Adhesive failure is a separation at the interface between the adhesive and the substrate. Cohesive failure is a break within the adhesive itself. The difference tells a formulator whether to work on the surface or on the formula, so a lap shear or peel value recorded without its failure mode leaves out the most useful part of the test.
Most adhesive labs already inspect the break. A technician looks at both coupons, estimates how much of the area shows adhesive on each side and writes a note. The problem is where that note ends up: in a comments column, a photo filename or a notebook page that nobody searches. The strength number goes into the results table, and the failure mode stays behind.
What adhesive, cohesive and substrate failure tell you
ISO 10365:2022 gives the common codes for describing a broken joint. The main ones:
- AF, adhesion failure: the joint gave way at the adhesive/substrate interface.
- CF, cohesion failure: the break ran within the adhesive layer.
- SCF, special cohesion failure: the break ran within the adhesive, very close to the interface.
- SF, substrate failure: one or both adherends broke.
- DF, delamination failure: a coating or primer tore away from the adherend.
When more than one pattern appears, the standard reports an approximate percentage for each, for example "AF (50 %) + CF (50 %)." That mixed result is common, and it is exactly the detail that gets lost when failure mode is written as a single word.
Why a strength number on its own misleads
A higher strength value does not always mean a better adhesive. Take an illustrative case. Two candidate formulations are tested on the same aluminum substrate. Formulation A reaches 14 MPa in lap shear with 80% adhesion failure. Formulation B reaches 11 MPa with 100% cohesion failure.
Sorted by strength, A wins. Read with its failure mode, A is the riskier choice. Its bond gave way at the interface, so its 14 MPa depends on how well that particular surface was cleaned and prepared. B broke inside the adhesive, which means the interface was stronger than the bulk material and the formula itself set the limit. A team that picks A and then meets a customer surface with a little more contamination, or a humid line, has chosen the result most sensitive to conditions it doesn't control.
This is why many adhesive specifications set a minimum percentage of cohesive failure alongside a strength value. The two numbers answer different questions.
What each failure mode points the formulator toward
The failure mode is a diagnosis as much as a measurement. Each pattern narrows where to look next.
- Adhesion failure points to the interface: surface contamination, inadequate cleaning or abrasion, a missing or mismatched primer, poor wetting, or a cure that did not develop fully at the bond line.
- Cohesion failure points to the adhesive: formula strength, filler loading, cure state, or degradation after aging. If strength is too low with clean cohesive failure, the formula is the lever.
- Substrate failure means the joint is stronger than the part. That is often a pass, but it also means the test no longer measures the adhesive, so a different specimen or substrate may be needed to compare formulations.
- Mixed or alternating patterns often point to variability: uneven bond-line thickness, inconsistent surface preparation, voids or incomplete mixing.
None of these readings is possible from a strength value alone. They all depend on the failure mode being stored next to the conditions that produced it, which is the same lesson the process context behind adhesive scale-up teaches at plant scale.
What to record with every bonded-joint test
A failure mode is only useful later if it can be compared across tests. For each specimen or test set, capture:
- the failure-mode code and the approximate percentage of each pattern
- a photo of both fracture surfaces, linked to the result
- substrate, grade and lot
- surface preparation, primer and the time between preparation and bonding
- bond-line thickness and how it was controlled
- cure schedule, measured at the joint where possible
- conditioning or aging before test (humidity, temperature, duration)
- test method, test rate and test temperature
- the formulation version and raw-material lots
The list looks long, but most of it already exists somewhere in the lab. The work is putting it into fields that sit with the result, so a future search can filter on them.
Can your team compare failure modes across past tests?
Only if the failure mode was recorded the same way each time. A quick check: pull the last 20 lap shear or peel results on one substrate. Count how many have a failure mode in a field you can filter on, rather than in a comment or a photo. Then try to answer one question, such as "which formulations gave more than 90% cohesive failure on this substrate after humidity aging?" If that takes more than a few minutes, the team is paying for tests it can't learn from.
Failed bonds belong in that search too. A test that broke at the interface is evidence about what a surface or primer won't tolerate, and it is learning the team already paid for.
What this means for adhesive R&D teams
- Is failure mode a structured field in your results, or a note?
- Do your specifications pair strength with a minimum cohesive-failure percentage?
- When a strength value looks good, does anyone check how the joint broke before it goes into a customer recommendation?
- Could a new formulator find every test where a given primer produced adhesion failure?
Making failure mode searchable in LIMS for R&D
Uncountable's LIMS for R&D logs each test result with its sample, substrate, surface preparation, cure and aging conditions, and the formulation that produced it. Failure mode and percentage can be captured as structured fields with fracture photos attached, so teams can filter past bonded-joint tests by how they broke as well as how strong they were. See how adhesive and sealant teams connect formulation to cure in one record.
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