Portable Hardness Testers: Rebound vs. Ultrasonic Based Testers

1 st in 4 Part Series: Selecting the Right Mechanical Testing Instrument

Selecting the right testing apparatus can be confusing.  In this four part series we'll examine what you need to consider when choosing an instrument for your application.  

Portable Hardness Testers: Rebound Based Tester vs. Ultrasonic Based Testers

Many clients call with questions regarding the selection of a portable hardness tester.  When deciding between Ultrasonic or a Rebound type testers there are a few parameters to consider.

Techniques

Each instrument gathers information on hardness from a certain zone within the sample.  Ultrasonic Testers, such as the SH-21 Handy Portable Tester, use a Vickers diamond to penetrate the surface to a predetermined load. Information on material hardness is determined from an area closer to the surface; however, penetration depth will depend on the specific model, with loads of 10N, 20N or 40N. 

Rebound Testers, such as the TH150 and TH130, throw a steel ball to the surface and measure the height of the rebound, so hardness depends on the material composition deeper inside the sample. 

Roughness

Rebound testers are not recommended for use on samples with surface roughness over 3.2 µm.   Standard deviation between measurements begins to increase when samples have a surface rougher than 0.8 µm and becomes too high at 3.2 µm (assuming a load of 20N, at 40N the maximum is 5 µm).

Thickness

Sample thickness also influences the hardness tester decision.  Rebound Testers require sample thickness of <13mm, while the SH-21 is suitable for <4mm.  However, the TH-160 Rebound Tester works on thinner samples down to 1 mm.  The TH-160 uses a smaller ball with a lower velocity as compared to the TH-130 and TH-150 models. However, generally repeatability and accuracy are not as high as the TH-130, TH-150 or SH-21 models.

Material Composition

Another important point when selecting an instrument is determining if the sample surface is different than the bulk.   If there is no difference, the limits explained above will work fine.  If they are different, it is important to decide where you want the information to come from.  Understanding this will explain why hardness variation exists between various tools and techniques.

Should I use a handheld tester?

Many companies use bench top systems as their quality control tools and want the ease of a portable tester for faster, on-site or in-field use.  No matter what type of hardness tester you prefer it is important to understand that the standard bench top Vickers & Rockwell Testers also get their hardness information from a specific area on the sample (as we will see in the next segment).  When surface and bulk are the same, comparative values are easy to be used but when they are different comparing portable testers to bench top systems can be a challenge, or even impossible.