Interface crack

Fig. 6.20. Ra(io of the strain energy release rates, Gl/Gt, plotted as a function of the material parameter, ot, for a doubly deflected interface crack. After Gupta et al. (1993).

Based on our observation, a membrane degradation and failure mechanism under the RH cycling, a pure mechanical effect is theorized as the following sequence electrode-microcracking- - crazing initiation at the electrode/electrolyte interface - crack growth under stress cycling- -fast fracture/instability. 

Ceramics and the matrix in ceramic composites exhibit cleavage fracture at room temperature as well as at elevated temperatures. This is fortunate since most of the theoretical developments in dynamic fracture are confined to linear elastic fracture mechanics which is then applicable to fracture of the ceramic matrix. However, the additional complexities of crack deflection and fiber-matrix interface cracking, as well as fiber/whisker/particulate pull-outs, are at this time yet to be addressed. 

Since is a real number, it alone cannot fully characterize the crack tip field. In order to specify K, which has two real components, the phase angle ip defined by Eq. (5) is also required. Using this definition, a point in the (( , ip) plane is uniquely related to a point in the (JCj, Ku ) plane. Following the concept of failure locus introduced by Rice [20] for a given phase angle ip, the interface crack will... 

The relationship between the force, Fc, the surface tension, a, (which in this case represents the work required to form a new interface), crack length, /, thickness, h, width, d, and Young s elasticity modulus, E, of the cleaved layer... 

After buckling, at a critical compressive stress in the scale, an interfacial crack starts from the periphery of the buckled area which leads to an increase of the delaminated area. The spalling of the scale occurs by the deflection of the interface crack toward... 

FIGURE 6-15 Scanning electron micrograph showing crack deflection at alumina-monazite interfaces. Cracks initiated by Vickers indentation. Source Morgan and Marshall, 1995. 

Figure 5 Typical interface crack patterns detected in Hi-Nicalon S and SA3 fiber reinforced minicomposites.

Figure 8.2 5. (a) Interface crack with strings tying the faces together (b) stress required to break the tied joint is Griffith stress plus the tying stress. 

Another way of increasing film adhesion is to adjust the interface adhesion energy so that it is comparable with the fracture energy of the polymer coating. Under such conditions, the interface crack prefers to deflect into the bulk of the coating material, as illustrated in Fig. 14.24(a). Once this happens, the interface crack has to be started again and this requires much extra force. It is rather like... 

F ifiure 16.17. Formation of interfacial disItKations (at the model laminate, (b) tcrwion causes tFie j>ap to open, (c) interface cracking starts, (d) healing begins, (e) healing is complete. 

Once the interface crack has initiated inside a composite material, then traveled some distance along the interface, there may be a tendency for the crack to heal, if the crack surfaces come into close proximity, so that the molecular adhesion forces can pull the smooth crack faces together again. The simplest situation in which this interface crack healing arises is shown in Fig. 16.17. This schematic diagram illustrates the observations made on a model composite laminate made from four strips of smooth, transparent rubber. 

The criterion for interfacial dislocations is that aacks start to move along the intaface, but the aack opening is constrained to cause healing. Thus the condition for dislocation formation is the same as that for interface cracking in the lap joint (Chapter 15). Dislocations will form at a stress... 

More direct measures of interface debonding energy are provided by the pull-out or push-out tests shown in Fig. 16.26. When a tensile force is applied to a fiber to extract it from its composite matrix, an interface crack eventually starts to run along the fiber. It is obvious from a simple fracture mechanics argument that the stress on the fiber to propagate the crack, assuming a very compliant matrix, must be given by an expression of the form... 

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