Process of Material Selection?

The choice of material for a mechanical element depends, to a large extent, on what are the physical phenomena that mainly affect it. For this purpose, it is useful to establish a temporal prioritization (which of them appear first).

The first basic requirement is that the piece meets the STATIC requirements of RESISTANCE, RIGIDITY and STABILITY. These requirements are calculated under a very unrealistic assumption, in which loads are applied extremely slowly (quasi-static loads) without being accompanied by additional energy. This type of calculation yields macroscopic parameters such as areas of the sections resistant to traction, compression or shear; bending and torsion resisting moments, etc. The properties of the material that are part of these first calculations are: the longitudinal modulus of elasticity "E" (Young's modulus), the lateral contraction modulus "nu" (Poisson's modulus) and the limit stresses of elasticity, yield and rupture. .

If a part passes this first test, almost in a laboratory, withstanding quasi-static loads, the next thing will be that it resists loads applied with IMPACT (from very moderate to levels compatible with its type of work: a fan or centrifugal pump works with low impact, but the ball mills, rolling mills and other machines have very high impacts). The impact is not about "the kilograms that the load has" but about the "potential or kinetic energy" that accompanies those kilos! The impact is not solved by enlarging the areas that were previously calculated by STATIC (that would actually make it worse) but by improving the part's ability to ABSORB ENERGY FROM IMPACTS. This leads to designing more elastic pieces, capable of deforming more to absorb energy, as well as supporting kilograms. The material participates in the impact through its stiffness (E modulus) and its "elongation at break" which, the higher it is, the more aptitude it shows to store energy before breaking.

If the piece passed the two previous tests (STATIC and IMPACT) the next thing that can appear are VIBRATIONS, which require handling not only the RIGIDITY of the piece but also its MASS. The relationship between RIGIDITY AND MASS determines the natural frequency at which parts tend to vibrate. Improving the response to vibrations implies tweaking these parameters "without changing the responses to STATIC and IMPACT".

The same happens later with the FATIGUE produced by cyclical loads... which is solved without touching the parameters of the STATIC response, the IMPACT or the VIBRATIONS...

...as it's a bit of a long story... I'll try to upload a PDF in which I explain it, at least partially. It can serve as a guide before reading very extensive and complicated books on the matter.

Kind regards,
Marcelo