Preloading, and How It Affects Your Mechanical Test

It is almost always recommended that the Bluehill^® Software preload feature be used when you’re performing a mechanical test. Preloading simply removes slack from the load string before a test begins. This is done by loading the specimen at a specified rate - called "preload speed" -  until a particular load limit is reached - called "preload". Note that this value should be high enough that it removes slack from your load string, but low enough that it doesn’t interfere with important early-test calculations like modulus or yield. Once the preload is reached, the software begins collecting data from the load cell, crosshead encoder, and any other measurement device you have in the system. That means your test begins collecting load data after it’s reached the preload value. It also means your first recorded load point will be the preload value and not zero—which is okay! There are also calculations, like slack correction, that allow you to remove what is called the “tow” section of the curve (Section B in the image below).

Once the preload is reached, Bluehill gives you the option to perform an auto-balance of your physical or virtual measurements. Any strain source, whether it is the crosshead extension or an extensometer, should be auto-balanced after the preload is reached. This ensures that all of the specimens in your sample start from the same load and strain point, making your data more accurate and much easier to process (especially if you are using extension and strain-based calculations).  If using extension as your strain source, keep in mind that your gauge length is now equal to the original gauge length plus the distance traveled to reach the preload. You should not auto-balance the load signal. We commonly see load being balanced incorrectly (see my post on balancing load cells). Once a specimen is gripped, it is common to see a bit of force on the load cell—either positive or negative. This is simply a function of how the grips hold the specimen, or it is due to the mass of the specimen. If the force recorded on the load cell is severe, it may be due to misaligned grips or a poorly-inserted specimen. If a specimen is brittle or fragile, a feature such as Specimen Protect can be used to ensure the load on the specimen does not exceed a specified amount (+/-) before beginning a test.

Effectively using the preloading and auto-balancing features of Bluehill can help keep your results much more consistent. If you have questions on how to incorporate these features into your tests, feel free to contact us.

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What Are Biodegradable Medical Devices?

What are biodegradable medical devices? They are the next hot medical device, one that only lasts a few months and then disappears without surgery or long-term side effects. Researchers around the globe are using new polymers and metals to design biodegradable medical devices. The devices do their jobs as long as needed and then dissolve, leaving the newly repaired tissue able to function normally.

Dr. Y. Yun at North Carolina Agriculture and Technical College (NACT) is using a combination LumeGen (flow/pressure) and CartiGen (compression) bioreactor system and an ElectroPuls™ E3000 to evaluate new biodegradable materials, design new components, and fatigue test new medical devices. He believes that biodegradable metal (magnesium) devices are greatly affected by their application in the human body; therefore, they should be tested in an environment that mimics this environment. Dr. Yun will be using Instron LumeGen system to evaluate biodegradable stents under physiologic pressure and flow conditions and the CartiGen to evaluate biodegradable screws under physiologic loading conditions. This in vitro test bed provides an alternative in vivo (animal testing) in which the experimental design is often limited due to high cost and results in inconclusive data. Dr. Yun hopes to reduce the gap between in vitro and in vivo testing with the Instron solutions. Read more

Automated XY Stage System Preventing Shattered Glass and Inaccurate Data

A pharmaceutical customer came to us with what they thought was a simple, low force, compression testing procedure that they wanted to automate to keep pace with increasing product testing demands.

What we found was an unreliable testing procedure that was time consuming, all manual, and completely operator dependent. The operator pulled a lever to set the applied force and manually record data. If the lever was pulled too quickly, the product would shatter and any obtained data was rendered useless (not to mention the safety hazard of cleaning up the broken pieces, including glass and potentially dangerous liquid).

We had a better solution that would not only be operator independent but also would allow more product testing in less time, leaving operators more time to perform other valuable tasks. The Automated XY Stage System provided the customer a reliable, repeatable, and safe way to test their product, complete with a product containment fixture that made the system easy to load. The solution resulted in increased efficiency and less wasted product. The customer was able to produce a better product due to a reliable testing procedure and more data in a shorter amount of time.

Now, an operator only needs to load a fixture, enter a few fields into the software, and press "run". While the system runs, the operator is free to walk away and address other needs. Read more

Oyster Glue Inspiring Stronger Adhesives

 
What adhesive is so strong that it sets wet? That's right: naturally produced oyster glue.

Chemist Jonathan Wilker from Purdue University focuses his research on this cement that allows oysters to attach to surfaces and each other, even throughout storms. Wilker's team also studies mussel glue, which is less strong compared to oyster glue, but has the same ability to set underwater. To evaluate the strength of these marine adhesives, Wilker and his team use an Instron electromechanical testing system.

Credit: Jonathan Wilker of Purdue University, NSF

Wilker is passionate about his findings because he knows how they may advance the biomedical world's development of surgical adhesives to seal body tissue and bone while a patient undergoes surgery.

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High-Frequency Testing of Nitinol Wire

Nitinol wire is a superelastic alloy with unique shape-memory properties that render it especially useful for a variety of different applications across industries. As nitinol requires a great deal of motion and flexibility in its applications, mechanical testing of nitinol must reach very high fatigue strains.

In an example of testing nitinol, the nitinol rod was clamped in place on the ElectroPuls™ E3000 using 3 kN pneumatic grips and flat serrated grip faces. Gripping pressure needed to be sufficient so the specimen did not slip during testing, but not so high that the grip faces created indentations in the nitinol. The test was run in load control between 10–700 N of tension with a triangular waveform. The frequency of the test was increased sequentially in blocks of 0.01Hz, 1Hz, 10Hz, 30Hz and 60 Hz, and the metal was expected to exert an elongation of approximately 7%.

