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Introduction of ultrasonic hardness tester measurement theory

2021-11-17 11:20:51

Introduction of ultrasonic Hardness Tester Measurement Theory (UCI Method)

1 Principle introduction - Ultrasonic contact impedance method (UCI method)

1.1 UCI method

In 1961, Dr. Claus Kleesattel, an American, invented the UCI ultrasonic impedance measurement principle in order to solve the problems of long test time of desktop Vickers hardness tester, high requirements for the operator to observe the indentation, and unable to measure the site. The hardness tester designed by the ultrasonic impedance measurement principle is called ultrasonic hardness tester.

The principle of UCI ultrasonic impedance measurement is to use a spring to produce the test force required for Vickers hardness measurement. The test force is applied to the user's workpiece through a vibration rod. The front end of the vibration rod is inlaid with a 136-degree standard diamond indenter, which forms a Vickers diamond indentation on the workpiece. The diagonal length of the indentation corresponds to the Vickers hardness, and the diagonal length of the indentation corresponds to the frequency change of the vibration rod, so the hardness of the workpiece can be known from the frequency change of the vibration rod.

Because of the use of standard Vickers indenter and test force, the ultrasonic hardness tester can directly measure the vickers hardness of the workpiece. Ultrasonic hardness tester is a kind of Vickers hardness tester.

1.2 UCI hardness test

Hardness is measured by applying a fixed pressure to a calibrated vibrating rod fitted with a specific indenter, such as a Vickers diamond.

1.3 the calibration

Specific values for important parameters of the UCI instrument are verified by comparison with conventional benchtop hardness test results or with a calibrated set of hardness blocks.

1.4 Surface quality

Surface roughness, that is, contour arithmetic mean deviation Ra.

2. Importance and applicability

The hardness value of the material depends on the method used. To avoid creating new hardness scales, the UCI method converts the results to common hardness values such as HV and HRC.

UCI hardness test can only measure the hardness of the contact surface, and the results of a specific position cannot represent other positions, let alone reflect the information inside the material.

The UCI hardness test can be used on a variety of surfaces of large and small parts, including hard-to-reach locations such as tooth sides or gear roots.

3 UCI hardness test instrument and method overview

3.1 the instrument

UCI instruments usually include :(1) a probe containing a vibrating rod with an indenter at the contact end of the rod, which may be a vickers diamond conforming to E 92 and E 384 standards; (2) excitation device; (3) vibration detection device; (4) Data processing circuit; (5) Digital display screen, showing measured hardness value.

3.2 UCI probe

Probes come in many sizes: they typically have static loads ranging from 1 N to 98 N, with different lengths of sensor rods depending on the application.

3.3 Method Overview

In conventional bench hardness tests (e.g., brinell or Vickers tests as per E 10, E 92, and E 384), the hardness value is determined by an optical measurement of the pit area after a certain load is applied to the material and then unloaded. In the UCI portable test, the pit area is measured not by optical methods, but by measuring frequency changes in ultrasonic resonance. When UCI measurement is performed, the vibration rod in the probe is excited by piezoelectric ceramics for longitudinal ultrasonic vibration with a frequency of about 70kHz. This is the zero frequency, which occurs when the head is in the air.

A spring in the probe provides a specific load, and the vibrating end is pressed into the material under test, creating an elastic contact that causes the rod frequency to change. This change is related to the indentation area (the contact area between the indenter and the material). This area corresponds to the hardness of the material given the elastic coefficient, as shown in Formula 1.

The solid line represents the longitudinal amplitude without contact and the dotted line represents the longitudinal amplitude with contact

T is a piezoelectric sensor, R is a receiver, O is a vibrating rod, V is a pressure head, and M is the metal to be measured

Figure 1 schematic diagram of UCI probe

Formula 1

As a result, the frequency change on harder materials is relatively small because the indentation is shallower. The deeper the indentation, the greater the frequency variation, with slightly larger indentations representing moderate hardness. Similarly, dents in soft materials are larger.

When the corresponding measuring frequency of the hand-held probe triggers the internal switch or automatic probe loading for a certain period of time, the loading of a specific load is completed, the instrument continuously monitors the resonance frequency, calculates the frequency change, and finally displays the hardness value.

When the elastic modulus of the measurement system is constant, the frequency change is a function of the notch size for a particular indender, such as dimensional diamond.

Formula 1 describes the relationship between vickers hardness and Vickers hardness. δ F = frequency change; A = dent area; Eeff = effective elastic modulus (including indenter and elastic constant of the material under test); HV = Vickers hardness number; F = load.

3.4 Influence of elastic modulus

According to Formula 1, the frequency change is not only related to the contact area, but also to the elastic modulus of the contact material. For materials with different Young's moduli, calibration instruments are required. The calibrated UCI method is suitable for all materials with elastic moduli. UCI instruments are usually calibrated artificially for both non-alloy and low-alloy steels, i.e. hardness blocks following the E 92 test method. In addition, some instruments can be used for other metals, such as high-alloy steel, aluminum and titanium, through quick calibration in the field.

4 Calibration for other materials

Prepare blocks of hardness of A particular material whose hardness values have been determined by one of conventional bench methods such as Vickers, Brinell or Rockwell, see A 370. At least 5 readings are required to calibrate the average hardness value. Make at least 5 UCI measurements on the material under test according to the equipment instructions. By adjusting the displayed average hardness value to the previously measured hardness value, a calibration value can be obtained to measure the hardness of the material within the desired hardness scale and range. For hardness testing of different materials, some instruments allow you to store all calibration data and adjustment parameters as needed.

5. Compare with other hardness testing methods

In contrast to the traditional low-load hardness test, the UCI method evaluates the dent size electronically rather than optically. UCI method relies on elastic modulus and is a comparative measurement method. After removing the load, the UCI probe using Vickers diamond produced dents that were almost identical to those in conventional bench vickers tests at the same load. If loaded as specified in E 92 and a dimensional UCI indenter is used, the indentation can be measured by the optical method of the standard Vickers test. In such cases, special preparations or probe attachments are required to ensure the accuracy of the actual load.

6. Characteristics of UCI method

Directly test the vickers hardness of the workpiece, without the need for conversion like The Richter and Shore hardness tester. The conversion error is eliminated.

ø Fast test speed, the fastest number of 1S

Standard Vickers indentation, small indentation, less damage to the workpiece

ø The test results are related to the elastic modulus of the workpiece. The replacement material needs to be calibrated according to the elastic modulus.

ø Small size, light weight, easy to carry

Easy to implement online measurement

Can be used for field measurement and large workpiece measurement. Include hard-to-reach areas, such as tooth sides or the root of gears

ø When measuring the hardness of the contact surface, the results of a specific position cannot represent other positions, let alone reflect the information inside the material

The development of SCM technology makes it easy to calibrate elastic modulus.

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