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Endurance Limit – Fatigue Failure

Endurance limit is another way of saying fatigue strength. It may be expressed in “Cycles to failure” as opposed to “PSI”. One of the most difficult questions to answer is a question relating to endurance limit. When someone asks about endurance limit, they are trying to find out how long a finished part will last in an application involving constant/repetitive motion or vibration. Failure may be anything from a small crack to an abrupt and catastrophic event.

While it is easy to see that this is a matter of great concern, there is unfortunately no formula for arriving at an answer based on a raw piece of steel. To accurately determine the response of a particular part in a particular application, the endurance test must be performed on the finished part in a simulation that duplicates as closely as possible the motion of the actual application.

The R.R. Moor Endurance Test, is an example of a test that utilizes bending and rolling contact to test torsional fatigue. [Variable introduced may be; vibration, compression, bending, twisting, rolling, etc.]. This test is extremely expensive and the evaluation period is lengthy. Individual companies, steel mills, and independent test labs, are unable to predict failure based solely on the chemical and physical properties of a type of steel. There are general guidelines published relative to standard SAE steel grades, but those are for general reference only. Steels that have been refined or otherwise modified to enhance toughness or to resist fatigue related failure (Steel produced to Clean Steel Production Standards) would not be adequately represented on a generic chart when it comes to endurance limit.

To repeat; in order to obtain any meaningful data, relative to endurance limits, the finished part must be tested under conditions that approximate actual service conditions. This is frequently done when production run parts are involved, because the quantity offsets the cost of testing. It is generally considered cost prohibitive to test steel for maintenance replacement parts for endurance limit.

Remember.

  1. Generally there is no accurate published data to indicate a universal endurance limit for shaft material.
  2. Reference data published on steel by grade is at best general. It is not an accurate reflection of the expected service life of our material.
  3. Endurance limit relates to “Toughness.” Maintenance steel grades that have been manufactured to Clean Steel Production Refinement have enhanced toughness over their generic SAE or AISI counterparts strongly address concerns about endurance limit.

Steel that fails in service through fatigue related circumstances would have lasted longer if it was “tougher”. Toughness is achieved through an orchestrated combination of core integrity refinement during the production of the steel, combined with a specifically targeted thermal treatment and stress relief.

MAKE IT CLEANER, MAKE IT HARDER, REMOVE STRESS – INCREASE SERVICE LIFE, INCREASE ENDURANCE LIMIT.

Howard Thomas, September 8th 2020