the strength of the materials being joined (Incure, 2026). A coating or laminate that tears off (DF) points to the coating's own adhesion.

No mode is acceptable or unacceptable by itself. ASTM D5573 "does not address the acceptability of any specific failure mode" (ASTM D5573-99(2019)), and Sika states that mixed or other failure types may be accepted depending on the application, the average joint strength and its repeatability (Sika, 2022).

Why do ISO and ASTM name adhesive failure modes differently?

ISO and ASTM name adhesive failure modes differently because they were written for different scopes: ISO 10365:2022 covers every bonded assembly, while ASTM D5573 was developed on glass fiber in a thermoset polyester matrix, sheet-molding compound (ASTM D5573-99(2019)). ASTM therefore splits failure in the adherend into fiber-tear, light-fiber-tear and stock-break modes, while ISO 10365:2022 covers adherend failure with three broader patterns: substrate failure (SF), cohesive substrate failure (CSF) and delamination failure (DF) (ISO 10365:2022).

The thin-layer case carries three names. ASTM D5573 calls it thin-layer cohesive failure; ISO 10365:2022 designates the pattern special cohesion failure (SCF); Sika's guideline, which uses ISO 10365 and the DVS 3302 technical code as references, expands SCF as surface-close cohesive failure (Sika, 2022).

The whitened failure also differs. ISO 10365:2022 added "failure with stress whitening of adhesive (SWCF)" in the 2022 edition (ISO 10365:2022), while Sika, following DVS 3302, calls it white failure (WF) (Sika, 2022).

ISO's definition of cohesion failure also reaches further than some guides. ISO 10365:2022 places the separation "in the adhesive or the adherend" (ISO 10365:2022), while maker guides such as Incure's list substrate failure as a separate third mode (Incure, 2026). When records from different labs or customers meet, convert them to one scheme and keep the original term in brackets.