A, N and C Certified Almen Strips

A: The thickness of the Almen strip determines its type:

• “N” = .031” (0.79 mm)

• “A” = .051” (1.3 mm)

• “C” = .0939” (2.39 mm)

The thickness determines a strip’s ability to arc during peening so each strip has an appropriate intensity range, as expressed by the arc height measurement from an Almen gage. The “A” strip is considered the standard strip and is used when the required intensity range is an arc height of .004” – .024”. When an arc height lower than .004” is required, the thinner “N” strip should be used. The thicker “C” strip is used to achieve an arc height above .024”.

The Almen strip is sensitive to thickness variations, as can be seen by the relationships among the three strip types. An “A” strip deflection of .005” would result in approximately .015” deflection of an “N” strip. The standard tolerance for thickness, ± .001”, would therefore allow approximately ± .0004” variation in expected curvature reading. For automotive applications this may be acceptable, but for aerospace applications it could be unacceptable. Therefore the ±.0005” thickness tolerance, which would translate into a ± .0002” curvature tolerance, is required for most aerospace applications.

A: For most general purpose shot peening applications, namely automotive and gears, you can rely upon the Grade A-2™ of Almen strips. For more critical applications, such as aerospace, a tighter tolerance may be specified for flatness (pre-bow) and hardness so you should use A-1S™ strips.

Electronics Inc. Almen strips are graded primarily by flatness (pre-bow) and hardness. The three most common flatness tolerances for the “A” strip are shown below.

The hardness requirements are halved for aerospace applications. The hardness tolerance of HRc 44-50 is standard for automotive applications and 45-48 HRc is required for aerospace applications.

The thickness tolerance ± .001″ is common for automotive applications and half of that tolerance (.0005”) is allowed for aerospace applications.

For example, the “A” strips have the following tolerances:

Flatness (Pre-Bow)
± .0015″ Grade A-3™ – bulk packaged (±0.0381 mm)
± .0015″ Grade A-2™ (±0.0381 mm)
± .0010″ Grade A-1™ (±0.0254 mm)
± .0005″ Grade A-1S™ (±0.0127 mm)

Hardness
44-50 Grade A-3™ – bulk packaged
44-50 Grade A-2™
45-48 Grade A-1S™

Thickness
± .001″ Grade A-3™ – bulk packaged (±0.0254 mm)
± .001″ Grade A-2™ (±0.0254 mm)
± .001″ Grade A-1™ (±0.0254 mm)
± .0005″ Grade A-1S™ (±0.0127 mm)

Note: A-3, A-2, A-1, A-1S, N-3, N-2, N-1, N-1S, C-3, C-2, C-1 are trademarks owned by Electronics Inc.

A: The important factor when using Almen strips is to determine the change in the arc height. Here is a case in point:

Peen a strip with .0000″ prebow until it arcs .010″. The change in arc height was .0010″.

Peen a strip with a +.001″ prebow using the same conditions and it will measure .0110″, but the “change” in arc height was .010″.

Peen a third strip with a -.001″ prebow using the same conditions and it will measure .009″, but the change in arc height was .010″.

All three strips had an increase in arc height of .0010″. Without prebow compensation, however, the recorded arc heights would have a range of .002″ (.009″ – .011″), which could be more than half of the allowable peening tolerance.

If prebow compensation is not used, use the 1-S grade Almen strip which limits the maximum range to .001″. (Electronics Inc. Almen strips are classified in grades based on prebow tolerances to meet a wide range of applications, from automotive to aerospace.)

A: It depends upon your quality procedures. You may be required to use the latest version of AMS-S-13165 which is the cancellation notice with a re-direct to AMS 2430. If that is the case, you should select strips that meet the requirements of SAE J 442. Otherwise, you may continue purchasing strips that meet the requirements of 13165.

A: It’s not very common but, if Almen strips are improperly handled or stored, they can develop rust. Fortunately, our packing methods and a few simple precautions will protect the surface of the test strip.

