Back in year 1996 Walter Schütz, in his work “A history of fatigue” told the history on how Gassner in 1941, measuring the fatigue strength of high strength aluminum alloys of the series 7XXX (Al-Zn-Mg), and finding this was not much different from the lower strength, lower cost, alloys of series 2XXX (Al – Cu), warned that if they were used, for example, to build an airplane, this would lead to premature fatigue failures.
This argument is tricky and seems illogical. Aren’t the alloys of series 7XXX of higher strength compared to alloys 2XXX? And didn’t Gassner find out the fatigue strength was about the same? So why would the fatigue live be smaller if the 7XXX alloys were used?
The explanation for this apparent contradiction is simple. The only possible reason to use a higher strength alloy (especially when it is more expensive) in place of a lower strength one, is to allow higher static stresses. In practice, one is able to reduce the thickness of a sheet, used to build, for example, the fuselage of an airplane. This leads to a lower mass of the structure, which is a good thing for airplanes. The loads acting on the structure, however, are approximately unchanged, since they depend mostly on the overall geometry of the structure. Since stress is the load divided by the cross section area, the material will work at a higher static strength level, and hence this will be inversely proportional to the thickness .
The problem is that the dynamic load (for example, the maximum and the minimum loads in function of time) are also inversely proportional to the reduction in thickness, therefore the difference between the maximum and the minimum stresses, in other words, the stress amplitude will also increase proportionally, leading to a more severe fatigue loading. Therefore fatigue life will be shorter, not because the fatigue strength (the material’s property) decreased, but because the loading became more severe, surpassing the fatigue strength.
This kind of elaborate argument is typical in Materials Science, a similar argument justifies why quenched steels should not be used in the maximum hardness is subject to a hydrogen-rich environment. Another tricky argument explains also why low stacking fault energy FCC metals present higher strain hardening.
I call this non-linear thinking. A present cause results in an effect as a consequence of a long chain of events, which is not self-evident in principle. As I already wrote, in my opinion the aim of superior education is to prepare the student for this way of thinking.
Non-linear thinking, however, is very common in Materials Science. I believe this is due to the highly interdisciplinary character of this subject. Of course, other scientific branches are also characterized by interdisciplinarity, but this seems to be more common in Materials Science.
It is this particular way of thinking which motivates the materials scientist. Non-linear thinking is addictive, the first time you get confronted, and understands, one of these long elaborate logical arguments, you will feel the need to learn more of the kind. So, be warned, if you are satisfied with thinking on thing in short cause-effect terms, stay away from Materials Science.