Analyzing Load History Dependence of Fracture in Structural Adhesives
Donald L. Hunston
The most common toughening mechanisms in structural adhesives are viscoelastic processes which means that the fracture enrgies for bulk adhesive specimens and bonded joints will vary with loading history. Although this is well known, relaviely few studies have examined this effect in detail, and those that do generally use simple constant cross-head speed tests which are a poor simulation for real world applications. The work here tests a model toughened epoxy using more complex loading histories and exaimes the dependence of fracture energy on loading rate, loading level, time under load, and recovery after unloading. The results suggest that until the load level reaches a substantial fraction of the failure load, the details of the loading histroy have little effect on the final fracture energy. When the loads are high, howver, the time under load becomes important and produces a significant increase in toughness. When a sample that has been exposed to high loads is unloaded for a short time before being loaded to failure. some but not all of the additional toughness introduced by the exposure to high loads is lost. These resutls are consistent with a viscoelastic toughening mechanism and can be qualitatively expalined with a simple crack-tip blunting model.