Testing Your Metal
Talking to a member of the committee on mechanical testing (E28) — which celebrates its 50th anniversary this year — is an education about something essential to structures we often take for granted: the bridges we cross, the buildings we work or live in, the parts in our cars and aircraft, and much more. That something is metal.
Before metal becomes part of a car, plane, building, or bridge, it has been through a lot. And along the way to becoming an engine or a beam, the metal has been tested over and over. These tests measure the metal’s essential properties and its readiness for its purpose. One of these properties is hardness.
“Hardness testing is probably the most used test for acceptance testing and process control for metallic materials and products. It’s fast and it’s cheap, and it doesn’t destroy the product to test it,” says Samuel R. Low, materials research engineer in the Mechanical Performance Group at the U.S. National Institute of Standards and Technology. He’s also chair of the subcommittee on indentation hardness testing (E28.06), which is part of E28. “Pretty much any metal that’s going out to a customer or coming in to a company will be hardness tested.”
Hardness testing reveals a lot about a material: strength, wear resistance, ductility, and more. The E28.06 subcommittee oversees several hardness standards, including two time-honored methods that were recently revised to include portable testers: the test method for Brinell hardness of metallic materials (E10) and test methods for Rockwell hardness of metallic materials (E18).
ASTM International, which has roots in metal for steel train rails, has long had tests for hardness. The E28.06 subcommittee oversees Vickers and Knoop hardness tests (E92) as well as the Brinell (E10) and Rockwell (E18) standards, among others.
The 1924 ASTM Proceedings, the year of E10’s initial publication, were already calling the tester “the standard Brinell machine.” The test had been proposed in 1900 by Swedish scientist Johan A. Brinell, Ph.D., who worked in the iron industry.
The E10 standard today — the test method for Brinell hardness of metallic materials — covers a two-step test using an indenter (a ball) that penetrates the test section with a particular force for a certain amount of time. In the second step, the resulting indentation diameter is measured, which is then used to derive the Brinell hardness value. The machine holds the test specimen and applies the force; however, the diameter measurements are often made using another independent measuring system.
Another significant standard, the test methods for Rockwell hardness of metallic materials (E18), also rely on indentation testing.
In 1919, Hugh Rockwell and Stanley Rockwell patented the tester design, which came from an idea of Paul Ludwik, who developed the differential depth hardness measurement several years earlier. Approval of the E18 standard came in 1932.
The Rockwell hardness test, Low says, has an advantage over the Brinell test. Although the test undergoes two force applications, the final Rockwell hardness value is directly determined and displayed by the tester. The indenter penetrates the test specimen, the indenter force is increased, and then that additional force is removed so there is a return to the initial force. This process helps reduce effects from any surface imperfections. Indentation depth measurements are made at specific steps during the test process (instead of the diameters as with the Brinell).
Both the E10 and E18 standards require testing machine verification with direct, indirect, and daily approaches:
- Direct verification checks that critical machine components fall in allowable tolerances.
- Indirect verification is a process to validate machine performance with standardized test blocks and indenters.
- Daily verification provides checks on hardness testing days and when indenter forces are changed.
Both E10 and E18 also provide information about specific test conditions, recommended hardness ranges, and statistical information. The details give anyone running the test what they need to know about a metal and whether its properties make it fit for its intended purpose.
Changing with the Times
Over the years, the E10 and E18 standards have been refined, although the basic test principles stay the same. Most recently, portable testers have become part of the E10 and E18 standards.
“It’s important to define the requirements because you potentially have a customer who requires Brinell testing, and it’s in fact being done with a portable tester when the customer was thinking it was being done with a bench tester,” says Jim O’Donovan, Calibration Laboratory manager for Instron. The latest changes to the standards help clarify the testing machine used.
Portable testers have been — and continue to be — covered by the test method for Rockwell and Brinell hardness of metallic materials by portable hardness testers (E110). What hasn’t always been clear is whether the metal testing happens with a portable or fixed testing machine and what standard the testing follows.
Fixed machine testing isn’t always a practical approach. For example, an airplane gear or an armored tank plate can more easily be checked with a portable tester. And it’s not always the case, according to Low, that the portable tester meets the E10 and E18 requirements.
To clear up any confusion, support the integrity of the tests, and ensure that the tests being run are the tests called for, portable test machine requirements have been added to E10 and E18. “Now when they use a qualified, verified, calibrated portable tester, it does meet E10 or E18,” says Low. The results simply need to reflect that a portable tester has been used.
Further calibration requirements may be added as the subcommittee continues to review the standards. “The committee’s standards are constantly looked at and reevaluated,” says Earl Ruth, special projects engineer at Tinius Olsen Testing Machine Co., a long-time E28 member, and a former member of the ASTM International board. “As things have moved from analog to digital, we’ve had to change calibration. We needed to look at and adjust the standards accordingly.”
Another significant calibration requirement has also been added to E10 and E18. Groups conducting direct and/or indirect Brinell and Rockwell machine verification must be accredited to the requirements of ISO 17025, general requirements for the competence of testing and calibration laboratories, or an equivalent. In addition, the accrediting group must be recognized by the International Laboratory Accreditation Cooperation.
“The groups have to meet the requirements of direct verification in order to state compliance to either E10 or E18,” O’Donovan adds.
A third recent and significant change has been a change in the actual indenter ball from steel to tungsten carbide in certain scales. Steel used on a material that is too hard could lead to a flat spot on the ball that might not be noticed. And while the steel was less expensive initially, it had to be replaced more frequently. Tungsten carbide holds up better and as a result provides more consistent data.
Shawn Byrd is technical manager at Tinius Olsen Testing Machine Co., and an E28 member since 2013. He ticks off some overall subjects of E28 metal test standards: tensile (stretching); compression (crush resistance); impact (material’s notch toughness), which can be tested at different temperatures (ambient, elevated, or cryogenic); and various hardness tests, all important platforms to obtain properties to measure to be measured in metals.
Work continues to ensure that the standards, in the end, provide metal materials that will work as intended and last. O’Donovan says, “The people that are involved with this are investing a significant amount of time to make sure that they’re right; that as things change they become more up to date, as feedback is provided on the standards as to questions or things that might not be clear or could use a little more explanation, these things are constantly evolving to provide the user with the best standard possible.”
Byrd adds, “We can’t be at every lab. We do the best we can to be sure that we’re fairly similar in how we test. What we can do as proponents of ASTM is give them [the users] the best platform available so that they have a good guideline or reference to follow.”
E10 and E18, and other E28 standards, are part of that.