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Stress Life Fatigue approach in CAEF
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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