# Superinsulation Materials for Energy-Efficient Train Envelopes
## 3 Methods
### Vibration Testing  
Both silica aerogel and VIPs are materials designed for industrial or building applications. Hence, there is little knowledge about their behaviour under continuous vibrations,as they occur in trains. The main concern was that silica aerogel boards might be compacted over time or release excessive amounts of dust and that the envelope of the VIPs would be compromised under constant vibration.  
A test on the basis of the standard IEC 61373 \"Railway applications--Rolling stock equipment--Shock and vibration tests\" [40] was implemented. This standard distinguishes different material classes and different application points within the train. Thermal insulation is classified as Class A and Category 1 (body mounted). For long-life tests, the test specimen is subjected to a random vibration with a defined acceleration spectral density. The highest amplitude of the three directions (vertical, transverse and longitudinal) was selected with 0.532 (m/s\({}^{2}\))\({}^{2}\)/Hz for a mass of the material of less than 500 kg. The limiting frequencies for this mass range were set at f\({}_{1}\) = 5 Hz and f\({}_{2}\) = 150 Hz (cf. Figure 2). According to the standard, the test duration is 15 h. Since the goal was to determine the released dust in case of the silica aerogel material, a much longer test duration of at least 55 h was applied (58 h for the silica aerogel and 55 h for the VIP).  
The vibration tests were performed using an electromagnetic shaker. A random signal with constant RMS (root mean square) between 5 and 150 Hz was produced by means of a signal generator and used as input to the power amplifier. The magnitude of the amplifier output was adjusted manually to obtain the desired overall vibration level of the shaker. The resulting vibration was measured with an accelerometer, and the acceleration density spectrum was determined with the MATLAB function pwelch. As shown in Figure 2, the shaker produced a spectrum within the limits of the standard IEC 61373. Note that the acceleration spectrum is modified by the dynamic characteristic of the shaker and decays with higher frequencies.  
The mounting of the samples on the shaker is shown in Figure 3. The samples were glued onto an aluminium plate, which was connected to the shaker via a steel bracket. In the case of the silica aerogel board, a polyurethane-based liquid glue was used. For the VIPs, double-sided adhesive tape was employed. For the silica aerogel, the sample was furthermore covered with a bag, which was weighed before, during and after testing in order to determine the weight of the dust that was released due to the vibrations. Like that, the vibration test period for the aerogel was split into three sub-periods of roughly equal duration in order to study changes in the rate of dust release. After the testing, both aerogel and VIP samples were checked for their integrity. The sample sizes were 500 \(\times\) 500 \(\times\) 20 mm\({}^{3}\) for the aerogel and 496 \(\times\) 496 \(\times\) 19.5 mm\({}^{3}\) for the VIP.  
Figure 2: (**a**) Shape of acceleration spectral density according to IEC 61373. (**b**) Acceleration spectral density as measured in our setup with an RMS output voltage of the amplifier of 9.6 V.