Simple Tools … Faster Solutions

Background to Fatigue -

At the dawn of the industrial revolution

The 1st major transportation disaster – Versailles accident of May 11th 1842

‘The Times’, May 11th 1842. “I have this day to announce to you one of the most frightful events that has occurred in modern times. … The train of the left bank was unusually long; … from 1500 to 1800 passengers. On arriving between Meudon and Bellevue the axle tree of the first engine broke. … The carriages arrived of course, and passed over the wreck, when six of them were … instantly ignited. Three were totally consumed, …without the possibility of escape to the unhappy inmates, who were locked up … The number killed is variously estimated (between 40 and 80).”

Typical broken axle of the 1840’s

Classic appearance of a fatigue cracked railway axle from Glynn, 1844

Background to Fatigue

19th Century 20th Century Year Name Contribution

1920 Griffith Fracture Mechanics

1945 Miner Cumulative Damage

1954 Coffin & Manson

Plastic Strains

1961 Paris Crack Growth

1963 Peterson Strain-Life Method

1967 Endo Cycle Counting

1980 Dirlik Rainflow Counting from PSDs

Year Name Contribution

1829 Albert Repeated Loads

1839 Poncelet “Fatigue”

1843 Rankine Stress Concentrations

1860 Wohler Systematic Investigations

1886 Baushinger Cyclic Deformation

1890 Goodman Mean Stresses

1903 Ewing & Humfrey

Fatigue Mechanisms

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

Fatigue (S-N) Curves (for metals?)

Alternative Approaches for Metals - Fracture mechanics – crack growth – still empirical and not integrated to CAE - Physics based approaches – science not established and no industrial acceptance

Can we get this and this right? In order to get

this right?

S-N Curves for Composites

Fracture Mechanics – Crack Growth and physics based approaches are not practical options at the current time.

Can we get this and this right?

In order to get this right?

•  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

Fatigue Technology The Three Basic Methods

• Fatigue (we must not over complicate this part)

• Dynamics (must work in time and frequency domain)

• CAE (the key element – must be fully integrated)

What is the Challenge?

So the key challenge is to manage the integration of fatigue calculations for dynamic systems done within a CAE environment

History of Fatigue Methods and their integration into the CAE Environment

S-N 175 yrs ago

Crack propagation 50 yrs ago

NASTRAN 45 yrs ago

MSC Fatigue (Dynamics & Fatigue) 14 yrs ago

Integration with MBD 10 yrs ago

1st major transportation disaster Versailles May

11th 1842

Failure Mechanisms 107 years ago

Use of non-linear (Marc) FE results 8 yrs ago

P_Fatigue 20 yrs ago

Fatigue

Dynamics

Nastran Embedded Fatigue TODAY

CAE