FIGURE 14 Delaminations induced in I-sections by applied bending moments. The load deflection response is indicated. SOURCE: Reprinted from Heredia et al. (1996) [31], copyright 1996, with permission from Elsevier Science.

tigue mechanisms operate, as elaborated in the following section on corrosion fatigue [73]. Composites are subject to a diminishing friction stress upon load cycling. The changes can be diagnosed by using hysteresis loop measurements [68] (Figure 16). The reductions in τ are attributed to interface wear. This process is not understood, but it exhibits a limiting friction, τs [23]. The diminished τ increases the stress intensity range at the matrix crack tip, ∆Ktip [70,71], as well as the mean value, tip. The consequences differ for TMCs and CMCs. For TMCs, the ∆Ktip causes the crack to extend in accordance with a Paris Law [70] (see Figure 16). But even though τ diminishes and increases ∆Ktip, the crack growth rates are considerably lower than those for the monolithic alloy, because of the strong fiber bridging (LSB) effect [70,71]. The principal problem arises from the notch sensitivity of TMCs, because they are embedded within a monolith. The TMC is subject to residual compression, whereas the monolith is in tension. This increases the R-ratio in the monolith, which accelerates crack formation. Should such a crack extend through the monolith and penetrate the TMC, the composite notch sensitiv-



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement