Hardness Test of UCI

(Ultrasonic Contact Impedance) Method

As in standard Vickers or Brinell hardness testing, the question as to the size of the test indentation in the material generated by a certain test load also arises in Vickers hardness testing according to the UCI (Ultrasonic Contact Impedance) method.

However, the diagonals of the test indentation, which have to be known in order to determine the Vickers Hardness value, are not evaluated optically as usual, but the indentation area is electronically detected by measuring the shift of an ultrasonic frequency.

A UCI probe typically consists of a Vickers diamond attached to the end of a metal rod (Fig. 1). This rod is excited into longitudinal oscillation at about 70 kHz by piezoelectric transducers.

When the test load is applied, a frequency shift occurs as the diamond penetrates into the material.

This frequency shift will become greater when the test indentation becomes larger, that means when the diamond penetrates deeper into “soft” material. Analogously, the smallest frequency shift is produced by hard test materials; the diamond penetrates only slightly into the material and leaves a small indentation.

This is the secret of UCI hardness testing: the frequency shift is proportional to the size of the test indentation produced by the Vickers diamond. Equation (1) describes this basic relation in comparison to the definition of the Vickers hardness value.

To carry out the UCI principle, a probe containing a rod with a Vickers diamond attached to the contact end is resonated by piezoelectric ceramics at an ultrasonic frequency.

A spring applies the load and the frequency of the rod changes in proportion to the contact area of the indentation produced by the Vickers diamond. Therefore, the hardness value is not visually determined by the diagonals of the indent, but by an electronic measurement of the frequency shift within seconds.

The instrument constantly monitors the frequency, performs the calculation and instantaneously displays the hardness value.

The frequency shift, nevertheless, also depends on the Young's modulus of elasticity, which is a material constant. For the practical application of the UCI-method, the Young's modulus therefore has to be considered. The instrument has to be calibrated when the hardness of different materials with different values of the Young's modulus has to be determined.

After completing the calibration, the UCI method can be used for all materials showing this modulus of elasticity. When being manufactured, the probes are calibrated on low- alloyed or unalloyed steels; however, modern test instruments can be calibrated quickly, also at the test location, to other materials as well, such as titanium or copper.