Metals Tensile Testing Standards: ISO 6892-1, ASTM E8/8M for Strain Control

Brief introduction into some of the changes and updates to both the ISO 6892-1 and ASTM E8/8M tensile testing standards for metals and ambient temperature.

Metals Tensile Testing Standards - ISO 6892-1 ASTM E8/8M - Strain Control from Instron

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Instron Year in Review

We've created a Year in Review to highlight exciting moments of 2013. We want to keep you up to date on all aspects of Instron® – from our community relations to our acquisitions to available webinars, and new products. There was never a dull moment or a period of time when we weren’t aiming to meet the expectations and needs of our customers. Every experience you have with Instron is invaluable to us as a company. Specifically within TechNotes, I work with our application engineers and product managers to bring you the most relevant and industry-specific news that will assist with your testing applications. Instron professionals are some of the best in the industry and hold seats on many of the ASTM and ISO committees. It’s my goal to share with you their knowledge and experience in order to better support your testing applications. So keep the feedback and questions coming! I welcome any chance I have to speak with you on ways to better communicate just exactly what you’d like to hear. Thank you for your continued support as we move forward into 2014 and I look forward to future communications with you! Best regards, Denise Czerpak TechNotes Editor

Download Now: A Year in Review

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Testing Metals to ASTM E8 with an Automatic Contacting Extensometer

 

The ASTM E8 standard describes tensile testing methods to determine yield strength, yield point elongation, tensile strength, elongation, and reduction of area for metals. It applies to metallic materials in any form: sheet, plate, wire, rod, bar, pipe, and tube.

When performing strain measurements, such as the strain corresponding to the yield strength, we typically recommend an automatic extensometer. Using an automatic extensometer, such as the AutoX750, helps reduce operator influence on the test while also ensuring accurate data. You can see this more clearly in the above video. Read more

Energy-controlled Impulse Testing of Shoes

Shoe and shoe material manufacturers alike must be able to prove that their products will withstand a substantial amount of wear and tear before releasing them to market. Useful in this analysis is the ability to simulate the impact of a runner on the sole of a shoe. During a typical gait cycle, these impacts can be higher than 3kN for an adult runner. In addition to controlling the load with which an impact is generated, researchers may also wish to control the energy which is generated as a result of the impact. A testing machine, which can successfully cater to these requirements, must be able to create and maintain a repeatable impact of a certain energy over a prolonged number of cycles.

Custom waveforms were created to replicate the impact of actual gait cycles. WaveMatrix™  software allows users to create and import their own custom waveform in the form of a CSV file, achievable in Microsoft® Excel. Through this custom waveform tab, customers also have the ability to specify a very precise impact duration. The up-to-5kHz acquisition rate of the 8800 Controller enables the waveform to be precisely controlled in the magnitude of milliseconds. As materials testing machines usually have two main controllable transducers, Instron established a method for achieving the given energy requirement for each impulse. The Advanced Amplitude Control is a feature of WaveMatrix that is key to conducting energy-controlled impulses, using a combination of controller gains to make sure that the energy requirement is met for each iteration of the impulse. The combination of this feature, in conjunction with the Calculations Module, allows the ElectroPuls™ to test footwear based on constant load, energy or any other requirement included in the calculations portion of WaveMatrix. Read more

Instron TGT at IMSS Exhibit in Chicago

Anna Wynn will be at the RX for Success exhibit at International Museum of Surgical Science (IMSS) in Chicago on December 5th, demonstrating the LigaGen L30-4c bioreactor instrument. This exhibit is focused on introducing and educating students on cutting-edge technology in bioengineering and the health care industry. The exhibit will be hosted by the Museum of Surgical Science and shared through both museum events and traveling exhibit venues. Read more

Question From a Customer: How to Report Strain at Break Following ASTM D638

Question: I am following ASTM D638-10, and my 'strain at break' results are nearly half of what other labs are reporting for the same material. What is wrong?

Reply: If your material exhibits necking or inhomogeneous strain, ASTM D638-10 specifies that the extensometer needs to be removed at specimen yield. Once the extensometer is removed, nominal strain is calculated to specimen failure. The standard specifies this to avoid tests where the specimen begins to neck outside of the gauge length. A common misinterpretation is that if your specimen necks within the extensometer gauge length that the extensometer can be left on until failure. However, to be compliant with ASTM D638-10, the extensometer must be removed at specimen yield regardless of where the necking occurs, and nominal strain must be calculated to specimen failure.

When the extensometer is left on the specimen until failure, strain at break is calculated by the change in extensometer gauge length divided by the original gauge length. The majority of specimen elongation will occur in the narrow section of the specimen where the extensometer is attached. When removing the extensometer at specimen yield, nominal strain is calculated by crosshead extension divided by the initial grip separation. This method is how ASTM D638-10 specifies strain at break to be calculated. The majority of specimen elongation will still occur in the narrow section of the specimen; however, the entire grip separation is now being used as the gauge length. Thus, this causes lower strain at break results. We have found in our lab that strain at break results are almost doubled when leaving the extensometer on until failure.

We recommended having the other labs remove their extensometers at specimen yield and check their method to make sure they are using nominal strain to calculate strain at break. Read more

What is Digital Image Correlation (DIC) and How Can It Help Me?

Digital Image Correlation (DIC) is an analytical technique that compares images of a specimen’s surface during testing to generate full-field strain maps. This technology gives you more information than a traditional point-to-point extensometer or a strain gauge and allows you to see the complete story of the material’s behavior beyond the stress strain curve.

Scientists and engineers have found dozens of useful applications for DIC including detecting cracks invisible to the naked eye, visualizing localized necking and discontinuous yielding, comparing differences in material behavior between two separate formulations, and analyzing strain on parts or components where a traditional extensometer is not feasible.

In our lab, we recently performed a test to ASTM standard D5766 for the open-hole tensile strength of polymer matrix composites and used DIC to see exactly where the strain was occurring. Using DIC, we were able to visualize where the strain on the specimen was concentrated and how it propagated through the material.

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