Corrosion fatigue environmental factors

General description. In incomplete fusion, complete melting and fusion between the base metal and the weld metal or between individual weld beads does not occur (Fig. 15.8). Incomplete fusion that produces crevices or notches at surfaces can combine with environmental factors to induce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). See Fig. 15.9. 

It is often difficult to conduct laboratory tests in which both the environmental and stressing conditions approximate to those encountered in service. This applies particularly to the corrosive conditions, since it is necessary to find a means of applying cyclic stresses that will also permit maintenance around the stressed areas of a corrosive environment in which the factors that influence the initiation and growth of corrosion fatigue cracks may be controlled. Among these factors are electrolyte species and concentration, temperature, pressure, pH, flow rate, dissolved oxygen content and potential (free corrosion potential or applied). 

Wear is the process of physical loss of material. In sliding contacts this can arise from a number of processes in order of relative importance they are adhesion, abrasion, corrosion and contact fatigue. Wear occurs because of local mechanical failure of highly stressed interfacial zones and the mode of failure is influenced by environmental factors. 

The fact that the response of polymer based materials is time dependent and/or viscoelastic has been mentioned in previous sections. Further, it has been indicated that this time dependence is inherent to polymeric materials due to their unique molecular structure and is quite different from time dependence induced in other materials such as metals by fatigue, moisture, corrosion or other environmental factors. In fact, these same environmental factors also affect polymers but manifest themselves differently than in other materials due to the intrinsic viscoelastic nature of the molecular structure. 

Environmental factors may also affect the fatigue behavior of materials. A few brief comments will be given relative to two types of environment-assisted fatigue failure thermal fatigue and corrosion fatigue. 

If the factor of safety against fatigue f, is less than ff = 1, then the shaft will fail. The suitability of factors of safety ff > 1 will depend on the operating conditions, i.e. the likelihood of corrosion, fretting or stress concentrations near the bottom bearing. The fatigue life of a shaft will be strongly dependent on environmental conditions and material properties. It is recommended that a materials specialist be consulted for advice on this matter. 

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