Fatigue environmental effects

In a previous section it has already been observed that high-strength 2000 and 7000 series alloys are sensitive to the presence of water vapour in corrosion fatigue tests. Stress-corrosion susceptibilities of these alloys in low temperature aqueous solutions and the effect of composition and heat treatment have been widely investigated . It is not surprising therefore that when subjected to corrosion fatigue in similar environments, substantial environmental effects can be observed particularly at low frequencies of less than 1 Hz and AA values above These environmental effects tend to be... 

In the case of the ASME codes for nuclear pressurised components, the questions of fatigue design and of flaw evaluation are dealt with separately in ASME Section III and Section XI Appendix A, respectively. The design S-A curve for machined butt welds typical of thick section pressurised components is set at a factor of two on stress range or twenty on cyclic life, whichever is more conservative, below the mean of S-N data developed on smooth cylindrical specimens in air. (A somewhat similar design curve obtained by a different method from experimental S-A data for machined butt welds is given in British Standard 5500.) These safety factors are intended to encompass any adverse influence of minor weld defects, size effects, data scatter and environment. As far as environmental effects are... 

Additional factors which must be taken into account are environmental effects (thermal as well as chemical), effects of defects, statistical variability of the material, long-term behavior, and cyclic versus static loading effects. Assessment of these effects requires the end user to conduct a large series of tests using multiple specimens. A typical series will examine a unidirectional material in tension in the 0, 90, and cross-ply directions 0, 90, and cross-ply in compression and 1-2, 1-3, and 2-3 shear at different temperatures ranging from —54°C to the expected service temperature creep rupture at temperatures up to the expected service temperature and fatigue at room and elevated temperature. This series of tests, shown in Table 12.1, may require over 400 specimens. 

W. J. Shack, T. F. Kassner, Review of environmental effects of fatigue crack growth of austenitic stainless steels. Office of Nuclear Reactor Regulation, U.S. Nuclear Regulatory Commission, Supt. of Docs, U.S. G.P.O. [distributor], Washington, DC, 1994. 

A point worthy of mention here, however, is that a close analogy may be drawn with fatigue and environmental effects since the effect of cycling is to reduce Oq to, say, a Oc- If a very simple model is used, then a single cycle can affect this reduction, so that if Kj is applied, the original zone length is ... 

Landry B, LaPlante G, LeBlanc LR. Environmental effects on mode II fatigue delamination growth in an aerospace grade carhon/epoxy composite. Compos Part A Appl Sci Manuf 2012 43 375-85. http //dx.doi.Org/10.1016/j.compositesa.2011.ll.015. 

In conjunction with the environmental effect, the loading frequency is an important factor. Since the environmental effect is due to the reaction between the environment and the freshly exposed material t the crack tip, the period of time for reaction is important. The degree of crack extension due to environmental crack tip damage during each cycle has been studied and reported by Wei [ ] and Bradshaw and Wheeler [" j. Also, test frequency can have a marked effect on results, if creep is a contributor to the fatigue processes [" ]. 

Note that outdoor weathering can involve all four factors simultaneously. The subject of weathering is considered in Chapter 6. Chemicals are considered in Chapter 3, and heat in Chapter 4. Mechanical stress involves several aspects fatigue is considered in Chapters 5 and 11. Chapter 12 links mechanical and environmental effects. 

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