Early Tests for Adhesive Strength

In the early years of structural bonding, say 1945 to 1965, testing of adhesives was solely for ultimate strength. This work provided little of value for stress analysis. The reasons for this 

Engineering Materials Department, American Cyanamid Company, Havre de 

The popular lap shear test (Fed. Spec. MMM-A-132) is widely used today, years after it was realized that its stress distribution was grossly uneven. The calculated adhesive stress, i.e., failing load divided by bond area, does not relate to the maximum stress at failure. It is clear that such data are limited for use in stress analysis. It is recognized that the test has other uses. These are (1) quality control of adhesives and bonding processes (2) comparing adhesives for superiority in a very broad sense (3) some insight into environmental resistance. 

Metal-to-metal peel tests quickly became popular, even mandatory, as a demonstration of toughness, yet the data had no use in direct stress analysis. Successful adhesives in service exhibit a range of peel strengths as high as nine to one. This strongly suggests that adhesive peel strength is not wholly relevant to the performance of bonded structure. 

Work of some value for stress analysis was done in the early years. This was the so-called long overlap shear testing. It was recognized early on that increasing the overlap in lap shear specimens did not result in a proportionate increase in strength. A length of lap is quickly reached where no further load increase is possible. 

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