Composite crack propagation

Because of the many types of damage and damage growth in composite materials, the initial state of the material is difficult, if not impossible, to characterize. Moreover, it is much more difficult to formulate a boundary-value problem to describe crack propagation in composite materials than in metals. 

New types of ceramic composites with high thermal shock resistance have recently been developed that show some promise for gas turbine applications. These composites consist of a ceramic matrix reinforced by ceramic fibers or platelets inside the matrix. The fibers pull out of the matrix during fracture to resist crack propagation. Such composites can be readily fabricated using a new process developed by Lanxide Corporation [18]. The process uses directed oxidation reactions of molten metals to grow a ceramic matrix around a reinforcing material. 

The interface between fiber (or whisker) and matrix is the key to the overall mechanical properties of a composite. A weak interface allows a propagating crack to be deflected, which increases the toughness of the composite. A strong interface allows transfer of the load from the matrix to the fiber and produces an increase in modulus and stiffness of the composite. In CMCs we are usually more interested in producing a weak interface so that debonding occurs, which often leads to fiber pull-out by frictional sliding and substantial absorption of energy. 

A general summary of typical physical characteristics of selected SiC whiskers is given in Table 1. The performance of the whiskers in a number of applications is dependent in part on the diameter and aspect ratio. For example, the toughening behavior in alumina matrix composites is dependent on the ability of the whiskers to bridge propagating cracks. This... 

Figure 8 shows a typical fracture surface of a glass matrix composite reinforced by short stainless steel fibres [16]. It is apparent that the fibre was intercepted and sectioned by the propagating crack and there is some evidence of plastic deformation. 

Norris C.J., Bond I.P. and Trask R.S. (2011b), Interactions between propagating cracks and bioinspired self-healing vascules embedded in glass fibre reinforced composites. Compos. Sci. TechnoL, 71,847-853. 

Mandell JF (1979) Origin of moisture effects on crack propagation in composites. Polym Eng Sci 19(5) 353-358... 

This type of break runs perpendicularly across the fiber, with a rough texture and no evidence of any propagating cracks the whole structure appears to be ready to fail at the same time, and the breaks are very similar in appearance to lower-magnification views of the fracture of fiber-reinforced composites (Fig. 7.8 and 7.9). 

Once first cracking has taken place in the brittle matrix, the fibres serve to inhibit unstable crack propagation. At this stage, the cracking patterns are complex, with discontinuous micro-cracks present ahead of the principal crack. This can be deduced from the various analytical models [28-30] and has also been observed microscopically (Figure 4.15) [31]. Thus, in the cracked composite, it is difficult to define the true crack tip. The simplistic definition of a traction-free crack (as assumed in LEFM) is not applicable to FRC. Stress is transferred across the crack by a variety of mechanisms, as can be seen from the idealization of a crack proposed by Wecharatana and Shah [32], in Figure 4.16. Three distinct zones can be identified ... 

A survey of much of the past work that has been done in this important area of study is given in Tsai. Other related topics of interest in the design of composites include the material response to impact on loading, fatigne nnder cyclic loading, fracture and crack propagation in composites, and the effects of environmental conditions on the material response. 

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