Global load sharing

The strength properties of fibers are statistical in nature. Consequently, it is necessary to apply principles of weakest link statistics, which define the properties of fibers within a composite. The initial decision to be made concerns the potential for interactions between failed fibers and matrix cracks. It has generally been assumed that matrix cracks and fiber failure are noninteracting and that global load sharing (GLS) conditions are obtained .16,63,64 In this case, the stress along a material plane that intersects a failed fiber is equally distributed among all of the intact fibers. Experience has indicated that these assumptions are essentially valid for a variety of CMCs. 

The ultimate failure of matrix infiltrated tows also involves a two step mechanism and a global load sharing when a fiber fails. In the presence of multiple cracks across the matrix and associated interfacial debond, the load carrying capacity of the matrix is tremendously reduced or annihilated. The matrix infiltrated tows can be assimilated to bundles of fibers... 

Compared with epoxy adhesives, polyimides share a very small part of the global market, limited to military, aerospace, and geothermal applications requiring long-term stability at elevated temperatures. Owing to their aromatic heterocyclic structure, virtually all polyimides are stable at 300 °C. When loaded with inorganic particles such as alumina, silica, silicon nitride, or aluminum, the thermal stability is still better. By contrast, some metals used in the composition of conductive adhesives, in particular silver and nickel, dramatically decrease the thermal resistance. This means that the adhesive strength of most polyimides is excellent at 200 °C,... 

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