THE UCI METHOD
12 The UCI method
The UCI method (Ultrasonic Contact Impedance) is successfully used in
hardness testing since many years.
A rod is excited into a longitudinal oscillation. The tip of the rod is equipped with
Vickers diamond. This diamond is pressed to the specimen with a discrete test
load. The test load F is mainly applied through a spring.
The rod oscillates with its self-resonant frequency which depends essentially on
its length. When the Vickers diamond penetrates the specimen, the oscillation
of the rod is damped. This causes a shift
f
of the resonance frequency, which
can easily be measured.
The damping of the rod and the resulting shift in resonance frequency depends
on the size of the area of contact between the diamond and the specimen and
therewith on the hardness of the material if the test load is constant. Beneath
the hardness, the elastic modulus of the material also affects the frequency
shift.
The hardness of the material can be calculated from the known test load, the
measured frequency shift and the material calibration factor (for taking the
elastic modulus into account).
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