by Kennedy Smith and Dirk Dusharme

If you’re not a hardness tester user, you’re likely not even aware of these tools or what they do. However, these instruments, which measure the hardness of materials, have been around for more than a century. They not only determine whether your product is within specification, but they may also help prevent potential major problems down the line.

A recent episode of the popular TV series CSI: Crime Scene Investigation illustrates this point. During the episode titled “Chasing the Bus,” a bus experiences a flat tire, veers off the road and crashes. The accident kills several passengers, leaving others with serious injuries--far more dire results than usually associated with a flat-tire incident. The following is an excerpt from the episode, in which investigators in the lab use a hardness tester to discover the real cause of the catastrophe:

Warrick: Three-quarter inch bolt. It matches the bolt on the specs for this suspension system. The hex head says it’s a grade eight.

(Warrick hands the bagged bolt to Grissom. He looks at it and notices the sheared end.)

Grissom: Snapped in half?

Sara: Bolts are graded according to their hardness. The higher the grade, the stronger the steel.

Nick: A grade-eight bolt shouldn’t snap like a toothpick.

Grissom: Did you do a Rockwell hardness test?

Nick: Yeah, I hit it with the Tru-Blue. It’s not a grade eight. It’s a grade five. (Note: Tru-Blue is a Rockwell hardness tester from United Calibration Corp.)

Sara: The bus company was using bogus bolts.

Grissom: Inferior bolts would cause the suspension to give and the bus to veer off the road.

Granted, this is a worst-case dramatization of what can happen when companies skimp on proper hardness testing. A more likely outcome is that without proper hardness testing of your materials, you’ll soon lose business to your competitors who will use this time-tested metrology tool to lure customers with better product.

So, what is a hardness tester? What’s its purpose on the shop floor? And what types of hardness testers are available?

Hardness testers’ purpose

“All hardness testers have the same thing in common,” explains Dick Ellis of David Ellis Co. “It’s a known load to a known penetrator. That’s it. That’s the whole story right there.” To elaborate, hardness testers are typically used to measure the resistance a piece of material will have against a set force. Often used to measure the hardness of heat-treated materials, including steel, they’re also used in testing plastics, rubber and other nonheat-treated substances.

According to Ellis, around 50 percent of the hardness testers used today were built before the 1960s. “There were some new developments when the computer came out, but a hardness tester is just like a micrometer or an optical comparator--its basic principles stay the same,” says Ellis. “Think of it this way: A screwdriver works just fine; you don’t need to add a graphical display and lasers to it. It’s the same with hardness testers.”

There are a number of hardness testers available on the market today. Which one to choose depends on your specific needs.

The Brinell method

According to Mark Antonik, hardness testing specialist at Sun-Tec Corp., the Brinell hardness tester first appeared on the market sometime near 1900.

Scott Robinson, a technician at The L.S. Starrett Co., elaborates. “Basically, a Brinell tester uses a round ball indenter to press into a piece of metal with a known force. That force will cause a dimple in the part. The harder the material, the smaller the dimple.” Once the dimple is established, it’s measured, and that measurement is assigned a certain value, as illustrated.

The problem with Brinell testers, however, is that the round ball indenter doesn’t necessarily make a clean, perfectly round indentation, making it difficult to measure exactly how hard the material really is.

Along comes Vickers

Sometime around 1924, the Vickers hardness tester was developed.

The difference between a Brinell and a Vickers hardness tester is the type of indenter used. Where Brinell uses a round ball indenter to press materials, Vickers utilizes a square or diamond-shaped indenter. It’s the same basic principle as the Brinell, but the user has a device to measure more defined indentations rather than Brinell’s harder-to-determine round indentation. (See illustration.)

Rockwell--a popular choice

In the United States, Rockwell is the hardness testing method of choice. Of course, there are exceptions, and Brinell and Vickers testers are abundant domestically. However, other countries, such as Japan and Germany, turn to Vickers and Brinell testing more often, according to Ellis.

The Rockwell hardness testing method was developed during World War II and uses a cone-shaped diamond indenter to penetrate material.

“Basically, developers take a known penetrator--the diamond point--and press it into a piece of material,” explains Robinson. “Then, they can measure the depth of indent. It’s a quicker system, and that’s what the United States settled on: the penetration system, rather than measuring across the distance on the material.” (See illustration)

To understand exactly how these tools correctly determine hardness, it’s important to learn about one of their most vital accessories: the load cell.

