When an adhesively bonded specimen breaks in a tensile or peel test, the failure load is not the only thing that matters. Where the failure occurred matters just as much. An experienced engineer looking at the failure surface can tell whether the problem lies in the adhesive or in the surface preparation. The naming of failure modes is defined in ISO 10365.

The main failure modes

• Adhesive failure: The adhesive has separated completely from the surface of one of the parts. It usually points to a surface preparation problem: oil, dirt, oxide, insufficient roughening or a missing primer.
• Cohesive failure: The failure runs through the adhesive itself, and adhesive remains on both surfaces. Adhesion to the surface is good; in most cases this is the desired failure mode.
• Substrate failure: The bonded part itself has broken. The joint is stronger than the part.
• Mixed failure: Adhesive failure over one portion of the surface and cohesive failure over another. The percentages are recorded separately.

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Why it matters

Even when the failure load passes the limit, if most of the failure is adhesive the process is not healthy: the strength was enough this time, but it may not be enough in the next batch. That is why many specifications ask for the failure mode and its percentage alongside the load.

Common mistakes in evaluation

1. Not photographing the failure surface
2. Estimating the percentages by eye and not recording them
3. Rounding a mixed failure to a single mode
4. Not linking the surface preparation records to the test result

Which of these is in your records?

Most of these mistakes slip past when records are read one by one by eye, and auditors look for them first. The real question is which one sits in your own records. Upload a photo of your failure surface to NTB SpecPro and quickly see which points may carry risk, the likely reason and a suggestion.

The NTB SpecPro bonding module marks the failure modes and their approximate percentages from a photo of the failure surface, and suggests likely causes and the steps to check. Final evaluation stays with the authorized engineer.

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Prepared by NTB SpecPro process quality engineering; reviewed by an engineer holding the European Adhesive Engineer (EAE) qualification. Questions: info@ntbspecpro.com