Polyurethane bond strength loadings

Thick-layer elastic-bonded joints first yield before they tear when subjected to peeling forces, allowing the load to be distributed over a wider area. Consequently, the stresses within the bonded materials are kept at a relatively low level. The high tear propagation strength of polyurethanes - even after tear initiation - usually prevents sudden and catastrophic failure of the joint. This yielding behaviour means that damaged adhesive joints can often be identified and repaired before... 

As shown in O Fig. 42.9a, and not the apparently more economical but weaker joint shown in O Fig. 42.9b. Some years ago, the author tried to stiffen an aluminum alloy honeycomb core by filling with a polyurethane foam. This was then scraped down to the level of the honeycomb and the bond made. Unfortunately, the foam prevented the formation of the fillet and the joint failed at a low load. With bonded honeycomb structure, the major defects consist of a lack of attachment between the core and the skin. This may be due to several causes such as locally crushed honeycomb O Fig. 42.10a), skin defects O Fig. 42.10b), or lack of adhesive (O Fig. 42.10c). In themselves, none of these defects may reduce the short-term joint strength. However, as for the lap joint, they may show poor preparation and may provide sites for fatigue crack propagation. 

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