Question

Three identical fatigue specimens (denoted A, B, and C) are fabricated from a nonferrous alloy. Each is subjected to one of the maximum-minimum stress cycles listed below; the frequency is the same for all three tests. a.Rank the fatigue lifetimes of these three specimens from the longest to the shortest. b.Now justify this ranking using a schematic S-N plot.

Answer 1

Complete Question:

attached

Answer:

B, A, C  

Explanation:

We will first calculate the stress for each sample :

σ_m = σ_max+σ_min/2

σ_ma = 450-150/2 = 150 Mpa

σ_mb = 300-300/2 = 0 Mpa

σ_mc = 500-200/2 = 150 Mpa

Now calculate the stress amplitudes:  

σ_a = σ_max-σ_min/2

σ_aA = 450+150/2 = 350 Mpa

σ_aB = 300+300/2 = 300 Mpa

σ_aC = 500+200/2 = 350 Mpa

Specimen B will have the longest fatigue life because it has the smallest mean stress. Next comes specimen A and finally specimen C. C is last because it has the highest stress amplitude and ties for the highest mean stress.  B, A, C