Never trust a glued joint in peel

The four ways a joint can be loaded, and the one that destroys adhesives.

A glued joint can be pulled straight apart, slid sideways, squeezed together or peeled back like a sticker, and of those four ways of loading it, peel is by far the weakest, because it concentrates the entire force onto a narrow line at the edge of the bond instead of spreading it across the whole glued area.

The short version

Pulling two glued surfaces directly apart, a tension load, or sliding one across the other, a shear load, both put the entire bonded area to work at once, since every square millimetre of the joint has to fail before the pieces separate, and a joint with a generous glued area resists these loads roughly in proportion to how much area there is. Peel loading works completely differently. Instead of asking the whole joint to fail together, it lifts one edge and rolls the separation forward a fraction of a millimetre at a time, so at any given instant only a thin strip right at that advancing edge is actually carrying the load, while the rest of the joint, still fully bonded further along, is doing nothing at all. Because that thin strip has to resist the whole force by itself, the stress concentrated there can be enormous even when the total force applied is small, and once that strip gives way the next strip along becomes the new edge and the process simply continues, unzipping the joint along its length with very little total effort required. Compression, squeezing the two glued surfaces together, sits apart from all three of these, since it is mostly resisted by the surfaces themselves pressing directly against each other rather than by the adhesive doing any real work, which is why compression is rarely the loading mode that decides whether a glued joint survives.

The plaster comparison

Removing an adhesive plaster from skin shows exactly how peel loading works, and how little force it actually needs. Trying to lift a plaster straight off, pulling it directly away from the skin all at once, would require fighting the full adhesive strength of the entire stuck-down area simultaneously, which is uncomfortable and takes real force. Nobody removes a plaster that way. Instead, lifting one corner and peeling it back at a shallow angle concentrates all the separating force onto the narrow line where the plaster is just starting to lift, and that line advances steadily across the skin with comparatively little effort, exactly the way a peel failure travels across a glued joint. The plaster's adhesive has not become weaker, the loading has simply changed from asking the whole area to resist at once to asking only a thin moving strip to give way, over and over, until nothing is left stuck down.

The one number worth remembering

A joint that can resist a substantial force pulled straight apart in tension may separate under a force many times smaller once that same joint is loaded in peel instead, since peel never asks more than a narrow strip at the advancing edge to actually resist anything, no matter how large the total bonded area happens to be.

What this does not explain

Because peel is so much weaker than the other loading modes, a joint's design matters as much as the adhesive chosen for it. A lap joint loaded so that any disturbance tends to lift one edge first, rather than pull the pieces straight apart or slide them evenly across each other, will fail at a fraction of the force the glue itself is genuinely capable of resisting, so a durable glued structure is usually one built so that the expected loads act in shear or tension across the joint, with any peeling tendency designed out, reinforced, or supported by fasteners at the vulnerable edge rather than left for the adhesive alone to withstand. Even the shape of the joint's edge matters, since a squared-off edge gives a peel front an obvious place to start, while a joint tapered gradually to nothing at its edge, or backed by a fillet of extra material, gives the load nowhere sharp to concentrate and makes a peel failure much harder to begin in the first place.