No abstract

The thermal oxidative degradation of carbon fiber/bismaleimide (Hexel T800H/F655-2) and carbon fiber/amine-cross-linked epoxy (Cytec Fiberite IM7/977-2) unidirectional composites and their corresponding neat polymers has been studied in air at temperatures ranging from 150 to 240 C by optical microscopy, using an interferential contrast, and IR microspectrophotometry.A coarse analysis of the damaged zone allowed the determination of main kinetic characteristics in the whole degradation process (oxidation, crack propagation, and their interaction) for each principal direction of the composite materials.It has been found that cracks initiate at the sample free surfaces when a critical value of embrittlement is reached in the oxidized layer. Then, cracks propagate differently toward the core according to both the fiber orientation and the nature of the composite system. The difference in behavior between T800H/F655-2 and IM7/977-2 systems can be explained by a difference in polymer toughness and oxygen permeability of the fiber-matrix interface.

a b s t r a c tThe mechanical behavior of a four-layer plain weave glass fiber/epoxy matrix composite is modeled at the mesoscopic scale, taking into account the dry fabric preforming before resin injection, the relative shift and nesting between fabric layers, and the characteristic damage mechanisms, i.e., intra-yarn cracking and decohesion at the crack tips. The surface strain fields obtained numerically are similar to the strain fields observed at the surface of the specimen. Damage is modeled by introducing discrete cracks in the FE mesh of the representative unit cell of the composite. The crack locations are determined using a stress based failure criterion. The predicted locations are similar to those observed experimentally. The effects of intra-yarn cracks on the macroscopic mechanical properties show the same trends as the experimental data. Good quantitative agreement is obtained if yarn/yarn or yarn/matrix decohesions at the crack tips are taken into account.