What makes Ultrasonic Contact Impedance (UCI) a good method for testing hardness? 

Learn how the Ultrasonic Contact Impedance method can be used for hardness testing

Last Updated: September 18th, 2023 First Published :

  

The UCI (Ultrasonic Contact Impedance) method is a portable and versatile modern technique for testing hardness. It operates on the premise that the frequency of an oscillating rod changes when it penetrates a sample. The UCI technique is ideal for fine-grained materials, narrow components, and components with hardened surfaces.

Understanding How UCI Hardness Testing Works

Claus Kleesattel created the UCI method (Ultrasonic Contact Impedance), often known as the modified Vickers method, and it has been used in the metal-processing sector for more than 50 years. Testing can be performed in all directions, in confined places, and while working with challenging material geometries or hard-to-reach locations.

The UCI principle uses an oscillating rod to measure an object’s frequency to assess its hardness. The rod consists of a pyramid-shaped diamond set on one end of a metal shaft with a vibrating element. The diamond is pushed into the material by spring pressure to perform a measurement.

A diamond indenter, typically in the shape of a Vickers pyramid, is affixed on a vibrating rod and pressed into the material’s surface with a predetermined amount of force.

Frequency Shift Measurement: As the diamond penetrates the material, there is a shift in the resonant frequency of the rod. This frequency shift is directly proportional to the material’s toughness.

The hardness value is calculated based on the frequency shift measurement. Depending on calibration and device capabilities, advanced UCI devices can convert and display the result in numerous hardness scales, such as Vickers, Brinell, or Rockwell.

 During mobile hardness testing, the manual measurement probe is placed manually and pressed on the material. The measured hardness values will be digitally displayed and recorded after one second. The operator can obtain even more exact results while using smaller test forces by using motorized measurement probes. 

Knowing the Science Behind UCI Method

Ultrasonic waves, contact mechanics, and material resistance form the foundation of the Ultrasonic Contact Impedance (UCI) technique for portable hardness testing. 

1. Ultrasonic Waves and Resonance Ultrasonic waves are sound waves with frequencies above the audible range of humans. When an object is subjected to an oscillating force, it vibrates at its resonant frequencies, which are its natural frequencies. A piezoelectric element generates ultrasonic vibrations in a rod using the UCI method.

2. Contact Mechanics and Diamond Indenter: A Vickers diamond indenter is located at the apex of the vibrating rod on a UCI probe. When this indenter is pressed against the surface of a material with the specified force, a small indentation is produced. This indentation’s depth and shape are determined by the material’s hardness and the force applied. Due to its pyramidal shape, the Vickers diamond is especially sensitive to changes in material hardness, resulting in distinct indentation geometries for various hardness values.

3. Change in Impedance: As the diamond indenter penetrates the material, the contact system’s stiffness or impedance changes. This change affects the vibrating rod’s resonant frequency. Harder materials result in shallow indentations, which increase the rigidity of the system and consequently the resonant frequency. In contrast, gentler materials produce deeper indentations, which decrease the resonant frequency.

4. Measurement of Frequency Shift: The instrument continuously measures the resonant frequency of the vibrating rod. The variation in this frequency caused by the indentation is directly proportional to the material’s hardness. Modern UCI devices use sophisticated electronics to detect even minute frequency changes, enabling accurate hardness measurements.

5. Calibration and Conversion: Through calibration processes, the frequency variations can be mapped to known hardness values. Once calibrated, the UCI device is able to convert frequency variations into hardness values on multiple scales (e.g., Vickers, Rockwell, Brinell). Calibration ensures the accuracy of measurements across a variety of materials and conditions.

Know How UCI Differs From Other Hardness Testing Methods 

The Ultrasonic Contact Impedance (UCI) method of hardness testing offers several advantages over traditional methods. It differs from Rockwell, Vickers & Micro-Vickers, Brinell, and Shore hardness testing in terms of principle, indentation size, speed, and application. Rockwell tests involve applying a preliminary force followed by an additional load, while UCI uses ultrasonic vibration and measures the frequency shift upon indentation. UCI’s indentation is often microscopic and less invasive, making it suitable for finished products or precision components.

