About vibration-damping materials and their performance measurement 3

In Japan, the main standards include JIS K6394 and JASO M306, M329, while internationally recognized standards include ASTM E756, BS AU125, DIN 53440, MIL P22581, SAE J671, and the international standard ISO 2856. Recently, in 1992, the SAE standard was partially revised in the United States, and in 1994, JIS G0602 concerning vibration-damping steel plates was established in Japan. These standards propose or recommend various testing methods to evaluate vibration-damping performance, depending on the differences in viscoelastic materials and the sizes of the test specimens. A representative testing method is the cantilever beam method, adopted in ASTM, DIN, and JASO standards, which uses a long rectangular specimen called a strip and fundamentally measures the half-value width to evaluate vibration-damping performance. Additionally, there is a method for measuring the vibration damping rate using a two-point support method, where both ends of the strip specimen are supported by knife edges, as seen in BS standards. Other methods include measuring the vibration damping rate by applying impact excitation or single-frequency excitation to the test panel, as in MIL and SAE standards, and the impedance method, commonly used in Japan, which involves applying sinusoidal or random excitation to the center of the specimen and measuring impedance at the drive point to evaluate vibration-damping performance. Furthermore, there are commercially available testing devices that utilize the relaxation phenomenon exhibited by viscoelastic materials to assess vibration-damping performance. However, the measurement frequency range of these devices is typically around several hundred Hz.

The cantilever beam method, central vibration method, two-point suspension, and two-point support method are representative testing methods for loss coefficients. The test specimens used in these methods are often of two-layer (bonded type) or three-layer (sandwich type). Here, we present the general characteristics of the cantilever beam method and central vibration method, the testing systems, error factors during testing, and the results of loss coefficient measurements.

It is a testing method adopted by DIN standards, ASTM standards, JASO standards, JIS standards, and recommended by SAE.

Generally, the choice of test specimens is determined by the differences in the magnitude of the Young's modulus of damping materials. Although a unique relationship between the Young's modulus of these damping materials and the measurable frequency range of the specimens cannot be established, a rough idea can be illustrated as shown in the figure below.

The measurement of loss coefficients is a very delicate test, and even when using carefully prepared specimens, it is often the case that the loss coefficients and resonance frequencies differ slightly. This is thought to be due to subtle differences in the physical properties of the damping materials and substrates used in the tests, as well as fabrication errors of the specimens. Therefore, to minimize these errors, it is effective to conduct tests using about three specimens and to calculate the average values of the obtained loss coefficients and resonance frequencies. The figure below shows the time variation of the loss coefficient. As shown in the figure, immediately after changing the temperature, the entire specimen has not reached the measurement temperature, so the loss coefficient is unstable. In the case of this damping material, it stabilizes in about 2 hours. The figure below shows the relationship between the loss coefficient of the specimen determined by the cantilever beam method and temperature, as well as the relationship between the loss coefficient and frequency.

The non-contact electromagnetic vibrator is mainly used in the cantilever beam method, and its structure is as follows.

It is advisable to wind coils with relatively thick wire (about 0.1 mm in diameter) with fewer turns (around 1000 turns). Winding more than this increases the DC resistance and reactance, making it ineffective for high-frequency vibrations. When considering the use of commercially available audio amplifiers, it is good to aim for a DC resistance of about 100 Ω when winding the coil. In this case, the reactance is expected to be around 20 mH. Additionally, the permanent magnet must be at its maximum value within the alternating magnetic field of the electromagnetic vibrator. If the maximum capacity of a non-contact electromagnetic vibrator is to be generated, there may be a lack of DC magnetic field. Furthermore, when vibrating small and lightweight test specimens, the DC magnetic field may be too strong. In each of these cases, it is possible to replace the permanent magnet with an electromagnet (by winding another set of coils) and apply a DC voltage.

Structurally, it is exactly the same as a non-contact electromagnetic vibrator. The number of coil windings and the wire material should also be exactly the same. Especially when inputting this signal directly into an FFT analyzer without using an amplifier, it is better to have a low output impedance. As mentioned regarding the non-contact electromagnetic vibrator, winding many thin wires is meaningless. The output is velocity-proportional, making it a user-friendly sensor capable of measuring up to high frequencies. However, it is important to be cautious when using it close to a non-contact electromagnetic vibrator, as there may occasionally be interference.

The principle of operation is as follows.

The accuracy between the sensor and the target is very good at about 0.1%, but there are practical difficulties due to the ratio of the sensor diameter (target size) to the distance with sensitivity being about 10:1 (a sensor size of 1 mm requires a target size of about 10 mm). Additionally, the target must be a strong dielectric (such as metal). Furthermore, due to its principle of being a displacement sensor, it has the disadvantage of being difficult to measure high frequencies. A benefit is that it does not interfere with electromagnetic exciters.

The widely used accelerometer is the piezoelectric type that utilizes the piezoelectric effect. Especially when it is necessary to measure up to high frequencies, it is currently essential to use piezoelectric accelerometers. Among various vibration sensors, it is the most user-friendly, but its weight often becomes an issue. The weight ratio of the test specimen to the accelerometer should ideally be less than 100:1 for loss coefficient and Young's modulus measurements. Additionally, care must be taken in handling the lead wires to ensure they do not become dampening. Furthermore, caution is required as contact resonance can occur due to the mounting method (such as adhesive). Small sensors often have low sensitivity, which can lead to a poor S/N ratio. It is desirable for charge sensitivity types to be above 1 pC/ms^-2 and for voltage sensitivity types to be above 1 mV/ms^-2.

It is currently the most advanced response sensor. The distance between the target and the sensor can be more than 100 mm, with a displacement accuracy of 10^-11 m, a speed accuracy of 10^-6 m/s, and a frequency reaching over 1 MHz. It covers most samples except in cases of complete reflection or complete transmission of the target. It is most convenient as a response sensor for the cantilever beam method, but can also serve as a replacement for the acceleration sensor in the impedance head of the central excitation method. A point to note when using it is that the measurement principle is based on the relative velocity of the sensor/target, so the sensor must be securely fixed to a solid base. When used in a constant temperature bath, care must be taken regarding the temperature limit. Rapid temperature changes can cause condensation, making measurement impossible. Another drawback of the laser vibrometer is its high price, and a reduction in cost is desired.