Creep Debonding

The previous paragraph has made it clear that if there are elastic fibers and a constant macroscopic stress is applied, the longitudinal creep rate will eventually fall to zero. With constant transverse stresses applied as well, the process of transient creep will be much more complicated than that associated with Eqns. (27) and (28). However, it can be deduced that the longitudinal creep rate will still fall to zero eventually. Furthermore, any transverse steady creep rate must occur in a plane strain mode. During such steady creep, the fiber does not deform further because the stress in the fiber is constant. In addition, any debonding which might tend to occur would have achieved a steady level because the stresses are fixed. 

The creep resistance of discontinuous magnesium composites can be significantly improved by comparison with the unreinforced matrix alloys in two separate ways. Direct strengthening of the composites arises mainly from effective load transfer provided that no debonding of the... 

The development of ceramic oxide composites has lagged behind the development of non-oxide composites because of the poor creep resistance of oxide fibers (compared to SiC fibers) and because of the lack of adequate oxide fiber coatings that promote fiber-matrix debonding. Recent advances in creep-resistant oxide fibers and progress on interface control has improved the potential for oxide ceramic composites in industrial and defense applications. However, an effective coating for oxide fibers that provides a weak fiber-matrix interface (and therefore tough composite behavior) remains to be demonstrated. As was discussed in Chapter 6, all oxide coating concepts discussed in the literature have been demonstrated with model systems rather than actual composite systems. 

At high temperatures, additional phenomena contribute to the extension of matrix damage and debond. Oxidation and creep are involved. 

For example, single-lap shear joints can have a known static load applied, then inserted into a specified environment, and the time-to-failnie of joints noted as a function of variables such as adhesive type, pre-treatment and stress (see Shear tests). Static loads can also be applied to other joint configurations. Many studies have shown that such tests exhibit good discrimination between different surface pre-treatments. The creep performance of adhesives can also be measured by means of static load testing (see Durability creep rupture and Durability sub-critical debonding). 

This short article begins by describing what is meant by creep and outlining typical creep behaviour and the role played by temperature. Supplementary articles include Durability creep rupture, Durability sub-critical debonding and Viscoelastidty. Various material laws that can be used to model such behaviour are then briefly presented. This article then addresses various aspects of creep in adhesively bonded stractures and finishes with the interaction between creep, fracture and fatigue. 

Long-term durability of adhesively bonded joints may require resistance to a number of individual or combined degradation modes, including environmental attack, fatigue and time-dependent failures. Time-dependent failure mechanisms are often characterized nsing either a strength approach, involving creep and creep-rupture tests, or a fracture approach, in which debond rate is determined. In creep-rupture tests, adhesive joints are subjected to... 

Further information is given in articles on Pre-treatment of metals prior to bonding, Durabiiity creep rupture, Durabiiity subcritical debonding. Weathering of adhesive joints and Weathering tests. 

Further consideration relating mechanics to joint design are given in Joint design strength and fracture perspectives, Durabiiity creep rupture. Durability subcritical debonding and Durability fatigue. 

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