UCI also uses a pyramid-shaped diamond indenter under a specific load, but the hardness value is determined by measuring the diagonals of the indentation. UCI’s indentation size is typically smaller than Micro-Vickers, making it ideal for on-site measurements and difficult-to-access areas. Brinell testing uses a spherical indenter and a significant load, while UCI uses frequency shifts upon indentation. Brinell leaves a much larger indentation compared to UCI, which is often used for bulk metals with a coarse or non-homogeneous grain structure.

UCI is primarily used for metals and hard materials, while Shore hardness is mainly used for elastomers and polymers. UCI devices are compact, ideal for in-field measurements and testing large or assembled components. They leave a nearly invisible indentation, allowing testing of finished or coated products without marring them. UCI can be applied to surfaces in any orientation and provides almost instantaneous results, making it ideal for high-throughput or quick on-site evaluations.

Key Applications of Portable Hardness Testing by UCI Method 

The UCI method has key applications across various fields, which are – 

  • Quality control is a process that involves making immediate assessments of product quality in industrial settings.
  • In the context of in-service inspections, hardness testing is conducted on structures, machinery, and equipment without the requirement of disassembly.
  • Heat Treatment Verification: The evaluation of heat treatments’ efficacy by the measurement of surface hardness.
  • Forensic and failure analysis uses the determination of component hardness in order to investigate forensic cases or analyze failures.

Advantages of Portable Hardness Testing by UCI Method 

  • Portability: UCI devices exhibit a compact and lightweight design, rendering them highly suitable for conducting in-field measurements and testing of sizable or installed components.
  • Minimal surface preparation refers to a testing method that involves creating a small depression, which is typically imperceptible to the unaided human eye. This technique enables the examination of finished products.
  • The instrument exhibits versatility as it may be effectively utilized in a wide range of applications, encompassing the assessment of hardness in welds, pipes, tanks, frames, and small components.
  • Velocity: The examination is characterized by its expeditious nature, often requiring only a few seconds, hence facilitating the quick assessment of numerous areas.
  • The adaptability of this technology allows for its application on surfaces that are vertical, horizontal, or overhead, providing it with a distinct advantage over conventional tabletop hardness tests.

Limitations of Portable Hardness Testing by UCI Method 

  • Although UCI is somewhat less stringent than alternative methods, it still imperative for the surface to have a sufficiently smooth texture and be devoid of any contaminants such as dirt, oil, or oxide coatings in order to provide precise measurements.
  • The process of calibrating is of utmost importance in order to retain precision.
  • UCI may not be an optimal choice for materials that are coarse-grained or possess diverse compositions.

How to select the right Portable hardness testing Provider

While selecting the  Portable hardness testing provider it is important to have a checklist on parameters like:

  • Experience: The  Portable hardness testing provider should have experience in dealing with different types of materials and applications. 
  • Accreditation: The provider should be accredited by the American Society for Testing and Materials (ASTM) or the International Organization for Standardization (ISO)
  • Cost: It is one of the pertinent questions to ask while finding a  Portable hardness testing provider. 

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Future Trends in  Portable Hardness Testing

  • The field of portable hardness testing is undergoing significant advancements, with the development of compact, lightweight, and user-friendly instruments. These instruments are being integrated with digital technology, such as touchscreen interfaces, wireless connectivity, and mobile apps, to improve accuracy and precision. They are also multi-functional, offering a broader range of material characterization in a single device. 
  • Advanced data analysis algorithms and software are being implemented, and they are being designed for use in challenging environmental conditions. 
  • The testing methods are also being optimized for non-destructive evaluation (NDE) applications. The testing equipment is also being customized to meet specific industry requirements and user preferences. The field is also focusing on sustainability.

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