BOXED ALMEN STRIPS

• Do not removed the shrink wrap covering the box of strips until the strips will be used.
• Do not discard the VCI (Volatile Corrosion Inhibitor) paper after opening the box. VCI paper is a very effective material for preventing rust and corrosion.
• Always return unused strips to the original EI box and replace the VCI paper, making sure the unprinted side of the paper is in contact with the strips.

BULK ALMEN STRIPS IN BAGS

• Electronics Inc. packages bulk Almen strips in blue VCI polyethylene bags. In addition, EI places a 12” x 12” sheet of VCI paper in the bags.
• The Almen strips should remain in the blue bag until the time of use. Always return unused strips to the VCI bag and replace the VCI paper, making sure the unprinted side of the paper is in contact with the strips.
• Before returning Almen strips to storage, close the blue VCI bag. Fold the leaves of the box to close it tightly or tape the box shut. Do not leave the strips open to the environment.

RECOMMENDED CLIMATE CONTROL

• Avoid major temperature changes when storing or using Almen strips. In particular, do not move strips from a warm environment to a cold one. This can cause condensation on the surface of the strips
• Recommended storage conditions are 65°-75°F (18˚C-24˚C) and a 20% – 60% relative humidity.

A: The strips are named after John Almen, an engineer at Buick Motor Division of General Motors in the 1940s. Mr. Almen discovered that automotive valve springs had fatigue life improvements after shot blasting. He realized that the periodic measurement of the shot blast would be necessary to achieve reliable and repeatable results.

Knowing that shot blasting would distort and bend sheet steel, Mr. Almen developed a small test coupon that reacted to the intensity of the shot blast stream. The coupon curves upwards in the middle in response to the intensity of the shot stream. The amount of upward deflection, the arc height, is measured by a gage that is now called the Almen gage.

Today’s Almen strips are precision springs made with tight tolerances from SAE grade 1070 steel. Electronics Inc. recognized the importance of these tolerances and developed a family of graded strips with tolerances matched to industry applications, from low-cost automotive applications (grade A-3™) to critical aerospace applications (grade A-1S™).

A: by Charles Barrett, former Chairman of the SAE Surface Enhancement Division – Fatigue, Design and Evaluation Committee
We all know that there are three strips used to qualify the intensity of a shot peening machine. Each of the strips has a different thickness. The “A” strip .050″ thick was probably named for J.O. Almen who conceived the idea of using a strip of C-1070 cold rolled spring steel to qualify the intensity of peening on coil springs. The “N” strip .030″ thick was developed by Charles Noble as the “A” strip was not sensitive enough to measure low intensities used on jet engine parts. The “C” strip is .094″ thick and used to measure high intensity shot peening.

I attended a dinner at an SAE Fatigue, Design & Evaluation Committee meeting some years ago. The entertainment for the evening was an amateur magician named John Straub. What some of you may not know is that John Straub was J.O. Almen’s assistant at the GM laboratories, and was involved in much of the early experimental shot peening. Later on he directed the Wheelabrator R&D shot testing laboratory, where he developed the Wheelabrator shot testing machine. Among other things, John was a proponent of excluding fines in operating shot mix for optimum peening results. He also authored many papers in the late 1940s and also holds a patented dual intensity peening process.

When he finished his astounding demonstration of magic, John was asked by Dr. Ralph Stevens, University of Iowa, to relate some of the early events of shot peening in which he was involved. The question was asked why there was no “B” strip. John replied that during World War II the government was investigating the attributes of the German “Tiger” tank over U.S. tanks at the Aberdeen proving grounds. It was found the Tiger had shot peened torsion bars, which gave them a greater fatigue life over U.S. tanks. A deep depth of compression was required in the surface of the bar. The peening intensity was too high for the “A” strip, which warped under the shot impact. So a “B” strip of approximately .078” thick was tried. But it was also too thin.

The “C” strip of .094 was finally developed for the application. The “B” strip was superfluous as the “A” and “C” strips cover the intensify range. The “B” strip was discarded and not used again. There was another “B” strip used by the Douglas Aircraft Company. However, it was made from aluminum and used primary to determine coverage on aluminum aircraft parts.