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Calculating damage with Miner’s Rule

by Siemens Experimenter Siemens Experimenter ‎07-11-2016 11:00 AM - edited ‎05-27-2017 02:20 PM

Calculating damage with Miner’s Rule

 

Miner’s Rule is used to calculate damage caused by cyclic/time variant loading.  It is a linear damage accumulation model that uses a load time history and SN-Curve as inputs to calculate damage.

 

When damage is equal to “1”, failure occurs. The definition of the failure of part varies. It could mean that a crack has initiated on the surface of the part.

 

Applying a constant amplitude, cyclical stress to a metal coupon causes it to fail (a crack appears) after a specific number of cycles. Stress is defined as Force (F) divided by Area (A) or F/A.

 

loads.png

By repeating this test at different stress levels, one could develop a material SN-curve.  For example the SN-curve* may look like this:

 

SN-Curve: Cyclic Stress Level versus Number of Cycles to FailureSN-Curve: Cyclic Stress Level versus Number of Cycles to Failure

SN-Curves are developed with testing machines that apply constant amplitude loads.  They can be axial loads, torsional loads, bending loads, etc.  Different stress levels are tested and the number of cycles to failure are recorded.

 

Axial fatigue test machine for material couponsAxial fatigue test machine for material coupons

 

Damage Accumulation

 

When a physical part undergoes stress cycles**, Miner’s Rule works like this:

 

base.png

 

 

On the left graph, there is a loading time history.  The SN-Curve is the middle graph, and a damage tally is kept on the right side.

 

 

base2.png

 

In this case, two cycles at a specific amplitude are applied to the part. At this amplitude the part could take 6 cycles before it would fail. Dividing two cycles by six cycles, the accumulated damage is 0.33.  A third of the life of the part has been used.

 

base3.png

 

Two more cycles of a higher amplitude are applied.  At this higher amplitude, four cycles would be required for failure to occur.  Dividing two cycles by four cycles, an additional 0.5 of damage has occured.  The total accumulated damage is now 0.83.  According to Miner’s Rule, no failure has occurred.

 

base4.png

 

One more cycle of the higher amplitude is now applied.  The accumulated damage is now 1.08.  Failure has occurred!

 

History

 

In 1945, M. A. Miner popularized a rule that had first been proposed by A. Palmgren in 1924. The rule is variously called Miner's rule or the Palmgren-Miner linear damage hypothesis.

 

Limitations of Miner's Rule

 

Miner's Rule does not take into account the sequencing or order in which in the cyclic loads are applied.  For example, if loads are applied in the plastic region, the endurance limit is no longer in effect.

 

Other Notes

 

*Note: Real SN curves for metals are log-log curves and easily range into millions of cycles.

 

**Note: The loads applied to product in the real world are usually not constant amplitude cycles.  To break down a real world load history into cycles, a cycle counting method called 'Rainflow Analysis' is used.

 

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