Application of Fracture Mechanics to Composite Materials

Composite materials have many distinctive characteristics reiative to isotropic materials that render application of linear elastic fracture mechanics difficult. The anisotropy and heterogeneity, both from the standpoint of the fibers versus the matrix, and from the standpoint of multiple laminae of different orientations, are the principal problems. The extension to homogeneous anisotropic materials should be straightfor-wrard because none of the basic principles used in fracture mechanics is then changed. Thus, the approximation of composite materials by homogeneous anisotropic materials is often made. Then, stress-intensity factors for anisotropic materials are calculated by use of complex variable mapping techniques. 

Wu [6-12] derives the stress distribution around a crack tip in an anisotropic material. He finds the intensities of the stresses a, Oy, and 

Other researchers have substantially advanced the state of the art of fracture mechanics applied to composite materials. Tetelman [6-15] and Corten [6-16] discuss fracture mechanics from the point of view of micromechanics. Sih and Chen [6-17] treat the mixed-mode fracture problem for noncollinear crack propagation. Waddoups, Eisenmann, and Kaminski [6-18] and Konish, Swedlow, and Cruse [6-19] extend the concepts of fracture mechanics to laminates. Impact resistance of unidirectional composites is discussed by Chamis, Hanson, and Serafini [6-20]. They use strain energy and fracture strength concepts along with micromechanics to assess impact resistance in longitudinal, transverse, and shear modes. 

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K. PBedrich., Application of fracture Mechanics to Composite Materials, Elsevier Science Pubhshers, Amsterdam, the Netherlands, 1989. 

Davidge, R.W. (1989). In Application of Fracture Mechanics to Composite Materials. Composite Materials Series, Vol. 6 (K. Friedrich ed.), Elsevier, Amsterdam, pp. 547-569. 

K Schulte and W W Stinchcomb, Damage mechanisms - including edge effects - in carbon fibre-reinforced composite materials . Application of Fracture Mechanics to Composite Materials, Amsterdam, Elsevier Science, 1989, pp 273-3 25. 

J. Karger-Kocsis, Application of fracture mechanics to composite materials. Composite Materials Series, Vol. 6, (EdK.Friedrich), Elsevier Sci. Publ., Amsterdam-... 

The discussion of fracture mechanics will be divided in two parts. First, basic principles of fracture mechanics will be described. Second, the application of fracture mechanics concepts to composite materials will be discussed. In both parts, the basic approach is that of Wu [6-12],... 

Nevertheless, if materials arc to be evaluated in more than a simple comparative manner, then tests are needed as a function of at least the amplitude of deformation. Perhaps because they are newer materials, it is more common to find this being allowed for in procedures for plastics and composites. Amplitude effects can also be studied in the more recently introduced tension fatigue test for rubbers. This allows the application of fracture mechanics concepts to the results, as will be discussed in a later section. 

Silicon carbide has attracted considerable interest because of its good mechanical and physical properties and chemical inertness. One of the most important applications of SiC is to produce a matrix reinforced by fibres, forming ceramic-matrix composites. These composite materials exhibit much better fracture toughness than monolithic ceramics. Compared with carbon/carbon composites, fibre-reinforced SiC matrix composites possess superior oxidation resistance and mechanical properties. The Si-C-H-Cl system (e.g. methyltrichlorosilane, CH3SiCl3) has been used for SiC deposition because it is easy to produce stoichiometric SiC deposits. 

Currently available data indicate that 3D reinforcements typically result in high values of Gc compared with the unidirectional laminate made from the same fibre and matrix. Whether the high delamination resistance of 3D FRP laminates does represent material values or apparent values, for example depending on specific test and specimen parameters, is still debated (see e.g. Ref. [92]). More generally, the applicability of linear elastic fracture mechanics to through-thickness reinforced FRP composites is questionable [98]. 

Eringen s interest in the micromorphic theory continued when he joined the faculty of Princeton University in 1966. During the early Princeton years, he concentrated on the application of this theory to turbulence, liquid crystals, polymers, suspensions, biomechanics, and composite materials. He has always kept a deep interest in questions related to the foundations of continuum mechanics and thermodynamics. In recent years, Eringen has been the most articulate and active proponent of the nonlocal theory of continua with applications to dislocation theory, fracture problems, surface physics, composite materials, and turbulence. 

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