High-frequency, high-capacity testing creates several challenges around specimen gripping, achieving load peaks, and quick increases in frequency. Slippage can become a problem if nitinol specimens are in the form of rounded rods or wire and relatively high loads are being applied. Instron has solved the issue of specimen slippage with wedge action pneumatic grips, which apply an increased gripping force as tensile load is increased. This ensures that the nitinol remains secure for the duration of the test.

E3000 with Pneumatic Grips and a Metal Rod

It is also essential to achieve the desired load peaks of 700 N for this test. Attaining load peaks can become more of a challenge as higher frequencies come into effect. The Amplitude Control feature of WaveMatrix™ Software ensures that the load peaks are met for each cycle and the desired 700 N of tension is achieved. Without this useful Amplitude Control feature, load peaks would likely drop below the desired 700 N of tension, especially as the frequency was increased. Read more

Consumer Reports Uses Instron for Bend Testing Smartphone

Consumer Reports sought to test LG's G Flex smartphone, which claims to bend to life's curves. The material properties of rigid plastic, aluminum, and glass typically are used in mobile devices, and one would assume they would be difficult to bend.

LG has claimed that the smartphone can withstand 88 pounds (about 40 kilograms), but the bendability is limited: "Do not bend inward or twist." Watch Consumer Reports prove that the G Flex phone can withstand more than LG indicated.

This video reveals the amount of load placed on the electronic device with an Instron system:

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Quick Tips for Balancing Load Cells

Whether you use bathroom scales, digital kitchen scales, spring scales – or any other measuring instrument, for that matter – you know the importance of zeroing it before it’s used. Failure to do so results in a shift in weight and thus, an incorrect reading on the scale. Now let’s think about the load cell in your testing system, which is just like a scale …

Prior to testing a batch of specimens, you should ensure that your load cell is properly calibrated and balanced – both actions are done right in the software. In Bluehill^® 3, this can be done from the console at the top of the screen. Simply click on the load cell icon, and then click "Calibrate". We refer to this as a software calibration (or “soft-cal”), which also balances the load cell.

Here are some soft-cal quick tips:

1. We recommend that the machine and load cell be switched on for about 15 minutes prior to performing a soft calibration. This allows the device to warm up, reducing the chances of drift.
2. Make sure you are not gripping a specimen when you perform a soft-calibration. It is okay to leave the upper grip or fixture attached to the load cell during the calibration – but it is also okay to leave it off. Just remember to perform an additional balance once the grip or fixture is added to the load cell.
3. Again, do not balance the load cell once the specimen is loaded into a grip or fixture. The force you see is real mass of the specimen, or force on the load cell. Balancing out this load would be like balancing a scale after you have put one foot on it (Note: this means the load cell should not be auto-balanced after preload). 
4. Though many systems will automatically recognize a load cell and restore its previous calibration every time it is plugged into the system, it is good practice to perform this soft-cal regularly, especially if swapping load cells. What does regularly mean for your system? Contact us, and we’ll help you figure it out.

Remember that a soft-calibration is not a replacement for on-site verifications by Instron Professional Services on a regular basis. For most laboratories, this means at least once a year, but your industry standards or internal protocols may differ.

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What is the Best Way to Grip Thin-walled Steel Tubes for Tension Testing?

Q. What is the Best Way to Grip Thin-walled Steel Tubes for Tension Testing?

A. For testing most tubular specimens, you should use vee-serrated grip jaws. They hold the specimen securely and the vee face ensures that the specimen is centered. However, the grip forces can deform thin-walled tubes. You should manufacture close-fitting mandrels with a tapered end to insert into the tube ends before gripping the tube. The mandrel will ensure the tube cross-section does not deform, and the tapered ends will accommodate any necking of the tube during testing. Read more

WPI Students Test ACL Graft Pretension Methods

At Worcester Polytechnic Institute's Biomedical Engineering Symposium, biomedical engineering students presented their research on anterior cruciate ligament (ACL) grafts. With an Instron electromechanical testing system, the students were able to analyze stress relaxation and creep techniques used for pretensioning before attaching ACL grafts in reconstructive surgery.

After ACL injuries, the wounded tend to undergo reconstructive surgery to insert grafts for knee rehabilitation. Over time, the ACL grafts lose tension strength. To prevent this, creep and stress relaxation methods are used on the grafts. Although relaxation techniques are common, universal standards are not used in evaluation of the best method.

Therefore, the students tested various protocols. Stress ratios between 15-minute pretensioning methods were significantly different: stress relaxation (0.47 ± 0.008) and creep (0.62 ± 0.005). This demonstrated that a creep protocol offers greater load retention after ACL reconstructive surgery. Interestingly, no significant difference was found when comparing 5-minute creep (0.63 ± 0.017) to 15-minute stress relaxation (0.47 ± 0.008). These results demonstrated the potential for physicians to save time in the operating room.

Instron was proud to attend symposium and watch WPI students Spencer Keilich, Allison Indyk, Jeremy Kibby, and Shreyas Renganathan present their fascinating research.

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Extensometer Slippage

Some of our customers are frustrated when they experience slippage of their strain gauge extensometers. Close examination of their extensometer knife-edges often reveals wear that has rounded the edge, increasing the likelihood of slippage. If you are experiencing extensometer slippage, take a close look at your knife-edges. For replacement parts, please contact your local Instron representative.

Note that the knife-edges are precision engineered to ensure an accurate gauge length, so we do not recommend that you attempt to sharpen the edges yourself. Read more