Ultrasonic Contact Impedance Hardness Testing

The non-destructive UCI hardness tester with a vibration bar with a Vickers diamond detects the material contact resistance without touching the test object. The frequency shift of the vibrational sensor rod relative to its stand-by frequency is exactly measured after a predetermined test force has been applied and the corresponding Vickers hardness value can be determined immediately.

Theoretical Background

Ultrasonic Testing (UT) uses high-frequency sound energy to evaluate various materials and detect flaws. It can also be used to measure the dimensions and surface thickness of an object. It is non-invasive and provides accurate results.

In this method, the vibration bar with a Vickers diamond at its end is pressed against the test material and a mechanical resonator in the probe produces longitudinal vibratory frequencies in the test material. These frequencies can be measured and the hardness value can be determined from the resonance curve generated by each individual electrode in the measurement system.

The UCI method does not produce the large short-term force of rebound testing, minimizing the risk of damage to the workpiece. It can be used on thin and lightweight objects, or on surfaces with a complex geometry. It can be used on surfaces with a textured finish or coating and is suitable for ion-nitride stamping dies and molds, welded inspection of the heat-affected zone (HAZ) and other challenging component geometries.

Objects to be Tested

The measurement probe is placed manually on the object to be tested. After one second the measuring series is automatically saved in the device so that the operator can evaluate and record the hardness value.

At the first contact point of the test sample with the probe, the probe generates a frequency jump and changes in vibrational amplitude due to dynamic coupling impedance. Once the test force is reached, this change in vibrational amplitude corresponds to the measured hardness value of the sample.

The UCI method offers significant advantages over other surface hardness testing methods. It allows for quick inspection tasks on serial parts after heat treatment or surface processing, weld inspection and coating hardness measurement. This significantly simplifies and accelerates the test process. Moreover, the measurement results can be reliably compared to other standard hardness tests. For this reason the UCI method is particularly suitable for quality assurance in mass production. The portable hardness testers NOVOTEST T-U2 and T-UD2 based on the UCI method are ideal for such applications.


There are many different methods for hardness testing: Brinell, Rockwell, Vickers and Leeb. These testing methods are often time-consuming, expensive, and require special training of the technician. Moreover, these tests are destructive and may damage the sample.

The UCI method, as used by NOVOTEST mobile hardness testers, provides a quick and reliable test. The measurement probe, a vibration bar with a 136-degree diamond at the end (also known as a ‘Vickers’) is pushed onto the material by hand or motorized depending on the model of the tester.

The measured hardness values are displayed digitally and saved immediately. The result is a hardness value that can be converted to all common hardness scales. The measured values are therefore comparable to those of the traditional test methods, but without damaging the material. This is a significant advantage over the other tested methods, especially in difficult-to-reach areas and challenging materials geometries. The acoustic measurements are also highly reproducible.


The UCI method works in a similar way to classic Vickers testing: a diamond at the end of a vibrating rod is depressed into the material and the base frequency changes. This change in frequency is subsequently converted into a hardness value.

The adjustment is carried out on MPA-calibrated steel hardness reference blocks. The measured frequencies of the test specimens are compared with the adjustment on the steel block, and an appropriate correction is determined. This adjustment number CAL is then entered into the UCI system and becomes the basis for all subsequent measurement results.

The result is a UCI-based Vickers hardness value of HVUCI. This is correlated with other hardness scales or tensile strength using conversion functions (see the SONODUR 3 material table). The resulting values are then displayed on the screen. This allows quick decisions on-site in areas where classic hardness testing is impractical, such as in incoming goods inspection or production control.

Mr. Yasir Asif at strongestinworld is team member who loves to write informational articles, find information and share the learning with the community.

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