Fibers composite failure

The strength of laminates is usually predicted from a combination of laminated plate theory and a failure criterion for the individual larnina. A general treatment of composite failure criteria is beyond the scope of the present discussion. Broadly, however, composite failure criteria are of two types noninteractive, such as maximum stress or maximum strain, in which the lamina is taken to fail when a critical value of stress or strain is reached parallel or transverse to the fibers in tension, compression, or shear or interactive, such as the Tsai-Hill or Tsai-Wu (1,7) type, in which failure is taken to be when some combination of stresses occurs. Generally, the ply materials do not have the same strengths in tension and compression, so that five-ply strengths must be deterrnined ... 

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. 

A part, consisting of a carbon fiber composite tube, was initially adhered to the inside of a short titanium coupling by a silica-filled epoxy. The bond failed and a fluorinated mold release was believed to be the cause of the failure and was the purpose for this investigation. A sample of the epoxy (Sample A) and the part (Sample B) were submitted for X-ray photoelectron spectroscopy (XPS) to analyze for the presence of both fluorine and silicon. 

Drzal, L.T., Rich, M.J., Camping, J.D. and Park, W.J. (1980). Interfacial shear strength and failure mechanisms in graphite fiber composites. In 35th Annual Tech. Conf., Reinforced Plast. Compo.sites Inst., SPI, Paper 20C. 

Hashemi, S., Kinloch, A.J. and Williams, J.G. (1989). Corrections needed in double-cantilever beam tests for assessing the interlaminar failure of fiber-composites. J. Mater. Sci. Lett. 8, 125-129. 

Zweben, C. (1968). Tensile failure of fiber composites. AIAA J. 6, 2325-2331. 

Beaumont P.W.R. and Anstice P.D. (1980). A failure analysis of the micro-mechanisms of fracture of carbon fiber and glass fiber composites in monotonic loading. J. Mater. Sci. 15, 2691-2635. 

Finally, we turn our attention to the prediction of strength in continuous, undirec-tional fiber composites. Consider again the case in which a tensile stress acts parallel to the fibers (cf. Figure 5.86a). The sequence of events varies, depending on which of the two components is more brittle—that is, which extension at fracture is smaller, or e, where the asterisk indicates fracture (failure). Let us consider each case independently. 

As the load increases, the fibers fail systematically, resulting in a characteristic fiber fragment length. At composite failure, there can be multiple cracks within some fibers. The existence of many fiber fragments is still... 

As previously noted, this chapter has been concerned mainly with those models for the creep of ceramic matrix composite materials which feature some novelty that cannot be represented simply by taking models for the linear elastic properties of a composite and, through transformation, turning the model into a linear viscoelastic one. If this were done, the coverage of models would be much more comprehensive since elastic models for composites abound. Instead, it was decided to concentrate mainly on phenomena which cannot be treated in this manner. However, it was necessary to introduce a few models for materials with linear matrices which could have been developed by the transformation route. Otherwise, the discussion of some novel aspects such as fiber brittle failure or the comparison of non-linear materials with linear ones would have been incomprehensible. To summarize those models which could have been introduced by the transformation route, it can be stated that the inverse of the composite linear elastic modulus can be used to represent a linear steady-state creep coefficient when the kinematics are switched from strain to strain rate in the relevant model. 

Linear elastic fracture mechanics (LEFM) approach can be used to characterize the fracture behavior of random fiber composites. The methods of LEFM should be used with utmost care for obtaining meaningful fracture parameters. The analysis of load displacement records as recommended in method ASTM E 399-71 may be subject to some errors caused by the massive debonding that occurs prior to catastrophic failure of these composites. By using the R-curve concept, the fracture behavior of these materials can be more accurately characterized. The K-equa-tions developed for isotropic materials can be used to calculate stress intensity factor for these materials. 

Depending on x, Gf, and g, two failure modes are possible for aligned short brittle fiber composites fiber pullout and fiber fracture. Let us consider a composite in which > e. If / < 4, the average stress in the fiber will be given by Eq. (15.47), so the strength in the composite can be written... 

Experiments will be necessary to prove the existence, behavior, and engineering of a new elass of physical instability. Tensor solitary waves have been hypothesized that are related to debonding instabilities first deteeted in particulate eomposites in the early 1980 s. Figure 4 shows the eharacteristies of that simpler instability. Figure 5 eaptures a mysterious, ultrafast failure mode first observed in 2000, whose explanation may be similar to Figure 4 s partieulate (not fiber) composite results, but whose... 

Figure 21.8 Micrographs of the failure surface by shear of LDPE-henequen ceUulosic fiber composite with 20 and 30% by volume of ceUulosic fiber (a) untreated ceUulose (b) LDPE preimpregnated ceUulose and (c) silane-treated ceUulose. Source Reproduced with permission from Herrera-Franco PJ, Aguilar-Vega M. J Appl Polym Sci 1997 65 197 [25]. Copyright 1997 John Wiley and Sons.

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