Back to Basics: Hardness Testing 101

Consider ultrasonic contact impedance for nondestructive inspection of components.

Hardness testing is one of the most popular, reliable and yet diversified material testing methods. Most hardness testing is performed by using the Rockwell, Brinell or Vickers method. When selecting the correct method there are a number of criteria to be considered, including test load, hardness scale, accuracy of results, adaptability to specimen shape, budget and portable or tabletop configuration.

The ultrasonic contact impedance (UCI) method allows operators to test the hardness on large or small components at various locations. It can be used to make hardness measurements on positions difficult to access, such as tooth flanks or roots of gears.

The testers that adopt UCI technology provide quick and precise measurements of metal hardness, plated coating (chrome) and mechanical strength estimation. The UCI method hardness testers provide the hardness ranges of main hardness scales, such as HB, HRC, HV, as well as HRA, HRB, HSD scales and tensile strength.

When using the UCI method, the diagonals of the test indentation, which have to be known in order to determine the Vickers hardness value, are not evaluated optically, but the indentation area is electronically detected by measuring the shift of an ultrasonic frequency. A UCI probe typically consists of a Vickers diamond indenter attached to a metal rod, which is subjected to oscillation by piezoelectric transducers.

After 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, or 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. Thus, the hardness value is determined by the frequency shift within seconds. This method of hardness testing is standardized by ASTM A1038.

The instrument that uses this technology constantly monitors the frequency and instantly displays the hardness value based on the difference in ultrasonic vibration frequency. Even though the UCI test procedure is slower than the dynamic impact method, it is portable, easy and accurate. UCI testers are not restricted to large mass and these units can test metals as thin as 1 millimeter and at a hardness value as low as 20 HRC (75 HB). They also can perform hardness tests on larger, harder metals, as well.

Another reason for the rise in popularity is due to the fact that the UCI method is categorized as nondestructive. The UCI method is intended not to replace traditional Vickers testing, but rather to complement it with a quick, field portable test method.

Additional changeable sensors of different design for measuring in slots, blind holes and in other areas are available from several manufacturers.

The tester has a flexible memory for recording results and allows the stored results to be transferred to a PC at a later stage with the help of software.

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