Fatigue failure of metals

In this section, we discuss the mechanisms of fatigue damage and failure in metals. 

In the following sections, we will discuss these stages in turn. 

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Fatigue failure of metal under conditions of repeated alternating stress. 

Kocanda, S. Fatigue Failure of Metals. Sijthoff Noordhoff Int Pubs, Alphena/d Rijd... 

FATIGUE (Metals). Failure of metal parts by progressive cracking caused by repeated application of stress. Most fatigue failures start at the surface where discontinuities in section such as square shoulders, screw threads, or even tool marks cause a high concentration of stress. 

Fatigue failure in metals is known to consist of crack initiation and growth. Crack initiation starts with dislocation movement. Submicrocracks are then formed at slip bands. They subsequently grow and coalesce to form a crack of detectable size (short crack) to complete the crack initiation process [12]. This is then followed by the growth of a single crack until final rupture. The period of crack initiation and submicrocrack growth covers most of the fatigue life. 

Sachs, G., Residual Stresses, Their Measurement and TEeir Effects on Structural Parts, Sym/msium on the Failure of Metals by Fatigue, Melbourne University Press, Melbourne, Australia, 1947, pp. 237-247. 

Welding. Considerable difficulty has been experienced in the design of welds subject to cyclic pressure stresses. Extreme conservatism with regard to metal thickness is helpful in eliminating fatigue failure of welds. Nozzles welded to pump heads have heavy sections at the weld.. I ull-penetration welds are used throughout, and butt wolds are used if possible. The inside surfaces of welds are machined smooth when they are accessible. 

The bellows is formed from a length of thin-walled tubing by extmsion in a die. The metals used in the constmction of the bellows must be ductile enough for reasonably easy fabrication and have a high resistance to fatigue failure. Materials commonly used are brass, bronze, beryllium copper, alloys of nickel and copper, steel, and Monel (5). 

Perhaps the most important stress factor affecting corrosion fatigue is the frequency of the cyclic stress. Since corrosion is an essential component of the failure mechanism and since corrosion processes typically require time for the interaction between the metal and its environment, the corrosion-fatigue life of a metal depends on the frequency of the cyclic stress. Relatively low-stress frequencies permit adequate time for corrosion to occur high-stress frequencies may not allow sufficient time for the corrosion processes necessary for corrosion... 

What of the corrosion resistance of new turbine-blade alloys like DS eutectics Well, an alloy like NiaAl-NisNb loses 0.05 mm of metal from its surface in 48 hours at the anticipated operating temperature of 1155°C for such alloys. This is obviously not a good performance, and coatings will be required before these materials are suitable for application. At lower oxidation rates, a more insidious effect takes place - preferential attack of one of the phases, with penetration along interphase boundaries. Obviously this type of attack, occurring under a break in the coating, can easily lead to fatigue failure and raises another problem in the use of DS eutectics. 

This represents the locus of all the combinations of Ca and Om which cause fatigue failure in a particular number of cycles, N. For plastics the picture is slightly different from that observed in metals. Over the region WX the behaviour is similar in that as the mean stress increases, the stress amplitude must be decreased to cause failure in the same number of cycles. Over the region YZ, however, the mean stress is so large that creep rupture failures are dominant. Point Z may be obtained from creep rupture data at a time equal to that necessary to give (V cycles at the test frequency. It should be realised that, depending on the level of mean stress, different phenomena may be the cause of failure. 

The set burst pressure should be selected to permit a sufficiently wide margin between it and the vessel s used or design operating pressure and temperature to avoid premature failure due to fatigue or creep of metal or plastic coatings. 

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