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

Measurements and Tolerances

Measurement science is its own language. These few notes barely scratch the surface. If you are new to this, or will just be peripherally involved, perhaps as a sales support person, it is suggested that you learn decimal equivalents down to the sixteenths, and key metric sizes, such as 5mm, 10mm, 20mm, 50mm, etc. It will make your life a lot easier.

15/16″ is fifteen sixteenths. Not fifteen sixteens.

.010″ is ten thousandths, .003″ is three thousandths, .0005″ is; half a tenth, or five ten thousandths.

In all methods of measurement tolerances nothing is an absolute single number. Tolerances are represented in a range. It is important to both parties involved in a transaction understand what that range involves. If you are not sure, ask. It may vary from company to company, product to prodcut, or even from material to material. In the case of machine tolerances, the spread may be just a few thousandths. Bar length ranges may involve many feet.

The above is not intended to be all-inclusive but a random exposure to the topic. We will try to broaden the scope in future posts.

August 5th, 2020 – Howard Thomas

Exactly the Same, Only Different

When steel flat products are thin, they are typically referred to as sheet and strip; as opposed to flats and plate. There are specific delineations separating the two types, however in general use distinction of the terms is pretty loose.

It is customary to describe flats and plate, or strip and sheet, from the smallest dimension through to the largest dimension. Thuse, 1/4″ x 48″ x 240″ would be how a quarter inch thick piece of 4ft wide 20ft long plate would be listed. It is fairly typical for the “grain” direction of the steel to run parallel to the length of the item. There is room for caution when grain is an important consideration; such as when forming is involved, since the plate may have subsequently been cut to a smaller size. In those instances, you can not be sure about grain direction unless it has been marketed. It is good practice to mark grain direction on remnants, or “drops”, once a plate has been cut. Grain is important for several reasons. First, when forming a steel plate it is generally advisable to form against (perpendicular to) the grain. In the case of wear plate, abrasive wear due to flow, may be diminished when the plate is installed so that the flow pattern is perpendicular to the grain. Unfortunately, many times this is theoretical rather than practical due to size and shape required.

Hollow materials may be pipes and tubes, or something like structural rectangular tubing. Tubes are generally more exacting in size, shape, and quality. Pipes are “big and ugly” hollow sections. Pipes are categorized as NPS, or National Pipe Size. Sizes up to 14″ NPS are described by their I.D. Or inside Diameter. At 14″ O.D. (outside diameter), the NPS refers to the O.D. Important information required would involve; O.D. I.D., and wall thickness. Generally, you would use only two of those measurements; not three. Caution must be exercised when “telescoping” one tube inside of another. Where a closer fit is desired, the two parts must be made, or fabricated in such a way to ensure that they are completely compatible; insured compatibility.

When dealing with pipe and tube you will doubtless encounter peripherals, such as elbows, flanges, laterals, T’s, etc. Configurations and cautions here are too numerous to list. If you will be involved with tubing or pipe it is recommended that you take the time to familiarize yourself with the materials and parts you will be encountering. Complex configurations require as much artistry and experience as technical knowledge.

Solid Steel Bars (Long bar products) are referred to as “bars” or “lengths”, or each (Ea). Hydraulic Pistons or shafts are referred to as “rods”.

Typically, there is a difference between the physical properties of a grade of steel and the mechanical properties. Generally, the physical properties are those properties that will be fairly uniform (common) to the grade. Those might include; thermal conductivity, thermal expansion, % elongation (%EL), %reduction in area (%RA), etc.

When discussing properties of steel, such as flatness, straightness, or out-of-round, it is best to avoid superlatives such as “perfectly flat”, or, “perfectly straight”. Presenting your requirements in that manner may result in a “No-Quote” from a potential supplier.

While there are many methods of determining hardness, it is fairly common in carbon and alloy steel to encounter Rockwell “C” hardness testing, or, Brinell (Bhn) testing. In sheet and coil you may encounter the Olson test, which involves an even larger ball impression than the Brinell ball. The Rockwell test is more appropriate for high surface finishes or finished parts, whereas the Brinell test method is used on “Big and Ugly” materials, such as hot rolled wear plate or bar.

Remember that surface decarb must be removed completely to obtain an accurate reading.

-Howard Thomas, July 8th 2020

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Howard Thomas

Howard Thomas

Experience

Sr. Acct. Mgr. (US Southwest) / Metallurgical Consultant
Associated Steel Corporation
Jan 2017 – Present

Past Vice President / General Manager
Associated Steel Corporation
Apr 1998 – Jan 2017

Past Vice President / General Manager
Baldwin International
Apr 1974 – Mar 1997

Education

Cleveland State University
Kent State University
University of Denver

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