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Question 3. [30] Fatigue is a progressive, localized, permanent structural change that occurs in materials subjected to fluctuating stresses and strains that may result in cracks or fractures after a sufficient number of fluctuations. Therefore, Laboratory fatigue tests are essential in understanding fatigue behavior. In figure 3 below, three materials have undergone fatigue testing and the S-N curve below represents their fatigue behaviors. Use this information to answer the following questions to understand more about the Fatigue test features. a. What are the three classes or stages of fatigue failure? (6) (4) b. From S-N Curve, depict the maximum stress for each material c. From S-N Curve, depict the minimum stress for each material d. Calculate the mean stress for each material (4) (4) e. Calculate the mean amplitude for each material (4) f. Calculate the stress range for each material (4) g. Calculate the stress ratio for each material (4) 1100 1000 2340 steel 48 HRC (unnotched) 900 800 Fatigue limit S 700 600 2340 steel 48 HRC (notched) 500 Fatigue limit, S, 400 300 200 Aluminum alloy 7075-T6 100 Stress ratio (R) = 1 1 0 10³ 107 108 104 105 106 Number of cycles to fracture, N Figure 3: S-N Curves Stress amplitude (S₂), MPa