This is a modified excerpt from an essay produced while at West Dean College, ‘Understanding and Treating Adhesive Failure in a 19th Century Mahogany Chair’
When adhesives and glues fail, it is important to understand where the failure has occurred, and also understand the appropriate language in regards to describing that failure.
An adhesive substance has both adhesive and cohesive properties. Adhesion is the bond between the substance and another material, while cohesion is the bond between a substance and itself. If one were to pull on the top and bottom of an Oreo, there are 3 general ways in which it could come apart (illustrated in Figure 1).
Figure 1 Adhesion and cohesion failure in Oreo biscuits 
The biscuit could break: This would be a cohesive failure in the ‘substrate’.
The white stuff inside could separate cleanly from the chocolatey biscuit: This is a failure in the adhesion between the ‘adhesive’ and the ‘substrate’.
The white stuff could tear in half: This would be a failure in the cohesion of the ‘adhesive’.
It is also likely to have some combination of the above. Often there will be both adhesive and cohesive failure across the material, whereby some of the adhesive remains intact and some of it detaches (Down, 2015).
Jane L. Down, M. A. M. J. T. R. S. W., 1996. Adhesive Testing at the Canadian Conservation Institute: An Evaluation of Selected Poly(vinyl acetate) and Acrylic Adhesives. Studies in Conservation, 41(1), pp. 19-44.