Composite microcrack

Suo, Z., Failure of brittle adhesive joints. Appl. Mech. Rev., 43(5), S276-S279 (1990). Naim, J.A., The strain energy release rate of composite microcracking a variational approach. J. Compos. Mater., 23, 1106-1129 (1989). 

Most recent studies (69) on elevated temperature performance of carbon fiber-based composites show that the oxidation resistance and elevated temperature mechanical properties of carbon fiber reinforced composites are complex and not always direcdy related to the oxidation resistance of the fiber. To some extent, the matrix acts as a protective barrier limiting the diffusion of oxygen to the encased fibers. It is therefore critical to maintain interfacial bonding between the fiber and the matrix, and limit any microcracking that may serve as a diffusion path for oxygen intmsion. Since interfacial performance typically deteriorates with higher modulus carbon fibers it is important to balance fiber oxidative stabiHty with interfacial performance. 

In investigations of the failure of fiber compositions (PETP — short glass fibers) [251] it was found that the main process responsible for composite failure under load is the rupture at the matrix-fiber interface. The author of [251] observed formation of microvoids in loaded samples, both at the interphases and in the bulk. The microvoids, or cavities) grow in size and become interconnected by microcracks, and this results in fiber separation from the binder. However, when the matrix-fiber bond is strong enough, the cavities appear mostly in the bulk of matrix, the failure of the specimen does not over-power cohesion and traces of polymer remain on the fibers. 

Figure 25. Effect of radiation fluence on microcrack formation in composite specimens subjected to 100 thermal cycles between 80 C and -150 C (Reproduced from reference 19.)...

Hiemstra, D.L. and Sottos, N.R. (1993). Thermally induced interfacial microcracking in polymer matrix composites.. /. Composite Mater. 27, 1030-1051. 

Flexural strengths tend to be higher than tensile strengths in SMC composites. Elexural load-deflection modulus values are nonlinear, which indicates the occurrence of microcracking even at low loading. In general, flexural properties follow the same trends as the tensile properties and are affected by fiber content, fiber lengths, type, and orientation. 

Arrhenius plots of conductivity for the four components of the elementary cell are shown in Fig. 34. They indicate that electrolyte and interconnection materials are responsible of the main part of ohmic losses. Furthermore, both must be gas tight. Therefore, it is necessary to use them as thin and dense layers with a minimum of microcracks. It has to be said that in the literature not much attention has been paid to electrode overpotentials in evaluating polarization losses. These parameters greatly depend on composition, porosity and current density. Their study must be developed in parallel with the physical properties such as electrical conductivity, thermal expansion coefficient, density, atomic diffusion, etc. 

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