## 2 Mechanical Vibration Tests
### Standard Test Fixtures Fastening Methods  
The device under test must be fastened to the fixture and the fixture has to be fastened to the head expander or the slip table. Technically, this is not the best choice when it comes to preventing relative motion, but it is the best choice if the fixture has to be used for different DUTs.  
Head expanders and slip tables contain stainless steel threaded inserts, arranged according to a standard or client pattern--an example is shown in Figure 6. The choice of insert pattern depends on certain factors, such as the type of test object, its shape and size, the required attachment method, and the desired test conditions. Manufacturers of vibration test equipment often provide a range of options for inserts, allowing users to customize the setup based on their specific testing needs. Common grid patterns that are often used in vibration test equipment are in millimeters (25 \(\times\) 25, 50 \(\times\) 50, or 100 \(\times\) 100) or in inches (1'' \(\times\) 1'', 2'' \(\times\) 2'', or 4'' \(\times\) 4'').  
Figure 7 shows that the common thread sizes that are often used in vibration test equipment are the metric thread sizes M6, M8, M10, M12, and M16, and 1/4''-20, 3/8''-16, 1/2''-13, and 5/8''-11.  
Figure 6: Examples of the (a) head expander’s and (b) slip table’s insert pattern.  
Using M8 grade 10.9 bolts for attaching the fixture to a sliding table or head expander, achieving a clamping force of 24 kN for a single bolt is theoretically possible. Even assuming that the clamping force is reduced due to insert properties to 70% of the mentioned value, this still amounts to about 16.8 kN. For a particular fixture design, the number of clamping screws can vary. For example, for a fixture base of 10 \(\times\) 15 cm, using a 50 \(\times\) 50 mm insert spacing pattern, it is possible to use 10 bolts to fasten the fixture only around the perimeter or 12 bolts to also fasten it with the internal bolts of the insert pattern. In the first case, a total force of 240 kN is obtained, and in the second case, the value is 288 kN. For a fixture base area of 150 cm\({}^{2}\), a total average clamping force of either 1920 N/cm\({}^{2}\) or 1600 N/cm\({}^{2}\) is obtained. That force could be smaller if the fixture base were bigger and only the wedges on the perimeter were used.  
The disadvantage of bolted connections is their large number (sometimes dozens) and, thus, the time required to precisely tighten so many bolts, fastening the DUT to the testing stand. Based on the vibration test equipment supplier's experience in assembling these types of large specimens (1500 mm/2000 mm), the screw fasteners are placed every 100 mm on the perimeter of the specimen. Then, 70 bolts are used (2 \(\times\) 1500 + 2 \(\times\) 2000)/100). Assuming manual disassembly with an electric screwdriver (10 s for every bolt), the total disassembly time will be 700 s (almost 12 min). Manual assembly is more difficult, as it requires the use of a torque wrench (30 s for every bolt). This increases the assembly time to 35 min.  
In the case of complex fixture shapes and difficult access to the bolts, the necessary time might be even longer. While the need to disconnect the fixture at the end of the test only means extended downtime for the vibration testing equipment, in the case of an emergency situation, in which the device under test requires immediate removal from the head expander or slip table, this is a critical disadvantage that precludes the rapid evacuation of the device under test.