Maximum Strength of End-Grain to End-Grain Butt Joints

A reservoir method of adhesive application compensating for end-grain penetration was used to investigate variables influencing tensile strength of end-grain butt-joints joined with epoxy adhesive. The variables studied were wood species, surface quality, adhesive flexibility, thickness of glueline. Results were interpreted by fracture theory using a new concept – “apparent flaw size” – the equivalent effect of actual flaws (bent fibers, air bubbles, and debris). Apparent flaw size gives a useful index of end-grain gluability, and emphasises the major influence of surface preparation. Flexibilizers enhance multiaxial straining effects but lessen cohesion. As wood porosity increases the apparent flaw size needed to initiate fracture decreases. The best ways to improve the butt-joint strength using epoxy resin are to have undamaged surfaces, to use adhesives which are flexible but have good cohesion, and to ensure a good supply of adhesive during the flow phase prior to gelation.

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