Fatigue limit strength

High temperature materials which exhibit the greatest resistance to high cycle fatigue on a strength basis, ie, fatigue limit/tensile strength vs are... 

Example 2.21 A rod of plastic is subjected to a steady axial pull of 50 N and superimposed on this is an alternating axial load of 100 N. If the fatigue limit for the material is 13 MN/m and the creep rupture strength at the equivalent time is 40 MN/m, estimate a suitable diameter for the rod. Thermal effects may be ignored and a fatigue strength reduction factor of 1.5 with a safety factor of 2.5 should be used. 

Proof stress (MPa) Tensile strength (MPa) Elongation on 50 mm (<7o) Fatigue limit (% of T.S.) Bend radius Young s modulus (GPa) Density (g/cm )... 

Fatigue data are normally presented as a plot of the stress (S) versus the number of cycles (N) that cause failure at that stress the data plotted defined as an S-N curve (Fig. 2-43). The use of an S-N curve is used to establish a fatigue endurance limit strength. The curve asymptotically approaches a parallel to the abscissa, thus indicating the endurance limit as the value that will produce failure. Below this limit the material is less susceptible to fatigue failure. 

Fig. 2-43 S-N curve establishes fatigue endurance limit strength.

Fig. 6.5 Cyclic stress-strain behavior observed during the room temperature fatigue of unidirectional SiQ/LAS-II at a maximum stress below the 105 fatigue limit (loading frequency = 10 Hz, crmin/crmax = 0.1). The cyclic stress-strain curves show very limited, if any, hysteresis. Note that the strength and strain capability is retained after cyclic loading below the proportional limit stress. After Prewo.42...

There are also many composites where the 106 cycle fatigue limit is above the proportional limit stress. Examples include Nicalon SiCf/172344 (Fig. 6.7), Nicalon SiQ/CAS45 (Fig. 6.8), and 2-D Nicalon SiQ/SiC46 (run-out at 2.5 x 105 at 1 Hz) (Fig. 6.9). 2-D T-300 Q/SiC, which does not possess a proportional limit, has a fatigue limit (106 cycles, 10 Hz) slightly above 300 MPa, which is close to 75% of the ultimate strength of this composite... 

Limonov VA, Razin AF, Mikel sons MYa. Strength and fatigue limit of fabric base composites under combined static shear and cyclic compressive stresses. Mech Compos... 

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