Ultrasonic Contact Impedance Hardness Testing

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UCI hardness testing based on the invention of Claus Kleesattel is particularly suitable for applications in which classic hardness measurement using an indenter is not possible or impractical. These include incoming goods inspection, production control and maintenance on integrated components.

The measurement device determines the comparative hardness value of a test sample based on the measured change in frequency of the vibrating rod. The values of the elastic modulus E and cross-contraction number can be calculated out of this.

Frequency Change

A mechanical resonator vibrates when it is brought into contact with a surface. This vibration changes the resonance frequency (resonator impedance) and displacement magnification. The resulting difference in frequency between the test force and the stand-by frequency f0 of the resonator is proportional to the hardness of the material.

The UCI method, developed by Claus Kleesattel, allows for an entirely new approach to hardness testing: it is non-destructive and comparatively easy. It is suitable for a large range of materials and products: small forgings, cast materials, welds, heat-affected zones, ion-nitrided stamping dies and molds, thin-walled parts and bearings, etc.

Qualitest offers a portable instrument using the UCI method, which is called SONODUR 3. Its mass storage capabilities and simple calibration make it ideal for applications where classic, destructive methods such as Brinell, Rockwell or Vickers are not possible or impractical. This includes incoming goods inspection, production control and maintenance on integrated components. It also enables fast, selective decision-making on-site.

Young’s Modulus

Young’s modulus is a property of the material that determines how easily the material can be bended or stretched. and young’s modulus and shear modulus G are related by Hooke’s law which describes the linear relationship between stress and strain in elastic materials.

The adjustment of a UCI hardness tester to a specific material requires determining a conversion formula based on the frequency change df of the resonator and a number of known values such as the modulus of elasticity E, the rod property k, the contact area A, and the transverse contraction ratio g(m) of the test sample.

These values are stored in a material table. Using this, an instrument adjustment to a new material is quickly possible via comparative measurement and only needs to be performed once. This is especially advantageous for materials with a deviating Young’s modulus. Such a material is called an anisotropic material, and its elastic constants sij (compliances) or cij (stiffnesses) depend on the lattice symmetry.

Contact Area

The resulting frequency shift is measured and converted into hardness values (Vickers, Leeb, Rockwell, Brinell) in real time. This results in a measurement that is a superficial determination of the material hardness condition at the location of contact.

Ultrasonic testing is non-destructive to the test sample and therefore can be used on components with complicated geometry, difficult-to-reach areas or materials that are sensitive to traditional methods such as abrasion or impact. This allows for a much faster and more accurate evaluation of the component or surface in comparison with other testing techniques.

The UCI hardness tester consists of a probe with a slim pin-shaped measuring tip and an electronic device to assess and save the test results. This makes the tester very easy to handle and enables tests to be performed quickly, in all directions. It is also possible to take measurements at positions that are difficult to access, for example on tooth flanks or the roots of gears.

Measurement Force

The measurement force is variable depending on the sensor. This enables the device to measure different surface conditions, for example, a lubricated or unlubricated surface, and also allows for the conversion into common hardness values HV, HB, etc. Furthermore, the need for a third hardness comparison plate which is necessary when using the conversion formula used until now has been omitted.

The device is easy to use and has the flexibility of measuring a wide range of materials. This is important as the production process often involves multiple steps of hardness testing and quality assurance. The device is non-destructive and allows the operator to accurately test parts without damage, thereby saving discarded or scrap parts and manufacturing costs due to subsequent inspections.

It is also possible to adjust the instrument for materials with a deviating Young’s modulus via a comparative measurement. This adjustment can be carried out with the help of a material table and only needs to be done once.

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