304SS fatigue limit?

[ttdmt]

Does anyone know where to get data on 304SS fatigue limit? The number of load cycles the material can take at a certain stress? My college text book Mechanics of Materials by E.P. Popov has a graph for 18-8 which is a generic term for 304 but does not say if the specimens were annealed or drawn or hot/cold rolled or what. It also has the curve flattening out toward infinite life at about 95,000psi which is way higher than I expected. This is above the tensile strength for commercial plate. The unguaranteed value for tensile in my steel catalog was only 75,000psi. It does not seem logical for materials capability to increase with cyclic loading. Can anyone help me?

 

[sreid]

The general rule for all steels is that if you keep the peak repeated stresses below 50% of the tensile stress, the fatigue life is infinite.

We have used 0.01 inch 304SS rolled shim stock in flexure applications with peak loads of +/- 75,000 PSI (in bending, peak surface stress) and tested to 10,000,000 cycles with no failures and no evidence of surface problems under microscopic examination. We did have faiures from surface scratch stress risers. As you are certainly know, surface finish is critical and shot peening helps.

 

[YLWEA]

"Fatigue of Metals" by P.G. Forest, 1962 has tables of fatigue data. Included is:
0.08%C, 0.8 Mn, 18.0 Cr, 8.75 Ni, softened, 18 KSI YS, 40 KSI UTS, 170 BHN, 24KSI rotating bending fatigue strength at 10 million cycles. They cite Firth-Vickers Stainless Steel Ltd as a source.

ASM Metals Handbook is often a good source, but my copy of the volume related to stainless is at home today.

 

[TVP]

ttdmt,

Austenitic stainless steels are strengthened by strain hardening, due to entanglement among dislocations, as well as dislocation-grain boundary interactions. Also, strain-induced martensite formation occurs, which further enhances strength and fatigue resistance. For your large thickness plate, the properties are essentially that of the annealed product. The data that you are referencing from Popov is definitely for a thinner, cold-rolled product, such as shim stock or strip for spring applications. Th data from YLWEA is representative for an annealed microstructure.