Theoretically, hardness is defined as the resistance offered by the metal to plastic deformation. The term may also refer to stiffness or resistance to abrasion or cutting.
Hardness testing can be defined on a macro, micro, or nano scale, according to the forces applied and displacements obtained. Testing of macro-hardness of materials is a quick and simple method, wherein mechanical property data for the bulk material is obtained from a small sample.
Such testing is also widely used for the quality control of surface treatments processes. However, where materials have a fine microstructure, are multi-phase, non-homogeneous, or prone to cracking, macro-hardness testing will be highly variable and will not identify individual surface features.
Therefore, here micro-hardness measurements are more appropriate. Micro-indenters works by pressing a tip into a sample metal and continuously measuring the reaction of the metal to the applied load, penetration depth, and cycle time.
Nano-indentation testing is the measure of the stiffness by indenting using very small, on the order of 1 Nano-Newton indentation forces, and measuring the depth of the indention that was made on the metal.
The metal industry uses three types of tests with accuracy:
- Brinell Test
- Rockwell Test
- Vickers Test.
Portable Hardness Tester
The inconvenience of going to a laboratory has been sorted almost completely by portable hardness testers, which have been developed so that it permits on location measurements by offering quick and economical supplements to conventional, stationary testing machines.
There are two main methods adopted worldwide, one is the UCI (Ultrasonic Contact Impedance) method, in which the methodology is to measure the frequency shift of a resonating rod with a Vickers - diamond tip that occurs when the diamond penetrates into the test material by applying a specific test load. The frequency shift is evaluated and is electronically converted to a stiffness value, which is displayed on the LCD.
Second well-known principle for portable testers is the rebound method. In this method, the device measures the velocity of a propelled impact body, directly before and after the impact onto the test material's surface. The ratio between both velocities indicates the stiffness of the material that can be converted into different scales by using conversion tables stored in the instrument for different materials.
Many devices combine these two most successfully applied testing principles in one instrument. Whether one wants to use the UCI principle or the dynamic Rebound testing method, all one needs is just a single instrument with all probes and impact devices can be plugged in for use.
There are many products available in the market and one can easily choose as per his or her requirement. The portable tester is very easy to use and requires no special knowledge in order to operate the instrument.
Portable hardness tester uses are immense and it comes handy because of its special features like quick measurement, which takes hardly a few seconds. It has built in memory for data and calibration points.
ASTM technical Committee A01 on Steel, Stainless Steel, and Related Alloys, has developed standards for portable hardness testing. While both methods - UCI as well as Rebound are successfully used for testing and work for many on-site applications, there are limitations concerning the kind of material under test, along with its size and weight, respectively.
Furthermore, because of the influence of Young's Modulus (which is a material property that describes its stiffness and is therefore one of the most important properties) most conventional testing methods do not allow to measure different materials without firstly calibrating or adjusting the instrument.
Mobile hardness testing instruments will not replace the conventional bench-top machines, but nevertheless, they solve plenty of the issues, however, each method is limited - more or less - to a specific application area.