Fast moving crack

The interference-optical method can easily be applied to static or very slow-moving cracks. Special experimental arrangements are necessary to investigate short-time problems of fast-moving cracks or high frequency fatigue. 

At present there is only very little and indirect information available on the crazing behavior at the tips of fast running cracks in glassy thermoplastics (e.g. s.se.wi) Hence this Section will confine itself to a discussion of the behavior of single crazes at the tips of stationary or slowly moving cracks. 

The theory of LEFM applies to brittle elastic materials, whereas gels are viscoelastic, so the theory would be expected to apply only when the strain rate is too fast for significant relaxation to occur. It can be shown [73] that there is an elastic region near the tip of a moving crack whose dimension, d, is given by... 

As the crack grows, the plate becomes less stiff, and relaxes so that the applied forces move and do work. 8W is therefore finite and positive. However, is now positive also (it turns out that some of 8W goes into increasing the strain energy of the plate) and our final result for fast fracture is in fact found to be unchanged. 

The flow of air into the regenerator on a catalytic cracking unit is controlled by two control values. One is a large, slow-moving value that is located on the suction of the air blower. The other is a small, fast-acting valve that vents to the atmosphere. 

Next, reactors may be designed to accommodate rapid deactivation-regeneration cycles. The best example is catalytic cracking where coke deposition is so fast that decay takes only minutes. Since, in this case, the feed is the precursor to coke, treatment or guard chambers are not practical. The only solution is to use fluidized beds, which provide reaction and regeneration in a continuous cycle. Other cases are slurry and moving bed reactors, w hen deactivation is not so rapid. 

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