When Joints Fail...
A properly executed and tight fitting glue joint will provide many years of structural integrity. All adhesives fail over time, but by taking several factors into account we can postpone premature failure as much as possible. The surfaces to be glued must be prepped accordingly, properly aligned, coated in the correct amount AND type of adhesive, clamped with adequate pressure, and be allowed to cure for the correct amount of time. It is the combination of these actions that provides the highest success rate for repairs that are both structural (broken or loose chair or table legs) and cosmetic (loose or missing veneer).
Failed glue joints can often be diagnosed as either one of or a combination of several problems:
This can best be described as the adhesive not properly adhering to the surface (or surfaces) being glued. This can be due to improper surface prep, use of an improper adhesive, or lack of good contact between the adhesive and the surfaces being adhered. Adhesive cleanly on one side of a glue joint can mean that adhesion failure is a likely culprit. We can also call this 'delaminating.'
A glue joint suffers from cohesion failure most likely due to a problem with the adhesive having difficulty bonding to itself. Think of a two part epoxy. Thoroughly mixed at the correct ratios, the epoxy will set as desired. Frugal mixing and/or incorrect ratios will result in an epoxy unfit for the job and the joint will fail. Using an improper adhesive to fill a gap between two surfaces can also be an issue. Fairly even amounts of adhesive on both sides of a failed joint usually indicates cohesion failure.
Substrate failure is the result of the surface being glued failing before the adhesive fails. An example being a chair leg breaking right along a previous break or an old glue line. There will be some wood embedded in the now exposed glue. This is evidence that the glue has successfully done its job: holding onto the available surface. Possible causes of substrate failure are misuse or improper handling (leaning back in a chair, dragging furniture across a floor, etc.).