Interphase joint, weak

Figure 2. Chain-link diagram of a weak interphase joint. This unanchored joint often results from premature hardening (dryout or precure) of the glue, which leads to inadequate transfer and penetration, especially with respect to the unspread surface. The glue film is not adequately attached to at least the one surface. However, a weak interphase can also arise when the wood has a damaged or deteriorated surface. This situation might very well occur with archaeological wood, (Reproduced with permission from ref, 1, Copyright 1950 American Furniture Manufacturers Association,)...

The adhesive strength of post-crossHnked samples was evaluated by a 180° peel test which measured the force of separation at a constant peel rate (5 mmmin ) at room temperature. The results reported in Fig. 24.3 show the effect of contact time (ranging from 1 h up to one month) in the second step of joint formation at room temperature. Up to approximately 300 h of contact, no effect of time is seen. For longer times, the peel strength increases slightly. Although the scatter is important, this weak trend can be related to crosslinks formed by irradiation in the molecular interphase. This is confirmed by the fact that no separation is observed for these joints immersed in a good solvent such as cyclohexane, whereas spontaneous delamination is observed for shorter contact times. 

The aging of adhesives and lacquers affects the durability of structural bonds and coatings. It depends on environmental influences such as humidity, temperature, irradiation, the surrounding media, and the network state of the polymer. In addition, the network properties and therefore the aging behavior are influenced by substrates that provoke the formation of an interphase during network formation. The properties of the interphase differ from the bulk and it is often the weak spot of structural adhesive joints. Hence, to improve the lifetime of such a joint it is crucial to understand the chemical processes both in the bulk polymer and in the adhesive-adherend interphase under given envi-... 

Together, the mode mixity and T-stress play critical roles in determining crack path selection and locus of failure in adhesively bonded joints. Hie tendencies induced by these two mechanics principles, combined with the spatial variation in mechanical properties within the adherends, adhesive, and interphase region, determine the ultimate failure mode. For bonded systems involving large spatial variations in mechanical properties, debonds may favor a weak interface rather than obey the tendencies imposed by the mechanics principles outlined in these two sections. For systems having reasonably adequate mechanical properties throughout the bond, however, these mechanics principles may control the failure event. 

This article is concerned with theories of fundamental adhesion, i.e., with theories concerned with the reasons for two materials holding together in an adhesive bond, however weak. In contrast, a weak boundary is a cohesively weak layer in the interfacial region (some authors use the term interphase ) of an adhesive joint, which may cause the joint itself to be weak, i.e., to fail at a low stress or with low fracture energy. So the question of weak boundary layers occurs when the level of practical adhesion is under consideration, and may form part of an answer to a question such as why did this joint fail at such and such a stress ... 

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