Hardness Tests Verify Material Properties
Hardness tests have been used for decades to verify material mechanical properties, validate a process, and give an indication of a material's wear resistance.Dynamic loading rate response of materials has been a focus of researchers for many years but recent events demand dynamic accurate material properties.
"A common problem that most people can relate to where dynamic material properties are concerned," said Adam Loukus, VP of REL, "is when we drop our cell phone and the screen glass shatters. We wonder what can be done to avoid this issue. Many solve this by putting an 'ugly' protective case around a very elegantly designed phone. This defeats the efforts of hundreds of engineers that have spent countless hours designing a phone and interface that feels, looks, and interacts with the user in the best possible way.”
Stronger, tougher glass has been developed by researchers and implemented in the latest smart phones, but there is still room for improvement.
Dynamic Hardness testing can directly impact this issue. Glass manufacturers can use this procedure to see the effects of their processing techniques and quantify these effects to make better, tougher glass.
Dr. Ghatu Subhash, then professor at Michigan Technological University, (currently at the University of Florida) developed a dynamic hardness procedure using many of the components of a Split Hopkinson Pressure bar (SHPB) or Kolsky Bar. Dr. Subhash authored an ASTM specification around this unique method to quantify dynamic hardness values of many common engineering materials.
REL’s SURE-Test platform
is designed to be flexible to handle a variety of loading conditions, so we developed a Dynamic Hardness module to add to the battery of tests currently available.
- Compression and Tension SHPB tests
- Direct Impact tests
- Momentum Transfer tests
- Servo mechanical testing (low to mid strain rates)
- Dynamic 3-point bend test
- Dynamic Pipe Burst test
- Acceleration sled testing (over 50000 g's)
This dynamic hardness test uses many of the components of a momentum trapped compression SHPB system. The main difference being a Vickers indenter mounted on the end of the incident bar. A view of this setup is seen in the images below.
A very similar signal process method to SHPB compression testing is used in REL’s SURE-Pulse Software to determine the dynamic hardness of a material. The loading duration and indent depth can be determined from the incident strain gage (or measurement of the indent under a microscope). The load on the sample is determined from a gage on the transmission bar or a load cell mounted behind the sample.