The answer is a little more complex than most people realize. In the world of adhesive bonding, most failures can be categorized into three distinct groups:
* Substrate Failure: Substrate fails before the adhesive.
* Cohesive Failure: Adhesive fails down the middle, adhesive remains on both substrates.
* Adhesive Failure: Adhesive fails from one substrate. Usually a customer would say, "The adhesive failed from the cold rolled steel but remained on the glass."
In many cases, however, customers experience a combination of all three types of failure.
When asked, "What would be the preferable mode of failure?", most customers will answer substrate failure. Is this an accurate answer?
A response to this question is that while the mode of failure is important, what should drive the adhesive manufacturer and the customer's selection, is choosing the adhesive that will meet the customer's specific requirements. For example, if a company wants their assembly to withstand rigorous environmental testing while being subjected to 2,500 psi of pressure, then the adhesive also needs to meet this requirement. Whether the customer experiences cohesive, adhesive, or substrate failure, for the most part, is irrelevant if the adhesive does not meet their requirements.
Is there any situation when the mode of failure is important? Yes, when everything is equal during the qualification process, it may be helpful to review the failure modes to narrow the field of adhesive candidates down. Also, in the case of failure analysis, the mode of failure is important to help determine the root cause. This information can be used to determine why a particular adhesive is or is not bonding to one or more of the substrates.
Ultimately, identifying the mode of failure is only important once the customer's requirements have been met. And even then, does it really matter that the adhesive failed from one of the substrates if the customer's requirements have been exceeded?