Presentation on theme: "CHAPTER 7 Fatigue Failure Resulting from Variable Loading"— Presentation transcript:
1CHAPTER 7 Fatigue Failure Resulting from Variable Loading 6/25/2007CHAPTER 7Fatigue Failure Resulting from Variable LoadingDr. A. Aziz BazouneKing Fahd University of Petroleum & MineralsMechanical Engineering DepartmentDr. Abdelaziz Bazoune
37-10 Stress Concentration Factor and Notch Sensitivity In fatigue: Stress concentration should always be taken into account.
4Some materials are not fully sensitive to notches and a reduced value of Kt is used and the maximum stress is calculated as follows:(7-29)Kf is the fatigue stress concentration factor, for simple loading: (Ex 7.7)or
5Notch 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 FigFor 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.
7In analysis or design work Find first from the geometry of the partSpecify the materialFindSolve 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.
8Equating Eqs. (7-31) and (7-32) gives the notch sensitivity equation (7-33)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.
9Table 7-8 Heywood’s Parameter for steels 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)Table 7-8 Heywood’s Parameter for steelswhere Table 7-8 gives values offor steels for transverse holes, shoulders and grooves.Feature(a)1/2 (in)1/2Sut in kpsi(a)1/2 (mm)1/2Sut in MPaTransverse hole5/Sut174/SutShoulder4/Sut139/SutGroove3/Sut104/Sut
10Example 7-3 (Textbook)A steel has a minimum strength of 520 MPa and a machined Surface. Estimate ka .SOLUTIONFrom Table 7-4, a=4.51 and b=-0.265From Eq.(7-18)therefore
11RemarkFrom this chart , it is seen that the surface factor Ka for machined or cold drawn is always less than 0.8.
15Example 7-6 (Textbook) SOLUTION See next page Figure 7-20 Equations (7-32) and (7-34)Equations (7-35)SOLUTIONFrom Figure A-15-9From Figure (7-20), forSut = 690 MPa and r =3 mmFrom Eq. (7-31)See next page