Strength of Elastic Adhesive Joints

In technical data sheets, the strength of an adhesive is generally stated in terms of its tensile lap-shear strength which is determined by performing tests on a singlelap adhesive joint. The test piece is subjected to a shearing stress by applying a tensile load axially to the two lapped substrates (Fig. 28). 

Since the effect of stress peaks at the ends of the overlaps can be neglected in bonds formed with elastic adhesives, it is not normally necessary to calculate the optimum ratio of substrate thickness to length of overlap. 

The reduction factor, /t, for the effects of temperature exposure on an elastic adhesive was determined with the aid of a lap-shear test (Fig. 29). The strength of the adhesive decreases with a rise in temperature. 

The results of creep rupture tests on single-lap joints yielded the reduction factor, /l, for an adhesive bond subjected to constant static loading (Fig. 30). The strength of the adhesive decreases with increasing exposure. In constant-load tests of this kind, particularly at higher temperatures, creep strain is observed in the adhesive layer where a certain initial load is exceeded. 

For this reason, a safety factor of at least 2 should always be included in the design calculations. 

---