Vibration damping materials and their performance measurement 3

In Japan, major standards include JIS K6394, JASO M306, and M329. Internationally, well-known standards include ASTM E756, BS AU125, DIN 53440, MIL P22581, SAE J671, and the international standard ISO 2856. More recently, the SAE standard was partially revised in the United States in 1992, and JIS G0602 for vibration-damping steel sheets was established in Japan in 1994. These standards propose or recommend various test methods for evaluating vibration-damping performance, depending on the differences in the viscoelastic material being tested and the size of the test specimens used. Among these, a representative test method is the cantilever beam method, which is adopted in ASTM, DIN, and JASO standards. It uses a long, narrow rectangular test specimen called a strip type, and basically measures the full width at half maximum to evaluate vibration-damping performance. Furthermore, there are methods for measuring vibration damping, such as the BS standard which uses a two-point support method where both ends of a strip-shaped test specimen are supported by knife edges; methods for measuring vibration damping by impact excitation or single-frequency excitation of a test panel, such as the MIL and SAE standards; and the impedance method, which is commonly used in Japan, where the center of the test specimen is subjected to sinusoidal or random excitation, and the impedance is measured at the drive point to evaluate the vibration damping performance. In addition, there are commercially available test devices that utilize the relaxation phenomenon exhibited by viscoelastic materials to determine vibration damping performance. However, the measurement frequency range of these devices is limited to around 100 Hz.

The cantilever method, central excitation method, two-point suspension method, and two-point support method are typical loss coefficient testing methods. The test specimens used in these methods are often two-layer (attached) or three-layer (sandwich) types. This section describes the general characteristics, test systems, error factors during testing, and loss coefficient measurement results for the cantilever method and central excitation method.

This test method is adopted by DIN, ASTM, JASO, and JIS standards, and is recommended by SAE.

Generally, the type of test specimen to use is determined by the difference in the Young's modulus of the vibration damping material. While it is not possible to uniquely determine the relationship between the Young's modulus of these vibration damping materials and the measurable frequency range of the test specimen, a general overview can be seen in the figure below.

Measuring the loss coefficient is a very delicate test, and even with carefully prepared test specimens, there are often slight differences in the loss coefficient and resonant frequency. This is thought to be due to subtle differences in the physical properties of the damping material and substrate used in the test, as well as errors in the production of the test specimens. Therefore, in order to avoid these errors as much as possible, it is effective to perform the test using about three test specimens and calculate the average value of the obtained loss coefficient and resonant frequency. The figure below shows the change in the loss coefficient over time. As shown in the figure, it can be seen that the loss coefficient is unstable immediately after the temperature is changed because the entire test specimen has not yet reached the measurement temperature. In the case of this damping material, it stabilizes in about 2 hours. The figure below shows the relationship between the loss coefficient and temperature, and the relationship between the loss coefficient and frequency, for a test specimen obtained by the cantilever beam method.

Non-contact electromagnetic vibrators are mainly used in cantilever beam construction, and their structure is as follows:

It is best to use relatively thick wire (around 0.1 mm Φ) and wind it a small number of times (around 1000 turns) for the coil. Winding it more than this will increase the DC resistance and reactance, making it pointless as it will prevent excitation at higher frequencies. When using a commercially available audio amplifier, it is a good idea to aim for a DC resistance of around 100 Ω when winding the coil. In this case, the reactance is expected to be around 20 mH. Also, the permanent magnet needs to be the maximum value within the AC magnetic field used by the electromagnetic exciter. When generating the maximum capacity of a non-contact electromagnetic exciter, the DC magnetic field may be insufficient. Conversely, when exciting a small, lightweight test specimen, the DC magnetic field may be too strong. In these cases, it is possible to use an electromagnet (winding another set of coils) instead of a permanent magnet and apply a DC voltage.

Structurally, it is exactly the same as a non-contact electromagnetic vibrator. The number of coil turns, wire material, etc., should also be exactly the same. In particular, when inputting this signal directly to an FFT analyzer, etc., without using an amplifier, a low output impedance is preferable, and as mentioned in the section on non-contact electromagnetic vibrators, winding many thin wires is pointless. As a sensor, it is easy to use because its output is velocity-proportional and it can measure up to high frequencies. However, it may occasionally interfere with a non-contact electromagnetic vibrator, so caution is needed when using them close together.

The operating principle is as follows:

The accuracy between the sensor and target is very good, at about 0.1%, but the ratio of the sensor diameter (target dimension) to the sensitive distance is about 10:1 (a 1 mm sensor dimension requires a target dimension of about 10 mm), which makes it difficult to use. Also, the target needs to be a ferroelectric material (metal, etc.). In addition, because it is fundamentally a displacement sensor, it has the disadvantage of being difficult to measure high frequencies. One advantage is that it does not interfere with electromagnetic vibrators.

Piezoelectric accelerometers, which utilize the piezoelectric effect, are widely used as Accelerometer Accelerometer. Currently, piezoelectric Accelerometer are necessary, especially when it is necessary to measure high frequencies. They are the easiest to use among various vibration sensors, but their weight is often a problem. The weight ratio of the test piece to Accelerometer should preferably be 100:1 or less when measuring the loss coefficient and Young's modulus. Care must also be taken in handling the lead wires, ensuring that the lead wires do not cause damping. Furthermore, contact resonance can occur depending on the mounting method (adhesion, etc.), so care must be taken. Small sensors often have low sensitivity, which can result in a poor signal-to-noise ratio. At least 1 pC/ms-2 or higher is desirable for charge-sensitive types, and 1 mV/ms-2 or higher for voltage-sensitive types.

This is currently the best response sensor available. The distance between the target and sensor can be over 100 mm, and it boasts displacement accuracy of 10⁻¹¹ m, velocity accuracy of 10⁻⁶ m/s, and a frequency of over 1 MHz. It covers most samples, except in cases where the target is either perfectly reflective or perfectly transmissive. While most convenient as a response sensor in the cantilever method, it can also replace Accelerometer in the impedance head of the central excitation method. A point to note when using it is that, since the measurement principle is the measurement of the relative velocity between the sensor and target, the sensor must be securely fixed to a sturdy base. When using it in a constant temperature bath, attention must be paid to the temperature limit, as rapid temperature changes can cause condensation and render measurements impossible. Another drawback of laser vibros is their high price; lower prices are desirable.