Previsione della durata a fatica su provino SAE utilizzando un approccio combinato di fatica oligociclica e meccanica della frattura

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Page 9 1.3. Crack Initiation 1.3.1. Fundamentals Basically , the problem of crack initiation in a ductile metallic material that is assumed not to have any internal flaws, is related to the presence of a free surface; here the structure of the metal (grains, or deeper crystals) is subject to a stronger effect from the external loads (where load is intended in a general meaning). The differences in the working environment between the internal and surface material can be pointed out in the following list [6]: (i) If during a fatigue test a thin layer of metal is removed from the test section surface stopping the test each some percent of the expected total life, this later may be exceeded many times. Hence something that causes the crack initiation was removed from the surface; for this it is understandable that what occurs on the surface before and during the test is related with the specimen life. (ii) A surface grain is not wholly surrounded by others grains that can support it; hence it is easier for it to deform plastically. The metallographic analysis of the surface of a component (or specimen) that undertakes a cyclic stressing test shows bands of slip that became wider and more dense as the test proceeds, although there are areas between the bands where no slips are present. This observation was made by Ewing and Humphrey in 1903, testing specimens of Swedish steel [6]. Fatigue cracks formed eventually in the broadened bands, but it was not possible to define precisely when this happened. Numerous slip bands were found to contain cracks at the end of a test, especially when a specimen had a long life. Successive analyses by other workers, confirmed the sequence of events described by Ewind and Humphrey; for example, some fifty years later, Thompson, Wadsworth, and Louat found that electropolishing the specimen surface and removing the roughness associated with a slip band, most of them became invisible, while a few became accentuated and were termed persistent slip bands; fatigue cracks grew eventually from these bands. During the last hundred years a lot of experimentation has been carried out, thanks also to improved techniques of metallurgical analysis. Actually the latest theories fall into two categories: those based on repeated slip that lead to some form of damage along a preferred slip plane and hence to a crack and those based on the repeated work-hardening of a soft element, i.e. a grain, surrounded by an elastic matrix, i.e. other grains having different elastic limits and strain-hardening characteristic; the work-hardened grain either reaches a stable state than does not lead to fracture, or continues its hardening process until it reaches its fracture stress. Other theories similar to the latter are based on a critical amount of plastic strain accumulated during the cycling. A brief review of the first theory, which is the widest spread and used especially for ductile material, follows now.

Anteprima della Tesi di Massimo Acciarini

Anteprima della tesi: Previsione della durata a fatica su provino SAE utilizzando un approccio combinato di fatica oligociclica e meccanica della frattura, Pagina 9

Tesi di Laurea

Facoltà: Ingegneria

Autore: Massimo Acciarini Contatta »

Composta da 224 pagine.

 

Questa tesi ha raggiunto 2828 click dal 20/03/2004.

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