Resistance of Casein Glue Joints to Shear Fatigue Stressing

The fatigue strength of casein glues determined from a specimen subjected to torsional shear indicates that at 10,000,000 cycles, the practical end point in fatigue testing, the strength is only 25 percent of the average static strength of the joint. In this study, in each cycle, the stress at all levels oscillated between zero and the maximum of the preassigned stress. This results in a shorter fatigue life than if the stress oscillated from 10 percent to the same maximum. It cannot be concluded from static shear tests alone that the fatigue characteristics of glue lines will show the ideal behavior hypothetically depicted, where the fatigue strength of the glue is higher than that of the wood used. If such a case is encountered, the glue bond itself is no longer of major concern since failure is very likely to occur in the wood and not in the glue line.

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