Keep Metal Parts Microstructure Stresses Below the Metal Fatigue Limit and You’ll Maximize Machinery Reliability

When metal parts fail early in their service life you can be certain they suffered microstructure loads well above the intended design value.

To get the utmost reliability from the metal parts in a machine, and thereby get the utmost reliability from the machine, you must ensure the sum of all cyclic loads on a part’s microstructure always produce stresses well below the metal fatigue limit.

 

Slide 12 – Staying Well Under the Metal Fatigue Limit of Steel and Alloy Parts Under Cycling Loads Maximizes the Part and Machine Reliability

Always keep metal parts material-of-construction stresses well below their metal fatigue limit

The image above shows two example metal fatigue limit failure curves. Curve A is typical for ferrous metals and titanium alloys. Curve B is the typical shape for non-ferrous materials such as aluminium, brass alloys, bronzes, etc. The metal fatigue diagram shows the cycles to failure at various stress levels in the laboratory samples used to generate the curves. The laboratory experiments use specially machined test pieces of the metal mounted in a machine that cycles a test piece back and forth. The machine is set to load the test piece to selected microstructure stress levels. That level of load is maintained during the cyclic movements until the test piece fails. The count of cycles to failure is plotted on the X-axis of the graph. The Y-axis of the graph shows the stress level applied during each run-to-failure test.

Curve A shows that at a high stress level, near the Ultimate Tensile Strength of the steel, the test piece failed in about 10,000 cycles. As the fatigue stress level is reduced the test piece lasts longer. When the imposed stress is limited to about 50% of the Ultimate Tensile Strength the cycles to failure for the steel test piece had no limit.

In the case of Curve B materials, the test pieces would have all eventually failed from fatigue. Nonetheless, Curve B materials exhibit the same outcome as stress reduces: decrease the fatigue stress and you increase the service lifetime before failure.

The metal fatigue limit curves tell us how to increase the reliability of our machines. Notice that the X-axis scale goes up by a magnitude (x 10) per graduation. From 10,000 cycles to failure for steel at a stress level just below its UTS, a fall of some 10% in stress delivers an extra 10-times the life. A drop of about 40% from the UTS delivers a service life of 100,000 cycles to failure. When the stress level is less than 50% of the UTS the steel test piece has an infinite life. As the stress is reduced the part’s reliability increases, not by 2, 3 or 4 times, but by 10, 100, and even 1,000 times. It’s a massive increase in service lifetime reliability for a small reduction in the amount of cyclic stress.

You now have a proven scientific solution for increasing the reliability of any machinery: decrease the stress in the working parts and you will get reliability growth of 10 and even 100 times more.

A designer of machinery intentionally chooses a material with stress carrying capability far above the service loads a part is meant to carry. The material strength distribution curve and the load distribution curve at the left side in the chart represent the designer’s intention that service loads will only ever cause low microstructure stresses. The designer wants parts to last many, many cycles before failure. They build reliability into a machine’s design by using materials that provide a large factor-of-safety, and by choosing the part’s shape and configuration to minimize stresses in its microstructure.

When your machines do not last for long service life times, it is almost certain that one or more of the working parts are suffering high microstructure cyclic stress — they are being failed by cycling loads and stresses they should not be carrying; or they are being forced to move back-and-forth when they are meant to be stationary; or are being forced to lengthen-and-shorten; or are being twisted one way and then back — thereby being fatigued and made to fail well short of the intended service lifetime.

 

This slide is a companion to the new Industrial and Manufacturing Wellness book. The book has extensive information, all the necessary templates, and useful examples of how to design and build your own Plant Wellness Way enterprise asset life cycle management system-of-reliability. Get the book from its publisher, Industrial Press, and Amazon Books.

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