An Introduction into Stress-Life Fatigue Techniques

Simple Tools … Faster Solutions

Crane failures resulting from crack growth   

Railway accidents can be very destructive

What is Fatigue?

•  Fatigue is: –  “Failure” under a repeated or otherwise

varying load which never reaches a level sufficient to cause failure in a single application. (deformation based approach)

–  The growth of an existing crack, or growth from a pre-existing material defect, until it reaches a critical size. (material behavior based approach)

What is Failure?

Crack too big

Crack Initiation

Glue Joint Breaking

Loss of Stiffness Delamination

Loss of Functionality

Durability Fatigue

Damage Tolerance

Total Separation

S-N curves

*  Grandfather of Fatigue

Between 1852 and 1870, August Wöhler studied the progressive failure of railway axles. He plotted nominal stress Vs. cycles to failure on what has become known as the S-N diagram. Each curve is still referred to as a Wöhler line.

Designing against Fatigue - Wöhler

Poor fatigue perfomance • Extreme stress concentration at P • Severe fretting corrosion at P

Designing against fatigue • Low stress concentration at A • Fretting at B is harmless at A

P B

A

Example: Using Paper Clips to Generate an S-N Curve

Take 5 paper clips each

Group 1 - apply 45 degree repeated strains

Group 2 - apply 90 degree repeated strains

Plot results on S-N curve

Estimate scatter and discuss!

Load or displacement control?

A Typical S-N Plot

mSKN =

Fatigue Technology...

■  Is very old technology (40-160 years old);

■  Is a collection of empirical rules that were induced to fit observed behaviour and are generally accepted to work;

■  The designer, or engineer, wishing to exploit it should be familiar with the concepts but not necessarily all of the theories

•  S-N (Stress-Life) Relates nominal or local elastic stress to fatigue life

•  ε-N (Strain-Life)

Relates local strain to fatigue life

•  LEFM (Crack propagation) Relates stress intensity to crack propagation rate

S

N

NSm=K LCF, say <10000

HCF, say >10000

Note that Strain-Life is OK for all N

Strain-Life OK

Stress-Life OK