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!