Accurate, repeatable test results are critical for any manufacturer to ensure that the components or materials used in its products will hold up under the varying forces and conditions they will face in real-world scenarios.

Whether a lab operator tests sutures for wound closure, rubber for tires, or thin films and foils for electric vehicle batteries, they need to avoid errors or variations in the testing results that could lead to missed product defects.

Ultimately, tensile testing errors can turn into potential hazards for the end user — and problems for the manufacturer. Among them:

• Lengthy quality investigations
• Late shipments
• Product recalls
• Financial losses
• Diminishing customer loyalty

By investing in the necessary equipment, procedures, and training to help deliver reliable results, a business helps to ensure that its products will meet expectations when they hit the market.

Granted, not every material is as critical as the steel that forms the framework of a skyscraper. But as Instron senior applications engineer Charlie Pryor explains, even the failure of a simple, everyday item can have serious ramifications.

“No one’s likely to get seriously injured by a pencil breaking,” he says, “but you might not want to buy that pencil again — and you might not buy other products from that company.”

Mistakes in tensile testing can happen at any phase — with the initial setup, during the test, or when the resulting data is interpreted. An error can stem from the method setup in the software, the choice of equipment, or an operator’s technique.

Below, we look at several common errors and some measures to help avoid them.

Sometimes, the problem occurs even before a test is underway. When a lab operator starts a test on a specimen that is not fully taut and then begins collecting data immediately, the results can be thrown off by that initial slack from the specimen.

“If you collect data before there is an adequate preload on your specimen, you will just be collecting noise — sometimes called the tow region,” says Kayla Thackeray, product marketing manager at Instron. “This data doesn’t need to be collected because your specimen isn’t seeing forces that can be calculated into results.”

How to avoid: Be sure the testing method in the software includes a defined preload rate and threshold to allow the slack to be taken up — then the test frame will start calculating force and collecting data after the initial force threshold has been reached.

It can be easy for an operator to enter specimen dimensions incorrectly or inconsistently — also a pre-test issue — and that can render the test unreliable.

“Specimen width and thickness and length — those are things that, if you get it wrong, then calculated results don’t come out as you expect them to,” says Instron applications manager Jon Camara, “and it can be jarring when you see something like that.”

An automated specimen measurement device

How to avoid: An automatic specimen measuring device enables operators to easily enter their measurements into the Bluehill Universal software, eliminating the need for manual data entry.

As Thackeray also explains, testing standards have rules about the number of measurements that should be taken and how exactly the operator should be taking them. The software makes it much easier to follow those standards accurately.

“Bluehill has the ability for you to say, ‘I’m taking three measurements. I want the median,’ ” she says. “Or, ‘I’m taking three measurements. I want the average.’ ”

Properly aligning a specimen to ensure it is perfectly vertical and centered is essential to producing accurate, repeatable test results. Incorrect alignment will pose a more critical problem with stiffer materials like metal, but it can have an impact on other types as well.

How to avoid: Be sure all lab operators are consistently trained in how to align specimens, and if necessary, invest in an alignment aid. Without that added assistance, newer operators will be especially hard-pressed to deliver repeatable results right out of the gate.

“You’re now relying on a new operator to get perfect alignment every single time when they’re just new to it,” says Pryor. “And I think it’s asking for a little too much at that point.”

A common misstep for lab operators happens when they place a grip on the load cell as they set up the test, but they forget to balance off the grip’s weight.

“So your test starts and it’s already reading that there’s 5 newtons of load on there, but that’s just the weight of the grip, which isn’t meaningful,” says Meredith Bernstein, senior applications engineer at Instron. “So that skews your results.”

Or the opposite can happen, where an operator balances the load after closing the grip and putting actual force on the specimen. In this case, real load being placed on the material is not being counted in the test results.

How to avoid: This kind of human error is primarily prevented by establishing standard operating procedures and having proper, uniform training across the lab’s staff to ensure procedures are followed consistently.

Different grips, jaw faces, or load cells may be needed for different test types, but operators may fall into the trap of using the same equipment across a range of specimens.

“Sometimes we’ll see customers who want to use a single set of grips to test all of their materials — when those grips are appropriate for some of them, but might be way oversized for testing something that’s much more delicate,” says Bernstein.

A few of the many types of jaw faces

In other cases, an operator may not have sufficient training to understand how different jaw faces — rubber versus serrated, for example — will affect varying specimen types.

The wrong load cell size will likely distort results as well. As Camara explains, if an operator uses a 50 kN load cell to test a specimen in the 1 to 2 N range, “you're really below the verifiable range of that load cell.”

How to avoid: Standards organizations such as ASTM and ISO can help identify the correct jaw faces, load cells, and other hardware for testing specific materials.

“Without that, you’re just shooting in the dark,” says Pryor. “You're just kind of going for what you think might work, but you don’t really know how it affects things, and you might not have the time or resources to figure out what’s best.”

Even with the right equipment, miscues can occur. One operator may tighten a manual grip to a certain clamping force, for example, but the next operator might apply twice as much — leading to different results in an otherwise similar test.

“So it’s all about using the right grips and fixtures, but then using them correctly for that material,” says Camara.

How to avoid: Defined operating procedures and proper training are important, but pneumatic or hydraulic grips can offer a semi-automated solution that makes it easier to set the correct clamping force every time. This can be an attractive option for manufacturers that prefer not to invest in a fully automated testing setup.

“We have this spectrum of automation that helps customers reduce their variation of results,” explains Thackeray.

The list goes on. Using an improper test speed or data rate, forgetting to return the crosshead to the correct starting point — even the temperature of the operator's hands can cause testing errors that might be difficult to pinpoint later.

Manufacturers have a few options that will help to narrow down this broad potential for error:

• Review all equipment manuals.
• Stay up to date with ASTM, ISO, or any other applicable standards for their industry.
• Properly train all staff who perform testing — and all engineers who set up methods in the software.
• Invest in automated or semi-automated equipment setups, if possible.
• Connect with Instron’s support team for guidance on how to optimize their testing setups.

Particularly in industries such as the biomedical or EV battery fields, where accurate test results ultimately impact people’s health and safety, a manufacturer needs to carefully follow all relevant standards and consistently train operators in proper procedure.

As an engineer who specializes in biomedical applications, Bernstein says, “In these industries — where these are products that affect patients directly — being able to ensure that there are no errors, and that the results you’re getting are reliable, really is critical to these customers.”