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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading Dr. A. Aziz Bazoune King Fahd University of Petroleum & Minerals Mechanical Engineering Department CH-07 LEC 23 Slide 1

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 2

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide Stress Concentration Factor and Notch Sensitivity 7-10 Stress Concentration Factor and Notch Sensitivity In fatigue: Stress concentration should always be taken into account.

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 4 Some materials are not fully sensitive to notches and a reduced value of K t is used and the maximum stress is calculated as follows: (7-29) K f is the fatigue stress concentration factor, for simple loading: (Ex 7.7) or

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 5 Notch sensitivity q index is defined by (7-30) To find use Fig for steel and Al alloys, for reversed bending or reversed axial load. For reversed torsion use Fig For cast iron use to be conservative. For, then and the material has no sensitivity at notch at all. For, then and the material has full notch sensitivity.

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 6

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 7 In analysis or design work Find first from the geometry of the part Specify the material Find Solve for from the following Equation (7-31) Figure 7-20 has its basis the Neuber equation, which is given by (7-32) Where is defined as the Neuber constant and is a material constant.

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 8 Equating Eqs. (7-31) and (7-32) gives the notch sensitivity equation For steel, with in kpsi, the Neuber equation can be approximated by a third polynomial fit of data as (7-34) Where is defined as the Neuber constant and is a material constant. (7-33)

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 9 A distinction in the configuration of the notch is accounted for in the modified Neuber equation (after Heywood), where the fatigue stress- concentration factor is given as (7-35) where Table 7-8 gives values of for steels for transverse holes, shoulders and grooves. Feature (a) 1/2 (in) 1/2 S ut in kpsi (a) 1/2 (mm) 1/2 S ut in MPa Transverse hole5/S ut 174/S ut Shoulder4/S ut 139/S ut Groove3/S ut 104/S ut Table 7-8 Heywood’s Parameter for steels

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 10

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 11

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 12 SOLUTION

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 13 SOLUTION

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 14

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 15 SOLUTIONSOLUTION See next page

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 16

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 17

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ME 307 Machine Design I Dr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Loading CH-07 LEC 23 Slide 18 With 2.5% lower than a) and b)

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