Tunnel cracking

The function g depends on whether the transverse fibers either remain in contact with the matrix upon loading, or separate. The relative unloading modulus associated with such tunnel cracks, E/E, depends primarily on the crack density, tp L, with L being the mean crack spacing in the 90° plies.94,96 At large crack densities, a limiting value is reached, given by,... 

Fig. 1.34 Simulated stress-strain response for a 2-D CMC subject to tunnel cracking.

Lateral extension of these tunnel cracks into the matrix of the 0° plies (Fig. 1.33) results in behavior similar to that found in 1-D material. Moreover, if the stress or0 acting on the 0° plies is known, the 1-D solutions may be used directly to predict the plastic strain. Otherwise, this stress must be estimated.94 For a typical 0/90 system, or0 must range between a and 2a, depending upon the extent of matrix cracking in the 90° plies and upon Et/El. Preliminary analysis has been conducted using, or0 = 2a, as implied by the comparison... 

Using this simplified approach, simulations of stress-strain curves have been conducted.89,97 These curves have been compared with experimental measurements for several 2-D CMCs. One result is summarized in Fig. 1.35. It is apparent that the simulations lead to somewhat larger flow strengths than the experiments, especially at small inelastic strains. To address this discrepancy, further modeling is in progress, which attempts to couple the behavior of the tunnel cracks with the matrix cracks in the 0° plies. 

The matrix cracks that form upon interlaminar shear loading and provide the plastic strains are material dependent. The simplest case, depicted in Fig. 1.36b, involves multiple tunnel cracks that extend across the layer and orient... 

When the interlaminar cracks form by tunneling, the solutions have a direct analogy within the transverse cracking results described above.98 In shear loading, the tunnel cracks evolve and orient such that a Mode II crack develops, as sketched on Fig. 1.36. There is a critical shear stress, r , at which interlaminar shear failure occurs, given by... 

The steady-state width of the tunnel crack is 2oc which may be less than, equal to or greater than 2hf. The amount of work per unit crack advance in the a —direction which must be supplied to overcome material resistance is... 

The energy release rate Q ri) at total crack width 2r] for each crack edge far behind the advancing portion of the tunnel crack, measured per... 

Fig. 4.42. Schematic diagram of steady advance of a tunnel crack in a strained layer embedded between two relatively thick substrates. Crack growth is driven by residual stress in the buried layer.

Note that this result applies only when the elastic properties of the film and the substrate are the same. The variation of Wm and Wq with crack depth ttc resembles that shown in Figure 4.39 for the case of a surface crack. A necessary condition for a tunnel crack to form when Fg > 2Ff at mismatch... 

Intake water tunnels are generally made from concrete, and absorption of water by concrete is the main reason for corrosion in reinforcement. In intake structures the problems are due to concrete failure from salts penetrating into the concrete and corroding the rebar. Hard, dense concrete with ample cover to reinforcement and without cracks and porosity has good resistance to corrosion against seawater. In Indian nuclear power plants, the experience with concrete intake tunnels with respect to corrosion behavior is not bad except that special care is required for protection against algae, clams, mussels, etc. which attach to the tunnel surface. 

Beginning with composite electrodes the simplest scheme here is to coat the surface of a small gap semiconductor that is well matched to the solar spectrum but which is electro-chemically unstable with a thin film of a wide gap, electrochemically stable semiconductor. To demonstrate the feasibility of using such a composite electrode, the film must be thin enough - of the order of 50-100 A or less - so that at least some of the photogenerated carriers in the small bandgap material can tunnel through to the electrolyte. Furthermore the film must not have any cracks or pinholes since these would... 

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