Cyclic crack propagation process

In the literature on fatigue crack propagation in metals and, later, in polymers it has been well documented that fracture surfaces can exhibit ripple markings which are due to the cyclic loading process. 

The figure shows the stress CF process, type B, purely time-dependent CF propagation, which is essentially a simple superposition of mechanical fatigue and SCC. Stress CF occurs only when > Ki qq. In this case, the cyclic character of loading is not important. The combination of true CF and stress CF results in type C, the most general form of CF crack propagation behavior (79). 

The strain necessary for initiation of multiple cracking and crack propagation is lower for a cyclic load than for a static and constant load. The process of fatigue in fibre-reinforced cement composites is similar to the static increase of load in the sense that debonding and pull-out also occur gradually. The fibres used as reinforcement for cement matrices usually do not show fatigue at all because the stress in fibres is maintained at quite a low level as related to their strength, in steel or carbon fibres, for example. 

Equation 7 does not in general agree with the Paris equation. A basic approach to account for both static and cyclic mode components of crack propagation involves superposition of the two processes [52] ... 

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