Composite failure

Strength predictions of composites are ia general quite complex and somewhat limited. This is particularly tme of compressive and shear strengths, which are needed, together with the tensile strengths, ia composite failure prediction. 

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. 

Although the reinforcements are intended to bear the loads, the matrices play a significant role in composite failure by damping sudden stress variations, by distributing the loads and by protecting the reinforcements. 

Wells J.K. and Beaumont P.W.R. (1982). Construction and use of toughness maps in a fracture analysis of the micromechanisms of composite failure. In Composite Materials Testing and Design. ASTM STP 787 (I.M. Daniel ed.), ASTM, Philadelphia, PA, pp, 147-162. 

The results of these experiments indicate that EE may be used to detect microfracture events on the composite surface, and signal the early stages of composite failure. It also clarifies the source of AE as a function of strain by the presence or absence of AE-EE correlations, and allows more details of failure mechanisms and composite characterization to be obtained. 

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... 

There are two bounding solutions for the function A (Fig. 1.18). Composite failure subject to multiple matrix cracking gives the upper bound. Failure in the presence of a single crack gives the lower bound. 

Materials Algorithms Project Source of algorithms usefiil for modeling material behavior, http //www. msm.cam.ac.uk/map/rrrapmain.html Materials Properties Handbooks Operation Distributes the Aerospace Structural Metals Handbook, the Structural Alloys Handbook, the Damage Tolerant Design Handbook, and the Composite Failure Analysis Handbook, http // www.purdue.edu/MPHO... 

Vogler TJ, Hsu S-Y, Kyriakides S. Composite failure under combined compression and shear. Int J Solids Struct 2000 37 1765-91. 

Hart-Smith LJ. The role of biaxial stresses in discriminating between meaningful and iUusaty composite failure theories. Compos Stmct 1993 25 3-20. 

Figure 12.21 Plot of the relationship between the predicted unidirectional (UD) carbon fibre composite failure strain and composite length. The data are presented for a viscoelastic TGAP-DDS matrix and a purely elastic matrix. The dotted line represents a typical experimental failure strain for a UD carbon fibre composite [49].

Loading Level 1 Matrix cracks begin to develop while the hbers remain intact. Under cyclic loading, these cracks propagate until they reach a hber—matrix interface or some other inhomogeneity. If the cyclic load is sufficiently low, these cracks will not propagate any further, and no more fracture surfaces will be created until the fatigue limit of the hbers is reached and overall composite failure occurs. 

This chapter has discussed the modes of damage that can occur with polymer composites. Failure occurs progressively under tension, compression, or shear, and polymer composites are particularly prone to low-velocity impacts that may result in damage that is not obvious to the naked eye. This emphasizes the need to test polymer composite aircraft structures both at the time of manufacture and while in service. The methodologies currently employed for NDT, NDI, and NDE of aircraft structures have been detailed, and examples have been shown. The precise method used for a given application will depend on the economic cost, subject to a sufficient probability of detection of any damage. 

Experience suggests that composite failure strains that exceed 0.6 percent provide robust thermostructural charac... 

The properties of composite materials cannot be predicted adequately by considering the fibre and resin constituents one by one. An important mechanism of composite failure under stress is delamination caused by differences between the engineering properties of successive plies or layers. These differences arise from the fact that successive layers may have different fibre orientations [34] or, occasionally, different fibres. It is a feature of laminates made by stacking pre-impregnated layers of reinforcement and is not an issue with, for example, unidirectional pultrusions. The process of delamination has been reviewed by Davies [35]. The fabrication of three-dimensional composites is an important step towards reducing or eliminating unwanted delaminations. Such materials are at an advanced stage of development. 

The seismic effects are generally considered as pseudo-static forces to be added to the other static forces. A detailed review ofthe literature regarding the computation of passive earth pressure using planar failure, composite failure mechanism and experimental investigations are presented in the following sections. In addition, the review of the literature for the computation of earthquake-induced displacements is also presented. 

Planar Failure Mechanism versus Composite Failure Mechanism... 

It can be noted from Table 2 that the values of K,. predicted by employing the composite failure mechanism are lower than the planar failure mechanism reported in Kramer (2003) for the case of <5 > 0.4 and the difference increases for higher values o 8 / 4>. Similar observations can... 

Cou- lomb Com- posite failure Cou- lomb Com- posite failure Cou- lomb Com- posite failure Cou- lomb Com- posite failure Cou- lomb Com- posite Coulomb Composite failure... 

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