1. Working at the heart of test & measurement Fatigue Life Analysis Suite

2. www.prosig.com Working at the heart of test & measurement History l Fatigue Analysis started in the early 1800’s l Early methods led to formulation of Whole Life Estimation –Measurement of the number of cycles N f for a specimen to fail at a stress level S. –Production of a set of stress-cycles curves known as SN curves. –Allowed estimation of Stress Life.

3. www.prosig.com Working at the heart of test & measurement Stress Life l May be applied to simple specimens or entire structure l Service life is measured and decomposed into fatigue cycles (Rainflow Analysis) l SN curve is used to determine –the Number of Cycles to failure N k for a given stress level S k –Damage Increment, D k, = 1/N k

4. www.prosig.com Working at the heart of test & measurement Stress Life l Damage increments summed linearly give measure of total damage l If the total damage is DT then the fraction of life used is 1/DT l This is known as Miners Rule l Like Rainflow counting it is common to ‘Whole Life’ and the ‘Crack Initiation’ method

5. www.prosig.com Working at the heart of test & measurement Stress Life l Stress Life method –limited to low stress regions where structure and materials behave elastically –need for life prediction under high stress regions led to ‘local strain’ method –proposed 1960 by Manson & Coffin –Smith Topper Watson is modern method l Determines the duration before a crack forms l Sometimes called Crack Initiation l More usually known as Strain Life

6. www.prosig.com Working at the heart of test & measurement Strain Life l Strain Life approach is similar to the Stress Life –uses Rainflow counting –uses the Miners Rule linear damage method l Principle difference is the use of a fatigue model –Instead of an SN curve Strain Life uses fatigue related properties of material –Only necessary to analyse material and not structure

7. www.prosig.com Working at the heart of test & measurement Strain Life l Important facet of strain life method… –relates the measured strains (nominal strains) to local strains at a notch or other high stress point l Strain Life method –handles both the elastic and the plastic regions –is the preferred method for highly stressed components –predicts when crack will be initiated. Highly stress components often fail when a crack is initiated.

8. www.prosig.com Working at the heart of test & measurement DATS Fatigue Life Analysis

9. www.prosig.com Working at the heart of test & measurement DATS Fatigue Life Analysis l Differences between Strain Life and Stress Life Stress Life Region Stress and Strain Number of Cycles Strain Life Region

10. www.prosig.com Working at the heart of test & measurement DATS Fatigue Life Analysis l Stage One - Capture data l Sufficiently long, representative signal l Data Recorded from strain gauges sited near critical point

11. www.prosig.com Working at the heart of test & measurement DATS Fatigue Life Analysis l Stage Two – Extract peaks & troughs

12. www.prosig.com Working at the heart of test & measurement DATS Fatigue Life Analysis l Stage Three - Select material

13. www.prosig.com Working at the heart of test & measurement DATS Fatigue Life Analysis l Stage Four - Calculate Fatigue Life

14. www.prosig.com Working at the heart of test & measurement DATS Fatigue Life Analysis l Summary & General Rules of Fatigue –High cycle low stress –Low cycle high stress More Severe Stress and Strain Number of Cycles

15. Working at the heart of test & measurement End