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Technical Report on Vibration-Damping Materials and Their Performance Measurement 4

12. Centralized Vibration Method

This testing method is often referred to as the mechanical impedance method in Japan and is adopted in the JIS standards for damping steel plates mentioned earlier; however, such testing methods are not common in Europe or the United States. The diagram above shows a loss coefficient measurement system using the central excitation method, which is commonly used in Japan, similar to the cantilever beam method.

In the United States, the impedance method refers to a technique that uses block-shaped damping materials to measure impedance and determine the loss coefficient, as shown in the next figure.

Recently, the central vibration method has been introduced in SAE 920406.

1. Characteristics of the Central Vibration Method.

strengths

  • Capable of measuring up to high frequency bands.

  • Compared to the cantilever beam method, there is a higher degree of freedom in the dimensions of the test specimens.

shortcomings

  • The testing device is complex.

  • The influence of the support department is unclear.

test piece

  • Strip-shaped, block-shaped.

test procedure

  • Sine or random vibration, the sweep of sine vibration is slow.

  • Responses are recorded using a frequency response analyzer, FFT analyzer, and level recorder. Pay attention to frequency resolution, truncation, reading errors, etc.

Assumptions and precautions during the exam.

  • It is desirable for the test specimen dimensions to have an aspect ratio of 20:1 or greater.

  • The test is to be conducted within a linear range. Pay attention to the vibration amplitude. It is desirable that the vibrations are at a level that is not visually noticeable.

  • The excitation force is always constant.

  • Be careful that bending vibration does not turn into torsional vibration.

  • Measurements should be limited to near the resonant frequency. → Range of fc / 10
    It is desirable to have more than 20 measurement points within the half-width.

  • The ideal adhesive width for the knife edge is ideally 'zero', but it is desirable to be less than 1/200 of the sample length (for a 100 mm specimen, this means less than 0.5 mm).

  • Mass cancellation of load cells.
    The effect of mass cancellation becomes significant when the loss coefficient of the test specimen is less than 0.03.

  • Zoom analysis must always be conducted.
    When the loss coefficient of the test specimen is below 0.03, the impact of whether or not to perform zoom analysis becomes extremely significant.

2. Impedance Head

Structurally, it can be considered as a combination of two sets of sensors: a load detector and an acceleration detector. It is essential as a detector for the central vibration method. It is fixed on the vibrator, and the measurement sample is vibrated through the impedance head, measuring the impedance at the vibration point. The impedance head should ideally be of the charge sensitivity type, considering it will be placed inside a constant temperature bath. The sensitivity of the force sensor and acceleration sensor should be at least 100 pC/N and 1 pC/ms-2, respectively, considering that the weight of the test material is at most about 100 g and the vibration force is at most about 10 N. If using a voltage sensitivity type, attention should be paid to the upper limit of the operating temperature, and the sensitivity of the force sensor and acceleration sensor should be at least 100 mV/N and 1 mV/ms-2, respectively. Additionally, since there may be occurrences of topple mode, it is preferable for the height of the impedance head to be as low as possible, and the lead wire extraction should be symmetrical, considering the balance in the front, back, left, and right, with short connectors being desirable.

3. Resonance and Anti-resonance of the Central Amplification Method

In the central vibration method, when measuring impedance (F [force] / V [velocity]), as shown in the figure below, resonance frequencies and anti-resonance frequencies appear alternately. At the resonance frequency, the applied force is very small, and the specimen vibrates significantly, while at the anti-resonance frequency, the applied force is large, but the specimen hardly vibrates.

Looking at the vibration modes, they will be as shown in the next figure. In this way, the vibration modes are completely different at the resonance frequency and the anti-resonance frequency.

Furthermore, the second central position of the anti-resonance in central vibration differs from the second resonance mode of two-point suspension due to different boundary conditions, and although they may seem similar, they are entirely different, sharing the same boundary conditions as the resonance mode of a cantilever that is twice the length.

The figure below shows the measurement results of the loss coefficient of a damping steel plate, representing cases with different specimen lengths and test temperatures. As shown in the figure, it can be seen that there are differences in the resonance frequency and anti-resonance frequency.

As shown in the figure, for frequency-dependent upward-sloping measurements, the anti-resonance side is larger, while for downward-sloping measurements, the resonance side is larger. Additionally, this phenomenon is not observed in the two-layer type (Oberst beam).

4. Error factors of the central vibration method.

  1. Problems in test specimen production → Inconsistencies in physical properties, dimensional errors, and poor adhesion.

  2. Type of adhesive → Pay attention to the thickness of the adhesive; some are insensitive to temperature changes.

  3. Temperature control → Pay attention to the uniformity of the temperature distribution within the test specimen.

  4. Parser error → Noise removal from response signals, sweep speed, etc.

  5. It is necessary to reduce energy loss in the support department.