Fatigue: Fully Reversed Lecture 28 ME EN 372 Andrew Ning - - PowerPoint PPT Presentation

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Fatigue: Fully Reversed Lecture 28 ME EN 372 Andrew Ning aning@byu.edu Outline Stress Concentrations Fully Reversed Load Examples Stress Concentrations Static stress concentrations Sec. 3-13, Tables A-15 K t = max 0 Purely

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Fatigue: Fully Reversed

Lecture 28

ME EN 372 Andrew Ning aning@byu.edu

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Outline

Stress Concentrations Fully Reversed Load Examples

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Stress Concentrations

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Static stress concentrations

Sec. 3-13, Tables A-15

Kt = σmax σ0 Purely geometric

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Notch Sensitivity

q: Figs. 6-20 and 6-21. Eqns. 6-34 and 6-35 Based on material and geometry.

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Fatigue stress-concentration factor

Kf = 1 + q(Kt − 1) similarly for shear stresses Kfs = 1 + qs(Kts − 1) Multiplier against nominal stress reversal (will discuss next)

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Fully Reversed Load

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time stress

Apply stress concentration factor to the reversed load.

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S-N diagram

Fig. 6-10

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Infinite Life If σrev ≤ Se ⇒ Infinite Life

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Finite Life

Fatigue strength: Sf = aN b

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Finite Life

Fatigue strength: Sf = aN b Safety factor in fatigue: nf = Sf σrev

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Finite Life

Fatigue strength: Sf = aN b Safety factor in fatigue: nf = Sf σrev Number of cycles to failure: N = σrev a 1/b

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How do we get a and b?

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Examples

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Example 1

Steel shaft in bending has ultimate strength of 690 MPa. Shaft has a 38 mm diameter section and a 32 mm section, connected by a radius with a fillet of 3 mm. Estimate Kf

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Example 2

Same shaft has a fully corrected endurance limit of Se = 280 MPa. The fully reversed nominal stress in filet is σ = 260 MPa. Estimate number of cycles to failure.