Applying the right pressure

Load cells measure the amount of force being applied to the indenter, ensuring that the same amount of force will be applied from one measurement to the next. Before load cell technology, hardness testers used a dead weight system in which frictional and mechanical losses would cause variations in the amount of force from measurement to measurement. This would negatively affect the user’s gage R&R. In other words, with load cells, frictional and mechanical losses are continuously compensated for; in dead weight systems, they’re not.

“When a force is applied to the body of the cell, a sensor (referred to as the strain gage) undergoes a change in resistance proportional to the applied force,” explains Phil Eusebi of United Calibration Corp. That change in resistance can be translated into a numerical value that can be viewed by an electronic readout system.

“The load cell has replaced the use of dead weights, levers and pivoting points. The United Tru-Blue II Rockwell tester doesn’t require leveling, cleaning and oiling of its measurement system as does the dead weight system. The tester’s load cell is calibrated using a proprietary computerized calibration system and a primary standard load cell that has been certified in compliance to a national standard.”

Durometer and IRHD

In the plastics and rubber industries, the Durometer method is often used. Evolving from a hand-held measurement device, there are now more accurate benchtop models that can read to a tenth of a point with good repeatability, according to Newage Testing Instruments Inc. The Durometer method applies a predetermined test force to a spherical- or conical-shaped indenter. The depth of indentation is translated into hardness value by means of a dial gage. “Micro” scales are also available for thinner and more narrow specimens.

The International Rubber Hardness Degrees method was developed in Europe but has become more common in the United States. IRHD provides repeatable results on rubber parts of any shape or size--coming in handy for measuring the hardness of rubber O-rings.

Choosing the right tester

According to Newage, there are several questions that need answering before choosing the right tester. These include:

Is there a hardness scale specified (Rockwell, Vickers, Brinell, etc.)?

What is the material being tested, and is it suitable for a particular test?

How large is the part?

What volume of testing has to be done?

How accurate do the results need to be?

What is your budget?

What problems have occurred in the past?

For a more in-depth look at how to choose the right tester for your application, see the chart below.

Testing considerations

Newage offers some general tips to help your hardness testing experience go smoothly. Among these considerations are:

Large parts. Parts that aren’t easily supported on an anvil should be clamped into place or properly supported.

Small parts. The smaller the part, the lighter the load should be. Meet the minimum thickness requirements and properly space the indentations away from edges.

Cylindrical testing. A correction to a test result is needed when testing on small-diameter cylindrical shapes due to a difference between axial and radial material flow.

Space indentations. Maintain a spacing equal to 2.5 times the indentation’s diameter from an edge or another indentation.

Thickness. Maintain material thickness at least 10 times the indentation depth. n Scale conversions. Sometimes it’s necessary to test on one scale and report on another. Conversions have been established by the American Society for Testing and Materials, but unless an actual correlation has been completed by testing on different scales, established conversions may not provide reliable information.

Gage R&R studies. In hardness testing, there are variables that preclude using standard gage R&R procedures with test pieces. Material variation and the inability to retest the same area on depth measuring are two significant factors.

ASTM and NIST standards

A hardness test is only sound if it compares accurately to others’ tests. This is where ASTM and the National Institute of Standards and Technology come into play. Hardness standards have been established and maintained by ASTM, including ASTM E18, E384, E10, E110 and others. (Visit the ASTM Web site at www.astm.org.)

NIST’s role in hardness testing began in the early 1990s, when representatives became active in ASTM’s Indentation Hardness Test Task Groups. “Before the NIST program, there was variance in hardness levels among all the different manufacturers around the world,” notes Sun-Tec’s Antonik. “Now we line up with international standards.” NIST helped standardize test blocks and has since facilitated commercial test block manufacturers to make their test blocks NIST-traceable.

A hard choice

This article has provided a basic overview of the purpose and different types of hardness testers available on the market. Depending on the hardness tester you need, you could be looking at spending anywhere from $5,000 to more than $50,000. To find out specific details about the different testers available, visit the Web sites listed on page 26.

About the authors

Kennedy Smith is Quality Digest’s associate editor. Dirk Dusharme is Quality Digest’s technology editor.