Failures by fatigue

I-2.2.2 Takeoff connections and attaching bosses, fittings, or adapters shall be made of suitable material and shall be capable of withstanding the maximum service pressure and temperature of the piping or equipment to which they are attached. They shall be designed to satisfactorily withstand all stresses without failure by fatigue. 

Fretting corrosion is another phenomenon that occurs because of mechanical stresses, and, in the extreme, it may lead to failure by fatigue or corrosion fatigue. 

Failure by fatigue always involves cracking [11,12]. The process may be simplified into three steps ... 

Cohesive failure by fatigue of CFRP/epoxy joints (Ashcroft et al. 2001)... 

We shall now examine material selection for a pressure vessel able to contain a gas at pressure p, first minimising the weight, and then the cost. We shall seek a design that will not fail by plastic collapse (i.e. general yield). But we must be cautious structures can also fail by fast fracture, by fatigue, and by corrosion superimposed on these other modes of failure. We shall discuss these in Chapters 13, 15 and 23. Here we shall assume that plastic collapse is our only problem. 

It is worrying that a vessel which is safe when it enters service may become unsafe by slow crack growth - either by fatigue or by stress corrosion. If the consequences of catastrophic failure are very serious, then additional safety can be gained by designing the vessel so that it will leak before it breaks (like the partly inflated balloon of Chapter 13). Leaks are easy to detect, and a leaking vessel can be taken out of service and repaired. How do we formulate this leak-before-break condition ... 

Ceramics, without exception, are hard, brittle solids. When designing with metals, failure by plastic collapse and by fatigue are the primary considerations. For ceramics, plastic collapse and fatigue are seldom problems it is brittle failure, caused by direct loading or by thermal stresses, that is the overriding consideration. 

While lead of purity in excess of 99.99% is commercially available, it is very rarely used owing to its susceptibility to grain growth and fatigue failure by intercrystalline cracking, and indifferent mechanical properties. Because of its generally superior corrosion resistance, pure lead to BS 334 1982 type A, shown in Table4.13, is occasionally used in chemical plant, but only if there is no suitable alternative. 

Undue static or cyclic stressing and other features which give rise to stress concentrations should be avoided as these may lead to premature failure by stress-corrosion cracking or corrosion fatigue. 

In flowing water enviroments a tubular rather than a solid rod cantilever anode may be used to give improved resistance to fatigue failure, since the anode design may result in fatigue failure by vortex shedding at high water velocities. 

Corrosion Fatigue failure by cracking caused by reversing alternating stress in the presence of a corrosive environment. 

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