me 307 machine design i dr. a. bazoune chapter 7: fatigue failure resulting from variable loading...

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Dr. A. BazouneDr. A. Bazoune Chapter 7: Fatigue Failure Resulting from variable Chapter 7: Fatigue Failure Resulting from variable LoadingLoading

Dr. A. Aziz Bazoune

King Fahd University of Petroleum & Minerals

Mechanical Engineering DepartmentCH-07 LEC 23

Slide 1

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CH-07 LEC 23 Slide 2

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7-10 Stress Concentration Factor and 7-10 Stress Concentration Factor and

Notch Sensitivity Notch Sensitivity

In fatigue: Stress concentration should always be taken into account.

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Some materials are not fully sensitive to notches and a reduced value of Kt is used and the maximum stress is calculated as follows:

Some 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)

Kf is the fatigue stress concentration factor, for simple loading: (Ex 7.7)

or

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Notch sensitivity q index is defined by

(7-30)

To find use Fig. 7-20 for steel and Al alloys , for reversed bending or reversed axial load. For reversed torsion use Fig. 7-21.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.

0 1q

q

0.20q 0q 1fK

1q f tK K

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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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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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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

Sut in kpsi(a)1/2 (mm)1/2

Sut in MPa

Transverse hole

5/Sut 174/Sut

Shoulder 4/Sut 139/Sut

Groove 3/Sut 104/Sut

Table 7-8 Heywood’s Parameter for steels

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0.2604.51 520 0.859ak

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SOLUTIONSOLUTION

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SOLUTIONSOLUTIONSOLUTIONSOLUTION

See next page

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0.062 in=0.312 mma

With 2.5% lower than a) and b)

me 307 machine design i dr. a. bazoune chapter 7: fatigue failure resulting from variable loading...

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