The plot below shows fatigue curves for three metal samples. a) (2-pts) Which ma

Question

The plot below shows fatigue curves for three metal samples.

a) (2-pts) Which material exhibits a

The plot below shows fatigue curves for three metal samples. Which material exhibits a "fatigue limit"? Which material has the lowest fatigue strength? Calculate the minimum allowable radius of a red-brass bar so that it will not fail due to fatigue with a load amplitude of 22,000 N after 107 cycles. Repeat part c) and calculate a minimum allowable radius with the same conditions but assuming a factor of safety of N=2.

Explanation / Answer

a) Steel shows a fatigue limit as shown by constant value of stress after 10^6 cycles to failure

hence steel

b) Red Brass has lowest value of stress for a particular number of cycles to failure

hence red brass

c) from graph after 10^7 cycles the stress amplitude should be less than 100MPa

therefore, W/A<=100 MPa

=> A=22000/100*10^6

=2.2 cm^2

thus radius , r=sqrt(2.2/pi)

= 0.836 cm

d)taking factor of safety n=2, S'=S/2=50MPa

thus A'=2*A= 4.4 cm^2

thus r= 1.183 cm

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