Testing Metals? Don’t Over Strain Yourself!

When testing metals, the rate at which the specimen is being strained can affect important material properties, and thus affect important results such as offset yield and even tensile strength. Major metals testing standards such as ISO 6892 or ASTM E8 specify certain strain rates at which tests should be run. Pull a metal specimen at a constant rate throughout the test…sounds easy enough right?

In fact, achieving a constant strain rate can be difficult due to system compliance. As a continuously yielding specimen is being pulled in tension, the entire load string (frame, grips, load cell, adapters, etc.) are also deforming due to the typically high load seen in metals testing. Once the specimen yields the stiffness of the load string changes again and since the load plateaus, the deformation of the load string also plateaus. Once this occurs, all of the movement of the frame crosshead is translated into specimen elongation.

It is important for a testing machine to compensate for the changing stiffness seen in the load string throughout a test. If you are interested in learning more about strain control and how it can affect your testing results, please leave a comment below. 

Also, please stay tuned to for a video showing a strain control test in accordance to ISO 6892! Read more

Why Automatic Extensometry?

When testing the mechanical properties of a specimen, being able to accurately and repeatedly measure the strain the specimen sees is crucial. Many testing standards require that a separate device, commonly known as an Extensometer, be used to ensure the most accurate strain data possible.

So before you run a test, attach a device onto the specimen to measure the strain… that doesn't sound bad, now does it? Now think about having to test hundreds of specimens. Odds are, manually attaching an extensometer to every specimen will add significant time to your testing process. This is where an automatic extensometer should be used.

Automatic extensometers allow the user to simply place their specimen into the grips that are being used during the test and hit “start”. You don’t have to worry about attaching the extensometer and removing it once the test is done; it’s done for you! Automatic extensometers can save time, increase repeatability, and reduce operator influence on results.  If you are interested in automatic extensometry, check out the AutoX 750. Read more

What is R-Value?

R-value (also known as “Plastic Strain Ratio”) is a measurement of the drawability of a sheet metal. Simply put, it measures the resistance of a material to thinning or thickening when put tension or compression. R-value is a very important material property to understand for materials that will be formed into various shapes in their end use. When testing in accordance to popular standards such as ISO 10113 or ASTM E517, R-value is required.

There are two methods to calculate R-value, a manual version and an automatic version. We can assume that the volume of the specimen during the test remains constant to make the calculations easier (see below for details). The manual version of calculating R-value requires that the user measure the initial and final width and length of the specimen using calipers. The automatic method requires the user to measure the axial strain (as opposed to the strain along the thickness of the specimen) and transverse strain to calculate R-value.

The best range over which to calculate R-value is after yield point elongation and before ultimate tensile strength. The benefit of using the automatic method of measuring R-value is that the specimen can be pulled until failure, thus allowing the user to calculate properties such as ultimate tensile strength or strain at break. However, in the manual method, you are to pull the specimen beyond yield-point elongation, but the strain should not exceed the strain at ultimate tensile strength. Once you pull the specimen to this point, you are to unload the specimen to take the final measurements with your caliper. 

The equation to calculate R-value is shown below:

Since it is difficult to measure the thickness of the material as it is plastically deforming, we can assume that the volume of the specimen remains constant and thus:

To learn even more about R-value, check out ASTM E517 or ISO 10113.

Have any questions about plastic strain ratio? Leave a comment below and I'll respond to you soon~

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Plastics for Life

The interface between engineering and medicine has been expanding in recent decades - universities are now offering dedicated bio-engineering degrees. One particular application of modern engineering in medicine is the use of polymers as implants in order to aid in the healing process or replace damaged tissue.

Cranial Implants
Head injuries can be life-changing in a way that few other injuries can. Treatment can involve implantation of a skulls prosthesis, which is quite understandably a very delicate procedure requiring the utmost care and attention. However, it is also critical to select appropriate materials, for purposes of production and bio-compatibility. This is where plastics are proving to be extremely useful. In a recent research project, a team of dedicated professionals has created a process for manufacturing bespoke cranial implants with precision, speed and at minimal cost. The patient is taken for an MRI scan and the results are converted by a computer into a 3D model. Using a 3D production method like laser sintering, the implant is then generated. The whole process could take out a great deal of time and complexity involved in the surgical procedure.

Tracheal implants
Similar bio-technology is being used in throat repair. The Trachea is a long tube that connects the lungs to the mouth. When it becomes damaged, it can be very difficult to repair, and implanted materials can cause problems for the patient’s immune system. But using a 3D printed scaffold seeded with the patient’s own stem cells, an implant can be devised that is 100% compatible with the patient’s body. This technology is very transferable into many structures in the body, as the basic manufacturing technique is very flexible and the patient’s own cells are integrated into the implant. The bio-compatibility of the polymer structure ensures that the body integrates the implant, rather than rejecting it
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However, implantation of any foreign device is a risky procedure, and the doctors and engineers responsible need to be 100% certain of success before engaging in a procedure. Implants should be thoroughly tested on a quality basis in order to prevent deployment of deficient products.

With a combination of Tensile, Compressive, Fatigue and Impact machines (among many others), we have a slew of solutions for a variety of bio-mechanical tests. In particular, we have a well-established history in stent testing and experience with a number of other bio-medical applications. We also have a unique environmental control unit for testing at 37 degrees.

Why not contact us to find out about the testing capabilities that we can offer? Read more

Happy Holidays

To share a little holiday cheer, I've worked with some of our blog authors (aka application specialists) to put together a fun montage of testing various types of holiday materials from wrapping paper to ornaments to garland ....

Enjoy and happy holidays!

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Bluehill Updates

Parasar Kodati, our Bluehill^® Product Manager, wrote an exciting post on our Bluehill blog about new updates to the software, as well as announces a new product that makes managing and analyzing results so much easier.

Check it out~ Read more

Question from a Customer

Q. What is the difference between percentage of full-scale and percentage of reading?

A. The accuracy of a device describes how close a measurement is to the actual value. It is usually presented in one of two forms: percentage of full-scale or percentage of reading.

Percentage of full-scale, usually shown as %FS, is a fixed error, and therefore, has a greater influence at lower measurement values. For example, if a load cell has a 200 lbf capacity and an accuracy of 0.3%FS, the error is 0.6 lbf throughout the measurement range. Therefore, at a measurement of 20 lbf, the 0.6 lbf error is 3% of the reading. This is outside of the ASTM E4 requirement that measurements should be within 1% of reading.

Percentage of reading is usually shown as %RO. For example, if a load cell has a 200 lbf capacity and an accuracy of 1% RO, then at a measurement of 20 lbf the error is 0.2 lbf or 1% of the reading, and within ASTM E4 requirements. Devices that specify accuracy as a percentage of reading typically have a wider range of use, since this is a more difficult specification to meet. Read more