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

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 = σ max σ 0 Purely geometric

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

Fatigue stress-concentration factor K f = 1 + q ( K t − 1) similarly for shear stresses K fs = 1 + q s ( K ts − 1) Multiplier against nominal stress reversal (will discuss next)

Fully Reversed Load

stress time Apply stress concentration factor to the reversed load.

S-N diagram Fig. 6-10

Infinite Life If σ rev ≤ S e ⇒ Infinite Life

Finite Life Fatigue strength: S f = aN b

Finite Life Fatigue strength: S f = aN b Safety factor in fatigue: n f = S f σ rev

Finite Life Fatigue strength: S f = aN b Safety factor in fatigue: n f = S f σ rev Number of cycles to failure: � 1 /b � σ rev N = a

How do we get a and b ?

Examples

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 K f

Example 2 Same shaft has a fully corrected endurance limit of S e = 280 MPa. The fully reversed nominal stress in filet is σ = 260 MPa. Estimate number of cycles to failure.