/kloc-At the beginning of the 9th century, a group of Napoleon soldiers, under the command of their commanders, crossed a bridge in Anger, France with majestic and neat steps. Near the middle of the bridge, the bridge suddenly shook strongly and eventually broke and collapsed, causing many officers and soldiers and citizens to fall into the water and die. After investigation, it was * * * that caused the tragedy! Because when the soldiers in the brigade went to March in a hurry, a frequency coincided with the natural frequency of the bridge, which strengthened the vibration of the bridge. When its amplitude reaches the maximum, until it exceeds the withstand voltage of the bridge, the bridge is broken. Similar incidents occurred in Russia and the United States. In view of this, the armies of many countries later stipulated that when a large group of people crossed the bridge, they should change to quick march.

For this bridge, what can break it is not only the heavy and neat steps of a large group of people, but also the seemingly empty wind can pose a threat to it. 1940, the 860m Tacom Bridge in America collapsed due to the * * * vibration caused by strong wind. Although the wind speed at that time was less than 1/3 of the design wind speed limit, the actual anti-vibration strength of the bridge was unqualified, which led to the accident. Tropical storms that ravage coastal areas every year also destroy houses and crops with the help of * * *. Because the wind not only produces a wind force along the wind direction, but also produces a lateral force on the structure in the wind area, and the vortex on the wind surface falls off under certain conditions, thus vibrating the structure. If the vibration frequency generated by the lateral force of the wind is the same as or close to the fixed frequency of the structure, wind load vibration will occur. In recent decades, the United States, Europe and other countries and regions have repeatedly experienced high-rise buildings shaking violently due to the * * * vibration caused by strong winds. [2]

* * * Ground vibration

When the helicopter is working (or taxiing) on the ground, it is subjected to external vibration, and the rotor blades are far away from the stable position, such as rotor retreat and shimmy. At this time, the center of gravity of the blade deviates from the center of rotation, and the centrifugal excitation force of the center of gravity of the rotor causes the vibration of the fuselage on the landing gear. The vibration of the fuselage feeds back to the shimmy motion of the rotor, supports the vibration excitation of the rotor, and forms a closed-loop system, which makes the shimmy motion of the rotor bigger and bigger. When the backward frequency of the rotor is equal to or close to the frequency of a certain fuselage on the landing gear, the resistance of the system is not enough to consume the energy of their mutual excitation. At this time, the vibration of the whole system will be unstable, and the vibration amplitude (amplitude) will become larger and larger until the helicopter is destroyed, that is, the ground appears.

Machine damage

When the machine tool is running, there will always be some asymmetry in the moving parts, which will exert periodic force on other parts of the machine tool and cause forced vibration of these parts. When the frequency of this force is close to or equal to the natural frequency of the machine tool, it will produce vibration, affect the machining accuracy, increase the fatigue damage of mechanical steel and increase the mechanical damage.

Infrasound vibration

What is particularly harmful to people is the vibration caused by infrasound waves. Infrasound is a kind of sound wave, which vibrates very little every second and can't be heard by human ears. The sound frequency of infrasound is very low, generally below 20 Hz, but the wavelength is very long and it is not easy to weaken. Natural solar magnetic storm, roaring waves, thunder and lightning, sudden change of air pressure, volcanic eruption; Military atomic bomb, hydrogen bomb explosion test, rocket launch, aircraft flight, etc. , can produce infrasound waves. Around our work, study and life, there are many low-power devices that can generate infrasound waves, such as blowers, induced draft fans, compressors, vacuum pumps, diesel engines, electric fans, vehicle engines and so on. This magical function of infrasound has also attracted great attention of military experts. Some countries have developed infrasound weapons, infrasound guns and bombs by using the nature of infrasound. Whether it is an infrasonic gun or an infrasonic bomb, the infrasonic wave with the frequency of 16- 17 Hz vibrates with some organs in the human body, causing the organs of the victim to deform, shift or bleed, thus achieving the purpose of killing the enemy. Modern scientific research has proved that a large number of infrasound waves with the frequency of 16- 17 Hz can cause unbearable trembling of human body, leading to visual impairment, disorientation, nausea and other symptoms, and even visceral injury or rupture, which can lead to death. This kind of infrasound weapon can be said to be a technical extreme for human beings to use vibration to harm themselves [3].

other

It is also because of the power of vibration that huge glaciers can be split by "gentle" waves. Even huge waves as high as several hundred meters often appear in Lee Du Ya Bay, Alaska, USA, because * * * vibration plays a great role in it. Because the * * * vibration of this bay is too severe, many sailors "stay away from it".

Earthquakes that have caused heavy casualties and property losses also have "ghost" vibration: when a plate in the local crust breaks, the generated wave frequency will be transmitted to the ground, which will cause strong * * * vibration with buildings, resulting in the tragedy of house destruction and death.

Constant sound at a certain frequency will break the glass. A loud shout on the mountain can cause the snow on the top of the mountain to shake in an instant, causing a big avalanche. If the rotation period of a moving car is just synchronized with the natural rhythm of the spring, then the vibration can make the car out of control, which will lead to the destruction of the car.

People will come into contact with various vibration sources in life and production, which may be harmful to human body. Scientific tests show that different parts of the human body have different natural frequencies, such as the natural frequency of the eyeball at most about 60 Hz and the natural frequency of the skull at most about 200 Hz. Looking at the human body as a whole, if the natural frequency in the horizontal direction is about 3-6 Hz, the natural frequency in the vertical direction is about 48 Hz. Therefore, operators who are very close to vibration sources, such as tractor drivers, pneumatic picks, shovels, chainsaws and trowels, should try their best to avoid the natural frequencies of these vibration sources and the vibration of relevant parts of the human body. Moreover, in order to ensure the safety and health of workers, the relevant departments have also made corresponding regulations, requiring that the frequency of all kinds of vibrating machinery operated by hand must be greater than 20 Hz.

(from Baidu Encyclopedia)